Activation Energies for the Singlet Excited State Processes of Substituted Benzenes:  para, meta, and ortho Isomers of Methylbenzonitrile and Methylanisole in Acetonitrile

• Published in: Journal of the American Chemical Society Vol. 126; no. 29; pp. 8870 - 8871
• Main Authors: González, Carlos M, Pincock, James A
• Format: Journal Article
• Language: English
• Published: Washington, DC American Chemical Society 28-07-2004
• Subjects: Chemistry; Exact sciences and technology; General and physical chemistry; Photochemistry; Physical chemistry of induced reactions (with radiations, particles and ultrasonics); Reaction intermediate; Activation parameter; Radiative lifetimes; Experimental study; Quantum yield; Fluorescence spectrometry; Organic solvent; Near ultraviolet radiation; Organic free biradical; Acetonitrile; Benzonitrile derivatives; Benzenic compound; Kinetic parameter; Rate constant; Chemical reactivity
• ISSN: 0002-7863; 1520-5126
• DOI: 10.1021/ja048780p

The rate constants of decay of the excited singlet states of the methylbenzonitriles (1−3) and the methylanisoles (4−6) have been determined by the measurement of fluorescence lifetimes over a broad range of temperatures (−45 to +65 °C) in acetonitrile. By fitting this data to a nonlinear expression that includes the Arrhenius equation, rate constants for the activated process (reaction) and the unactivated ones (fluorescence and intersystem crossing) can be reliably obtained. Available literature data for benzene, toluene, and ortho-xylene were also analyzed. The results indicate that the excited singlet state of substituted benzenes is quite reactive and forms a prefulvene biradical intermediate efficiently (quantum yield = 0.69 for benzene itself) by an activated route. In contrast, the efficiency of isolable product formation is quite low because the dominant process for this intermediate is returned to starting material. These observations explain why Ermolaev's rule does not apply to benzene derivatives.