Substituent and charge distribution effects on the redox potentials of radicals. Thermodynamics for homolytic versus heterolytic cleavage in the 1-naphthylmethyl system

Abstract

Can. J. Chem. 70, 121 (1992). The electrochemical oxidation and reduction potentials of a number of substituted 1-methylnaphthalenes (la-1) and 1-naphthylmethyl radicals (2a-1') as well as 2-methylnaphthalene (3) and the 2-naphthylmethyl radical (4') have been measured by cyclic voltammetry and photomodulation voltammetry. The oxidation potentials correlate with a+ (p+ = -7.1 and -8.4 for 1 and 2' respectively) while the reduction potentials correlate with a (p-= 10.1 and 13.0 for 1 and 2' respectively). The relative magnitude of the p values can be rationalized when the charge density distribution in these systems is considered. This leads to the interesting conclusion that even though a full charge is placed in the IT-system of 1 when it is oxidized or reduced, the fraction of the charge that accumulates at C, is actually less than in 2+ or 2-where only 50-70% of the charge is delocalized into the ring. A correlation between p for the redox reactions of 1 , 2', benzyl, diphenylmethyl, and cumyl and the calculated (AMI) charge density at C, is established, implying that the sensitivity of the corresponding ions to substituent effects increases as the fraction of charge at that site increases. The redox data have been used in thermochemical cycles in order to estimate the substituent effect on the homolytic, mesolytic, and heterolytic cleavage reactions of 1 and its corresponding radical ions. The implication of these results on the C-C cleavage versus deprotonation of radical cations and on the photochemical homolysis versus heterolysis of naphthylmethyl halides and-acetates is discussed.

The photochemistry of 1-naphthylmethyl carbonates and carbamates

“…Variation in substituents (X) on the naphthalene ring varies the oxidation potential of the 1-naphthylmethyl radical and, thus, the rate of electron transfer. These oxidation potentials were shown to correlate with a+ (p+ = -8.4) (15). The rates of electron transfer agreed very well with Marcus theory in both the normal and inverted regions (1 1, 12).…”

Making Nitronaphthalene Fluoresce

“… a The reduction potentials of 1 , 2 , and 3 , E X | X •– (1/2) / V vs SCE, measured for acetonitrile (Figure ), are listed in the parentheses. For naphthalene and 1-methylnaphthalene, E X | X •– (1/2) ≈ −2.5 V vs SCE; , for dansylamide, E X | X •– (1/2) ≈ −2.05 V vs SCE; and for prodan, E X | X •– (1/2) ≈ −2.09 V vs SCE …”

Thermodynamic and kinetic characteristics of intermediates of electrode reactions: Determination by direct and combined electrochemical methods

“…The essence of the PVA method consists of that the intensity of light and, correspondingly, the concentration of intermediates that are generated uniformly throughout the solution bulk (the near-electrode layer) is modulated with the aid of a chopper in a certain range of frequencies (in works [15][16][17][18][22][23][24][25][26][27], this is 20-250 Hz) and the dependence of the current of intermediates approaching the electrode by means of diffusion on the electrode potential (photovoltammogram) is measured at the same frequency. In view of the bulk character of the generation of intermediates, the PVA method is applicable only for the determination of E 0 for relatively long-lived radicals and requires, correspondingly, a high degree of purification of a nonaqueous solvent (acetonitrile).…”