VIBRATIONAL ANALYSIS OF THE IN-PLANE VIBRATIONS OF PYRROLE

Abstract

The infrared and Raman spectrum of pyrrole is supposedly well understood. The authoritative vibrational assignment of Lord and $Miller^{1}$ has been modified by several workers in only minor respects. A VFF calculation by $Prima^{2}$ reproduces the Lord-Miller frequencies to within an average of $3 cm^{-1}$. This fit is achieved with 27 adjustable parameters. There are about 35 truly `independent’ frequencies, since the remaining 33 planar frequencies are practically (and trivially) identical to certain members of the first isotopic set. A Urey-Bradley calculation with 19 adjustable parameters fails to converge to the Lord-Miller assignment. A simple VFF calculation with 14 principal parameters and several interaction parameters likewise fails to converge. These calculations are discussed in the light of recent experimental measurements.