Conformations of dimethylhydrogen phosphonate (DMHP): a matrix isolation infrared and ab initio study

The infrared spectra of dimethylhydrogen phosphonate (DMHP) isolated in nitrogen, argon and krypton matrices using an effusive source at 298 and 373 K have been recorded. Experiments were also performed using a supersonic jet source to look for conformational cooling in the expansion process. As a result of these experiments, infrared spectral characteristics of the ground and higher energy conformers of the DMHP have been identified for the first time. The structures of DMHP were optimized at the hybrid B3LYP and Hartree fock (HF) levels of theory using the 6-31++G** basis sets. Computationally, four minima were obtained corresponding to DMHP conformers with G (+/-)G (-/+), G (-)G (-), TG (+) and TG (-) structures in the order of increasing energy. Frequency calculations were done to confirm that the structures were indeed minima on the potential energy surface (PES). The computed frequencies corroborated well with the experimental matrix isolation infrared frequencies leading to definite assignments of the infrared features of DMHP, for the G (+/-)G (-/+) and TG (+) conformers. At B3LYP/6-31++G** level, the energy difference between the G (+/-)G (-/+) and G (-)G (-) conformer was 1.53 kcal/mol, and that between G (+/-)G (-/+) and TG (+), G (+/-)G (-/+) and TG (-) were 1.65 and 1.95 kcal/mol. Transition-state calculations were also carried out at B3LYP/6-31++G** level connecting the G (+/-)G (-/+) to G (-)G (-), TG (+) and TG (-) conformers. Computations indicated that the conformer interconversion between G (-)G (-) --> G (+/-)G (-/+) is barrierless, whereas the barriers for TG (+) --> G (+/-)G (-/+) and TG (-) --> G (+/-)G (-/+) are 1.47 and 0.88 kcal/mol, respectively.