The microwave and millimeter spectrum of ZnCCH (2Σ+): A new zinccontaining free radical
J. Min, D. T. Halfen, M. Sun, B. Harris, and L. M. Ziurys
Citation: J. Chem. Phys. 136, 244310 (2012); doi: 10.1063/1.4729943
View online: http://dx.doi.org/10.1063/1.4729943
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THE JOURNAL OF CHEMICAL PHYSICS 136, 244310 (2012)
The microwave and millimeter spectrum of ZnCCH (X̃ 2 + ): A new
zinc-containing free radical
J. Min (), D. T. Halfen, M. Sun,a) B. Harris,b) and L. M. Ziurysc)
Department of Chemistry and Biochemistry, Department of Astronomy and Steward Observatory,
933 North Cherry Avenue, University of Arizona, Tucson, Arizona 85721, USA
(Received 12 April 2012; accepted 31 May 2012; published online 28 June 2012)
The pure rotational spectrum of the ZnCCH (X̃2 + ) radical has been measured using Fourier transform microwave (FTMW) and millimeter direct-absorption methods in the frequency range of 7–
260 GHz. This work is the first study of ZnCCH by any type of spectroscopic technique. In the
FTMW system, the radical was synthesized in a mixture of zinc vapor and 0.05% acetylene in argon,
using a discharge assisted laser ablation source. In the millimeter-wave spectrometer, the molecule
was created from the reaction of zinc vapor, produced in a Broida-type oven, with pure acetylene
in a dc discharge. Thirteen rotational transitions were recorded for the main species, 64 ZnCCH, and
between 4 and 10 for the 66 ZnCCH, 68 ZnCCH, 64 ZnCCD, and 64 Zn13 C13 CH isotopologues. The fine
structure doublets were observed in all the data, and in the FTMW spectra, hydrogen, deuterium,
and carbon-13 hyperfine splittings were resolved. The data have been analyzed with a 2 Hamiltonian, and rotational, spin-rotation, and H, D, and 13 C hyperfine parameters have been established for
this radical. From the rotational constants, an rm (1) structure was determined with rZn-C = 1.9083 Å,
rC-C = 1.2313 Å, and rC-H = 1.0508 Å. The geometry suggests that ZnCCH is primarily a covalent
species with the zinc atom singly bonded to the C≡C—H moiety. This result is consistent with the
hyperfine parameters, which suggest that the unpaired electron is localized on the zinc nucleus. The
spin-rotation constant indicates that an excited 2 state may exist ∼19 000 cm−1 in energy above the
ground state. © 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4729943]
I. INTRODUCTION
Zinc is one of the more relevant metals for organic chemistry and biochemistry. It is the second most abundant transition metal in living organisms, playing an important role
in many biological processes.1 The element is a key component of “zinc fingers”, for example, which are small protein
domains that are involved in a wide range of cellular functions, including replication, transcription, and metabolism.2
The “fingers” are structurally diverse, with one common
form resembling acetylene. Artificial zinc fingers have in
fact been used for therapeutic gene editing.3 Zinc is widely
used in organic synthesis, as well, as in the Barbier-Grignard
process. Here zinc is employed as the magnesium substitute, enabling the alkylation to be conducted in an aqueous
medium.4 Furthermore, zinc acetylide in situ formation is a
key step in the addition reaction of terminal acetylenes to
aldehydes.5, 6
The interaction of zinc with carbon is clearly fundamental, with many chemical and biological applications. One avenue by which theorists have investigated this interaction is
through computational studies of small species with Zn-C
bonds. The structure and properties of the 3d-metal monocarbides MC (M = Sc-Zn) and their negatively and positively
a) Present address: Department of Chemistry and Biochemistry, University of
Arizona, 1306 E University Blvd, Tucson, Arizona 85721, USA.
b) Present address: Department of Chemistry, University of Virginia, Char-
lottesville, Virginia 22904, USA.
c) Author to whom correspondence should be addressed. Electronic mail:
[email protected]. Fax 520-621-1532.
0021-9606/2012/136(24)/244310/10/$30.00
charged ions, for example, have been pursued using density functional theory (DFT).7 Further studies have been conducted on ZnC by Tsouloucha et al. in 2003, who employed
multi-reference variational methods coupled with large basis sets (Roos-ANO-TZ/aug-cc-pVQZ).8 In 2007, Barrientos
et al. investigated small ZnCn (n = 1–8) clusters theoretically at the B3LYP/6-311+G(d) level, calculating the electronic energies, vibrational frequencies, dipole moments, and
rotational constants.9 These authors found that ZnCn chains
have triplet ground states, but ZnC2 is cyclic and closedshell. They also examined the geometries and electronic properties of open-chain and cyclic ZnCn +/− (n = 1–8) ionic
molecules.
Experimentally, little is known about small species involving zinc and carbon. Only ZnCH3 , ZnCN, HZnCN,
and HZnCH3 have been studied in detail in the gas phase,
although carbon-chain type compounds have been investigated in argon matrices.10–13 In 1993, Cerny et al. successfully produced ZnCH3 in a supersonic jet expansion and
measured the Ã2 E ← X̃ 2 A1 electronic transition of the
radical using laser-induced-fluorescence (LIF).10 More recently, the Ziurys group have measured the pure rotational
spectrum of ZnCN radical in its X̃ 2 + state, as well as
HZnCN (X̃1 ) and HZnCH3 (X̃1 A1 ), using both millimeterwave direct absorption and Fourier transform microwave
(FTMW) techniques.11, 12 These species were either produced
by Broida-oven methods, or by using Zn(CH3 )2 as the zinc
donor. In addition, the Graham group have detected the bent
isomer of ZnC3 in its 1 A electronic state, trapped in solid
136, 244310-1
© 2012 American Institute of Physics
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244310-2
Min et al.
argon, in the infrared using dual laser ablation of carbon and
zinc rods.13
One zinc-bearing molecule of interest is the monoacetylide, ZnCCH. ZnCCR-type compounds are known intermediates in the addition of terminal acetylenes to aldehydes, as mentioned.5, 6 ZnCCH is a model system for such
intermediates, and representative of the bond between zinc
and an unsaturated organic ligand. Unfortunately, there are
no past studies of ZnCCH, experimentally or theoretically.
In fact, little is known about 3d monoacetylide species in
general. The two exceptions are CrCCH, studied by Brugh
et al. using R2PI methods,14 and CuCCH, which has recently
been characterized using pure rotational spectroscopy by
Sun et al.15
Here we present the first spectroscopic study of ZnCCH.
The pure rotational spectrum of this molecule in its X̃ 2 +
ground electronic state has been recorded using a combination
of FTMW and millimeter direct absorption methods across
the 7–260 GHz frequency range. The spectra of five isotopologues were recorded with zinc, carbon, and hydrogen substitutions. In the FTMW data, hyperfine splittings due to the
hydrogen, deuterium, and carbon-13 nuclei were resolved. In
this paper, we present these data, their analysis, and interpret
the derived spectroscopic constants in terms of the bonding
and structure in ZnCCH.
II. EXPERIMENT
Measurements of ZnCCH were conducted using one of
the direct absorption spectrometers of the Ziurys group.16
The instrument consists of a phase-locked Gunn oscillator/Schottky diode multiplier source, a double-pass gas cell
incorporating a Broida-type oven, and a liquid helium-cooled
hot electron bolometer. The radiation is directed from the
source, through the cell, and into the detector using a scalar
feedhorn, a series of Teflon lenses, a rooftop reflector, and a
polarizing grid. Frequency modulation of the Gunn oscillator is employed for phase-sensitive detection and a secondderivative spectrum is obtained.
The FTMW spectrometer used in this study is a BalleFlygare-type, consisting of a vacuum chamber containing a
Fabry-Pérot cavity with two spherical aluminum mirrors. Antennas are embedded in both mirrors for injecting and detecting microwave radiation. A supersonic jet expansion is
used to introduce the gas mixture, produced by a General
Valve pulsed nozzle. To create metal radicals such as ZnCCH,
a discharge-assisted laser ablation source (DALAS) is employed. Microwaves are pulsed into the cavity, allowed to interact with the jet expansion, and then free induction decay
(FID) emission signals from the molecules are collected. Frequency domain spectra are generated with 4 kHz resolution by
a fast Fourier transform. Each transition appears as a Doppler
doublet due to the orientation of the jet expansion relative to
the electric field of the cavity. The transition frequency is the
average of the two doublets.
For the millimeter measurements, ZnCCH was synthesized in a dc discharge by the reaction of zinc vapor, produced
in the Broida oven, with acetylene in argon carrier gas. Approximately 5 mTorr of acetylene and 15 mTorr of argon were
J. Chem. Phys. 136, 244310 (2012)
introduced underneath the oven with a discharge of 900 mA at
350 V. About 15 mTorr of argon was also continuously flowed
over the cell lenses to help prevent coating by the metal vapor. The plasma exhibited a bright purple color due to atomic
emission from zinc. The 64 Zn, 66 Zn, and 68 Zn isotopologues
of ZnCCH were all observed in their natural abundance ratio
of 48.6:27.9:18.8.
Transitions of 64 ZnCCH, 66 ZnCCH, and 68 ZnCCH were
measured by averaging 20 to 50 scans, each 5 MHz wide.
Half of the scans were taken in increasing frequency and
the other half in decreasing frequency. The line widths varied from 630 to 750 kHz across the range 206 to 263 GHz.
Gaussian profiles were fit to the observed lines to determine the center frequency with an experimental uncertainty of
±50 kHz.
In the FTMW system, ZnCCH was produced with
DALAS from a mixture of 0.05% acetylene in argon and the
ablation of a zinc rod (ESPI Metals). DALAS consists of a rotating/translating metal rod, contained in a housing, which is
ablated by an Nd:YAG laser (Continuum Surelite I-10). The
jet expansion from the nozzle is sent through the ablation region, entraining metal vapor, and then through a dc discharge.
More details can be found in Ref. 15. The gas pulse from
the nozzle, which is 800 μs in duration, was introduced into
the chamber at a stagnation pressure of 50 psi. Simultaneously, a dc discharge was activated for 1100 μs using a voltage of 1500 V. The laser, with a flash-lamp voltage of 1.29 kV
(240 mJ/pulse), was fired 970 μs after the initial opening of
the gas valve. The spectra of 64 ZnCCH and 66 ZnCCH were
obtained in their natural abundance, while for the D and 13 C
isotopologues, 0.1% DCCD (Cambridge Isotopes, 99% enrichment) or 0.05% H13 C13 CH (Sigma-Aldrich 99%) reaction
mixtures in argon were used. Typically, 250–500 pulses were
accumulated to achieve an adequate signal-to-noise ratio for
the main isotopologue, while for the other species, 1000–5000
pulses were necessary.
III. RESULTS
Because there was no previous theoretical work on
ZnCCH, standard B3LYP/aug-cc-pVTZ calculations were
first performed to establish the geometry and estimate the
rotational constant of the molecule, as a starting point for
the spectroscopy study.17–19 Based on the calculated rotational constant of B = 3759 MHz, the search for ZnCCH
was initially conducted with the millimeter-wave spectrometer over the frequency range 210–220 GHz. Two lines at
219 796.3 MHz and 219 684.1 MHz were first found, with
a frequency separation similar to the spin-rotation splitting
in ZnCN, ZnF, and ZnCl.11, 20, 21 Other harmonically-related
doublets were then located and identified as 66 ZnCCH. By
searching nearby frequency ranges, doublets arising from
64
ZnCCH and 68 ZnCCH were also found. Location of harmonic patterns from all three zinc isotopologues aided in
the identification. Lower energy transitions of 64 ZnCCH and
66
ZnCCH near 7–38 GHz were subsequently recorded using
the FTMW instrument, in which small proton hyperfine splittings were observed. Following these measurements, spectra of 64 ZnCCD and 64 Zn13 C13 CH were also measured in the
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244310-3
Min et al.
7–38 GHz range; smaller hyperfine interactions were found in
the case of the deuterated analog, while 64 Zn13 C13 CH showed
a much more extensive hyperfine pattern, presumably generated by the two spins of 13 C. These data solidly confirmed the
identity of the molecule as ZnCCH.
As shown in Table I, a total of 13 and 10 transitions
were recorded in the range 7–260 GHz for 64 ZnCCH and
66
ZnCCH, respectively, each consisting of spin-rotation doublets. In the FTMW data, i.e., the lower N transitions, each
spin-rotation component is split into two hyperfine lines, arising from the proton spin of I = 12 . This hyperfine structure
is completely collapsed in the millimeter data. For 68 ZnCCH
(also see Table I), seven transitions were measured but only
in the millimeter region (218–264 GHz).
Representative FTMW spectra of 64 ZnCCH and
66
ZnCCH are shown in Figure 1. Here the N = 2 → 1
transition near 15 GHz for each species is displayed. There
are frequency breaks in each spectrum such that both
spin-rotation components, indicated by J, can be shown. As
illustrated in the figure, each transition is composed of four
prominent hyperfine components (two for each spin-rotation
doublet), labeled by quantum number F, as expected for a
nuclear spin of I = 12 . Each spectral feature exhibits Doppler
doublets, indicated by brackets. The signal-to-noise ratio is
slightly lower for the 66 ZnCCH lines as a result of the natural
zinc isotope abundances.
Representative millimeter-wave spectra are shown in
Figure 2. Here the N = 29 ← 28, 30 ← 29, and 31 ← 30 rotational transitions are presented near 221–237 GHz. The fine
structure doublets, labeled by quantum number J and separated by ∼110 MHz, are clearly evident in each spectrum.
The signal-to-nose ratio is not nearly as high as for the FTMW
data, and reflects the difficulty in making 3d monoacetylides
with Broida-oven methods.
Five and four rotational transitions were measured for
64
ZnCCD and 64 Zn13 C13 CH, respectively, with the FTMW
spectrometer, as shown in Tables II and III. For the deuterated
isotopologue, hyperfine splittings arising from the D nucleus
(I = 1) produces triplets per spin-rotation doublet, neglecting
weaker F = 0 lines. Across the five observed transitions,
15 out of the 30 possible individual hyperfine components
were recorded (see Table II). For the 13 C doubly-substituted
species, the hyperfine pattern is quite complicated because of
the contribution of the two 13 C nuclei (I = 12 ). As later determined in the analysis, for 64 Zn13 Cα 13 Cβ H, hyperfine interactions arising from 13 Cα are dominant, and thus indicated by
quantum number F1 , where F1 = J + I1 (13 Cα ). The next important coupling is from 13 Cβ , such that F2 = F1 + I2 (13 Cβ ).
The proton hyperfine interaction, defined by F = F2
+ I3 (H), is smallest. Note that the magnetic moments of the
H and 13 C nuclei are 2.79285 μN and 0.70241 μN .22 A given
rotational transition for 64 Zn13 C13 CH thus potentially consists of 16 hyperfine individual components, eight for each
spin-rotation doublet (F = −1). Typically, 2-10 components were actually measured per transition, as illustrated in
Table III.
In Figure 3, the FTMW spectrum of the N = 2 → 1 transition of 64 ZnCCD near 14 GHz is displayed. There is a frequency break in the data in order to show both spin-rotation
J. Chem. Phys. 136, 244310 (2012)
doublets. Four of the six possible deuterium hyperfine components that comprise this transition are shown. Three of the
lines arise from the J = 2.5 → 1.5 spin doublet, two of which
are blended together. The Doppler doublets are indicated by
brackets.
A representative FTMW spectrum of 64 Zn13 C13 CH is
shown in Figure 4. Here nine of the 16 possible hyperfine
components of the N = 2 → 1 transition are visible, four and
five from the two fine structure components, J = 1.5 → 0.5
and J = 2.5 → 1.5, respectively. There are seven frequency
breaks in the spectrum to show all nine features. The Doppler
doublets are indicated by brackets. The F = 4 → 3 and
3 → 2 lines in the J = 2.5 → 1.5 component (F2 = 3.5
→ 2.5) are separated only by about 300 kHz, and illustrate
the smaller splitting due to the proton spin. The 13 C interactions are significantly larger, as much as 15 MHz (see
Table III).
IV. ANALYSIS
The data for 64 ZnCCH, 66 ZnCCH, and 64 Zn13 C13 CH
were analyzed by using the non-linear least squares routine SPFIT with a 2 Hamiltonian containing rotation, spinrotation, and magnetic hyperfine terms, respectively,23
H = H rot + H SR + H mhf .
(1)
For 64 ZnCCH and 66 ZnCCH, both FTMW and
millimeter-wave data were analyzed in combined fits.
The magnetic hyperfine Hamiltonian in these two cases
considers only the proton spin. For the 13 C isotopologue, only
FTMW data were analyzed, but three terms in Hmhf were
required to fit the hyperfine interactions corresponding to
the two 13 C spins, as well as the proton. The 64 ZnCCD data,
recorded with only the FTMW instrument, were fit with a
Hamiltonian which includes one additional term, the electric
quadrupole interaction,
H = H rot + H SR + H mhf + H eqQ .
(2)
The resulting constants from the analysis of these five
isotopologues of ZnCCH are presented in Table IV. The rms
values of the fits are 13 kHz (64 ZnCCH), 40 kHz (66 ZnCCH),
96 kHz (68 ZnCCH, mm-wave data only), 7 kHz (64 ZnCCD),
and 4 kHz (64 Zn13 C13 CH). The rms values are within the
experimental accuracy, considering that some fits include
millimeter-wave data, as well as FTMW spectra.
V. DISCUSSION
From this study, the structure of the ZnCCH radical has
been accurately established. Both r0 and rm (1) structures have
been determined from the rotational constants established for
the five isotopologues. The r0 bond lengths were obtained
directly from a least-squares fit to the moments of inertia,
while the rm (1) geometry was derived by a mass-dependent
method developed by Watson.24 The Watson rm (2) structure would be optimal, but could not be calculated because
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Min et al.
244310-4
J. Chem. Phys. 136, 244310 (2012)
TABLE I. Measured rotational transitions of 64 ZnCCH, 66 ZnCCH and 68 ZnCCH (X̃ 2 + ).a
64 ZnCCH
66 ZnCCH
68 ZnCCH
N
J
F
N
J
F
ν obs
ν o-c
ν obs
ν o-c
ν obs
ν o-c
1
1
2
2
2
2
3
3
3
3
4
4
4
4
5
5
5
5
27
27
27
27
28
28
28
28
29
29
29
29
30
30
30
30
31
31
31
31
32
32
32
32
33
33
33
33
34
34
34
34
35
35
35
35
1.5
1.5
1.5
1.5
2.5
2.5
2.5
2.5
3.5
3.5
3.5
3.5
4.5
4.5
4.5
4.5
5.5
5.5
26.5
26.5
27.5
27.5
27.5
27.5
28.5
28.5
28.5
28.5
29.5
29.5
29.5
29.5
30.5
30.5
30.5
30.5
31.5
31.5
31.5
31.5
32.5
32.5
32.5
32.5
33.5
33.5
33.5
33.5
34.5
34.5
34.5
34.5
35.5
35.5
2
1
2
1
3
2
3
2
4
3
4
3
5
4
5
4
6
5
27
26
28
27
28
27
29
28
29
28
30
29
30
29
31
30
31
30
32
31
32
31
33
32
33
32
34
33
34
33
35
34
35
34
36
35
0
0
1
1
1
1
2
2
2
2
3
3
3
3
4
4
4
4
26
26
26
26
27
27
27
27
28
28
28
28
29
29
29
29
30
30
30
30
31
31
31
31
32
32
32
32
33
33
33
33
34
34
34
34
0.5
0.5
0.5
0.5
1.5
1.5
1.5
1.5
2.5
2.5
2.5
2.5
3.5
3.5
3.5
3.5
4.5
4.5
25.5
25.5
26.5
26.5
26.5
26.5
27.5
27.5
27.5
27.5
28.5
28.5
28.5
28.5
29.5
29.5
29.5
29.5
30.5
30.5
30.5
30.5
31.5
31.5
31.5
31.5
32.5
32.5
32.5
32.5
33.5
33.5
33.5
33.5
34.5
34.5
1
0
1
0
2
1
2
1
3
2
3
2
4
3
4
3
5
4
26
25
27
26
27
26
28
27
28
27
29
28
29
28
30
29
30
29
31
30
31
30
32
31
32
31
33
32
33
32
34
33
34
33
35
34
7697.428
7700.893
15 223.875
15 224.891
15 338.119
15 338.760
22 864.809
22 865.148
22 978.718
22 978.995
30 505.420
30 505.600
30 619.216
30 619.372
38 145.852
38 145.972
38 259.596
38 259.706
206 145.183
206 145.183
206 258.451
206 258.451
213 774.762
213 774.762
213 888.000
213 888.000
221 403.488
221 403.488
221 516.663
221 516.663
229 031.386
229 031.386
229 144.556
229 144.556
236 658.424
236 658.424
236 771.527
236 771.527
244 284.467
244 284.467
244 397.595
244 397.595
251 909.628
251 909.628
252 022.710
252 022.710
259 533.807
259 533.807
259 646.886
259 646.886
...
...
...
...
− 0.001
0.000
0.001
0.000
0.001
0.001
0.000
− 0.001
0.000
0.001
− 0.003
0.000
− 0.004
− 0.002
− 0.004
0.006
− 0.004
0.007
0.016
0.013
0.011
0.008
0.026
0.023
0.022
0.019
0.007
0.003
− 0.029
− 0.032
0.011
0.008
0.004
0.001
0.038
0.035
− 0.003
− 0.005
− 0.019
− 0.022
0.001
− 0.002
− 0.017
− 0.020
− 0.007
− 0.010
− 0.027
− 0.030
0.017
0.015
...
...
...
...
7637.621
7641.085
15 105.581
15 106.597
15 218.942
15 219.591
22 687.149
22 687.488
22 800.175
22 800.446
30 268.397
30 268.576
30 381.312
30 381.470
37 849.465
37 849.582
...
...
...
...
...
...
...
...
...
...
219 684.087
219 684.087
219 796.299
219 796.299
227 252.697
227 252.697
227 364.923
227 364.923
234 820.627
234 820.627
234 932.861
234 932.861
242 387.546
242 387.546
242 499.698
242 499.698
249 953.749
249 953.749
250 065.749
250 065.749
...
...
...
...
...
...
...
...
0.000
− 0.001
0.001
0.000
− 0.001
0.007
0.000
0.000
− 0.001
− 0.006
− 0.001
0.001
0.000
0.003
− 0.003
0.004
...
...
...
...
...
...
...
...
...
...
0.012
0.009
− 0.020
− 0.023
− 0.097
− 0.100
− 0.076
− 0.079
− 0.017
− 0.019
0.053
0.050
− 0.050
− 0.052
− 0.021
− 0.023
0.128
0.126
0.048
0.046
...
...
...
...
...
...
...
...
...
...
...
...
...
...
...
...
...
...
...
...
...
...
...
...
...
...
...
...
...
...
...
...
...
...
218 061.710
218 061.710
218 173.161
218 173.161
225 574.432
225 574.432
...
...
233 086.679
233 086.679
233 197.920
233 197.920
240 597.643
240 597.643
240 709.192
240 709.192
248 108.139
248 108.139
248 219.446
248 219.446
255 617.242
255 617.242
255 728.635
255 728.635
263 125.677
263 125.677
263 236.813
263 236.813
...
...
...
...
...
...
...
...
...
...
...
...
...
...
...
...
...
...
...
...
...
...
...
...
...
...
0.001
0.001
0.079
0.079
− 0.158
− 0.158
...
...
0.064
0.064
− 0.067
− 0.067
− 0.113
− 0.113
0.064
0.064
0.154
0.154
0.088
0.088
− 0.032
− 0.032
− 0.012
− 0.012
0.084
0.084
0.153
0.153
a
In MHz.
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244310-5
Min et al.
J. Chem. Phys. 136, 244310 (2012)
FIG. 1. Representative FTMW spectra of the N = 2 → 1 transition of 64 ZnCCH (upper panel) and 66 ZnCCH (lower panel) near 15 GHz, showing both
spin-rotation doublets, labeled by quantum number J. Each doublet is split into two hyperfine components, indicated by quantum number F, arising from the H
nuclear spin (I = 1/2). There are two frequency breaks in the spectra in order to show all the data. Doppler doublets are indicated by brackets. Each spectrum is
a compilation of four 600-kHz scans, with 800 pulses per scan.
spectra of 64 Zn13 CCH and 64 ZnC13 CH were not measured.
The resulting bond lengths of ZnCCH are listed in Table V,
along with the bond distances of related molecules.
From the rm (1) structure, the Zn-C bond length is
1.9083 Å, the C-C bond distance 1.2313 Å, and the C-H bond
length is 1.0508 Å. As the table shows, the C-C bond and CH bond lengths for ZnCCH are very similar to those in acetylene, although the C-C is slightly longer in the zinc compound
by about 0.029 Å. The C-C bond length in zinc monoacetylide
is about 0.02 Å longer than that in CuCCH (1.213(2) Å),
which also contains a triple carbon-carbon bond. Among
zinc-bearing species, the rm (1) Zn-C bond in ZnCCH is
0.04 Å shorter than that of the ZnCN radical (1.9496
Å), but lies between those of the closed-shell molecules
HZnCH3 (1.9281(2) Å) and HZnCN (1.8966(6) Å).
ZnCN, HZnCN, and HZnCH3 are thought to have single zinc-
carbon bonds.11, 12 Given these comparisons, the proposed
structure of zinc monoacetylide is Zn—C≡C—H, namely,
the acetylene moiety retains its basic properties, and links
to zinc with a single bond. The presence of the unpaired
electron on zinc slightly lengthens the Zn-C bond relative to
closed-shell zinc species; it also appears to somewhat increase
the C-C triple bond, as well, versus that in acetylene and
CuCCH.
The spin-rotation constant γ has a value of 113.676 MHz
for ZnCCH. The Zn-bearing molecules ZnCN, ZnF, and
ZnCl have γ = 104.062, 150.93, and 126.48 MHz,
respectively,11, 20, 21 in their 2 + ground states. To first order, the spin-rotation constant γ should scale with the rotational constant B. The γ /B ratios of these molecules are 0.014,
0.026, 0.027, and 0.030 for ZnF, ZnCl, ZnCN, and ZnCCH.
Note that the ratio for ZnCCH is close to the values of the
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Min et al.
244310-6
J. Chem. Phys. 136, 244310 (2012)
FIG. 2. Millimeter-wave spectra of the N = 29 ← 28, N = 30 ← 29, and N = 31 ← 30 transitions of 64 ZnCCH near 221–237 GHz. Each transition is composed
of fine structure doublets, indicated by quantum number J, as the hyperfine splittings are collapsed at these frequencies. Each spectrum is an average of twenty
5-MHz wide scans, each 70 s in duration.
heavier molecules ZnCl and ZnCN, and ZnF, the lightest
molecule of the group, has the smallest ratio. The variation
in γ /B ratios suggests the contribution of second-order spinorbit coupling to the spin-rotation constant in ZnCCH, likely
arising from a nearby 2 state.
γ
(2)
=
The second-order spin-rotation coupling arises from
cross terms between Hrot and HSO . Assuming that the major contribution to γ is second-order spin-orbit coupling, the
energy of the nearby 2 state can be estimated from formula
given by Brown et al.25
+ B(r)L − 2 1/2 2 1/2 H SO 2 + + 2 + H SO 2 1/2 2 −1/2 B(r)L − 2 +
2 2 −1/2
1/2
−1/2
1/2
E S|S− |S + 1
Here HSO is the one electron operator and is effectively equal
to a± s± , where a is the atomic spin-orbit constant. Evaluation of matrix elements in Eq. (3) for ZnCCH requires the
approximation that the unpaired electron in this molecule has
.
(3)
some pσ character; else, the matrix elements vanish. This assumption is justified because the hydrogen spin-dipolar constant c is non-zero, indicating some non-s orbital character of
the unpaired electron, as is discussed below. Also, the B(r)L-
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244310-7
Min et al.
J. Chem. Phys. 136, 244310 (2012)
TABLE II. Observed rotational transitions of 64 ZnCCD(X̃2 + ).a
N
J
F
N
J
F
ν obs
ν o-c
1
1
1
1
2
2
2
2
3
3
3
4
4
5
5
0.5
0.5
1.5
1.5
1.5
2.5
2.5
2.5
2.5
2.5
3.5
3.5
4.5
4.5
5.5
0.5
1.5
1.5
2.5
2.5
3.5
2.5
1.5
3.5
2.5
4.5
4.5
5.5
5.5
6.5
0
0
0
0
1
1
1
1
2
2
2
3
3
4
4
0.5
0.5
0.5
0.5
0.5
1.5
1.5
1.5
1.5
1.5
2.5
2.5
3.5
3.5
4.5
1.5
0.5
1.5
1.5
1.5
2.5
1.5
0.5
2.5
1.5
3.5
3.5
4.5
4.5
5.5
6904.511
6906.182
7060.362
7061.447
13 966.173
14 070.730
14 070.949
14 070.968
20 975.507
20 975.525
21 079.964
27 984.700
28 089.144
34 993.796
35 098.197
0.000
0.004
0.004
0.000
0.011
− 0.007
0.003
0.003
0.001
− 0.016
− 0.005
− 0.004
0.013
0.004
− 0.005
a
In MHz
operator arises from Hrot , which is defined with respect to a
coordinate system with the origin at the molecular center of
mass. For ZnCCH, the molecular center of mass is close to,
but not exactly at, the Zn nucleus, bringing additional uncertainty to the calculation.
FIG. 3. FTMW spectrum of the N = 2 → 1 transition of 64 ZnCCD near
14 GHz, showing four hyperfine components, labeled with the appropriate
F quantum number, arising from the deuterium spin of I = 1. There is a
frequency break in the spectrum in order to show hyperfine lines from both
spin-rotation doublets, indicated by quantum number J. Brackets indicate the
Doppler doublets in each spectral feature. Two hyperfine lines are closely
blended. The spectrum is a compilation of two 600-kHz scans, with 1000
pulses per scan.
TABLE III. Observed rotational transitions of 64 Zn13 C13 CH (X̃ 2 + ).a
N
J
F1 F2 F
N
J
F1 F2 F
ν obs
ν o-c
1
1
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
0.5
1.5
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
3.5
3.5
3.5
3.5
3.5
4.5
4.5
4.5
4.5
4.5
4.5
1
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
4
4
4
3
3
4
4
4
4
5
5
5
5
1.5
2.5
2.5
2.5
1.5
1.5
2.5
1.5
3.5
3.5
2.5
3.5
3.5
2.5
3.5
3.5
2.5
4.5
4.5
3.5
3.5
3.5
4.5
4.5
4.5
4.5
5.5
5.5
4.5
4.5
2
2
3
2
2
1
2
1
4
3
3
4
3
3
4
3
2
5
4
4
4
3
5
4
5
4
6
5
5
4
0
0
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
0.5
0.5
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
2.5
2.5
2.5
2.5
2.5
3.5
3.5
3.5
3.5
3.5
3.5
0
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
3
3
3
2
2
3
3
3
3
4
4
4
4
0.5
1.5
1.5
1.5
0.5
0.5
1.5
0.5
2.5
2.5
1.5
2.5
2.5
1.5
2.5
2.5
1.5
3.5
3.5
2.5
2.5
2.5
3.5
3.5
3.5
3.5
4.5
4.5
3.5
3.5
1
1
2
1
1
0
1
0
3
2
2
3
2
2
3
2
1
4
3
3
3
2
4
3
4
3
5
4
4
3
7224.714
7305.802
14 464.670
14 465.393
14 467.851
14 468.628
14 543.606
14 548.588
14 558.264
14 558.568
14 559.805
21 711.160
21 711.530
21 712.779
21 802.415
21 802.545
21 803.220
21 810.887
21 811.051
21 811.600
28 955.254
28 955.449
28 960.523
28 960.733
29 057.979
29 058.062
29 063.287
29 063.384
29 063.700
29 063.845
− 0.003
− 0.004
− 0.004
0.003
0.002
− 0.009
0.001
0.002
0.002
0.003
− 0.002
0.003
0.003
0.002
0.000
− 0.001
− 0.009
0.005
0.003
0.002
0.002
− 0.003
0.002
0.001
− 0.005
0.005
0.003
− 0.005
− 0.002
0.002
a
In MHz.
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244310-8
Min et al.
J. Chem. Phys. 136, 244310 (2012)
FIG. 4. FTMW spectrum of the N = 2 → 1 transition of 64 Zn13 C13 CH near 14 GHz, showing the complex hyperfine pattern generated by three nuclear spins.
There are seven frequency breaks in the spectrum, and the hyperfine lines arise from both spin-rotation components, labeled by J. The hyperfine splittings
originating with the 13 Cα nucleus (I = 1/2), indicated by F1 , are the largest, followed by those from the 13 Cβ nucleus, labeled by F2 . The smallest interactions,
which are indicated by F, arise from the H nuclear spin, The spectrum is a compilation of eight 600-kHz scans, with 2500–5000 shots per scan.
Assuming that the orbital of the unpaired electron in
ZnCCH is an equal mixture of sσ and pσ character, namely,
= √12 [|sσ ± |pσ ]), and using B(r) ∼ B(X̃2 + ), Equation
(3) reduces to the simple expression,
γ (2) = Ba/E .
(4)
Using the experimentally-determined values of B (0.127
cm−1 ) and γ (0.00379 cm−1 ), and assuming a = 583 cm−1 ,
the spin–orbit constant of Zn+ ,26 E can be calculated,
if γ ≈ γ (2) . Equation (4) then yields E ≈ 19 535 cm−1 .
This value is similar in magnitude to the experimentally determined 2 -2 energy difference of ZnCl (E ≈ 18 000
cm−1 ),27 and the calculated value for ZnF (E ≈ 20 400
cm−1 ).20
For ZnCCH, the Fermi contact and dipolar parameters
have been determined for the hydrogen nucleus. In general,
the Fermi contact constant, bF , is proportional to both the nuclear g-factor and the electron density of the unpaired electron
at the coupling nucleus,22
bF =
8π
gS μB gN μN
|(0)i |2 S .
3
i
(5)
The dipolar coupling constant, c, is proportional to the
g-factor, as well as the field gradient around the coupling
TABLE IV. Spectroscopic constants of ZnCCH (X̃2 + ).a
64 ZnCCH
66 ZnCCH
68 ZnCCH
64 ZnCCD
64 Zn13 C 13 C H
α
β
Parameter
MW, MMW
MW, MMW
MMW
MW
MW
B
D
γ
γD
bF (H)
c(H)
bF (D)
c(D)
bF (13 Cα )
c(13 Cα )
bF (13 Cβ )
c(13 Cβ )
eqQ (D)
rms
3820.33898(52)
0.00122571(42)
113.6760(86)
−0.000185(27)
9.356(49)
2.468(96)
3790.65511(60)
0.00120752(50)
112.7956(93)
−0.000220(33)
9.354(50)
2.471(96)
3762.6425(55)
0.0011877(26)
111.372(84)
3504.6477(15)
0.000925(41)
104.3825(71)
3626.2245(17)
0.001100(61)
107.943(16)
a
9.37(11)
2.44(13)
1.440(19)
0.351(37)
270.03(10)
26.43(68)
41.98(13)
11.09(30)
0.013
0.040
0.096
0.253(91)
0.007
0.004
In MHz; values in parenthesis are 3σ errors.
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244310-9
Min et al.
J. Chem. Phys. 136, 244310 (2012)
TABLE V. Bond lengths of ZnCCH and related species.a
Molecule
ZnCCH (X̃2 + )
ZnCN (X̃ 2 + )
HZnCN (X̃1 + )
HZnCH3 (X̃ 1 A1 )
CuCCH (X̃ 1 + )
HC≡CH
a
b
r(Zn-C) (Å)
r(C-C) (Å)
r(C-H) (Å)
Method
Ref.
1.902(3)
1.9083(3)
1.9496
1.8966(6)
1.9281(2)
...
...
1.238(4)
1.2313(3)
1.048(1)
1.0508(1)
1.213(2)
1.20241(9)
1.058(1)
1.0625(1)
r0
rm (1)
rm (1)
rm (1)
r0
rm (1)
re , Infrared, Raman
This work
This workb
Ref. 31
Ref. 11
Ref. 12
Ref. 15
Ref. 32
In MHz; values in parentheses are 1σ uncertainties.
cb = −0.0163(2).
nucleus, arising specifically from the electrons with electronic
orbital angular momentum,22
3 cos2 θi − 1
3
c = gS μB gN μN
.
2
r3i
i
(6)
S
In ZnCCH, the summation is over the one unpaired electron.
Both constants for the hydrogen nucleus are relatively
small for 64 ZnCCH (bF = 9.356 MHz and c = 2.468 MHz: see
Table IV), indicating that limited electron density is present
at the hydrogen nucleus. Furthermore, bF is almost four times
larger than c. The electron density at the H nucleus is therefore primarily in an s orbital. The nonzero value of c, however, indicates sp hybridization of the orbital, which is consistent with the proposed acetylene-like structure for ZnCCH.
The hydrogen s character of this orbital can be estimated by
comparing the ratio of atomic vs. molecular Fermi contact
terms, i.e., bF (molecule)/bF (atom), where bF (atom) = 1420
MHz.28 The percent hydrogen s character is 0.66%.
The location of the unpaired electron can be further elucidated by the 13 C hyperfine constants. For
64
Zn13 Cα 13 Cβ H, the analysis suggests bF (13 Cα ) = 270.03(10)
MHz, c(13 Cα ) = 26.43(68) MHz and bF (13 Cβ ) = 41.98(13)
MHz, c(13 Cβ ) = 11.09(30) MHz. The amount of electron density on the two carbon nuclei thus decreases with distance
from the zinc atom. It is notable that the Fermi contact term
on the carbon atom bonded to the zinc is rather large, relative
to that of the proton; presumably, the additional electron density on the alpha carbon is what lengthens the C-C bond relative to acetylene. However, the Fermi contact and spin dipolar
parameters for the two 13 C nuclei are all much smaller than
those analogue constants in 13 CCH (bF = 900.7(6) MHz and
c = 142.87(3) MHz) and C13 CH (bF = 161.63(10) MHz and
c = 64.07(5) MHz).29 These results indicate that the unpaired
electron resides principally on the zinc nucleus.
From the dipolar constant, the expectation value of 1/r3 for the unpaired electron can be derived for ZnCCH, where
r is the distance of this electron from the respective nucleus.
The dipolar constants of 64 Zn13 Cα 13 Cβ H and 13 Cα Cβ H are
shown in Table VI, as well as the resulting 1/r3 values for
the 13 C and H nuclei for each molecule. For the proton in
64
Zn13 Cα 13 Cβ H, the value found is 1/r3 = 2.30 × 1028 m−3 ,
while 1/r3 = 1.01 × 1030 m−3 and 4.23 × 1029 m−3 for 13 Cα
and 13 Cβ . Therefore, the electron-nucleus distance increases
steadily from 13 Cα, to 13 Cβ, and then to H, as expected.
For CCH, the analogous numbers are 1.19 × 1029 m−3 ,
5.45×1030 m−3 , and 2.44 × 1030 m−3 for H, 13 Cα , and
13
Cβ , respectively. The addition of the zinc thus significantly
changes the field gradient across the CCH moiety. In comparison, 1/r3 for ZnCN has been found to be 8.43 × 1029 m−3 ,
as calculated from the dipolar constant of the nitrogen nucleus. The electron is thus closer to the nitrogen nucleus in
ZnCN than the proton in ZnCCH. The unpaired electron of
the cyanide species is also thought to be primarily on the zinc
nucleus.11 This result is consistent with the structures of two
linear molecules, one having two carbons between zinc and
the nucleus with the spin (ZnCCH), and the other having one
carbon (ZnCN).
The deuterium hyperfine constants in 64 ZnCCD are
smaller than those of the proton for ZnCCH, consistent with
the relative magnetic moments (0.857392 μN vs 2.792670
μN ),30 and the proposed structure. Moreover, the eqQ constants for 64 ZnCCD and CuCCD are quite close in value:
0.253(91) and 0.214(23) MHz. The two different metal atoms
and the addition of one unpaired electron do not appear to
significantly alter the field gradient across the terminal deuterium. The model of the metal bonded to an acetylenic
TABLE VI. Expectation values 1/r3 for 64 Zn13 C13 CH, 13 CCH, and C13 CH.
64 Zn13 C 13 C H
α
β
13 C
c(MHz)
r13 (m−3 )
α
26.44(68)
1.01 × 1030
13 C
β
11.09(30)
4.23 × 1029
13 CCH
C13 CH
H
13 C
H
13 C
H
2.40(16)
2.30 × 1028
142.87(3)
5.45 × 1030
12.17(5)
1.17 × 1029
64.07(5)
2.44 × 1030
12.64(5)
1.21 × 1029
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244310-10
Min et al.
ligand appears to apply to both zinc and copper. It would be
interesting to see if other 3d metals exhibit similar acetylenic
structures.
VI. CONCLUSION
A new zinc-containing radical, ZnCCH, has been synthesized in the gas phase and characterized by rotational spectroscopy. ZnCCH is a model system for acetylenic zinc compounds such as ZnCCR. The data clearly indicate that the
molecule has a 2 + ground electronic state, with a possible
2
state lying ∼19 000 cm−1 higher in energy. From the five
isotopologues of ZnCCH investigated in this work, the molecular structure for this radical appears to be a Zn-C single bond
with an acetylene ligand. The proton and the two 13 C hyperfine structures are consistent with the zinc nucleus being the
main location of the unpaired electron, although some density
is clearly shared with the α-carbon. Studies of other 3d acetylene species, such as TiCCH, would aid in understanding the
role of transition metals in synthesis and catalysis.
ACKNOWLEDGMENTS
This work was supported by NSF (Grant No. CHE1057924). We thank Professor Dennis Clouthier for carrying
out the initial theoretical predictions for ZnCCH.
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