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 range, respectively. The effects of hydrogen bonding on vibrational spectra of THF were analyzed by comparing spectra of aqueous solution with the spectra of liquid and solid THF reported previously by Cadioli et al. [J. Phys. Chem. 1 993, 97, 7844]. More regular band shapes and smaller bandwidths of ring stretching modes indicate that the barrier for pseudorotation of the furanose ring increases in aqueous solution. This finding is in agreement with the results of our ab initio calculations using the Langevin dipoles (LD) solvation model, which predicted that the pseudorotational barrier of gaseous THF increases in aqueous solution 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 harmonic force fields of the C 2 conformer of THF were calculated by using the Hartree−Fock (HE), density functional theory (DFT), and Møller−Plesset perturbation theory of the second order (MP2). The scale factors for the S-VWN, B3-LYP and B-LYP density functional, and HF force constants 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 agreement between the calculated and experimental infrared intensities, nonresonant Raman intensities, and depolarization ratios supports the proposed spectral assignment. The scale factors calculated here for THF augment scale factors determined previously for other nucleotide components: dimethyl phosphate and nucleic acid bases. Consequently, reliable ab initio interpretations of the vibrational spectra of nucleic acids in aqueous solution can be obtained in the future by using the concept of the transferability of scale factors from these nucleic acid constituents.