Getting the Shape of Methyl Internal Rotation Potential Surfaces Right

Abstract

The effect of the multidimensional character of methyl internal rotation on torsional potential functions in small conjugated methyl molecules is discussed. Partitioned energetics obtained from different ab initio calculation levels are divided into σ and π symmetry components and then further dissected by breakdown of the fully relaxed internal rotation process into separate relaxation steps and their nuclear virials. Important contributions to barrier shape, height, and origin are thus disclosed. The analysis reveals closely balanced antagonistic π and σ contributions as determinants for the barrier height and identifies the dominant contributions to the barrier origin as σ-bonding changes largely resulting from bond lengthening during methyl group rotation. Skeletal hydrogen out-of-plane motion components of the torsional coordinate (e.g. CH 2 ethylenic twisting in propene) provide a general mechanism for barrier narrowing in small conjugated methyl molecules. Rydberg jet experiments are discussed as a general means for obtaining accurate torsional frequency information (including ground state) to test internal rotation potential surface models.

Torsional Angle Definitions and Linear and Quadratic Force Field Variations along the Torsional Coordinate for CH3OH and CH3CHO

“…For these various reasons, comparisons of higher-order terms such as those found for V 6 , for example, in Ref. (27) may warrant reexamination after the precise meaning of γ in the various phenomenological fitting Hamiltonians is better understood.…”

Single‐Crystal X‐Ray Diffraction, Isolated‐Molecule and Cluster Electronic Structure Calculations, and Scanning Electron Microscopy in an Organic solid: Models for Intramolecular Motion in 4,4′‐Dimethoxybiphenyl

“…[7,11,[13][14][15][16] They are also good test cases for comparing calculated and measured barriers to intramolecular motion. [17][18][19][20][21][22] In this paper we bring together single crystal X-ray crystallography, isolated molecule and molecular cluster ab initio electronic structure calculations, and field emission scanning electron microscopy with 4,4'-dimethoxybiphenyl (Figures 1 and 2). This allows us to relate the properties describing methyl group rotation and methoxy group libration to the structure on both the microscopic and macroscopic scales.…”

Ab initio anharmonic force field and equilibrium structure of propene

“…There have been many ab initio calculations on propene but most of them are devoted to the calculation of the methyl internal rotation potential [20] or to the heat of formation [21]. In this last work, however, there is a coupled cluster calculation of the geometry and of the harmonic force field.…”