Conformational Analysis of Octopamine and Synephrine in the Gas Phase

Four and six conformers of the neurotransmitters octopamine and synephrine, respectively, have been identified in the gas phase using a laser ablation device in combination with a molecular-beam Fourier transform microwave spectrometer operating in the 4-10 GHz frequency range. The identification of all of the conformers was based on a comparison of the experimental rotational and N-14 quadrupole coupling constants with those predicted by ab initio calculations, as well as the relative values of their electric dipole moment components. The conformational preferences have been rationalized in terms of the various intramolecular forces operating in the different conformers of the studied molecules. All observed species are characterized by an intramolecular hydrogen bond of the type O-H center dot center dot center dot N established in the side chain of the neurotransmitters, which adopts an extended disposition in their most stable forms. For conformers with a folded side chain, an extra N-H center dot center dot center dot pi hydrogen-bond-type interaction is established between the amino group and the pi system of the aromatic ring.