Slow monomer vibrations in formic acid dimer: Stepping up the ladder with FTIR and Raman jet spectroscopy.

In an effort to extend the cold gas phase spectroscopic database of the cyclic formic acid dimer (FAD), we present and analyze the jet-cooled vibrational infrared and Raman spectrum of (HCOOH)2 in the monomer fingerprint region between 600 and 1500 cm-1. The present study bridges the gap between the intermolecular dimerization-induced and the carbonyl stretching fundamentals that have already been reexamined using jet-cooled or high-resolution spectroscopy. This completes the characterization of the jet-cooled vibrational (HCOOH)2 spectrum below the complex OH (CH) stretching fundamentals, and we report resonance-induced FAD combination/overtone transitions that will serve as a valuable reference for a theoretical modeling of its vibrational dynamics. As a by-product, several new formic acid trimer fundamentals are identified in the jet spectra and assigned with the help of second-order vibrational perturbation theory (VPT2). The polar formic acid dimer still eludes detection in a supersonic jet, but we are able to estimate an experimental upper-bound of the polar dimer-to-trimer-to-cyclic dimer intensity ratio to about 1:10:100 under typical expansion conditions. Using VPT2 with resonance treatment (VPT2+K), we reinvestigate the notorious ν22 resonance triad. Generally, we find that VPT2, which is, of course, inadequate for modeling the resonance-rich OH stretching spectrum of FAD, is performing very satisfactorily in predicting fundamental and two-quantum state term values for the slower modes below 1500 cm-1. As these modes are the building blocks for the ultrafast energy dissipation in the OH stretching region, the present work opens the door for its quantitative understanding.

A Symmetric Recognition Motif between Vicinal Diols: The Fourfold Grip in Ethylene Glycol Dimer.

Ethylene glycol has a transiently chiral, asymmetric global minimum structure, but it favors a highly symmetric, achiral dimer arrangement which has not been considered or found in previous quantum-chemical studies. Complementary FTIR and Raman spectroscopy in supersonic jets allows for the detection and straightforward assignment of this four-fold hydrogen-bonded dimer, which introduces an interesting supramolecular binding motif for vicinal diols and provides a strong case for transient chirality synchronization.

Microscopic roots of alcohol-ketone demixing: infrared spectroscopy of methanol-acetone clusters.

Infrared spectra of isolated methanol-acetone clusters up to tetramers are experimentally characterized for the first time. They show evidence for a nanometer-scale demixing trend of the cold species. In combination with quantum calculations, the mutual repulsion is demonstrated to start beyond three molecular units, whereas individual molecules still prefer to form a mixed complex.

Alcohol dimers--how much diagonal OH anharmonicity?

The OH bond of methanol, ethanol and t-butyl alcohol becomes more anharmonic upon hydrogen bonding and the infrared intensity ratio between the overtone and the fundamental transition of the bridging OH stretching mode decreases drastically. FTIR spectroscopy of supersonic slit jet expansions allows to quantify these effects for isolated alcohol dimers, enabling a direct comparison to anharmonic vibrational predictions. The diagonal anharmonicity increase amounts to 15-18%, growing with increasing alkyl substitution. The overtone/fundamental IR intensity ratio, which is on the order of 0.1 or more for isolated alcohols, drops to 0.004-0.001 in the hydrogen-bonded OH group, making overtone detection very challenging. Again, alkyl substitution enhances the intensity suppression. Vibrational second order perturbation theory appears to capture these effects in a semiquantitative way. Harmonic quantum chemistry predictions for the hydrogen bond-induced OH stretching frequency shift (the widely used infrared signature of hydrogen bonding) are insufficient, and diagonal anharmonicity corrections from experiment make the agreement between theory and experiment worse. Inclusion of anharmonic cross terms between hydrogen bond modes and the OH stretching mode is thus essential, as is a high level electronic structure theory. The isolated molecule results are compared to matrix isolation data, complementing earlier studies in N2 and Ar by the more weakly interacting Ne and p-H2 matrices. Matrix effects on the hydrogen bond donor vibration are quantified.

Dimers of cyclic carbonates: chirality recognition in battery solvents and energy storage.

Dimers of ethylene carbonate and propylene carbonate are created in supersonic jet expansions and characterized by FTIR spectroscopy. Fermi resonances are switched on and off by dimerization. There is a unique centrosymmetric dimer of ethylene carbonate in a pronounced case of complementary chirality synchronization, contributing to its energy storage capacity at melting. Two chiral propylene carbonate molecules combine in more intricate ways. If they have the same handedness, one of them is forced into an axial conformation and the binding partner stays in the more stable equatorial structure. If they have opposite handedness, centrosymmetric dimers of either axial or equatorial conformations are formed. This suggests the usefulness of chirality control in elucidating ionic transport mechanisms in battery solvents and asymmetric catalysis in such solvents.