THE JOURNAL OF CHEMICAL PHYSICS 136, 144312 (2012)
The microwave and millimeter rotational spectra of the PCN radical (X̃3 − )
D. T. Halfen,1 M. Sun,1,a) D. J. Clouthier,2 and L. M. Ziurys1
1
Departments of Chemistry and Astronomy, Arizona Radio Observatory and Steward Observatory,
University of Arizona, Tucson AZ 85721, USA
2
Department of Chemistry, University of Kentucky, Lexington, Kentucky 40506, USA
(Received 18 January 2012; accepted 1 March 2012; published online 13 April 2012)
The pure rotational spectrum of the PCN radical (X̃3 − ) has been measured for the first time using a combination of millimeter/submillimeter direct absorption and Fourier transform microwave
(FTMW) spectroscopy. In the millimeter instrument, PCN was created by the reaction of phosphorus
vapor and cyanogen in the presence of an ac discharge. A pulsed dc discharge of a dilute mixture
of PCl3 vapor and cyanogen in argon was the synthetic method employed in the FTMW machine.
Twenty-seven rotational transitions of PCN and six of P13 CN in the ground vibrational state were
recorded from 19 to 415 GHz, all which exhibited fine structure arising from the two unpaired electrons in this radical. Phosphorus and nitrogen hyperfine splittings were also resolved in the FTMW
data. Rotational satellite lines from excited vibrational states with v2 = 1–3 and v1 = 1 were additionally measured in the submillimeter range. The data were analyzed with a Hund’s case (b)
effective Hamiltonian and rotational, fine structure, and hyperfine constants were determined. From
the rotational parameters of both carbon isotopologues, the geometry of PCN was established to be
linear, with a P–C single bond and a C–N triple bond, structurally comparable to other non-metal
main group heteroatom cyanides. Analysis of the hyperfine constants suggests that the two unpaired
electrons reside almost exclusively on the phosphorus atom in a π 2 configuration, with little interaction with the nitrogen nucleus. The fine structure splittings in the vibrational satellite lines differ
significantly from the pattern of the ground state, with the effect most noticeable with increasing v2
quantum number. These deviations likely result from spin-orbit vibronic perturbations from a nearby
1 +
state, suggested by the data to lie ∼12 000 cm−1 above the ground state. © 2012 American
Institute of Physics. [http://dx.doi.org/10.1063/1.3696893]
I. INTRODUCTION
Metal and non-metal cyanides have three possible geometries: linear cyanide, linear isocyanide, or T-shaped. For
example, the alkali metal cyanides NaCN and KCN exhibit
a T-shaped structure with a poly-topic bond between the
metal and the CN moiety.1, 2 The alkaline-earth metals and
aluminum prefer the linear isocyanide form, with MgNC,
CaNC, SrNC, BaNC, and AlNC being the lowest energy
structures,3–7 although the higher-lying isomers MgCN and
AlCN have also been observed in the laboratory.8 In contrast,
most non-metal main-group elements favor the linear cyanide
structure (M-CN). This geometry has been found for CCN,
NCN, OCN, FCN, SiCN, SCN, and ClCN.9–15 The degree of
covalent relative to ionic character of the bond between the
metal/non-metal atom and the cyanide moiety apparently determines the choice of geometry.16 The most ionic form is
the T-shaped arrangement, while the most covalent structure
is the cyanide isomer, with the isocyanide geometry being an
intermediate type.16
For the main-group element phosphorus, linear PCN and
PNC are the predicted structures. Several theoretical calculations have been performed on these species using SCF,
HF, DFT, MR-SCDI, CASSCF, and CCSD(T) methods.17–22
a) Present address: Department of Chemistry, University of Manitoba,
Winnipeg, Manitoba R3T2N2, Canada.
0021-9606/2012/136(14)/144312/12/$30.00
These studies indicate that the phosphorus cyanide form is
the lower energy isomer, as opposed to the isocyanide, with
a 3 − ground state. Bond lengths and vibrational frequencies
have also been calculated, as well as the dipole moment, reported to be 2.4–3.2 D.18, 21 The only experimental work on
these species was reported by Basco and Lee in 1968. These
authors detected PCN in the laboratory for the first time, observing an electronic band system tentatively assigned as a
3
-3 − transition.23 This assignment was supported by the
theoretical work of Cai and Xiao.19
Recently, a series of phosphorus-containing species have
been observed in the interstellar medium. For example, PN
has been identified in several molecular clouds,24, 25 and CP
has been discovered in circumstellar gas.26 In the past four
years, PN, HCP, PO, CCP, and PH3 have also been detected
in circumstellar envelopes of late-type AGB and supergiant
stars.27–31 Therefore, PCN would appear to be a viable candidate for further interstellar searches.
Because of the interest in PCN chemically and astrophysically, we have measured its pure rotational spectrum at
microwave, millimeter, and submillimeter wavelengths using
direct absorption and Fourier transform microwave (FTMW)
methods. Transition frequencies have been recorded for the
ground state and in several vibrationally excited states. From
these data, the spectroscopic constants and the molecular
structure of PCN have been determined. Interesting variations
in the fine structure as a function of vibrational state were also
136, 144312-1
© 2012 American Institute of Physics
144312-2
Halfen et al.
J. Chem. Phys. 136, 144312 (2012)
found. Here we present the data, its analysis, and interpretation of the bonding in this free radical.
II. EXPERIMENTAL
The pure rotational spectrum of PCN was measured using two instruments of the Ziurys group. For measurements
in the range 137–415 GHz, a millimeter/submillimeter direct
absorption spectrometer was employed.32 This system consists of a radiation source, a single-pass free space gas cell,
and a detector. The frequency source is a suite of Gunn oscillator/Schottky diode multiplier combinations that produce
radiation from 65 to 850 GHz. The molecular chamber is a 4
in. diameter glass cell chilled to −65 ◦ C with liquid methanol.
The detector is an InSb hot electron bolometer that is cooled
to 4 K with liquid helium. The radiation, modulated at 25 kHz,
is directed through the system to the detector by a series of
Teflon lenses, and is detected at 2f using a lock-in amplifier.
PCN was produced in the cell by the reaction of phosphorus vapor and cyanogen. Solid red phosphorus was heated
to ∼500 ◦ C by a heating mantle to create the vapor. About
10 mTorr of (CN)2 and 35 mTorr of Ar gas were then introduced into the chamber, and the mixture subjected to an
ac discharge, produced longitudinally by two ring electrodes
with an input power of 200 W at an inductance of 600 .
This discharge mixture exhibited a blue glow in the chamber.
In addition to the main isotopologue, P13 CN was observed in
natural abundance (12 C/13 C ∼ 90).
The final millimeter/submillimeter frequencies were
measured by averaging pairs of spectra in 5 MHz wide
scans, one increasing in frequency and another decreasing
in frequency. Usually 1–3 such pairs were necessary to
obtain good signal-to-noise ratios for the ground and first
excited state lines (v1 = 1 and v2 = 1), with 3–6 scan pairs
typically needed for higher-lying vibrational states and for
the P13 CN data. The absorption features were fitted with a
Gaussian-shaped line profile to determine the rest frequency
as well as the line width, which ranged from 600 to 1300
kHz across 137–415 GHz. The measurement accuracy is
estimated to be ±100 kHz.
Measurements in the range of 19–40 GHz for
PCN were conducted with a Balle-Flygare-type FTMW
spectrometer.33, 34 This instrument consists of a large vacuum
chamber with an unloaded pressure of ∼10−8 torr, achieved
using a cryopump. Inside the cell is a Fabry-Perot cavity consisting of two spherical mirrors; antennas are imbedded in
both mirrors for injecting and detecting radiation. A pulsedvalve nozzle, which lies at a 40◦ angle relative to the longitudinal axis, is used to create a supersonic jet expansion. The
nozzle contains a pulsed dc discharge source consisting of two
copper ring electrodes. Data is acquired at a nozzle pulse rate
of 10 Hz. The time domain signals are processed with an FFT
to create spectra with 3 kHz resolution. The emission features
appear as Doppler doublets with a full width at half maximum (FWHM) of 10 kHz per feature; the rest frequencies are
simply taken as the average of the two Doppler components.
More details can be found in Ref. 34.
For the FTMW experiments, PCl3 was used as the phosphorus precursor. PCN was created in a pulsed discharge from
a mixture of approximately 3% PCl3 and 1% (CN)2 in Ar (200
psi). The mixture was pulsed into the chamber with a backing
pressure of ∼10 psi with a mass flow of 20–30 sccm. A dc
discharge voltage of ∼1000 V at 50 mA was used.
III. RESULTS
The region between 360 and 397 GHz, a range of ∼6B,
was initially scanned for transitions of PCN. After this search,
a series of intense, harmonically-related triplet patterns were
identified in the data, as would be expected for a molecule
with a 3 − ground state. Chemical tests proved that these features were produced only in the presence of phosphorus vapor,
cyanogen gas, and the ac discharge. Hence, these lines were
assigned to PCN. The data contained additional triplets with
weaker intensities that were identified as vibrational satellite
lines of the heavy atom stretch (v1 = 1) and bending mode
(v2 = 1–3) of PCN, including v2 l-type components. Transitions of P13 CN were then searched for and found, based
on predicted frequencies scaled from the main isotopologue
data. Additional spectra were subsequently recorded covering
the range 137–415 GHz: a total of 24 rotational transitions
for the main isotopologue and six for the carbon-13 species.
Doublets arising from phosphorus hyperfine interactions
(I(31 P) = 1/2) were observed in one or more of the spin components up to the N = 24 ← 23 transition near 280 GHz; at
higher frequencies, the hyperfine splittings could not be resolved. The rest frequencies of PCN and P13 CN in the ground
vibrational state (000) are listed in Table I.
TABLE I. Transition frequencies of PCN and P13 CN (X̃3 − : v = 0).
P13 CN
PCN
N
J
F1 F
↔
N
J
F1 F
ν obs (MHz)
ν obs -ν calc (MHz)
1
2
2
2
2
2
2
2
2
2
1.5
1.5
1.5
1.5
2.5
2.5
2.5
2.5
2.5
1.5
0.5
1.5
2.5
2.5
1.5
1.5
2.5
3.5
→
→
→
→
→
→
→
→
→
2
1
1
1
1
1
1
1
1
1
0.5
0.5
0.5
0.5
1.5
1.5
1.5
1.5
1.5
1.5
0.5
0.5
1.5
2.5
0.5
1.5
1.5
2.5
19326.171
19328.700
19329.544
19332.112
19369.123
19371.401
19371.745
19372.998
19374.928
0.002
0.002
0.000
0.001
− 0.001
0.000
0.004
0.000
− 0.001
ν obs (MHz)
ν obs - ν calc (MHz)
144312-3
Halfen et al.
J. Chem. Phys. 136, 144312 (2012)
TABLE I. (Continued.)
P13 CN
PCN
N
J
F1 F
↔
N
J
F1 F
ν obs (MHz)
ν obs -ν calc (MHz)
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
11
11
12
12
13
13
12
12
13
13
14
14
13
13
14
14
15
15
14
14
15
15
16
16
15
15
16
16
17
17
1.5
1.5
1.5
1.5
2.5
2.5
2.5
2.5
2.5
2.5
2.5
2.5
3.5
3.5
3.5
3.5
3.5
2.5
2.5
3.5
3.5
3.5
3.5
3.5
3.5
4.5
4.5
4.5
11.5
10.5
11.5
12.5
13.5
12.5
12.5
11.5
12.5
13.5
14.5
13.5
13.5
12.5
13.5
14.5
15.5
14.5
14.5
13.5
14.5
15.5
16.5
15.5
15.5
14.5
15.5
16.5
17.5
16.5
1.5
0.5
2.5
1.5
2.5
1.5
3.5
2.5
1.5
1.5
2.5
3.5
3.5
2.5
2.5
3.5
4.5
2.5
3.5
3.5
2.5
4.5
2.5
3.5
4.5
3.5
4.5
5.5
→
→
→
→
→
→
→
→
→
→
→
→
→
→
→
→
→
→
→
→
→
→
→
→
→
→
→
→
←
←
←
←
←
←
←
←
←
←
←
←
←
←
←
←
←
←
←
←
←
←
←
←
←
←
←
←
←
←
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
10
10
11
11
12
12
11
11
12
12
13
13
12
12
13
13
14
14
13
13
14
14
15
15
14
14
15
15
16
16
0.5
0.5
0.5
0.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
2.5
2.5
2.5
2.5
2.5
1.5
1.5
2.5
2.5
2.5
2.5
2.5
2.5
3.5
3.5
3.5
10.5
9.5
10.5
11.5
12.5
11.5
11.5
10.5
11.5
12.5
13.5
12.5
12.5
11.5
12.5
13.5
14.5
13.5
13.5
12.5
13.5
14.5
15.5
14.5
14.5
13.5
14.5
15.5
16.5
15.5
0.5
0.5
1.5
1.5
1.5
0.5
2.5
2.5
1.5
0.5
1.5
2.5
3.5
2.5
1.5
2.5
3.5
1.5
2.5
2.5
1.5
3.5
1.5
2.5
3.5
2.5
3.5
4.5
23039.743
23041.927
23042.184
23043.513
23088.016
23089.387
23090.680
29332.686
29336.299
29337.146
29338.626
29340.037
29368.765
29371.983
29373.247
29374.572
29375.970
34603.808
34604.905
34622.017
34622.152
34622.999
39628.686
39629.837
39630.868
39657.607
39658.680
39659.735
137119.675
137123.898
138452.835
138453.948
139449.597
139445.867
148840.787
148844.268
149988.162
149989.071
150859.444
150856.354
160527.234
160530.062
161522.840
161523.622
162289.668
162287.071
172186.421
172188.783
173056.904
173057.541
173735.847
173733.627
183824.001
183825.961
184590.141
184590.687
185194.773
185192.839
0.001
− 0.001
0.003
0.000
0.000
0.001
− 0.002
0.001
− 0.001
0.000
0.000
− 0.001
− 0.001
− 0.005
0.002
0.001
0.001
− 0.006
− 0.005
0.000
− 0.002
0.001
− 0.002
− 0.001
− 0.001
0.000
0.001
0.001
0.037
− 0.037
− 0.038
0.065
0.035
0.017
0.019
0.015
− 0.019
0.030
0.006
0.012
0.021
− 0.004
− 0.015
0.027
0.027
0.036
− 0.007
− 0.001
0.052
0.044
− 0.004
− 0.011
0.001
0.000
0.012
− 0.008
0.024
− 0.017
3
12
13
14
15
16
b
b
b
b
b
b
b
b
b
b
b
b
b
b
b
b
b
b
b
b
b
b
b
b
b
b
b
b
b
b
2
11
12
13
14
15
b
b
b
b
b
b
b
b
b
b
b
b
b
b
b
b
b
b
b
b
b
b
b
b
b
b
b
b
b
b
ν obs (MHz)
ν obs - ν calc (MHz)
144312-4
Halfen et al.
J. Chem. Phys. 136, 144312 (2012)
TABLE I. (Continued.)
P13 CN
PCN
N
J
F1 F
↔
N
J
F1 F
18
17
17
18
18
19
19
18
18
19
19
20
20
19
19
20
20
21
21
20
20
21
21
22
22
21
21
22
22
23
23
22
22
23
23
24
24
23
23
24
24
25
25
24
24
25
25
26
26
25
25
26
26
27
27
26
26
27
27
28
28
17.5
16.5
17.5
18.5
19.5
18.5
18.5
17.5
18.5
19.5
20.5
19.5
19.5
18.5
19.5
20.5
21.5
20.5
20.5
19.5
20.5
21.5
22.5
21.5
21.5
20.5
21.5
22.5
23.5
22.5
22.5
21.5
22.5
23.5
24.5
23.5
23.5
22.5
23.5
24.5
25.5
24.5
24.5
23.5
24.5
25.5
25.5
26.5
25.5
24.5
25.5
26.5
26.5
27.5
26.5
25.5
26.5
27.5
27.5
28.5
b
←
←
←
←
←
←
←
←
←
←
←
←
←
←
←
←
←
←
←
←
←
←
←
←
←
←
←
←
←
←
←
←
←
←
←
←
←
←
←
←
←
←
←
←
←
←
←
←
←
←
←
←
←
←
←
←
←
←
←
←
17
16
16
17
17
18
18
17
17
18
18
19
19
18
18
19
19
20
20
19
19
20
20
21
21
20
20
21
21
22
22
21
21
22
22
23
23
22
22
23
23
24
24
23
23
24
24
25
25
24
24
25
25
26
26
25
25
26
26
27
27
16.5
15.5
16.5
17.5
18.5
17.5
17.5
16.5
17.5
18.5
19.5
18.5
18.5
17.5
18.5
19.5
20.5
19.5
19.5
18.5
19.5
20.5
21.5
20.5
20.5
19.5
20.5
21.5
22.5
21.5
21.5
20.5
21.5
22.5
23.5
22.5
22.5
21.5
22.5
23.5
24.5
23.5
23.5
22.5
23.5
24.5
24.5
25.5
24.5
23.5
24.5
25.5
25.5
26.5
25.5
24.5
25.5
26.5
26.5
27.5
b
19
20
21
22
23
24
25
26
27
b
b
b
b
b
b
b
b
b
b
b
b
b
b
b
b
b
b
b
b
b
b
b
b
b
b
b
b
b
b
b
b
b
b
b
b
b
b
b
b
b
b
b
b
b
b
b
b
b
b
b
b
b
b
b
b
b
b
b
18
19
20
21
22
23
24
25
26
b
b
b
b
b
b
b
b
b
b
b
b
b
b
b
b
b
b
b
b
b
b
b
b
b
b
b
b
b
b
b
b
b
b
b
b
b
b
b
b
b
b
b
b
b
b
b
b
b
b
b
b
b
b
b
b
b
b
b
ν obs (MHz)
ν obs -ν calc (MHz)
207050.051
207051.506
207654.590
207654.590
208140.864
208139.365
218644.302
218645.572
219185.406
219185.406
219624.491
219623.149
230228.756
230229.835
230715.330
230715.330
231113.284
231112.086
241804.905
241805.811
242244.289
242244.289
242606.228
242605.192
253374.039
253374.760
253772.297
253772.297
254102.488
254101.607
264937.011
264937.596
265299.201
265299.201
265601.444
265600.717
276494.722
276494.722
276825.087
276825.087
277102.367
277101.744
288047.401
288047.401
288349.788
288349.788
288604.521
288604.521
299595.823
299595.823
299873.309
299873.309
300108.205
300108.205
311140.258
311140.258
311395.587
311395.587
311612.629
311612.629
− 0.054
0.009
0.248
− 0.199
0.003
− 0.080
− 0.040
0.045
0.220
− 0.181
0.039
− 0.066
− 0.031
0.033
0.203
− 0.159
0.036
− 0.075
− 0.041
− 0.011
0.171
− 0.157
0.017
− 0.059
0.029
− 0.009
0.185
− 0.114
− 0.004
− 0.033
0.091
0.015
0.138
− 0.135
0.052
0.085
0.292
− 0.287
0.164
− 0.087
0.039
0.097
0.255
− 0.254
0.142
− 0.089
0.323
− 0.290
0.269
− 0.181
0.126
− 0.088
0.333
− 0.221
0.209
− 0.190
0.098
− 0.100
0.313
− 0.188
ν obs (MHz)
ν obs - ν calc (MHz)
144312-5
Halfen et al.
J. Chem. Phys. 136, 144312 (2012)
TABLE I. (Continued.)
P13 CN
PCN
N
J
F1 F
↔
N
J
F1 F
28
27
27
28
28
29
29
28
28
29
29
30
30
29
29
30
30
31
31
30
30
31
31
32
32
31
31
32
32
33
33
32
32
33
33
34
34
33
33
34
34
35
35
34
34
35
35
36
36
35
35
36
36
37
37
27.5
26.5
27.5
28.5
28.5
29.5
28.5
27.5
28.5
29.5
29.5
30.5
29.5
28.5
29.5
30.5
30.5
31.5
30.5
29.5
30.5
31.5
31.5
32.5
31.5
30.5
31.5
32.5
32.5
33.5
32.5
31.5
32.5
33.5
33.5
34.5
33.5
32.5
33.5
34.5
34.5
35.5
34.5
33.5
34.5
35.5
35.5
36.5
35.5
34.5
35.5
36.5
36.5
37.5
b
←
←
←
←
←
←
←
←
←
←
←
←
←
←
←
←
←
←
←
←
←
←
←
←
←
←
←
←
←
←
←
←
←
←
←
←
←
←
←
←
←
←
←
←
←
←
←
←
←
←
←
←
←
←
27
26
26
27
27
28
28
27
27
28
28
29
29
28
28
29
29
30
30
29
29
30
30
31
31
30
30
31
31
32
32
31
31
32
32
33
33
32
32
33
33
34
34
33
33
34
34
35
35
34
34
35
35
36
36
26.5
25.5
26.5
27.5
27.5
28.5
27.5
26.5
27.5
28.5
28.5
29.5
28.5
27.5
28.5
29.5
29.5
30.5
29.5
28.5
29.5
30.5
30.5
31.5
30.5
29.5
30.5
31.5
31.5
32.5
31.5
30.5
31.5
32.5
32.5
33.5
32.5
31.5
32.5
33.5
33.5
34.5
33.5
32.5
33.5
34.5
34.5
35.5
34.5
33.5
34.5
35.5
35.5
36.5
b
29
30
31
32
33
34
35
36
a
b
b
b
b
b
b
b
b
b
b
b
b
b
b
b
b
b
b
b
b
b
b
b
b
b
b
b
b
b
b
b
b
b
b
b
b
b
b
b
b
b
b
b
b
b
b
b
b
b
b
b
b
b
b
Blended line; not included in fit.
Hyperfine collapsed.
28
29
30
31
32
33
34
35
b
b
b
b
b
b
b
b
b
b
b
b
b
b
b
b
b
b
b
b
b
b
b
b
b
b
b
b
b
b
b
b
b
b
b
b
b
b
b
b
b
b
b
b
b
b
b
b
b
b
b
b
b
ν obs (MHz)
ν obs -ν calc (MHz)
ν obs (MHz)
ν obs - ν calc (MHz)
322681.126
322681.126
322916.619
322916.619
323117.478
323117.478
334218.563
334218.563
334436.328
334436.328
334622.597
334622.597
345753.142
345753.142
345954.641
345954.641
346127.601
346127.601
357284.607
357284.607
357471.551
357471.551
357632.396
357632.396
368813.260
368813.260
368986.970
368986.970
369136.808
369136.808
380339.247
380339.247
380500.890
380500.890
380640.633
380640.633
391862.595
391862.595
392013.230
392013.230
392143.686
392143.686
403383.403
403383.403
403523.968
403523.968
403645.906
403645.906
414901.738
414901.738
415033.058
415033.058
415147.141
415147.141
0.174
− 0.181
0.103
− 0.081
0.253
− 0.203
0.041
− 0.276
0.112
− 0.060
0.263
− 0.152
0.172
− 0.112
0.098
− 0.063
0.191
− 0.190
0.140
− 0.115
0.102
− 0.049
0.146
− 0.202
0.108
− 0.122
0.083
− 0.058
0.140
− 0.181
0.111
− 0.097
0.080
− 0.053
0.130
− 0.165
0.088
− 0.100
0.060
− 0.065
0.082
− 0.191
0.067
− 0.103
0.048
− 0.070
0.069
− 0.185
0.062
− 0.093
0.045
− 0.066
0.062
− 0.173
355566.181
355566.181
355754.619
355754.619
355916.802
355916.802
367039.619
367039.619
367214.728
367214.728
367365.777
367365.777
378510.310
378510.310
378673.299
378673.299
378814.206
378814.206
− 0.024
− 0.024
− 0.014
− 0.014
− 0.015
− 0.015
0.016
0.016
0.012
0.012
− 0.024
− 0.024
0.010
0.010
0.013
0.013
− 0.003
− 0.003
a
a
a
a
390261.965
390261.965
401443.926
401443.926
401585.713
401585.713
401708.717
401708.717
412906.987
412906.987
413039.437
413039.437
413154.513
413154.513
0.074
0.074
− 0.003
− 0.003
0.009
0.009
0.004
0.004
0.001
0.001
− 0.019
− 0.019
− 0.037
− 0.037
144312-6
Halfen et al.
In addition, the N = 1 → 2, 2 → 1 and 3 → 2 transitions were measured in the range 19-40 GHz with the FTMW
spectrometer. Here both phosphorus and nitrogen (I = 1) hyperfine splittings were resolved. A case bβJ coupling scheme
is appropriate in this case, such that F1 = J + I1 (P), F = F1
+ I2 (N). The phosphorus splitting is clearly larger, generating doublets separated by 20-50 MHz, which are then further
split by about 1–6 MHz by the nitrogen spin. Transitions with
F1 = ±1 and F = 0, ±1 were recorded. The microwave
data are also listed in Table I.
The millimeter/submillimeter transition frequencies for
the excited vibrational state satellite lines are given in
Table II. Six transitions, each consisting of three fine structure components, were measured for the v1 = 1 and v2 = 1–3
states. All expected l-type components, labeled c and d, where
levels with parity +(–1)J are called c and –(–1)J are d levels,
were observed for the v2 mode. The F1 fine structure components of the (022d 0) state, however, appear perturbed, and
that in the N = 33 ← 32, J = 34 ← 33 transition was sufficiently shifted that it could not be identified in the data. In
addition, one line of the (031c 0) state was blended with another unknown feature.
The vibrational pattern of PCN is illustrated in Figure 1
with a stick spectrum of the N = 33 ← 32 transition between
378 and 385 GHz, ignoring the fine and hyperfine structure.
The vibrational lines of the v2 state progress to higher frequency, and are further split by l-type doubling and l-type
resonance (also see Table II). The v1 = 1 transition appears
at lower frequency relative to the ground state. A transition of
P13 CN is also shown in the spectrum, illustrating its intensity
relative to the main isotopologue.
Representative spectra of PCN and P13 CN are given in
Figure 2. The three fine structure components of the N = 34
← 33 transition, labeled by J, are shown near 392 GHz (upper panel). The three components of the N = 33 ← 32 transition of P13 CN near 378 GHz is displayed in the lower panel.
P13 CN was observed in natural abundance.
A representative spectrum of the microwave data of PCN
is displayed in Figure 3. Here the nitrogen hyperfine components from F1 = 2.5 → 1.5 phosphorus doublet are displayed, arising from the J = 2 → 1 fine structure component in the N = 1 → 2 transition near 19 GHz. Several image
(lines from the other sideband, generated by the mixer) and
unknown lines, marked by asterisks, are also visible in the
data. The spectrum covers a range of 7 MHz and is comprised
of 24, 300 kHz wide scans. Each hyperfine component is split
into Doppler doublets, indicated by brackets, arising from the
orientation of the electromagnetic field supported by the cavity relative to the molecular expansion.
The fine structure pattern in the vibrational states of PCN
varies with v and l quantum numbers, as shown in Figure 4.
Here a stick spectrum of the N = 32 ← 31 transition is plotted
for all of the vibrational states observed in the study. The fine
structure pattern of the (000) and (100) states appear similar,
while in the (011c 0), (011d 0), and (020 0) states, the middle
component is shifted to higher frequency with respect to the
outer two lines. In the (022c 0), (022d 0), (031c 0), and (031d 0)
states, this effect is even more prominent, with the largest shift
occurring in the (033cd 0) state. The magnitude of this pertur-
J. Chem. Phys. 136, 144312 (2012)
bation appears to follow a v + l rule, where the states with the
same value for the sum of the vibrational and l-type quantum
numbers have a similar fine structure pattern.
IV. ANALYSIS
The spectra of PCN (v = 0–3) and P13 CN were analyzed
using the non-linear least squares routine SPFIT.35 The data
were fit to a case (b) effective Hamiltonian that included rotation, spin-rotation, spin-spin, magnetic hyperfine, electric
quadrupole, and l-type (v2 = 0) interactions:
Ĥeff = Ĥrot + Ĥsr + Ĥss + Ĥmhf + ĤeQq + Ĥl-type .
(1)
In the analysis, the lowest frequency spin component of the
triplet was labeled F3 (J = N − 1), while the highest one was
assumed to be F1 (J = N + 1). This assignment matches that
of HCCP (X̃3 − ), a related molecule with the same ground
state and similar geometry.36 Reversing these designations results in an unacceptable rms value of several 100 MHz. Analysis of the (011 0), (022 0), and (031 0) states also included p
and p terms, as well as the pD centrifugal distortion correction ((011 0) and (031 0) only), in order to account for the
parity dependence of the spin-rotation splitting of the l-type
doublets. (The parameter q is for l-type doubling itself.) The
p and p terms have the same functional form as the lambdadoubling constant p in or states.38–40 In addition, the oD
constant was introduced into the analysis of the (031 0) state,
which has the same form as the oD term in states. The o
parameter itself could not be determined, and was not used in
the fit.
Some difficulties were encountered in fitting the vibrationally excited satellite lines. All F1 (J = N + 1) components of the (022d 0) state appear to be shifted from their expected frequencies, with the perturbation increasing with N
up to the N = 33 ← 32, J = 34 ← 33 line, which could not be
identified in the data, as mentioned. After this transition, the
frequency shifts decrease with increasing N. These data could
not be fit without high residuals of 1–4 MHz, and thus were
not included in the analysis; see Table II. This effect is likely
due to a near chance degeneracy with states not observed in
the current data set. Higher-order spin-spin constants λH and
λL were needed for the v2 excited vibrational states as well,
indicative of other perturbations.
The spectroscopic constants determined for PCN and
P13 CN in the ground vibrational state are listed in Table III.
The final rms values were 31 kHz for PCN and 24 kHz
for P13 CN (v = 0). The parameters for the vibrationally excited states are given in Table IV. The rms values of the fits
for these states varied between 49 and 146 kHz. As seen in
Tables III and IV, the (100) state has similar rotational and
fine structure constants to the (000) state, with B, D, γ , and λ
being slightly smaller. The fine structure parameters are similar in the (011 0) state as well, although B is higher in magnitude. For the states with v2 ≥ 2, γ and λ increase significantly
relative to the ground state.
The constants of HCCP are also listed in Table III, for
comparison. The fine structure and phosphorus hyperfine parameters of PCN are similar to those of HCCP. For example, for PCN, γ = −27.986 MHz and bF = 155.413 MHz,
144312-7
Halfen et al.
TABLE II. Transition frequencies of vibrational states of PCN (X̃3 − )
(100)
N J ← N J
31 30
31
32
32 31
32
33
33 32
33
34
34 33
34
35
35 34
35
36
36 35
36
37
←
←
←
←
←
←
←
←
←
←
←
←
←
←
←
←
←
←
ν obs
(MHz)
30 29 356985.345
30 357168.172
31 357329.020
31 30 368508.734
31 368678.407
32 368828.456
32 31 380029.894
32 380187.639
33 380327.780
33 32 391548.951
33 391695.731
34 391826.823
34 33 403065.848
34 403202.662
35 403325.447
35 34 414580.893
35 414708.510
36 414823.584
(011c 0)
(011d 0)
(022c 0)
ν obs -ν calc
(MHz)
ν obs
(MHz)
ν obs -ν calc
(MHz)
ν obs
(MHz)
ν obs -ν calc
(MHz)
ν obs
(MHz)
ν obs -ν calc
(MHz)
−0.001
0.046
0.015
−0.011
−0.007
−0.023
−0.010
0.010
−0.018
0.026
−0.007
−0.005
−0.040
−0.045
−0.002
0.033
0.008
0.031
358261.793
358460.392
358602.007
369821.889
370006.007
370138.106
381379.203
381550.159
381673.572
392933.817
393092.808
393208.214
404485.825
404633.932
404741.886
416035.251
416173.291
416274.517
−0.016
0.100
−0.041
−0.042
0.066
−0.047
−0.042
0.026
−0.017
−0.031
0.003
0.002
0.007
0.037
−0.004
0.036
−0.053
0.016
358784.060
358984.852
359128.899
370361.183
370547.353
370681.675
381935.503
382108.371
382233.793
393507.079
393667.868
393785.201
405075.975
405225.779
405335.440
416642.290
416782.008
416884.290
−0.001
−0.102
−0.026
0.025
−0.062
−0.012
0.055
−0.040
0.027
0.053
−0.012
0.185
0.005
0.019
0.135
−0.049
0.018
−0.222
359741.853
359988.943
360087.664
371351.659
371579.635
371672.528
382958.425
383169.129
383256.563
394562.171
394757.350
394839.640
406163.058
406344.162
406421.656
417761.213
417929.549
418002.319
−0.006
0.087
−0.026
0.009
−0.075
0.041
0.024
−0.066
0.012
−0.028
0.029
−0.034
−0.049
0.056
−0.001
0.050
−0.030
0.009
(022d 0)
ν obs
(MHz)
ν obs -ν calc
(MHz)
(020 0)
ν obs
(MHz)
359745.032 −0.040 359862.251
359991.720 0.037 360060.321
a
360198.286
360091.191
371355.107 0.028 371468.879
371582.671 −0.082 371652.699
a
371781.116
371676.975
382962.117 0.060 383072.225
383172.432 −0.033 383243.112
b
383362.793
394566.126 0.031 394672.375
394760.905 0.074 394831.553
a
394943.064
394846.316
406167.130 −0.125 406269.297
406347.922 0.053 406417.812
a
406521.970
406427.027
417765.623 0.046 417863.399
417933.559 −0.049 418002.319
a
418099.240
418007.198
(031c 0)
(031d 0)
(033cd 0)
ν obs -ν calc
(MHz)
ν obs
(MHz)
ν obs -ν calc
(MHz)
ν obs
(MHz)
ν obs -ν calc
(MHz)
ν obs
(MHz)
ν obs -ν calc
(MHz)
−0.008
0.010
0.005
0.025
0.006
0.021
0.020
0.001
0.020
0.001
0.023
−0.066
−0.109
−0.108
−0.022
0.072
0.069
0.043
360670.908
360913.488
361007.071
372306.269
372530.387
372618.368
383938.445
384145.536
384228.253
0.027
0.111
−0.096
−0.066
−0.003
0.105
0.024
−0.126
−0.006
395759.230
395836.941
407192.685
407371.021
407444.049
418815.178
418980.951
419049.612
0.043
−0.002
−0.075
0.052
−0.059
0.087
−0.069
0.054
360693.671
360933.347
361029.852
372331.304
372552.220
372642.965
383965.459
384169.426
384254.836
395596.483
395785.246
395865.777
407224.454
407399.409
407475.273
418849.302
419011.670
419083.158
−0.049
0.000
0.018
0.099
−0.037
0.093
0.017
−0.120
−0.094
−0.027
0.036
−0.019
−0.008
0.150
0.004
−0.028
−0.037
0.001
360943.227
361270.487
361299.986
372595.511
372896.033
372925.707
384244.263
384520.912
384550.411
395889.673
396144.856
396173.973
407531.821
407767.774
407796.283
419170.926
419389.473
419417.209
−0.013
0.273
−0.051
0.067
−0.274
0.149
0.013
−0.197
0.044
−0.058
0.066
−0.139
−0.102
0.233
−0.168
0.093
−0.100
0.165
c
a
J. Chem. Phys. 136, 144312 (2012)
Perturbed; not included in fit, see text.
Not identified, see text.
c
Blended line; not included in fit.
b
Halfen et al.
144312-8
J. Chem. Phys. 136, 144312 (2012)
~
Stick Spectrum of PCN (X 3 ): N = 33
~
PCN (X 3 ): N = 1
32
(000)
J=2
1, F1 = 2.5
1.5
F = 3.5
F = 2.5
2
2.5
1.5
(0110)
F = 1.5
(0220)
P13CN
(100)
(000)
(0330)
(0200)
(0310)
F = 2.5
*
2.5
*
378
379
380
381
382
383
384
0.5
Image
*
Image
Image
385
Frequency (GHz)
19369.5
FIG. 1. Stick spectrum of the N = 33 ← 32 transition of PCN, showing the
pattern of the ground and excited vibrational states, neglecting fine structure
splittings. The vibrational progression of the v2 bending mode is shown to the
right of the ground state lines, with satellite features up to v2 = 3, and the v1
stretching mode is to the left. In addition, a line of P13 CN is displayed to show
the relative intensity of this isotopologue, observed in natural abundance.
~3
PCN (X
J = 33
32
N = 34
J = 34
33
33
J = 35
19371.5
19373.5
19375.5
Frequency (MHz)
FIG. 3. FTMW spectrum of the N = 1 → 2, J = 2 → 1, F1 = 2.5 →
1.5 transition of PCN near 19 GHz, showing multiple hyperfine components
generated by the nitrogen spin, and labeled by the F quantum number. (F1 indicates the larger, phosphorus splitting, which creates doublets.) The Doppler
components for each line are indicated by brackets. There are several image
lines, as well as unknown features in the spectrum, marked by asterisks. This
spectrum is composed of 24 scans, each 600 kHz wide, an average of 2000
pulses, displayed over a 7.2 MHz range.
34
while γ = −30.092 MHz and bF = 145.7 MHz for HCCP.
Hence, these two molecules apparently have very similar
electronic structures and nuclear environments. The value of
the nitrogen electric quadrupole coupling constant for PCN,
eQq = −4.6423 MHz, is also very similar to that of HCN
(−4.70903 MHz).37
V. DISCUSSION
391860
391960
13
~3
P CN (X
J = 32
392060
N = 33
32
31
J = 33
378510
392160
378610
32
378710
J = 34
33
378810
Frequency (MHz)
FIG. 2. Laboratory spectra of PCN and P13 CN observed in this work. In the
top panel, the N = 34 ← 33 transition of PCN near 392 GHz is displayed,
consisting of three distinct fine structure components, labeled by the J quantum number. The P13 CN spectrum of the N = 33 ← 32 transition near 378
GHz is shown in the lower panel, measured in natural abundance, also consisting of three fine structure lines. Each spectrum is a composite of four,
single, 110 MHz wide scans, each about 70 s in duration.
A. Structure and geometry of PCN
The r0 structure of PCN was determined by a nonlinear least-squares analysis to the rotational constants using
the STRFIT routine.41 The resulting structural parameters,
as well as those from theoretical calculations, are listed in
Table V. From the fit, the P–C bond length was established
to be 1.732(2) Å, while the C–N bond distance was found to
be 1.167(2) Å. The bond lengths determined from the spectra agree well (within 0.008 Å) with the theoretical values,
especially those predicted using the DFT/B3LYP method.20
Also listed in Table V are several other molecules with
P–C bonds and C–N bonds for comparison. As seen in the
table, molecules with single P–C bonds, such as H2 PCN and
H2 PCCCN, have lengths of 1.770–1.787 Å,42, 43 while double P–C bonds have distances between 1.6576 and 1.685 Å,
e.g., HCCP and H2 CP.36, 44 Triple P–C bonds have r(P–C)
∼ 1.5402–1.549 Å, as seen in HCP and NCCP.45, 46 Therefore, in PCN, the P–C bond (r0 = 1.732(2) Å) is principally
a single bond. Comparison with the CN radical suggests that
the C–N bond in PCN is a triple bond. The geometry of this
species is thus P–C≡N.
The rotation-vibration dependence of the PCN rotational
constants can be determined by fitting the experimental data
Halfen et al.
144312-9
J. Chem. Phys. 136, 144312 (2012)
~3
Stick Spectrum of PCN (X
): N = 32
TABLE III. Ground state spectroscopic constants of PCN, P13 CN, and
HCCP (X̃3 − ).a
31
v2 = 0
F13
Parameter
F22
368.8
368.9
369.0
368.6
368.7
v2 = 1
369.8
369.9
368.8
1c
370.0
v2 = 1
Theory
B
5769.45738(51)
5711b ,
5783c ,
5799d
D
γ
γD
λ
λD
bF (P)
c (P)
bF (N)
c (N)
eQq (N)
rms
0.00197253(34)
−27.986(33)
0.000088(16)
73783.49(45)
0.00556(60)
155.413(20)
−447.934(34)
5.6225(69)
−14.194(14)
−4.6423(91)
0.031
369.1
v1 = 1
368.5
PCN
F31
370.1
1d
P13 CN
HCCPe
5741.7571(53) 5623.11558(58)
0.0019684(23) 0.00153731(43)
−27.33(45)
−30.092(22)
73779(89)
0.0063(13)
63429.4(1.1)
0.02447(52)
145.7(1.5)
−418.9(5.9)
0.024
a
370.4
370.5
v2 = 2
371.5
371.4
370.7
371.7
371.8
In MHz; errors are 3σ in the last quoted decimal places.
Be from Ref. 20.
c
Be from Ref. 18.
d
Be from Ref. 21.
e
Ref. 36.
b
0
371.6
v2 = 2
370.6
stretching mode α 1 is small and positive, and does not appear
to be affected by Fermi resonance with the v2 = 2 state.49
The vibrational frequencies for the heavy-atom stretch
and the bending mode of PCN can be estimated from its derived spectroscopic constants. Using the Kratzer relation,
4B3
ω1 ∼
,
(3)
D
2c
371.5
371.6
371.7
v2 = 22d
371.4
371.5
371.6
v2 = 3
372.3
372.4
372.5
v2 = 3
372.3
372.4
372.5
372.7
the frequency of the stretching mode was calculated to be ω1
∼ 658 cm−1 , assuming the CN moiety is a single unit. This
number compares well to theoretical values of this quantity
(635–681 cm−1 ), see Table VI, and is very similar to that predicted using DFT/B3LYP methods (649 cm−1 ). The bending
frequency can be determined from the l-type doubling term q
via Neilsen’s formula:49
ξ 2 ω2
2B2e
2i 2
q∼
1+4
.
(4)
2
ω2
ω
− ω22
i
i
372.6
1d
v2 = 3
372.6
371.7
1c
372.6
3cd
372.8
372.9
Frequency (GHz)
FIG. 4. Stick spectrum of the N = 32 ← 31 transition of PCN in all of the
vibrational modes observed in the study. Each spectrum is 400 MHz wide.
The (000) and (100) states have similar fine structure splittings, while those
of the (011c 0), (011d 0), and (020 0) levels are comparable. The F2 component
dramatically shifts to higher frequency for the (022c 0), (022d 0), and (031 0)
states, with the pattern for (033cd 0) level the most perturbed. The data are
suggestive of spin-vibronic couplings with the nearby 1 + state.
to the power series
48
Bv = B̃e − α1 (v1 + 1/2) − α2 (v2 + 1).
(2)
Here B̃e = Be −α 3 /2. The derived constants are B̃e
= 5752.380(58) MHz, α 1 = 5.980(21) MHz, and α 2
= −20.067(21) MHz. The value for α 2 , the rotation-vibration
constant of the bending mode, is negative, as is typically
found for linear triatomic species.49 The parameter for the
The summation (x4) is the Coriolis term that is usually around
0.1–0.3 for most small molecules.49, 50 Assuming these values
for this term, ω2 frequency falls in the range 290–343 cm−1 ,
in good agreement with the predicted values (312–333 cm−1 );
see Table VI.
B. Fine structure interactions and vibronic effects
As described previously (Figure 4), the fine structure pattern varies significantly with v2 quantum number, with the F2
(J = N) component apparently shifted relative to the F1 and
F3 lines. The shifts in the fine structure pattern are reflected
in the γ and λ constants of these vibrational states, as shown
in Table IV. The value of γ increases as a function of vibrational level, from about −27 MHz in the (100) and (011 0)
states to −176 MHz in the (033 0) level. The spin-spin constant λ also increases with v2 quantum number, from ∼73 000
MHz to 91 600 MHz. Furthermore, the γ and λ parameters of
144312-10
Halfen et al.
J. Chem. Phys. 136, 144312 (2012)
TABLE IV. Spectroscopic constants for vibrational states of PCN (X̃3 − )a .
(100)
(011 0)
(020 0)
(022 0)
(031 0)
(033 0)
5763.4775(53)
0.0013969(23)
− 26.85(44)
5789.5887(38)
0.0020344(16)
− 27.49(32)
5812.311(11)
0.0026389(34)
−43(11)
0.0016(16)
74800(1100)
1.18(13)
− 0.00070(11)
1.60(33) × 10−7
5809.6598(89)
0.0020870(28)
− 70.5(8.1)
0.0042(12)
78570(800)
3.202(99)
− 0.001897(86)
4.19(25) × 10−7
5825.4795(81)
0.0024829(25)
− 69.8(7.9)
0.0042(12)
77820(780)
3.104(89)
− 0.001824(78)
3.99(23) × 10−7
− 0.0300(11)
0.69(60)
− 0.00023(18)
− 0.0532(75)
− 0.0001554(33)
5829.657(11)
0.0021483(34)
−176(12)
0.0145(17)
91600(1100)
9.52(12)
− 0.00562(11)
1.233(32) × 10−6
Parameter
B
D
γ
γD
λ
λD
λH
λL
oD
p
pD
q
qD
p
rms
a
73203(89)
− 0.0246(12)
73126(63)
0.0275(78)
− 5.2(3.4) × 10−6
5.37(60)
− 0.00095(18)
− 8.4802(75)
0.0000113(33)
0.024
0.066
− 0.0159(92)
− 0.0000155(40)
− 0.38(12)
0.051
0.049
0.067
0.146
In MHz; errors are 3σ in the last quoted decimal places.
the (022 0) and (031 0) levels are very similar in value, as reflected in their fine structure patterns seen in Figure 4. These
effects are most likely due to spin-orbit vibronic coupling, as
explained in more detail below.51 Similar perturbations have
been seen in the bending mode of several vibrational states of
CrCN.40 Mishra et al. in fact investigated these interactions in
the A3 state of PNC, but not in the ground state of PCN.52
In heavy molecules, the major contribution to the spinspin parameter λ is second-order spin-orbit coupling, resulting from perturbations with nearby excited states. The pure
microscopic electron spin-spin interactions in contrast, is only
a small effect, such that λ ≈ λso .53, 54 The second-order spinorbit contribution can be estimated based on the following
equation:55
λso =
−30(2S − 2)!
(2S + 3)!
×
[3 2 − S(S + 1)] n , , Ĥso |n,,2
En − En
n , , Here the quantum numbers n , , and sum over nearby
perturbing states. The selection rules for spin-orbit coupling
are S = 0, ±1, = 0, = − = 0, ∓1, and ±
↔ ∓ .53 Therefore, the excited states of PCN that can interact with the ground 3 − state through this coupling are 1 +
and 3 , as well as 5 , 5 + , 3 + , and 1 .
The 3 − ground state has an electron configuration of
(core) 2π 4 9σ 2 3π 2 or (core) 2π 4 9σ 2 (3π + α3π − α).19 Of the
possible perturbing states, the 1 + state is predicted to lie
lowest in energy, with a proposed configuration of (core)
2π 4 9σ 2 (3π + α3π − β).19 Also, the 1 + state would perturb
only the = 0 fine structure component (F2 ), as observed.
(1 would affect the = 1 sub-level, for example.) Therefore, assuming that the 1 + state is the primary perturber,
Eq. (3) becomes
λso =
−30(2S − 2)!
(2S + 3)!
.
×
[3 2 − S(S + 1)]|1 + |Ĥso |X3 − |2
(5)
E(1 ) − E(X̃3 − )
,
(6)
TABLE V. Structural parameters of PCN and related molecules.
Molecule
r(CP) (Å)
r(CN) (Å)
PCN
1.732(2)
1.7237
1.167(2)
1.1697
1.724
1.174
1.7403
1.1744
HCP
NCCP
HCCP
H2 CP
H2 PCN
H2 PCCCN
C≡N
1.5402
1.549(3)
1.685
1.6576(28)
1.787(1)
1.770
1.1631(8)
1.1577(1)
1.161
1.172
Method
λso =
Ref.
r0
This work
20
re , ab initio
B3LYP/cc-pVTZ
18
re , ab initio
UHF/6-31G*
21
re , ab initio
CCSD(T)/aug-cc-pVQZ
45
re , MW
46
r0
36
r0
r0 , MMW
44
r0
42
r0
43
47
re
1 |1 0+ |Ĥso |X3 0− |2
.
2 E(1 ) − E(X̃3 − )
(7)
TABLE VI. Vibrational frequencies of PCN.a
ω1
∼658
ω2
ω3
290–343
649
333
2037
681
332
1771
635
312
2046
a
In cm−1 .
Method
Ref.
Kratzer relation or
Neilsen’s formula
re , ab initio
B3LYP/cc-pVTZ
re , ab initio
UHF/6-31G*
re , ab initio
CCSD(T)/aug-ccpVQZ
This work (see text)
20
18
21
144312-11
Halfen et al.
J. Chem. Phys. 136, 144312 (2012)
Here only the = = 0 component of the 3 − state connects with the 1 0+ term. The Slater determinants of the X̃3 0−
and 1 0+ states are
1
|X̃3 0− = √ [|π + απ − β| + |π + βπ − α|],
2
(8)
1
|1 0+ = √ [|π + απ − β| − |π + βπ − α|].
2
(9)
The matrix element therefore reduces to:
1
0+ |Ĥso |X̃3 0− − π + βπ − α|a3p lz sz |π + βπ − α]
(10)
Assuming that a3p is the atomic spin-orbit constant of phosphorus, 275.2 cm−1 ,51 Eq. (5) becomes
2
a3p
1
λ =
.
2 E(1 + ) − E(X̃3 − )
Ground State
1 A
2+
1+
2
r
3−
2
i
1+
r(XC) (Å)
r(CN) (Å)
Ref.
2.379(15)
2.064(1)
2.005(1)
1.844(1)
1.732(2)
1.6301(14)
1.627(1)
1.170(4)
1.166b
1.166b
1.166b
1.167(2)
1.1831(18)
1.166(1)
1
8
7
15
This work
12
14
All structures calculated are r0 .
Held fixed.
unpaired electrons. These results are additional evidence for
localization of the unpaired electrons on the phosphorus atom.
1
[π + απ − β|a3p lz sz |π + απ − β
2
so
Na(CN)
MgCN
AlCN
SiCN
PCN
SCN
ClCN
b
− π + βπ − α|Ĥso |π + βπ − α]
= a3p .
Molecule
a
1
= [π + απ − β|Ĥso |π + απ − β
2
=
TABLE VII. Structural parameters of second row cyanide species.a
(11)
Using the value of λ of 91 600 MHz for the (033 0) state for
λso , the energy of the 1 + state is estimated to be ∼12 400
cm−1 . Cai and Xiao propose that this state lies higher than
∼8300 cm−1 ,19 which is consistent with this calculated value.
C. Hyperfine interactions
The Fermi contact and dipolar constants were determined
for both the phosphorus and nitrogen nuclei in PCN, providing insight into the bonding of this radical. The phosphorus
Fermi contact term bF = 155.413 MHz in PCN, compared
to the atomic value of 13 306 MHz,56 indicates that the unpaired electrons on this nucleus have only ∼1.2% s character. The dipolar parameter c = 447.9 MHz is approximately
three times larger than the bF constant, indicating that most
of the electron density arises from orbitals with non-spherical
(i.e., p) character. The hyperfine constants thus support the
proposed electron configuration, where the two unpaired electrons are located in the 3π orbital, with mostly phosphorus
3pπ character.21
The Fermi contact term for the nitrogen nucleus is 5.6225
MHz, while that for the free atom is 1811 MHz.57 Also, the
value of the nitrogen dipolar constant is c = −14.194 MHz,
slightly larger than the nitrogen bF term, but smaller than the
P hyperfine constant by a factor of ∼32. Note that the nuclear magnetic moments for the P and N nuclei differ by approximately three, however (1.132 μN for P and 0.404 μN
for N). Nonetheless, there appears to be little unpaired electron density on the nitrogen nucleus. In addition, the electric quadrupole coupling constant eQq of the nitrogen nucleus in PCN (−4.6423 MHz) is very similar to that of HCN
(−4.70903 MHz).37 Hence, the environment around the N nucleus appears to resemble a closed-shell species, not one with
D. Bonding in main group cyanides/isocyanides
The structures for all of the second-row main group
cyanides are listed in Table VII. The geometry of these species
progresses from the T-shaped NaCN to the linear cyanides
from magnesium to chlorine.1, 7, 8, 12, 14, 15 The trend in the XC bond lengths, where X is the second-row element, shows
a steady decrease from Na to Cl of 2.379 Å to 1.627 Å.
This trend curiously follows the decrease in atomic radii from
sodium to chlorine, which indicates that there is a large covalent component in the bonding of these species. Furthermore,
where structural information has been experimentally determined, the CN bond distance ranges from 1.166 to 1.183 Å
in these species.12, 14 The C–N bond lengths in CN and HCN
are 1.172 Å and 1.153 Å, respectively.47, 58 These results suggest that across the second row, a CN triple bond is favored
independent of the heteroatom.
VI. CONCLUSIONS
The pure rotational spectrum of the PCN radical has
been measured both in the ground and several vibrationally
excited states, and spectroscopic parameters determined
for the first time. The geometry of PCN was confirmed as
the linear cyanide structure, with a P–C single bond and a
C–N triple bond. Significant perturbations were found in the
satellite lines of the v2 bending mode, in particular for the F2
(J = N) fine structure component, although there are other
local effects. Spin-orbit vibronic coupling between the X̃3 −
state and a nearby 1 + excited state is likely occurring. The
spin-spin constant in the (033 0) state suggests the 1 + state
lies ∼12 400 cm−1 above the ground state. The hyperfine
constants for both P and N in PCN indicate that most of the
unpaired electron density resides on the phosphorus atom in
π orbitals. The structure and electronic properties of PCN
were found to resemble those of HCCP, which suggests that
phosphorus bonds similarly to the CN and CCH moieties.
ACKNOWLEDGMENTS
The work here is supported by the NSF (Grant
No. AST 09-06534) and NASA Exobiology (Grant No.
144312-12
Halfen et al.
NNX10AR83G). D.J.C. also acknowledges NSF support
(Grant No. CHE-1106338).
1 J.
J. Van Vaals, W. L. Meerts, and A. Dymanus, Chem. Phys. 86, 147
(1984).
2 T. Törring, J. P. Bekooy, W. L. Meerts, J. Hoeft, E. Tiemann, and A.
Dymanus, J. Chem. Phys. 73, 4875 (1980).
3 K. Kawaguchi, E. Kagi, T. Hirano, S. Takano, and S. Saito, Astrophys. J.
406, L39 (1993).
4 T. C. Steimle, S. Saito, and S. Takano, Astrophys. J. 410, L49 (1993).
5 M. Douay and P. F. Bernath, Chem. Phys. Lett. 174, 230 (1990).
6 V. Milhailov, M. D. Wheeler, and A. M. Ellis, J. Phys. Chem. A 107, 4367
(2003).
7 K. A. Walker and M. C. L. Gerry, Chem. Phys. Lett. 278, 9 (2000).
8 M. A. Anderson, T. C. Steimle, and L. M. Ziurys, Astrophys. J. 429, L41
(1994).
9 Y. Ohshima and E. Endo, J. Mol. Spectrosc. 172, 225 (1995).
10 S. A. Beaton, Y. Ito, and J. M. Brown, J. Mol. Spectrosc. 178, 99 (1996).
11 S. Saito and T. Amano, J. Mol. Spectrosc. 34, 383 (1970).
12 A. Maeda, H. Habara, and T. Amano, Mol. Phys. 105, 477 (2007).
13 M. Bogey, A. Farkhsi, F. Remy, I. Dubois, H. Bredohl, and A. Fayt, J. Mol.
Spectrosc. 170, 417 (1995).
14 W. J. Lafferty, D. R. Lide, and R. A. Toth, J. Chem. Phys. 43, 2063 (1965).
15 A. J. Apponi, M. C. McCarthy, C. A. Gottlieb, and T. Thaddeus, Astrophys.
J. 536, L55 (2000).
16 V. M. Rayón, P. Redondo, H. Valdés, C. Barrientos, and A Largo, J. Phys.
Chem. A 111, 6334 (2007).
17 C. Thomson, Int. J. Quant. Chem. 10, 85 (1976).
18 A. Largo and C. Barrientos, J. Phys. Chem. 95, 9864 (1991).
19 Z.-L. Cai and H.-M. Xiao, J. Mol. Struct.: THEOCHEM 279, 267 (1993).
20 J. El-Yazal, J. M. L. Martin, and J.-P. François, J. Phys. Chem. A 101, 8319
(1997).
21 S. Wang, Y. Yamaguchi, and H. F. Schaefer III, J. Theor. Comput. Chem.
5, 281 (2006).
22 N.-N. Pham-Tran, X. J. Hou, and M. T. Nguyen, J. Phys. Org. Chem. 19,
167 (2006).
23 N. Basco and K. K. Lee, Chem. Commun. 152 (1968).
24 L. M. Ziurys, Astrophys. J. 321, L81 (1987).
25 B. E. Turner and J. Bally, Astrophys. J. 321, L75 (1987).
26 M. Guélin, J. Cernicharo, G. Paubert, and B. E. Turner, Astro. Astrophys.
230, L9 (1990), http://adsabs.harvard.edu/abs/1990A%26A...230L...9G.
27 M. Agúndez, J. Cernicharo, and M. Guélin, Astrophys. J. 662, L91
(2007).
28 S. N. Milam, D. T. Halfen, E. D. Tenenbaum, A. J. Apponi, N. J. Woolf,
and L. M. Ziurys, Astrophys. J. 684, 618 (2008).
J. Chem. Phys. 136, 144312 (2012)
29 E.
D. Tenenbaum, N. J. Woolf, and L. M. Ziurys, Astrophys. J. 666, L29
(2007).
30 E. D. Tenenbaum and L. M. Ziurys, Astrophys. J. 680, L121 (2008).
31 D. T. Halfen, D. J. Clouthier, and L. M. Ziurys, Astrophys. J. 677, L101
(2008).
32 C. Savage and L. M. Ziurys, Rev. Sci. Instrum. 76, 043106 (2005).
33 T. J. Balle and W. H. Flygare, Rev. Sci. Instrum. 52, 33 (1981).
34 M. Sun, A. J. Apponi, and L. M. Ziurys, J. Chem. Phys. 130, 034309
(2009).
35 H. M. Pickett, J. Mol. Spectrosc. 148, 371 (1991).
36 I. K. Ahmad, H. Ozeki, and S. Saito, J. Chem. Phys. 107, 1301 (1997).
37 V. Ahrens, F. Lewen, S. Takano, G. Winnewisser, Š. Urban, A. A. Negirev,
and A. N. Koroliev, Z. Naturforsch. 57a, 669 (2002).
38 A. J. Apponi, M. A. Anderson, and L. M. Ziurys, J. Chem. Phys. 111, 10919
(1999).
39 M. A. Brewster and L. M. Ziurys, J. Chem. Phys. 117, 4853 (2002).
40 M. A. Flory, R. W. Field, and L. M. Ziurys, Mol. Phys. 105, 585 (2007).
41 Z. Kisiel, J. Mol. Spectrosc. 218, 58 (2003).
42 L. Kang and S. E. Novick, J. Mol. Spectrosc. 225, 66 (2004).
43 L. Kang, A. J. Minei, and S. E. Novick, J. Mol. Spectrosc. 240, 255 (2006).
44 S. Saito and S. Yamamoto, J. Chem. Phys. 111, 7916 (1999).
45 P. Dréan, J. Demaison, L. Poteau, and J.-M. Denis, J. Mol. Spectosc. 176,
139 (1996).
46 L. Bizzochi, C. D. Esposti, and P. Botschwina, J. Chem. Phys. 113, 1465
(2000).
47 D. R. Lide, Jr and C. Matsumura, J. Chem. Phys. 50, 3080 (1968).
48 C. H. Townes and A. L. Schawlow, Microwave Spectroscopy (Dover, New
York, 1975).
49 D. A. Fletcher, M. A. Anderson, W. L. Barclay, Jr., and L. M. Ziurys, J.
Chem. Phys. 102, 4334 (1995).
50 S. Mishra, W. Domcke, and L. V. Poluyanov, Chem. Phys. 327, 457 (2006).
51 S. Mishra, W. Domcke, and L. V. Poluyanov, Chem. Phys. Lett. 446, 256
(2007).
52 H. Lefebvre-Brion and R. W. Field, The Spectra and Dynamics of Diatomic
Molecules (Elsevier, Amsterdam, 2004).
53 J. M. Brown and A. Carrington, Rotational Spectroscopy of Diatomic
Molecules (Cambridge University Press, Cambridge, 2003).
54 J. M. Brown, E. A. Colbourn, J. K. G. Watson, and F. D. Wayne, J. Mol.
Spectrosc. 74, 294 (1979).
55 E. A. Colbourn and F. D. Wayne, Mol. Phys. 37, 1755 (1979).
56 J. R. Morton and K. F. Preston, J. Magn. Reson. 30, 577 (1978).
57 K. P. Huber and G. Herzberg, Molecular Spectra and Molecular Structure.
IV. Constants of Diatomic Molecules (Van Nostrand Reinhold, New York,
1979).
58 R. C. Woods and R. J. Saykally, Philos. Trans. R. Soc. London, Ser. A 324,
141 (1988).
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