Stereodynamics of triethylamine: Molecular mechanics calculations on the direct rotational racemization of the C₃‐symmetric conformers

Abstract

The direct interconversion of the two Ca-symmetric enantiomeric conformations of triethylamine, via C -N bond rotation, has been studied by molecular mechanics (MM2) calculations. The M M 2 calculations have been used to characterize the minima (equilibrium geometries) and first-order saddle points (transition states) for this process. For one interconversion, there are five saddle points and six minima. The highest energy saddle point results from the uncoupled rotation of one ethyl group to eclipse the lone pair. Two of the barriers result from coupled rotation of two ethyl groups in close passage.

The effect of high pressure on the dynamics of doped organic glasses: A study by spectral hole-burning

“…49 Note that the distance from the central N atom in the TEA molecule to any of the outer C atoms is 2.5 Å. 48 Taking a spherical structure of radius 2.5 Å containing one molecule, we find that a decrease in the radius of 0.005 Å yields a ⌬Vϳ Ϫ0.4 Å 3 per molecule. This rather small volume change shows up as a measurable shift in the potential barrier, demonstrating that the use of high pressure in spectral holeburning adds a new and highly sensitive parameter for the study of glass dynamics.…”

“…It is known that TEA exists in the gas and liquid phases in a number of conformations, 47 and a molecular mechanics calculation shows barrier heights between the various conformers of 1500 cm Ϫ1 , 150 cm Ϫ1 , and 90 cm Ϫ1 . 48 This suggests a scenario for the formation of TLSs by the ''downhill'' energy mechanism: as the excitation wanders through the glass via Förster energy transfer, 46 it may find itself close to a TEA molecule ͑or cluster of molecules͒ that can, with the input of ϳ135 cm Ϫ1 , be transformed into another conformation that will act as a TLS.…”

Stereodynamics of N-Allyl-N-methyl-2-aminopropane. 1H and 13C{H} DNMR Studies. Molecular Mechanics Calculations

“…The vinyl moiety is “locked” in the G position by the isopropyl methyl group that is gauche to the lone pair. This is reflected by the large chemical shift difference between the methylene protons (Δδ 1.03). ,,,, While isolated rotation about the N−CH 2 bond that interconverts GG and G‘G forms (vinyl group passes lone pair) is expected to have a barrier lower than 4.5 kcal/mol (exchange is fast at 100 K), ,, any significant concentration of the G‘G conformer is precluded by highly destabilizing syn-1,5-repulsions between isopropyl methyl and vinyl groups. Any exchange between the GG conformer and a minuscule concentration of the G‘G conformer at 100 K will not result in any perceptible time-averaging of the methylene protons signals. , The large coupling of the vinyl methine proton to the methylene proton that is anti to the lone pair shows that these two protons prefer to be anti to each other, thus establishing the conformational preference of the vinyl group in the GG conformation.…”

“…The k 3 conversion (GG‘ to GA) involves C-methyl passing N-methyl, C−H passing N-methylene, and C-methyl passing the loan pair. During all of these processes, at least one C−C bond must pass an N−C bond, leading to rotation barriers high enough to be DNMR-visible (5.1−5.5 kcal/mol). ,,− The direct k 5 conversion (i.e., GG to GG‘) must occur via a pathway that involves essentially two simultaneous C-methyl/N-methyl and C-methyl/N-methylene eclipsings. The GG to GG‘ conversion can also occur via the GA form as an unstable intermediate; this requires two sequential 120° rotations, each of which involves just one C−C/N−C passing.…”

“…A number of factors will determine the relative energies of the equilibrium conformations of a tertiary aliphatic amine including the pyramidality at nitrogen and the steric and/or electronic requirements of various substituents. , In particular, previous studies of tertiary aliphatic amines showed that the methyl group of an N-ethyl substituent shows a strong, but not exclusive, preference for the position gauche to the nitrogen lone pair, as compared to the position anti to the lone pair. ,,, The phenyl group on the N-benzyl substituent in tertiary aliphatic amines shows a conformational preference similar to methyl. , While the stereodynamics of allylamine have been studied by microwave spectroscopy, by gas phase electron diffraction, and by ab initio SCF calculations, the conformational preference of the N-allyl substituent in a tertiary amine has not been assessed. This paper reports 1 H and 13 C{ 1 H} DNMR studies of N -allyl- N -methyl-2-aminopropane (AMAP).…”