DFT studies of the structure and vibrational spectra of 8-hydroxyquinoline N-oxide.

The geometry, frequency and intensity of the vibrational bands of 8-hydroxyquinoline N-oxide (8-HQNO) and its deuterated derivative (8-DQNO) were obtained by the density functional theory (DFT) with the BLYP and B3LYP functionals and 6-31G(d,p) basis set. The optimized bond lengths and bond angles are in good agreement with the X-ray data. The IR and INS spectra of 8-HQNO and 8-DQNO computed at the DFT level reproduce the vibrational wavenumbers and intensities with an accuracy, which allows reliable vibrational assignments.

Tautomerism and infrared spectra of 2-thiopurine: an experimental matrix isolation and theoretical ab initio and density functional theory study.

Infrared spectra of 2-thiopurine (2-mercaptopurine, 2-purinethiol ) isolated in low-temperature Ar and N2 matrixes are reported. These spectra indicate that the compound adopts exclusively the thiol N9H tautomeric form. The theoretical calculations of relative energies of 2-thiopurine tautomers have been carried out at the MP4(SDTQ)//HF level using the 6-31G(d,p) basis set. The thiol N9H tautomer was predicted to be the most stable of all isomers of 2-thiopurine. The infrared spectra of the tautomers of 2-thiopurine have been calculated at the DFT(B3LYP)/6-31G(d,p) level. Good agreement between the experimental spectra and the spectra calculated for thiol N9H tautomer supported the identification of the dominant tautomer. It has also allowed for the reliable assignment of the bands observed in the experimental IR spectrum.

Molecular structure and vibrational raman and infrared spectra of bromochlorofluoromethane and its silicon and germanium analogs: quantum-mechanical DFT and MP2 calculations.

DFT(B3LYP) and MP2 calculations with the 6-311G(2d, 2p)-type basis set have been carried out for the prediction of molecular parameters (bond distances, bond angles, rotational constants, and dipole moments) and vibrational Raman and infrared spectra (harmonic wavenumbers, absolute intensities, Raman scattering activities, and depolarization ratios) of bromochlorofluoromethane (HCBrCIF) and its silicon and germanium analogs (HSiBrClF and HGeBrCIF). The predicted geometry and vibrational Raman and infrared spectra of HCBrClF agree well with the available experimental data for this molecule and their deuterated derivatives. This agreement allows one to believe that the predicted molecular parameters and vibrational spectra of HSiBrClF, HGeBrClF, and their deuterated derivatives will guide their future experimental studies.

Spectroscopic identification of 2,4-pyrimidinedithiol; an experimental matrix isolation and ab initio Hartree-Fock and density functional theory study.

2,4-Pyrimidinedithiol (the dithiol form of 2,4-dithiouracil) was generated by UV (lambda > 335 nm) irradiation of the dithione form of 2,4-dithiouracil isolated in low-temperature argon or nitrogen matrices. The IR and UV spectra of the photoproduct are reported. The dithiol form of 2,4-dithiouracil was identified by comparison of the experimental spectrum with the spectrum theoretically predicted for this form at the HF/6-31 (d,p) and DFT(B-3LYP)/6-31 (d,p) levels. This comparison resulted also in assignment of the bands observed in the IR spectrum of 2,4-pyrimidinedithiol to the theoretically predicted normal modes.

Monte Carlo simulation of hydration of the guanine-uracil pairs with guanine in two tautomeric forms: contribution of water bridging to relative stability of mispairs.

Results are presented from Monte Carlo simulation of hydration of guanine-uracil mispairs by 25 and 50 water molecules. The hydration shells of three mispairs formed between "normal" dioxo form of uracil (U) and three forms of guanine ("normal" amino-oxo tautomer G and two rotamers of the "rare" amino-hydroxy tautomer G*) depend on the tautomeric forms of the guanine molecule. The simulation shows the important role of hydration effects on the relative stability of the mispairs.

On tautomerism of the cytosine molecule.

Tautomerism of the cytosine molecule is discussed in connection with recent experimental matrix-isolation infrared spectroscopic measurements and recent ab initio calculations of relative stabilities of tautomers and of IR spectra for different tautomeric forms of the compound. Experimental IR spectra in the N-H and O-H stretching regions and in the C = O stretching region are presented for cytosine and for its several derivatives considered as model compounds. This experimental evidence, as well as the quantum-mechanical calculations (including both electron correlation and zero-point vibrational contributions), clearly indicate that two tautomers of cytosine, i.e. the amino-hydroxy and amino-oxo forms with the hydrogen atom at the N(1) position, exist in equilibrium when the cytosine molecule is isolated in an inert environment. The effect of the environment on the relative stabilities of several tautomers is also discussed briefly.