McWeeny's modification of London's molecular orbital theory of π-electron ring currents has been used to calculate the individual currents in the 15 pentacyclic hydrocarbons. The results, useful in themselves for the calculation of NMR chemical shifts, are also significant in the light of what they shed on the question of the range of validity of the assumption that the current in each ring is approximately equal to the benzene ring current.

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Articles you may be interested inHigh-resolution electron spectroscopy, preferential metal-binding sites, and thermochemistry of lithium complexes of polycyclic aromatic hydrocarbonsThe McWeeny theory has been used to calculate the shielding parameters for polycyclic hydrocarbons up through four rings. These results are presented along with experimental values, some of which come from new analyses of lOO-Mc/sec NMR spectra of these compounds, and semiquantitative agreement is observed. The Jonathan-Gordon-Dailey semiempirical equation for spin-spin coupling constants is also subjected to further test by new data. Absence of inter-ring spin-spin coupling is reconfirmed.

synopsisCrosslinked epoxy resins, tetraglycidyl 4,4diamino diphenyl methane cured with 4,ediamino diphenyl sulfone, were soaked in water at either 25°C or 70°C for varying lengths of time. The infrared spectra and DSC thermograms were obtained for samples that were soaked, or soaked and dried. There was a monotonic decrease in exothermic reaction energy with water content. The glass transition was also lowered, although samples soaked at 70°C showed a leveling in the T,around 115°C. When the soaked samples were dried, the exothermic reaction energy showed near reversibility for samples soaked at 25°C while the 70°C samples were highly irreversible. IR of the latter samples showed that the 70°C water soaking resulted in reaction of some of the unreacted epoxide groups that remained after the initial cure.

The McWeeny theory has been used to calculate chemical shielding parameters for all even, alternant, pentacyclic hydrocarbons, and comparison with experiment was made where data were available. The spectra of two pentacyclic molecules, 1,2,3,4-dibenzanthracene and picene, not reported previously, were obtained and analyzed, and a complete analysis of the 1,2-benzpyrene spectrum is given. Agreement between theory and experiment is satisfactory for protons not subject to large van der Waals contact, and it is found that steric effects contribute an extra downfield shift of 0.54 ppm to angular phenanthrenic type protons. Additions and new interpretations are made for previously reported results of similar data on three- and four-ring molecules.