Paramagnetic resonance experiments of the triplet state of 1,5hyphen;naphthyridine in mixed crystals of durene and perdeuterated durene at 80deg;K are reported. The fine structure observed at 35 GHZ is described by the spin Hamiltonian,Hthinsp;equals;thinsp;bgr;Hmiddot;gmiddot;Sthinsp;plus;thinsp;Dlpar;Sz2thinsp;minus;thinsp;2thinsp;sol;thinsp;3rpar;thinsp;plus;thinsp;Elpar;Sx2thinsp;minus;thinsp;Sy2rpar;, in whichSthinsp;equals;thinsp;1, Dthinsp;sol;thinsp;hcthinsp;equals;thinsp;plus;thinsp;0.1030thinsp;plusmn;thinsp;0.0008cmminus;1, Ethinsp;sol;thinsp;hcthinsp;equals;thinsp;minus;thinsp;0.0167thinsp;plusmn;thinsp;0.0007cmminus;1, andgxxthinsp;equals;thinsp;2.0043thinsp;plusmn;thinsp;0.0005, gyythinsp;equals;thinsp;2.0032thinsp;plusmn;thinsp;0.0008, gzzthinsp;equals;thinsp;2.0018thinsp;plusmn;thinsp;0.0006. The sign ofDis determined by the analysis of weak satellites in the hyperfine structure with the magnetic field along theXaxis. The spin densities determined from the anisotropic hyperfine interaction along the three molecular axes arergr;nthinsp;ap;thinsp;0.113, rgr;2thinsp;ap;thinsp;0.101, rgr;3thinsp;ap;thinsp;0.051, rgr;4thinsp;ap;thinsp;0.227, rgr;9thinsp;ap;thinsp;0.009.
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