Physics behind the Barrier to Internal Rotation of an Acetyl Chloride Molecule: A Combined Approach from Density Functional Theory, Car–Parrinello Molecular Dynamics, and Time-Resolved Wavelet Transform Theory

The physics behind the barriers to internal rotation of acetyl chloride (AC) molecule has been reported. The AC molecule closely resembles the molecular structure of acetaldehyde; the only subtle difference is the presence of a heavy chlorine atom in place of the hydrogen atom of the aldehyde group for the latter. This paper aims to study the effect of substitution of the heavy chlorine atom on the barrier energetics of the AC molecule. The reason behind the barrier for the AC molecule has been estimated for the first time from the unified approach using barrier energetics, natural bond orbital, nuclear virial, and relaxation analyses using density functional theory, Car–Parrinello molecular dynamics, and wavelet transform theory. Complete analyses reveal the concomitant relaxations of both the in-plane C_methyl–C₁ and C_methyl–H₄ bonds toward understanding the origin of the barrier due to internal rotation for the AC molecule. The large negative value of “V₆” further suggests that both the abovementioned degrees of freedom are coupled with the −CH₃ torsional vibration of the molecule. The coupling matrix (H₁₂) element has also been estimated. Time-resolved band stretching frequencies of C_methyl–C₁ and C₁–Cl₃ bonds of the AC molecule, as obtained from wavelet transformation analysis, primarily preclude the possibility of coupling between the C₁–Cl₃ bond and the torsional motion associated with the methyl group of the molecule.