A Molecular Orbital Study of the Conformational Properties of Tyramine and Phenethylamine

• Published in: The journal of physical chemistry. A, Molecules, spectroscopy, kinetics, environment, & general theory Molecules, spectroscopy, kinetics, environment, & general theory, 2004-02, Vol.108 (7), p.1233-1241
• Main Authors: Richardson, Patricia R, Bates, Simon P, Jones, Anita C
• Format: Article
• Language: English
• ISSN: 1089-5639; 1520-5215
• DOI: 10.1021/jp036900u

The conformational behavior of tyramine and phenethylamine has been explored using a combination of MP2/6-31G(d,p) and CIS//MP2/6-31G(d,p) ab initio calculations. Seven stable conformer structures have been calculated for tyramine and four for phenethylamine. The increase in the number of conformers in going from phenethylamine to tyramine is due to syn and anti orientations of the hydroxyl and ethylamine substituents and can be related to an asymmetry in the electron density in the HOMO, similar to that found in phenol. In phenethylamine, the direction of the transition dipole moment (TDM) is predicted to vary significantly between different conformers, in agreement with experimental measurements. In both molecules, the electron density distribution in the frontier orbitals is sensitive to the orientation of the substituents and can be described by orbital mixing of the HOMO/HOMO-1 and the LUMO/LUMO+1. In phenethylamine, the electron density in the LUMO and LUMO+1 is particularly sensitive to the conformation of the alkylamine tail and this accounts for the change in the orientation of the TDM between different conformers. The presence of the hydroxyl group in tyramine is predicted to greatly reduce rotation of the TDM between different conformers. The OH group fixes the electron density distribution in the frontier orbitals into a phenol-like pattern. The effect of the OH group can be rationalized in terms of orbital mixing.