A. Navarro

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 2,2,2-trifluoroethyl trifluoromethanesulfonate, CF3SO2OCH2CF3

The infrared spectra of 2,2,2-trifluoroethyl trifluoromethanesulfonate (CF3SO2OCH2CF3) were obtained in the gaseous, liquid and solid states as well as the Raman spectrum of the liquid. Quantum chemistry calculations using the density functional theory were used to predict the most stable geometry and conformation of the studied molecule. Subsequently, the harmonic vibrational frequencies and force field were calculated. An assignment of the observed spectral features made after comparison with the related molecules and with the predicted frequencies was used as the basis of a scaling of the original force field in order to reproduce as well as possible the experimental frequencies. With this purpose a set of scale factors was calculated by a least square procedure, leading to a final root mean square deviation (RMSD) of 9.7 cm(-1).

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.

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.

The molecular force field of 4-fluorostyrene: an insight into its vibrational analysis using inelastic neutron scattering, optical spectroscopies (IR/Raman) and theoretical calculationsElectronic supplementary information (ESI) available: Structural parameters, frequences and force constants of 4-fluorostyrene (4 tables); observed and calculated IR spectra and INS spectra (6 figures). See http://www.rsc.org/suppdata/cp/b2/b212620f/

This paper reports for the first time a molecular force field of the 4-fluorostyrene molecule making a joint use of data from optical spectroscopies (IR/Raman), neutron scattering (INS) and theoretical calculations, ab initio n(HF, MP2) and DFT with different basis sets. Harmonic molecular force fields were calculated following two different procedures, scaling and refinement. Also, proposed is a complete set of valence force constants obtained via the so-called pure vibrational force field. In a parallel way, some structural features such as the vinyl torsion barriers, the CC vinyl bond, and Ph-vinyl CC bond, among others, were analysed. A comparison with available experimental data for some standard and related molecules allows conclusions about the aromaticity of these systems.

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.

Theoretical and experimental study of a novel psolaren derivate: (E)-9-(3,4-dimethylpent-2-enyloxy)-7H-furo[3,2-g]chromen-7-one

A new psolaren derivate, (E)-9-(3,4-dimethylpent-2-enyloxy)-7H-furo[3,2-g]chromen-7-one, has been isolated and characterized by experimental and theoretical methodologies. The solid state molecular structure has been determined by X-ray diffraction methods. The substance crystallizes in the monoclinic P21/c space group with a=4.2389(5), b=26.090(3), c=12.482(1)Å, β=96.990(9)°, and Z=4 molecules per unit cell. The crystal structure shows the molecule fused phenyl and hetero-cycle rings to be coplanar with each other. Ab initio(MP2) and DFT methods have been used to predict the molecular structure in the isolated molecule approximation and the results compared with the experimental data. The MP2/6-311G(d,p) calculations are in good agreement with the X-ray results. The calculated HOMO-LUMO energy gap shows that the intra-molecular charge transfer could easily occur, a prediction closely related to the observed bioactivity of this new compound. In addition, the infrared absorption and Raman dispersion spectra were recorded and an assignment of the observed spectral features to molecular vibrations was made. The vibrational study was assisted by quantum chemistry calculations at the MP2 and DFT level, which provided theoretical mode frequencies. The study was completed by natural bond orbital (NBO) analysis.

Force Field for the S-Triazine Molecule by using the Inelastic Neutron Scattering Spectrum and Quantum Mechanical Calculations

The vibrational spectrum of the s-triazine molecule (1, 3, 5-triazine) has been widely studied both experimental and theoretically. This system has D3h symmetry with 21 normal modes distributed as: 3A1′+2A2′+5E′+2A2″+2E″. All the normal modes are actives in IR and/or Raman techniques except those of A2′ character, i.e. v4 and v5due to the selection rules. Both normal modes have not been observed up to date and therefore all force fields carried out for this molecule have not included data for these ones in their refinement.