Benchmarking the anisotropy of nitroxyl radical solvation with IR spectroscopy.

Two simple nitroxyl radicals, di-tert-butyl nitroxyl (DTBN) and 2,2,6,6-tetramethylpiperidinyloxyl (TEMPO) are solvated by one or two water, methanol, tert-butyl alcohol or phenol molecules. The resulting low temperature IR spectra of the vacuum-isolated microsolvates in the OH stretching range are assigned based on harmonic DFT predictions for closed shell solvent dimers and trimers and their offset from experiment, to minimise theory-guided assignment bias. Systematic conformational preferences for the first and second solvent molecule are observed, depending on the conformational rigidity of the radical. These assignments are collected into an experimental benchmark data set and used to assess the spectral predicting power of different DFT approaches. The goal is to find inexpensive computational methods which provide reliable spectral predictions for this poorly explored class of microsolvates.

Raising the benchmark potential of a simple alcohol-ketone intermolecular balance.

2-Butanone offers two hydrogen bond docking variants to a solvating methanol which are cleanly separated by supersonic jet infrared absorption spectroscopy in the OH-stretching range, resolving earlier action spectroscopy indeterminacies for this elementary case of an intermolecular alcohol-ketone balance. The solvent preference for the shorter chain side is unambiguously derived from the spectra of homologous compounds. It is analysed in terms of competing steric and dispersion interactions and the resulting energy differences across a low interconversion barrier. Fortuitous cancellations are discussed and quantitative energy deficiencies of the employed DFT approaches are suggested. Some benchmarkable experimental observations: at low temperature, a single methanol molecule prefers the methyl-sided oxygen lone pair of 2-butanone over the ethyl-sided lone pair by 1-2 kJ mol-1, the trans butane backbone is conserved in both low-lying isomers, the OH-stretching fundamentals differ by 47(2) cm-1.

Halogens in Acetophenones Direct the Hydrogen Bond Docking Preference of Phenol via Stacking Interactions.

Phenol is added to acetophenone (methyl phenyl ketone) and to six of its halogenated derivatives in a supersonic jet expansion to determine the hydrogen bonding preference of the cold and isolated 1:1 complexes by linear infrared spectroscopy. Halogenation is found to have a pronounced effect on the docking site in this intermolecular ketone balance experiment. The spectra unambiguously decide between competing variants of phenyl group stacking due to their differences in hydrogen bond strength. Structures where the phenyl group interaction strongly distorts the hydrogen bond are more difficult to quantify in the experiment. For unsubstituted acetophenone, phenol clearly prefers the methyl side despite a predicted sub-kJ/mol advantage that is nearly independent of zero-point vibrational energy, turning this complex into a challenging benchmark system for electronic structure methods, which include long range dispersion interactions in some way.

Pinacolone-Alcohol Gas-Phase Solvation Balances as Experimental Dispersion Benchmarks.

The influence of distant London dispersion forces on the docking preference of alcohols of different size between the two lone electron pairs of the carbonyl group in pinacolone was explored by infrared spectroscopy of the OH stretching fundamental in supersonic jet expansions of 1:1 solvate complexes. Experimentally, no pronounced tendency of the alcohol to switch from the methyl to the bulkier tert-butyl side with increasing size was found. In all cases, methyl docking dominates by at least a factor of two, whereas DFT-optimized structures suggest a very close balance for the larger alcohols, once corrected by CCSD(T) relative electronic energies. Together with inconsistencies when switching from a C4 to a C5 alcohol, this points at deficiencies of the investigated B3LYP and in particular TPSS functionals even after dispersion correction, which cannot be blamed on zero point energy effects. The search for density functionals which describe the harmonic frequency shift, the structural change and the energy difference between the docking isomers of larger alcohols to unsymmetric ketones in a satisfactory way is open.