Chirality, photochemistry and the detection of amino acids in interstellar ice analogues and comets

The primordial appearance of chiral amino acids was an essential component of the asymmetric evolution of life on Earth. In this tutorial review we will explore the original life-generating, symmetry-breaking event and summarise recent thoughts on the origin of enantiomeric excess in the universe. We will then highlight the transfer of asymmetry from chiral photons to racemic amino acids and elucidate current experimental data on the photochemical synthesis of amino and diamino acid structures in simulated interstellar and circumstellar ice environments. The chirality inherent within actual interstellar (cometary) ice environments will be considered in this discussion: in 2014 the Rosetta Lander Philae onboard the Rosetta space probe is planned to detach from the orbiter and soft-land on the surface of the nucleus of comet 67P/Churyumov-Gerasimenko. It is equipped for the in situ enantioselective analysis of chiral prebiotic organic species in cometary ices. The scientific design of this mission will therefore be presented in the context of analysing the formation of amino acid structures within interstellar ice analogues as a means towards furthering understanding of the origin of asymmetric biological molecules.

Dissipative geometric phase and decoherence in parity-violating chiral molecules

Within a generalized Langevin framework for open quantum systems, the cyclic evolution of a two-level system is analyzed in terms of the geometric phase extended to dissipative systems for Ohmic friction. This proposal is applied to the dynamics of chiral molecules where the tunneling and parity violating effects are competing. The effect of different system-bath coupling functions in the dissipated energy is shown to be crucial to understand the behavior of the geometric phase as well as the decoherence displayed by the corresponding interference patterns.

Frontiers in spectroscopy

We review the frontiers of spectroscopy from a historical perspective, starting with the development of atomic spectroscopy about 150 years ago, followed by some comments on selected previous Faraday Discussions. As the spectrum of frontiers at the Faraday Discussion 150 is very broad, we give only a brief survey providing a map of the various frontiers approached today. This is followed by an exemplary discussion of one particular frontier towards the spectroscopic detection of symmetry violations in fundamental physics. In particular the understanding of parity violation in chiral molecules has recently made great progress. We briefly describe the advances made in recent decades as well as the current status of theory and experiments in this exciting field of research. We conclude with an outlook on open questions and frontiers of the future in spectroscopy.

Novel coherent two-dimensional optical spectroscopy probes of chirality exchange and fluctuations in molecules

We demonstrate how stochastic transitions between molecular configurations with opposite senses of chirality may be probed by 2D optical signals with specific pulse polarization configurations. The third-order optical response of molecular dimers (such as biphenyls) with dynamical axial chirality is calculated to order of k(2) in the wavevector of light. Spectroscopic signatures of equilibrium chirality fluctuations are predicted for three dynamical models (Ornstein-Uhlenbeck, two-state jump, and diffusion in double well) of the dihedral angle that controls the chirality.

Absolute frequency list of the ν3-band transitions of methane at a relative uncertainty level of 10(-11)

We determine the absolute frequencies of 56 rotation-vibration transitions of the ν(3) band of CH(4) from 88.2 to 90.5 THz with a typical uncertainty of 2 kHz corresponding to a relative uncertainty of 2.2 × 10(-11) over an average time of a few hundred seconds. Saturated absorption lines are observed using a difference-frequency-generation source and a cavity-enhanced absorption cell, and the transition frequencies are measured with a fiber-laser-based optical frequency comb referenced to a rubidium atomic clock linked to the international atomic time. The determined value of the P(7) F(2)((2)) line is consistent with the International Committee for Weights and Measures recommendation within the uncertainty.

Perspectives of relativistic quantum chemistry: the negative energy cat smiles

Given the remarkable advances in relativistic quantum chemistry, some conceptual aspects still remain to be addressed. Among others, the role of negative energy states (NES) in electron correlation and other properties requires most attention. Based on critical assessments of the configuration space (CS), no-photon (and no-time) Fock space (FS) and quantum electrodynamics (QED) approaches, it is concluded that only QED provides the correct prescription for the contributions of NES to correlation, while both CS and FS give rise to wrong results. This essentially means that one should work either with the no-pair approximation (which has an intrinsic error of order (Zα)(3)) or with QED. Whether a consistent relativistic many-electron theory does exist in between remains an open question. Even under the no-pair approximation, there still exists an issue arising from that the no-pair Hamiltonian is incompatible with explicitly correlated methods. It turns out that this can nicely be resolved by introducing the concept of extended no-pair projection. Apart from these take-home messages, other immediate prospects of relativistic quantum chemistry are also highlighted for guiding future developments and applications.

Stabilization of chiral molecules by decoherence and environment interactions in the gas phase

We study the tunnel dynamics of a chiral molecule between its left (L) and right (R) conformations, under the global effect of collisional decoherence together with the effect of a mean-field generated by the environment where an energetic difference, K, between homochiral and heterochiral interactions is assumed. We show that this decoherence leads unavoidably to equal populations of the L and R chiral conformations even for a high enough value of K which tends to keep localized an initial chiral state. However, we also show that K contributes to the stabilization of an initial L or R state for times that could be many orders of magnitude larger than the tunneling time, in the case the decoherence rate is much greater than the tunneling rate. In this case, an estimation of this stabilization time and a critical tunneling time is made. Even in the case in which the tunneling rate is greater than the decoherence rate, the effect of K is to keep localized the initial chiral state for times greater than the tunneling time. A possible slight chiral asymmetry is also considered.

Parity-violation energy of biomolecules-IV: protein secondary structure

The parity-violation energy difference between enantiomeric forms of the same amino acid sequence, from the amyloid β-peptide involved in Alzheimer's disease, in both α-helix and β-sheet configurations, is investigated with ab-initio techniques. To this end, we develop an extension of the N2 computational scheme that selectively includes neighboring amino acids to preserve the relevant H-bonds. In agreement with previous speculations, it is found that the helical α structure is associated with larger parity-violation energy differences than the corresponding β form. Implications for the evolution of biological homochirality are discussed as well as the relative importance of various effects in determining the parity-violation energy.

Sympathetic cooling of polyatomic molecules with S-state atoms in a magnetic trap

We present a rigorous theoretical study of low-temperature collisions of polyatomic molecular radicals with (1)S(0) atoms in the presence of an external magnetic field. Accurate quantum scattering calculations based on ab initio and scaled interaction potentials show that collision-induced spin relaxation of the prototypical organic molecule CH(2)(X(3)B(1)) (methylene) and nine other triatomic radicals in cold (3)He gas occurs at a slow rate, demonstrating that cryogenic buffer-gas cooling and magnetic trapping of these molecules is feasible with current technology. Our calculations further suggest that it may be possible to create ultracold gases of polyatomic molecules by sympathetic cooling with alkaline-earth atoms in a magnetic trap.

Quantum stochastic resonance in parity violating chiral molecules

In order to explore parity violating effects in chiral molecules, of interest in some models of evolution towards homochirality, quantum stochastic resonance (QSR) is studied for the population difference between the two enantiomers of a chiral molecule (hence for the optical activity of the sample), under low viscous friction and in the deep quantum regime. The molecule is described by a two-state model in an asymmetric double well potential where the asymmetry is given by the known predicted parity violating energy difference (PVED) between enantiomers. In the linear response to an external driving field that lowers and rises alternatively each one of the minima of the well, a signal of QSR is predicted only in the case that the PVED is different from zero, the resonance condition being independent on tunneling between the two enantiomers. It is shown that, at resonance, the fluctuations of the first order contribution to the internal energy are zero. Due to the small value of the PVED, the resonance would occur in the ultracold regime. Some proposals concerning the external driving field are suggested.

Progress toward the first observation of parity violation in chiral molecules by high-resolution laser spectroscopy

Parity violation (PV) effects in chiral molecules have so far never been experimentally observed. To take up this challenge, a consortium of physicists, chemists, theoreticians, and spectroscopists has been established and aims at measuring PV energy differences between two enantiomers by using high-resolution laser spectroscopy. In this article, we present our common strategy to reach this goal, the progress accomplished in the diverse areas, and point out directions for future PV observations. The work of André Collet on bromochlorofluoromethane (1) enantiomers, their synthesis, and their chiral recognition by cryptophanes made feasible the first generation of experiments presented in this article.

Analysis of parity violation in chiral molecules

In order to guide the experimental search for parity violation in molecular systems, in part motivated by the possible link to biomolecular homochirality, we present a detailed analysis in a relativistic framework of the mechanism behind the tiny energy difference between enantiomers induced by the weak force. A decomposition of the molecular expectation value into atomic contributions reveals that the effect can be thought of as arising from a specific mixing of valence s(1/2) and p(1/2) orbitals on a single center induced by a chiral molecular field. The intra-atomic nature of the effect is further illustrated by visualization of the electron chirality density and suggests that a simple model for parity violation in molecules may be constructed by combining pre-calculated atomic quantities with simple bonding models. A 2-component relativistic computational procedure is proposed which bridges the relativistic and non-relativistic approaches to the calculation of parity violation in chiral molecules and allows us to explore the single-center theorem in a variational setting.

Torsion-rotation coupling and the determination of the torsional potential energy function of HSOH

Hindered torsional motion in a molecule such as HSOH leads to tunnelling splittings. Lying between the simpler limiting cases of strongly hindered torsion (the "we can simply neglect the tunnelling" limit) and that of free internal rotation (the "tunnelling pair of levels are just different vibrational states" limit) HSOH is particularly challenging due to strong and inherent torsion-rotation coupling. The low symmetry due to the different terminal moieties, SH and OH, further enriches the energy level pattern by allowing a systematic mixing within each quadruplet of torsion- and asymmetry-doubled levels. This mixing, which varies in a systematic manner with angular momentum quantum numbers, can reach 50% and is in addition to the mass-dependent and quasicyclic variation of the tunnelling splittings. We show that a carefully implemented reduced-dimension model, the Generalised SemiRigid Bender (GSRB), combined with a few modest ab initio calculations, is sufficient to account for the bulk of these physical effects. We also point out the reversal of energy pairing of torsional levels with increased energy. That is, at low torsional energy the lowest of the torsional level pair is the symmetric level while at high torsional energy it is the antisymmetric level which is the lowest. Finally, we present a purely empirical determination of the tunnelling splittings for K = 0, 1, 3, 4, and 5.

Jahn-Teller effects in molecular cations studied by photoelectron spectroscopy and group theory

The traditional "ball-and-stick" concept of molecular structure fails when the motion of the electrons is coupled to that of the nuclei. Such a situation arises in the Jahn-Teller (JT) effect which is very common in open-shell molecular systems, such as radicals or ions. The JT effect is well known to chemists as a mechanism that causes the distortion of an otherwise symmetric system. Its implications on the dynamics of molecules still represent unsolved problems in many cases. Herein we review recent progress in understanding the dynamic structure of molecular cations that have a high permutational symmetry by using rotationally resolved photoelectron spectroscopy and group theory. Specifically, we show how the pseudo-Jahn-Teller effect in the cyclopentadienyl cation causes electronic localization and nuclear delocalization. The fundamental physical mechanisms underlying the vaguely defined concept of "antiaromaticity" are thereby elucidated. Our investigation of the methane cation represents the first experimental characterization of the JT effect in a threefold degenerate electronic state. A special kind of isomerism resulting from the JT effect has been discovered and is predicted to exist in all JT systems in which the minima on the potential-energy surface are separated by substantial barriers.

Chemical applications of laser- and sympathetically-cooled ions in ion traps

Ensembles of cold atomic and molecular ions in ion traps prepared at millikelvin temperatures by laser and sympathetic cooling have recently found considerable interest in both physics and chemistry. At very low temperatures the ions form ordered structures in the trap also known as "Coulomb crystals". Ion Coulomb crystals exhibit a range of intriguing properties which render them attractive systems for novel experiments in chemical dynamics, ultrahigh-resolution spectroscopy and quantum-information processing. In this article we review the methods used to prepare atomic and molecular ion Coulomb crystals and discuss some recent studies in mass spectrometry, low-temperature chemistry and precision spectroscopy to illustrate their scientific potential for chemical applications. Finally, we conclude with an outlook on outstanding challenges and prospective further developments in the field.

Chirality recognition between neutral molecules in the gas phase

Noncovalent interactions are particularly intriguing when they involve chiral molecules, because the interactions change in a subtle way upon replacing one of the partners by its mirror image. The resulting phenomena involving chirality recognition are relevant in the biosphere, in organic synthesis, and in polymer design. They may be classified according to the permanent or transient chirality of the interacting partners, leading to chirality discrimination, chirality induction, and chirality synchronization processes. For small molecules, high-level quantum chemical calculations for such processes are feasible. To provide reliable connections between theory and experiment, such phenomena are best studied in vacuum isolation at low temperature, using rotational, vibrational, electronic, and photoionization spectroscopy. We review these techniques and the results which have become available in recent years, with special emphasis on dimers of permanently chiral molecules and on the influence of conformational flexibility. Analogies between the microscopic mechanisms and macroscopic phenomena and between intra- and intermolecular cases are drawn.