¹H NMR spectra of alcohols in hydrogen bonding solvents: DFT/GIAO calculations of chemical shifts

Abstract

Proton nuclear magnetic resonance (NMR) shifts of aliphatic alcohols in hydrogen bonding solvents have been computed on the basis of density functional theory by applying the gauge-including atomic orbital method to geometry-optimized alcohol/solvent complexes. The OH proton shifts and hydrogen bond distances for methanol or ethanol complexed with pyridine depend very much on the functional employed and very little on the basis set, provided it is sufficiently large to give the correct quasi-linear hydrogen bond geometry. The CH proton shifts are insensitive to both the functional and the basis set. NMR shifts for all protons in several alcohol/pyridine complexes are calculated at the Perdew, Burke and Ernzerhof PBE0/cc-pVTZ//PBE0/6-311 + G(d,p) level in the gas phase. The results correlate with the shifts for the pyridine-complexed alcohols, determined by analysing data from the NMR titration of alcohols against pyridine. More pragmatically, computed shifts for a wider range of alcohols correlate with experimental shifts in neat pyridine. Shifts for alcohols in dimethylsulfoxide, based on the corresponding complexes in the gas phase, correlate well with the experimental values, but the overall root mean square difference is high (0.23 ppm), shifts for the OH, CHOH and other CH protons being systematically overestimated, by averages of 0.42, 0.21 and 0.06 ppm, respectively. If the computed shifts are corrected accordingly, a very good correlation is obtained with a gradient of 1.00 ± 0.01, an intercept of 0.00 ± 0.02 ppm and a root mean square difference of 0.09 ppm. This is a modest improvement on the result of applying the CHARGE programme to a slightly different set of alcohols. Some alcohol complexes with acetone and acetonitrile were investigated both in the gas phase and in a continuum of the relevant solvent.

Origin of Hydrocarbons Stability from a Computational Perspective: A Case Study of Ortho‐Xylene Isomers

“…Furthermore, Myburgh and co‐workers have also found, in line with our observations herein, a negative exchange‐correlation term for the sterically congested H−H atoms in cyclic hydrocarbons . Proton NMR chemical shifts have appeared to loosely correlate with the exchange‐interaction energies for the considered cyclic hydrocarbons . Although, the nature of close C−H⋅⋅⋅H−C contacts is still controversial, it shall be noticed, that more and more suggestions appear in the recent literature which highlight the importance of rather attractive C−H⋅⋅⋅H−C contacts …”

Association of symmetrical alkane diols with pyridine: DFT/GIAO calculation of 1H NMR chemical shifts

“…The OH and CH shifts of alkanol/pyridine complexes are well reproduced by calculations at the PBE0/6-311 + G(d,p) level in the gas phase. [4] More simply, shifts for a wider range of alkanol/pyridine complexes correlate with values observed in neat pyridine. [4] For the symmetrical diols considered in the present study, even after empirical scaling, the RMSD for overall shifts is still high (0.18 ppm).…”

“…[4] More simply, shifts for a wider range of alkanol/pyridine complexes correlate with values observed in neat pyridine. [4] For the symmetrical diols considered in the present study, even after empirical scaling, the RMSD for overall shifts is still high (0.18 ppm). This perhaps reflects the fact that the shifts, even in the very simplest case of diol 1, are derived from values computed for some 30 different structures, of which about ten are important.…”

“…The longer OH···N distance is accompanied by an OH proton shift enhancement about 0.6 ppm lower, as was observed in previous work on alcohols. [4] The shortening of the intramolecular OH···OH distance on going from the free diol to the 1 : 1 complex (Δ(3)) is slightly smaller for the most stable conformer, tGg′ (0.10 Å), than for the next most stable, gGg′ (0.12 Å), but does not depend on the functional. Calculations on the butane-2,3-diols, 2 and 3, pinacol, 4, and propane-1,3-diol, 5, were run only at the B3LYP/6-311 + G(d,p) level.…”

“…Solvent effects on proton nuclear magnetic resonance (NMR) chemical shifts can be handled in density functional theory (DFT) [1][2][3] calculations by the solvent continuum approach or by modelling solute/solvent complexes in various ways, either with one or more discrete solvent molecules or by molecular dynamics simulations with a much greater number of, usually, small molecules. [4] These two approaches may be combined by putting discrete complexes into a solvent continuum, but this is possible only for solute species that have just one important conformer. [5] Failing this, shifts can be computed by performing discretecomplex calculations in the 'gas phase', ignoring the effect of the solvent continuum.…”