Conformational stability, r 0 structural parameters, ab initio calculations, and vibrational assignment for cyanocyclopentane § James R. Durig a, *, Rachel M. Ward a , Arindam Ganguly a , Ahmed M. El Defrawy a,1 , Keegan G. Nelson a , Todor K. Gounev a , Mamdouh S. Soliman a,2 , Gamil A. Guirgis b a Department of Chemistry, University of Missouri-Kansas City, 5100 Rockhill Road, Kansas City, MO 64110-2499, USA b Department of Chemistry and Biochemistry, College of Charleston, Charleston, SC 29424, USA 1. Introduction Mono-substituted cyclopentanes have been of interest for many years since cyclopentane undergoes a ring vibration designated [1] as pseudorotation since the two ‘‘out-of-plane’’ vibrational modes which are usually described qualitatively as ring-puckering and ring-twisting modes have nearly equal frequencies. This motion was originally treated [1] for cyclopen- tane in terms of an amplitude coordinate q and a phase angle coordinate f. This concept for cyclopentane was questioned [2] when the low frequency ring mode of this molecule appeared ‘‘normal’’ but the fundamentals could not be assigned on the basis that cyclopentane had D 5h symmetry. The authors [2] concluded that a decision among the models with pseudorotation or rigid structures could not be made and there was further reluctance [3,4] to accept the concept of pseudorotation of the puckering motion and the consequent indefiniteness of the cyclopentane conformation. However, a later infrared study [5] of the CH 2 deformation of cyclopentane clearly showed that the ring was undergoing pseudorotation and the motion was nearly barrier free. After the initial prediction [1] of pseudorotation in saturated five- membered rings, a study followed [6] wherein the authors proposed that fluorocyclopentane as well as some other mono- substituted cyclopentanes should have the bent conformation (envelope) as the preferred form. The barrier to pseudorotation for fluorocyclopentane was estimated [6] to be 700 cal/mol (245 cm 1 ) by employing the torsional barriers in ethane and ethyl fluoride and a similar value was predicted for methylcyclopentane. Vibrational Spectroscopy 53 (2010) 45–53 ARTICLE INFO Article history: Received 17 September 2009 Received in revised form 24 January 2010 Accepted 28 January 2010 Available online 4 February 2010 Keywords: Xenon solutions r 0 structural parameters Vibrational assignment Pseudorotation Cyanocyclopentane ABSTRACT The infrared spectra (3200–400 cm 1 ) of the gas, xenon solutions, and solid and the Raman spectrum (3200–100 cm 1 ) of liquid cyanocyclopentane, c-C 5 H 9 CN, have been recorded. From infrared spectra of xenon solutions at 10 different temperatures, the enthalpy difference between the more stable axial conformer and the equatorial form has been determined to be 109  37 cm 1 (312  106 cal/mol). The percentage of the equatorial conformer present at ambient temperatures is estimated to be 41  7%. The ab initio MP2(full) average predicted energy difference from a variety of basis sets is 182  23 cm 1 (520  66 cal/ mol) whereas the energy difference obtained from the corresponding density functional theory calculations by the B3LYP method predict the equatorial conformer to be more stable by 164  11 cm 1 (469  31 cal/mol). By utilizing previously reported microwave determined rotational constants for both conformers combined with ab initio MP2(full)/6-311+G(d,p) predicted structural values, adjusted r 0 parameters have been obtained. The determined heavy atom structural parameters for the axial[equatorial] conformer are: the distances C 1 – C 2 = 1.542(5)[1.543(5)], C 1 –C 3 = 1.542(5)[1.543(5)], C 2 –C 4 = 1.542(5)[1.544(5)], C 3 –C 5 = 1.542(5)[1.544(5)], C 4 –C 5 = 1.559(5)[1.564(5)], C 1 –C 6 BN 8 = 1.467(5)[1.462(5)], C 6 BN 8 = 1.156(5)[1.159(5)] (A ˚ ) and angles in degrees nC 3 C 1 C 2 = 102.0(5)[102.3(5)], nC 1 C 2 C 4 = 103.6(5)[103.2(5)], nC 1 C 3 C 5 = 103.6(5)[103.2(5)], nC 2 C 4 C 5 = 105.7(5)[105.8(5)], nC 3 C 5 C 4 = 105.7(5)[105.8(5)], nC 6 C 1 C 2 = 111.5(5)[113.1(5)], nC 6 C 1 C 3 = 111.5(5)[113.1(5)] (8) and tC 1 C 2 C 4 C 5 = 25.8(5)[26.1(5)], tC 1 C 2 C 4 C 5 = 0.0(5)[0.0(5)] (8). Vibrational assign- ments have been provided for most of the observed bands which have been supported by MP2(full)/6-31G(d) ab initio calculations to predict harmonic force fields, frequencies, infrared intensities, Raman activities and depolarization ratios for both conformers. The results are discussed and compared to the corresponding properties of some similar molecules. ß 2010 Elsevier B.V. All rights reserved. § Presented in preliminary form at the 5th International Conference on advanced vibrational spectroscopy, Melbourne, Australia. * Corresponding author. Tel.: +1 816 235 6038; fax: +1 816 235 2290. E-mail address: durigj@umkc.edu (J.R. Durig). 1 Taken in part from the Thesis of A.M. El Defrawy, which has been submitted in partial fulfillment of the Ph.D. degree. 2 Permanent address: Department of Chemistry, Mansoura University, Mansoura, Egypt. Contents lists available at ScienceDirect Vibrational Spectroscopy journal homepage: www.elsevier.com/locate/vibspec 0924-2031/$ – see front matter ß 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.vibspec.2010.01.022