Ab initioand regularized force fields of haloethanes: CH3CH2Cl, CH3CHCl2, CH3CF2Cl, and CH3CFCl2

Scaled in Cartesian Coordinates Ab Initio Molecular Force Fields of DNA Bases: Application to Canonical Pairs

The model of Regularized Quantum Mechanical Force Field (RQMFF) was applied to the joint treatment of ab initio and experimental vibrational data of the four primary nucleobases using a new algorithm based on the scaling procedure in Cartesian coordinates. The matrix of scaling factors in Cartesian coordinates for the considered molecules includes diagonal elements for all atoms of the molecule and off-diagonal elements for bonded atoms and for some non-bonded atoms (1–3 and some 1–4 interactions). The choice of the model is based on the results of the second-order perturbation analysis of the Fock matrix for uncoupled interactions using the Natural Bond Orbital (NBO) analysis. The scaling factors obtained within this model as a result of solving the inverse problem (regularized Cartesian scale factors) of adenine, cytosine, guanine, and thymine molecules were used to correct the Hessians of the canonical base pairs: adenine–thymine and cytosine–guanine. The proposed procedure is based on the block structure of the scaling matrix for molecular entities with non-covalent interactions, as in the case of DNA base pairs. It allows avoiding introducing internal coordinates (or coordinates of symmetry, local symmetry, etc.) when scaling the force field of a compound of a complex structure with non-covalent H-bonds.

High-Resolution Infrared Study of the nu(7), nu(8), and nu(15) Bands and Millimeter-Wave Investigation of the v(8) = 1 State of CF(2)Cl-CH(3)

High-resolution (Deltavarsigma = 2.3 and 2.9 x 10(-3) cm(-1)) FTIR spectra of natural and (35)Cl monoisotopic CH(3)CF(2)Cl have been recorded at -70 degrees C in the 600-1400 cm(-1) range. The bands nu(7), nu(8), and nu(15) have been rotationally analyzed for both isotopic varieties. With the help of predictions based on nu(8) parameters, the millimeter-wave spectrum of the (35)Cl species in the v(8) = 1 state has been observed and jointly fitted with the IR data. Only a small number of local perturbations have been detected in the spectra. Altogether more than 8000 IR transitions have been fitted with an experimental precision of ca. 3 x 10(-4) cm(-1). Copyright 2000 Academic Press.