No abstract

The self-consistent field molecular-orbital method, including adjustment of coulomb and resonance integrals using spectroscopic data, is applied to conjugated heterocyclic systems with the further elaboration that the effective nuclear charge for each 2pn-orbital is varied according to the electron density around the corresponding nucleus. Since the effective nuclear charge is a measure of the electronegativity of the 2pnorbita1, the method has been termed the " self-consistent electronegativity " (s.c.e.) method. The formaldehyde molecule has been studied to illustrate the new method.Calculations of spectroscopic intervals, already successfully achieved for heterocycles using fixed nuclcar cliarges, are little altered, but the calculated r-electron distribution is much more uniform than has been found in previous calculations on heterocycles.The dipole moment of formaldehyde can be accounted for entirely in terms of hybridization moments plus a small n-electron moment; no polarization of the C-0 a-bond need be invoked. The assumption of an unpolarized a-core, made in the present calculations, thus seems more reasonable than is widely believed. It is emphasized that an appreciablc portion of the molecular dipole in some hydrogen compounds may arise from hybridization of the hydrogen valence orbital.The oxygen lone-pair ionization potential cannot be explained merely in terms of the charge gained by the oxygen through carbonyl bond polarization. The present results suggest the values a 0 = a + O*lp and pco = 1.13p for the simple 111.0. method when applied to carbonyl compounds.

No abstract

Theoretical considerations indicate that the electron distribution in 3,4,-dimethylenecyclobutene is non-uniform and a dipole moment of 0.71 D has been predicted by inclusion of non-neighbour core resonance integrals. Preliminary studies of the microwave spectrum of this compound provide a dipole moment of 0.618 � 0.011 D. The term "pseudo-alternant" is proposed to describe this phenomenon. In the present paper additional physical and chemical properties of 3,4,-dimethylene- cyclobutene are described. The observed low-field proton chemical shifts for 3,4-dimethylenecyclobutene are not accounted for using a ring current model and a small calculated paramagnetic current indicates properties usually associated with H�ckel 4n-hydrocarbons. A local anisotropy model provides a better account of the observed shift. The carbon-13- proton satellite spectrum of 3,4-dimethylenecyclobutene indicates similarities in the geometry of the four-membered ring to that in cyclobutene. In addition, infrared and ultraviolet spectra support the conclusion that this hydrocarbon be regarded as a cross-conjugated diene. A detailed discussion of the products arising from the bromination of 3,4-dimethylenecyclobutene under a variety of conditions indicates consecutive 1,4-addition of bromine across the four-membered ring, a property which has been associated with cross-conjugation. Hydrogenolysis, chlorination, iodination, and attempted Diels-Alder addition reactions are also described. 3,4-Dimethylenecyclobutene was prepared by the thermal rearrangement of hexa-1,5-diyne. The mechanisms of thermal and photolytic rearrangement of hexa-1,5-diyne are discussed using Woodward-Hoffmann postulates. 1,2-Dideutero-,1-ethyl-, and 1-n-propyl-3,4-dimethylenecyclobutene were also prepared by the thermal rearrangement of the corresponding diyne. In addition, the thermal rearrangement of hexa-1,5-diyne over the temperature range 290-700� to give variable quantities of 3,4-dimethylenecyclobutene, fulvene, and benzene is described. 3,4- Dimethylenecyclobutene is thermally rearranged to benzene at 680�.

The proton magnetic resonance spectra of benzofuran, dibenzofuran, indole, 7-azaindole, carbazole, and 4-azacarbazole (α-carboline) have been investigated at 100 Mc/s, and the spectral parameters for these molecules have been obtained. The presence of cross-ring coupling constants involving the NH proton of indole and carbazole is reported.