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Study of molecular motion on main group III and V addition complexes by broadline and pulse NMR. Ang, Tian Tse

Abstract

The proton magnetic resonance absorption and spin-lattice relaxation measurements have been carried out on Me₃N-BMe₃, Me₃N-AℓMe₃, Me₃P-BMe₃, Me₃P,Me₃B, Me₂NH-BMe₃, Me₂NH- AℓMe₃, Me₃NBBr₃, Me^NBB^, and trimethylamine-halogen complexes in order to study the molecular motion and phase transitions of these complexes in the solid state between 77 K to their melting points. The activation energies and rate parameters associated with these motional processes are reported. The second moment data of all the trimethyl amine addition complexes indicate the methyl groups attached to nitrogen are all rigid (as far as the nmr second moment is concerned) at liquid nitrogen temperature and at higher temperature the methyl groups as well as the trimethylamine moiety rotate with a frequency greater than about 10 kHz. The methyl groups rotate prior to the rotation of the trimethylamine moiety in all but the Me₃N-Br₂ and Me₃N-BMe₃ complexes. The present study concludes that if the intermolecular contributions to the activation energies for methyl group reorientations are of the same order, the activation energies for the methyl groups follow the trend of the stability of the trimethyl amine complexes. The second moment data show that the methyl groups attached to boron, phosphorus and aluminium are all reorientating at 77 K at a rate faster than about 10 kHz. The relaxation studies conclude that the activation energies for the three methyl groups attached to various central atoms decrease in the order N > C > B > Si > Aℓ. Comparison of the calculated and experimental minimum for methyl group reorientation suggests that there is small distortion of the C₃ potential well, and comparison of the calculated and experimental pre-exponential factors suggests that the activation energy for rotation of the whole molecule or the moiety is temperature dependent.

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