Location of the Fourth, Forbidden Factor Group Component of the 1B2u State of Crystalline Benzene

There have been two different experimental techniques used to locate the position of the fourth, forbidden factor group component of the ${}^1{B}_{\text{2u}}$ state of crystalline benzene. The method of variation of energy denominators gives rise to a very wide exciton band, placing the forbidden $A_u$ component at 37 996 cm^− 1. More recently, the density‐of‐states function for the ${}^1{B}_{\text{2u}}$ exciton band has been determined and used to predict that the $A_u$ component lies between 37 815 and 37 875 cm^− 1. As both of these techniques rely upon the use of first‐order Frenkel theory, their disagreement might be interpreted to mean that a higher‐order theory, perhaps involving ion‐pair states, is needed to explain the experiments. In order to shed more light on this problem, the method of variation of energy denominators is re‐examined in this paper through improved and more extensive experiments. The variation of energy denominators method, which assumes the spectral shift terms to be independent of isotopic substitution, is found to be inappropriate for the study of the ${}^1{B}_{\text{2u}}$ state of crystalline benzene. In addition to this finding, evidence based upon vibronic spacings is presented that indicates no strong mixing between the ion‐pair states and the ${}^1{B}_{\text{2u}}$ state of the crystal. In fact, this same evidence is used independently to predict the position of the forbidden $A_u$ component to be very close to 37 860 cm^− 1, in agreement with the limits set by the experimental density‐of‐states function. The density function, calculated using first‐order Frenkel theory and the assumption that nearest‐neighbor interactions dominate, is found to be in excellent agreement with the experimentally observed function. It is therefore concluded that mixing with ion‐pair states need not be considered to explain the band structure of the lowest excited singlet state of benzene crystals.