The relative intensities of the members of a progression (Franckndash;Condon factors) can be used to determine changes in molecular geometry upon electronic excitation. A detailed examination of the normal coordinates shows that for benzene and similar species only one vibrational modelpar;ngr;1, sim;990cmminus;1rpar;is expected to form progressions, even for deuterated benzenes having many totally symmetric vibrations which, according to group theory, might be expected to form progressions. For aD2hdistorted benzene a second coordinatelpar;ngr;8, sim;1600cmminus;1rpar;is expected to form progressions. The phosphorescence of all isotopically substituted benzenes has only one main progression frequency (950ndash;990 cmminus;1), while the dominant progressions for the methylbenzenes involve the 1600hyphen;cmminus;1mode. Quantitative calculations show that in the lowest triplet state of benzene the difference between long and short bonds is less than 0.01 Aring;, while for toluene a value of sim;0.07 Aring; is found.
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