A Study of Non-Rigid Aromatic Molecules by Supersonic Molecular Jet Spectroscopy: Observation and Spectroscopic Analysis of the Stable Conformations of Various Alkylbenzenes

The technique of supersonic molecular jet laser spectroscopy was used to determine the stable conformations of a series of alkylbenzenes. This study demonstrates, for the first time, the sensitivity of molecular jet spectroscopy in determining both the number of stable conformations as well as the geometry of various ethyl, propyl and butyl substituents relative to the aromatic ring. Different rotamers with low barriers to interconversion, 5 kcalmole, can be isolated in the supersonic jet expansion. Each observed conformation exhibits its own spectroscopic origin S1 S0 transition is a 2-color time-of-flight mass spectrum TOFMS. The number of stable conformations is then used to determine the minimum energy geometries of the substituent group. Previous identification of individual molecular conformations for such low barriers to interconversion has not been attainable with conventional techniques such as variable temperature NMR.