We report an experimental investigation of homochiral cluster formation in seeded molecular beam expansions of (2R,3R)-butanediol. Synchrotron radiation vacuum ultraviolet photoionization measurements have been performed using a double imaging electron-ion spectrometer in various configurations and modes of operation. These include measurements of the cluster ion mass spectra, wavelength scanned ion yields, and threshold electron spectra. Protonated cluster ions ranging up to n = 7 have been observed and size-selected photoelectron spectra and photoelectron circular dichroism (PECD) have been recorded by velocity map imaging, recorded in coincidence with ions, at a number of fixed photon energies. Translation temperatures of the cluster ions have been further examined by ion imaging measurements. As well as the sequence of protonated clusters with integral numbers of butanediol monomer units, a second series with half-integral monomer masses is observed and deduced to result from a facile cleavage of a butanediol monomer moiety within the nascent cluster. This second sequence of half-integral masses displays quite distinct behaviours. PECD measurements are used to show that the half-integral mass cluster ions do not share a common parentage with whole integer masses. Using an analogy developed with simple theoretical calculations of butanediol dimer structures, it is inferred that the dissociative branching into integral and half-integral ion mass sequences is controlled by the presence of different butanediol monomer conformations within the hydrogen bonded clusters.
Most large molecules are chiral in their structure: they exist as two enantiomers, which are mirror images of each other. Whereas the rovibronic sublevels of two enantiomers are almost identical (neglecting a minuscular effect of the weak interaction), it turns out that the photoelectric effect is sensitive to the absolute configuration of the ionized enantiomer. Indeed, photoionization of randomly oriented enantiomers by left or right circularly polarized light results in a slightly different electron flux parallel or antiparallel with respect to the photon propagation direction-an effect termed photoelectron circular dichroism (PECD). Our comprehensive study demonstrates that the origin of PECD can be found in the molecular frame electron emission pattern connecting PECD to other fundamental photophysical effects such as the circular dichroism in angular distributions (CDAD). Accordingly, distinct spatial orientations of a chiral molecule enhance the PECD by a factor of about 10.
A competition between chiral characteristics alternatively attributable to either conformation or to absolute configuration is identified. Circular dichroism associated with photoexcitation of the outer orbital of configurational enantiomers of 1,3- and 2,3-butanediols has been examined with a focus on the large changes in electron chiral asymmetry produced by different molecular conformations. Experimental gas-phase measurements offer support for the theoretical modeling of this chiroptical effect. A surprising prediction is that a conformationally produced pseudo-enantiomerism in 1,3-butanediol generates a chiral response in the frontier electron dynamics that outweighs the influence of the permanent configurational handedness established at the asymmetrically substituted carbon. Induced conformation, and specifically induced conformational chirality, may thus be a dominating factor in chiral molecular recognition in such systems.
The valence shell photoionization of the simplest proteinaceous chiral amino acid, alanine, is investigated over the vacuum ultraviolet region from its ionization threshold up to 18 eV. Tunable and variable polarization synchrotron radiation was coupled to a double imaging photoelectron/photoion coincidence (i(2)PEPICO) spectrometer to produce mass-selected threshold photoelectron spectra and derive the state-selected fragmentation channels. The photoelectron circular dichroism (PECD), an orbital-sensitive, conformer-dependent chiroptical effect, was also recorded at various photon energies and compared to continuum multiple scattering calculations. Two complementary vaporization methods-aerosol thermodesorption and a resistively heated sample oven coupled to an adiabatic expansion-were applied to promote pure enantiomers of alanine into the gas phase, yielding neutral alanine with different internal energy distributions. A comparison of the photoelectron spectroscopy, fragmentation, and dichroism measured for each of the vaporization methods was rationalized in terms of internal energy and conformer populations and supported by theoretical calculations. The analytical potential of the so-called PECD-PICO detection technique-where the electron spectroscopy and circular dichroism can be obtained as a function of mass and ion translational energy-is underlined and applied to characterize the origin of the various species found in the experimental mass spectra. Finally, the PECD findings are discussed within an astrochemical context, and possible implications regarding the origin of biomolecular asymmetry are identified.
Alanine/chemistry , Gases/chemistry , Photons , Aerosols , Photochemical Processes , Photoelectron Spectroscopy , Stereoisomerism , Synchrotrons , Thermodynamics , Ultraviolet Rays , Vacuum , Volatilization
A detailed study of the valence photoionization of small homochiral glycidol (C3O2H6) clusters is carried out with the help of circularly-polarized VUV synchrotron radiation by recording photoionization-based spectroscopic data detected by velocity map electron imaging with coincidence ion selection. We show that information on the stability of cationic as well as neutral chiral clusters can be obtained with enhanced sensitivity by examining the chiral fingerprint encapsulated in Photoelectron Circular Dichroism (PECD) spectra. In particular, by varying the clustering conditions we demonstrate that the PECD signal effectively carries the signature of the neutral precursor species, prior to any fragmentation of the ion, as may be inferred from the below-threshold monomer measurements (including ion imaging). Here the monomer's direct ionization channel is closed and the monomer ion hence must result exclusively as a fragment from dissociative ionization of the dimer (or higher) clusters. At higher photon energies, the mass-selection on the electron spectroscopy data, achieved through filtering the electron images in coincidence with selected ion masses, evidently succeeds in providing a degree of size-selection on the neutral clusters being ionized with, in particular, a clear differentiation of monomer and dimer PECD, showing the strong sensitivity of this chiroptical effect to the non-local long-range molecular potential.
