Detailed Assignment of the CH Chromophores in Methyl Mandelate and Mandelic Acid: A Multi-Experimental Approach Using Polarized FTIR Microspectroscopy of Sublimated Crystals

A sublimation cell for the characterization of microcrystals using Fourier transform infrared (FTIR) microspectroscopy was constructed and applied to methyl mandelate and mandelic acid. It was possible to assign different CH chromophores in the FTIR spectra. Comparison to quantum chemical calculations, attenuated total reflectance infrared (ATR-IR) spectra of related compounds, and isotope labeling was used to support the assignments. Vibrational transition dipole vectors were deduced from crystal rotation and polarization experiments. The direction of these vectors can be used to constrain the absolute orientation of the molecules in the crystal as well as for the assignment of further vibrational bands

Molecular Docking via Olefinic OH···π Interactions: A Bulky Alkene Model System and Its Cooperativity

Complexes of <i>t</i>-butyl alcohol with norbornene and its monocyclic constituents cyclopentene and cyclohexene are studied via their OH stretching fundamental transitions in supersonic jet expansions. Compared to OH···OH hydrogen bonds, the spectral shifts due to OH···π bonding in the mixed dimers are reduced by a factor of 2. Mixed trimers show substantially different spectral signatures due to cooperative effects. Regioselective docking on the two sides of the double bond in norbornene is observed. Harmonic modeling of the spectra using dispersion-corrected hybrid functionals is quite successful, suggesting a high predictive power for this poorly explored class of complexes between alcohols and alkenes

Phenyl- vs Cyclohexyl-Substitution in Methanol: Implications for the OH Conformation and for Dispersion-Affected Aggregation from Vibrational Spectra in Supersonic Jets

The monomers and hydrogen-bonded dimers of benzyl alcohol, cyclohexyl­methanol, and 2-methyl-1-propanol are investigated by jet-FTIR spectroscopy, complemented by Raman spectra and quantum chemical calulations, including CCSD­(T) corrections. A large variety of London dispersion effects from the interacting carbon cycles is revealed, sometimes adding to and sometimes competing with the alcoholic hydrogen bonds. Conformational (in-)flexibility provides the key for understanding these effects, and this requires accurate predictions of monomer conformational preferences, which are shown to be subtly at variance with experiment even for some triple-ζ MP2 calculations. In some observed dimers, cooperative OH···OH···π patterns are sacrificed to optimize σ–π dispersion interactions. In other competitive dimers, dispersion interactions are far from maximized, because that would imply a substantial weakening of the hydrogen bond. In the series from methanol dimer to 1-indanol dimer, which this contribution bridges, B3LYP-D3 appears to switch from an overestimation to a slight underestimation of cohesion, but overall it provides a very useful modeling tool for vibrational spectra of systems affected by both hydrogen bonds and London dispersion

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

Helium Nanodroplet Study of the Hydrogen-Bonded OH Vibrations in HCl–H₂O Clusters

Mixed (HCl)_<i>N</i>(H₂O)_<i>M</i> clusters have been assembled in He droplets from the constituting molecules. Spectra of the clusters were obtained in the range of hydrogen-bonded OH vibrations (3100–3700 cm^–1) by infrared laser depletion spectroscopy. The observed bands were assigned to cyclic hydrogen-bonded aggregates containing up to two HCl and three H₂O molecules. The obtained frequencies are in good agreement with the results of harmonic quantum chemical calculations upon appropriate uniform shifts mimicking anharmonic corrections. Although larger clusters containing up to six water molecules were also produced in the droplets, their spectra were found to contribute to the unresolved signal in the range 3250–3550 cm^–1. The fact that no narrow bands could be unambiguously assigned to the mixed clusters containing more than three water molecules may indicate that such clusters exist in many isomeric forms that lead to overlapped and unresolved bands giving rise to broad structureless features. Another possible explanation includes the formation of elusive zwitterionic clusters, whose bands may have considerable breadth due to electrostatic coupling of different vibrational modes and concomitant intramolecular vibrational relaxation

Adaptive Aggregation of Peptide Model Systems

Jet-cooled infrared spectra of acetylated glycine, alanine, and dialanine esters and their dimers are reported in the amide A and amide I–III regions. They serve as particularly simple peptide aggregation models and are found to prefer a single backbone conformation in the dimer that is different from the most stable monomer backbone conformation. In the case of alanine, evidence for topology-changing chirality discrimination upon dimer formation is found. The jet spectroscopic results are compared to gas phase spectra and quantum chemical calculations. They provide reliable benchmarks for the evaluation of the latter in the field of peptide interactions