Why betaine crystallizes in high local C-s symmetry. An ab initio MO and DFT study of anhydrous betaine and betaine monohydrate

A theoretical study of the structure, charge distribution, rotational barri er and fundamental Vibrations of anhydrous betaine (CH3)(3)NCH2COO (trimeth ylglycine) was carried out and compared with available experimental data. C alculations were carried out at HF, MP2 and B3LYP levels using a 6-31+G(d,p ) basis set. The calculated rotational barrier of the betaine carboxylic gr oup is 40.5 kJ/mol at the MP4(SDQ)/6-311G(d,p)//HF/6-31+G(d,p) level of the ory. The rotation of the carboxylic group changes the molecule from a highl y symmetric (C-8) conformation into a twisted conformation resulting in sho rtening of the molecule by about 50pm. Natural population analysis (NPA) in dicates intramolecular interaction between the carboxylic oxygen and the ne arest methyl hydrogens resulting in internal hydrogen bonding. MP4(SDQ)/6-3 11G(d,p) single-point NPA calculations on a betaine monohydrate model taken from the X-ray geometry show an expected weakening in the internal hydroge n bond. Calculations explain why betaine preferentially crystallizes in hig h local C-8 symmetry.