ABSOLUTE SIGNS OF THE ZERO-FIELD PARAMETERS FROM MICROWAVE-OPTICAL DOUBLE RESONANCE (MODR) SPECTROSCOPY: 1,2,4,5-TETRACHLOROBENZENE
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Date
1972
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Ohio State University
Abstract
Absorption and stimulated emission of microwaves has been observed in the 3 cm MODR spectrum of the photoexcited triplet state of symmetric tetrachlorobenzene (s-TCB) in a durene host crystal at liquid helium temperatures. Studies of the relative intensities of the ``$\Delta m_{s} = \pm 1$’’ transitions as a function of temperature, with the magnetic field directed along molecular fine structure axes, have been used to determine the absolute signs of the zero-field (ZF) parameters. Combining these results with those from experiments done in zero field, it is found that D = +0.1518 and $E = +0.02908 cm^{-1}$. The signs of the ZF parameters in the $\Pi,\Pi^{*}$ triplet states of aromatic molecules will be discussed, and it will be shown that the sign of E in s-TCB and other substituted benzenes can be predicted using simple molecular orbital considerations. This work was supported by AEC Contract No. AT(11-1)-3435 and by a grant from the Research Corporation. C. R. Chen is a Postdoctoral Research Associate supported by NSF Science Development Grant No. GU-3184. J. A. Mucha is an Air Pollution Fellow supported by PHS Grant No. 5-TO1-AP-00041-04.
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Author Institution: Department of Chemistry, University of Pittsburgh