Determination of binding energy in molecular clusters by ion imaging methods: A test on the phenol–water 1:1 cluster

Ionization energies and ionization-induced structural changes in 2-phenylethylamine and its monohydrate

We report the resonance-enhanced two-photon ionization combined with various detection approaches and quantum chemical calculations of biologically relevant neurotransmitter prototypes, the most stable conformer of 2-phenylethylamine (PEA), and its monohydrate, PEA-H₂O, to reveal the possible interactions between the phenyl ring and amino group in the neutral and ionic species. Extracting the ionization energies (IEs) and appearance energy was achieved by measuring the photoionization and photodissociation efficiency curves of the PEA parent and photofragment ions, together with velocity and kinetic energy-broadened spatial map images of photoelectrons. We obtained coinciding upper bounds for the IEs for PEA and PEA-H₂O of 8.63 ± 0.03 and 8.62 ± 0.04 eV, within the range predicted by quantum calculations. The computed electrostatic potential maps show charge separation, corresponding to a negative charge on phenyl and a positive charge on the ethylamino side chain in the neutral PEA and its monohydrate; in the cations, the charge distributions naturally become positive. The significant changes in geometries upon ionization include switching of the amino group orientation from pyramidal to nearly planar in the monomer but not in the monohydrate, lengthening of the N-H⋯π hydrogen bond (HB) in both species, C_α-C_β bond in the side chain of the PEA⁺ monomer, and the intermolecular O-H⋯N HB in PEA-H₂O cations, leading to distinct exit channels.

Vibrational predissociation of the phenol-water dimer: a view from the water

The vibrational predissociation (VP) dynamics of the phenol-water (PhOH-H₂O) dimer were studied by detecting H₂O fragments and using velocity map imaging (VMI) to infer the internal energy distributions of PhOH cofragments, pair-correlated with selected rotational levels of the H₂O fragments. Following infrared (IR) laser excitation of the hydrogen-bonded OH stretch fundamental of PhOH (Pathway 1) or the asymmetric OH stretch localized on H₂O (Pathway 2), dissociation to H₂O + PhOH was observed. H₂O fragments were monitored state-selectively by using 2+1 Resonance-Enhanced Multiphoton Ionization (REMPI) combined with time-of-flight mass spectrometry (TOF-MS). VMI of H₂O in selected rotational levels was used to derive center-of-mass (c.m.) translational energy (E_T) distributions. The pair-correlated internal energy distributions of the PhOH cofragments derived via Pathway 1 were well described by a statistical prior distribution. On the other hand, the corresponding distributions obtained via Pathway 2 show a propensity to populate higher-energy rovibrational levels of PhOH than expected from a statistical distribution and agree better with an energy-gap model. The REMPI spectra of the H₂O fragments from both pathways could be fit by Boltzmann plots truncated at the maximum allowed energy, with a higher temperature for Pathway 2 than that for Pathway 1. We conclude that the VP dynamics depends on the OH stretch level initially excited.