Amatzya Y. Meyer

Paratropicity and antiaromaticity: role of the homo-lumo energy gap

Abstract Doubly charged systems derived from fused benzenoid polycycles reveal an unquenched delocalization of 4 n π-electrons and hence are predicted to possess antiaromatic character. The magnitude of the paratropic 1 H NMR chemical shifts, due solely to the paramagnetic secondary field sustained in these species, was found to depend linearly upon the magnitude of LUMO-HOMO energy gaps of the corresponding systems. The existence of such a correlation enables a comprehensive treatment of the various factors which determine the antiaromatic character and the subtle interrelations between those factors. This, in turn, leads to a deeper understanding of antiaromaticity.

Molecular mechanics of saturated organic halides: Part I. Monochlorides and non-geminal dichlorides

Abstract A molecular-mechanical hydrocarbon force field is extended to apply to saturated organic chlorides, including non-geminal dichlorides. Simultaneous computation of molecular geometries, conformer energies, barriers to internal rotation, and dipole moments, is provided for. Results are reported and discussed for 17 molecules, acyclic and cyclic, including crowded structures.

Metalations—XVI : Polylithiation at benzylic positions

Abstract Metalation of mesitylene with butyllithium in hexane in the presence of tetramethylethylenediamine yields mono-, di- and trilithiated benzylic compounds ( 1–5 ), characterized as their silylated derivatives. Dilithiation occurred partly at the same but preferentially at different methyls. The symmetrical tris-1,3,5-(lithiomethylene)benzene was the main product. Similar metalation occurred with m -xylene. o -Xylene gave also dilithiated products, but at a slower rate, and p -xylene gave slowly and exclusively the gem -dilithio-derivative. The results were explained by greater stabilization of polylithio derivatives with charges all on the starred carbons of the conjugated system, rather than those with charges uniformly dispersed on all carbons. CNDO/2 calculations of the energy of the anions account only partially for the order of stability of most of the studied compounds, but a good correlation between this experimental order and the energies of the lithium compounds was found.

Metalation reactions—XVII : The dimetalation of branched olefins. The question of the aromaticity of the trimethylenemethane dianion and related derivatives☆

Abstract Metalation of methallylbenzenes with BuLi gave dilithio derivatives which were dialkylated with methyl bromide. Derivatives of cross conjugated dianions are obtained preferentially to linear ones. The parent trimethylenemethane dianion 2 was obtained by metalation of isobutene. Energies and atomic net charges of these and related molecules were calcualted. The question of the aromaticity of these systems is discussed.

Molecular mechanics of saturated organic halides: Part II. Fluorides

Abstract A molecular-mechanical hydrocarbon force field is extended to apply to saturated organic fluorides, including non-geminal and geminal polyfluorides. Electrostatic interactions are evaluated internally, by a formulation based on the dipole approximation. Simultaneous calculation of molecular geometries, conformational energies, barriers to internal rotation, and dipole moments, is provided for. Results are reported for 16 molecules, including crowded structures.

QUANTIFYING THE DEGREE OF MOLECULAR SHAPE DISTORTION. A CHIRALITY MEASURE

Abstract We suggest that the “black or white” attitude towards chirality (either chiral or non-chiral) overlooks fine details of the shape of molecules. We argue that a measure is in order to quantitate the intuitive feeling that, say, 2-deuteriobutane is “less chiral” than 2-iodobutane. Such a measure, X, which is based on relative molecular shape distortions, is proposed and its properties demonstrated for a series of chiral monohalogenated n-alkanes. For instance, the X values for 2-F-, 2-Cl-, 2-Br-, and 2-I-butane (anti conformation) are 0.17, 6.67, 15.46 and 29.76, respectively, emphasizing that inducing chirality in butane by substitution with F causes only a minor shape distortion, while distortion due to iodine is considerable. The gradual change in chirality of rotamers is manifested by a continuous gradual change in the X value of the rotamers of 2-halogenobutanes. The application of the χ scale to two-dimensional chirality is demonstrated for monosubstituted naphthalenes. An outlook for applications of the shape-distortive scale to issues other than chirality is given.

1H NMR patterns of antiaromatic systems: Paratropic displacement dependence upon LUMO-HOMO energy gap

Abstract 1 H NMR chemical shifts of antiaromatic species reveal an enhanced paratropic displacement. The high-field shifts exhibited by doubly charged benzenoid polycycles were shown to be strongly related to the LUMO-HOMO energy gaps in these antiaromatic systems. As the gap decreases, a larger paratropic shift was observed.

Planar and non-planar unsaturation—II : Twisting-distortions in fulvalenic structures

Abstract A π-electronic method, adapted to the study of non-planar unsaturated molecules, is used to examine the consequences of twisting about the exocyclic bond in derivatives of fulvene and fulvalene. The geometry, electronic spectrum, ionization and reduction potentials of fulvene, 6,6-disubstituted fulvenes, fulvalene, octachlorofulvalene, tetrabenzofulvalene and (9-fluorenylidene)cyclopentadiene are considered. The last-mentioned molecule and its dimer have been prepared and their properties are reported. Incidental comments on the applicability of the Huckel rule (“4n + 2”) to annelated odd-membered rings and on charge-distribution in fused non-alternants are also made.

Molecular mechanics and molecular shape: Part II. Beyond the van der Waals descriptors of shape☆

Abstract Propositions are delineated that lead from the van der Waals descriptors of shape (volume of the molecule and surface area) to a characterisation of the room that molecules occupy in the bulk. A way to partition the volume and surface area into segments is also described. Numerical results for a large number of alkanes, cycloalkanes, and chloro- and bromo-derivatives thereof, are discussed in terms of the heat of vaporisation, solubility, connectivity function, and the “conformational rule”.

Molecular mechanics and molecular shape: Part VII. Structural factors in the estimation of solvation energies

Abstract Pathways are proposed to facilitate the estimation of parameters that enter the computation of solvation energies: the cavity radius and the set of bond moments. The radius can be estimated in ways similar to the estimation of molecular van der Waals radii, except that the atomic radii are different. Bond moments can be obtained by Del Res procedure of charge distribution, since it works through the intermediacy of bond charges. If the procedure is modified to include “pseudobonds”, bond moments can be estimated for molecules in an aggregate. These modes of estimation were tried in the context of Abrahams solvation energies. Three test-cases, which could not be tackled by previous versions of the method, are described.