Methyl Group Internal Rotation Dynamics:  Overtone Study of Gaseous Methylpyridine-2-αd₂ and -3-αd₂

Abstract

Conventional infrared absorption and Raman spectroscopy have been used to record the vapor phase spectra of methylpyridine-2-Rd 2 ,6-d 1 , methylpyridine-2-Rd 2 , and methylpyridine-3-Rd 2 ,-d 4 in the ∆V CH ) 1-4 regions. The spectra are analyzed with a theoretical model that takes into account, in the adiabatic approximation, the coupling between the internal rotation of the methyl group and the methyl CH stretching vibration. The principal parameters used in this model have been determined by ab initio calculations at the HF/6-31G** level of theory. A good agreement between experimental and calculated spectra is found. This indicates that this coupling is at the origin of the majority of the observed spectral profiles. A comparison of these results with those previously obtained for similar methylated molecules reveals that the change in type and size of the barrier to internal methyl rotation is at the origin of significant spectral differences. These changes are particularly important for methylpyridine-2-Rd 2 , revealing that the methyl group experiences increasingly different internal dynamics with increasing energy. These spectral changes can be well explained by the deformation of the effective internal rotation potential in the vibrational excited states. The overtone spectra of the aryl CD stretching of methylpyridine-3-Rd 2 ,-d 4 have also been studied.

SERS investigation of neocuproine adsorption on silver: Influence of electrode potential on methyl groups

“…Apparently, the attractive interaction between the methyl group hydrogen and the in‐plane lone‐pair electrons of the nitrogen atom is weaker than the attraction exerted by the π ‐ electronic cloud of the phenanthroline rings. This explains the equilibrium conformation of the methyl groups in neocuproine in full analogy with what is known for 2‐methylpyridine . The identical orientation of the methyl group in the same position of the aromatic fragment was found for phenanthrene .…”

“…This explains the equilibrium conformation of the methyl groups in neocuproine in full analogy with what is known for 2-methylpyridine. [67] The identical orientation of the methyl group in the same position of the aromatic fragment was found for phenanthrene. [65] The fact that phenanthrene does not contain heteroatoms indicates that the nitrogen atoms have minor influence on the conformation of the methyl groups compared to the influence from the aromatic rings.…”

Internal Methyl Rotation in the CH Stretching Overtone Spectra of 2-, 3-, and 4-Methylpyridine

“…The minimum energy in 3-methylpyridine occurs at a torsional angle of zero (the in-plane methyl H atom faces the nitrogen atom). 21,26 The minimum energy in 2-methylpyridine occurs at a torsional angle of 60°(the in-plane methyl H atom is opposite the nitrogen atom). 21,26 It is interesting to note that when the methyl group is in close proximity to the nitrogen atom in 2-methylpyridine, the in-plane hydrogen is opposite the nitrogen at the minimum energy conformer.…”

“…21,26 The minimum energy in 2-methylpyridine occurs at a torsional angle of 60°(the in-plane methyl H atom is opposite the nitrogen atom). 21,26 It is interesting to note that when the methyl group is in close proximity to the nitrogen atom in 2-methylpyridine, the in-plane hydrogen is opposite the nitrogen at the minimum energy conformer. When the methyl group is further away from the nitrogen atom in 3-methylpyridine, the in-plane hydrogen is toward the nitrogen at the minimum energy conformer.…”

“…16,17 Cavagnat et al use a Morse oscillator-rigid rotor basis model within the adiabatic approximation and obtain good agreement between the observed and calculated spectra for the fundamental and overtone CH stretching regions of nitromethanes, toluenes, and methylpyridines. [18][19][20][21][22] Zhu et al simulated the methyl region of the overtone spectrum of 2,6-difluorotoluene with a nonadiabatic model that included all three CH oscillators of the methyl group within the HCAO approximation and described methyl torsion in a rigid rotor basis. 23 We have successfully reproduced the observed methyl band profiles in the CH stretching overtone spectra of toluene-d 0 , -Rd 1 , and -R-d 2 .…”