The infrared and Raman spectra of vapor-phase and liquid-phase benzocyclobutane (BCB) were recorded and assigned. The fluorescence excitation spectra of jet-cooled BCB were also collected and together with its ultraviolet absorption spectra were used to assign its vibrational frequencies in its S1(p,p*) electronic excited state. The experimental work was complemented by theoretical ab initio and DFT computations. The results were compared to those of the related molecules indan (IND), tetralin (TET), 1,4-benzodioxan (14BZD), 1,3-benzodioxan (13BZD), and 1,4-dihydronaphthalene (14DHN). The focus of the work was to understand the changes in structures and vibrations between the ground and excited states. In each case the decreased p bonding in the excited states results in longer carbon-carbon bonds in the benzene ring and lower vibrational frequencies. The out-of-plane skeletal modes were of special interest as their large decreases in frequencies arise from the increased floppiness of these bicyclic molecules in their S1(p,p*) excited states.

Spectroscopic and theoretical studies of the structures and vibrations of benzocyclobutane and related molecules in their ground state and excited electronic states

247th National Meeting (2014)

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