Internal Rotation of Methylcyclopropane and Related Molecules: Comparison of Experiment and Theory

Abstract

The internal rotation about the single bond connecting a cyclopropyl ring to a CH3, SiH3, GeH3, NH2, SH, or OH group was investigated. Both CCSD/cc-pVTZ and MP2/cc-pVTZ ab initio calculations were performed to predict the structures of these molecules and their internal rotation potential energy functions in terms of angles of rotation. The barriers to internal rotation for the CH3, SiH3, and GeH3 molecules from the calculations agree well with the experimental ones, within -11% to +1% for CCSD/cc-pVTZ and -4% to +9% for MP2/cc-pVTZ. Comparisons between theory and experiment were also performed for propylene oxide and propylene sulfide, and the agreements were very good. Theoretical calculations were performed to compute the internal rotation potential energy function for cyclopropanol, and these were used to guide the determination of a potential function based on experimental data. This molecule has two equivalent synclinal (gauche) conformers with an estimated barrier of 759 cm(-1) (9.1 kJ/mol) between them. The minima are at internal rotation angles of the OH group of 109° and 251°. The theoretical potential functions for cyclopropanethiol and cyclopropylamine were also calculated, and these agree reasonably well with previous experimental studies.

The Conformational Landscape, Internal Rotation, and Structure of 1,3,5-Trisilapentane using Broadband Rotational Spectroscopy and Quantum Chemical Calculations

“…Methyl rotor internal rotation is a topic that has been heavily studied for over 60 years. − Over the past 45+ years, the molecular structure and internal rotation dynamics of species with one or two −SiH 3 groups have been of significant interest. ,,− This interest and area of study has been significantly driven from the differences in molecular structure upon substitution of a carbon atom by a silicon atom. The general elongation of the bond created from the Si substitution lowers torsional energy strain, providing more freedom for the −SiH 3 group to rotate.…”