Torsional anharmonicity in the conformational analysis of tryptamine

In this paper we calculate the relative conformer populations of the tryptamine molecule. Our approach combines high level electronic structure conformer energies with harmonic frequencies and an anharmonic treatment of the torsional motions using the torsional path integral Monte Carlo method. We have developed a 3-D potential energy surface as a function of the torsional coordinates at the B3LYP/6-31+G(d) level using 2535 grid points. Eight conformers of tryptamine were found to be significantly populated at 430 K as opposed to the experimental observation of seven. This, along with further comparisons with various experimental data, leads us to suppose that conformer interconversion occurs during the cooling phases of many of the experiments. The ordering of the calculated populations fits well with available experimental data. Torsional anharmonicity is found to affect conformer populations more significantly at 430 K than at 100 K (although overall the effects are small), while quantum mechanical effects are not important at either temperature.