The Interaction of Nonbonding Orbitals in Carbonyls

Abstract

Zusammenfassung. In Dicarbonylen wie Glyoxal oder Benzochinon bilden die nichtbindenden Sauerstofforbitale delokalisierte Molekelorbitale mit verschiedenen Energien. Wir berichten iiber EHT-und CNDO/Z-Berechnungen einiger Dione und Trione. Die berechnete Aufspaltung zwischen den Nnichtbindenden 0 Kombinationen wird auf der Grundlage der u through-space 1) und H throughbond )) Wechselwirkung analysiert. Bei den tiefliegenden nichtbindenden Orbitalen der Carbonylgruppen scheint der zweite Faktor zu dominieren. Gerust-cr-Orbitale und nichtbindende Sauerstofforbitale sind stark miteinander gemischt.An isolated carbonyl group is characterized by two 'lone-pair' orbitals, described as equivalent in the valence structure 1. In any molecular orbital description of a carbonyl group, symmetry adapted combinations of these two equivalent lone-pairs must be taken, resulting in the s and fi type lone-pairs of 2. The latter orbitals are D 1 2 nonequivalent and possess widely differing energies. There is little doubt that the highest occupied u molecular orbital of simple aldehydes and ketones is p type. It is this orbital which is most directly involved in (n, n*) electronic transitions and in the mass spectral reactions of such compounds.In molecules containing several equivalent carbonyl groups the 'nonbonding ' p orbitals must be combined into more delocalized wave functions. These molecular orbitals, degenerate in the absence of any interaction, may as a result of interaction between the individual nonbonding orbitals be differentiated in energy. The energy splitting between these molecular orbitals is the prime operational measure of the extent of orbital interaction. Experimentally, this splitting may be obtained by ascertaining the ionization potentials of the appropriate orbitals. Through the technique of photoelectron spectroscopy [l] such measurements have become feasible. Whenever nonbonding orbital interaction was considered in the past, that interaction was judged small1). This is now not in accord with either theoretical or experimental results. Lone-pairs, double bonds and other isolated subunits of a molecule interact significantly with other such units by direct through-space and indirect through-bond mechanisms. These interactions are easily analyzed See for example the considerations for glyoxal and biacetyl in [Zj.

Electronic structure of dicarbonyls. Ground state of glyoxal

“…Swenson and Hoffmann, using CNDO calculations, have concluded that the interaction which removes the n orbital degeneracy is a through-bond interaction with the carbons. 30 In n general sense, if the interaction were through-space instead of through-bond, then bringing the oxygens spatially closer shoui<l increase the na -nb separation. Since this separation does not increase when the oxygens are spatially closer in the ci~ form, our results support a through-bond interpretation.…”

Pyridine, Pyrimidine, and Pyrazine Derivatives with an Isocyanato Substituent Vicinal to Nitrogen

“…2-Isocyanatopyridine could be obtained only as dimer"'; and 2-isocyanato-3-methylpyridine (2) is stable only in its dimeric form (3), m. p. 212-215°C. This dimer (3) is obtained in quantitative yield by reaction of 3-methyl-2-pyridinamine with diphenyl carbonate via the intermediates ( I ) and (2).…”

“…Finely powdered s-triazinamine (0.1 mol) is suspended in a mixture of oxalyl chloride (0.3-0.5 mol per amino group) and anhydrous toluene or chlorobenzene (1-2 mol) [but distilled thiophane dioxide for ( 2 ) ] , and the mixture is boiled under reflux until evolution of gas is complete (7-8 h). After filtration and removal of solvent by distillation, the residue is distilled in a vacuum.…”

“…Yellow 2-chloro-3-isocyanato-1,4-naphthoquinone (2) was obtained analogously in 75% yield from 2-amino-3-chloro-1,4-naphthoquinone (50 g) and oxalyl chloride (200 g) in anhydrous o-dichlorobenzene.…”

Lone pair interactions in dimethoxymethane and anomeric effect

“…We will show how a change in these interactions with rotation influences the geometric and conformational properties of the acetal segment. This procedure has been extensively used in the theoretical analysis of the interaction of nonbonding orbitals in different kinds of molecules along with EHT and C N D 0 / 2 semiempirical methods (19,20). Through-space and through-bond orbital interactions of lone pairs in an antiperiplanar conformation are analyzed using a simple perturbation molecular orbital (PMO) method (21) in the first part of the paper.…”