Yves G. Smeyers

An ab initio structural and spectroscopic study of acetone—An analysis of the far infrared torsional spectra of acetone‐h6 and ‐d6

The far infrared torsional spectra of acetone (CH3)2CO and (CD3)2CO have been determined from ab initio calculations, and the main features of the experimental data assigned. For this purpose, the potential energy surface for the double methyl rotation was determined with fully relaxed geometry into the RHF and RHF+MP2 approximations using a 6–31G(p,d) basis set. The energy values, as well as the kinetic parameters obtained from the optimized geometry, were fitted to double Fourier expansions as functions of the rotational angles in seven terms. The torsional solutions were developed on the basis of the symmetry eigenvectors of the G36 nonrigid group, which factorize the Hamiltonian matrix into 16 boxes. The energy levels and torsional wave functions for each symmetry specie were then obtained diagonalizing each blocks separately. Intensities were obtained from the calculated electric dipole moment variations and the nuclear statistical weights, and were combined with the torsional frequencies to predict ...

Introduction to Group Theory for Non-Rigid Molecules

Publisher Summary This chapter provides a general description of Group Theory for Non-rigid Molecules, and reviews the Longuet–Higgins and Altmanns points of view. A new theory, the Non-Rigid Molecule Group (NRG), is proposed. In contrast to the Longuet–Higgins Theory, which resorts to permutations and permutations inversions, this new theory retains the Schijnfliess formalism of physical operations, just as Altmann does, to conserve an easy physical interpretation. The concepts of full and restricted NRGs are defined. The full NRGs, which consider the overall rotations, are seen to be entirely equivalent to the molecular symmetry groups of Longuet–Higgins, whereas the restricted NRGs, limited to the interconversion motions, may be compared with the Isodynamic groups of Altmann, and the isometric groups of Giinthard.To introduce the NRG formalism, the necessary mathematical machinery is introduced in a very simple and gradual way considering a lot of examples. Finally, the concept of local group, which cannot be introduced easily into the Longuet–Higgins formalism, is proposed for simplifying still more the very involved non-rigid problems and classifying in more detail their solutions. The main applications of the rNRGs are reviewed. So, for example, symmetry adapted minimal expansions are proposed for determining potential energy functions. Symmetry eigenvectors, which factorize the Hamiltonian matrix in boxes are given to construct the torsional solutions of the Schrodinger equation for the nuclear motion. Internal dynamics of non-rigid systems is studied in the case of acetone, and conformational probability density map are drawn by using symmetry eigenvectors. Selection rules for torsional transitions in the far-infrared region, as well as the most relevant features of the FIR acetone spectrum were determined theoretically. Finally, torsional band structures in the triplet-singlet spectra of thioacetaldehyde and thioacetone are determined, classiflying the torsional states of both electronic states into a same NRG.

Ab initio determination of the far infrared spectra of some isotopic varieties of ethanol

The far infrared (FIR) spectra of various isotopic species of ethanol (-h6, -d1, and -d3) are analyzed from MP4((SDQT) ab initio calculations using models in one and two dimensions. From the calculated frequencies and intensities, previous assignments of ethanol-h6 and -d1 bands are reviewed. The position of several combination bands are predicted. Ethanol shows two conformers, trans and gauche, and two interacting torsional modes. The torsional barriers have been calculated to be V3(trans)=1226.7 cm−1, V3(gauche)=1296.3 cm−1, VOH(α=62°)=404.1 cm−1, and VOH(α=180°)=423.3 cm−1. The flexible models in one and two dimensions yields the same OH torsional frequencies, whereas they differ in the methyl group state calculations. The fundamental bands of the ethanol-h6 have been evaluated at 205.5 cm−1 (OH torsion) and 257.0 cm−1 (CH3 torsion) and their corresponding intensities to be 18.650×10−4 and 0.662×10−4.

Quantum mechanical and QSAR study of some α-arylpropionic acids as anti-inflammatory agents

Abstract The optimal structures of a series of α-arylpropionic acids with anti-inflammatory activity are established by using the semi-empirical quantum mechanical procedures, AMI and AMSOL, in the gas phase and in water solution, respectively. In these calculations, the arylpropionic acids are considered in their neutral and ionized forms. As expected, these compounds exhibit two preferred conformations in which the α-hydrogen atoms of the propionic acid group lies approximately in the plane of the central aryl ring. The deprotonation energies are then determined as the difference between the formation energies of the protonated and deprotonated forms. A Quantitative Structure Activity Relationship (QSAR) study reveals that only the gas phase results compare to some extent favorably with the anti-inflammatory activity. As expected, the smaller the deprotonation energy, the larger the anti-inflammatory activity. Satisfactory relationships between the in vivo activities and deprotonation energies, the HOMO energies and lipophilicities were found.

Relationships between the activity of some H2-receptor agonists of histamine and their ab initio molecular electrostatic potential (MEP) and electron density comparison coefficients

Abstract From an analysis of the ab initio molecular electrostatic potential (MEP) maps of some H 2 -receptor agonists of histamine in their essential trans—trans conformations, for both neutral and cationic species, a good relationship between H 2 -activity data and the MEP minima located at the N π nitrogen atom of the imidazole ring is predicted. From these data it appears easy to define a threshold value according to which the H 2 -agonists may be classified as being strongly or weakly active. While the MEP values appear to be a good parameter for activity prediction, in this case, the comparison of electron densities does not give any additional useful information.

Potential energy surface determination for non rigid molecules. Two-rotor molecules

Abstract The potential energy surface determination problem is considered in the case of two rotor molecules from two points of view: 1) The calculation of the energy of each molecular configuration; 2) The application of Group Theory for non-rigid molecules in order to reduce to the strict minimum the configurations to be calculated. From the Group Theory symmetry adapted functional forms are deduced for the potential energy of a set of molecules of different internal symmetry: p-xylene, acetone, dimethylamine, cis-N-methylethylidenimine and trans-1, 2-dimethylcyclopropane. In the same way, minimal expansions, useful for describing the main features of the potential energy surfaces are deduced. From the minimal expansion, the different surfaces are determined num e rically by calculating the energy of a very limited number of nuclear conform a tions. For these last calculations, - barrier height calculations - the CNDO/2 method is found to be the more suitable one, even when compared with the ab initio methods.

An ab initio determination of the bending-torsion-torsion spectrum of dimethyl ether, CH3OCH3 and CD3OCD3

We have calculated the potential energy hypersurface of dimethyl ether with respect to the COC bending coordinate α and the torsional angles of the two methyl groups, θ1 and θ2. Two sets of ab initio calculations were carried out. The first was made at the level MP2/6‐31G(d,p) in which the structural coordinates were fully relaxed except for the grid points on the hypersurface. More extensive calculation were carried out with MP4 corrections for electron correlation with the same molecular structure. The torsional bending Hamiltonian matrix was symmetrized by the operations of the G36 nonrigid group and was solved variationally. The effect of explicitly considering the bending mode in the three‐dimensional treatment was determined by a comparison to the two‐dimensional model in which the flexibility of the frame was absorbed into the calculation by the fully relaxed method. It was found that the three‐dimensional calculation gave a much better account of the sin(3θ1)sin(θ2) intermode coupling than the two...

Ab initio theoretical study of the methyl and phosphine torsion modes in ethylphosphine

In this paper, the far infrared (FIR) methyl and phosphine torsional frequencies and intensities are determined theoretically in ethylphosphine from ab initio calculations. For this purpose, the potential energy function for the double rotation of the methyl and phosphine groups in the electronic ground state is determined in a standard calculation by using the MP2/RHF and a6-31G(3df,p) basis set, with full optimization of the geometry. The numerical results are fitted to a symmetry adapted analytical form and introduced together with the kinetic parameters into the Hamiltonian operator. The Schrodinger equation for these two motions is solved by developing the solutions on the basis of products of trigonometric functions. From the energy levels, the torsional functions and the dipole moment variations the FIR spectrum is synthesized. A new assignment is proposed for some trans-trans and the gauche-gauche transitions between the phosphine levels. Additional transitions between the methyl levels are also p...

Theoretical study of some H2-receptor agonists of histamine. The H2-essential conformation of histamine

Abstract The conformational energy maps and charge distributions of histamine and six H2-receptor agonists of histamine were determined in the CNDO/2 approach. When several tautomeric forms were possible only the NτH was considered. A judicious analysis of the conformational results, as a function of the H2-receptor activity, allows us to propose, as “essential” for the H2-receptor agonists, an open conformation, trans—trans, for the side chain. The analysis of the relationship between the mean charges on the aromatic ring and the activities supports this conclusion.

Dynamical and spectroscopic studies of nonrigid molecules. Application to the visible spectrum of thioacetaldehyde

The methyl torsion and aldehydic hydrogen wagging modes are studied theoretically in thioacetaldehyde, in both ground and first triplet excited states. For this purpose, the potential energy surfaces were determined by ab initio restricted Hartree Fock and unrestricted Hartree Fock calculations with 4–31 basis set +d orbitals on the sulphur atom. The two electronic states were found to have different preferred conformations. The singlet state exhibits a planar eclipsed conformation, whereas the structure of the triplet state is anti‐eclipsed and pyramidal. It was found that a potential function, which was constructed from a symmetry adapted double Fourier expansion of the wagging and torsional coordinates, gave a reasonable fit to the energy points. The two‐dimensional Schrodinger equations for the torsion and wagging motions were solved for both singlet and triplet states, taking into account the internal symmetry and the appropriate basis. The relative band locations and the intensities (Franck–Condon f...