Knowledge Bank

We have measured electronic absorption spectra of benzene in both the vapour and crystalline phases, for the molecules $\mathrm{<mrow\; data-mjx-texclass="ORD">12</mrow>}C6\mathrm{<mrow\; data-mjx-texclass="ORD">1</mrow>}H6,13C6\mathrm{<mrow\; data-mjx-texclass="ORD">1</mrow>}C6,\mathrm{<mrow\; data-mjx-texclass="ORD">12</mrow>}C6\mathrm{<mrow\; data-mjx-texclass="ORD">2</mrow>}H6$, and $\mathrm{<mrow\; data-mjx-texclass="ORD">13</mrow>}C6\mathrm{<mrow\; data-mjx-texclass="ORD">2</mrow>}H6$. Although spectra in the region between 48000 and $54000cm-1$ are vibrationally diffuse, the use of isotopic substitution permits the deduction that there are (at least) two electronic transitions which overlap in this region. In the vapour phase, the two transitions with origins about 49000 and $52500cm-1$ are of apparently similar intensity, but much less intense than the transition with origin near $55000cm-1$. The vibrational structure possibly indicates that the state reached near $49000cm-1$ may have a non-planar equilibrium conformation.