IR and Raman spectra, conformational flexibility, and scaled quantum mechanical force fields of sodium dimethyl phosphate and dimethyl phosphate anion

Quantum chemical calculations, involving Hartree-Fock (HF), perturbation (MP2), and density functional (DFT) theories, are carried out for the dimethyl phosphate anion (DMP) and sodium dimethyl phosphate (NaDMP), model systems for the DNA phosphate group. Energies, geometries, and harmonic force fields of different conformations of DMP and NaDMP are compared. In addition, atomic charges derived from the HF/6-31G* and MP216-31+G* electrostatic potential of DMP and NaDMP are calculated in order to determine the effects of counterions upon the charge distribution. Finally, IR and Raman spectra of solid and aqueous NaDMP, recorded here in the 80-4000 cm(-1) (IR) and 150-3100 cm(-1) (Raman) spectral regions, are assigned using differentially scaled Hartree-Fock, MP2, and B3-LYP force fields of DMP and NaDMP. Our interpretation of the individual vibrational bands confirms the results of the previous empirical normal-coordinate analysis of DMP of Thomas et al. (Biophys. J. 1994, 66, 225). Also, the predicted frequencies, IR and Raman intensities, depolarization ratios, and C-13 iSotopic frequency shifts agree well with the experimental data: Among the computational methods, the best results are obtained using the B3-LYP gradient corrected density functional. The proposed scale factors for the HF/3-21G(*), HF/6-31G*, MP2/6-31+G*, and B3-LYP/6-31G* force fields of DMP or NaDMP are transferable to larger systems involving phosphodiester moiety, for example to nucleotides or phospholipids.