Printed in Great Britain oo40-4039/86 $3.00 +.OO Perqamon Journals Ltd. SPECTROSCOPIC AND CHEMICAL EVIDENCE FOR METHYLENE SINGLET-TRIPLET INTERSYSTEM CROSSING IN SOLUTION

Singlet-triplet intersystem crossing of methylene in perfluorohexane solution has been investigated by CIDNP. Evidence of this process is shown by the polarized absorption 1 H-NMR signal of ClxC-CHZD, a triplet product, which is obtained upon direct irradiation of diazirine in perfluorohexane solution containing saturated deuteriochlo-reform. These results are confirmed by product analysis as a function of dilution by perfluorohexane. The rate constant for intersystem crossing for singlet methylene to triplet methylene is evaluated as ca. 8 x lo8 S-1 from the data.-Methylene (CH2), the parent carbene, has been studied extensively by chemists, 1,2 spectrosco-pists, 3 and chemical physicists. 4,5 Its unique chemistry is primarily due to the substantially different chemical behavior of two low-lying electronic states, and the lowest singlet state (lCH2). the ground triplet state c3C~,) The energy difference between the two states has been cal-culated theoretically, and estimated experimentally to be about 9-10 kcal/mol. 6 Intersystem crossing (ISC) of methylene in the gas phase has been reported. 5,7 The rate of methylene ISC has been obtained directly in the gas phase by monitoring the absorption decay of 1 CH2 at 610 nm,5 and by detecting the growth of the triplet absorption at 141.5 nm, 7 respectively. In contrast to the extensive studies of methylene ISC in the gas phase, the deactivation of singlet-to-triplet CH2 has not been fully established in solution. 8 The reactrvity and the ISC o