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 13 C and the 18 O isotopologues. The argon- 12 C 5 H 8 16 O van der Waals complex has rotational constants of A = 2611.6688, B = 1112.30298, and C = 971.31969 MHz. The 20 Ne- 12 C 5 H 8 16 O complex has rotational constants of A = 2728.8120, B = 1736.5882, and C = 1440.4681 MHz. In addition, the five unique, singly substituted 13 C and 18 O isotopologues of the argon complex are reported. The five single-substituted 13 C of the 20 Ne complex and the 22 Ne- 12 C 5 H 8 16 O 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. ■ INTRODUCTION The microwave spectrum of cyclopentanone was first reported in 1954 by Erlandsson. 1 He determined that the planar moment, 17.5 amu Å 2 , 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 Gwinn 3 studied cyclopentanone and 2-d-cyclopentanone, discovering two conformations for the latter molecule. They determined that cyclopentanone had C 2 symmetry, because the out-of-plane dipole moment μ c was nearly zero. This ruled out the other likely nonplanar C s structure. The microwave spectra of the three 13 C isotopomers of cyclopentanone by Mamleev et al. 4 directly confirmed the C 2 structure through Kraitchman analysis. 5 Far infrared 6 and microwave studies 3 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 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 C γ . 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 four- membered 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 Article pubs.acs.org/JPCA © 2014 American Chemical Society 856 dx.doi.org/10.1021/jp410381r | J. Phys. Chem. A 2014, 118, 856-861