Deuterium isotope effects in the principal electronic transition of nitrobenzene and aniline and their p-alkyl derivatives

Abstract

Deuterium Isotope Effects in Electronic Transition 951 defined charge-transfer band was observed and whenever there was little distortion of the spectrum owing to reaction-product formation. Only absorbance data from the first two increments of donor could be used because significant amounts of reaction product had accumulated by the time the third addition of donor was made. Considering these limitations, the equilibrium constants obtained are questionable. Results for the complexes are given in Table I.Although valid comparisons cannot be made from the equilibrium constants, some qualitative differences are evident from the optical spectra. There certainly appears to be some hindrance to complex formation by methyl substitution in the positions adjacent to the nitrogen atom, and the effect is more pronounced with disubstitution than with monosubstitution. Also, if the assumption is made that all of the complexes have approximately equal extinction coefficients, the amount of complexation may be estimated from the intensity of the charge-transfer band after the initial addition of donor, since the concentrations of reactants were the same for each system. The initial spectrum was used, since only small amounts of reaction products had distorted the spectrum and consumed TONE at this point. The peaks decreased in intensity in the order 3,4-lutidine > 3,5-lutidine > 4-picoline > pyridine. Using this method of analysis, the intensities of the complex bands of the donors which are not sterically hindered parallel closely their basicities. Although comparisons of the steric effect and order of donor strength are not conclusive, these data indicate that the donors act as n-electron donors toward TONE.Esr investigations of the TCNE-pyridine systems showed the nine-line spectrum of the TONE anion radical, but no evidence was found for any other paramagnetic species. The lack of evidence for the presence of the pyridine cation radical implies that the formation of the TONE anion radical may not be due to the dissociation of the charge-transfer complex.The radical anion is formed in good yields by the reaction of TONE with cyanide ion,14 and this method was used to synthesize the potassium salt of the anion radical in this laboratory. Since the formation of both pentacyanopropenide and tricyanoethenolate ions in the donor-TCNE systems liberates cyanide ion, this ion is very likely responsible for the reduction of TONE to the anion radical. This possibility is supported by our finding that the concentration of radical increased slowly over a period of hours.

Reactions of the Cumyloxyl and Benzyloxyl Radicals with Strong Hydrogen Bond Acceptors. Large Enhancements in Hydrogen Abstraction Reactivity Determined by Substrate/Radical Hydrogen Bonding

“…The observation of k H / k D ratios of 0.91 and 1.0 clearly indicates that in these substrate/radical couples C–H bond cleavage does not occur in the rate-determining step of the reaction. The observation of an inverse kinetic deuterium isotope effect in the reaction of BnO • with DMSO/DMSO- d 6 may be a consequence of the slightly larger electron releasing effect displayed by CD 3 groups as compared to CH 3 in some processes, an effect that would results in the formation of a more stable prereaction complex and in a corresponding increase in the rate of complex formation. However, this is at most a working hypothesis that would require support from additional studies.…”

Ultraviolet spectrometry

“…The use of electronic spectra to distinguish between carbonyl, thiocarbonyl, and azomethine chromophores was discussed (383). A study of the deuterium isotope effects in the principal transition of derivatives of nitrobenzene and aniline was reported (309). Spectral shift data of indol derivatives with aluminum chloride distinguish those with a skatole radical from those with other types of substitution (235).…”