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On the possibilities of π-ylides
Canadian Journal of Chemistry
Abstract
A systematic survey of substituted annulenes, using zeroth order Hückel theory, has uncovered a subclass of such structures which are predicted to have very substantial charge separation. An examination of selected structures by MNDO semi-empirical molecular orbital computations provided support for the Hückel descriptions. Such compounds may be of immediate interest to those pursuing synthetic or mechanistic problems which involve cycloaddition reactions. Keywords: π-ylides.
Perturbation theory is applied to fully π-bonded non-alternant monocycles at the Hückel level. An effective "back-of-the-envelope" approach is developed that permits one to anticipate Hückel charge patterns for these monocycles. Very little modification is required to alter the basic approach so that it will permit rapid anticipation of AM1 -calculated charge patterns for such compounds. Keywords: hydrocarbon polarity, perturbation theory.
Heats of formation, molecular geometries, ionization potentials, and dipole moments are calculated by the MNDO method for a large number of molecules. The MNDO results are compared with the corresponding MINDO/3 results on a statistical basis. For the properties investigated, the mean absolute errors in MNDO are uniformly smaller than those in MINDO/3 by a factor of about 2. Major improvements of MNDO over MINDO/3 are found for heats of formation of unsaturated systems and molecules with NN bonds, for bond angles, for higher ionization potentials, and for dipole moments of compounds with heteroatoms.
The basic approximations of the MNDO (modified neglect of diatomic overlap) method are described including a semiempirical model for the two-center repulsion integrals. Parametric functions for the various terms in the MNDO Fock matrix are then chosen which contain atomic parameters only (no bond parameters). Using a nonlinear least-squares iterative optimization technique, numerical values of the parameters are determined for the elements H, C, N, O. Finally, the main differences between the MNDO and MINDO methods are discussed.
The pattern of charge densities in a molecule is determined, at least in part, by the connectivity or the topology of the molecule. For the class of planar alternant hydrocarbons it is well-known that the calculated π-electron charge densities are all equal. But in nonalternant systems or in alternant systems for which the number of π electrons is not equal to the number of atomic orbitals that make up the system, the charge densities are not uniform. Many examples suggest that nature prefers to place atoms of greater electronegativity in those positions where the topology of the structure and the electron-filling level tend to pile up extra charge in the isoelectronic hydrocarbon. Since such heteroatomic systems are preferentially stabilized by molecular topology, the effect can be called the rule of topological charge stabilization. That such a rule indeed operates can be seen by comparing trends in calculated and empirical resonance energies, experimental heats of formation, and known relative molecular stabilities and reactivities to the patterns of charge densities calculated for the isoelectronic hydrocarbons. The topological charge stabilization rule has great potential value as a guide to synthetic efforts and as a quick way to rank the stabilities of positional isomers. It is easy to apply. Although more limited in its applicability than are energy quantities, the rule is often more direct because while energy is the property of a molecule as a whole, charge density is a property of an atom in a molecule. From a pattern of charge densities one can see immediately what is favorable or destabilizing about a particular arrangement of atoms. The simplicity and utility of this topological charge stabilization rule have gone largely unappreciated, although the idea was noticed at least as early as 1950 by Longuet-Higgins, Rector, and Platt.
The Dewar-Dougherty approach to aromaticity estimates, based on approximate perturbation molecular orbital (PMO) estimated Dewar resonance energies, is shown to be unreliable. Difficulties are particularly serious for cyclic molecules containing 4n π electrons. A proposed modification of the Dewar-Dougherty method is shown to furnish more reasonable estimates of Dewar resonance energies.