Size-selected methyl lactate clusters: fragmentation and spectroscopic fingerprints of chiral recognition

Understanding the Formation and Growth of New Atmospheric Particles at the Molecular Level through Laboratory Molecular Beam Experiments

Atmospheric new particle formation (NPF), which exerts comprehensive implications for climate, air quality and human health, has received extensive attention. From molecule to cluster is the initial and most important stage of the nucleation process of atmospheric new particles. However, due to the complexity of the nucleation process and limitations of experimental characterization techniques, there is still a great uncertainty in understanding the nucleation mechanism at the molecular level. Laboratory-based molecular beam methods can experimentally implement the generation and growth of typical atmospheric gas-phase nucleation precursors to nanoscale clusters, characterize the key physical and chemical properties of clusters such as structure and composition, and obtain a series of their physicochemical parameters, including association rate coefficients, electron binding energy, pickup cross section and pickup probability and so on. These parameters can quantitatively illustrate the physicochemical properties of the cluster, and evaluate the effect of different gas phase nucleation precursors on the formation and growth of atmospheric new particles. We review the present literatures on atmospheric cluster formation and reaction employing the experimental method of laboratory molecular beam. The experimental apparatuses were classified and summarized from three aspects of cluster generation, growth and detection processes. Focus of this review is on the properties of nucleation clusters involving different precursor molecules of water, sulfuric acid, nitric acid and NxOy, respectively. We hope this review will provide a deep insight for effects of cluster physicochemical properties on nucleation, and reveal the formation and growth mechanism of atmospheric new particle at the molecular level.

Rovibronic signatures of molecular aggregation in the gas phase: subtle homochirality trends in the dimer, trimer and tetramer of benzyl alcohol

Molecular aggregation is of paramount importance in many chemical processes, including those in living beings. Thus, characterization of the intermolecular interactions is an important step in its understanding. We describe here the aggregation of benzyl alcohol at the molecular level, a process governed by a delicate equilibrium between OH⋯O and OH⋯π hydrogen bonds and dispersive interactions. Using microwave, FTIR, Raman and mass-resolved double-resonance IR/UV spectroscopic techniques, we explored the cluster growth up to the tetramer and found a complex landscape, partly due to the appearance of multiple stereoisomers of very similar stability. Interestingly, a consistently homochiral synchronization of transiently chiral monomer conformers was observed during cluster growth to converge in the tetramer, where the fully homochiral species dominates the potential energy surface. The data on the aggregation of benzyl alcohol also constitute an excellent playground to fine-tune the parameters of the most advanced functionals.

Mass spectrometry of aerosol particle analogues in molecular beam experiments

Nanometer-size particles such as ultrafine aerosol particles, ice nanoparticles, water nanodroplets, etc, play an important, however, not yet fully understood role in the atmospheric chemistry and physics. These species are often composed of water with admixture of other atmospherically relevant molecules. To mimic and investigate such particles in laboratory experiments, mixed water clusters with atmospherically relevant molecules can be generated in molecular beams and studied by various mass spectrometric methods. The present review demonstrates that such experiments can provide unprecedented details of reaction mechanisms, and detailed insight into the photon-, electron-, and ion-induced processes relevant to the atmospheric chemistry. After a brief outline of the molecular beam preparation, cluster properties, and ionization methods, we focus on the mixed clusters with various atmospheric molecules, such as hydrated sulfuric acid and nitric acid clusters, N_x O_y and halogen-containing molecules with water. A special attention is paid to their reactivity and solvent effects of water molecules on the observed processes.

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.

Computational Study on the Properties and Structure of Methyl Lactate

A theoretical study on the properties and molecular level structure of the very important green solvent methyl lactate is carried out in the gas phase and methanol and water solutions, with the solvent treated both explicitly and as a continuum. Torsional barriers giving rise to different conformers by rotation of the hydroxyl and methyl groups were analyzed using density functional theory (DFT) to establish the most stable conformer both in gas phase and solution. DFT computations on lactate dimers were also done to study short-range features, and the effect of the surrounding solvent on intra- and intermolecular hydrogen bonding was analyzed according to the polarizable continuum model approach. We have also studied lactate/water and lactate/methanol small clusters together with the corresponding binding energies. Moreover, classical molecular dynamics simulations (MD) were carried out to study medium- and large-range effects at lower computational cost. MD simulations at different pressure and temperature conditions on pure lactate were carried out, and mixtures with water and methanol of different compositions were also studied. Structural information, analyzed through the radial distribution functions, together with dynamic aspects of pure and mixed fluids were considered. The intramolecular hydrogen bonding ability of methyl lactate together with the possibility of homo- and hetero-intermolecular association determines the behavior of this molecule in pure fluids or in mixed.

Spontaneous resolution, whence and whither: from enantiomorphic solids to chiral liquid crystals, monolayers and macro- and supra-molecular polymers and assemblies

One of the great challenges in stereochemistry is the explanation of why some molecules resolve spontaneously while others do not. In this critical review the recent advances in the creation of chiral systems from achiral and racemic compounds in three-, two- and one-dimensional systems are discussed. There are some groups of molecules in some systems that do tend to display conglomerates, which may suggest that there are enantiophobic and enantiophilic molecules whose assembly is guided by the structural and thermodynamic properties of the systems in question.

Conformational preferences of chiral molecules: free jet rotational spectrum of 1-phenyl-1-propanol

The rotational spectra of normal and O-d species of the two most stable conformers of chiral 1-phenyl-1-propanol, obtained by free jet millimetre-wave absorption spectroscopy reveal that both conformers are stabilized by a O-H[dot dot dot]pi interaction, and have the Calpha-Cbeta-bond oriented nearly perpendicular to the plane of the benzene ring. The methyl group is trans with respect to the phenyl group for the most stable conformer (T), while it is gauche with respect to the phenyl group and entgegen with respect to the hydroxyl group for the second most stable conformer (GE). The energy difference (E(GE)-E(T)) was estimated to be 50(50) cm(-1) from relative intensity measurements.

Rotational spectrum of a chiral α-hydroxyester: conformation stability and internal rotation barrier heights of methyl lactate

High resolution spectrum of methyl lactate, a chiral alpha-hydroxyester, has been investigated using a molecular jet Fourier transform microwave spectrometer. High level ab initio calculations were employed to study the conformational isomerism of methyl lactate. The observed rotational spectrum confirms that the most stable conformer has an intramolecular hydrogen bond of OH...O==C type, as predicted by the ab initio calculations. The internal rotation barrier heights of the ester methyl group and the alpha-carbon methyl group were calculated to be 5.4 and 14.5 kJ mol(-1) at the MP2/aug-cc-pVDZ level of theory for the most stable conformer. The internal rotation splittings due to the ester methyl group were observed and analyzed and the ester methyl group tunneling barrier height was determined experimentally to be 4.762 (3) kJ mol(-1).

Chirality transfer through hydrogen-bonding: Experimental and ab initio analyses of vibrational circular dichroism spectra of methyl lactate in water

The infrared vibrational absorption (VA) and vibrational circular dichroism (VCD) spectra of methyl lactate were measured in the 1000-1800 cm(-1) region in the CCl(4) and H(2)O solvents, respectively. In particular, the chirality transfer effect, i.e. the H-O-H bending bands of the achiral water subunits that are hydrogen-bonded to the methyl lactate molecule exhibit substantial VCD strength, was detected experimentally. A series of density functional theory calculations using B3PW91 and B3LYP functionals with 6-311++G(d,p) and aug-cc-pVTZ basis sets were carried out to simulate the VA and VCD spectra of the methyl lactate monomer and the methyl lactate-(H(2)O)(n) complexes with n = 1, 2, 3. The population weighted VA and VCD spectra of the methyl lactate monomer are in good agreement with the experimental spectra in CCl(4). Implicit polarizable continuum model was found to be inadequate to account for the hydrogen-bonding effect in the observed VA and VCD spectra in H(2)O. The methyl lactate-(H(2)O)(n) complexes with n = 1, 2, 3 were used to model the explicit hydrogen-bonding. The population weighted VA and VCD spectra of the methyl lactate-H(2)O binary complex are shown to capture the main spectral features in the observed spectra in aqueous solution. The theoretical modeling shows that the extent of chirality transfer depends sensitively on the specific binding sites taken by the achiral water molecules. The observation of chirality transfer effect opens a new spectral window to detect and to model the hydrogen-bonding solvent effect on VCD spectra of chiral molecules.

Intra- vs. intermolecular hydrogen bonding: dimers of alpha-hydroxyesters with methanol

Intermolecular hydrogen bonding competes with an intramolecular hydrogen bond when methanol binds to an alpha-hydroxyester. Disruption of the intramolecular OH...O=C contact in favour of a cooperative OH...OH...O=C sequence is evidenced by FTIR spectroscopy for the addition of methanol to the esters methyl glycolate, methyl lactate and methyl alpha-hydroxyisobutyrate in seeded supersonic jet expansions. Comparison of the OH stretching modes with quantum-chemical harmonic frequency calculations and 18O labelling of methanol unambiguously prove the insertion of methanol into the intramolecular hydrogen bond. This is in marked contrast to UV/IR hole burning studies of the homologous system methyl lactate: (+/-)-2-naphthyl-1-ethanol, where only addition complexes were found and the intramolecular hydrogen bond was conserved. This switch in hydrogen bond pattern from aliphatic to aromatic heterodimers is thought to reflect not only a kinetic propensity but also a thermodynamic preference for addition complexes when dispersion forces become more important in aromatic systems.