J. Phys. Chem. A 2010, 114, 1427–1431
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Determination of the Structure of Cyclopentene Oxide and the Argon-Cyclopentene Oxide van der Waals Complex† Andrea J. Minei,‡ Jennifer van Wijngaarden,§ Stewart E. Novick,‡ and Wallace C. Pringle*,‡ Department of Chemistry, Wesleyan UniVersity, Middletown, Connecticut 06459, and Department of Chemistry, UniVersity of Manitoba, Winnipeg, Manitoba, Canada R3T 2N2 ReceiVed: July 28, 2009; ReVised Manuscript ReceiVed: September 22, 2009
Rotational spectra of cyclopentene oxide and the argon-cyclopentene oxide van der Waals complex were studied using pulsed-jet Fabry-Perot Fourier transform microwave (FTMW) spectroscopy. Spectra of the parent along with those of the 13C and 18O singly substituted isotopologues, in natural abundance, of the monomer and of the complex were measured in the frequency region of 5-26.5 GHz. The complete heavy atom substitution structure was determined for the monomer and complex. The boat structure for cyclopentene oxide was confirmed with naturally abundant 13C and 18O isotopes. For the argon cyclopentene oxide complex, both a and b-type transitions were observed and the rotational constants for the all-12C 16O isotopologue were determined to be A ) 3268.254(2), B ) 993.345(1), and C ) 950.430(9) MHz. The r0 coordinates of the argon in the principal axis system of cyclopentene oxide are a ) 0.27, b ) 0.42, and c ) 3.91 Å, such that the argon is exo to the boat of the ring and on the opposite side of the ring from the oxygen and is 0.42 Å off to the side and 0.27 Å from the center of mass toward the back end of the ring (again away from the oxygen). Large amplitude van der Waals bending vibrations require an averaging model to account for differences between the observed complex and monomer planar moments of inertia. Introduction Six-membered ring compounds such as cyclohexane assume a chair conformation in the ground vibrational state and have a higher energy boat conformation. In the chair conformation, the C-C-C angles are nearly tetrahedral and the torsional interactions are a minimum due to a staggered conformation. In cyclopentene oxide (CPO), 6-oxabicyclo[3.1.0]hexane, there is added ring strain due to the cross ring C-C bond in the five-membered carbon ring. Lafferty1 studied the microwave spectrum of the ground vibrational state and the first two excited states of the out-of-plane ring vibration of CPO. He also measured the dipole moments in the ground state. The measured planar moments and projections of the bond dipole moments were modeled by Lafferty against the dihedral ring angles, and this confirmed the boat conformation for CPO.1 Carriera and Lord2 studied the far-infrared spectrum of CPO and determined that their data could be best fit with a single minimum, asymmetrical potential, suggesting the chair conformation has an energy approximately 1000 cm-1 higher than the boat. Antolinez et al.3 using FTMW spectroscopy determined the heavy atom substitution structure of CPO. They assigned the hydrogenbonded complexes of HCl and DCl, including 35Cl and 37Cl isotopes, with CPO. They found the HCl hydrogen bonded in an equatorial position to the lone pair of electrons on the epoxide oxygen. They assigned 12 transitions of the parent monomer and 6 for each 13C isotopologue and 5 for 18O. Since the isotopologue transitions were among J ) 0, 1, and 2 only, the centrifugal distortion constants were fixed at the values for the parent. In the parent, since the maximum J and Ka values were 5 and 2, respectively, ∆JK and δJ were fixed at zero. We extended the †
Part of the “W. Carl Lineberger Festschrift”. * To whom correspondence should be addressed. E-mail: wpringle@ wesleyan.edu. ‡ Wesleyan University. § University of Manitoba.
Figure 1. Structure of the argon-cyclopentene oxide complex in the principal axis system of the monomer (with superscript of m for the monomer as in text). The alpha, beta, and gamma carbons are labeled. The polar coordinates corresponding to the mixed r0, rs, re structure given in Table 3 are shown; R0 ) 3.94, θ0 ) 7.3°, Φ0 ) 57.3°(not shown but defined from the am axis toward b).
assigned spectra of the monomer to 60 a and c transitions for the parent up to J ) 7, Ka and Kc ) 6. We also extended their assignments to 18, 21, 12, and 9 transitions for the CR, Cβ, 18O, and Cγ isotopologues, respectively, see Figure 1. The structures of the van der Waals (vdW) complexes of argon with methylene cyclobutane4 and cyclobutanone5 have been studied in our laboratory. By observing the 13C isotopomers in natural abundance, we have shown that the large amplitude bending motion of the argon across the rings leads to equivalence in the beta cross ring carbons with an intensity relative to the parent of 2%. The r0 positions of the argons are about 0.5 Å to the side of the ac planes
10.1021/jp907214a 2010 American Chemical Society Published on Web 10/15/2009
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TABLE 1: Spectral Assignment and Frequencies for 12C, 13C, and 18O Isotopologues of Cyclopenteneoxide frequency (MHz) J′
K a′
K c′
J′′
Ka′′
Kc′′
all- C
4 6 3 3 1 2 2 5 3 3 5 1 4 3 4 4 4 4 3 7 2 4 3 5 5 3 2 6 5 2 5 6 6 4 5 5 5 6 7 5 2 4 3 4 6 7 7 6 2 6 3 2 3 3 2 5 3 2 3 3
2 4 3 1 0 2 2 4 3 2 2 1 4 3 1 3 3 4 2 3 0 2 0 4 3 1 1 2 5 0 5 5 4 0 1 4 2 3 4 3 1 4 0 1 6 2 3 1 1 2 1 2 1 0 2 1 2 2 1 2
2 3 1 2 1 1 0 2 0 2 3 0 1 1 3 2 1 0 1 4 2 3 3 1 3 3 2 4 1 2 0 1 3 4 4 2 4 4 4 2 1 1 3 4 1 5 5 5 1 5 2 0 3 3 1 5 2 0 2 1
4 6 3 3 0 2 2 5 3 3 5 0 4 3 4 4 4 4 3 7 1 4 2 5 5 2 1 6 5 1 5 6 6 3 5 5 5 6 7 5 1 4 2 3 6 7 7 6 1 6 2 1 2 2 1 4 2 1 2 2
2 3 2 1 0 0 1 3 2 0 2 0 3 1 1 1 2 3 1 3 1 0 2 3 1 2 1 2 4 0 4 4 2 2 1 2 0 1 2 2 1 2 1 2 5 2 1 1 0 0 2 1 1 0 1 2 2 0 1 2
3 3 1 3 0 2 2 2 2 3 4 0 1 2 4 3 3 2 3 5 0 4 0 3 4 1 1 5 1 1 2 3 4 1 5 3 5 5 5 4 0 2 1 2 1 6 6 6 1 6 0 0 2 2 1 3 1 1 1 0
5806.678 6256.212 6853.370 7527.981a 7790.103a 7892.129 8015.952 8809.529 9630.943 9810.306 10012.889 10250.540a 10731.585 11532.036 11614.641a 11733.970 12089.077 12103.478 12206.632a 12419.051 12485.097a 12605.394 12925.321 13138.447 13280.767 13716.415a 14288.064a 14464.633 14569.730 14945.535a 15103.830 15284.159 15289.405 15477.891 15552.830a 15567.337 15883.609 15900.234 16568.284 16770.698 16872.316a 17013.981 17064.856a 18069.992 18187.331 18611.357 19116.306 19275.783 19332.750 19368.483 20609.727 21011.884 21097.709 21452.087 21669.359 21695.661 23370.249 23472.317 24749.295 25288.394
a
12
γ- C
β or β′-13C
R or R′-13C
7664.128a
7728.943a
7486.894 7739.361a
13
7889.342
10155.809a
18
O
7564.495a 7932.418
9667.961
9730.453
10134.579a
10177.887a
10030.049a
11679.726
12379.778a 12521.923
12406.046a
14080.715a
14146.480a
14193.250a
13933.211a
14748.353a
14785.409a
14844.573a
14593.209a
16575.760a
16769.264a
16764.159a
16324.744a
19174.897
19202.684
18790.295
20594.248 20724.434 20867.014 21191.012 21363.372
20493.095 20857.488 20955.918 21304.894 21508.817
21329.856
23186.772 23130.068 24549.419 25182.517
23218.021 23296.014 24586.423 25131.133
22693.433 23135.396 24032.975 24372.158
16801.524
19067.441
20813.757 21200.564 22992.329
Transitions from ref 3.
perpendicular to the rings. If this were the equilibrium position of the argon, there would have been different spectra for the two β 13 C, since they would no longer be equivalent. The fact that there is only one spectrum for the β 13C species means that the equilibrium position of the argon, re, is in the Cs plane perpendicular to the ring as also supported by ab initio calculation and Watson’s
rm(1) method.6 In the argon complex of chlorocyclobutane,7 we observed different spectra for the β 13C and β′ 13C since the argon is 2.8 Å from the ac plane of the chlorocyclobutane monomer. Thus a bending vibration across the ring in chlorocyclobutane would require an amplitude of about 5.6 Å to make the β 13C and β′ 13C isotopologues equivalent and that does not happen. In the
Structure of CPO and Ar-CPO van der Waals Complex
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TABLE 2: Spectroscopic Constants for the all-12C, 13C, and 18O Isotopologues of Cyclopentene Oxide A (MHz) B (MHz) C (MHz) ∆J (kHz) ∆K (kHz) ∆JK (kHz) δJ (kHz) δK (kHz) N σ (kHz) Pam(amu Å2) Pbm Pcm
all-12C
γ-13C
β or β’-13C
R or R’-13C
5709.4227(5) 4541.1203(4) 3248.9873(4) 0.84(2) 1.02(1) -0.71(2) 0.149(2) 0.31(1) 60 5 89.1613 66.388 22.128
5699.980(3) 4455.8310(8) 3208.3011(5) 0.75(8) 1.02* -0.71* 0.149* 0.31* 9 3 91.139 66.383 22.280
5614.4084(4) 4520.1735(2) 3208.7751(2) 0.81(5) 1.02* -0.71* 0.149* 0.31* 21 2 89.645 67.854 22.160
5665.4778(6) 4512.4140(4) 3226.9508(4) 0.76(5) 1.02* -0.71* 0.149* 0.31* 18 4 89.703 66.909 22.294
18
O
5649.9149(7) 4380.1377(5) 3184.363(6) 0.77(8) 1.02* -0.71* 0.149* 0.31* 12 3 92.317 66.388 23.061
* Centrifugal distortion constants fixed at parent value.
TABLE 3: Structure of Cyclopentene Oxide and the Argon Atom in the Argon-Cyclopentene Oxide in the Principal Axis System of the Parent Monomer (Å) O CR Cβ Cγ HR Hβ Hβ’ Hγ Hγ′ Ar(rs) Ar(r0, rs, re) Ar(rm(1)) Ar(re, ab initio)
am
bm
cm
-1.261 -0.731 0.675 1.411 -1.469 0.731 1.113 1.356 2.494 0.268 0.246 0.157 0.332
0.000 (0.733 (1.228 0.000 (1.327 (2.106 (1.541 0.000 0.000 (0.415 (0.419 0.000 -0.009
-0.706 0.411 0.184 -0.397 0.977 -0.485 1.159 -1.501 -0.142 3.906 3.906 3.870 3.744
CPO complex, two pairs of equivalent 13C result since the equilibrium position of the argon is also in the ac plane of CPO monomer. Spycher et al.8 and Klots et al.9 have developed models to examine the large amplitude motion of the argon that results in the planar moments of the complex, Pbc and Pcc, to be slightly smaller than the planar moment of the monomer, Pam and Pbm. The superscripts m and c refer to the monomer and complex, respectively. Planar moments are defined in a cyclic manner as
Pξ)(Iζζ + Iηη - Iξξ)/2 )
∑ miξi2
where
ξηζ ) a,b,c (1)
We show below that this model for the averaging of the cross ring argon bending motion is equivalent to the extreme Kraitchman structure and the mixed r0, rs, re structure determined using the structural fit program by Schwendeman.15 Experiment The spectra of CPO, argon-CPO complex, and their corresponding isotopologues were measured using a pulsed-jet Fourier transform microwave spectrometer based on the original design by Balle and Flygare,10 and modifications to that have been described elsewhere.11 Cyclopentene oxide (98%, Aldrich) was prepared as a gas mixture of 0.5% with argon as the carrier gas. The backing pressure for both the monomer and complex were approximately 2 atm. The spectra of all the isotopic species for both the monomer and the complex were observed in natural abundance.
Cyclopentene Oxide Monomer. Lafferty1 has previously assigned the a,c-type conventional microwave spectrum of the parent CPO monomer. As described above, Antolinez et al.3 fit a limited number of FTMW transitions for the parent and 13C and 18O isotopologues and determined the Kraitchman substitution structure for the monomer. We observed their transitions, labeled with an asterisk in Table 1, and added 50 more parent and twice as many isotopologue transitions. Results are presented in Table 1. Assigned frequencies were fit to a semirigid rotor Hamiltonian in Ir representation with a Watson A reduction,16 using Pickett’s spectral fitting programs12 with three rotational constants and five quartic centrifugal distortion constants that are reported in Table 2. The rotational constants for the monomer, CPO, and Kraitchman substitution structures are only modestly improved over those of ref 3, but the centrifugal distortion constants were fit better since we observed transitions up to J ) 7 and Ka and Kc ) 6. The planar moments are also reported in Table 2. We also observed spectra from three unique 13C isotopologues of CPO, of which two pairs of these isotopologues had intensities that were about twice as large as the lone 13Cγ isotopologue, consistent with the observations in ref 3. This reconfirms that the pairs of β and R carbons are equivalent to their symmetric β′ and R′ counterparts, respectively, see Figure 1. The heavy atom Kraitchman substitution structure13 confirms the Cs symmetry of the molecule and boat structure of the ring as in refs 1 and 3. Although, Kraitchman substitution coordinates are absolute values, only the coordinates with the signs given in Table 3 are consistent with all the isotopologue substitution coordinates and the re structure from a DFT calculation with a basis set, B3LYP/6-311++g(2df,2pd), using the Gaussian suite of programs.14 A mixed ro, re, and rs structure was determined by fitting the ground state rotational constants of all isotopologues, fixing the heavy nuclei at their Kraitchman substitution positions. The re bond lengths and angles for the hydrogens, determined from above, were added as constraints in the leastsquares structural determination of the structure maintaining Cs symmetry as given in Table 3. The structural program was written by Schwendeman and modified by Hillig.15 The 21 observed moments of inertia for the isotopologues are calculated with an rms error of 0.002 amu Å2. The fit C-H bond lengths changed at most 0.01 Å, and the C-C-H bond angles changed less than 1° from the constrained re values from the DFT structure. Cyclopentene Oxide-Argon van der Waals Complex. The assigned spectra for the argon complexes with CPO of the parent, consisting of 143 a, b transitions, and all isotopologues are available in the Supporting Information. Observation of
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TABLE 4: Spectroscopic Constants for the All -12C, 13C, and 18O Isotopologues of Argon-Cyclopentene Oxide 12
A (MHz) B (MHz) C (MHz) ∆J (kHz) ∆K (kHz) ∆JK (kHz) δJ (kHz) δK (kHz) N σ (kHz) Pac (amu Å2) Pbc Pcc a
C
3268.254(2) 993.345(1) 950.430(9) 2.234(4) -16.1(1) 14.8(2) 0.095(2) 4.7(4) 143 5 442.934 88.803 65.830
γ-13C
β or β′-13C
R or R′-13C
3225.756(1) 988.467(2) 942.301(1) 2.198(4) -16.03(8) 14.67(4) 0.098(2) 4.7(7) 42 3 445.465 90.859 65.810
3227.746(2) 988.491(2) 947.760(2) 2.210(5) -15.9(1) 14.63(4) 0.087(3) 4.4(9) 71 5 443.962 89.273 67.299
3245.744(1) 990.916(1) 948.141(1) 2.216(3) -16.11(7) 14.76(2) 0.092(2) 4.3(4) 70 3 443.664 89.358 66.347
18
O
3209.202(8) 977.6950(6) 931.1366(9) 2.097(6) -16.1a 14.8a 0.095a 4.7a 11 5 451.093 91.661 65.815
Fixed to those of the parent.
equivalent R and β 13C’s in the argon-CPO complex spectra indicates that the equilibrium symmetry of the complex is Cs with the argon in the Cs plane (see Figure 1). The vibrational averaging of large amplitude van der Waals bending displacement across the ring in the ground vibrational state leads to an ro position of the argon that is not in the ac plane of the ring, while the equilibrium position is in the ac plane. Analysis of relative intensities showed that the 13Cβ and 13CR isotopomers are approximately twice as intense as the 13Cγ isotopomer. Thus the β and R carbons are equivalent, not only in the structure of the monomer, CPO, but in the argon complex as well. Spectroscopic constants for the all-12C, three 13C and the 18O isotopologues of the argon CPO complex are listed in Table 4. A consequence of the large amplitude vdWs bending motions is that the planar moments of the complex Pbc and Pcc, are approximately 0.56 and 0.31 amu Å2 smaller than the planar moment of the monomer, Pam and Pbm. Klots et al.9 and Spycher et al.8 developed models to explain this reduction in the planar moments in the rare gas-planar phenyl complexes. The rings were rotated about the a and b axes of the monomer to angles to account for the large amplitude argon bending vibrations. This led to averaged moments of inertia in the complexes that reduced the planar moments of the complexes until these matched the planar moments of the monomers. The rotated monomers and argon were treated as a pseudodiatomic with a separation of the center of mass of the monomer and argon of R and an angle, γ, that R makes with the monomers original c axis in the Cs plane. This was done for each isotopologue to further investigate the averaged position of the argon and ring. We determined the position of the argon, reported in the principal axis system of the monomer, in four ways. First, the mixed r0, rs, re structure of the monomer given in Table 3 and described above is assumed to be unchanged in the complex. Only the three argon coordinates are varied to minimize the deviation of the calculated and observed moments of inertia for all the isotopologues of the complex. This leads to the mixed coordinate values labeled Ar(r0, rs, re) in Table 3; amAr ) 0.246, bmAr ) (0.419, cmAr ) 3.906 Å. Note that the coordinates of the oxygen are amOx ) -1.260, bmOx ) 0.000, cmOx ) -0.707 Å for reference. In the second method, extreme Kraitchman, the complex is considered an isotope of the monomer with an argon mass change from 0 to 40.0. There are errors for this method since the vibrational averaging in the complex is much different from the monomer. The result is labeled Ar(rs) in Table 3 with a, b, c (0.268, 0.415, and 3.906 Å). This position of the argon is drawn in Figure 1 with polar spherical coordinates of R0 ) 3.94 Å, θ0 ) 7.3° and φ0 ) 57.3° that are relative to the cm axis of the monomer. Both of these methods are vibrationally
TABLE 5: Kraitchman Substitution Coordinates for the Position of Argon in the All-12C, 13C1, and 18O Isotopologues of the Complex in the PAS of the Same Cyclopentene Oxide Isotopologue a b c
all-12C
γ-13C
β or β′-13C
R or R′-13C
0.268 0.415 3.906
0.233 0.423 3.912
0.271 0.406 3.904
0.262 0.414 3.902
18
O
0.357 0.426 3.915
averaged over the large amplitude stretch and the two bends of the van der Waals bond. The fits once again have an rms deviation of 0.001 amu Å2. The differences in the planar moments from the monomers to the vdW complexes for all the isotopologues are naturally satisfied. The model of ring bending about the monomer’s a axis described in ref 8 and ref 9 is clearly shown in the b coordinate of the argon that is 0.4 Å out of the monomer’s Cs plane. The extreme Kraitchman coordinates for the argon in each isotopologue are given in Table 5. Since the spectral isotopic evidence is conclusive that the equilibrium position of the argon is in the Cs plane of the monomer and complex, we fit the isotopologue rotational constants of the complex using Watson’s rm(1) mass-dependent structural analysis16 with a program, STRFIT, from Kisiel.17 The Watson structure has been shown to be closer to the equilibrium structure than the r0 or rs structures. Indeed, the structure obtained when fitting the three argon coordinates, keeping the monomer structure fixed, and Watson’s three rm(1) parameters places the argon in the Cs plane of the monomer, as given in the next to last row of Table 3. The values are labeled Ar(rm(1)) and are amAr ) 0.157, bmAr ) 0.000, cmAr ) 3.870 Å. In ab initio calculation, we optimized only the argon position, fixing the monomer structure as optimized in the DFT calculation described above with the B3LYP/6-311++g(2df,2pd) basis set. This was carried out at the MP2 level with 6-311++g(2df,2pd) basis set on the Wesleyan computer cluster.18 The last row of Table 3 reports this result that is also approximately the equilibrium position of the argon in the PAS of cyclopentene oxide. The values are labeled Ar(re, ab initio) amAr ) 0.332, bmAr ) -0.009, cmAr ) 3.744 Å. The Watson and ab initio results for the a coordinate of the argon leads to an estimate for the coordinate, γe, in the vibrational rotation of the ring to match the planar moments of the monomer and complex as derived in refs 8 and 9 (see Figure 2). From Table 3, the bending angle of the monomer ring about its a axis is approximately formed with the bm and the cm coordinates of the vibrationally averaged argon, such that Rx ) arctan(0.419/3.906) ) 6.12° and that about the b axis is Ry ) arctan(0.247/3.906) - arctan(0.157/3.870) ) 1.32° for rm(1) as shown in Figure 2. Using the ab initio
Structure of CPO and Ar-CPO van der Waals Complex
J. Phys. Chem. A, Vol. 114, No. 3, 2010 1431 Gaussian14 shows that the most positive portions of the CPO molecule are the hydrogens on the CR and CR′ on the opposite side of the ring from the oxygen. Argon, though chemically neutral and inert except for dispersion interactions, acts as a Lewis base partially sharing a pair of electrons to the most positive part of the ring. This avoids the polar C-O bonds. In the structure given in Table 3 using the argon position labeled, Ar(rs), the argon-alpha proton distance, 3.08 Å, is slightly larger than the sum of the vdWs radii of Ar and H, 2.97 Å.19 Our previous work on argon ring complexes has the argon in a position that is in van der Waals contact with relatively positive hydrogens. In the argon CPO complex, the position of argon is exo to the ring avoiding the polar C-O bonds and the oxygen lone pairs to which the HCl hydrogen is bound in the CPO-HCl complex.3
Figure 2. Argon-cyclopentene oxide structure indicating the equilibrium position of the argon, Ar(re), at the circle in the middle of the horizontal dashed line that is in the Cs plane. The angle, Ry, is the rotation of the argon from the re position about the y/bm axis. The rotation of the argon about the x/am axis takes the argon out of the Cs plane and lands the argon at its r0 position, Ar(r0). All the dashed lines are in the Cs plane of cyclopentene oxide and the angles are exaggerated for clarity.
calculation for the argon equilibrium position, Rx is the same 6.12° and Ry is arctan(0.247/3.906) - arctan(0.332/3.744) ) -1.46°. The numerators of these arctangents are the displacements of the argon from the equilibrium positions given in Table 3. The alphas are the same as those in ref 8 and ref 9, except the equilibrium position of the argon is known from the Watson and ab initio calculations. The rotational angles, exaggerated for clarity, are shown in Figure 2 where the dashed lines are in the Cs plane of the monomer and the x axis is the principle a axis of the monomer and the y axis is the PAS b axis. Also, the hydrogens have been removed for clarity. In addition, the vibrationally averaged distance from the center of mass of the monomer to the argon is 3.936, labeled Ar(r0) in Figure 2, and the equilibrium value is 3.873 and 3.759 Å for rm(1) and ab initio, respectively. This is the increase in the vdW bond due to the stretching vibration. A calculation of the vibrationally averaged moments of inertia of the complex similar to those in refs 8 and 9 was carried out, where we rotate the ring about the monomer a and b axis by trial angles Rx and Ry. Then we calculate the moment of inertia matrix in the nonrotated axis system and add the argon with a trial R and angle, γe, that the angle R makes with the monomer original cm axis in the Cs plane (see ref 9). In this work, the angle γe is known from the equilibrium argon position that is given by Watson’s rm(1) fit or the ab initio result and is the in-plane angle of the argon. Thus the moments of inertia for the complex are calculated for a grid of values of three variables, R, Rx, and Ry to find the minimum in the residuals to the observed values from Table 4. The minimum was R ) 3.956 Å, Rx ) 3.0°, and Ry ) -0.3°. The minimum residual is fairly large at 0.9 amu Å2. The errors in the fits of the moments of inertia of all the complex isotopologues resulting in the structure given in Table 3 with the argon position in the row labeled Ar(r0, rs, re) are on the order of 0.001 amu Å2. Thus the planar moments that result from this complex structure are identical to the observed planar moments given in Table 4. Conclusions The Mulliken charge distribution from an B3LYP/6311++g(2df,2pd) density functional theory calculation using
Acknowledgment. S.N. thanks the Petroleum Research Fund of the American Chemical Society for support. We gratefully acknowledge computer resources provided by Wesleyan University Computer Cluster supported by the NSF Grant CNS-0619508. Supporting Information Available: Spectroscopic data for the isotopologues of argon-cyclopentene oxide. This information is available free of charge via the Internet at http:// pubs.acs.org. References and Notes (1) Lafferty, W. J. J. Mol. Spectrosc. 1970, 36, 84. (2) Carreira, L. A.; Lord, R. C. J. Chem. Phys. 1969, 51, 2735. (3) Antolinez, S.; Lessari, A.; Lopez, J. C.; Alonso, J. L. Chem.sEur. J. 2000, 6, 3345. (4) Lin, W.; Gayle, J. A.; Pringle, W. C.; Novick, S. E. J. Mol. Spectrosc. 2008, 251, 210. (5) Munrow, M. R.; Pringle, W. C.; Novick, S. E. J. Phys. Chem. A 1999, 103, 2256. (6) Watson, J. K. G. Vibrational Spectra and Structure; Durig, J., Ed.; Elsevier: Amsterdam, The Netherlands, 1977; Vol. 6, p 1. (7) Subramanian, R.; Szarko, J. M.; Pringle, W. C.; Novick, S. E. J. Mol. Struct. 2005, 165, 742. (8) Spycher, R. M.; Petitprez, D.; Bettens, F. L.; Bauder, A. J. Phys. Chem. 1994, 98, 11863. (9) Klots, T. D.; Emilsson, T.; Ruoff, R. S.; Gutowsky, H. S. J. Phys. Chem. 1989, 93, 1255. (10) Balle, T. J.; Flygare, W. H. ReV. Sci. Instrum. 1981, 52, 33. (11) Hight Walker, A. R.; Chen, W.; Novick, S. E.; Bean, B. D.; Marshall, M. D. J. Chem. Phys. 1995, 102, 7298. (12) Pickett, H. M. J. Chem. Phys. 1991, 49, 371. (13) Kraitchman, J. Am. J. Phys. 1953, 21, 17. (14) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Montgomery, J. A., Jr.; Vreven, T.; Kudin, K. N.; Burant, J. C.; Millam, J. M.; Iyengar, S. S.; Tomasi, J.; Barone, V.; Mennucci, B.; Cossi, M.; Scalmani, G.; Rega, N.; Petersson, G. A.; Nakatsuji, H.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Klene, M.; Li, X.; Knox, J. E.; Hratchian, H. P.; Cross, J. B.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Ayala, P. Y.; Morokuma, K.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Zakrzewski, V. G.; Dapprich, S.; Daniels, A. D.; Strain, M. C.; Farkas, O.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.; Ortiz, J. V.; Cui, Q.; Baboul, A. G.; Clifford, S.; Cioslowski, J.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, I.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng, C. Y.; Nanayakkara, A.; Challacombe, M.; Gill, P. M. W.; Johnson, B.; Chen, W.; Wong, M. W.; Gonzalez, C.; Pople, J. A. Gaussian 03, revision B.04; Gaussian, Inc.: Wallingford, CT, 2004. (15) Schwendeman, R. Hillig, K. Private communication, January 1987. (16) Watson, J. K. G.; Roytburg, A.; Ulrich, W. J. Mol. Spectrosc. 1999, 196, 102. (17) Kisiel, Z., PROSPE, http://info.ifpan.edu.pl/-kisiel/prospe.htm. Accessed May 2006. (18) Wesleyan University Computer Cluster supported by the NSF Grant CNS-0619508. (19) (a) Bondi, A. J. Phys. Chem. 1964, 68, 441. (b) Roland, R; Taylor, R. J. Phys. Chem. 1996, 100, 7384 (for hydrogen).
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