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

Abstract

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.

Keywords: benchmark; density functional theory; dispersion; gas phase; hydrogen bonds; ketone–alcohol complexes; molecular recognition; pinacolone; vibrational spectroscopy.

Figure 1

Schematic representation of the two possible docking sides 4 and 4${}^{\prime }$ in a pinacolone molecule (tBu and Me) with different alcohols (R-OH, with the abbreviations Me for methyl, tBu for tert-butyl and Cp for cyclopentyl as R).

Figure 2

Harmonically zero-point corrected energy differences $\Delta E_{\mathrm{Me}-t\mathrm{Bu}}^0$ plotted against the electronic energy differences $\Delta E_{\mathrm{Me}-t\mathrm{Bu}}^{\mathrm{el}}$ referenced to the tBu side, computed at B3LYP-D3 (green) and TPSS-D3 (black) level, each with a def2-TZVP (empty symbols) and def2-QZVP (filled symbols) basis set. The electronic energy differences are seen to be a good approximation to experimentally relevant ZPVE-inclusive differences and the methyl docking side is systematically preferred (see also Table S3).

Figure 3

Computed OH wavenumber difference between the two docking sides $\Delta {\omega }_{\mathrm{Me}-t\mathrm{Bu}}$ relative to the number of C-atoms of the corresponding alcohol. This shows that the employed computational methods predict the same spectral trends for MeOH and tBuOH, indicated by dashed lines. For CpOH a somewhat larger discrepancy can be observed, with the smaller basis set TPSS result differing most from the experimental trend (blue) (see Table S4 for details).

Figure 4

FTIR jet OH stretching spectra of Pin with the three alcohols. The 1:1 complexes are marked with O, indexed by the assigned docking preference. Both docking sides are observed. Pin is only a stronger OH shifting partner than the alcohol itself for MeOH.

Figure 5

Experimental (anharmonic) downshift of the 1:1 complexes $\Delta {\tilde{\nu }}_{\mathrm{M},\exp }$ plotted against the harmonically computed downshifts $\Delta {\omega }_{\mathrm{M},\mathrm{theo}}$ for four computational variants. The harmonic DFT overestimation and the trend between docking sides (dashed arrows from tBu to Me docking) are uniform.

Figure 6

Experimental tBu docking fraction $x_{t\mathrm{Bu}}=c_{t\mathrm{Bu}}/(c_{t\mathrm{Bu}}+c_{\mathrm{Me}})$ (based on 95% confidence intervals and the mean value for the ratio $I_{\mathrm{Me}}/I_{t\mathrm{Bu}}$ from Table 1 and Tables S7 and S8) plotted against the computed ZPVE corrected energy differences $E_{\mathrm{Me}-t\mathrm{Bu}}^0$. Grey areas indicate inconsistency between experiment and theory, when allowing for an estimated anharmonic ZPVE error of $\pm 0.2$ $\mathrm{k}$$\mathrm{J}$${\mathrm{mol}}^{-1}$ and assuming correct cross section ratios form the respective theoretical model. (a) DFT energies, where all models predict the correct qualitative docking preference, but the correlation of energy and abundance is non-uniform. (b) As in (a), but with the electronic energy being replaced by the corresponding DLPNO-CCSD(T) value (see Section 2.3).