Knowledge Bank

The absorption spectra of pyrazine and some methyl and chloropyrazines have been measured and compared in the region of the $\mathrm{<mrow\; data-mjx-texclass="ORD">1</mrow>}B3u-\mathrm{<mrow\; data-mjx-texclass="ORD">1</mrow>}Ag$, n$\Pi \ast$ transition of pyrazine. The vapor spectrum of 2, 6-dimethyl pyrazine shows a weak but allowed band system (0,0 at 31 $158cm-1$) in addition to the stronger, but very broad pyrazine $n\Pi \ast$ transition ($0,0\sim 31380$ $cm-1$). Rotational contours of the vibronic bands of the weak system and symmetry considerations are consistent with an assignment of this system as the $\mathrm{<mrow\; data-mjx-texclass="ORD">1</mrow>}B2g(n-\to \Pi 4)$ transition of pyrazine. Vapor absorption spectra, and $4\circ K$ absorption, emission, and excitation spectra of several of the pyrazine derivatives provide additional indirect evidence for an additional low-lying state.