IR and Raman spectra are measured to elucidate both the conformation and vibrational modes of bis(4,4-dimethyl-2,5-cyclohexadien-1-ylidene) in the fundamental state. It is found that the rule of mutual exclusion holds for the IR and Raman spectra. Ab initio Hartree-Fock and density functional theory (DFT) calculations with scaling were carried out to study the molecular structure and vibrational spectra. The HF and DFT calculations predict that the molecule has a planar D-2h structure, showing a little discrepancy between theory and experiment. In the Raman spectra, the most intense line is observed at 1640 cm(-1), which is consistent with the DFT value of 1647 cm(-1) and is found to be a totally symmetric ring stretching along the lone axis and in-plane C-H bending. In the IR spectra, however, the most outstanding line is 780 cm(-1), in excellent agreement with the theoretical value of 775 cm(-1) and also found to be a z-polarized out-of-plane C-H and C-C-C bending (b(1u)). The results of analysis have been confirmed by measuring the depolarization ratios from the solution Raman scattering. Comparison of the calculated and experimental vibrational spectra reveals that the DFT calculations are quite accurate in predicting the fundamental frequencies and intensities in the region below 1700 cm(-1).