ABSTRACT
The effects of the formation of hydrogen-bonded networks on the important osmolyte trimethylamine N-oxide (TMAO) are explored in a joint Raman spectroscopic and electronic structure theory study. Spectral shifts in the experimental Raman spectra of TMAO and deuterated TMAO microsolvated with water, methanol, ethanol, and ethylene glycol are compared with the results of electronic structure calculations on explicit hydrogen-bonded molecular clusters. Very good agreement between experiment and theory suggests that it is the local hydrogen-bonded geometry at TMAO's oxygen atom that dominates the structure of the extended hydrogen-bonded networks and that TMAO's unique stabilizing abilities are a result of the "indirect effect" model. Natural bonding orbital (NBO) calculations further reveal that hyperconjugation results in vibrational blue shifts in TMAO's C-H stretching region when solvated and a red shift in methanol's C-H stretching region when hydrogen bonding with TMAO.
Subject(s)
Methylamines/chemistry , Solvents/chemistry , Deuterium/chemistry , Hydrogen Bonding , Models, Theoretical , Spectrum Analysis, Raman , ThermodynamicsABSTRACT
The effects of hydration on vibrational normal modes of trimethylamine N-oxide (TMAO) are investigated by Raman spectroscopy and electronic structure computations. Microsolvated networks of water are observed to induce either red or blue shifts in the normal modes of TMAO with increasing water concentration and to also exhibit distinct spectral signatures. By taking advantage of the selective and gradual nature of the water-induced shifts and using comparisons to theoretical predictions, the assignments of TMAO's normal modes are re-examined and the structure of the hydrogen-bonded network in the vicinity of TMAO is elucidated. Agreement between experiment and theory suggests that the oxygen atom in TMAO accepts on average at least three hydrogen bonds from neighboring water molecules and that water molecules are likely not directly interacting with TMAO's methyl groups.
