Knowledge Bank

Unique $3s\leftarrow n$ transition origins are observed in the 2 + 1 resonance-enhanced multiphoton ionization (REMPI) spectra of methyl-substituted cyclohexanones and cyclopentanones. Furthermore, a simple set of empirical rules can be formed as a function of ring size and methyl group position and orientation on the ring. Methyl groups can shift the $3s\leftarrow n$ transition origin band by as much as $2000cm-1$ relative to the unsubstituted ring. Supersonic expansion efficiently cools the sample such that only the lowest vibrational state and the first few rotational levels in the ground state are significantly populated providing sharp spectral bands $(FWHM=5cm-1)$. Thus, spectra containing configurational and even conformational isomers may be assigned by the electronic transition origin band. The temperature dependence on the equilibrium between conformational isomers has been extracted for several of these compounds and experimental $\Delta Hf\prime s$ are reported here for the first time.