The electronic spectrum of pyrrole in the vapour and crystal

Abstract

The vapour spectrum of pyrrole was investigated below 39 000 cm^-1. Four Rydberg-like series were observed with a vibrational pattern that appears to be similar to the one previously found for other simple heterocycles, and to involve excitations of v ₈, v ₇, v ₅, v ₄. The series lead to the lowest ionization potential (8·215 eV). No evidence was found of Rydberg series leading to the next ionization potential. The weak system near 46 000 cm^-1 is tentatively assigned to a ¹ B ₁ ← [Xtilde]¹ A ₁ valence-shell transition, while the very weak absorption in the range 40 000–44 000 cm^-1 may partly or wholly be due to a 3s excitation of mixed character. The crystal spectrum in polarized light was measured in the range 45 000–55 000 cm^-1. It is slightly dichroic, and shows two diffuse absorption systems. Optical observations and spectral data indicate that the crystal structure of pyrrole is orthorhombic, probably with four molecules in the unit cell.