A study of hydrogen bonded vibrational spectra of (R)-(+)-Methylsuccinic acid, as aided by DFT dimer analysis.

Infrared and Raman spectral measurements in the region 4000-400cm(-1) have been carried out for (R)-(+)-Methylsuccinic acid. The vibrational band structures near 3100-3040cm(-1) in the IR and near 1650cm(-1) in the Raman spectra have indicated the presence of an inter-molecular hydrogen bonding. A DFT dimer model has been proposed that involves O-H⋯OC type of hydrogen bonding. The proposed dimer model has been derived from the three stable monomers computed at RHF/3-21G and B3LYP/6-311+G(d,p) levels of theory. A total of six dimer structures have been considered with a Boltzmann population of 38% for the most stable dimer and 62% for the remaining five dimer populations. A Boltzmann population weighted vibration spectrum has predicted bands, among others, for O-H⋯OC group that are in very good agreement with experiment. All the dimers have the same structure in that the two pairs of -O-H and -OC form a closed cyclic structure with a local center of inversion. This dimer geometry has given rise to one asymmetric mode at 1683 and one symmetric -CO mode at 1637cm(-1) corresponding to mutually exclusive an experimental IR band at 1700 and a Raman band at 1651cm(-1). Further, the bond length, H⋯O, for the most stable dimer is 1.686Å, being shorter than the sums of van der Waals radii, 2.72Å and the angle between O-H and H⋯O is almost linear (179°) suggesting that the hydrogen bonding is fairly strong.