Mass-Selected Circular Dichroism of Supersonic-Beam-Cooled [D4]-(R)-(+)-3-Methylcyclopentanone

UV spectroscopy and electronic circular dichroism (ECD) experiments on supersonic-beam-cooled deuterated (R)-(+)-3-methylcyclopentanone ([D-4]-(R)-(+)-3-MCP) have been performed by using a laser mass spectrometer. The spectral resolution not only allowed excitation and CD measurements for single vibronic transitions but also for the rotational P, Q, and R branches of these transitions. The investigated n ->pi*(18(0)(4) 25(0)(1)) transition showed the largest anisotropy factor ever observed for chiral molecules in the gas phase, which, due to residual satu-ration of the excited transition, represents only a lower limit for the real anisotropy factor. Furthermore, one-color (1+1+1) and two-color (1+ 1') resonance-enhanced multiphoton ionization (REMPI) measurements were performed and the effusive-beam (room temperature) and supersonic-beam results for [D-4]-(R)-(+)-3-MCP were compared. These results allowed a differentiation between single-step ECD (comparable to conventional ECD) and cumulative ECD (only possible in multiphoton excitation) under supersonic- beam conditions.