Experimental Study of the Higher π → π* Transitions of Benzene in Low-Temperature Matrices

Abstract

In this paper we present the results of an experimental study of the absorption spectrum of benzene and deuterated benzenes in solid Ar, Kr, Xe, and N₂ in the spectral region 2800–1700 Å, with special reference to the 2100‐ and to the 1850‐Å transitions. Our main results are: (a) On the basis of the observed vibrational structure the second excited singlet state of the benzene molecule is assigned to the ${}^1{A}_{\text{1g}}{\mathrm{\hspace{0.17em}\to \hspace{0.17em}}}^1{B}_{\text{1u}}$ rather than to the ${}^1{A}_{\text{1g}}{\mathrm{\hspace{0.17em}\to \hspace{0.17em}}}^1{E}_{\text{2g}}$ excitation. (b) Theoretical calculations of the dynamic electronic–vibrational coupling between the ${}^1{B}_{\text{1u}}$ and the ${}^1{E}_{\text{1u}}$ states support the ${}^1{B}_{\text{1u}}$ assignment of the 2100‐Å transition. (c) The vibrational structure of the 1850‐Å ${}^1{A}_{\text{1g}}{\mathrm{\hspace{0.17em}\to \hspace{0.17em}}}^1{E}_{\text{1u}}$ transition was resolved. (d) No experimental evidence for Jahn–Teller coupling in the ${\mathrm{\pi \hspace{0.17em}\to \hspace{0.17em}\pi *}}^1{E}_{\text{1u}}$ state was observed, in agreement with theoretical analysis. (e) Information on site splittings for the higher $\mathrm{\pi \hspace{0.17em}\to \hspace{0.17em}\pi *}$ excitation of benzene in rare‐gas solids has accumulated. (f) Analysis of matrix shifts for the ${}^1{A}_{\text{1g}}{\mathrm{\hspace{0.17em}\to \hspace{0.17em}}}^1{B}_{\text{1u}}$ and ${}^1{A}_{\text{1g}}{\mathrm{\hspace{0.17em}\to \hspace{0.17em}}}^1{E}_{\text{1u}}$ transitions indicates that the “solvent effect” is dominated by dispersion interactions. (g) Information on deuteration effects on the ${}^1{B}_{\text{1u}}$ and ${}^1{E}_{\text{1u}}$ energy levels was obtained. (h) Qualitative information on intramolecular radiationless decay in the two higher $\mathrm{\pi \hspace{0.17em}\to \hspace{0.17em}\pi *}$ excited states of the benzene molecule has been inferred from the linewidths in the absorption spectrum.