Fundamental and Torsional Combination Bands of Two Isomers of the

Jan 16, 2013 - Spectra of two isomers of the weakly bound complex OCS–CO2 are observed in the region of the CO2 ν3 fundamental vibration (∼2349 c...
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Fundamental and Torsional Combination Bands of Two Isomers of the OCS−CO2 Complex in the CO2 ν3 Region J. Norooz Oliaee,† M. Dehghany,† N. Moazzen-Ahmadi,† and A. R. W. McKellar*,‡ †

Department of Physics and Astronomy, University of Calgary, 2500 University Drive North West, Calgary, Alberta T2N 1N4, Canada ‡ National Research Council of Canada, Ottawa, Ontario K1A 0R6, Canada S Supporting Information *

ABSTRACT: Spectra of two isomers of the weakly bound complex OCS−CO2 are observed in the region of the CO2 ν3 fundamental vibration (∼2349 cm−1), using an infrared tunable diode laser to probe a pulsed supersonic slit-jet expansion. Two bands are measured and analyzed for each isomer, the fundamental asymmetric stretch of the CO2 component and a combination band involving this fundamental plus the intermolecular out-of-plane torsional mode. For one isomer, the corresponding torsional combination band is also detected in the OCS ν1 stretching region (∼2060 cm−1). The resulting torsional frequencies are found to be 18.8 and 15.9 cm−1 for isomers a and b of OCS−CO2, respectively. This may be the first time that such a combination band is observed for a higher-energy isomer of a weakly bound complex.

1. INTRODUCTION Accurate and detailed information on intermolecular potential energy surfaces can be obtained from high-resolution infrared spectra of weakly bound complexes. Among the knowledge that can be extracted from spectra, the measurement of lowfrequency intermolecular modes is especially useful in probing potential surfaces, and the results can serve as a sensitive test for ab initio calculations. However, the direct observation of intermolecular modes may pose experimental difficulties as these transitions tend to be weak and to fall in the difficult farinfrared range. Thus, only a few such direct measurements of intermolecular modes are available in the literature. However, these modes can also be measured indirectly in the mid-infrared region by observing combination bands involving the sum of intramolecular plus intermolecular vibrations. Many such observations have been reported, involving, for example, (DF)2,1 (N2O)2,2,3 (OCS)2,4 (CS2)2,5 (CO2)3,6 (N2O)3,7 CO2−C2H2,8−10 N2O−C2H2,9−11 and N2O−CO2.12 In the present article, we add to this list the first experimental determination of an intermolecular frequency for two isomers of the OCS−CO2 complex. Until recently, the experimental spectroscopic study of OCS−CO2 was limited to the pure rotational microwave transitions studied in 1988 by Novick et al.13 With the help of isotopic substitution, they determined a planar geometry with the OCS and CO2 molecules having a slipped near-parallel orientation with the S atom in the “inner” position. This structure, which we call isomer a, is illustrated at the top of Figure 1. A second form of OCS−CO2, isomer b, was detected recently by our group14 in the infrared and subsequently by Sedo and van Wijngaarden15 in the microwave region. Its structure, shown at the bottom of Figure 1, can be described as “O-interior, not quite as parallel”. Prior to our direct observation, the existence of isomer b had been suggested by © 2013 American Chemical Society

Figure 1. Experimentally derived structures of two isomers of the OCS−CO2 complex. Both isomers are planar, and isomer a is the lower in energy. Structure a is from Novick et al.,13 and structure b is from Dehghany et al.14, used with permission from AIP.

ab initio and DFT studies16 and also by the observed structure of the OCS−(CO2)2 trimer.17,18 The observation of isomer b helped to complete the rationalization of the geometries of trimers containing OCS and CO2 in terms of the related dimers. Peebles and Special Issue: Oka Festschrift: Celebrating 45 Years of Astrochemistry Received: December 14, 2012 Revised: January 16, 2013 Published: January 16, 2013 9605

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after observing the torsional combination bands (see below) that we were able to assign the OCS−CO2 fundamentals among all of the competing absorption bands. To identify the OCS−CO2 fundamentals, we carefully compared two spectra in the 2346−2350 cm−1 range, one recorded using a mixture of He and CO2 and the other when the gas mixture also contained OCS. New absorption lines were observed below the CO2 dimer band (which is centered at 2350.77 cm−1) and overlapping with the CO2−He band (centered at 2349.22 cm−1). In particular, Q-branch-like features were observed at 2346.86 and 2348.39 cm−1, which we were able to assign to isomers a and b of OCS−CO2, respectively, as explained in the following sections. 2.2.1. Isomer a Fundamental Band at 2346.857 cm−1. A part of the observed band at 2346.857 cm−1 is illustrated in the top trace of Figure 2. Knowing the isomer a lower-state

Kuckzkowski derived a distorted triangular cylinder structure for OCS−(CO2)2 from its rotational spectrum,17,18 which was in close agreement with the most stable form of this complex (isomer I) obtained from later calculations by Valdés and Sordo.16 The two faces of isomer I containing OCS and CO2 have structures very similar to those of the isolated OCS−CO2 dimers, isomers a and b. Interestingly, a less stable form of the OCS−(CO2)2 trimer (isomer II), in which both faces containing OCS and CO2 resemble isomer b, was also predicted by Valdés and Sordo, and this trimer has recently been detected by our group in the infrared region.19 Our original infrared observations of isomers a and b of OCS−CO 2 were made in the region of the OCS ν 1 fundamental stretch (∼2060 cm−1).14 The present paper reports the detection of the corresponding bands of OCS− CO2, which accompany the CO2 ν3 asymmetric stretch (∼2349 cm−1). In addition to these dimer fundamental bands, we also observe combination bands involving the out-of-plane intermolecular torsional mode for both isomers, and, returning to the OCS ν1 region, we detect one torsional combination band (for isomer a) there as well. Thus, by analyzing vibration− rotation bands for two isomers of the same complex (OCS− CO2) excited in different ways (OCS or CO2 fundamentals, alone or in combination with the low-frequency torsional mode), we obtain rather specific information about the dependence of intermolecular forces on intramolecular motions.

2. RESULTS AND ANALYSIS 2.1. Experimental Conditions. The spectra were recorded at the University of Calgary using a pulsed supersonic slit-jet apparatus, as described in previous publications.20,21 As a part of an ongoing process to improve our spectrometer, a second slit-shaped nozzle was added in order to increase the absorption path length through the expansion gas. Consequently, the signal-to-noise ratio of the recorded spectra was essentially improved by a factor of 2, enabling us to observe weaker transitions. The expansion gas consisted of an equal mixture of OCS and CO2 in helium. For the fundamental bands, the OCS (or CO2) concentration was ∼0.24%. For the combination bands, a more dilute mixture (∼0.14%) was found to produce the optimum signal while reducing the effect of intervening bands. The spectrometer was enclosed in a plastic tent purged with nitrogen gas to minimize the effects of strong atmospheric carbon dioxide absorption in the 2350 cm−1 region. The jet backing pressure was adjusted to 10 and 14 atm for the fundamental and combinations bands, respectively. Wavenumber calibration was carried out by simultaneously recording signals from a fixed etalon with a free spectral range of 0.00997 cm−1 and a reference gas cell containing CO2 or OCS. Line assignment, simulation, and fitting of the data were facilitated by Colin Western’s PGOPHER software.22 2.2. Fundamental Bands. The observation of the spectra of clusters containing at least one CO2 monomer in the proximity of the CO2 ν3 band origin (∼2349.1 cm−1) is rather challenging because of the inevitable presence within 5 cm−1 of relatively strong absorption bands due to CO2−He, (CO2)2, two isomers of (CO2)3, and (CO2)6. Previous observations have shown that fundamental bands due to heterodimers such as CO2−C2H2 and CO2−N2O fall in this region.8,12 In the case of OCS−CO2, early attempts to locate the fundamental bands failed, even with the help from our previous assignment of the corresponding bands in the OCS ν1 region. In fact, it was only

Figure 2. Part of the observed (top trace) and simulated spectra of isomer a of the OCS−CO2 complex in the CO2 ν3 region, showing the band origin (∼2346.85 cm−1) on the left and R-branch transitions to the right. Also shown is the simulation for the overlapping fundamental band of isomer b. A Gaussian line width of 0.0022 cm−1 and a rotational temperature of 2.5 K are used in the simulation. Blank regions in the experimental spectrum correspond to CO2 monomer absorption lines.

rotational parameters from previous work13,14 helped greatly in assigning the transitions. As expected from the orientation of CO2 within the dimer, these transitions were mostly b-type (ΔKa = ±1). However, the weaker a-type (ΔKa = 0) transitions are responsible for the Q-branch that is visible at 2346.86 cm−1. A total of 130 transitions were assigned, with J and Ka ranging up to 10 and 3, respectively, out of which 36 were a-type. The observed a-to-b-type intensity ratio was approximately 1:4 (i.e., 1:2 ratio for transition dipole moments). 2.2.2. Isomer b Fundamental Band at 2348.387 cm−1. The second observed band, centered at 2348.387 cm−1 and assigned to isomer b, is shown partly in the top trace of Figure 3. Again, the lower-state rotational parameters were known;14,15 therefore, rotational line assignments were not too difficult once the band had been recognized among all of the competing absorptions. This is also a hybrid band but with approximately equal a- and b-type contributions. A total of 113 transitions were assigned with J and Ka values ranging up to 13 and 3, respectively, with an almost equal number of a- and b-type transitions. 9606

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Table 1. Molecular Parameters for Isomer a of the OCS− CO2 Complex (in cm−1)a ν0 A B C 106 107 107 107 108

× × × × ×

ΔK ΔJK ΔJ δK δJ

ground state

CO2 ν3

OCS ν1

0.1485890(10) 0.05062772(50) 0.03768182(47) 0.54(20) 0.0b 1.613(60) 3.9(19) 4.70(34)

2346.85716(6) 0.148250(21) 0.0506583(69) 0.0376728(47) 4.7(15) 0.0b 3.80(35) 3.9c 10.8(29)

2066.12623(4) 0.1479757(49) 0.0505832(25) 0.0376112(14) 0.30(15) 0.0b 1.82(11) 7.8(20) 5.79(60)

a Uncertainties in parentheses are 1σ from the least-squares fits in the units of the last quoted digit. bThe parameter could not be determined with statistical significance. cParameter fixed at its ground-state value.

slightly different than those of ref 15 because the infrared data extend to higher values of J and Ka compared to the microwave data. The weighted rms deviation of the fit was 0.00020 cm−1 (unweighted 0.00027 cm−1), and results of the isomer b fit are given in Table 2. The observed line positions, rotational assignments, and relative weights for isomers a and b are provided as Supporting Information in Tables A1 and A2.

Figure 3. Part of the observed (top trace) and simulated spectra of isomer b of the OCS−CO2 complex in the CO2 ν3 region, showing the band origin (∼2348.39 cm−1) on the left and R-branch transitions to the right. Also shown are simulations for the overlapping fundamental bands of isomer a and the CO2 dimer. A Gaussian line width of 0.0022 cm−1 and a rotational temperature of 2.5 K are used in the simulation. Blank regions in the experimental spectrum correspond to CO2 monomer absorption lines.

Table 2. Molecular Parameters for Isomer b of the OCS− CO2 Complex (in cm−1)a ground state ν0 A B C 106 × ΔK 107 × ΔJK 107 × ΔJ 107 × δK 108 × δJ

2.2.3. Analysis. Simulations, line assignments, and fits were done using PGOPHER22 with a conventional A-reduced asymmetric rotor Hamiltonian in the Ir representation.22 Data from previous microwave and infrared studies were included in the fits, with microwave transitions given a relative weight of 10000 to reflect their higher precision. Weaker or blended infrared lines were given reduced relative weights. For isomer a, 15 microwave transitions from Novick et al.13 as well as 264 transitions from our previous infrared observation14 of this isomer in the OCS ν1 region were included in the fit. For the ground state, we varied all rotational and quartic centrifugal distortion parameters except for ΔJK, which was not well determined and fixed to zero. For the newly observed CO2 ν3 fundamental band, the band origin, the three upper-state rotational parameters, and three of the five quartic distortion constants were determined, while ΔJK and δK were not well determined and were fixed to their ground-state values. For the corresponding band in the OCS ν1 region, refined parameters were obtained by varying all upper-state parameters except for ΔJK. The quality of the fit was rather good, with a weighted rms deviation of 0.00024 cm−1 (unweighted 0.00029 cm−1). Results for isomer a are given in Table 1. For isomer b, 33 microwave transitions measured by Sedo and van Wijngaarden15 and 169 previous infrared transitions14 were included in the fit. For the new CO2 ν3 fundamental band of this isomer, the band origin, upper-state rotational parameters, and two of the upper-state quartic centrifugal distortion constants were determined, while ΔJK, ΔK, and ΔJ were fixed at their ground-state values. For the OCS ν1 fundamental band of this isomer, the upper-state parameters were significantly improved, compared to those in ref 14, thanks to the availability of the microwave data to help determine the ground state. Our ground-state parameters are

0.25965871(71) 0.03632409(114) 0.03180295(112) 21.28(18) −25.51(28) 1.618(36)

CO2 ν3

OCS ν1

2348.38749(5) 0.258177(18) 0.0363587(20) 0.0317982(18) 13.3(14)

2055.72191(4) 0.257768(12) 0.0364201(20) 0.0318500(20) 19.81(67)

−25.51b

−26.7(11)

1.041(83) b

5.9(56)

5.9

2.75(19)

2.75b

1.512(89) 5.9b 2.75b

Uncertainties in parentheses are 1σ from the least-squares fits in the units of the last quoted digit. bParameter fixed at its ground-state value. a

2.3. Torsional Bands. Because both isomers of the OCS− CO2 dimer belong to the Cs point group and taking into account all of the intermolecular modes (van der Waals stretch, in-plane symmetric and asymmetric bends, and out-of-plane torsion), we know that c-type combination bands can only arise from the torsional mode, which has A″ upper-state symmetry. The situation is entirely analogous to the case of the CO2−N2O complex,12 and we expected the resulting c-type torsional combination bands to have a distinctive appearance similar to that shown in Figure 3 of ref 12. Searching in the CO2 ν3 fundamental region for such bands, we came across two weak Q-branch features that required the presence of OCS in the expansion gas mixture. One of them occurs at 2364.24 cm−1, just above and overlapping with the torsional combination band of the cyclic CO2 trimer.6 The other is at 2365.78 cm−1. Preliminary fits using ground-state parameters from ref 14 revealed that these could indeed be assigned as combinations of the intramolecular CO2 ν3 vibration plus the intermolecular 9607

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out-of-plane torsional vibration for OCS−CO2 isomers b and a, respectively. 2.3.1. Isomer a Torsional Combination Band at 2365.783 cm−1 in the CO2 ν3 Region. The top trace of Figure 4 shows a

Figure 5. Q-branch and part of the R-branch of the observed (top trace) and simulated torsional band of isomer b in the CO2 ν3 region. Simulations for the overlapping torsional band of cyclic (CO2)3 and the torsional band of isomer a are also shown. A Gaussian line width of 0.0022 cm−1 and a rotational temperature of 2.5 K are used in the simulation.

power and other experimental parameters, it was not possible to decide which combination band was stronger. However, at least it can be seen that isomer b (Figure 5) is comparable in intensity to isomer a (Figure 4). 2.3.3. Isomer a Torsional Combination Band at 2084.858 cm−1 in the OCS ν1 Region. Knowing the fundamental and torsional combination band centers for the two isomers in the CO2 ν3 stretching region, the (upper-state) torsional frequency for each isomer is simply given by their differences, which are 18.92 cm−1 for isomer a and 15.86 cm−1 for isomer b. With this information, it was natural to search for the analogous torsional bands in the OCS ν1 region,14 assuming that the intermolecular modes do not depend too strongly on the intramolecular vibrations. This assumption is supported by experiments such as the measurement of the out-of-plane torsional frequency of the N2O−CO2 dimer, where the torsional frequency is measured to be 25.802 and 25.707 cm−1 in the CO2 ν3 and N2O ν1 stretching regions, respectively.12 Therefore, the corresponding torsional bands of OCS−CO2 in the OCS ν1 region should appear at around 2085.05 and 2071.58 cm−1 for isomers a and b, respectively. Fortunately, we had laser coverage around these regions, and the torsional combination band of isomer a was detected at 2084.86 cm−1, only about 0.2 cm−1 lower than anticipated. Figure 6 shows the central part of this band, which partly overlaps with an OCS dimer combination band23 (shown by the red trace). Because of limited coverage and low laser power in this region, it was only possible to assign 21 transitions (see Table A5 of the Supporting Information). Unfortunately, the isomer b torsional band could not be detected in this region, most likely because of the masking effect of strong absorption lines of the fundamental band of the nonpolar OCS dimer.24,25 2.3.4. Analysis. The results of the analyses of the three torsional combination bands are summarized in Table 3. Ground-state parameters were fixed at the values from Tables 1

Figure 4. Q-branch and part of the R-branch of the observed (top trace) and simulated torsional band of isomer a in the CO2 ν3 region. A simulation for the overlapping torsional band of isomer b is also shown. A Gaussian line width of 0.0022 cm−1 and a rotational temperature of 2.5 K are used in the simulation.

portion of the isomer a torsional combination band. It also contains residual contributions from the R-branch of the isomer b combination band, as indicated by the simulation (red trace). This band is fairly weak and required extensive signal averaging (over 2000 scans), but the distinctive c-type Q-branch is still quite evident. A total of 112 transitions were assigned, with J and Ka values ranging up to 11 and 6, respectively. Of these, 19 transitions were given reduced relative weights in the analysis because of blending with transitions of isomer b and other unknown species. The observed line positions, assignments, and weights are given in Table A3 of the Supporting Information. 2.3.2. Isomer b Torsional Combination Band at 2364.244 cm−1 in the CO2 ν3 Region. The observed and simulated torsional combination band spectra for isomer b are illustrated in Figure 5, which includes a simulation of the interfering CO2 trimer torsional band6 (green trace). We assigned 117 transitions, of which 42 were given a lower relative weight due to blending with transitions from other species. The J and Ka values ranged up to 13 and 5, respectively. The line positions and assignments are given in Table A4 of the Supporting Information. We believe this may be the first example of such a combination band being observed for a higher-energy isomer of a weakly bound complex. Comparing Figures 4 and 5, the most apparent difference between these two combination bands is the separation of the two lobes of the central Q-branch (rQ and pQ). This separation is proportional to the rotational parameter A, and the significantly larger A-value of isomer b explains the wider splitting of the Q-branch in Figure 5. Due to variations in laser 9608

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assignments with Ka′ = 2 were less certain but fitted quite well when we fixed all distortion parameters to their groundstate values and simply varied the band origin and the three upper-state rotational parameters in the fit. It is fair to say that this is the least well characterized of the five bands studied in the present work, but it still gives us a reliable value for the most important parameter, the band origin.

3. DISCUSSION AND CONCLUSIONS Interestingly, all of the vibrational changes in rotational parameters determined here for the fundamental bands involving the CO2 ν3 excitation are smaller than those determined for the OCS ν1 excitation. The changes in rotational parameters for the combination bands (Table 3) are considerably larger, which is not surprising because these involve the intermolecular torsional modes, which almost certainly have larger amplitudes. By far, the largest change is that of the A-value for the 2364.24 cm−1 torsional band of isomer b in the CO2 ν3 region, which decreases by 0.033 cm−1, or 1000 MHz. At the same time, this upper state also had a very large value of ΔK and required inclusion of ΦK. Such “anomalous” values for the parameters describing the Ka rotational motion are a signature of an a-type Coriolis interaction, and all of the other three intermolecular modes (van der Waals stretch and in-plane symmetric and asymmetric bends) have the correct symmetry (A′) for such an interaction with the torsional mode. Since the effective A-value is reduced, the perturbing state must lie above the observed torsional combination state. For example, tests using PGOPHER show that a state lying 2.2 cm−1 higher and having an a-type Coriolis interaction parameter of 0.25 cm−1 could account well for the observed anomalies in A′, ΔK′, and ΦK′. This interaction parameter corresponds to ζa ≈ 0.5 for the dimensionless Coriolis zeta parameter. A similar, but considerably smaller, atype Coriolis interaction was observed previously for an intermolecular antigeared bend combination band of the nonpolar N2O dimer.3 Ab initio calculations16 suggest that isomer a of OCS−CO2 is lower in energy than isomer b, and this is confirmed by experiment in the sense that isomer b seems only to appear when helium is used as the supersonic expansion carrier gas. However, in previous infrared14 and microwave15 experiments, the isomer b spectra tended to be stronger (using helium). The same appears to be the case in the present study; we estimate that the fundamental band of isomer b in the CO2 ν3 region was stronger than that of isomer a by almost a factor of 2, though the determination is not very precise. Does this indicate that isomer b really has a higher abundance in the jet, or could it simply mean that the microwave dipole moment and infrared transition moments are enhanced in isomer b relative to isomer a? The latter is certainly possible because microwave Stark effect measurements15 show that the dipole moment of isomer a is reduced relative to that of the OCS monomer by about 16%. An improved high-level ab initio calculation would help to illuminate this apparent anomaly, as would a microwave measurement of the isomer b permanent dipole moment. The vibrational shifts observed for the fundamental bands of OCS−CO2 relative to the corresponding monomer band origins are shown in Table 4. In the CO2 ν3 region, the bands of the two isomers are considerably closer together and closer to the monomer, indicating that the geometry shift shown in Figure 1 has less effect on the CO2 vibration than on the OCS vibration.

Figure 6. Central part of the observed (top trace) and simulated (dark blue trace) torsional band of isomer b in the OCS ν1 region. Simulation of an overlapping OCS dimer combination band is also shown. A Gaussian line width of 0.0017 cm−1 and a rotational temperature of 2.5 K are used in the simulation. (The slightly narrower width here, compared to that in the other figures, is believed to be an instrumental effect, not due to upper-state lifetime.).

Table 3. Molecular Parameters for Torsional Combination Bands of the OCS−CO2 Complex (in cm−1)a

ν0 A B C 106 × ΔK 107 × ΔJK 107 × ΔJ 107 × δK 108 × δJ 106 × ΦK

isomer ab CO2 ν3 + torsion

isomer bc CO2 ν3 + torsion

isomer ab OCS ν1 + torsion

2365.78257(10) 0.146612(14) 0.0500526(64) 0.0378175(67) 0.54d

2364.24469(10) 0.226294(39) 0.035912(22) 0.032138(21) −380.7(41)

2084.8580(2) 0.14614(11) 0.050157(17) 0.0377349(84) 0.54d

1.6(36) 1.68(63) 7.(17) 4.70d 0

36.1(35) 1.34(58) −239.(96) 7.0(29) −4.60(11)

0.0d 1.613d 3.9d 4.70d 0

a Uncertainties in parentheses are 1σ from the least-squares fits in the units of the last quoted digit. bGround-state parameters are fixed at the values reported in Table 1. cGround-state parameters are fixed at the values reported in Table 2. dThese parameters were fixed at groundstate values.

and 2 as determined from the microwave data and fundamental bands. For isomer a in the CO2 ν3 region, seven upper-state parameters were varied, and ΔK and δJ were fixed at their ground-state values. However, for isomer b, all five upper-state quartic distortion parameters were varied, and it was also necessary to include the sextic parameter ΦK. This special case is discussed below. The weighted root-mean-square deviations of the fits were about 0.00027 and 0.00043 cm−1 for isomers a and b, respectively. For the combination band of isomer a in the OCS ν1 region, one transition, 505 ← 515, was apparently perturbed and excluded from the fit. Most of the remaining 20 transitions involved upper-state levels with Ka′ = 0 and 1. Three 9609

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Table 4. Vibrational Shifts of the Fundamental Bands of Two Forms of the OCS−CO2 Complex in the Regions of OCS ν1 and CO2 ν3 Vibrations with Respect to the Corresponding Monomer Band Origin (in cm−1) isomer a isomer b

OCS ν1 region

CO2 ν3 region

+3.9254 −6.4789

−2.2856 −0.7553

Table 5. Band Centers and Torsional Frequencies for the Two Isomers of the OCS−CO2 As Measured in the OCS ν1 and CO2 ν3 Regions (in cm−1)

isomer a (CO2 ν3 region) isomer b (CO2 ν3 region) isomer a (OCS ν1 region) isomer b (OCS ν1 region)

torsional band

torsional frequency

2346.85716

2365.78257

18.9254

2348.38749

2364.24469

15.8572

2066.12623

2084.8580

18.7318

2055.72191

18.7318 cm−1, depending on whether ν3 of CO2 or ν1 of OCS is excited. For isomer b, it is 15.8572 cm−1 for CO2 ν3. The similarity of these values for isomer a in the two vibrational states supports the general assumption that intermolecular modes of weakly bound van der Waals complexes do not depend strongly on the intramolecular vibrations. Thus, it is reasonable to expect that the ground-state torsional frequencies of OCS−CO2 should be close to the values reported here. In conclusion, five new rotationally resolved infrared bands of the OCS−CO2 complex have been studied using a tunable diode laser spectrometer to probe a supersonic jet expansion through a dual-slit nozzle. Two fundamental bands arising from the ν3 asymmetric stretch of the carbon dioxide subunit of the two OCS−CO2 isomers yielded band origins and upper-state rotational parameters. Three combination bands yielded the first experimental values for the out-of-plane torsional frequency of this complex. The results of this work should be useful in testing and refining the intermolecular potential energy surface that explains the forces between two important molecules, carbonyl sulfide and carbon dioxide.

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ASSOCIATED CONTENT

S Supporting Information *

Detailed line measurements and assignments. This material is available free of charge via the Internet at http://pubs.acs.org.

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REFERENCES

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A key result of this study is the determination of torsional vibration frequencies for the OCS−CO 2 complex, as summarized in Table 5. For isomer a, this is 18.9254 or

fundamental band

Article

AUTHOR INFORMATION

Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENTS We thank F. Mivehvar for help with the measurements and L. Murdock for technical assistance. The financial support of the Natural Sciences and Engineering Research Council of Canada is gratefully acknowledged. 9610

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