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The concept of chirality dates back to 1848, when Pasteur manually separated left-handed from right-handed sodium ammonium tartrate crystals 1 . Crystallization is still an important means for separating chiral molecules into their two different mirror-image isomers (enantiomers) 2 , yet remains poorly understood 3 . For example, there are no firm rules to predict whether a particular pair of chiral partners will follow the behaviour of the vast majority of chiral molecules and crystallize together as racemic crystals 4 , or as separate enantiomers. A somewhat simpler and more tractable version of this phenomenon is crystallization in two dimensions, such as the formation of surface structures by adsorbed molecules. The relatively simple spatial molecular arrangement of these systems makes it easier to study the effects of specific chiral interactions 5 ; moreover, chiral assembly and recognition processes can be observed directly and with molecular resolution using scanning tunnelling microscopy 6 , 7 , 8 , 9 . The enantioseparation of chiral molecules in two dimensions is expected to occur more readily because planar confinement excludes some bulk crystal symmetry elements and enhances chiral interactions 10 , 11 ; however, many surface structures have been found to be racemic 12 , 13 , 14 , 15 , 16 , 17 , 18 . Here we show that the chiral hydrocarbon heptahelicene on a Cu(111) surface does not undergo two-dimensional spontaneous resolution into enantiomers 19 , but still shows enantiomorphism on a mesoscopic length scale that is readily amplified. That is, we observe formation of racemic heptahelicene domains with non-superimposable mirror-like lattice structures, with a small excess of one of the heptahelicene enantiomers suppressing the formation of one domain type. Similar to the induction of homochirality in achiral enantiomorphous monolayers 20 by a chiral modifier, a small enantiomeric excess suffices to ensure that the entire molecular monolayer consists of domains having only one of two possible, non-superimposable, mirror-like lattice structures.

Chirally sensitive measurement techniques have generally been restricted to bulk samples. Here, we report the observation of fluorescence-detected circular dichroism (FDCD) from single (bridgedtriarylamine) helicene molecules by using an excitation wavelength (457 nanometers) in the vicinity of an electronic transition that shows circular dichroism in bulk samples. The distributions of dissymmetry (g) parameters by analysis of signals from pure M- and P-type diastereomers are almost perfect mirror images of one another, each spanning a range of both positive and negative values. In addition, we observe a well-defined structure in the histogram of dissymmetry parameters suggestive of specific molecular orientations at the polymer interface. These single-molecule results highlight strong intrinsic circular dichroism responses that can be obscured by cancellation effects in ensemble measurements of a randomly oriented bulk sample.

Vibrational optical activity (VOA) spectra, such as vibrational circular dichroism (VCD) and Raman optical activity (ROA) spectra, of aliphatic amino acids are simulated using density functional theory (DFT) methods in both gas phase (neutral form) and solution (zwitterionic form), together with their respective infrared (IR) and Raman spectra of the amino acids. The DFT models, which are validated by excellent agreements with the available experimental Raman and ROA spectra of alanine in solution, are employed to study other aliphatic amino acids. The inferred (IR) intensive region (below 2000 cm-1) reveals the signature of alkyl side chains, whereas the Raman intensive region (above 3000 cm-1) contains the information of the functional groups in the amino acids. Furthermore, the chiral carbons of the amino acids (except for glycine) dominate the VCD and ROA spectra in the gas phase, but the methyl group vibrations produce stronger VCD and ROA signals in solution. The C-H related asymmetric vibrations dominate the VOA spectra (i.e., VCD and ROA) > 3000 cm-1 reflecting the side chain structures of the amino acids. Finally the carboxyl and the C(2)H modes of aliphatic amino acids, together with the side chain vibrations, are very active in the VCD/IR and ROA/Raman spectra, which makes such the vibrational spectroscopic methods a very attractive means to study biomolecules.

The enantiomers of a racemic photoresponsive material represent two distinct states that can be modulated with irradiation at a single wavelength by changing the handedness of the light. Dynamic control over molecular chirality was obtained by the interconversion of enantiomers of helically shaped molecules with either left or right circular polarized light (CPL). Photoresolution of the bistable compound as a dopant in a nematic liquid crystalline phase by CPL irradiation led to a chiral mesoscopic phase. The chiral information inherent to CPL is therefore transmitted to the bistable molecule, followed by amplification and macroscopic expression of the chirality.

A general method of cooling polyatomic molecules to ultracold temperatures is reported; the optoelectrical cooling technique removes kinetic energy via a Sisyphus effect, effectively causing the molecules to continually ‘climb’ a hill of potential energy. Optoelectrical cooling of polar molecules Ultracold polar molecules are of interest for various fundamental studies, including quantum-information science, ultracold chemistry and physics beyond the standard model. However, a general method for cooling polyatomic molecules to ultracold temperatures has been lacking. This paper demonstrates an optoelectrical cooling technique that can reduce the temperature of about a million methyl fluoride (CH 3 F) molecules by a factor of more than ten. The scheme removes kinetic energy by means of a 'Sisyphus effect' that causes the molecules to continually 'climb' a hill of potential energy. In contrast to other cooling mechanisms, it proceeds in a trap, cools in all three dimensions and should work for a large variety of polar molecules. The chip-like trap and guide architecture used in this work are well suited to use in quantum-information processing with cold and ultracold molecules. Polar molecules have a rich internal structure and long-range dipole–dipole interactions, making them useful for quantum-controlled applications and fundamental investigations. Their potential fully unfolds at ultracold temperatures, where various effects are predicted in many-body physics 1 , 2 , quantum information science 3 , 4 , ultracold chemistry 5 , 6 and physics beyond the standard model 7 , 8 . Whereas a wide range of methods to produce cold molecular ensembles have been developed 9 , 10 , 11 , 12 , 13 , the cooling of polyatomic molecules (that is, with three or more atoms) to ultracold temperatures has seemed intractable. Here we report the experimental realization of optoelectrical cooling 14 , a recently proposed cooling and accumulation method for polar molecules. Its key attribute is the removal of a large fraction of a molecule’s kinetic energy in each cycle of the cooling sequence via a Sisyphus effect, allowing cooling with only a few repetitions of the dissipative decay process. We demonstrate the potential of optoelectrical cooling by reducing the temperature of about one million CH 3 F molecules by a factor of 13.5, with the phase-space density increased by a factor of 29 (or a factor of 70 discounting trap losses). In contrast to other cooling mechanisms, our scheme proceeds in a trap, cools in all three dimensions and should work for a large variety of polar molecules. With no fundamental temperature limit anticipated down to the photon-recoil temperature in the nanokelvin range, we expect our method to be able to produce ultracold polyatomic molecules. The low temperatures, large molecule numbers and long trapping times of up to 27 seconds should allow an interaction-dominated regime to be attained, enabling collision studies and investigation of evaporative cooling towards a Bose–Einstein condensate of polyatomic molecules.

Stereodynamic complexes of copper were found to undergo inversion of a helical chiral element upon oxidation or reduction. The amino acid methionine was derivatized by the attachment of two chromophores to the nitrogen atom. The resultant ligands formed stable complexes with CuIand CuIIsalts. For a derivative of a given absolute chirality, the complexes afford nearly mirror image circular dichroism spectra. The spectral changes originate from reorientation of the nitrogen-attached chromophores due to a conformation interconversion driven by the exchange of a carboxylate for a sulfide ligand. The electrically induced chirality inversion coupled with strong interactions with polarized light is unique and may lead to novel chiral molecular devices.

Evaporative cooling of molecules has not been achieved so far, owing to unfavourable collision properties and trap losses; microwave-forced evaporative cooling of hydroxyl molecules loaded in a magnetic quadrupole trap is now reported. Ultracool OH molecules approach quantum regime Evaporative cooling is the process that makes a cup of steaming hot coffee grow cold: the temperature of a substance is decreased by the removal of particles with energies much greater than the average total energy per particle. In the form of forced evaporative cooling of magnetically trapped atoms, it is used to produce Bose–Einstein condensates and other ultracold states of matter in which the quantum regime rules. Ultracold quantum gases of molecules—as opposed to atoms—may have even richer physics, but evaporative cooling of molecules has not been achieved so far because of unfavourable collision properties and trap losses. The paper reports microwave-forced evaporative cooling of hydroxyl (OH) molecules loaded in a magnetic quadrupole trap. This unexpected result occurs because of a long-range, repulsive interaction in the OH system that prevents short-range inelastic losses. Much colder temperatures are expected to be reachable, which may enable a large number of molecular species—including chemically interesting ones—to enter the quantum regime. Atomic physics was revolutionized by the development of forced evaporative cooling, which led directly to the observation of Bose–Einstein condensation 1 , 2 , quantum-degenerate Fermi gases 3 and ultracold optical lattice simulations of condensed-matter phenomena 4 . More recently, substantial progress has been made in the production of cold molecular gases 5 . Their permanent electric dipole moment is expected to generate systems with varied and controllable phases 6 , 7 , 8 , dynamics 9 , 10 , 11 and chemistry 12 , 13 , 14 . However, although advances have been made 15 in both direct cooling and cold-association techniques, evaporative cooling has not been achieved so far. This is due to unfavourable ratios of elastic to inelastic scattering 13 and impractically slow thermalization rates in the available trapped species. Here we report the observation of microwave-forced evaporative cooling of neutral hydroxyl (OH • ) molecules loaded from a Stark-decelerated beam into an extremely high-gradient magnetic quadrupole trap. We demonstrate cooling by at least one order of magnitude in temperature, and a corresponding increase in phase-space density by three orders of magnitude, limited only by the low-temperature sensitivity of our spectroscopic thermometry technique. With evaporative cooling and a sufficiently large initial population, much colder temperatures are possible; even a quantum-degenerate gas of this dipolar radical (or anything else it can sympathetically cool) may be within reach.

Polar molecules are desirable systems for quantum simulations and cold chemistry. Molecular ions are easily trapped, but a bias electric field applied to polarize them tends to accelerate them out of the trap. We present a general solution to this issue by rotating the bias field slowly enough for the molecular polarization axis to follow but rapidly enough for the ions to stay trapped. We demonstrate Ramsey spectroscopy between Stark-Zeeman subleveis in ¹⁸⁰Hf¹⁹F⁺ with a coherence time of 100 milliseconds. Frequency shifts arising from well-controlled topological (Berry) phases are used to determine magnetic g factors. The rotating-bias-field technique may enable using trapped polar molecules for precision measurement and quantum information science, including the search for an electron electric dipole moment.

We derive an analytical expression for the magnetochiral birefringence of a dilute diamagnetic chiral molecular medium subjet to a constant magnetic field. We use the single-oscillator model of Condon [Rev. Mod. Phys. 9, 432 (1937)] and Condon et al. [J. Chem. Phys. 5, 753 (1937)] to describe the optical properties of the individual molecules. The result is a function of the refractive index and the rotatory power. This result is compared to experimental data.

Elementary chemistry distinguishes two kinds of strong bonds between atoms in molecules: the covalent bond, where bonding arises from valence electron pairs shared between neighboring atoms, and the ionic bond, where transfer of electrons from one atom to another leads to Coulombic attraction between the resulting ions. We present a third, distinct bonding mechanism: perpendicular paramagnetic bonding, generated by the stabilization of antibonding orbitals in their perpendicular orientation relative to an external magnetic field. In strong fields such as those present in the atmospheres of white dwarfs (on the order of 10 5 teslas) and other stellar objects, our calculations suggest that this mechanism underlies the strong bonding of H 2 in the $^ 3\\sum \\display _{u} ^{+} (1 \\sigma _g 1 \\sigma _{u} ^{*})$ triplet state and of He 2 in the $^ 1\\sum \\display _{g} ^{+} (1 \\sigma _{g} ^{2} 1 \\sigma _{u} ^{*2} )$ singlet state, as well as their preferred perpendicular orientation in the external field.