On the assignment of the lowest triplet state of 1,4hyphen;dihalobenzenes. MODR spectrum of the3B1ustate ofphyphen;dichlorobenzene in aphyphen;dibromobenzene host crystal

摘要

A careful examination of the orientational dependence of the microwavehyphen;optical double resonance (MODR) spectrum of the lowest (pgr;,pgr;*) triplet state ofphyphen;dichlorobenzene (DCB) in a host crystal ofphyphen;dibromobenzene at liquid helium temperatures yields accurate values of the principal components of the zerohyphen;field (ZF) splitting tensor (Xequals; mnplus; 0.0987,Yequals; plusmn; 0.0780, andZequals; plusmn; 0.0207 cmminus;1) and thegtensor (gxxequals; 1.986,gyyequals; 2.001, andgzzequals; 2.002). Formally forbidden transitions involving simultaneous electronsol;nuclear spin flips are also observed at high microwave power levels, and these are attributed to a transferred hyperfine interaction between the guest triplet state and the bromine atoms of the host. When combined with the results of previous PMDR studies on this system, these experiments show conclusively that the ordering of the ZF levels of the lowest triplet state of DCB is tgr;y tgr;zGt;tgr;xand that the orbital symmetry of this state isB1u. This is the state derived from the accepted assignment of the lowest triplet state of benzene itself.