A σ and not a π complex has been identified for the prototypical arenium intermediate, protonated benzene. The activation of the infrared multiphoton dissociation of C6H7+ ions (obtained by the gas‐phase protonation of benzene) by the absorption of free electron laser radiation shows a wavelength‐dependent dissociation of H2 (see spectrum) that matches the transitions calculated for the σ complex benzenium ion.

Infrared multiple photon dissociation (IRMPD) spectroscopy of cis-[Pt(NH(3))(2)(G)Cl](+) and cis-[Pt(NH(3))(2)(A)Cl](+) ions (where A is adenine and G is guanine) has been performed in two spectral regions, 950-1900 and 2900-3700 cm(-1). Quantum chemical calculations at the B3LYP/LACV3P/6-311G** level yield the optimized geometries and IR spectra for the conceivable isomers of cis-[Pt(NH(3))(2)(G)Cl](+) and cis-[Pt(NH(3))(2)(A)Cl](+), whereby the cisplatin residue is attached to the N7, N3, or carbonyl oxygen atom, (O6), of guanine and to the N7, N3, or N1 position of adenine, respectively. In addition to the conventional binding sites of native adenine, complexes with N7-H tautomers have also been considered. In agreement with computational results, the IR characterization of cis-[Pt(NH(3))(2)(G)Cl](+) points to a covalent structure where Pt is bound to the N7 atom of guanine. The characterized conformer has a hydrogen-bonding interaction between a hydrogen atom of one NH(3) ligand and the carbonyl group of guanine. The experimental C═O stretching feature of cis-[Pt(NH(3))(2)(G)Cl](+) at 1718 cm(-1), remarkably red-shifted with respect to an unperturbed C═O stretching mode, is indicative of a lengthened CO bond in guanine, a signature that this group is involved in hydrogen bonding. The IRMPD spectra of cis-[Pt(NH(3))(2)(A)Cl](+) are consistent with the presence of two major isomers, PtAN3 and PtAN1, where Pt is bound to the N3 and N1 positions of native adenine, respectively.

Phenylalkylamines of the general formula C6H5(CH2)nNH2 (n = 1-4) have been delivered to the gas phase as protonated species using electrospray ionization. The ions thus formed have been assayed by IRMPD spectroscopy in two different spectroscopic domains, namely, the 600-1800 and the 3000-3500 cm(-1) regions using either an IR free electron laser or a tabletop OPO/OPA laser source. The interpretation of the experimental spectra is aided by density functional theory calculations of candidate species and vibrational frequency analyses. Protonated benzylamine presents a relatively straightforward instance of a single stable conformer, providing a trial case for the adopted approach. Turning to the higher homologues, C6H5(CH2)nNH3(+) (n = 2-4), more conformations become accessible. For each C6H5(CH2)nNH3(+) ion (n = 2-4), the most stable geometry is characterized by cation-π interactions between the positively charged ammonium group and the aromatic π-electronic system, permitted by the folding of the polymethylene chain. The IRMPD spectra of the sampled ions confirm the presence of the folded structures by comparison with the calculated IR spectra of the various possible conformers. An inspection of the NH stretching region is helpful in this regard.

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(+)-π interaction is observed. The dispersion-corrected density functional theory calculations reveal the important effects of dispersion on the cation-π interaction and the large vibrational anharmonicity of the NH3(+) group involved in the NH(+)-π hydrogen bond. They allow for assigning overtone and combination bands and explain anomalous intensities observed in previous IR multiple-photon dissociation spectra. Comparison with neutral PEA reveals the large effects of protonation on the geometric and electronic structure.