In this paper we show that a pseudo-Jahn−Teller (PJT) coupling between the 1A1g ground state and the 1B2uexcited states along the Kekulé mode of b2u symmetry is responsible for the surprisingly low frequency ofthis mode in the ground state and its remarkable upward shift of 261 cm-1 upon excitation to the first 1B2uexcited state.