In this paper we show that a pseudo-Jahn-Teller (PJT) coupling between the (1)A(1g) ground state and the B-1(2u) excited states along the Kekule mode of b(2u) symmetry is responsible for the surprisingly low frequency of this mode in the ground state and its remarkable upward shift of 261 cm(-1) upon excitation to the first B-1(2u) excited state.