J . Am. Chem. SOC.1986,108, 5074-5078
5074
Table 111. state
CI Energies (eV) of Some Low-Lying States'' SDTqb MRSD' state SDTQb MRSDc 0.22 0.29 3BI, 0 0 IA, 2.77 2.46 3B3u 3.45 2.98 1B3u 3.08 3B2g 3.28 3.78 3.40 'BZg 3.92 3.94 2'A, 3Ag 4.44 d d Z3BI, 4.76 5.08 4.94 lBIu 'Calculations carried out at the optimized UHF geometry for 'B,,, using K orbitals generated from 3B,u R H F MOs with the SV basis set. b~ CI wave function with excitations through quadruples in the 10 orbital space of a conceptual minimal basis set and one reference configuration for all states except lAg, for which two reference configurations were employed. Energies are relative to that (-384.3820 hartrees) of IA,. e~ CI wave function with excitations through doubles in the full 20 orbital space of the basis set from all configurations with coefficients >0.2 in the SDTQ-CI wave function, except for the lowest singlet and triplet states, for which the reference configuration cutoff was a coefficient >0.1. Energies are relative to that (-384.4096) of 'A,. dMRSD-CI energy not computed. K
the low-lying singlet and triplet states of 1, so that the singlet can be identified by UV spectroscopy. The calculations were performed with the SV basis set at the U H F geometry. Two types of CI calculations were carried out. The first included SDTQ-CI in the 10 a orbitals of the conceptual minimal basis set. The second consisted of MRSD-CI calculations with the full set of 20 a orbitals, in which all configurations with coefficients larger than 0.2 in the SDTQ-CI were included in the (13) A singlet ground state could be revealed indirectly by EPR if the singlet-triplet separation were small enough to permit detection of the signal from the triplet and if the Curie plot (signal intensity vs. 1/T) were found to be convex.
reference space for the excited states of each multiplicity. The results are shown in Table 111. Not surprisingly, the C I calculations that utilize the full set of 20 a orbitals give lower excitation energies, since they allow greater flexibility in the description of the excited states. However, even these MRSD-CI calculations give excitation energies that are probably too high, since the reference space for the lowest singlet and triplet included all configurations with coefficients larger than 0.1, instead of the 0.2 cutoff used for the excited states. Inclusion of u-K correlation would probably also tend to lower at least some of the excitation energies by selectively stabilizing those excited states that are more ionic than the lowest singlet and triplet. Therefore, the vertical excitation energies given in Table 111 should be regarded as upper limits. The data shown in Table 111 suggest that 'A, should have an absorption spectrum that is distinguishable from that of 3Blu. Both states are predicted to have an allowed absorption around or slightly below 3.0 eV, polarized along the long molecular axis.I4 However, the triplet is expected also to have an absorption at lower energy, corresponding to the dipole-forbidden excitation to 3B3u.
Acknowledgment is made to the donors of the Petroleum Research Fund, Administered by the American Chemical Society, and to the National Science Foundation for support of this research. Registry No. 1,2,4,5-Tetramethylenebenzene,91 130-22-2. (14) The oscillator strength calculated for the excitation in the singlet manifold is 7.6 X lo-*, while that for the triplet is 5.9 X The lowest energy transitions that are polarized along the short molecular axis IB,, and 3B,u )A,] are calculated, respectively, to havef= 3.0 X 10- and 3.5 x 10-5.
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The Quenching of Aromatic Ketone Triplets by Oxygen: Competing Singlet Oxygen and Biradical Formation? A. A. Corman*+and M. A. J. Rodgers*f Contribution from the Chemistry Department, University of Manchester. Manchester M13 9PL, U.K., and the Center for Fast Kinetics Research, University of Texas at Austin, Austin, Texas 78712. Received September 3, 1985
Abstract: Singlet oxygen, IA8, has been produced by pulsed laser excitation of benzophenone, acetophenone, 3'-methoxyacetophenone, and 2-acetonaphthonein benzene and acetonitrile. In some cases second-order decay of the 02('$)luminescence, monitored at 1270 nm, has been observed and a similar effect was apparent when 02(lAg) was produced in the initial presence of ketyl radicals. It is proposed that the general inefficiency of Oz(lAg) production on sensitization by aromatic ketone triplets is a consequence of competitive biradical formation. Subsequent reaction of these biradicals gives rise to one or more long-lived species which react with Oz(lAg).
I. Introduction The mechanism of the oxygen quenching of triplet states is a subject of some discussion and controversy. The collision complex can have singlet, triplet, or quintet multiplicity (eq 1-3), and it was originally concluded on both theoreticall and experimental2 grounds that quenching proceeds exclusively via the singlet com-
3S* + 02(3zi)i=! I[S***O2]* S + Ot(lAJ -+
3s*
+ Oz(3C;) a 3[s-.02]* s + 02(3C
