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The various rotamer conformations associated with the hindered ethyl group rotor in ethyl-substituted cyclohexanones and cyclopentanones have been trapped in a supersonic expansion and spectroscopically analyzed by 2 + 1 resonance-enhanced multiphoton ionization via the 3s -n Rydberg transition. The rapid cooling in the supersonic expansion effectively freezes many of the conformations at their room temperature equilibrium concentrations. The various rotamers, including some in the axial orientation, were identified by comparing peak intensities with those calculated by both molecular mechanics and ab initio MO methods.Excellent agreement between the calculations and the experimental results is noted.

Luminescence experiments are colorful, dramatic, and can be very popular with students. However, sophisticated instrumentation or lasers (necessary for some experiments) are not always available. In this paper we describe a simple procedure for sem(quantitatively examining luminescence quenching that can be performed using the "chemicals" and "equipment" found in many discos: tonic water, table salt, a shot glass, volumetric glassware (glasses), and a black light. The experiment can also be performed quantitatively if a filter fluorimeter or spectrofluorimeter is available. It demonstrates a number of features of luminescence and can be used as the basis for a number of discussions or student special projects.

Structural analysis of 17 methyl-substituted cyclopentanones (CPOs) was conducted using 2 -I-1 resonanceenhanced multiphoton ionization (REMPI) spectroscopy via the 3s Rydberg state. The sharp spectra obtained with supersonically cooled samples show that methyl substitution leads to transition origin shifts with respect to CPO which depend on the position and orientation of the methyl group. Furthermore, multiple methyl substitutions give rise to shifts that are nearly equal to the addition of composite monomethyl shifts. Molecular mechanics and ab initio molecular orbital calculations indicate that CPO and most of the methyl-substituted derivatives have twisted geometries. The calculated potential energy barrier between various conformations was found to be no greater than 2.0 kcal/mol for the methyl-substituted derivatives, which is considerably smaller than the 3.8 kcaYmol barrier in unsubstituted CPO. One molecule, cis-2,Sdimethyl CPO, is predicted to be stable only in an envelope (bent) form, while others were found to have stable envelope and twist potential wells. A correlation was found between the 3s -n REMPI spectra and 13C NMR carbonyl carbon chemical shifts. The I3C NMR shifts could be calculated from local van der Waals energies and electrostatic potentials at the carbonyl carbon. It was possible to calculate the energy of the 3s excited state from these nonbonded interactions on the basis of the observed correlation.

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