Conformational stability from temperature-dependent FT-IR spectra of krypton solutions, vibrational spectra and assignment, ab initio calculations, and r0 structural parameters of 2-chlorobutane

Variable temperature (2100 to 2150 8C) studies of the infrared spectra (3500–60 cm21) of 2-chlorobutane, CH3CH2CHClCH3, dissolved in liquid krypton, have been carried out. By utilizing six different conformer pairs, the enthalpy difference between the rotational isomers CH3-trans/Cl-trans where CH3-trans indicates a planar carbon skeleton has been determined to be 166 ^ 17 cm21 (1.99 ^ 0.20 kJ/mol) and between the rotational isomers CH3-trans/H-trans to be 245 ^ 25 cm21 (2.93 ^ 0.30 kJ/mol) with the CH3- trans conformer the most stable form. At ambient temperature there is approximately 57 ^ 2% of the CH3-trans form, 26 ^ 2% of the Cltrans form, and 17 ^ 2% of the H-trans conformer present at ambient temperature. The optimal geometries, conformational stabilities, harmonic force fields, infrared intensities, Raman activities, depolarization ratios, and vibrational frequencies are reported for all three conformers from MP2/6-31G(d) ab initio calculations. Nearly complete vibrational assignments are provided for all three conformers. The potential function governing the conformational interchanges has been obtained from DFT calculations by the B3LYP method with the 6- 311 þ G(d,p) basis set and compared to the one obtained from MP2/6-311 þ G(2d,2p) ab initio calculations. The order of the conformational stabilities were the same from all of the ab initio calculations with the ones from the MP2/6-311 þ G(d,p) ab initio calculation giving the CH3-trans conformer more stable than the Cl-trans form by 189 cm21 and the H-trans form by 229 cm21 which are the most consistent with the experimental results. The adjusted r0 parameters have been obtained by combining MP2/6-311 þ G(d,p) ab initio predicted structural parameters with previously reported microwave rotational constants for the ethyl, isopropyl, and t-butyl chlorides. These data were used to predict the r0 parameters for 2-chlorobutane, which are compared to those reported from an electron diffraction study. The results of these spectroscopic and theoretical studies are discussed and compared to the corresponding results for some similar molecules. q 2003 Elsevier B.V. All rights reserved