The spinndash;orbital coupling mechanism for3npgr;*larr;1Awas investigated for several substituted benzophenones by use of polarized absorption spectra and highhyphen;field Zeeman studies and was found to be consistent with the scheme already determined for benzophenone itself S. Dym, R. M. Hochstrasser, and M. Schafer, J. Chem. Phys.48,646 (1968). Spinhyphen;polarization experiments on anthrone innhyphen;heptane at 4.2 and 2deg;K and benzophenone in pyridine at 2deg;K showed that excitation into the singlet resulted in the population of the predominantly active spin state,verbar;Tzrang;. Examination of lowhyphen;temperature absorption spectra of a variety of acetophenones and benzophenones revealed band broadening within the triplet manifold, suggesting the presence of higher triplets in many cases. No such evidence for a higher triplet state below the singlet origin could be detected in the spectra of benzophenone and perdeuterobenzophenone. The difference in linewidths for the triplet and singlet origins of benzophenone was measured and interpreted as uncertainty broadening due to strong intersystem mixing; the rate of this process was calculated to be about 1011secminus;1. Additional broadening of the1npgr;*, 0ndash;1 carbonyl stretching band was observed and attributed to vibrational relaxation and anincreasedrate of intersystem crossing from this level. The use ofC2ugr;symmetry labels as an appropriate basis for discussion of aromatic carbonyl transitions is discussed. Finally a mechanism for intersystem crossing in aromatic ketones is given involving direct coupling of modified1npgr;*and3npgr;*states.
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