Knowledge Bank

The absorption spectrum of pyridine in gas phase and rare gas matrices was studied in the spectral region extending from 1700 {\AA} to 2900 {\AA}. The main results show: (a) The 2900 {\AA} $(n\to \pi \ast )$ transition is similar to the corresponding gas phase transition. Only one $n\to \pi \ast$ transition is observed. (b) In the absorption spectrum of the 2600 {\AA} $\pi \to \pi \ast$ transition $(1A1g\to 1B2u$ in benzene symmetry) progressions are found with intervals of $900cm-1(\nu 2)$ starting at the origin and at distance of $330cm-1[\nu 20(b1)],460cm-1[\nu 18(b1)],550cm-1[\nu 18(a1)]$ and $725cm-1[\nu 11(a1)]$ from the origin. (c) Two progressions were observed for the second $\pi \to \pi \ast$ transition $(1A1g\to ;1B2u$ in benzene symmetry) the intervals being $900cm-1(\nu 20)$ starting at the origin and at 500cm$-1(\nu 18)$ from the origin. (d) One progression of 900cm$-1(\nu 2)$ was observed for the third $\pi \to \pi \ast$ transition $(1A1g\to 1E1u$ in benzene symmetry). The spectrum represents no evidence for the splitting of the $\mathrm{<mrow\; data-mjx-texclass="ORD">1</mrow>}E1u$ state. (e) The transition to the $\mathrm{<mrow\; data-mjx-texclass="ORD">1</mrow>}E2g$ state (in benzene symmetry) was not observed.