Microwave Spectra and Structure of the Argon–Cyclopentanone and

Article pubs.acs.org/JPCA

Microwave Spectra and Structure of the Argon−Cyclopentanone and Neon−Cyclopentanone van der Waals Complexes Wei Lin,† Andrew H. Brooks,‡ Andrea J. Minei,§ Stewart E. Novick,‡ and Wallace C. Pringle*,‡ †

Department of Chemistry and Environmental Sciences, University of Texas at Brownsville, One West University Blvd., Brownsville, Texas 78520, United States ‡ Department of Chemistry, Wesleyan University, 51 Lawn Avenue, Middletown, Connecticut 06459, United States § Department of Chemistry and Biochemistry, Division of Natural Sciences, College of Mount Saint Vincent, 6301 Riverdale Avenue, Riverdale, New York 10471, United States S Supporting Information *

ABSTRACT: The rotational spectra of cyclopentanone and its van der Waals complexes with argon and neon have been observed with a Balle−Flygare type pulsed jet Fourier transform microwave spectrometer in the 6 to 20 GHz region. This work improves the rotational constants and quartic centrifugal distortion constants for cyclopentanone and its five 13C and the 18O isotopologues. The argon−12C5H816O van der Waals complex has rotational constants of A = 2611.6688, B = 1112.30298, and C = 971.31969 MHz. The 20Ne−12C5H816O complex has rotational constants of A = 2728.8120, B = 1736.5882, and C = 1440.4681 MHz. In addition, the five unique, singly substituted 13C and 18O isotopologues of the argon complex are reported. The five single-substituted 13C of the 20Ne complex and the 22Ne−12C5H816O complex are reported. The rare gases are in van der Waals contact with the carbonyl α carbon and nearly in contact with the hydrogen on β and γ carbons toward the back of the ring.

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INTRODUCTION The microwave spectrum of cyclopentanone was first reported in 1954 by Erlandsson.1 He determined that the planar moment, 17.5 amu Å2, is too large to be due to the out-of-plane hydrogens. Thus, the carbons of the ring are not planar. This was followed by a microwave spectrum measured to frequency standard accuracy by Burkhalter.2 Kim and Gwinn3 studied cyclopentanone and 2-d-cyclopentanone, discovering two conformations for the latter molecule. They determined that cyclopentanone had C2 symmetry, because the out-of-plane dipole moment μc was nearly zero. This ruled out the other likely nonplanar Cs structure. The microwave spectra of the three 13C isotopomers of cyclopentanone by Mamleev et al.4 directly confirmed the C2 structure through Kraitchman analysis.5 Far infrared6 and microwave studies3 have determined that the barrier to pseudorotation in cyclopentanone is relatively high. Thus, the complications observed in the microwave spectrum of the low pseudorotational barrier molecule, tetrahydrofuran,7,8 are not present in the rotational spectrum of cyclopentanone. Microwave structural studies using numerous isotopologues of the monomer ring compounds and their rare gas complexes allow us to determine the site on the host ring at which the rare gas forms a van der Waals bond. We have determined that the argon occupies a position above the ring and on the opposite side of the epoxide in both the five-membered ring, cyclopentene oxide (CPO),9 and the six-membered ring, cyclohexene oxide.10 In the argon−cyclobutanone complex,11 we found that the argon is in van der Waals (vdWs) contact © 2014 American Chemical Society

with the carbonyl carbon but not the oxygen. Argon is further back on the ring and just inside the positively charged hydrogen on one of the equivalent cross-ring carbons. In the argon− methylene cyclobutane complex,12 the argon is close to vdWs contact with the ring double-bonded carbon. It is also 0.1 Å further from vdWs contact with the hydrogen on Cγ. Thus, the argon complexes to a position on the ring that has a relatively large electrostatic positive charge. The argon avoids the more polarizable and dipolar substituents. In this manner, as first described by Klemperer and co-workers,13 the argon is acting as a Lewis base and partially shares electrons at the acceptor positions of the rings. This interaction is in addition to the dispersive interactions. The planarity or nonplanarity of ring molecules is governed by a competition between the planar driving force, ring strain, and the nonplanar driving force, torsional repulsion. Ring strain in four-membered rings with ring angles near 90° is much larger than that in five-membered rings with ring angles near 105°. For example, in cyclobutanone,10 the unstrained ring angles would be 120° for the carbonyl angle and the tetrahedral angle, 109°, for the highly strained C−C−C ring angles. In this case, competition leads to a very small barrier to planarity, and the structure in the ground ring-puckering state is planar because the level lies above the barrier. In less strained fourmembered rings such as trimethylene sulfide,14 the torsional barrier dominates and the ring-puckering potential is a double Received: October 20, 2013 Revised: January 3, 2014 Published: January 15, 2014 856

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Table 1. Spectroscopic Constants for 12C, 13C, and 18O Isotopologues of Cyclopentanone A (MHz) B (MHz) C (MHz) ΔJ (kHz) ΔK (kHz) ΔJK (kHz) δJ (kHz) δK (kHz) N σ (kHz) a

all-12C

α-13C

β-13C

γ-13C

6620.0563(9) 3351.5304(3) 2410.4155(3) 0.3369(50) 0.455(95) 1.1567(214) 0.05410(83) 0.4671(155) 43 4

6620.4472(4) 3336.0047(3) 2402.4290(3) 0.28(1) 0.46a 1.16a 0.0541a 0.47a 16 2

6490.6778(4) 3351.2183(3) 2393.2710(3) 0.26(1) 0.46a 1.16a 0.0541a 0.47a 16 2

6570.9023(6) 3304.4345(4) 2380.7552(4) 0.27(2) 0.46a 1.16a 0.0541a 0.47a 17 3

18

O

6620.0401(3) 3177.6996(3) 2319.1607(3) 0.259(8) 0.46a 1.16a 0.0541a 0.47a 15 2

Fixed at the values for the all-12C isotopomer.

O isotopologue was five times weaker than the 13C α transitions. The assignment and observed frequencies for the parent and isotopologues of CPO are available in Table S1 of Supporting Information. A mixed r0, re structure of the monomer was determined by fitting the Cartesian coordinates of the monomer nuclei using the structural fitting program written by Schwendeman.18 The 14 equilibrium bond lengths, 23 nonredundant equilibrium bond angles of the molecule, and 15 ground vibrational state moments of inertia for all isotopologues were used to fit the 42 Cartesian coordinates of the monomer maintaining the C2 symmetry of the molecule. The equilibrium bond lengths and angles were calculated using the MP2 method with a Dunningtype aug-cc-pVDZ basis set with the Gaussian 09 program.19 The nonlinear, weighted least-squares fits were carried out in two steps. First, the coordinates of all nuclei except the carbonyl oxygen were fit; then those results were used to fit the coordinate of the oxygen. The root-mean-square deviation (rmsd) of the 15 ground-state moments of inertia of the isotopogues for this structure, calculated from the rotational constants reported in Table 1, is 0.003 amu Å2. The largest deviation in the fit versus equilibrium bond lengths was 0.001 Å, and the largest angle deviation was 0.7°. The Cartesian coordinates for the monomer resulting from this analysis are reported in Table 2. The errors in the coordinates are from this fit. The output of the final perturbation of the Schwendeman fit18 is given in Table S2 of Supporting Information. This lists the input parameters (observed in the output) including the re, αe, and ground-state moments of inertia for all isotopologues and the weighting factors used in the nonlinear least-squares fit. The observed minus calculated (fit) values of all parameters are also given, as are the calculated rotational constants and the fit Cartesian coordinates of the monomer (Table 2). The Kraitchman substitution structures, rs, for each 13C and 18 O are given in Table 3. Several coordinates are nearly zero, and the slightly negative squared coordinates lead to the small imaginary values reported in Table 3. The substitution coordinates from ref 4 are also given because the original paper is in Russian. The structure reported in Table 2 is consistent with the traditional Kraitchman coordinates reported in Table 3. Argon−Cyclopentanone. A preliminary structure of the argon−cyclopentanone complex was predicted by placing the argon in a position that is analogous to the argon in the cyclobutanone complex11 and calculating the moments of inertia. This position is just inside the ring near the β carbon and 3.7 Å above the center of mass of the monomer. The argon is close to the positive proton on the carbon adjacent to the 18

minimum. The unstrained C−S−C ring angle is 90°. Because the first four vibrational states are below the barrier, the ring is nonplanar in the ground state. In five-membered rings, the ring strain is much lower and the eclipsed torsional repulsion at the planar structure dominates the ring strain. The rings relieve this repulsion by becoming nonplanar with staggered torsional conformation.

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EXPERIMENTAL SECTION The spectra of cyclopentanone, argon−cyclopentanone, and neon−cyclopentanone were measured using a pulsed-jet Fourier transform microwave (FTMW) spectrometer based on the Balle−Flygare design with modifications.15,16 The fullwidth-half-maximum (fwhm) of the observed lines were 5−7 kHz. With this fwhm, we can estimate peak centers on transitions to within 1 kHz. Cyclopentanone (99+%, Aldrich) was prepared as a gas mixture of 0.5% with argon or neon as the carrier gases. The backing pressures were approximately 0.3 and 2 atm, respectively. The spectra for all of the isotopic species were observed in natural abundance. Spectra for cyclopentanone, argon−cyclopentanone, and neon−cyclopentanone and their corresponding isotopologues were fit using Pickett’s SPCAT and SPFIT programs17 and in particular were fit to a Hamiltonian in Ir representation, Watson A reduction, using three rotational constants and five quartic centrifugal distortion constants.

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RESULTS Cyclopentanone Monomer. The microwave spectrum of the parent cyclopentanone and its 13C isotopologues have been previously studied and assigned using conventional Stark modulated microwave spectroscopy.1−4 We have added 32 new transitions to the 10 previously assigned for the parent.3 These new a-type transitions have a maximum J = 13 and Ka = 4. With the higher resolution and sensitivity of FTMW spectroscopy, we improved the accuracy of the rotational constants and quartic centrifugal distortion constants as presented in Table 1. In the fits for the 13C isotopologues, the higher resolution led to improvement in the rotational constants and ΔJ was determined. The other quartic centrifugal distortion constants were fixed at the values of the parent. In addition, we have observed and assigned the spectrum of the 18 O isotopologue in its natural abundance of 0.2%. The intensities of the transitions for the 13Cβ, 13Cβ′, and analogous γ and γ′ isotopologues were approximately 2% of the intensity of the parent, and the 13Cα intensity was approximately 1% of the parent. As originally reported in ref 4, this confirms the C2 symmetry of cyclopentanone monomer. The intensity of the 857

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Table 2. Structure of Cyclopentanone from a Fit to the Mixed Ab Initio Equilibrium Bond Length and Angle Parameters and the Observed Isotopologue Moments of Intertia.a Cα Cβ Cβ′ Cγ Cγ′ O Hβ1 Hβ2 Hβ′1 Hβ′2 Hγ1 Hγ2 Hγ′1 Hγ′2

a (Å)

b (Å)

c (Å)

−0.846(4) 0.049(4) 0.049(4) 1.451(3) 1.451(3) −2.054(4) −0.315(5) −0.042(4) −0.042(4) −0.315(5) 1.579(6) 2.251(5) 2.251(5) 1.579(6)

0.0000 −1.233(1) 1.233(1) −0.734(1) 0.734(1) 0.0000 −1.976(5) −1.665(7) 1.665(7) 1.976(5) −0.772(8) −1.321(3) 1.321(3) 0.772(8)

0.0000 −0.141(1) 0.141(1) 0.238(4) −0.238(4) 0.0000 0.577(11) −1.137(9) 1.137(9) −0.577(11) 1.323(4) −0.212(9) 0.212(9) −1.323(4)

Figure 2. Top view of argon−cyclopentanone van der Waals complex. The sizes of the atoms are 75% of their van der Waals radii.

transition. The transitions of the 18O complex are approximately five times weaker than those of the 13C. In order to determine the structure of the complex, the ground-state moments of inertia for the monomer and the argon complex were used in a Kraitchman analysis.5 The position of argon in the principal axis system (PAS) of the monomer is calculated by assuming the argon in the monomer has a mass of zero and in the complex has a mass of 40. This “extreme” Kraitchman will have some errors due to the large amplitude van der Waals vibrations that are present in the complex and not in the monomer. This results in the absolute values of the argon coordinate, in Table 5 labeled as Ar (|rs|, extreme Kraitchman), |a| = 0.945, |b| = 0.804, |c| = 3.459 Å. To determine the signs of the argon coordinates, each of the eight possible ± coordinates were added to the fixed monomer coordinates in Table 2 and the moment of inertia for each isotopologue was calculated. The only combination that agreed with the observed ground-state moments of inertia calculated from the rotational constants reported in Table 4 was a = −0.945, b = −0.804, and c = 3.459 Å. For reference, the coordinates of oxygen are −2.0537, 0.0, and 0.0, respectively. Traditional Kraitchman analysis5 of the position of each 13C and 18O in the complex is also consistent with the carbon positions in Table 2 (after converting the complex coordinates into the monomer coordinate system). A side-on view and top view of this structure are shown in Figures 1 and 2 as Gaussian captured images with the nuclei of radii equal to 75% of the vdWs radii.19 Assuming the monomer structure is not changed by complexation, the position of the argon was varied to fit the 21 ground-state moments of inertia of the parent and all isotopologues from Table 4 using Schwendeman’s structural fitting program.18 The monomer structure was fixed at the values given in Table 2. The resulting position of the argon in the monomer PAS is a = −0.946, b = −0.803, and c = 3.459 Å. This position is labeled Ar (fixed monomer, vary Ar) in Table 5. The output of the Schwendeman fit program,18 varying only the coordinates of the argon with the monomer structure fixed at the values in Table 2, is reported in Table S4 of Supporting Information. The errors in the argon position are from the fit. The coordinates of the argon−cyclopentanone complex in the PAS of the complex are given in Table 6. The Kraitchman substitution coordinates of the five 13C and 18O isotopologues are given in Table 7. These match the coordinates of the heavy atoms of the complex given in Table 6. The rotational constants and moments of inertia for all the isotopologues calculated from the structure given in Table 6 with the proper isotopic mass substituted are given in the Schwendeman structural fit output18 included as Table S4 of Supporting Information.

Errors are from the fit, described in text with results presented in Table S2 of Supporting Information. C2 symmetry was maintained in the fit. a

Table 3. Kraitchman Substitution Coordinates for Cyclopropanone Monomer from the Rotational Constants in Table 1a 13

Cα Cβ 13 Cγ 18 O 13

|a|

|b|

|c|

0.8143 (0.8395(7)) 0.05i (−) 1.4483 (1.4484(4)) 2.0528

0.06i (0) 1.2317 (1.2319(9)) 0.7318 (0.7315(8)) 0.0168

0.003i (0) 0.1329 (0.139(8)) 0.2364 (0.236(2)) 0.01i

a

The imaginary values result from atomic coordinates that are very small, and the square of the calculated coordinate was slightly negative. The values in parentheses are from ref 4. They did not observe the 18O isotopologue.

carbonyl carbon (Figures 1 and 2). This is consistent with our observation that the rare gas acts as a Lewis base and seeks a

Figure 1. Side view of the argon−cyclopentanone van der Waals complex. The sizes of the atoms are 75% of their van der Waals radii.

position near positively charged ring atoms. We assigned 106 transitions for the argon−CPO parent complex. Observed lines for the parent and isotopologues were primarily b-type, with a few weak a-type and much weaker c-type lines. The spectral frequencies and assignments are given in Table S3 in Supporting Information. The rotational molecular constants for the parent, five 13C, and 18O isotopologues are given in Table 4. The intensity of the 13C isotopologue transitions are approximately 1% of the intensity of the corresponding parent 858

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Table 4. Spectroscopic Constants for 12C, 13C, and 18O Isotopologues of Argon−Cyclopentanone A (MHz) B (MHz) C (MHz) ΔJ (kHz) ΔK (kHz) ΔJK (kHz) δJ (kHz) δK (kHz) N σ (kHz)

all-12C

α-13C

β-13C

β′-13C

γ-13C

γ′-13C

2611.6688(2) 1112.30298(6) 971.31969(6) 2.5732(4) 1.321(8) 7.151(3) 0.3919(3) 4.89(1) 102 2

2600.4210(3) 1110.5405(3) 968.4116(2) 2.553(4) 1.03(1) 7.27(1) 0.390(2) 4.88(6) 21 1

2590.5509(3) 1105.884(4) 969.3973(3) 2.534(4) 1.04(1) 7.09(2) 0.378(3) 4.61(8) 28 1

2592.4591(4) 1104.7614(4) 968.0575(2) 2.534(4) 1.17(2) 7.00(2) 0.373(2) 4.70(8) 26 1

2592.0986(5) 1100.8140(6) 961.4532(4) 2.533(7) 2.53(2) 6.43(2) 0.392(3) 4.87(9) 25 2

2581.0381(2) 1107.2212(2) 964.5622(2) 2.548(3) 1.43(1) 6.85(1) 0.398(1) 4.67(5) 26 1

Table 5. Coordinates of Rare Gases in the Complexesa rare gas

a (Å)

b (Å)

c (Å)

Ar (|rs|, extreme Kraitchman) Ar (fixed monomer, vary Ar) 20 Ne (|rs|, extreme Kraitchman) 20 Ne (fixed monomer, vary 20 Ne) 22 Ne (|rs|, extreme Kraitchman)

0.945 −0.9456(4) 0.914

0.804 −0.8031(1) 0.783

3.459 3.4592(5) 3.261

−0.9132(5)

−0.7827(2)

3.2603(10)

0.913

0.785

13

Cα 13 Cβ 13 Cγ 13 Cβ′ 13 Cγ′ 18 O

3.257

The coordinates (Å) are in the principal axis system of cyclopentanone. The errors are from the complex isotopologues fit.

a (Å)

b (Å)

c (Å)

−0.9262 0.0459 1.0016 1.3956 −0.2424 −2.0441 −0.3145 0.0736 −0.9431 −0.0497 1.8953 2.0731 1.8062 0.7398 0.3027(4)

−0.1006 −1.2713 0.6602 −0.6090 1.1684 −0.1670 −1.9750 −1.7969 1.6540 1.8621 −0.3274 −1.2539 1.3930 0.3925 0.1136(12)

|a|

|b|

|c|

0.8513 1.0236 1.3081 1.3089 2.0757 0.3784

0.9198 0.11i 1.3928 0.2106 1.0224 2.0432

0.05i 1.2656 0.6183 1.1849 0.6674 0.07i

a

The imaginary values result from atomic coordinates that are very small, and the square of the calculated coordinate was slightly negative.

Table 6. Structure of Ar−Cyclopentanone in PAS of Complexa −0.8569 −1.0156 −2.1114 −1.3253 −1.3694 −0.4037 −1.7660 −0.0549 −2.0574 −0.5511 −0.3941 −1.8832 −2.1539 −3.1389 2.4689(1)

O

2505.3269(4) 1111.6130(5) 955.6824(3) 2.491(6) 0.38(3) 7.47(2) 0.409(2) 5.0(1) 17 1

Table 7. Kraitchman Substitution Coordinates for Ar− Cyclopentanone in PAS of Complex Calculated from the Rotational Constants in Table 4a

a

Cα Cβ Cγ′ Cγ Cβ′ O Hβ1 Hβ2 Hβ′1 Hβ′2 Hγ1 Hγ2 Hγ′1 Hγ′2 Ar

18

backing gas to single out transitions belonging to the neon− cyclopentanone complex. We measured 70 and 57 transitions for the 20Ne− and 22Ne− cyclopentanone parent complexes. These transitions were fit to three rotational constants and five quartic centrifugal distortion constants in the ground state to a semirigid rotor Hamiltonian in an Ir representation Watson A reduction. We have observed a-, b-, and c-type transitions as expected. The intensities of the a- and b-type transitions are comparably strong, whereas the ctype transitions are much weaker. For the five 20Ne singly substituted 13C isotopologues, we measured between 21 and 28 transitions. The transition frequencies and assignments are listed in Table S5 of Supporting Information. The molecular rotational constants for these neon complexes are listed in Table 8. Structural analysis of the neon complexes was carried out in the same manner as that of the argon complexes. Extreme Kraitchman analysis was followed by simultaneous fits of all the isotopologues of the 20Ne complex. We did not observe the 18O isotopologue in the neon complexes. The results of these calculations are given in Table 9 with the same labeling as in the argon complexes. The extreme Kraitchman result is 20Ne (|rs|, extreme Kraitchman), |a| = 0.914, |b| = 0.783, |c| = 3.261 Å. Extreme Kraitchman analysis was also applied to the 22Ne isotopologue resulting in 22Ne (|rs|, extreme Kraitchman), |a| = 0.913, |b| = 0.788, |c| = 3.257 Å. Using the monomer structure given in Table 2 and varying the position of the 20Ne results in position of the argon in the monomer PAS of a = −0.9132(5), b = −0.7827(2) and c = 3.2603(10) Å. The errors are from the structural fit with all isotopologues. This position is labeled 20 Ne (fixed monomer, vary 20Ne) in Table 5. For this structure, the moments of inertia of the parent 20Ne, five 13C, and 22Ne isotopologues were simultaneously fit varying only the coordinates of the 20Ne using the structural fitting program by Schwendeman.18 The results of this fit of 21 moments of

a The structure of the monomer was fixed at the values in Table 2, and the coordinates of argon were fit to the 21 moments of inertia of all isotopologues of the complex.

Referring to Figures 1 and 2, the argon is above the Cβ. This carbon is twisted slightly below the plane that would exist if the ring were planar. Because the monomer has such low symmetry, C2, this placement of the argon results in a complex that has only the identity symmetry element. Thus, none of the carbons in the complex are equivalent, resulting in five distinct 13 C spectra and isotopologue rotational constants reported in Table S3 of Supporting Information and Table 4, respectively. Neon−Cyclopentanone. A preliminary structure of the neon−cyclopentanone complex was predicted based on the structure of the argon−cyclopentanone complex. Scan files with neon as the backing gas were compared to those with argon as 859

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Table 8. Spectroscopic Constants for 12C and 13C Isotopologues of Neon−Cyclopentanone 22

Ne all-12C

A (MHz) B (MHz) C (MHz) ΔJ (kHz) ΔK (kHz) ΔJK (kHz) δJ (kHz) δK (kHz) N σ (kHz)

2707.7492(6) 1658.3352(4) 1381.4216(3) 13.883(5) −7.01(3) 39.29(3) 8.64(7) 3.175(2) 57 4

20

Ne all-12C

2728.8120(5) 1736.5882(3) 1440.4681(3) 15.050(5) 49.99(4) −13.96(3) 6.53(6) 3.604(3) 70 4

20

Ne α-13C

20

2717.011(6) 1736.488(8) 1437.113(6) 15.00(2) 49.7(7) −13.9(8) 3.63(7) 6.7(2) 26 2

2705.776(6) 1726.074(7) 1439.714(5) 14.75(1) 47.7(7) −13.2(7) 3.47(7) 6.2(1) 27 2

Table 9. Structure of 20Ne−Cyclopentanone in PAS of Complexa Cα Cβ Cγ Cγ′ Cβ′ O Hβ1 Hβ2 Hβ′1 Hβ′2 Hγ1 Hγ2 Hγ′1 Hγ′2 20 Ne

a (Å)

b (Å)

c (Å)

−0.1553 −0.4501 −1.0676 −1.8208 −0.8649 0.5228 −1.0711 0.5104 −1.4172 −0.1422 −0.2767 −1.7191 −2.0682 −2.7561 2.6483(1)

−0.9076 0.0698 1.2919 0.6809 −0.4090 −1.9040 −0.4003 0.3205 −1.2595 −0.0884 1.9567 1.8727 1.4213 0.2282 0.8840(5)

−0.1269 −1.2664 −0.5725 0.6289 1.1335 −0.2084 −2.0289 −1.7296 1.5476 1.8838 −0.2141 −1.2239 1.3885 0.2878 0.2938(15)

The structure of the monomer was fixed at the values in Table 2, and the coordinates of neon were fit to the 21 moments of inertia of all isotopologues of the complex.

inertia to the 3 Cartesian coordinates of 20Ne are given in Table S6 of Supporting Information. The structure of the Ne− cyclopentanone complex in the PAS of the complex is given in Table 9. The results of the Kraitchman substitution structure of the 22Ne and five 13C isotopologues are given in Table 10. Table 10. Kraitchman Substitution Coordinates for 20Ne− Cyclopentanone in PAS of Complex Calculated from the Rotational Constants in Table 8a Cα 13 Cβ 13 Cγ 13 Cβ′ 13 Cγ′ 22 Ne

|a|

|b|

|c|

0.1252 0.4350 1.0346 0.7798 1.7781 2.6200

0.8992 0.07i 1.2933 0.3613 0.7218 0.8816

0.0327 1.2627 0.5883 1.1630 0.6468 0.2235

20

Ne β′-13C

2707.349(8) 1725.051(1) 1437.428(7) 14.75(2) 47.7(1) −13.1(1) 3.44(1) 6.4(2) 21 3

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DISCUSSION

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ASSOCIATED CONTENT

20

Ne γ-13C

2699.844(7) 1728.184(7) 1429.316(6) 14.90(2) 51.1(9) −15.5(7) 3.63(8) 6.2(2) 28 3

20

Ne γ′-13C

2715.460(6) 1715.546(7) 1425.552(4) 14.72(1) 54.2(7) −16.3(5) 3.54(8) 5.9(1) 23 1

The argon and neon form van der Waals bonds at very nearly the same position on the cyclopentanone. Because the neon has a smaller vdWs radius, it is approximately 0.2 Å closer to the ring. In the argon complex, the two hydrogens that are closest to the argon are Hβ and Hγ at 3.17 and 3.31 Å. The distance from the argon to the carbonyl carbon is 3.55 Å. The sum of the van der Waals radii for Ar−H is 2.97 Å and that for Ar−C is 3.55 Å. Thus, the argon is in vdWs contact with the carbonyl carbon and approximately 0.2 Å outside vdW radius of the closest hydrogen. In the neon complex, the distances to the same two hydrogens are 3.00 and 3.16 Å, respectively. The extreme Kraitchman analyses and the structural fits lead to coordinates for each rare gas that differ by at most 0.001 Å as shown in Table 5 by comparing rows 1 and 3 to 2 and 4, respectively. The rare gases act as Lewis bases and seek out a positive electrostatic part of the ring. Mullikan population analysis of the charges of the ring nuclei using the ab initio Gaussian calculation19 for CPO described above supports this theory of contact with the carbonyl carbon and ring hydrogens. The charges on Cα, Hβ, and Hγ are 0.13, 0.18, and 0.15 e, respectively. These positively charged atoms in the molecule have polarizability lower than that of negatively charged atoms. In addition, the position of the rare gases in the cyclopentanone complexes is consistent with the empirical model for rare gas ring complexes put forth by Kisiel.20 The rare gas is above the center of mass of the ring molecule tilted toward the heteroatom of the ring. In ref 20, his model predicted the structures of argon complexes with furan, pyrrole, and pyridine. The rare gas is displaced from the center of mass of these rings that consist of comparable polarizabilities toward the smallest heavy atom in the ring.20 The model includes an attractive dispersive interactions and uses the cube of the covalent radii of the atoms to determine the polarizabilities for the dispersive attractive interactions.20 Thus, the vdWs interaction in raregas−cyclopentanone complex is likely a combination of dispersive and electrostatic interactions. We are currently investigating rings with larger dipolar substituents to determine the effect of polar dispersion.

a

13

Ne β-13C

a

The imaginary values result from atomic coordinates that are very small, and the square of the calculated coordinate was slightly negative.

S Supporting Information *

Comparison of the Kraitchman coordinates to the fit structure in Table 9 indicates the validity of the assumption that the structure of cyclopentanone is not modified significantly by the van der Waals complexation by the rare gas Ne or Ar.

Spectroscopic data and structural fits for the isotopologues of cyclopentanone, argon−cyclopentanone, neon−cyclopentanone. This material is available free of charge via the Internet at http://pubs.acs.org. 860

dx.doi.org/10.1021/jp410381r | J. Phys. Chem. A 2014, 118, 856−861

The Journal of Physical Chemistry A

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Article

(18) Schwendeman, R. H. In Critical Evaluation of Chemical and Physical Structural Information; Lide, D. R., Paul, M. A., Eds.; National Academy of Science: Washington, D.C., 1974. (19) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.;Robb, M. A.; Cheeseman, J. R.; Scalmani, G.; Barone, V.; Mennucci, B.; Petersson, G. A.; et al. Gaussian 09, revision A.02; Gaussian Inc.: Wallingford, CT, 2009. (20) Kisiel, Z. A. Simple Model for Predicting Structures of GasPhase van der Waals Dimers Containing a Rare-Gas Atom. J. Phys. Chem. 1991, 95, 7605−7612.

AUTHOR INFORMATION

Corresponding Author

*Tel: 860-685-2728. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS S.E.N. thanks the National Science Foundation for support from grant CHE-1011214. We acknowledge computer resources provided by Wesleyan Computer Cluster supported by NSF Grant CNS-0619508.

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REFERENCES

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dx.doi.org/10.1021/jp410381r | J. Phys. Chem. A 2014, 118, 856−861