Experimental Determination of Even-Odd Character of Excited Electronic States of Molecules with a Center of Symmetry

Even-even transition intensities increase with monosubstitution but decrease with opposed disubstitution. Even-odd transition intensities which increase with monosubstitution must increase further with opposed disubstitution. Determination of parity in this way may settle disputed assignments. Thus the 1A1g—1E2g π-transition of benzene is probably at 1700 A. The first singlet absorption of naphthalene is 1A1g—1B1u or 1A1g—1B2u and cannot be 1A1g—1A1g even though the cancellation of transition matrix elements (from configuration interaction, in the one-electron approximation) gives it an intensity and vibrational structure similar to those in a forbidden transition. The visible bands of porphine are even-odd (a2u→eg 1Ag—1Qu0). Intensities in the 2100 A transition of phenyl derivatives are being remeasured to settle its assignment in benzene, since existing data give conflicting results.