Journal of Molecular Spectroscopy 244 (2007) 153–159
www.elsevier.com/locate/jms
The pure rotational spectrum of ZnCl (X2R+):
Variations in zinc halide bonding
E.D. Tenenbaum, M.A. Flory, R.L. Pulliam, L.M. Ziurys
*
Department of Chemistry, Steward Observatory, University of Arizona, 933 N. Cherry Avenue, Tucson, AZ 85721, USA
Department of Astronomy, Steward Observatory, University of Arizona, 933 N. Cherry Avenue, Tucson, AZ 85721, USA
Received 16 March 2007; in revised form 24 May 2007
Available online 2 June 2007
Abstract
The radical ZnCl (X2R+) has been studied using millimeter-wave direct-absorption techniques. Pure rotational spectra of 67Zn35Cl,
Zn37Cl, 68Zn35Cl, 64Zn35Cl, 64Zn37Cl, and 66Zn35Cl were measured in the vibrational ground state and data were also recorded for
the latter three in the v = 1 and v = 2 states. Every rotational transition was found to be split into a doublet due to spin–rotation interactions. For 67Zn35Cl, each doublet exhibited additional splittings arising from hyperfine coupling of the 67Zn (I = 5/2) nucleus. Rotational, fine structure, and hyperfine constants have been determined from these data, and equilibrium parameters calculated. The
equilibrium bond length of 64Zn35Cl is found to be 2.13003305(24) Å, in good agreement with recent theoretical predictions. Interpretation of hyperfine constants indicates that the 12r orbital is 70% Zn(4s) in character, suggesting that the zinc chloride bond is relatively ionic.
Published by Elsevier Inc.
66
Keywords: Zinc chloride (ZnCl); Microwave spectroscopy; Rotational spectroscopy; Hyperfine structure
1. Introduction
Understanding the bonding behavior of zinc is valuable
to many scientific disciplines. Zinc plays an important role
in synthetic organic chemistry as well as in biological systems. For example, zinc dichloride is reported to be a
highly efficient catalyst in the alkylation of ketones [1]. Furthermore, as the second most abundant transition metal in
the body, zinc has been linked to neurological conditions
such as Alzheimer’s disease [2].
Millimeter-wave spectroscopy offers an avenue in which
to study properties of zinc-bearing species by establishing
electronic ground states, bond lengths, and highly accurate
rotational, vibration–rotation, fine structure, and hyperfine
constants. In addition, interpretation of the hyperfine constants leads to information about the bonding and the participation of d orbitals. A number of zinc-containing
*
Corresponding author. Fax: +1 520 621 5554.
E-mail address: [email protected] (L.M. Ziurys).
0022-2852/$ - see front matter Published by Elsevier Inc.
doi:10.1016/j.jms.2007.05.011
molecules have been studied with gas-phase high-resolution
spectroscopy. ZnH has been investigated using electronic,
vibrational, and rotational methods (see [3] and references
therein). Vibrational transitions of HZnCl [4] and ZnH2 [5]
have been measured, and ZnCH3 [6] has been studied with
electronic spectroscopy. In addition, pure rotational spectra were recorded for ZnF [7], ZnCN [8], Zn(CH3)2 [9],
and, most recently, HZnCH3 [10]. Surprisingly, no rotationally-resolved spectroscopic work on zinc chloride
(ZnCl) has been published.
ZnCl has been the subject of several other investigations. Givan and Lowenschuss [11] measured the fundamental vibrational frequency of this species using matrixisolation Raman spectroscopy. Sureshkumar et al. [12]
and Cornell [13] recorded vibrationally-resolved emission
bands of gaseous ZnCl in the range of 2070–6550 Å, and
derived values of xe and xexe for the ground and excited
states of the observed transitions. Sureshkumar et al.
assigned the emission bands at 5100–6550 Å to the
A2P fi X2R transition and the spectra recorded by Cornell
154
E.D. Tenenbaum et al. / Journal of Molecular Spectroscopy 244 (2007) 153–159
at 2950 and 2075 Å were subsequently assigned by Herzberg to the C2P fi X2R and E2R fi X2R transitions,
respectively [14]. The dissociation energy of ZnCl was
determined by Hildenbrand et al. [15] using mass spectrometric measurements. In addition to experiment, computational investigations of this radical have been done by
Boldyrev and Simons [16], and by Kerkines, Mavridis
and Karipidis [17], which yielded predictions of the equilibrium bond length, the dipole moment, the dissociation
energy, and the fundamental vibrational frequency.
Here we report a millimeter-wave study of ZnCl in the
gas-phase. The pure rotational spectra of six isotopologues
of this species were recorded and the 2R+ ground state was
confirmed. Least-squares fitting of the data yielded rotational, fine, and hyperfine constants which are reported,
in addition to estimates of vibrational parameters and equilibrium bond lengths. An interpretation of the bonding in
this radical is also given.
2. Experimental
The pure rotational spectrum of ZnCl was measured
using one of the spectrometers of the Ziurys group. Instrumental details are available in an earlier publication [18].
In summary, the spectrometer is a double-pass, directabsorption system with a steel-walled, water-cooled, lowpressure reaction chamber. Millimeter-wave radiation is
generated by phase-locked Gunn oscillators combined with
Schottky diode multipliers giving frequency coverage from
65 to 640 GHz. The radiation is focused through the reaction
chamber by a Gaussian-beam optics scheme which includes a
rooftop reflector, and then into a He-cooled InSb hot electron bolometer detector. Frequency modulation of the Gunn
oscillator is employed to help eliminate background noise,
and data is processed at 2f using a lock-in amplifier.
ZnCl was synthesized by reacting zinc vapor with chlorine gas. Pieces of zinc (99.9% Aldrich) were melted in a
Broida-type oven using an Alumina crucible to produce
metal vapor, and chlorine gas was introduced into the reaction chamber in the region above the oven. A chlorine gas
pressure of approximately 5 mtorr was found to optimize
signal strength.
Due to the lack of any prior high-resolution spectroscopic
studies of ZnCl, an initial scan of 30 GHz, which is six times
the value of the predicted rotational constant, was done to
locate and assign transitions. The ZnCl lines were identified
based on their characteristic doublet pattern arising from
spin–rotation coupling and on theoretical predictions of
the ZnCl bond length [16]. The strongest series of doublets
was assigned to 64Zn35Cl and the weaker doublet patterns
were identified as excited vibrational states and minor isotopologues. Actual transition frequencies were measured by
scanning the radiation source over a 5 MHz range centered
at a given transition frequency. The Gunn phase-lock is
maintained throughout these scans and is continuously referenced to the 10 MHz output of a rubidium standard. The
10 MHz signal of the standard is accurate to ±0.01 Hz. Sev-
eral 5 MHz scans were usually averaged with half of the
scans taken in increasing frequency and half taken in
decreasing frequency. Typically between two to six of the
5 MHz wide scans were needed to achieve the necessary signal-to-noise level. Center frequencies were determined by fitting Gaussian curves to the line profiles. The measurement
accuracy for transition frequencies is approximately
±50 kHz and observed linewidths ranged from 1 to
1.5 MHz over the frequency range 337–528 GHz.
3. Results and analysis
Rotational transitions within the ground vibrational
state were measured for six isotopologues of zinc chloride:
64
Zn35Cl, 64Zn37Cl, 66Zn35Cl, 66Zn37Cl, 67Zn35Cl, and
68
Zn35Cl, all in natural abundances of the two elements
64
( Zn:66Zn:67Zn:68Zn = 49:28:4:19; 35Cl:37Cl = 3:1). In
addition, vibrational satellite lines were recorded for
64
Zn35Cl, 64Zn37Cl, and 66Zn35Cl.
Ten rotational transitions were measured for the
64
Zn35Cl isotopologue, while five, six, seven, and seven
transitions were recorded for the 66Zn37Cl, 68Zn35Cl,
64
Zn37Cl, and 66Zn35Cl species, respectively. Table 1 lists
the observed frequencies (v = 0) in the range 440–
529 GHz arising from all species, except for 67Zn35Cl. Five
rotational transitions were measured in the v = 1 states of
64
Zn35Cl, 64Zn37Cl, and 66Zn35Cl and in the v = 2 state of
64
Zn35Cl. The respective frequencies are available online
as supplementary information. Six rotational transitions
of 67Zn35Cl were recorded in the range 337–405 GHz,
and frequencies are given in Table 2.
For all species except 67Zn35Cl, each rotational transition
consisted of two fine structure components with a splitting
ranging from 118 to 123 MHz. In the 67Zn35Cl spectrum,
hyperfine interactions were observed as well; each fine structure component was found to be split into a sextet by the 67Zn
nuclear spin (I = 5/2). At frequencies above 463 GHz, the
sextets were not fully resolved. Hyperfine interactions due
to the nuclear spin of chlorine (I = 3/2 for 35Cl and 37Cl) were
not apparent in any of the recorded data.
A representative spectrum of the N = 45 ‹ 44 transition
of 64Zn35Cl (v = 0) is displayed in Fig. 1. Here the fine
structure doublet with a splitting of 122 MHz is apparent, indicating a 2R ground electronic state. Fig. 2 shows
a spectrum of the N = 40 ‹ 39 transition of 67Zn35Cl.
The sextet pattern in each spin–rotation component is
clearly visible in the data, which consists of two 50 MHz
wide scans, separated by a frequency gap.
Molecular constants for ZnCl were determined by fitting
the data using the code ‘‘Hund b’’ developed by J.M.
Brown. Individual vibrational states were modeled with
the effective case (bbJ) Hamiltonian:
H eff ¼ H rot þ H sr þ H mhfðZnÞ
ð1Þ
This Hamiltonian includes terms describing molecular
frame rotation (Hrot; operator form: N2), spin–rotation
interactions (Hsr), and zinc hyperfine coupling (Hmhf(Zn)).
Table 1
Transition frequencies for ZnCl (X2R+: v = 0)a
N0
J0
45
45
46
46
47
47
48
48
49
49
50
50
51
51
52
52
53
53
54
54
55
55
44.5
45.5
45.5
46.5
46.5
47.5
47.5
48.5
48.5
49.5
49.5
50.5
50.5
51.5
51.5
52.5
52.5
53.5
53.5
54.5
54.5
55.5
a
In MHz.
‹
Zn35Cl
N00
J00
v
44
44
45
45
46
46
47
47
48
48
49
49
50
50
51
51
52
52
53
53
54
54
43.5
44.5
44.5
45.5
45.5
46.5
46.5
47.5
47.5
48.5
48.5
49.5
49.5
50.5
50.5
51.5
51.5
52.5
52.5
53.5
53.5
54.5
440
441
450
450
460
460
470
470
479
479
489
489
499
499
509
509
518
518
528
528
66
vobscalc
922.959
046.548
664.690
788.127
402.654
525.870
136.539
259.718
866.521
989.559
592.433
715.320
314.186
436.907
031.683
154.259
744.849
867.283
453.610
575.883
0.048
0.019
0.016
0.009
0.073
0.006
0.009
0.012
0.003
0.017
0.007
0.019
0.015
0.007
0.008
0.003
0.007
0.002
0.020
0.021
Zn35Cl
v
455
455
465
465
474
474
484
484
493
494
503
503
513
513
68
Zn35Cl
vobscalc
493.929
615.955
124.798
246.684
751.752
873.518
374.749
496.350
993.630
115.103
608.377
729.694
218.879
340.055
0.010
0.010
0.003
0.008
0.005
0.008
0.024
0.013
0.005
0.009
0.002
0.003
0.015
0.011
v
Zn37Cl
vobscalc
450869.194
450990.008
469 932.800
470 053.358
488
489
498
498
508
508
517
517
64
980.746
101.022
498.659
618.790
012.430
132.407
521.964
641.790
v
66
Zn37Cl
vobscalc
v
vobscalc
793.051
912.059
187.019
305.886
577.187
695.928
0.004
0.007
0.001
0.007
0.001
0.004
458
458
467
467
070.637
188.233
357.783
475.260
0.003
0.011
0.000
0.004
491 345.835
491 464.300
0.005
0.017
500 842.462
510 098.362
510 216.520
0.006
0.004
0.003
485
486
495
495
504
504
920.643
037.851
196.205
313.262
467.762
584.638
0.003
0.015
0.003
0.009
0.005
0.016
519 586.394
0.011
0.017
0.024
0.012
0.018
0.007
0.011
0.003
0.007
0.003
0.000
0.008
0.012
453
453
463
463
472
472
E.D. Tenenbaum et al. / Journal of Molecular Spectroscopy 244 (2007) 153–159
64
Transition
155
156
E.D. Tenenbaum et al. / Journal of Molecular Spectroscopy 244 (2007) 153–159
Table 2
Transition frequencies for
N0
J0
F0
35
34.5
34.5
34.5
34.5
34.5
34.5
35.5
35.5
35.5
35.5
35.5
35.5
36
37
a
67
ZnCl (X2R+: v = 0)a
N00
J00
F00
v
vobscalc
30
37
36
35
34
33
33
34
35
36
37
38
34
33.5
33.5
33.5
33.5
33.5
33.5
34.5
34.5
34.5
34.5
34.5
34.5
31
36
35
34
33
32
32
33
34
35
36
37
Blend
337 896.970
337 901.333
337 906.108
337 911.336
337 916.752
337 992.276
337 997.528
338 002.812
338 007.619
338 011.914
338 015.924
0.020
0.001
0.012
0.026
0.070
0.016
0.035
0.024
0.061
0.006
0.024
35.5
35.5
35.5
35.5
35.5
35.5
36.5
36.5
36.5
36.5
36.5
36.5
33
38
37
36
35
34
34
35
36
37
38
39
35
34.5
34.5
34.5
34.5
34.5
34.5
35.5
35.5
35.5
35.5
35.5
35.5
32
37
36
35
34
33
33
34
35
36
37
38
347
347
347
347
347
347
347
347
347
347
347
347
514.767
518.413
522.675
527.202
532.138
536.834
615.139
619.860
624.806
629.369
633.524
637.448
36.5
36.5
36.5
36.5
36.5
36.5
37.5
37.5
37.5
37.5
37.5
37.5
34
39
38
37
36
35
35
36
37
38
39
40
36
35.5
35.5
35.5
35.5
35.5
35.5
36.5
36.5
36.5
36.5
36.5
36.5
33
38
37
36
35
34
34
35
36
37
38
39
357
357
357
357
357
357
357
357
357
357
357
357
133.456
136.932
141.025
145.388
149.987
154.174
235.078
239.300
243.805
248.242
252.300
256.047
‹
N0
J0
F0
N00
J00
F00
v
40
39.5
39.5
39.5
39.5
39.5
39.5
40.5
40.5
40.5
40.5
40.5
40.5
37
42
41
40
39
38
38
39
40
41
42
43
39
38.5
38.5
38.5
38.5
38.5
38.5
39.5
39.5
39.5
39.5
39.5
39.5
36
41
40
39
38
37
37
38
39
40
41
42
385
385
385
385
385
385
386
386
386
386
386
386
971.338
974.505
978.152
981.967
985.781
988.638
076.034
078.929
082.769
086.621
090.204
093.638
0.018
0.009
0.006
0.008
0.018
0.069
0.024
0.024
0.011
0.043
0.008
0.020
0.008
0.007
0.053
0.009
0.022
0.022
0.027
0.031
0.008
0.019
0.024
0.005
41
40.5
40.5
40.5
40.5
40.5
40.5
41.5
41.5
41.5
41.5
41.5
41.5
38
43
42
41
40
39
39
40
41
42
43
44
40
39.5
39.5
39.5
39.5
39.5
39.5
40.5
40.5
40.5
40.5
40.5
40.5
37
42
41
40
39
38
38
39
40
41
42
43
Blend
395 580.762
395 584.248
395 587.856
395 591.474
395 594.048
395 683.289
395 685.708
395 689.270
395 693.077
395 696.502
395 699.798
0.055
0.037
0.003
0.002
0.024
0.064
0.057
0.095
0.077
0.012
0.010
0.001
0.040
0.005
0.007
0.032
0.059
0.005
0.028
0.076
0.013
0.030
0.009
42
41.5
41.5
41.5
41.5
41.5
41.5
39
44
43
42
41
40
41
40.5
40.5
40.5
40.5
40.5
40.5
38
43
42
41
40
39
405
405
405
405
405
405
0.014
0.002
0.016
0.070
0.008
0.031
‹
vobscalc
180.675
183.603
187.003
190.410
193.888
196.142
In MHz.
The 67Zn hyperfine interactions were modeled with the
Fermi contact term bF and the spin-dipolar constant c.
Because of the large magnitude of bF, centrifugal distortion
effects were also accounted for using bFD. Attempts to fit
the 67Zn35Cl data with a quadrupole coupling term only
resulted in a statistically undetermined eqQ constant. This
result is understandable given the high N values of this
work. The constants determined for the v = 0 states are
listed in Table 3.
The equilibrium parameters Be, De, ce, c0e , be, and ae for
64
Zn37Cl, 66Zn35Cl, and 64Zn37Cl were established from linear fits of the spectroscopic constants to the equations [14]:
1
Bv ¼ B e a e v þ
ð2Þ
2
1
ð3Þ
Dv ¼ De þ be v þ
2
1
ð4Þ
cv ¼ ce þ c0e v þ
2
The harmonic vibrational frequency xe and anharmonic correction xexe were estimated using the approximations derived by Kratzer [19] and Pekeris [20],
respectively. From the anharmonic potential terms, equilibrium dissociation energy, DE, e, and ground state dissociation energy, DE, v=0, were calculated using the relations
[14]:
DE;e ¼
x2e
4xe xe
DE;v¼0 ¼ DE;e ð5Þ
xe xe xe
þ
2
4
ð6Þ
Table 4 lists the equilibrium parameters and dissociation
energies determined for 64Zn37Cl, 66Zn35Cl, and 64Zn37Cl.
The errors reported in the equilibrium rotational parameters reflect the uncertainties in Bv, Dv, and cv. The reported
error in the equilibrium bond length re is propagated from
the uncertainties in Be, atomic masses, and Planck’s
constant.
E.D. Tenenbaum et al. / Journal of Molecular Spectroscopy 244 (2007) 153–159
64
J = 44.5
Zn35Cl(X2Σ +) : N = 45
44
43.5
J = 45.5
440920
440970
44.5
441020
441070
Frequency (MHz)
Fig. 1. Spectrum of the N = 45 ‹ 44 transition of 64Zn35Cl (v = 0) near
441 GHz. The two fine structure components, indicated by quantum
number J, are prominent in the data and verify the 2R electronic ground
state for ZnCl. The scan is 150 MHz wide and was acquired in 60 s.
Zn35Cl(X2Σ +) : N = 40
67
38.5
386070
41
40
42
F = 42
F = 43
39
39.5
F = 41
38
F = 40
F = 38
37
38
37
F = 39
385985
F = 39
39
F = 40
385970
F = 38
41
40
F = 42
J = 40.5
F = 41
36
F = 37
J = 39.5
39
386085
Frequency (MHz)
Fig. 2. Spectrum of the N = 40 ‹ 39 transition of 67Zn35Cl (v = 0) near
386 GHz. Each fine structure doublet, labeled by J, is additionally split
into six hyperfine components by the nuclear spin of 67Zn (I = 5/2). The
spectrum is composed of two scans separated by a frequency gap. Each
scan is 50 MHz wide and was created by averaging eleven scans, 30 s in
duration.
4. Discussion
Patterns exhibited in the ZnCl spectrum confirm the 2R+
ground state, as predicted by theoretical studies [16] and
observed in optical experiments [13]. Each rotational transition appears as a doublet with a splitting of 120 MHz.
The doublets can be attributed to spin–rotation interac-
157
tions as opposed to lambda-doubling, because their separation remains fairly constant with N. The isovalent molecule
ZnF (X2R+) showed the same pattern in its pure rotational
spectrum, with doublet splittings of 150 MHz [7].
The 64Zn35Cl equilibrium bond length of 2.13003305(24)
Å compares reasonably well to the highest-level theoretical
value of 2.122 Å given by Kerkines et al. [17], which was
calculated by the coupled cluster method with inclusion
of relativistic effects. The fundamental vibrational frequency found here is xe 392 cm1. This value is in good
agreement with those determined from gas-phase electronic
emission spectra by Cornell [13] and Sureshkumar et al.
[12] (390.5 and 389.30 cm1, respectively).
The unpaired electron of ZnCl resides in the antibonding 12r molecular orbital. A simple molecular orbital diagram analysis suggests that the atomic orbitals Zn(4s),
Znð3dz2 Þ, Zn(4pz) and Cl(3pz) are potential contributors
to this orbital. The amount of s, p, and d zinc character
in the 12r orbital can be estimated by comparing the
molecular and atomic hyperfine parameters of the 67Zn
nucleus. Calculations done by Koh and Miller [21] using
the Roothan–Hartree–Fock method predict the Fermi contact term of Zn+ to be 1683.96 MHz. Using this value, a
ratio of 0.71 for bF(molecule)/bF(atom) is found, suggesting that the 12r orbital is 71% Zn(4s) in character. The fact
that the dipolar constant has a small, positive value
(44.0 MHz) implies that there are additional contributions
to this orbital from the Znð3dz2 Þ and Zn(4pz) orbitals.
Assuming that the 3dz2 orbital of Zn is non-bonding [22],
the dipolar constant can be interpreted as the Zn(4pz) contribution to the 12r orbital.
For ZnF, the Fermi contact constant of 67Zn is reported
to be 1291.4(1.6) MHz, arising from the analogous
unpaired electron in the 10r orbital [7]. Comparing this
value with the same atomic Zn+ bF constant from Koh
and Miller [21] indicates that the 10r orbital is 77%
Zn(4s) in character. In addition, the fluorine hyperfine constants have also been measured in ZnF, showing that the
10r orbital is 12% F(2pz) in composition, with negligible
contribution from the F(2s) orbital. The remaining 11% of
the 10r orbital is thus Zn(4pz) in character. The values of
the dipolar constant of 67Zn for the two halides are equal
within the 3r error; in ZnF, c(67Zn) is 39.1(3.1) MHz [7],
compared to 44.0(9.1) MHz in ZnCl, suggesting that the
contribution of Zn(4pz) to the 12r orbital is the same in
both species. In this case, in ZnCl the Zn(4pz) orbital contributes 11% and the Cl(3pz) contributes 18% to the
12r orbital. The 12r orbital in ZnCl is therefore more
covalent in character than its fluoride counterpart. It
should be noted that while splitting due to the nuclear spin
of F (I = 1/2) was observed in ZnF, no hyperfine interactions were resolved in ZnCl arising from the chlorine
nucleus. This result can be attributed to differences in
nuclear g-factors and the fact that ZnF was studied at
lower rotational transitions than ZnCl.
The pr contribution can also be tested by examining the
value of the spin–rotation constant c. In heavy molecules
158
E.D. Tenenbaum et al. / Journal of Molecular Spectroscopy 244 (2007) 153–159
Table 3
Spectroscopic constants for ZnCl (X2R+: v = 0)a
Constant
64
64
66
B0
D0
c0
cD
bF(Zn)
bFD(Zn)
c(Zn)
rms
4913.86656(17)
0.00346561(30)
126.48(30)
0.000481(40)
4742.50431(87)
0.00322777(16)
122.22(18)
0.000465(23)
4861.3137(13)
0.00339170(25)
125.17(25)
0.000474(33)
4689.9479(14)
0.00315656(27)
120.93(29)
0.000461(36)
0.025
0.0090
0.013
0.011
Zn37Cl
Zn35Cl
Zn37Cl
67
Zn35Cl
68
Zn35Cl
4836.1403(14)
0.00335486(48)
124.56(25)
0.000471(54)
1192.9(2.6)
0.00099(30)
44.0(9.1)
0.037
4811.8046(11)
0.00332302(27)
123.92(28)
0.000467(36)
0.015
In MHz; values in parentheses are 3r errors.
Table 4
Equilibrium parameters for ZnCl (X2R+)a
Constant
64
Be (MHz)
ae (MHz)
De (MHz)
be (MHz)
ce (MHz)
c0e (MHz)
re (Å)
xe (cm1)
xexe (cm1)
DE, v=0 (eV)
4927.78483(31)
27.83659(39)
0.00346361(40)
0.00000404(29)
127.63(40)
2.28(31)
2.13003305(24)
392.123(23)
1.73189(14)
2.72752(39)
a
66
Zn35Cl
66
Zn35Cl
64
Zn37Cl
4875.0113(28)
27.3953(43)
0.00338978(47)
0.00000384(63)
126.22(49)
2.12(68)
2.13003234(65)
390.020(27)
1.71376(41)
2.72702(76)
4755.7021(15)
26.3957(19)
0.00322594(29)
0.00000367(35)
123.42(31)
2.39(38)
2.13003185(41)
385.215(17)
1.67179(19)
2.72732(41)
Values in parentheses are 3r errors.
such as zinc chloride, the second order spin–orbit contribution dominates the first order spin–rotation contribution to
this parameter. In ZnCl, the likely source of the second
order coupling is the nearby A2P state. Under the assumption of pure precession, the spin–rotation constant can be
expressed as [23]:
cð2Þ ¼
2Av Bv ‘eff ð1 þ ‘eff Þ
EP ER
Cl equilibrium bond length of 2.079 Å [17], in good agreement with experimental work [4]. When a hydrogen atom is
included in the ZnCl system, the unpaired electron in the
12r antibonding orbital is moved into a bonding orbital
between zinc and hydrogen, explaining why the zinc–chlorine bond length in HZnCl is 0.05 Å shorter than in ZnCl.
Further evidence of the stabilizing effect of the hydrogen is
found in the dissociation energies of ZnCl and HZnCl.
From Eq. (6), we calculated a ground state dissociation
energy of 2.7 eV for 64Zn35Cl, somewhat larger than the
most recent experimental value of 2.4 eV [15] and the highest-level theoretical value of 2.1 eV [17]. In comparison, the
equilibrium dissociation energy of HZnCl going to Cl (2P)
and ZnH (X2R+) is calculated by Kerkines et al. to be
4.79 eV [17].
This work is the final high-resolution spectroscopic
study for the 3d-transition metal monochloride group.
Trends in ground state equilibrium bond length in this series and the monofluoride species are shown in Fig. 3. As the
figure illustrates, the bond lengths do not change significantly in going from cobalt to copper for both series. At
ð7Þ
Here Bv is the rotational constant in the ground state, Av
is the spin–orbit parameter in the excited P state, EP ER
is the energy difference between the ground state and the
excited P state, and ‘eff is the effective orbital angular
momentum in the 12r orbital. The 2P–2R system in ZnCl
is not a pr–pp complex, and therefore pure precession does
not apply in the strictest sense. However, the hyperfine constants imply that some pr character is present in the orbital
of the unpaired electron. Using the spin–orbit constant of
Zn+ (583 cm1) as Av [23], and the experimentally determined value of 18 000 cm1 for EP ER [12], then Eq.
(7) yields ‘eff 0.3 or a 30% pr contribution to the 12r
orbital. This result reinforces the assumption that the
Znð3dz2 ) orbital is basically non-bonding, and the remaining contributions to the 12r orbital are from the Cl(3pz)
and Zn(4pz) atomic orbitals.
Comparing the results of this experiment with studies of
HZnCl shows that the addition of a hydrogen to the zinc
atom in zinc chloride leads to a stabilization of the Zn–
Cl bond. High-level ab initio calculations predict a HZn–
2.3
4
Φ
1
Σ
2.2
Bondlength, re (A)
a
Zn35Cl
7
5
Δ
6
Σ
Σ
6
Δ
Chlorides
2
Σ
2.1
3
Φ
2
3
Φ
2
Π
1
Co
Ni
Cu
Π
1
Σ
2.0
1.9
1.8
7
4
1
Σ
Φ
5
Δ
Σ
6
Σ
2
6
Δ
Σ
Σ
Fluorides
1.7
Sc
Ti
V
Cr
Mn
Fe
Zn
Fig. 3. Experimentally determined ground state equilibrium bond lengths
of 3d-transition metal monochlorides and monofluorides. The trends in
bond distances are similar for these two series, but there are subtle
differences, particularly in going from copper to zinc. References: ScF [24],
ScCl [25], TiF [26], TiCl [27], VF [28], VCl [29], CrF [30], CrCl [31], MnF
[32], MnCl [33], FeF [34], FeCl [35], CoF [36], CoCl [37], NiF [38], NiCl re
was estimated by Zou and Liu [39] from r0 in Hirao et al. [40], CuF [41],
CuCl [42], ZnF [7], ZnCl (this work).
E.D. Tenenbaum et al. / Journal of Molecular Spectroscopy 244 (2007) 153–159
zinc, however, re increases by 3.8% for the chlorides, while
for the fluoride group, re increases by only 0.8%. This difference can be attributed to the variation in ionic character
of the zinc species. Analysis of hyperfine constants shows
ZnCl to be less ionic than ZnF. In the covalent scheme,
the electron that is added in going from the copper halide
to the zinc halide occupies an antibonding orbital; consequently, this addition should increase the covalent bond
length. Because the covalent contribution in ZnCl is larger
than in ZnF, the antibonding effect is more severe in ZnCl
and the bond length increase follows accordingly.
5. Conclusions
The pure rotational spectrum of ZnCl has been
recorded, confirming the molecule has a 2R+ ground state.
From these data, rotational, vibrational, fine structure, and
hyperfine constants have been determined. Interpretation
of the 67Zn35Cl hyperfine constants indicates that zinc chloride is primarily an ionic species, but with more covalent
character than its fluoride analog.
Acknowledgments
This work was supported by NSF Grant No. CHE 0411551. The authors thank Prof. J.M. Brown for use of
his fitting code. E.D.T. acknowledges financial support
from the National Science Foundation Graduate Research
Fellowship Program.
Appendix A. Supplementary data
Supplementary data for this article are available on
ScienceDirect (www.sciencedirect.com) and as part of the
Ohio State University Molecular Spectroscopy Archives
(http://msa.lib.ohio-state.edu/jmsa_hp.htm).
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