IR spectrum of the protonated neurotransmitter 2-phenylethylamine: dispersion and anharmonicity of the NH3+-π interaction

The structure and dynamics of the highly flexible side chain of (protonated) phenylethylamino neurotransmitters are essential for their function. The geometric, vibrational, and energetic properties of the protonated neutrotransmitter 2-phenylethylamine (H(+)PEA) are characterized in the N-H stretch range by infrared photodissociation (IRPD) spectroscopy of cold ions using rare gas tagging (Rg = Ne and Ar) and anharmonic calculations at the B3LYP-D3/(aug-)cc-pVTZ level including dispersion corrections. A single folded gauche conformer (G) protonated at the basic amino group and stabilized by an intramolecular NH+-pi interaction is observed. The dispersion-corrected density functional theory calculations reveal the important effects of dispersion on the cation-pi interaction and the large vibrational anharmonicity of the NH3+ group involved in the NH+-pi hydrogen bond. They allow for assigning overtone and combination bands and explain anomalous intensities observed in previous IR multiplephoton dissociation spectra. Comparison with neutral PEA reveals the large effects of protonation on the geometric and electronic structure.