ARTICLE pubs.acs.org/JPCA
Pure Rotational Spectrum and Ring Inversion Tunnelling of Silacyclobutane Jennifer van Wijngaarden,* Ziqiu Chen, Cody W. van Dijk, and John L. Sorensen Department of Chemistry, University of Manitoba, Winnipeg Manitoba, R3T 2N2 Canada ABSTRACT: The ground state pure rotational spectrum of silacyclobutane (SCB) (c-SiH2C3H6) has been investigated using both Fourier transform microwave (FTMW) and chirped pulse Fourier transform microwave (cp-FTMW) spectroscopies. Spectra of the 13C, 29Si, and 30Si singly substituted isotopologues, in natural abundance, were recorded in the 624 GHz region along with those of the normal species. The ring inversion tunnelling splitting in the ground vibrational state was resolved and analyzed to determine the energy splitting of the two states: 75.7260(19) MHz. Structural analysis based on heavy atom substitution provided accurate geometric parameters including the bond lengths, bond angles, and ring puckering angle of the SCB ring backbone.
I. INTRODUCTION Silacyclobutane (SCB) has been explored as a precursor for the production of device quality SiC films via chemical vapor deposition techniques.14 The principal advantage of SCB over other more commonly used starting materials is that it is both carbon and silicon bearing. This simplifies film production by eliminating the need for process control over multicomponent gas mixtures that contain potentially toxic constituents such as silanes and chlorosilanes. Compared with other single-source gas precursors that have been studied, SCB is an ideal prototype for SiC film production because its ring strain energy facilitates its decomposition at lower temperatures.4 From the spectroscopic point of view, small heterocyclic molecules such as SCB provide interesting model systems to probe the connection between the unique spectroscopic signature of a molecule and its underlying potential energy surface.57 SCB has been the subject of numerous spectroscopic and theoretical investigations since the first mid-infrared and NMR spectra were reported in 1967.8 Within a few years of this initial study, the farinfrared spectra corresponding to the ring puckering modes of the normal, deuterated, and halogenated versions of SCB were reported.9,10 Based on the observed far-infrared spectra, SCB is characterized by a double minimum potential along the ring puckering coordinate with a barrier to planarity of 440 cm1 as shown in Figure 1. As the frequency of the ring puckering fundamental of SCB is only 157 cm1, the lowest three vibrational states fall below this barrier, which leads to inversion doubling of these levels. The puckered geometry and double minimum potential function were subsequently confirmed and refined by Pringle via microwave spectroscopy of the ground and first excited ring puckering vibrational states of SCB.11 Analysis of the microwave spectrum revealed that the energy levels of the inversion pair in the ground vibrational state are split by ∼75.75 MHz (labeled states 0 and 1 by Pringle), while those in the first excited ring puckering state are split by ∼7800 MHz (labeled states 2 and 3) (Figure 1). As the resolution of the r 2011 American Chemical Society
microwave spectrometer used in Pringle’s study was 0.2 MHz, the a-type transitions within states 0 and 1 were completely coalesced; however, the energy difference between these states was estimated (∼75.75 MHz) via the observation of c-type transitions that connect states 0 and 1. More recently, the structure of SCB has been investigated using ab initio and density functional theory (DFT) methods,12,13 and the results compare well with the latest gas phase electron diffraction (ED) experiments.13 As expected based on the earlier spectroscopic work, the geometry of SCB is that of a flexed ring (Cs symmetry) with a puckering angle of 34.5 (MP2/ cc-pVTZ)12 or 33.5 (ED).13 On the basis of the ab initio calculations,12 the previously reported IR and Raman bands10 were partially reassigned, although none of these have been investigated with rotational resolution. Rotationally resolved infrared spectra would provide solid evidence for final, irrefutable assignment of the various fundamental vibrations, hotbands, and combination bands of SCB. In this paper, we report the first high-resolution microwave spectroscopic investigation of the ground vibrational state of normal SCB and four minor isotopologues in natural abundance (29Si-4.7%, 30Si-3.1%, 13C-1.1%). The observed spectra include heavy atom substitution at each of the three unique ring sites (Si, R-C, β-C), which are reported here for the first time. The spectrum of the parent species was extended to lower frequency and measured at a higher resolution than in the previous microwave study.11 This enabled the accurate experimental determination of the positions of the heavy atoms in the SCB ring. Furthermore, the high resolution of our instrument allowed the observation of the ground state ring inversion tunnelling splitting of states 0 and 1 for the first time. The spectral analysis has led to an improved set of ground vibrational state spectroscopic constants in support of Received: May 29, 2011 Revised: July 5, 2011 Published: July 05, 2011 8650
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Figure 1. Potential energy curve along the ring puckering coordinate Φ of SCB. The barrier to planarity and splitting of levels 2 and 3 are taken from ref 11. The splitting of levels 0 and 1 was determined in this work, and the ground state ring puckering angle was derived: 31.1(4)o. The energy level splittings are not to scale.
rotationally resolved infrared investigations of SCB and its ring puckering mode.
II. EXPERIMENTAL SECTION SCB was prepared by a one-step reduction8 of the precursor 1,1-dichlorosilacyclobutane (GELEST, 97%) with lithium aluminum hydride (LiAlH4). The precursor and LiAlH4 were mixed in a 2:1 ratio in n-butyl ether for 24 h, and the SCB product was then isolated via an 8 h distillation at 110 C with a dry ice and acetone-cooled receiving flask. The product purity was verified by GC/MS and was used without further purification. The sample for spectroscopic study was prepared as a gas mixture of 1% SCB using neon as the carrier gas. The total backing pressure was maintained at approximately 5 atm. The spectra of all isotopologues were observed in natural abundance. The pure rotational spectra of the parent and four minor isotopologues of SCB were recorded using a Balle-Flygare type pulsed-jet Fourier transform microwave (FTMW) spectrometer, which has been previously described.14 The FTMW technique typically yields spectral linewidths of ∼7 kHz (full width at halfmaximum (fwhm)), which was critical for the resolution of the ring inversion tunnelling splitting of SCB. With this technique, line positions are determined with an accuracy of (1 kHz. The FTMW experiments were carried out in tandem with measurements using our broadband chirped pulsed-jet Fourier transform microwave (cp-FTMW) spectrometer,15 which permits the collection of survey spectra over a 6 GHz bandwidth. For SCB, the cp-FTMW spectrometer proved most useful for locating weak transitions, including those of the minor isotopologues. As the linewidths tend to be considerably broader (∼150 kHz fwhm) with this instrument, all transition frequencies identified in the broadband surveys were subsequently verified using the higher-resolution FTMW instrument.
Figure 2. Structure of SCB in its principal axis system.
Figure 3. FTMW spectrum of the Doppler-doubled 101000 transition of the parent isotopologue of SCB after 200 averaging cycles. The additional 10 kHz splitting of this transition is due to ring inversion tunnelling. The lower frequency transition corresponds to the symmetric inversion tunnelling component (state 0), and the higher corresponds to the antisymmetric component (state 1).
III. SPECTRAL ASSIGNMENT AND ANALYSIS The ground state structure of SCB is that of a puckered ring as shown in Figure 2. Although the point group symmetry is Cs, the molecular symmetry group is C2v as a result of ring inversion tunnelling arising from the double-well nature of SCB’s potential 8651
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Table 2. Experimental Spectroscopic Constants for the Inversion Tunnelling States (States 0 and 1) of Normal SCB state 0
state 1
A
8815.6917(12)
8815.6945(12)
B C
6288.9936(15) 4245.2692(17)
6288.9897(11) 4245.2811(11)
Rotational Constants/MHz
Centrifugal Distortion Constants / 103 MHza ΔJ
2.10(14)
ΔJK
4.41(16)
4.41
ΔK
1.91(16)
1.91
δj
0.313(19)
0.313
δk
1.36(21)
1.36
2.10
ΔE01 /MHz
75.7260(19)
rms error/kHz # transitions
3.0 26
a
The centrifugal distortion constants of state 1 were fixed to those of state 0 during the fit.
Figure 4. Energy level diagram of some of the observed transitions of the parent SCB species. The observed a-type (vertical lines) and c-type (diagonal lines) transitions form closed loops, which were used to unambiguously assign transitions to the ring inversion tunnelling states 0 (dashed lines) and 1 (solid lines).
Table 1. Observed Microwave Transitions of SCB in MHz a-type
state 0
state 1
J0 Ka0 Kc0 J00 Ka0 0 Kc0 0
νobs
νobs νcalc
νobs
101000
10534.2524
0.0020
10534.2624
0.0000
202101 212111
20234.6555 19024.7465
0.0067 0.0042
20234.6818 19024.7771
0.0003 0.0037
211110
23112.1797
0.0016
23112.1908
0.0002
211212
6131.1597
0.0013
6131.1314
0.0057
221202
15028.0274
0.0013
15028.0088
0.0047
312313
12027.3755
0.0007
12027.4009
0.0031
322303
17728.2627
0.0022
17728.2321
0.0025
νobs νcalc
c-type J0 Ka0 Kc0 v0 J00 Ka0 0 Kc0 0 v0 0
νobs
νobs νcalc
110(0)000(1) 110(1)000(0)
15028.9525 15180.4040
0.0008 0.0028
202(0)110(1)
15588.5019
0.0000
202(1)110(0)
15739.9866
0.0046
221(0)211(1)
7504.3328
0.0047
221(1)211(0)
7655.8097
0.0043
220(0)212(1)
14469.2710
0.0017
220(1)212(0)
14620.7660
0.0013
331(0)321(1) 331(1)321(0)
14580.8274 14732.3218
0.0006 0.0008
energy surface. Due to the 440 cm1 barrier to planarity, the vibrational ground state is split into two (labeled as 0 and 1 here
Figure 5. Example cp-FTMW spectrum of the 101000 transitions of three minor isotopologues of SCB after 40 000 average cycles.
following Pringle’s notation) corresponding to the symmetric (A1) and antisymmetric (B1) inversion tunnelling components of the wave function. As a result, state 0 has rotationvibrationinversion energy levels (KaKc) of symmetry A1(ee), A2(eo), B1(oo) and B2(oe), while state 1 has rotationvibrationinversion energy levels of symmetry B1(ee), B2(eo), A1(oo), and A2(oe). SCB has dipole moment components along both the a-axis (0.439 D) and c-axis (0.146 D),11 and by symmetry, the microwave spectrum should be composed of a-type transitions within a particular inversion state (0 or 1) and c-type transitions that cross inversion states (0 to 1, 1 to 0). (a). Normal Isotopologue. Based on the previously reported rotational constants,11 26 transitions of normal SCB were observed in the range from 6 to 24 GHz. These include 10 weak c-type transitions that cross between states 0 and 1 and eight stronger a-type transitions that are inversion tunnelling doubled. 8652
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Table 3. Observed Microwave Transitions of the 29Si and 30Si Isotopologue of SCB in MHz 29
a-type J0 Ka0 Kc0 J00 Ka0 0 Kc0 0
30
Si
state 0
state 1
νobs
νobs νcalc
Si
state 0
νobs
νobs νcalc
νobs
state 1
νobs νcalc
νobs
νobs νcalc
101000
10419.4088
0.0050
10419.4225
0.0024
10310.3711
0.0043
10310.3843
0.0015
202101
20049.2484
0.0029
20049.2758
0.0029
19871.2627
0.0000
19871.3889
0.0000
212111
18837.9585
0.0013
18837.9899
0.0019
18660.2092
0.0011
18660.2510
0.0004
211110
22839.6112
0.0012
22839.6225
0.0005
22581.1958
0.0011
22581.2058
0.000
c-type J0 Ka0 Kc0 v0 J00 Ka0 0 Kc0 0 v0 0
νobs
νobs νcalc
110(0)-000(1)
14950.1750
0.0006
110(1)-000(0)
15100.4981
0.0006
νobs
νobs νcalc
15024.8449
0.0000
Table 4. Observed Microwave Transitions of the 13C of SCB in MHz R-13C a-type J0 Ka0 Kc0 J00 Ka0 0 Kc0 0
a
β-13C
state 0
state 1
νobs
νobs νcalc
νobs
state 0
state 1
νobs νcalc
νobs
νobs νcalca
νobs
νobs νcalca 0.0000
101000
10462.0680
0.0007
10462.0822
0.0010
10330.2530
0.0000
10330.2628
202101
20018.1617
0.0000
20018.1881
0.0000
19904.4344
0.0000
19904.4626
0.0000
212111
18840.3275
0.0002
18840.3574
0.0003
18694.5167
0.0000
18694.5488
0.0000
211110
23007.8605
0.0002
23301.8797
0.0003
The fit of the β-13C isotopologue involved six transitions and six unknowns as described in the text.
Figure 3, for example, shows the observed 10 kHz splitting of the 101-000 a-type transitions of states 0 and 1. The assignment of the individual transitions to states 0 and 1 was unambiguously determined by the presence of two closed loops of transitions as depicted in the energy level diagram shown in Figure 4. The transition frequencies of all assigned transitions of normal SCB are summarized in Table 1. The transitions involving states 0 and 1 for SCB were fit together using Watson’s A-reduced Hamiltonian in Ir representation in Pickett’s spectral fitting program.16 The observation of c-type transitions allowed the determination of the energy spacing between these two inversion tunnelling components (ΔE01) of the ground vibrational state of normal SCB. The rotational and centrifugal distortion constants for each state plus the ΔE01 term are summarized in Table 2. As the inversion tunnelling states lie very close in energy (ΔE01 = 75.7260(19) MHz), the centrifugal distortion constants were assumed to be equal in states 0 and 1 so that fewer parameters were varied. (b). 29Si, 30Si and 13C Isotopologues. Transitions due to the less abundant isotopologues of SCB were sought using the broadband capability of the cp-FTMW spectrometer. Figure 5 shows a typical survey spectrum of SCB featuring the 101000 transitions of the normal species and its 29Si and R-13C analogues. Note that the resolution does not allow the observation of the tunnelling splitting, but each new a-type transition found with this instrument was later investigated with the higher resolution FTMW spectrometer. In the end, 10 transitions were observed for 29 Si including two c-type transitions and four tunnelling split a-type transitions. For the 30Si species, one c-type transition and four
tunnelling split a-type transitions were assigned. These are summarized in Table 3. The weak c-type transitions were not observed for the 13C species, but, as in the other isotopologues, the a-type transitions were tunnelling split. In total, four tunnelling split a-type transitions were assigned for the R-13C species along with three for the β-13C species. These are listed in Table 4. The rotational spectra of each of the four minor isotopologues were fit as described above for the normal species. As fewer lines were observed overall, the centrifugal distortion constants were held fixed to the values determined from the normal species. The rotational and centrifugal distortion constants determined for the 29Si and 30Si isotopologues along with the ΔE01 term are summarized in Table 5. As only a-type transitions were observed for the 13C species, the ΔE01 term could not be directly determined. As a result, this term was held fixed to the value determined for the 29Si isotopologue, which has the same mass. As only three transitions for each state were observed for the β-13C species, the fit of the rotational constants for this species yielded a root-mean-square (rms) error of zero. In the subsequent structural calculations, the uncertainties determined in fitting the rotational constants of the R-13C isotopologue were used to approximate those of the β-13C species. The rotational and centrifugal distortion constants determined for 13C isotopologues are summarized in Table 5.
IV. DISCUSSION The microwave spectrum of SCB was measured with high resolution and consisted of strong a-type and weak c-type 8653
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Table 5. Experimental Spectroscopic Constants for the Inversion Tunnelling States (States 0 and 1) of 29Si, 30Si and 13C SCBa 29
30
Si
R-13C
Si
state 0
state 1
state 0
β-13C state 0c
state 1
state 1c
state 0
state 1
A/MHz
8815.251(11)
8815.186(11)
8814.294(11)
8814.771(11)
8599.3515(25)
8599.2956(25)
8810.3526(25)
8810.3256(25)
B
6210.1282(9)
6210.124(9)
6135.4392(6)
6135.4358(6)
6272.92537(18)
6272.92542(17)
6148.10728(18)
6148.10903(18)
C
4209.2940(8)
4209.3091(8)
4174.9447(6)
4174.9585(6)
4189.15183(18)
4189.16427(17)
4182.15429(18)
4182.16678(18)
75.198(11)
74.647(11)
75.198b
75.198b
rms error/kHz
2.4
1.6
0.5
0.5
# transitions
10
9
8
6
ΔE01 /MHz
a
The centrifugal distortion constants of both states of 29Si, 30Si and 13C SCB were held fixed to the values determined for the normal isotopologue as reported in Table 2. b The ΔE01 parameter for both was held fixed to the value reported or the 29Si species. c The uncertainties reported here are those obtained for the equivalent parameter in the R-13C isotopologue, as the fit of the β-13C species contained only six transitions and six parameters.
Table 6. Substitution Coordinates (Å) from the Kraitchman Equations for SCBa
a
R-13C
β-13C
a b
0.4121(36) (1.2003(13)
1.3490(11) 0b
1.0160(15) 0c
1.0154(15) 0c
c
0.167(9)
0.204(7)
0.053(28)
0.063(24)
29
Si
30
Table 7. Substitution (rs) and Effective (ro) Structural Parameters of SCB rea
EDb,c
rs (this work)
r0c (this work)
R(SiRC) (Å) R(RCβC) (Å)
1.893 1.558
1.885(2) 1.571(3)
1.879(6) 1.567(5)
1.8848(8) 1.5646(21)
— RCSiRC ()
78.2
77.2(9)
79.4(5)
79.10(4)
— SiRCβC ()
85.9
87.9(12)
86.4(9)
86.28(6)
— RCβCRC ()
100.0
97.0(15)
100.0(5)
100.18(18)
Φ ()
34.5
33.5(27)
30.5(36)
31.1(4)
Si
Note that while the Kraitchman equations provide only absolute values of the coordinates, the signs are inferred based on the orientation of the conformer in its principal axis system based on the ab initio predictions and the observed selection rules. b Small imaginary number as the β-C atom lies on the a-axis. c Small number as the Si atom lies on the a-axis. The Costain error is larger than the coordinate.
transitions, which is consistent with Pringle’s original work in which he determined the dipole moment contributions along the a- and c-axes to be 0.439 and 0.146 D, respectively.11 The Hamiltonian employed appears to provide a good model of the ground state, as the rms error of the fit was only 3.0 kHz when the two inversion components were fit together for the normal SCB isotopologue. (a). Ring Inversion Tunnelling. Using FTMW spectroscopy, the ring inversion tunnelling splitting of the a-type spectrum was resolved for the first time, and this splitting is on the order of 1030 kHz for the observed transitions. This splitting was too small to be sensitive to the inclusion of a Coriolis-type interaction term in the Hamiltonian as Pringle used for the excited ring puckering vibrational state of SCB (states 2 and 3).11 In the excited vibrational state, the inversion levels are separated by ∼7790 MHz, which is 2 orders of magnitude greater than the separation in the ground state (ΔE01 = 75.7260(19) MHz). In fact, comparison of the rotational constants for states 0 and 1 (Table 2) reveals that they are essentially identical to within the 1σ uncertainties of these parameters. Although the splitting is small, the tunnelling components corresponding to states 0 and 1 have been unambiguously assigned due to the observation of closed loops connecting the asymmetric rotor energy levels (Figure 4). Confirmation that the observed splitting arises from ring inversion tunnelling is found by comparing the energy differences (ΔE01) determined for the three Si isotopologues (28Si: 75.7260(19) MHz, 29Si: 75.198(11) MHz, 30Si: 74.647(11) MHz). As expected, the heavier isotopologues exhibit a smaller energy gap due to decreased tunnelling probability, as they experience a higher barrier to planarity. (b). Structural Determination. The selection rules corresponding to the observed microwave spectra of SCB are
τCCSiC
a
-19.891(23)
Ab initio values (MP2/cc-pVTZ) from ref 12. b ED data from ref 13. c The parameters in bold were fit (uncertainty in parentheses) to reproduce the rotational constants. Other parameters were calculated from these using trigonometric relationships.
consistent with the puckered ring structure shown in Figure 2. The geometry shown corresponds to the Cs point group and is in agreement with previous ab initio and DFT calculations.12,13 Although there are three carbon atoms in SCB, only two unique 13 C spectra were observed due to the equivalence of the two R carbon atoms. This is confirmed by comparison of the relative intensities of the 29Si and R-13C transitions shown in Figure 5. The relative intensities are approximately 2:1, while the natural abundance of 29Si and 13C are 4.7% and 1.1%, respectively. The rotational constants of the five isotopologues were used to derive structural information for the ground state of SCB. As the rotational constants are very similar for states 0 and 1, the results for the two states match to within the parameter uncertainties. The analysis involving the rotational constants for state 0 is described below. (i). rs Geometry. A Kraitchman analysis17,18 was performed to obtain the coordinates of the heavy atoms in the SCB ring. These are summarized in Table 6. The uncertainties of the substitution coordinates were calculated according to the Costain rule.19 The nonzero c-coordinates (particularly for the carbon atoms) clearly demonstrate that the ring backbone is nonplanar. The Kraitchman equations produced a small imaginary number for the b-coordinate of the β-C atom as it lies in the ac-plane. This coordinate was set to zero in the subsequent structural analysis as were the b-coordinates for the Si atom. In the latter, the values were small, real numbers, but the Costain uncertainties were larger than the coordinates themselves. In fact, the Si atom lies very close to the a-axis as seen upon comparison of the A rotational constants of the three Si 8654
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The Journal of Physical Chemistry A isotopologues (Tables 2 and 5), which differ by less than 1 MHz. On the basis of the ab initio geometry, the R-C lies on the opposite side of the ab-plane relative to the β-C and Si atoms. As such, appropriate signs have been added to the Kraitchman coordinates listed in Table 6. Using the substitution coordinates, the bond lengths and angles involving the heavy atoms within the SCB ring were calculated using trigonometric relationships. These are summarized in Table 7. (ii). ro Geometry. The r0 geometry of the SCB ring was obtained by fitting the 15 experimentally determined rotational constants to five key structural parameters involving the heavy atoms of SCB using Kisiel’s STRFIT program.18 The geometric parameters involving the hydrogen atoms were fixed at the ab initio values reported in ref 12. The fit parameters include the SiRC (1.8848(8) Å) and RCβC (1.5646(21) Å) bond lengths, the R CSiRC (79.10(4) o) and SiRCβC (86.28(6)o) bond angles, and the βCRCSiRC (19.891(23)o) dihedral angle. The maximum discrepancy between the observed and calculated rotational constants from the structural fit was better than 0.01%. The parameters obtained from the fit are listed in bold in Table 7 along with other angles calculated using trigonometric relationships. The r0 structural parameters derived in the present study are compared to the theoretical equilibrium values12 and those found from ED13 experiments in Table 7. The bond lengths and angles derived via microwave spectroscopy in the present study match previous structural estimates to within 0.5% and 3%, respectively. The largest discrepancy is in the ring puckering angle of 31.1(4)o (the angle between the planes containing RCSiRC and RCβCRC), which differs by 7% from earlier reported values. It should be noted that the ED results represent an average over multiple energy states, which creates an interpretation challenge for such a fluxional molecule. The ED data was most recently analyzed by invoking a dynamic model to treat the large amplitude ring puckering motion. Compared with the structural estimates derived from earlier ED experiments,20 the use of this dynamic model led to considerable changes in the estimated geometry of SCB; particularly for the RCβC bond length, which is shorter by 0.04 Å in the more recent analysis. In the present microwave study, the ring puckering angle is derived from spectra sensitive only to the ground vibrational state of SCB, and the reported uncertainty (0.4) in Table 7 is that calculated via propagation of errors from the least-squares fit of 15 spectroscopic constants to five structural parameters. The Costain error for the same parameter from the rs structural analysis is considerably larger (3.6), and this error is meant to reflect the effects of vibrational averaging. Comparison of the ring puckering angle derived from the Kraitchman analysis (30.5(36)o) with that obtained from the ED data (33.5(27)o) shows agreement to within these uncertainties. Such comparisons of ab initio, ED, and microwave results highlight the value of a multifaceted approach for characterizing the structure and dynamics of fluxional molecules such as SCB.
V. CONCLUSIONS In conclusion, we observed and assigned the pure rotational spectra of the normal species of SCB and four isotopologues involving 13C, 29Si, and 30Si substitution. The spectra are consistent with a puckered ground state structure (Cs point group symmetry) that undergoes ring inversion through a planar ring intermediate of C2v symmetry. The components of this
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tunneling splitting were unambiguously assigned due to the observation of closed loops of a-type and c-type transitions, and the energy difference between the inversion states of the ground vibrational state of SCB was precisely determined to 75.7260(19) MHz. The derived rotational constants were used to accurately determine the structure of the SCB ring backbone. Analysis of the spectra has led to an improved set of ground-state spectroscopic constants in support of rovibrational investigations of ring puckering and other low frequency vibrations in SCB.
’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected]. Phone: (204) 474-8379. Fax: (204) 474-7608.
’ ACKNOWLEDGMENT J.v.W. would like to extend her deep gratitude to W. C. Pringle for the many illuminating discussions over the past few years. This research is funded by the Natural Sciences and Engineering Research Council of Canada (NSERC) through the Discovery Grants, University Faculty Award (J.v.W.), and Undergraduate Students Research Award (C.W.v.D.) programs. We are grateful to the University of Manitoba for research fellowship support through the Faculty of Science (USRA - C.W.v.D.) and Faculty of Graduate Studies (UMGF - Z.C.) and for equipment purchased through the University Research Grants program (URGP). ’ REFERENCES (1) Steckl, A. J.; Yuan, C.; Li, J. P.; Loboda, M. J. Appl. Phys. Lett. 1993, 63, 3347–3349. (2) Johnson, A. D.; Perrin, J.; Mucha, J. A.; Ibbotson, D. E. J. Chem. Phys. 1993, 93, 12937–12948. (3) Loboda, M. J.; Seifferly, J. A.; Dall, F. C. J. Vac. Sci. Technol. A. 1994, 12, 90–96. (4) Shi, Y. J.; Lo, B.; Tong, L.; Li, X.; Eustergerling, B. D.; Sorensen, T. S. J. Mass. Spectrom. 2007, 42, 575–583. (5) Legon, A. C. Chem. Rev. 1980, 80, 231–262. (6) Laane, J. Annu. Rev. Phys. Chem. 1994, 45, 179–211. (7) Laane, J. Int. Rev. Phys. Chem. 1999, 18, 301–341. (8) Laane, J. J. Am. Chem. Soc. 1967, 89, 1144–1147. (9) Laane, J.; Lord, R. C. J. Chem. Phys. 1968, 48, 1508–1513. (10) Laane, J. Spectrochim. Acta 1970, 26A, 517–540. (11) Pringle, W. C. J. Chem. Phys. 1971, 54, 4979–4988. (12) Al-Saadi, A. A.; Laane, J. Organometallics 2008, 27, 3435–3443. (13) Novikov, V. P.; Dakkouri, M.; Vilkov, L. V. J. Mol. Struct. 2006, 800, 146–153. (14) Sedo, G; van Wijngaarden, J. J. Chem. Phys. 2009, 131, 044303–044307. (15) Evangelisti, L.; Sedo, G.; van Wijngaarden, J. J. Phys. Chem. A. 2011, 115, 685–690. (16) Pickett, H. M. J. Mol. Spectrosc. 1991, 148, 371–377. (17) Kraitchman, J. Am. J. Phys. 1953, 21, 17–24. (18) Kisiel, Z. PROSPE Programs for Rotational SPEctroscopy, . (19) Costain, C. C. Trans. Am. Crystallogr. Assoc. 1966, 2, 157–164. (20) Mastryukov, V. S.; Dorofeeva, O. V.; Vilkov, L. V.; Cyvin, B.; Cyvin, S. Zh. Struckt. Khim. 1975, 16, 438–440.
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