Linear and nonlinear circular dichroism of R-(+)-3-methylcyclopentanone

Abstract

Linear and nonlinear circular dichroism of R-(+)-3-methylcyclopentanone (R-3MCP) is reported in the gas and liquid phases. Measurements of (2+1) resonance-enhanced multiphoton ionization circular dichroism (REMPICD) for nozzle-jet expanded molecular beams of the equatorial conformer of R-3MCP are presented. Monitoring either mass-selected cations or photoelectrons produced via (2+1) REMPI through the n --> 3s Rydberg transition yielded a REMPICD of +1.5+/-0.5% [REMPICD identical with 2(I(L)-I(R))(I(L)+I(R))], where I(L/R) refers to the ion/electron signal for left/right circularly polarized light. A racemic mixture of 3-methylcyclopentanone showed no significant CD; however, the signal fluctuations were much larger than that observed for the resolved R-(+)-3-methylcyclopentanone as might be expected for the small number of ions produced from slightly unequal numbers of enantiomers in each laser shot. Gas phase, vibrationally resolved, one-photon CD for vapor phase R-(+)-3-methylcyclopentanone (i.e., admixture of five axial and equatorial forms) was measured to be approximately 0 and -0.004 at photon energies corresponding to the one- (nonresonant) and two-(3s resonance) photon energy levels. The one-photon CD (of the room temperature population of conformers) at an energy corresponding to the ionization step was measured previously to be approximately +0.0011 which is of the same sign as the REMPICD. The first step is also near a positive CD region. This suggests that the (2+1) REMPICD is determined primarily by both the initial and continuum steps. The one-photon CDs for the equatorial and axial forms of 3MCP are calculated, using GAUSSIAN03, to be approximately equal but having opposite sign for the transitions of interest. The CD for 3MCP in cyclohexane is found to be strongly temperature dependent as a result of the presence of both the axial and equatorial conformers. The energy difference between the two conformers is determined from a van't Hoff plot of these data to be 3.50+/-0.05 kJ/mole in cyclohexane and is approximately 1 kJ/mole smaller than measurements employing other methods.