Abstract
Extensive work in this laboratory has been devoted to the study of intermolecular interactions from scattering experiments, in order to provide ingredients for modelling forces acting in systems involving hydrocarbons, the components of atmospheres, and water. Our detection of aligned oxygen in gaseous streams and further evidence on simple molecules has been extended to benzene and various hydrocarbons. Chiral effects can be seen in the differential scattering of oriented molecules, in particular from surfaces. It is pointed out that it may be of pre-biotical interest that we focus on possible mechanisms for chiral bio-stereochemistry of oriented reactants, for example when flowing in atmospheres of rotating bodies, specifically the planet earth, as well as in vortex motions of celestial objects. Molecular dynamics simulations and experimental verifications are in progress.
Similar content being viewed by others
Introduction
In the recent discussions of the origin of homochirality in biology many previously advanced hypotheses are under scrutiny and none has yet received a global consensus. Evidence of the role of circular polarized light and in general of magnetic and electric fields, although experimentally demonstrated, appears circumstantial, because intensity of such fields has not been proved to be sufficient to induce substantial effects in the production of a specific enantiomer of a chiral species. For a review, see Avalos et al. (1998), and also Rikken and Raupach (2000) for asymmetric synthesis. The latest experimental observations [see, for example Stranges et al. (2005)] of dichroic effects in photoionization required very intense circularly polarized synchrotron radiation, probably not available in nature. Recent investigations of the origin of asymmetry based on accurate quantum chemical calculations have also shown that parity violation due to weak forces leads to extremely small energy difference for enantiomers (see the review by Quack (2002), the latest paper by Faglioni et al. (2005), and references therein).
The present contribution is a move in an alternative direction. Chiral separation can arise in vortices, and is exploited in centrifugal separation chromatography. These phenomena occur in the liquid phases. Our recent experimental investigation proves interesting alignment and orientation effects in the gas phase, simply due to microscopic events–namely molecular collisions naturally occurring in streams, typically when in a gaseous mixture a “heavier” molecular component is seeded in a lighter one. These observations are briefly reviewed in the first part of this paper. The second part investigates the possibility that similar microscopic events occurring in vortices and surface scattering may lead to chiral discrimination. Some molecular dynamics arguments are given, awaiting for detailed model calculations. Most interestingly, experimental demonstrations appear to be within reach both for vortex motion (Su, private communication) and for scattering experiments at surfaces.
Alignment in Streams
In this section we will illustrate the state-of-the-art with examples from this laboratory for alignment in gaseous streams and then proceed with an overview of current progress in experimental and theoretical approaches and of further perspectives. Some recent achievements in experimental techniques for controlling the spatial orientation of molecules [see the reviews Aquilanti (2005); Aquilanti et al. (2005)] represent, for the time being, tools for investigations of the basic mechanisms, but perspectives are open to also exploit them for applied purposes in chemical stereodynamics.
Aligning and orienting molecules
Recent papers (Pirani et al. 2001, 2003) provide our starting point for this account of advances in the production of intense and continuous beams of aligned molecules, demonstrating that in the prototypical case of a seeded supersonic expansion of a beam of the disc-shaped molecule benzene, besides acceleration and cooling, orientation of the molecular plane also occurs because of the anisotropy of the intermolecular forces which govern collisions. Previous studies on the collisional alignment of the rotational angular momentum of diatomic molecules regarded O2, for which the effect was probed by magnetic analysis (Aquilanti et al. 1994, 1995) and N2, for which the probe was molecular beam scattering (Aquilanti et al. 1997). Extensions to other hydrocarbons, particularly ethylene, have been also demonstrated (Cappelletti et al. 2006).
Up to now, we have been using terms like orientation and alignment as synonyms. According to current usage, we should consistently use alignment here, leaving orientation only for cases where one also specifies “heads” or “tails” spatial features of a molecule. However, “alignment of a molecular plane” is somewhat contrary to common linguistic usage and we will often employ “orientation” to obtain a more immediate picture of the phenomenon. We refer to collisional alignment techniques as “natural” ones, as compared to those where external fields induce a “forced” alignment. We mention the focusing in electric fields through the Stark effect, which can be either second order for linear molecules or first order for symmetric top molecules (the latter stronger than the former), the use of polarized absorption (limited to optically favorable transitions in the molecular manifold), the brute force techniques, which use strong electrical or magnetic fields and are applicable only to rotationally relaxed molecules with permanent electric or magnetic dipole moments, and the alignment in intense non-resonant laser fields. Further details and updated developments toward dramatic improvement in intensity will be referred to in the next section, but interest for the situation of naturally occurring fields in uncertain.
Natural alignment in supersonic seeded beams
A natural and effective molecular alignment technique involves microscopic collisions in environments exhibiting anisotropic velocity distributions, such as supersonic expansions of seeded molecular beams. Historically, this phenomenon was suggested long ago to occur in transport processes but it was observed much later and shown to be less trivial and much more interesting. Simple inorganic molecules, like Na2, Li2, I2, and CO2, had been found to align their rotational angular momentum in supersonic expansions when seeded with lighter carriers. Considerable efforts were devoted to the characterization of the dependence of the phenomenon on the probed rotational levels and on the beam source conditions, such as the stagnation pressure, the gas carrier composition, and the angular displacement off the molecular beam axis.
Aquilanti et al. (1994), by measuring the variation of the paramagnetism of O2 in continuous supersonic seeded molecular beams of molecular oxygen relaxed in the lowest rotovibrational states reported the first experimental evidence of the strong dependence of the alignment on the final molecular speed. Further probing of alignment by scattering cross-section measurements, performed downstream of the beam source and using as projectiles velocity selected O2− and N2−seeded molecular beams and rare gas as targets (Aquilanti et al. 1997), confirmed the correlation between molecular alignment and molecular velocity and allowed both an accurate determination of the involved interaction potential energy surfaces and characterization of the collisional dynamics of aligned molecules. The quantum mechanical theory is outlined by Aquilanti et al. (1999). These experiments suggested that measurements of anisotropy effects in the scattering cross-sections, combined with a proper velocity selection of the molecular beams, are an alternative source of information on the molecular alignment degree if the topography of the potential energy surface and details of the involved collisional dynamics are available.
Recently (Pirani et al. 2001, 2003), our interest has been addressed to the demonstration that the disk-shaped benzene molecule in supersonic seeded molecular beams would act similarly. Note that in this case the alignment of the rotational angular momentum corresponds to a preferential orientation of the molecular plane along a particular direction. Benzene is a favorite target of organic chemists for studies of steric effects, so we could imagine a wide range of applications for an oriented benzene molecular beam, in particular for investigation of the stereodynamics of elastic, inelastic, and reactive events. The probing of the orientation has been carried out through two complementary experiments – direct IR laser absorption and molecular beam scattering.
Other methods and applications
Several molecular beam techniques to force the molecular orientation have been developed using electrostatic or optical methods. References have been listed in Aquilanti et al. (2005), where progress is described on how to improve hexapole electrostatic state-selectors, which are frequently employed to study the stereodynamics in chemical reactivity, such as reactive gas-phase scattering, surface scattering, photodissociation, and electron scattering. Applications have also been listed in Aquilanti et al. (2005).
In a recent experiment (Vattuone et al. 2004) the sticking of ethylene molecules on a 001 Ag surface, at 80 K and saturated with O2 molecules, has been studied as a function of the degree of alignment of ethylene as produced in a velocity selected supersonic seeded beam. It has been found that the sticking coefficient strongly depends on such an alignment: when the molecules arrive on the surface rotating “helycopter”-like, they have a sticking tendency about 30% larger than that when they arrive rotating “cartweel”-like. This is a new twist in nano-catalysis, namely steric control at the level of microscopic elementary processes: this is anticipated to make possible practical applications in chemistry, and so will necessitate further research effort at the nano-scale.
Chiral Scattering of Oriented Molecules in Vortices and at Surfaces
A link to the previous section is suggested from the recent astrophysical discovery of aromatic molecules, particularly benzene (Cernicharo et al. 2001): this supports the study of their role in building up species of prospective biochemical interest. So our detection of aligned benzene and hydrocarbons in gaseous streams points out that as our future work-plan we focus on possible mechanisms for chiral biostereochemistry (or indeed photobiology ) of oriented reactants – for example when flowing in atmospheres of rotating bodies, specifically the planet earth – and particularly investigate vortices, where under rarefied gas dynamics conditions, not only alignment and orientation can be produced, but possibly even chirality discrimination (Lee et al. 2004, and private communication).
A two dimensional analogue to illustrate observations detailed in the previous sections can be pointed out. Logs floating down a river would show up on the average aligned along the stream. Dynamically, they would be modeled by a linear object with two equal moments of inertia, and the third one zero. Alignment in streams would occur for an object whose moments of inertia would be those a planar lamina ( two moments of inertia adding up to the third one), idealized as an isosceles triangle (Aquilanti et al. 2000). A scalene triangle is chiral in the plane: a stream of floating logs would not discriminate enantiomers, but if the river bends, it might. And it will do it differently if it bends to the right or to the left! In a vortex, adding another dimension to the problem, a three dimensional physical objects with three different moments of inertia – idealized as a fully irregular tetrahedron – may be discriminated in the two possible enantiomeric forms.
Ray et al. (1999) had shown that when chiral molecules are given a specific orientation in a film, asymmetry results for the scattering by polarized electrons. More recently Kim et al. (2005) studied photoemission by absorbed chiral molecules. As seen in “Alignment in Streams”, Vattuone et al. (2004) and Gerbi et al. (2005a,b) demonstrate stereodynamic effects in scattering from surfaces of molecules aligned according to the technique described in the previous section.
Among the possibility of chiral physical fields, circularly polarized photons, and the magnetochiral effect induced by the magnetic field and unpolarized light, have been shown to be enantioselective in photochemical reactions. But in general translation–rotation motions are true chiral force fields: recently, liquid vortex motions have been shown to induce chiral discrimination in the formation of mesophase aggregates of achiral porphyrins (Ribó et al. 2001).
Busalla et al. (1999) have given a theoretical proof that in collisions between unpolarized projectiles and chiral molecules, the differential cross-sections for a molecule and its enantiomer differ if the molecules are oriented. They also showed (Musigmann et al. 2001a, b) that left- and right-handed molecules can scatter unpolarized electrons differently if a chiral framework is provided by at least three non-coplanar polar vectors defined in the collision processes (see also Thompson and Blum 2000; Thompson 2004). This concept extended to translation–rotation collision conditions provides a molecular mechanism for chirality generation. Taking the seeded supersonic beams of the previous section as experimental evidence, inside a vortex, the major gas component is seen to drive the seeded molecules in the form of a directed flow. Classical trajectory calculations (Lee et al. 2004) show that a seeded beam of an organic molecule (1-bromo-2-chloroethane with argon) is oriented such that the bromo-end is pointing in the Ar flow direction. If it is now considered that the oriented molecules are colliding within a vortex, with their velocity direction being perpendicular to the orientation of the molecules, the macroscopic translation–rotational motion would set up a chirality generating environment. Su et al. (to be published) show by molecular dynamics calculations that preferential excitation of the conformational motion can come into play, most effectively by applying a torque along the C–C bond to the chloro-end, the farthest fragment from the mass center of molecules – for example, through collisions with a surface. This may result in a preferential accumulation of one of the rotamers in the properly screw sense of motion, leading to observations of molecular chirality enrichment (Lee et al. unpublished).
Conclusion
Macroscopic translation–rotational motion could induce molecular chirality through the processes of molecular orientation and preferential energy transfer in the differential scattering of enantiomers. It will be of interest to study the molecular dynamics and the scaling with physical sizes of vortices and with the type of molecules in a stream. A particularly fascinating scenario is in the formation of a typical low-mass star, such as our solar system (Hartmann 2000). In the process of stellar accretion, a rotating stellar core is formed together with a co-rotating circumstellar disk. The disk accretion process would generate highly collimated bipolar jets/winds in the direction perpendicular to the disk mimicking a seeded molecular beam situation, according to the discussion in “Alignment in Streams”. These translation–rotational jets/winds arising from the rotating disk surface, and the accompanying slower molecular flows, would collide with the matter which surround the disk, and this situation contains the basic ingredients for the generation of chiral matter. Su et al. (private communication), providing experimental evidence of the basic mechanism in laboratory gas flows are led to suggest that during stellar formation, chirality-enriched matter of opposite sign could be separately generated and distributed in the south and north regions of the stellar disk. In the final formation of planets such as the earth, it is plausible that large portions of the early planet could be dominated by just one type of chirality-enriched matter, setting the stage for homochirality at the emergence of life. Indeed, the Coriolis forces which act in the terrestrial atmosphere (Gladyshev 1992) lead to tornadoes with opposite chiralities in the northern and southern hemispheres. Present knowledge of possible chirality in streams is in its infancy and has to be further investigated to prove or disprove any role in prebiotic issues. In the agenda for future work, this paper suggests that we list study of properties of scattering by aligned or oriented chiral molecules and accompanying molecular dynamics simulations.
References
Aquilanti V (2005) Molecular alignment in gaseous expansions and anisotropy of intermolecular forces (Lloyd Thomas Lecture) in rarefied gas dynamics. Amer Inst Phys 762:26–31
Aquilanti V, Ascenzi D, Cappelletti D, Pirani F (1994) Velocity dependence of collisional alignment of oxygen molecules in gaseous expansions. Nature 371:399–402
Aquilanti V, Ascenzi D, Cappelletti D, Pirani F (1995) Rotational alignment in supersonic seeded beams of molecular oxygen. J Phys Chem 99:13620–13626
Aquilanti V, Ascenzi D, Cappelletti D, Fedeli R, Pirani F (1997) Molecular beam scattering of nitrogen molecules in supersonic seeded beams: a probe of rotational alignment. J Phys Chem, A 101:7648–7656
Aquilanti V, Ascenzi D, de Castro Vitores M, Pirani F, Cappelletti D (1999) A quantum mechanical view of molecular alignment and cooling in seeded supersonic expansion. J Chem Phys 111:2620–2632
Aquilanti V, Beddoni A, Cavalli S, Lombardi A, Littlejohn R (2000) Collective hyperspherical coordinates for polyatomic molecules and clusters. Mol Phys 98:1763–1770
Aquilanti V, Bartolomei M, Pirani F, Cappelletti D, Vecchiocattivi F, Shimizu Y, Kasai T (2005) Orienting and aligning molecules for stereochemistry and photodynamics. Phys Chem Chem Phys 7:291–300
Avalos M, Babiano R, Cintas P, Jiménes JL, Palacios JC (1998) Absolute asymmetric synthesis under physical fields: facts and fictions. Chem Rev 98:2391–2404
Busalla A, Blum K, Thompson DG (1999) Differential cross section for collisions between electrons and oriented chiral molecules. Phys Rev Lett 83:1562–1565
Cernicharo J, Heras AM, Tielens AGGM, Pardo GR, Herpin F, Guelin M, Waters LBFM (2001) Infrared space observatory’s discovery of C4H2, C6H2 and benzene in CRL 618. Astrophys J 546:123–126
Cappelletti D, Bartolomei M, Aquilani V, Pirani F, Demarchi G, Bassi D, Iannotta S, Scotoni M (2006) Alignment of ethylene in supersonic seeded expansion probed by infrared polarized laser absorption and by molecular beam scattering. Chem Phys Lett 420:42–53
Faglioni F, D’Agostino PS, Cadioli B, Lazzeretti P (2005) Parity violation energy of biomolecules – II: DNA. Chem Phys Lett 407:522–526
Gerbi A, Vattuone L, Rocca M, Valbusa U, Pirani F, Cappelletti D, Vecchiocattivi F (2005a) New insights on the stereodynamics of ethylene adsorption on an oxygen pre-covered silver surface. J Chem Phys 123:224709
Gerbi A, Vattuone L, Rocca M, Valbusa U, Pirani F, Cappelletti D, Vecchiocattivi F (2005b) Stereodynamic effects in the adsorption of propylene molecules on Ag(001). J Phys Chem, B 109:22884–22889
Gladyshev GP (1992) Oriented hydrodynamics flows in rotational medium and asymmetry in bioworld. Ukrainian Polymer Journal 1:55–62
Hartmann L (2000) Accretion processes in star formation, Cambridge University Press, Cambridge
Kim JW, Carbone M, Dil JH, Tallarida M, Flammini R, Casaletto MP, Horn K, Piancastelli MN (2005) Atom-specific identification of adsorbed chiral molecules by photoemission. Phys Rev Lett 95:107601
Lee H-N, Su T-M, Chao I (2004) Rotamer dynamics of substituted simple alkanes. I. A classical trajectory study of collisional orientation and alignment of 1-bromo-2-chloroethane with argon. J Phys Chem, A 108:2567–2575
Musigmann M, Busalla A, Blum K, Thompson DG (2001a) Enantio-selective collisions between unpolarized electrons and chiral molecules. J Phys, B 34:79–85
Musigmann A, Blum K, Thompson DG (2001b) Scattering of polarized electrons from anisotropic chiral ensembles. J Phys, B 34:2679–2696
Pirani F, Cappelletti D, Bartolomei M, Aquilanti V, Scotoni M, Vescovi M, Ascenzi D, Bassi D (2001) Orientation of Benzene in supersonic expansions, probed by IR-Laser absorption and by molecular beam scattering. Phys Rev Lett 86:5035–5038
Pirani F, Bartolomei M, Aquilanti V, Cappelletti D, Scotoni M, Vescovi M, Ascenzi D, Bassi D, Cappelletti D (2003) Collisional orientation of the benzene molecular plane in supersonic seeded expansions, probed by infrared polarized laser absorption spectroscopy and by molecular beam scattering. J Chem Phys 119:265–276
Quack M (2002) How important is parity violation for molecular and biomolecular chirality. Angew Chem (Int Ed) 41:4618–4630
Ray K, Ananthavel SP, Waldeck DH, Naaman R (1999) Asymmetric scattering of polarized electrons by organized organic films of chiral molecules. Science 283:814–816
Ribó JM, Crusats J, Sagués F, Claret J, Rubires R (2001) Chiral sign induction by vortices during the formation of methophases in stirred solutions. Science 292:2063–2066
Rikken GLJA, Raupach E (2000) Enantioselective magnetochiral photochemistry. Nature 405:932–935
Stranges S, Turchini S, Alagia M, Alberti G, Contini G, Decleva P, Fronzini G, Stener M, Zema N, Prosperi T (2005) Valence photoionization dynamics in circular dichroism of chiral free molecules: the methyl-oxirane. J Chem Phys 122:244303
Thompson DG (2004) Chiral effects in the ionization of chiral molecules by electron impact. J Phys, B 37:1013–1024
Thompson DG, Blum K (2000) Chiral effects in the ionization of HCl by electron impact. J Phys, B 33:L773–L777
Vattuone L, Gerbi A, Rocca M, Valbusa U, Pirani F, Vecchiocattivi F, Cappelletti D (2004) Stereodynamic effects in the adsorption of ethylene onto a metal surface. Angew Chem (Int Ed) 43:5200–5203
Acknowledgements
We thank T-M Su (Taiwan) for preprints and correspondence. The achievements on molecular alignment reviewed in this account were especially carried out jointly with Professors Fernando Pirani, David Cappelletti, Massimiliano Bartolomei and the other coworkers mentioned in the bibliography. Italian MIUR and ASI and European collaborative grants are acknowledged.
Author information
Authors and Affiliations
Corresponding author
Rights and permissions
About this article
Cite this article
Aquilanti, V., Maciel, G.S. Observed Molecular Alignment in Gaseous Streams and Possible Chiral Effects in Vortices and in Surface Scattering. Orig Life Evol Biosph 36, 435–441 (2006). https://doi.org/10.1007/s11084-006-9048-z
Published:
Issue Date:
DOI: https://doi.org/10.1007/s11084-006-9048-z