SCALED QUANTUM-MECHANICAL FORCE-FIELDS AND VIBRATIONAL-SPECTRA OF NUCLEIC-ACID CONSTITUENTS - 9 - TETRAHYDROFURAN

Infrared and Raman spectra of 1 M aqueous solution of tetrahydrofuran (THF) were recorded in the 850-3050 and 560-3050 cm(-1) frequency rang e, respectively. The effects of hydrogen bonding on vibrational spectr a of THF were analyzed by comparing spectra of aqueous solution with t he spectra of liquid and solid THF reported previously by Cadioli et a l. [J. Phys. Chem. 1993, 97, 7844]. More regular band shapes and small er bandwidths of ring stretching modes indicate that the barrier for p seudorotation of the furanose ring increases in aqueous solution. This finding is in agreement with the results of our ab initio calculation s using the Langevin dipoles (LD) solvation model, which predicted tha t the pseudorotational barrier of gaseous THF increases in aqueous sol ution by 0.25 +/- 0.1 kcal/mol. Considering available gas-phase data. the free energy barrier for the pseudorotation in aqueous solution was estimated to be 0.5 +/- 0.2 kcal/mol. The geometric structure and har monic force fields of the C-2 conformer of THF were calculated by usin g the Hartree-Fock (HE), density functional theory (DFT), and Moller-P lesset perturbation theory of the second order (MP2). The scale factor s for the S-VWN, B3-LYP and B-LYP density functional, and HF force con stants of THF were determined. These scaled force fields were found to reproduce the observed frequencies with the overall 1% accuracy, with the B3-LYP method providing the most accurate results. The obtained a greement between the calculated and experimental infrared intensities, nonresonant Raman intensities, and depolarization ratios supports the proposed spectral assignment. The scale factors calculated here for T HF augment scale factors determined previously for other nucleotide co mponents: dimethyl phosphate and nucleic acid bases. Consequently, rel iable ab initio interpretations of the vibrational spectra of nucleic acids in aqueous solution can be obtained in the future by using the c oncept of the transferability of scale factors from these nucleic acid constituents.