Knowledge Bank

The discrete banded absorption spectra of the oxalyl dihalides, $(COX)2$ where $X=F,Cl$ and Br, in the region 2800 to $4400\backslash AA$ have been examined under high resolution. Electronic transitions of symmetry $\mathrm{<mrow\; data-mjx-texclass="ORD">1</mrow>}Au\leftarrow \mathrm{<mrow\; data-mjx-texclass="ORD">1</mrow>}Ag$ and $\mathrm{<mrow\; data-mjx-texclass="ORD">2</mrow>}Au\leftarrow \mathrm{<mrow\; data-mjx-texclass="ORD">1</mrow>}Ag$ have been identified, the upper state resulting from $n\to \pi \ast$ electron promotion. The molecules remain planar and trans in the excited state. Main features of the vibrational structure can be explained in terms of progressions in the totally symmetric $C\approx O$ stretching, OCX scissors and OCX rocking vibrations and sequences in the torsional frequency.