Ab initio MO and quasi-classical direct ab initio MD studies on the nitrogen inversion of trimethylamine

Abstract

A coupled motion of the skeletal inversion and methyl torsion in trimethylamine is investigated theoretically at various levels up to MP4(SDQ)/aug-cc-pVTZ. Among possible structures, the pyramidal C 3v is the only minimum energy structure, and the planar C s is the transition state structure of the nitrogen inversion. Our best estimate of the barrier height (without zero-point energy) is 3290 cm-1. The quasi-classical direct ab initio MD using HF/6-31G* starting from the pyramidal C 3v structure indicates that the coupled methyl torsional motion is the primary factor for the nitrogen inversion of trimethylamine.

A theoretical study of the inversion and rotation barriers in Methyl-substituted Amines

“…22,23 The experimental result obtained from microwave spectra is 4.4 ± 1 kcal mol -1 , 16,17 which has an error limit somewhat large. A better comparison with the experimental data should include the ZPE correction obtained from the B3LYP/cc-pVQZ calculations, which are 3.24, 4.12 and 4.63 kcal mol -1 calculated using B3LYP/ cc-pVQZ, CCSD(T)/equation 2 and CCSD(T)/cc-pVQZ, respectively.Trimethylamine has its ground state equilibrium geometry well determined 24,26,[29][30][31] as a pyramidal structure with C 3v point group symmetry with an eclipsed conformation. Figure 2 gives the C 3v equilibrium geometry and the two other conformations with C s point group symmetry, the staggered and planar structures, which correspond respectively to the internal rotation of the methyl groups and to the skeletal inversion.…”

“…28 Theoretical results were also obtained using ab initio methods up to MP4 level. [29][30][31] It has been adequately demonstrated that a simultaneous consideration of basis set saturation and core correlation effects using coupled cluster method and extended basis set provide very accurate values of many molecular properties. 32,33 Recently, using this methodology, we have investigated the geometries, harmonic vibrational frequencies, and the barrier heights of methylamine.…”

“…The calculations were carried out using the degree of inversion d, as defined by Tanaka and Aida. 31 As analyzed for methylamine and dimethylamine, the structural stability is essentially due to the stereo electronic effect.Calculated trimethylamine barrier heights are presented in Table 3 together with the experimental data. 24,25,27,28 The inclusion of core correlation is more important for the inversion barrier, as already discussed for the methylamine and dimethylamine.…”

“…The previous MP4(SDQ)/ aug-cc-pVTZ result without ZPE is 4.04 kcal mol -1 . 31 For the inversion, the B3LYP/cc-pVQZ without and with ZPE are 7.45 and 6.29 kcal mol -1 , respectively. Our best result without ZPE is 9.51 kcal mol -1 , which is similar to the previous theoretical study at MP4(SDQ)/aug-cc-pVTZ level, equal to 9.42 kcal mol -1 .…”

“…[20][21][22][23] The trimethylamine molecule has also received various experimental and theoretical studies. [24][25][26][27][28][29][30][31] The experimental data of the geometry, vibrational frequencies and internalinversion and rotational barriers were obtained using microwave, 24,26 infrared and Raman,25,27 techniques and employing analysis of the ultraviolet absorption and fluorescence spectra. 28 Theoretical results were also obtained using ab initio methods up to MP4 level.…”