The potential energy landscape of noradrenaline.  An electronic structure study

The relaxed potential energy pro. les for interconversion between the conformers in the two most stable noradrenaline families (AG1 and GG1, each containing four members) have been calculated at the density functional B3LYP/6-31+G* level of theory. Rigid rotation ( in which the molecule is kept frozen during variation of the torsional angle) is found to introduce large errors in the relative energies and geometries of the stationary points; it appears to be best to compute relaxed potential energy surfaces wherever possible, even if this can only be accomplished by using a lower level of theory. The barriers for interconversion between the noradrenaline conformers ( from least to most stable structure, with inclusion of scaled zero-point vibrational effects) range from 7 to 15 kJ mol(-1). The barrier for conversion between the two most stable noradrenaline conformers, AG1a and GG1a, is among the highest of those computed: 15.4 kJ mol(-1) at the B3LYP/6-31+G* level and 21.7 kJ mol(-1) at the MP2/ 6-31+G* level. Temperature effects further increase the barrier between AG1a and GG1a. The high transition barriers indicate that conformer conversion is unlikely to occur during collisional cooling of noradrenaline in a supersonic expansion.