POLARIZED ELECTRONIC-SPECTRA OF SINGLE-CRYSTALS OF GLUTARIMIDE

Reflection spectra for radiation polarized along the a, b, and c crystal axes of the (100) and (001) faces of single crystals of glutarimide have been measured between 500 and 140 nm. Corresponding absorption spectra are obtained through Kramers-Kronig analysis of the reflection data. Three transitions are assigned as follows: I, 40 200 cm(-1) (249 nm), f = 0.001, n pi*, polarized perpendicular to the plane of the imide linkage; II, 44 800 cm(-1) (223 nm), f = 0.007, n pi*, polarized perpendicular to the mirror plane; and III, 49 800 cm(-1) (201 nm), f = 0.30, pi pi*, polarized perpendicular to the mirror plane. Additional absorption is observed around 70 000 cm(-1) in the crystal spectra but the spectra cannot be uniquely decomposed into individual bands owing to its diffuse nature. The two weak, lower energy transitions (I and II) are derived mainly from carbonyl n pi?* transitions. Semiempirical MO calculations (INDO/ S-CI) predict two weak n pi* bands followed by a moderately intense pi pi* transition with the observed excited state symmetries. The calculations lead to a satisfactory fit for the energy of the pi pi* band but underestimate the energies of the n pi* bands by about 10 000 cm(-1).