Effects of aromatic fluorine substitution on protonated neurotransmitters: the case of 2-phenylethylamine

Fluorination of pharmaceutical compounds is a common tool to modulate their physiochemical properties. We determine the effects of site-specific aromatic fluorine substitution on the geometric, energetic, vibrational, and electronic properties of the protonated neurotransmitter 2-phenylethylamine (xF-H(+) PEA, x=ortho, meta, para) by infrared multiphoton photodissociation (IRMPD) in the fingerprint range (600-1750 cm(-1) ) and quantum chemical calculations at the B3LYP-D3/aug-cc-pVTZ level. The IRMPD spectra of all ions are assigned to their folded gauche conformers stabilized by intramolecular NH(+) ⋅⋅⋅π hydrogen bonds (H-bonds) between the protonated amino group and the aromatic ring. H→F substitution reduces the symmetry and allows for additional NH(+) ⋅⋅⋅F interactions in oF-H(+) PEA, leading to three distinct gauche conformers. In comparison to oF-H(+) PEA, the fluorination effects on the energy landscape (energy ordering and isomerization barriers) in pF-H(+) PEA and mF-H(+) PEA with one and two gauche conformers are less pronounced. The strengths of the intramolecular NH(+) ⋅⋅⋅F and NH(+) ⋅⋅⋅π bonds are analyzed by the noncovalent interaction (NCI) method.