Evidence and understanding of sulfur-centered hydrogen bonding, especially where the donor is a thiol, lags far behind that for conventional OH interactions. To help address this deficiency, conformer specific IR spectra of 2-phenylethanethiol (PET) and associated 1:1 solvent complexes have been measured in SH, OH, and CH stretch regions using resonant-two-photon-ionization (R2PI) and IR-UV ion dip spectroscopic techniques. The aromatic and aliphatic CH stretch regions show signature differences between anti and gauche conformers. Supported by ab initio calculations, a PET-water cluster with an OH···S arrangement and a PET-diethyl ether cluster expressing an SH···O interaction were identified. The SH stretch band of the SH···O complex is red-shifted and undergoes significant intensity enhancement compared to the bare molecule, which is characteristic of hydrogen bonding. These findings offer insight into the nature of the thiol functional group as a potential hydrogen bond donor and acceptor.

The electronic spectra of 2-bromoethylbenzene and its chloro and fluoro analogues have been recorded by resonant two-photon ionisation (R2PI) spectroscopy. Anti and gauche conformers have been assigned by rotational band contour analysis and IR-UV ion depletion spectroscopy in the CH region. Hydrate clusters of the anti conformers have also been observed, allowing the role of halocarbons as hydrogen bond acceptors to be examined in this context. The donor OH stretch of water bound to chlorine is red-shifted by 36 cm, or 39 cm in the case of bromine. Although classed as weak H-bond acceptors, halocarbons are favourable acceptor sites compared to π systems. Fluorine stands out as the weakest H-bond acceptor amongst the halogens. Chlorine and bromine are also weak H-bond acceptors, but allow for more geometric lability, facilitating complimentary secondary interactions within the host molecule. Ab initio and DFT quantum chemical calculations, both harmonic and anharmonic, aid the structural assignments and analysis.

A gas-phase electronic spectrum of nicotine in a supersonic expansion has been recorded using two-color resonant two-photon ionization spectroscopy. Efficient photoionization was achievable only via the pyridine chromophore owing to poor Franck−Condon overlap in the N-methylpyrrolidine moiety. Two conformers of nicotine have been characterized and assigned by infrared−ultraviolet (IR−UV) ion depletion and IR−UV hole-burning spectroscopy, in combination with quantum chemical techniques. Trans-A with nitrogen atoms further apart is more stable by 2 kJ mol −1 and the most populated conformer in the supersonic jet, owing this stability to a stronger inter-ring CH•••N hydrogen bond than the trans-B counterpart.

Nicotine has many physiological effects due to its neurotransmitter binding properties, associated with its usage as a recreational drug. Despite the great number of solution phase studies, very little has been published about gas phases spectroscopic properties. We report the first electronic spectra, obtained via two colour resonant two-photon ionisation spectroscopy. Evidence of two distinct conformers is supported by conformer specific IR-UV ion depletion spectra, interpreted with the aid of quantum chemistry calculations. Unconventional CH⋯N type hydrogen bonds between the rings play a key role in determining the conformational structures and giving rise to "blue-shift" perturbations in their IR spectra that facilitate their assignment. It is striking that the conformers are structurally very similar in the ground state, but upon excitation one of them undergoes a significant change in pyrrolidine puckering giving rise to an electronic spectrum of quite different character. Nicotine analogue 2-phenylpyrrolidine serves as a useful comparison, having a much more accessible conformational space defined by pyrrolidine puckering. It is also one of the few such molecules in which both gas and solution phase conformational preferences are analysed. Resonant two-photon ionization (R2PI), microwave, and NMR spectroscopic techniques were employed, supported by ab initio calculations. There is a significant change in the conformational preference between the gas and solution phases, with the CH⋯N type hydrogen bonds affected by the presence of solvent molecules.