The 3,3-isopyrazoles Ila-e have been prepared. Irradiation of Ila-e produces the vinyl diazo compounds IXa-e. When the vinyl diazo compounds IXa-e are irradiated at 10 °C the cyclopropenes Xa-d result in high yields. When the irradiation of the vinyl diazo compounds IXa-d is carried out at ca. 5 K in rigid medium (MCIP), the ESR spectrum indicates a stable triplet species is produced. The vinylmethylene structure Ia-d ha's been assigned to these triplet species. The temperature dependence (5-30 K) of the ESR signal due to Ia-d indicates the triplet is the ground state and that the singlet is not thermally populated within this temperature range. In contrast, no triplet signal was detected upon irradiation of lie under these conditions and it is proposed that Ie is a ground-state singlet species. The involvement of vinylcarbene and/or vinylmethylene as intermediates in the preparation and photochemical reaction of cyclopropenes is discussed. Some limitations of the temperature-dependent ESR signal intensity approach for the determination of the energy separation of equilibrating singlet and triplet states are pointed out.
CHEONG WONG. Can. J. Chem. 58, 2537 (1980). The half-wave reduction potentials, measured by cyclic voltammetry, and n,x' triplet energies, measured by phosphorescence spectroscopy, were determined for a series of eighteen symmetrically and unsymmetrically substituted benzophenones. Attempts are made to correlate the results with Hammett substituent constants. Better correlations are observed when the data are correlated with a two-parameter function consisting of Hammett substituent constants and a set of substituent parameters describing variations in free radical stability. Significant deviations from "normal" behaviour are observed for benzophenones substituted by both electron-donating and electron-withdrawing substituents. These deviations are attributed to merostabilization of the radical-like species, and an empirical approach designed to evaluate the importance of this effect is developed. Ab initio calculations of molecular orbital energies in meta-and para-substituted benzaldehydes are used to evaluate the substituent effects on E,12red and ETn"* in terms of the effect on the energies of the n-and n:*-orbitals.WILLIAM J. LEIGH, DONALD R. ARNOLD, ROBERT W. R. HUMPHREYS et P o CHEONG WONG. Can. J. Chem. 58, 2537 (1980). On a determine les potentiels de reduction de demi-vague mesurts par voltametrie cyclique et les Cnergies de l'etat triplet, n,n' mesurees par spectroscopic de phosphorescence, pour une sOrie de 18 benzophenones substituees symttriquement et asymktriquement. On a essaye d'bablir une correlation entre les resultats et les constantes de substituant de Hammett. On a observe les meilleures correlations quand les donnees sont relikes a une fonction adeux paramktres se composant des constantes de substituant de Hammett et a un ensemble de paramktres de substituant decrivant les variations de stabilitk du radical libre. On a observe des deviations importantes par rapport au comportement "normal" dans le cas des benzophenones substitutes a la fois avec des substituants donneurs et des substituants accepteurs d'electrons. Ces deviations sont attribuees a une merostabilisation de I'espltce radicalaire et on a developpk une approche empirique pour tvaluer I'importance de cet effet. On a utilise des calculs a b initio d'energies d'orbitales moltculaires dans des benzaldehydes substitues en me'ta et enpara pour &valuer les effects de substituants sur E112rrdetE~nprr* en fonction de l'effet sur les energies des orbitales n et x'.[Traduit par le journal] Introduction stituents were, in fact, influencing the energy of the One of the early successes of molecular orbital 7~'-0rbita1(3). theory was the recognition that the half-wave reIt was also found that there is a correlation beduction of polynuclear aromatic hy-tween the n,nf triplet energy of a series of substidrocarbons correlates with the energy calculated tuted aromatic carbony1 compounds and their for the lowest unoccupied molecular orbital half-wave reduction potentials (3). This was attri-(LUMO; that is, the n*-orbital) of the aromatic buted to ...
An analysis of the temperature dependence (4–30 K) of the electron spin resonance (esr) spectra of substituted diphenylmethylenes (e.g. para-, para′-; methoxy, cyano, dimethylamino, and nitro) indicates all of these species have a triplet ground state. The variation in the zero-field splitting parameter D for this series of methylenes provides evidence that the average separation of the unpaired electrons is greatest for the unsymmetrically substituted derivatives, those having a strong para-electron-withdrawing substituent and a strong para′-electron-donating substituent. This result is explained in terms of a favourable contribution of charge-separated valence-bond structures, i.e., merostabilization.
The electron paramagnetic resonance (epr) spectra of the diarylmethylenes: diphenylmethylene, p-cyanodiphenylmethylene, p-methoxydiphenylmethylene, p,p′-dimethoxydiphenylmethylene, p,p′-dicyanodiphenylmethylene, p-cyano-p′-methoxydiphenylmethylene, have been studied. The zero-field splitting parameters have been determined. The signal intensity as a function of temperature (ca. 4–20 K) indicates these methylenes are ground state triplet species.
Suminary An analysis of the e.s.r. spectra of unsymmetrically substituted diphenylmethylenes (e.g. p,p'methoxy, cyano, dimethylamino, or nitro) indicate these species have a triplet ground state which is delocalized by merostabilization.
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