MOLECULAR-STRUCTURE, VIBRATIONAL-SPECTRA AND QUANTUM-MECHANICAL FORCE-FIELDS OF MODIFIED OLIGONUCLEOTIDE LINKAGES .1. METHYL METHOXYMETHANPHOSPHONATE

Methyl methoxymethanphosphonate (CH3OCH:P(O-2)OCH3-) (MMP) was studied by vibrational spectroscopy and by ab initio calculations. IR and Ram an spectra were measured from 1 M aqueous solutions and crystalline sa mples of sodium and potassium salts of MMP. In these spectra, a superi mposition of the counter-ion and conformational effects was observed. The observed spectra were interpreted by using the Hartree-Fock (HF) a nd double harmonic approximations and the 6-31G set of atomic orbital s. The full gradient optimization and vibrational analysis of the g(-) gg and g(-)gt conformations of MMP in the anionic forms and also compl eted with the Na+ and K+ ions were carried out. The harmonic frequenci es and IR and Raman intensities were predicted by using the scale fact ors for the HF/6-31G force field that were transferred from the dimet hyl phosphate anion (DMP) and its sodium salt. The good agreement betw een the experimental and calculated spectra in the 200-1800 cm(-1) fre quency region evidenced the transferability of scale factors between D MPT and MMP and also between compounds completed with different metal ions. (C) 1997 Elsevier Science B.V.