Conformer-selective photoelectron circular dichroism: Experimental development and application to nitrogen chirality

Here, we describe recently developed upgrades to our experimental scheme for obtaining the photoelectron spectrum, the anisotropy parameter, and the photoelectron circular dichroism (PECD) of jet-cooled flexible molecules with conformer selectivity. The one-color resonance-enhanced multiphoton ionization process used allows ionizing selectively the different conformers present in the supersonic expansion by selecting their S₀-S₁ transition. We first describe the experimental setup with emphasis on the data acquisition and processing. Then, we apply this ionization scheme to a flexible molecule, 1,2,3,4-tetrahydro-3-isoquinoline methanol (THIQM). This molecule shows two stereogenic centers, namely, an asymmetric carbon and a nitrogen atom. It exists in two conformers, THIQM_I and THIQM_II, which differ by the direction of the intramolecular hydrogen bond and the absolute configuration of the nitrogen atom. Therefore, these two conformers are also diastereomers, endowed with slightly different ionization energies. The ionization energy of THIQM_I, which shows an OH…N hydrogen bond, is slightly higher than that of THIQM_II. Their PECD spectra, although of identical signs, differ in shape and magnitude. Surprisingly, the anisotropy parameter is more sensitive than the PECD to the conformational isomerism at play in this system.