Shape of biomolecules by free jet microwave spectroscopy: 2-amino-1-phenylethanol and 2-methylamino-1-phenylethanol.

We report a free jet microwave absorption study of 2-amino-1-phenylethanol and 2-methylamino-1-phenylethanol, which are analogues of noradrenaline and adrenaline, respectively. The spectra, recorded under different expansion conditions and with different carrier gases, show the presence of several conformational species: two conformers for 2-amino-1-phenylethanol and three for 2-methylamino-1-phenylethanol. The assignment is based on the comparison of the experimental rotational constants and the orientation of the molecular dipole moment with the ones predicted by theoretical methods and allows the univocal identification of all observed conformers while intensity measurements give information on their relative stability. All of the observed conformational species are stabilized by an intramolecular hydrogen bond between the hydroxyl hydrogen atom and the amino nitrogen atom, whereas the alkylamino side chain can be in the extended anti or folded gauche conformation. The presence of the bulky methyl group substituent on the alkylamino side chain of 2-methylamino-1-phenylethanol influences the relative stability of the conformers.