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

Abstract Infrared and Raman spectral measurements in the region 4000–400 cm − 1 have been carried out for (R)-(+)-Methylsuccinic acid. The vibrational band structures near 3100–3040 cm − 1 in the IR and near 1650 cm − 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 1637 cm − 1 corresponding to mutually exclusive an experimental IR band at 1700 and a Raman band at 1651 cm − 1 . Further, the bond length, H ⋯ O, for the most stable dimer is 1.686 A, being shorter than the sums of van der Waals radii, 2.72 A and the angle between O–H and H ⋯ O is almost linear (179°) suggesting that the hydrogen bonding is fairly strong.