THE FIRST SINGLET STATE OF PYRIDINE

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1974

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Ohio State University

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Absorption spectra of the $^{1}B_{1} \leftarrow ^{1}A_{1} (\Pi^{*} \leftarrow$ n) systems of pyridine $–h_{5}$ and $–d_{5}$ in solid solutions of benzene, cyclohexane and p-dioxane at 4.2$^{\circ} K$ have been analyzed. Of particular interest was the fate of a moderately intense band in the vapor spectrum lying $138 cm^{-1}$ to higher energy from the 2875 {\AA} $band.^{1}$ Jesson, Kroto, and $Ramsay^{2}$ have recently assigned this band as representing a two quantum excitation of the out-of-plane $\nu_{16b}$ vibration in the excited state. These authors have also shown that the $\nu_{16b}$ coordinate in the $^{1}B_{1}$ state contains a double minimum, with a barrier height of about $4 cm^{-1}$. Analysis of polarized absorption spectra of pyridine in benzene implies that the excited state $\nu_{16b}$ frequency is much higher in the solid solutions relative to the vapor phase ($155 cm^{-1}$ in benzene versus approximately $60 cm^{-1}$ in the vapor). Bands of similar energy are also observed in cyclohexane and p-dioxane matrices, but $\nu_{16b}$ transitions in these solvents are broader. The energies and relative intensities of the $\nu_{16b}$ progressions suggest that the corresponding potential is harmonic (or nearly so) in these solid solutions, and that the double minimum in the $\nu_{16b}$ coordinate of the free molecule has been largely removed.

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$^{1}$H. Sponer and H. Stucklen, J. , l Che\""{u}. Phys. 14, 101 (1946). $^{2}$J. P. Jesson, H. W. Korto, and D, A. Ramsay, J. Chem. Phys. 56, 6257 (1972).
Author Institution: Baird-Atomic, Inc.; Frick Chemical Laboratory, Princeton University

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