THEORETICAL VIBRATIONAL SPECTRA STUDIES: THE EFFECT OF RING SIZE ON THE CARBONYL VIBRATIONAL FREQUENCIES

Abstract

In this paper, molecular structures and vibrational frequencies of cycloketone, cyclopropanone, cyclobutanone, cyclopentanone, and cyclohexanone have been investigated by density functional theory (DFT) and the second order Møller and Plesset (MP2) levels of theory, using 6-311G, 6-311G**, 6-311++G, and 6-311++G** basis sets. The calculations predict a planar structure for cyclopropanone. The cyclobutanone ring is puckered and according to the calculations at B3LYP/6-311++G** and MP2/6-311++G** levels of theory, it deviates from planarity by 5.0° and 15.7°, respectively. The calculated barrier height between puckered and planar conformer, at MP2/6-311G** level of theory, is 196.7 cm-1. The puckering frequency at B3LYP/6-311G** and MP2/6-311G** levels are 47.5 cm-1 and 121.2 cm-1. Cyclopentanone has twisted conformations ( C 2 symmetry) in which carbons C1 and C2 are out-of-plane with a torsional angle of 11.5° (MP2/6-311G**) and the molecule goes from one conformation to the other via an envelope transition conformation ( C s symmetry). or planar hilltop conformation ( C 2v symmetry). The most stable conformation of the cyclohexanone is C s chair, which has no ring strain, is used as land mark.