Jorge A. Medrano

On the quantum theory of valence and bonding from the ab intio standpoint

Abstract Ab initio calculations of valence, degree of bonding and anisotropy were performed according to the definitions by Armstrong et al. Some numerical results are given for selected molecules.

Reply to “comments on the quantum theory of valence and bonding: Choosing between alternative definitions”

Abstract In this Reply, to the preceding, Comment of Mayer, we indicate that there is no reason to prefer Mayers definitions to ours (or conversely) when in the usual range of validity.

On the theory of valence and bonding: A further generalization

Abstract Ab initio and semiempirical definitions of atomic valence, anisotropy, and degree of bonding between pairs of atoms have previously been applied to LCAO-MO RHF, UHF or GVB wavefunctions. It is shown that the same definitions can be applied to a CI wavefunction. Numerical examples are given as well as a comparison of the result of “opening shells” by means of CI or GVB.

A novel interpretation of bonding in [1.1.1]propellane and theoretical calculation of its unimolecular rearrangement

Abstract The quasi ab-initio PRDDO method, with and without the generalized valence bond (GVB)-type correlation of one pair of orbitals is used to examine some interesting features of the bonding in [1.1.1] propellane. It is found that the singlet, partial diradical configuration is the most stable one and that the central, interbridgehead bond is a weak one, i.e., it is characterized by a low degree of bonding. We thus integrate apparently contradictory evidence from former calculations. We also used the PRDDO approximation together with the synchronous transit method to study the rearrangement of [1.1.1]propellane to 3-methylene-cyclobutene, in theoretical detail.

Unexpected chirality in trans-1,4-dicyanocyclohexane

Abstract The experimental non-zero dipole moment of trans -1,4-dicyanocyclohexane was verified through a procedure little sensitive to solute/solvent interactions. This value cannot be explained on the basis of the generally accepted centrosymmetry for trans -1,4-derivatives of cyclohexane. NMR data of both this compound and its C-1, C-4 di-deuteroderivative, as well as semi-empirical theoretical calculations with the MNDO methods show that besides a centro-symmetric form there are also low energy stable non-centrosymmetric geometries. The latter exist as a helical deformation that can account for the non-zero value of the experimental dipole moment.

A PRDDO-GVB theoretical SCF-MO study of the torsion of ethylene in its ground state

Abstract A detailed study for different torsion angles of the HF-PRDDO and GVB-PRDDO wavefunctions of ethylene is made, including the correlation energy obtained through the GVB formalism, non-analiticities in certain HF-predicted curves, valency and degree of bonding, and the ability of the methods to describe the diradical character of the system. GVB corrects all the inadequacies of the HF description, allows a far better prediction of the torsion barrier and describes correctly the diradical character of ethylene.

The UHF extension of the quantum theory of valence and bonding

Abstract General definitions of the empirical notions of classical chemistry, such as valence and degree of bonding, have been given before. These definitions use the density matrix of the system and can be applied to any type of SCF-LCAO-MO wavefunction, whether ab initio or semiempirical. However, we find in this paper that the discussion that has been provided before for the open-shell unrestricted Hartree-Fock (UHF) case [M.A. Natiello, H.F. Reale and J.A. Medrano, J. Comput. Chem., 6 (1985) 108] is incomplete and faulty. We now give the correct derivation for this important case. We show how this statistical population analysis (SPA) relates to the problem of calculating mean occupation numbers of molecular orbitals, and we also give definitions for other magnitudes related to the charge associated with different regions within the molecule. Finally we report the result of application to some selected molecules.

A comparison of CNDO and eht-type methods when applied to the study of rotational isomerism in oxalyl halides: I. Oxalyl chloride

Abstract CNDO/2, EHT and IEHT were used to make a conformational investigation of oxalyl chloride, comprising the study of the barrier to internal rotation around the C-C bond, and dipole moments, charges, overlap populations and E core of CNDO/2 as functions of the internal rotation angle. The results of the three methods compared with each other and with experimental data, whenever possible, confirmed trends observed recently. EHT proved to be the best suited for the study of rotational isomerism in this molecule.

Theoretical Predictions of Nonlinear Optical Properties of Molecules and Polymers

The prediction of nonlinear optical properties of molecules and polymers is an important goal toward which a considerable effort has been recently applied. It entails the calculation of the polarizability and first and second hyperpolarizability tensor, for the systems under study, using their quantum wavefunctions. In principle several approaches are possible. The wavefunctions themselves can be calculated at the ab initio or semi-empirical levels of approximation and then the calculations can proceed via the sum over states (SOS) or the finite field (FF) formalisms. Since we intend to calculate large systems we decided on the semi-empirical MNDO/AMI approaches for the calculation of the wavefunction. We also used the FF approach and numerical methods for finding the necessar) derivatives. The comparison between calculated and available experimental results is very encouraging. We also calculated the tensors for considerably large molecules, providing the first theoretical prediction of these values. The effect of different substituents on a given structure and the result of doping, were also studied. The polarizabilities of some oligomers as functions of the number of repeat units were calculated to assess the convergence properties. Because of the required periodicity of the perturbations, calculations cannot be made for infinite polymers in the same fashion as for small molecules. This problem is currently being addressed.

A comparison of CNDO and EHT-type methods applied to the study of rotational isomerism in oxalyl halides: Part II. Oxalyl fluoride

Abstract In continuation of the work reported in the first paper of this series, CNDO/2, INDO, EHT and IEHT were used to make a conformational investigation of oxalyl fluoride. The barrier to internal rotation around the CC bond, dipole moments, atomic charges, overlap populations, and E core of CNDO/2 were calculated as functions of the internal rotation angle. The results of the four methods are compared with each other, with available experimental data and with conclusions obtained in the first paper. EHT again proved to be the method best suited to the study of rotational isomerism in this molecule. Failures of CNDO and INDO-type and of IEHT methods are discussed.