J. Phys. Chem. 1987,91, 6373-6380 these curves. Models based on 6-31 1G**E1 with and without BSSC showed long-range REP when dipolescaled COU was used. This apparent long-range R E P disappeared when COU was optimally scaled. Overall, the results obtained by fitting E1 were consistent with those from fitting EX, showing hydrogen diameters H(C) > H(N) > H ( 0 ) . The results obtained by fitting EX were more consistent and physically reasonable. However, we recommend a cautious interpretation of the repulsion parameters listed in Table IV. The
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establishment of differing diameters for bonded hydrogen by theoretical methods lays groundwork for improving methods of quantitative derivation of empirical nonbonded potential parameters from experimental data. Acknowledgment. This research was supported by National Institutes of Health Research Grant No. GM37453. Registry No. Methane, 74-82-8; ammonia, 7664-41-7; water, 773218-5; methanol, 67-56-1.
Photochemistry of 3,4,9,1O-Perylenetetracarboxylic Dlanhydrlde Dyes. 3. Singlet and Triplet Excited-State Properties of the Bls(2,5-di-ferf -butylphenyl)imide Derivative' William E. Ford*+and Prashant V. Kamat Radiation Laboratory, University of Notre Dame, Notre Dame, Indiana 46556 (Received: February 27, 1987; In Final Form: July 10, 1987)
Fluorescence lifetime and triplet-state spectral, kinetic, and energetic properties are reported for the first time for a perylenebis(dicarboximide) dye, N,N'-bis(2,5-di-tert-butylphenyl)-3,4,9,lO-~~lenebis(dicarboximide) (DBPI). The fluorescence lifetime (3.8 i 0.2 ns), quantum yield (10.95), and singlet-state energy (54 f 1 kcal mol-') were determined for DBPI in a variety of organic solvents (24 "C). Triplet sensitization was used to characterize the triplet state of DBPI by laser flash photolysis and pulse radiolysis. The triplet-triplet absorption spectrum of DBPI had a maximum in the range 495-505 nm with an extinction coefficient of (6.3 i 0.7) X lo4 M-' cm-' , and the triplet-state energy and lifetime were 27.5 f 2 kcal mol-' and -100 ~ s respectively. , Direct photoexcitation produced the DBPI triplet with very low quantum yields (
