Infrared and Raman spectra and theoretical calculations for benzocyclobutane in its electronic ground state.

The infrared and Raman spectra of vapor-phase and liquid-phase benzocyclobutane (BCB) have been recorded and assigned. The structure of the molecule was calculated using the MP2/cc-pVTZ basis set and the vibrational frequencies and spectral intensities were calculated using the B3LYP/cc-pVTZ level of theory. The agreement between experimental and calculated spectra is excellent. In order to allow comparisons with related molecules, ab initio and DFT calculations were also carried out for indan (IND), tetralin (TET), 1,4-benzodioxan (14BZD), 1,3-benzodioxan (13BZD) and 1,4-dihydronaphthalene (14DHN). The ring-puckering, ring-twisting, and ring-flapping vibrations were of particular interest as these reflect the rigidity of the bicyclic ring system. The infrared spectra of BCB show very nice examples of vapor-phase band types and combination bands.