Effects of fluorine substitution on the shape of neurotransmitters: the rotational spectrum of 2-(2-fluorophenyl)ethanamine.

The effects of fluorination on the conformational landscape of adrenergic neurotransmitters is exemplified trough the conformation analysis of 2-(2-F-phenyl)ethanamine (2FPEA) carried out by microwave spectroscopy and quantum chemical calculations. Five different conformers of the nine possible stable ones for 2FPEA are observed by molecular-beam Fourier-transform microwave spectroscopy. Their unambiguous identification is possible by comparing the experimental rotational constants and the quadrupole coupling constants with those obtained by quantum chemical calculations carried out at the MP2/6-311++G(d,p) level of theory. The relative abundances of the conformers in the jet are estimated from the relative intensities in the observed spectra. A qualitative agreement between experimental and theoretical energies was found, and the remaining deviations are explained by population transfer taking place during the adiabatic expansion. The energy landscape, which also takes the interconversion barriers between the conformers into consideration, is thus characterized completely by the strong interplay of quantum chemical methods and precise experimental data. Significant changes in energy and structure of the 2FPEA conformers are found compared to those obtained for the prototype molecule 2-phenylethanamine (PEA).