The basis for this study comes from the 1H NMR spectra in binary systems of acetic acid (AA) and dimethyl sulfoxide (DMSO). According to experimental studies [1], the correlation between NMR chemical shift of the acidic proton in acetic acid and the molar part of acid in a AA/DMSO binary system is non-monotonic. When the mole fraction of acetic acid in a binary system increases from 0 to ~0.7, the chemical shift of hydrogen drops around 0.5ppm (from 11.9ppm to 11.4ppm), then rises, reaching around 11.7ppm in pure acetic acid. This phenomenon is caused by the shifting equilibrium between various molecular (hydrogen bond) aggregates formed between acetic acid molecules themselves as well as between acetic acid and DMSO when the molar fraction of the binary mixture is changing. It is nearly impossible to find out the nature of these molecular aggregates in particular their population experimentally, however, these problems can be very effectively dealt with by using advanced molecular modelling techniques such as molecular dynamics (MD) simulations and combined quantum mechanics/molecular mechanics (QM/MM) approaches. These techniques have also been very recently applied to study molecular aggregation in glacial acetic acid [2].
There are two main objectives of this study: carrying out a qualitative and quantitative analysis of molecular aggregates and calculating the chemical shift of the acidic proton in AA/DMSO systems of different composition. Comparison between computational and experimental results allows for a detailed understanding of the reasons of curious concentration dependence of the chemical shift of the acidic proton. A different binary system of acetic acid and a non-polar solvent – cyclohexane, also displays a non-monotonic fashion [1], but it is caused only by interactions between acetic acid molecules themselves. However, when analysing AA/DMSO systems it is important to also account for interactions between acetic acid and DMSO because of polarity of the solvent.
The analysis of molecular aggregates in a binary system was carried out for three different AA:DMSO mixtures: 1:3, 1:1 and 3:1 (molar ratios). MD simulations were executed using an all-atom OPLS force field and standard Coulomb plus 12-6 type Lennard-Jones potential. System's equilibrium was achieved by allowing system's density to converge in NPT simulations, then switching to NVT ensemble, running another equilibration run and completing with 2ns long production run. Atomic point charges were calculated for pure AA and DMSO, according to the CHelpG scheme, and then averaged considering mole fractions of components in each system. NMR shielding constants were calculated for all three AA/DMSO systems using QM/MM calculations. These calculations were performed for a set of molecular configurations captured during MD simulations of a given system. The analysis of hydrogen bonds in different AA/DMSO systems was based on geometric definition of a hydrogen bond which allowed to identify different types of molecular aggregates forming in these systems.
The presentation will include a comparison between experimental and computed 1H NMR spectra of acidic proton of acetic acid and results of the analysis of molecular aggregates in AA/DMSO binary systems.