M. Fernández Gómez

A new insight into the vibrational analysis of pyridine

A new proposal of vibrational assignment for pyridine is reported. Infrared spectra for the liquid and gas phases as well as Raman spectra for the liquid have been recorded and analyzed for -d(0), -d(5) and, for the first time to our knowledge, for 15N isotopomers as well. The proposal of assignment has been assessed by the calculation of a number of force fields, theoretical (ab initio, density functional theory) approaches as well as by a set of simple valence internal coordinates force constants transferred from benzene using the pure vibrational force field approximation. In all cases, the root mean square (rms) for the wavenumbers turn out to be lower than the best obtained so far, i.e. 6.6 cm(-1), as stated by Wiberg et al.

Unambiguous formalism of molecular vibrations: Use of redundant coordinates and canonical matrices

This paper introduces a new approach to the problem of describing molecular vibrations in redundant valence coordinates. A consistent definition for canonical force field, which is different of that by Kuczera and Pupyshev et al., is outlined. This new definition is also generalized to other matrices like kinetic-energy matrices and transformation matrices between different sets of vibrational coordinates. The importance of such canonical matrices follows from the fact that they are uniquely determined and allow us to consider the redundant coordinates as though they were independent.

Vibrational spectra of s-triazine and its halogenated derivatives. Harmonic scaled ab initio force fields for s-triazine, trifluoro-s-triazine and trichloro-s-triazine

The vibrational spectra of s-triazine, trifluoro-s-triazine and trichloro-s-triazinc were calculated by using the 3-21G basis set and the program system GAUSSIAN 90. The ab initio force fields were scaled to try to reproduce the best values assigned for experimental frequencies for the above-mentioned molecules. The transferability of the scale factors among these molecules was also studied. Taking into account the results obtained, and with the help of new IR and Raman spectra, the normal mode v12 of trifluoro-s-triazine has been reassigned. Likewise, the assignment proposed by Lancaster et al. [J.E. Lancaster, R.F. Stamm and N.B. Colthup, Spectrochim. Acta, 17 (1961) 1551 for the normal mode v,, of s-triazine has been confirmed. New values for the inactive modes in IR and Raman spectra, uq and z+, have also been calculated.

Experimental and theoretical vibrational study of silyl trifluoromethanesulfonate, CF3SO2OSiH3

Infrared and Raman spectra were obtained for liquid silyl trifluoromethanesulfonate, a silylating agent of limited stability. The molecular geometry was optimized by means of density functional theory and Möller-Plesset second order perturbation theory methods, using different basis sets. The optimized structure presents a gauche conformation, similar to that adopted by methyl trifluoromethanesulfonate, which was determined experimentally a short time ago. The wavenumbers for the normal modes of vibration and the corresponding force constants were also calculated, facilitating the interpretation of the vibrational data. The harmonic force constants given by theory were scaled to reproduce adequately the experimental wavenumbers.

Symmetrised quantum-mechanical force-fields and INS spectra: s-triazine, trichloro-s-triazine and pyrazine

Abstract A vibrational analysis of the Inelastic Neutron Scattering (INS) spectra of s-triazine, trichloro-a-triazine and pyrazine has been carried out starting from different ab initio calculations levels (HF/3-21G, HF/6-31G ∗ and MP2/6-31G ∗ ). Resulting force fields were transformed to the symmetry-coordinate system and refined to the observed INS spectral profile. Similar final force fields result regardless of the level of the ab initio calculation. The importance of INS intensities in determining the values for the interaction constants is demonstrated.

Theoretical study of the structure and vibrational spectra of VO2X2− (X=F, Cl, Br, I) anions

Abstract We have carried out a structural and vibrational theoretical study for the four members of the series VO 2 X 2 − (X=F, Cl, Br, I). In order to assess the behaviour of such theoretical calculations (ab initio and DFT) we have made a comparative work for the fluorine and chlorine derivatives, for which vibrational data exist, to evaluate not only the best level of theory but also the best basis set to be used to reproduce the experimental wavenumbers. The results were then used to predict the vibrational spectra and molecular geometry of the other members of the series, for which there are no experimental data. The theoretical vibrational calculations allowed us to obtain a set of scaled force constants fitting the observed wavenumbers.

ASTROPHYSICAL MOLECULES AlD AND CaH: TRANSITION PROBABILITIES AND DISSOCIATION ENERGY

The Franck-Condon (FC) factors and r-centroids for the bands systemC1Σ+→ X1Σ+ of AlD and E2Π → X2Σ+ of CaH have been evaluated by means of a reliable numerical integration procedure by using a suitable potential. The dissociation energy, De, for the electronic ground states of AlD and CaH have been estimated by the curve fitting method to the RKRV experimental potential curve turning out to be 3.01 eV and 2.32 eV, respectively.

Franck-Condon factors and r-centroids for a number of band systems of the astrophysical molecule AlF

Abstract Franck–Condon factors and r-centroids, which are very closely related to transition probabilities, have been evaluated by the more reliable numerical integration procedure for the bands of B 1 Σ + – X 1 Σ + , C 1 Σ + – X 1 Σ + , c 3 Σ + – b 3 Σ + , f 3 Π – b 3 Σ + , F 1 Π – B 1 Σ + and G 1 Σ + – B 1 Σ + systems of the AlF molecule. The species has been observed in circumstellar envelope around Asymptotic Giant Branch stars and is likely to be present in the umbral atmosphere. The molecular parameters, necessary for studying the physical and dynamical properties of the above said sources, are evaluated in the present work.

The force field of the triazinic ring C3N3

Abstract We have obtained a set of force constants for the symmetric triazinic ring C3N3, from data on the vibrational frequencies of the s-triazine, trifluoro-s-triazine and trichloro-s-triazine molecules. The procedure consists on several steps of refinement, concluding with the determination of the force constants of the non-common part of each individual species. The force field of the common block has been used to calculate the vibrational spectrum of tribromo-s-triazine, leading to satisfactory results.

A set of force constants common for SiH3–X molecules

Abstract A set of common force constants was obtained for the silyl group (SiH 3 –X) using as experimental data the frequencies from silyl cyanide (X=–CN), isocyanate (X=–NCO) and isothiocyanate (X=–NCS). Initial values of the force constants were obtained by ab initio methods, using three different levels of theory, Hartree–Fock, second order Moller–Plesset and Density Functional Theory. We have analysed the variation of the common block of force constants for the silyl group with respect to the nature of the X-substituent.