J.J. López González

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.

Theoretical and experimental vibrational spectrum study of 4-hydroxybenzoic acid as monomer and dimer.

Theoretical calculations on the molecular geometry and the vibrational spectrum of 4-hydroxybenzoic acid were carried out by the Density Functional Theory (DFT/B3LYP) method. In addition, IR and Raman spectra of the 4-hydroxybenzoic acid in solid phase were newly recorded using them in conjunction the experimental and theoretical data (including SQM calculations), a vibrational analysis of this molecular specie was accomplished and a reassignment of the normal modes corresponding to some spectral bands was proposed. The geometries of monomers and dimers in gas phase were optimized using the DFT B3LYP method with the 6-31G*, D95** and 6-311++G** basis sets. Also, both the vibrational spectra recorded and the results of the theoretical calculations show the presence of one stable conformer for the 4-hydroxybenzoic acid cyclic dimer. The B3LYP/6-31G* method was used to study the structure for cyclic dimer of 4-hydroxybenzoic acid and for a complete assignment our results were compared with results of the cyclic dimer of benzoic acid. A scaled quantum mechanical analysis was carried out to yield the best set of harmonic force constants. The formation of the hydrogen bond was investigated in terms of the charge density by the AIM program and by the NBO calculations.

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 analysis of the inelastic neutron scattering spectrum of s-triazine and trichloro-s-triazine

Abstract The inelastics neutron scattering (INS) spectra of s-triazine and trichloro-s-triazine at 5 K provide the first observation of the IR and Raman inactive modes, v 4 and v 5 . A full assingment is made by fitting a harmonic force field to the INS spectra profile. In the case of trichloro-s-triazine we demonstrate that this approach can be used successfully for a molecule without H atoms.

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.

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.

Self‐Assembly Structures of 1H‐Indazoles in the Solution and Solid Phases: A Vibrational (IR, FIR, Raman, and VCD) Spectroscopy and Computational Study

1H-indazoles are good candidates for studying the phenomena of molecular association and spontaneous resolution of chiral compounds. Thus, because the 1H-indazoles can crystallize as dimers, trimers, or catemers, depending on their structure and the phase that they are in, the difficulty in the experimental analysis of the structure of the family of 1H-indazoles becomes clear. This difficulty leads us to contemplate several questions: How can we determine the presence of different structures of a given molecular species if they change according to the phase? Could these different structures be present in the same phase simultaneously? How can they be determined? To shed light on these questions, we outline a very complete strategy by using various vibrational spectroscopic techniques that are sensitive (VCD) and insensitive (IR, FIR, and Raman) towards the chirality, together with quantum chemical calculations.

Condensation reactions in silanol–water clusters

Abstract Two types of complexes were found in the 2H 3 SiOH+ n H 2 O ( n =1,2) systems by B3LYP and MP2 methods. Global energy minima correspond to cyclic structures and less stable open isomers, in which one hydrogen bond is eliminated, are higher in energy by 3–7 kcal/mol. The decrease in the stability of open structures due to the rupture of one hydrogen bond is partly compensated by a weak silicon–oxygen intermolecular interaction. This interaction favors condensation and results in a substantially lower barrier height for the reaction path emerging from the open complex. The catalytic effect of water reveals in the substantial lowering of the barriers heights for both types of transition states in going from n =0 to n =2.

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.