H. P. Figeys

A CDOE/INDO LMO study of the nuclear spin-spin coupling constants between directly bonded C-H and C-C atoms

Müller-Pritchard (MP) type relations are used to study1JCH and1JCC coupling constants in a series of unsubstituted as well as in heterosubstituted hydrocarbons. In the case of the coupling constant between two atoms connected by a multiple bond, a generalised MP-type relation is derived.

Ab initio and LMO studies of integrated intensities of infrared absorption bands of polyatomic molecules. Part 1.—Method, application to cyanoacetylene and comparison with semi-empirical “all-valence” theories

A general scheme is described for a non-empirical quantum chemical interpretation of the integrated intensities of infrared absorption bands. The localized molecular orbital (LMO) technique is used to calculate bond moment derivatives with respect to the normal coordinates. A second general decomposition scheme of (ab initio) calculated molecular dipole moments, into point charge, hybridization and homopolar terms, is also presented.The theory is applied to the stretching modes of cyanoacetylene. The calculations are performed with three fundamentally different types of SCF-wavefunctions (INDO, INDO/D and STO-NG). The canonical molecular orbitals are localized with an INDO approximate charge density localization technique. Two different force fields were used in the calculation of the normal coordinates.As opposed to the STO-NG functions, neither INDO, nor INDO/D functions yield satisfactory results for the calculated intensities. The influence of the force field on the calculated intensity seems to be of minor importance compared with the influence of the wavefunction.Quantum chemical evidence is presented for the failure of the classical bond moment hypothesis for stretching modes. The dipole moment decomposition scheme provides sound evidence for the importance of the hybridization and homopolar terms in ab initio calculated intensities. LMO-analysis of the (∂µ/∂Qi)0 values led to an understanding of the combined effect of vibrational and electronic factors on the integrated intensity.

Ab initio and LMO studies of integrated intensities of infrared absorption bands of polyatomic molecules. Part 2.—Chloroacetonitriles

Minimal basis set ab initio(STO-6G) calculations are performed on the equilibrium dipole moments and dipole moment derivatives of the chloroacetonitriles CH3–nClnCN (n= 0, 1, 2, 3) in order to explain the experimentally observed behaviour of the CN and CC stretching intensities in this series. New force fields are presented for acetonitrile and dichloroacetonitrile. A normal coordinate analysis is perfomed for the corresponding modes. The experimental minimum in the CN intensity for the monochloro-derivative is not found theoretically. Equilibrium dipole moment decomposition schemes together with bond moment analyses of the various dipole moment derivatives permit rejection of the existing literature hypotheses and explanations on the CN intensity evolution in the chloroacetonitriles. The essential factor which governs the computed CN intensity evolution was seen to be vibrationally and electronically localized in the CN bond. A careful examination of the displacements of the nitrogen–atom nucleus and the centroid of negative charges in the nitrogen lone pair LMO led to an extension of the concept of incomplete orbital following, which is well known in bending vibrations, to the case of stretching modes. The different behaviour of the CN bond during the CN stretching vibration in CH3CN and CCl3CN was seen via this technique, to be due to a reversal of sign of the µCN bond moment variation during molecular distortions along the CN normal coordinate.

A gas electron diffraction and ab initio quantum-mechanical investigation into the molecular structure of tricyclo-(3.1.002,4)-hexane

Abstract The structure of tricyclo-(3.1.00 2,4 )exane has been determined by gas phase electron diffraction. The molecule has an inversion centre. The mean carbon—carbon bond length, averaged over both three- and four-membered rings is 1.508 A. A model with equal C-C bond lengths fits to the measured diffraction intensities. The four-membered ring is planar with valency angles of 90°, while the carbon atoms of the three-membered rings form isosceles triangles. Ab initio quantum mechanical calculations at the STO-3G level support this geometry. The valency angle CCC (between three- and four-membered rings) for the equilateral four-membered ring model has been found experimentally to be 109.9°. The average C-H bond distance (1.080 A) is small as a result of increased s -character in these bonds in agreement with reported INDO—LMO calculations.

Approximate charge density localization of molecular orbitals

A NDO approximate procedure based on the indirect intrinsic ab initio localization method of von Niessen is developed. It is shown that only when the NDO approximations are introduced at the two electron level, expressions are obtained which are the charge density counterpart of those found in the approximate energy localization methods. The results of these two methods are quite similar both in the CNDO and INDO approximations. The indeterminacies observed in the CNDO localization for unsaturated systems and for molecules with two or three lone pairs on the same atom, are removed by localizing up to an INDO level. The approximate charge density localization is however computationally much easier than the approximate energy localization method and should be more appropriate in LMO studies of large organic molecules.

On the use of NDO approximate wavefunctions in the evaluation of momentum density and radial momentum density distributions in polyatomic molecules

The use of single determinantal approximate molecular wavefunctions of the LCAO MO NDO type for the calculation of the momentum densityρ(p) and the radial momentum density distributionJ(p) is discussed. In each case, these expressions should be orientationally invariant and the momentum density should be normalized. Combining these two requirements, it is shown that only two approximations are physically significant:(1)NDO wavefunctions are used andρ(p) andI(p) are approximated respectively up to an INDO and a CNDO level;(2)Overlap integrals are explicitly taken into account when solving the Roothaan SCF equations or deorthogonalized NDO functions are employed, together with the unapproximated expressions.

Iterative variation of charge dependent atomic orbital exponents in approximate molecular all-valence electron linear combination of atomic orbitals (L.C.A.O.) methods

A new method for optimizing atomic orbital (a.o.) exponents in approximate all valence methods is proposed. The exponents are varied charge dependently in an iterative manner until self-consistency is reached. A new set of rules is set up to calculate screening constants depending on the electronic densities in the different a.o.s. As a consequence of the iteration procedure, net atomic charges are smoothed out and the calculated molecular dipole moments are in fair agreement with experimental data.

Orientational invariance and significance of electron densities obtained with approximate all-valence electron wavefunctions

The total electron density ρ(R), computed with approximate all-valence electron wavefunctions, at a point defined by the position vector R, should (i) obey the orientational invariance requirement, i.e., be independent of the orientation of the molecule in a general coordinate system provided the position vector R is transformed according to the same “reorientation” matrix as the molecular coordinates, and (ii) satisfy the normalization condition, i.e., ∫ρ(R) dR=N, where N is the total number of valence electrons. It is shown that with these two restrictions in mind, only two approximations are physically significant: (i) the use of NDO-type wave functions (CNDO/2, INDO, etc.) combined with the evaluation of ρ(R) up to an INDO-level; (ii) the use of wavefunctions obtained either by solving the complete Roothaan equations, i.e., where overlap integrals are explicitly taken into account, or by deorthogonalizing NDO-functions, together with the unapproximated expression for ρ(R). A comparison of the density maps for formaldehyde obtained by these two methods, with ab initio plots obtained with wavefunctions giving an energy close to the Hartree–Fock limit, leads to the conclusion that the second possibility is the more significant.