Ion-molecule complexes of the form HAr are produced in pulsed-discharge supersonic expansions containing hydrogen and argon. These ions are analyzed and mass-selected in a reflectron spectrometer and studied with infrared laser photodissociation spectroscopy. Infrared spectra for the n = 3-7 complexes are characterized by a series of strong bands in the 900-2200 cm region. Computational studies at the MP2/aug-cc-pVTZ level examine the structures, binding energies, and infrared spectra for these systems. The core ion responsible for the infrared bands is the proton-bound argon dimer, Ar-H-Ar, which is progressively solvated by the excess argon. Anharmonic vibrational theory is able to reproduce the vibrational structure, identifying it as arising from the asymmetric proton stretch in combination with multiple quanta of the symmetric argon stretch. Successive addition of argon shifts the proton vibration to lower frequencies, as the charge is delocalized over more ligands. The Ar-H-Ar core ion has a first solvation sphere of five argons.

The reaction between Ti and methanol (CH OH) is a model system for competition between activation of C-O, C-H, and O-H bonds and of the role of excited electronic pathways in catalytic processes. Herein, we use experimental kinetics, quantum chemical calculations, and statistical modeling to identify the critical features of the reaction's potential energy surface. Experimental kinetics data between 300 and 600 K shows the reaction largely proceeds through C-O bond activation, yielding TiOH and TiO . Products of the O-H activation pathway, TiOCH and TiOCH are minor, whereas C-H bond activation is not observed at thermal energies. Statistical modeling well-reproduces the experimental results and offers insight into the reaction mechanism. Notably, efficient spin-crossing along the C-O activation pathway is required to produce the observed product distribution, in contrast to a previous estimate of inefficient crossing based on calculation of a small spin-orbit coupling constant. This discrepancy highlights a potential limitation of simple models within the Landau-Zener framework, which are commonly used to calculate surface-crossing probabilities in reactive systems.

The OH cation is a well-known diatomic for which the triplet ( Σ ) ground state is 50.5 kcal mol more stable than its corresponding singlet ( Δ) excited state. However, the singlet forms a strong donor-acceptor bond to argon with a bond energy of 66.4 kcal mol at the CCSDT(Q)/CBS level, making the singlet ArOH cation 3.9 kcal mol more stable than the lowest energy triplet complex. Both singlet and triplet isomers of this molecular ion were prepared in a cold molecular beam using different ion sources. Infrared photodissociation spectroscopy in combination with messenger atom tagging shows that the two spin isomers exhibit completely different spectral signatures. The ground state of ArOH is the predicted singlet with a covalent Ar-O bond.

Near-infrared spectroscopy measurements are presented for protonated water clusters, H(HO) , in the size range of n = 1-8. Clusters are produced in a pulsed-discharge supersonic expansion, mass selected, and studied with infrared laser photodissociation spectroscopy in the regions of 3600-4550 and 4850-7350 cm. Although there is some variation with cluster size, the main features of these spectra are a broad absorption near 5300 cm, a sharp doublet near 7200 cm, as well as a structured absorption near 4100 cm for n ≥ 2. The vibrational patterns measured for the hydronium, Zundel, and Eigen ions are compared to those predicted by different forms of anharmonic theory. Second-order vibrational perturbation theory (VPT2) and a local mode treatment of the OH stretches both capture key aspects of the spectra but suffer understandable deficiencies in the quantitative description of band positions and intensities.

Both prominent CH isomers, the benzylium and the tropylium cations, were generated in an electrical discharge/supersonic expansion from toluene and cycloheptatriene precursors. Their infrared spectra were measured in the region of 1000-3500 cm using photodissociation of the respective argon- and nitrogen-tagged complexes with a broadly tunable OPO/OPA laser system. Spectral signatures of both isomers were observed independent of the precursor, albeit in different relative intensities. The spectra were assigned based on scaled harmonic B3LYP-D3/cc-pVTZ frequency computations and comparisons to previous experimental studies. Consistent with its high symmetry, only two bands were observed for the (nitrogen-tagged) tropylium ion at 3036 and 1477 cm, corresponding to C-H stretching and C-C-H deformation/C═C stretching vibrations, respectively. Furthermore, the C-H stretching region of the benzylium ion is reported for the first time.