Photoelectron spectroscopy (PES) in combination with computational chemistry has been used systematically over the past decade to elucidate the structures and chemical bonding of size-selected boron clusters. Small boron clusters have been found to be planar or quasi-planar, consisting of a monocyclic circumference with one or more interior atoms. The propensity for planarity has been found to be a result of both σ and π electron delocalisation over the molecular plane, giving rise to concepts of σ and π multiple aromaticity. In particular, the B 36 cluster has been found to possess a highly stable planar structure with a central hexagonal vacancy. This finding provides the first indirect experimental evidence that single-atom layer boron-sheets with hexagonal vacancies, dubbed 'borophene', are potentially viable. Another exciting discovery has been the observation and characterisation of the first all-boron fullerenes. PES revealed that the cluster consisted of two isomers with very different electron binding energies. Global minimum searches led to two nearly degenerate isomers competing for the global minimum: a quasi-planar isomer with a double hexagonal vacancy and an unprecedented cage isomer. In the neutral, the B 40 cage is overwhelmingly the global minimum, which is the first all-boron fullerene to be observed and is named 'borospherene'. Rapid progresses in our understanding of the structures and bonding of size-selected boron clusters have been made during the past decade, which will be the focus of this review. The recent findings about borophenes and borospherenes have stimulated growing interests in boron clusters and will accelerate the pace of discovery in boron chemistry and nanostructures.
We review recent progress in developing potential energy and dipole moment surfaces for polyatomic systems with up to 10 atoms. The emphasis is on global linear least squares fitting of tens of thousands of scattered ab initio energies using a special, compact fitting basis of permutationally invariant polynomials in Morse-type variables of all the internuclear distances. The computational mathematics underlying this approach is reviewed first, followed by a review of the practical approaches used to obtain the data for the fits. A straightforward symmetrization approach is also given, mainly for pedagogical purposes. The methods are illustrated for potential energy surfaces for , (H 2 O) 2 and CH 3 CHO. The relationship of this approach to other approaches is also briefly reviewed.
The expected depletion of fossil fuel reserves and its serious environmental impact have emphasised the issue of sustainable development of the human society. Solar hydrogen by photocatalytic water splitting is a promising alternative to conventional fossil fuels, which is of great potential to relieve the energy and environmental issues and bring an energy revolution in a clean and sustainable manner. This review is going to make a brief introduction of the basic principles of photocatalytic water splitting and the concept of different kinds of water splitting systems. Various engineering strategies for searching higher efficiency of water splitting based on the photocatalytic processes, including light harvesting, charge carriers separation and co-catalysts loading, have been outlined and discussed with selected typical examples on some elaborately designed semiconductor-based photocatalytic systems. Moreover, recent impressive progresses and advancements for photocatalytic water splitting with some promising materials are presented. Finally, this review is concluded with a summary and perspective in this hot area of research.
Carbonyl oxides, also known as Criegee intermediates, are key intermediates in both gas phase ozonolysis of unsaturated hydrocarbons in the troposphere and solution phase organic synthesis via ozonolysis. Although the study of Criegee intermediates in both arenas has a long history, direct studies in the gas phase have only recently become possible through new methods of generating stabilised Criegee intermediates in sufficient quantities. This advance has catalysed a large number of new experimental and theoretical investigations of Criegee intermediate chemistry. In this article we review the physical chemistry of Criegee intermediates, focusing on their molecular structure, spectroscopy, unimolecular and bimolecular reactions. These recent results have overturned conclusions from some previous studies, while confirming others, and have clarified areas of investigation that will be critical targets for future studies. In addition to expanding our fundamental understanding of Criegee intermediates, the rapidly expanding knowledge base will support increasingly predictive models of their impacts on society.
Biochemical reactions are subject to the particular environmental conditions of planet earth, including solar irradiation. How DNA responds to radiation is relevant to human health because radiation damage can affect genetic propagation and lead to cancer and is also important for our understanding of how life on earth developed. A reductionist approach to unravelling the detailed photochemistry seeks to establish intrinsic properties of individual DNA building blocks, followed by extrapolation to larger systems, to incorporate interactions between the building blocks and the role of the biomolecular environment. Advances in both experimental and computational techniques have lead to increasingly detailed insights in the excited state dynamics of DNA bases in isolation as well as the role of the solvent and intermolecular interactions. This review seeks to summarise current findings and understanding.
Many gas-phase chemical reactions proceed via reaction intermediates, supported by potential wells. The characteristics of such complex-forming reactions differ drastically from those for direct reactions that involve barriers. For example, the reaction path for a complex-forming reaction is often barrierless, which results in weak and sometimes negative temperature dependence for its rate constant. The product angular and internal distributions of such reactions also bear clear signatures. Specifically, the angular distribution (i.e. differential cross-section) of a complex-forming reaction is often dominated by scattering in the forward and backward directions, and the product rotational state distribution usually peaks near the highest accessible rotational state, while vibrational state distribution often decays monotonically. While the quantum dynamics of direct reactions is well established, our understanding of complex-forming reactions is still far from complete. Given the importance of such reactions in interstellar, atmospheric and combustion chemistry, much research effort has recently been devoted to understand their dynamics. In this review, we survey the recent progress in the quantum dynamics of several prototypical complex-forming reactions, particularly those involving three or four atoms. We will focus on methodological advances in quantum scattering theory, quasi-classical trajectory methods and statistical models.
Intermolecular hydrogen bonding, as an important site-specific interaction between hydrogen donor and acceptor molecules both in the gas phase and in solution, plays a significant role and has a remarkable influence on the photophysics and photochemistry of chromophores in the hydrogen-bonding surroundings. In recent years, the excited-state structures and dynamics of the intermolecular hydrogen bonding have been widely investigated by using both the experimental and theoretical methods. This review article focuses on the recent research progress of the important intermolecular hydrogen-bonding effects on the non-adiabatic photophysical processes and photochemical reactions. Firstly, the corresponding relationship between electronic spectral red-shift or blue-shift and excited-state intermolecular hydrogen bond strengthening or weakening has been clarified. A dynamic equilibrium induced by the intermolecular hydrogen bond strengthening in the electronically excited state of fluorenone chromophore was used to explain the steady-state spectral features. The stepwise mechanism of excited-state double proton transfer reaction for the 2-aminopyridine/acid systems was demonstrated. The role of intermolecular hydrogen bonding on the excited-state proton transfer in the sensing mechanism of fluorescent probes has also been discussed. Moreover, a dihydrogen-bonded complex formed by borane-trimethylamine and phenol showed interesting geometric structures and infared spectrum in electronically excited state.
Gaussian wavepacket methods are an attractive way to solve the time-dependent Schrödinger equation (TDSE). They have an underlying trajectory picture that has a natural connection to semi-classical mechanics, allowing a simple pictorial interpretation of an evolving wavepacket. They also have better scaling with system size compared to conventional grid-based techniques. Here we review the variational multi-configurational Gaussian (vMCG) method. This is a variational solution to the TDSE, with explicit coupling between the Gaussian basis functions, resulting in a favourable convergence on the exact solution. The implementation of the method and its performance will be discussed with examples from non-adiabatic photo-excited dynamics and tunneling to show that it can correctly describe both of these strongly quantum mechanical processes. Particular emphasis is given to the implementation of the direct dynamics variant, DD-vMCG, where the potential surfaces are calculated on-the-fly via an interface to quantum chemistry programs.
Recent advances in the gas phase vibrational spectroscopy of mass-selected ions are described, highlighting experiments on hydrogen-bonded (HBed) clusters relevant to atmospheric chemistry. The use of cryogenic ion traps in combination with the widely tunable and intense radiation from infrared free electron lasers has allowed for new molecular-level insights into the structure and other properties of HBed clusters. Advances and challenges in the interpretation of their vibrational action spectra, in particular, the importance of considering anharmonic effects, are described and discussed. The advantages of isomer-specific measurements relying exclusively on excitations within the vibrational manifold are also evaluated. The article concludes with an outlook on future challenges and perspectives.
While the marriage of mass spectrometry and laser spectroscopy is not new, developments over the last few years in this relationship have opened up new horizons for the spectroscopic study of biological molecules. The combination of electrospray ionisation for producing large biological molecules in the gas phase together with cooled ion traps and multiple-resonance laser schemes are allowing spectroscopic investigation of individual conformations of peptides with more than a dozen amino acids. Highly resolved infrared spectra of single conformations of such species provide important benchmarks for testing the accuracy of theoretical calculations. This review presents a number of techniques employed in our laboratory and in others for measuring the spectroscopy of cold, gas-phase protonated peptides. We show examples that demonstrate the power of these techniques and evaluate their extension to still larger biological molecules.
This review provides a computational chemist's perspective of rotational spectroscopy and discusses the theoretical background and application of state-of-the-art quantum-chemical methods for the accurate determination of the relevant spectroscopic parameters.
The time-dependent quantum wave packet approach has been improved and formulated to treat the multiple surface problems and thus provided a new simple, yet a clear quantum picture for interpreting the reaction mechanism underlying the nonadiabatic dynamical processes. The method keeps the salient feature of the original quantum wave packet theory developed for single surface problems, i.e. the introduction of the absorbing potential and the grid basis including the discrete variable representation and the fast Fourier transformation, which makes the present methodology a very efficient implement for the nonadiabatic quantum scattering calculations. Here, we review the theoretical basis of this approach and its applications to the fundamental triatomic chemical reactions, the latter include the nonadiabatic dynamics calculations on the F + H 2 , F + HD, F + D 2 , O( 1 D) + N 2 , O( 3 P, 1 D) + H 2 , D + + H 2 , and H + + D 2 reactions. We also present a thorough historical overview of the theoretically nonadiabatic dynamical investigations particularly on the triatomic systems, and show how the time-dependent wave packet approach complements the time-independent quantum scattering theory.
A micro-reactor system (approximately 0.5-1 mm inner diameter by 2-3 cm in length) coupled with photoionization mass spectrometry and matrix isolation/infrared spectroscopy diagnostics is described. Short residence time flow reactors (roughly ≤ 100 μs) combined with suitable diagnostic tools have the potential to allow observation of unimolecular decomposition processes with minimum interference from secondary reactions. However, achieving the short residence times desired requires very small micro-reactors that are difficult to characterise experimentally because of their size. In this article the benefits of using these micro-reactors are presented along with some details of the systems employed. This is followed by some general flow considerations and then some simple analyses to illustrate particular features of the flow. Finally, computational fluid dynamics simulations are used to explore the flow and chemical behaviour of the reactors in detail. Some findings include: (1) The reactor operates in the laminar domain. (2) Heating and large pressure differences across the reactor result in a compressible flow that chokes (meaning the velocity reaches the sonic condition) at the reactor exit. (3) When helium is the carrier gas, under some circumstances there is slip at the boundaries near the downstream end of the reactor that reduces the pressure drop and heat transfer rate; this effect must be accounted for in the simulations. (4) Because the initial reactant concentration is held to less than 0.1%, secondary reactions are minimised. (5) Although the fluid dynamical residence time from entrance to exit ranges from 25 to 150 μs, in practice the period over which reactions take place is much shorter. In essence, there is a 'sweet spot' within the reactor where most reactions take place. In summary, the micro-reactor, which has been used for many years to generate radicals or study unimolecular decomposition chemical mechanisms, can be used to extract kinetic information by comparing simulations and measurements of reactant and product concentrations at the reactor exit.
During the last decade, density function theory (DFT) in its static and dynamic time dependent forms, has emerged as a powerful tool to describe the structure and dynamics of doped liquid helium and droplets. In this review, we summarise the activity carried out in this field within the DFT framework since the publication of the previous review article on this subject [M. Barranco et al., J. Low Temp. Phys. 142, 1 (2006)]. Furthermore, a comprehensive presentation of the actual implementations of helium DFT is given, which have not been discussed in the individual articles or are scattered in the existing literature.
Laboratory studies of liquid-liquid phase separation in particles containing organic species and inorganic salts of atmospheric relevance are reviewed. The oxygen-to-carbon elemental ratio (O:C) of the organic component appears to be the most useful parameter for estimating, to a first approximation, the occurrence of liquid-liquid phase separation and the separation relative humidity (SRH) in these particles. A trend of decreasing SRH for increasing O:C was found for simple organic-inorganic mixtures (<11 species). Phase separation in particles composed of laboratory-produced secondary organic material and sulphate species and in ambient particles is generally consistent with this trend. A further constraint is that liquid-liquid phase separation was always observed for O:C < 0.5 and was never observed for O:C ≥ 0.8. For organic materials of intermediate O:C ranging from 0.5 to 0.8, phase separation in simple organic-inorganic mixtures was influenced by the organic functional groups represented. The organic-to-inorganic mass ratio (OIR) affected the occurrence of liquid-liquid phase separation in a small number of cases. A dependence on salt type was observed with 87% of the studied organics exhibiting the following trend in SRH values: (NH 4 ) 2 SO 4 ≥ NH 4 HSO 4 ≥ NaCl ≥ NH 4 NO 3 , consistent with previous salting-out studies and the Hofmeister series. Liquid-liquid phase separation does not appear to be strongly influenced by the number of species making up the organic material. The morphology of phase separated particles appears to depend on composition, including O:C of the organic material, the inorganic salt and the OIR.
It was in 'The Magellanic Cloud' (1955) - a science fiction novel by Stanislaw Lem - that engineers travelling to another star noticed that their spacecraft for unknown reasons overheated. The cause had to be outside the spaceship, but obviously there was only emptiness, at least compared to terrestrial conditions. The space between the stars, the interstellar medium (ISM), however, is not completely empty and at the high speed of the spacecraft the cross-section with impacting particles, even from such a dilute environment, was found to be sufficient to cause an overheating. Today, 60 years later, the ISM has been studied in detail by astronomical observations, reproduced in dedicated laboratory experiments and simulated by complex astrochemical models. The space between the stars is, indeed, far from empty; it comprises gas, dust and ice and the molecules detected so far are both small (diatomics) and large (long carbon chains, PAHs and fullerenes), stable and reactive (radicals, ions, and excited molecules) evidencing an exotic and fascinating chemistry, taking place at low densities, low temperatures and experiencing intense radiation fields. Astrochemists explain the observed chemical complexity in space - so far 185 different molecules (not including isotopologues) have been identified - as the cumulative outcome of reactions in the gas phase and on icy dust grains. Gas phase models explain the observed abundances of a substantial part of the observed species, but fail to explain the number densities for stable molecules, as simple as water, methanol or acetonitrile - one of the most promising precursor species for the simplest amino acid glycine - as well as larger compounds such as glycolaldehyde, dimethylether and ethylene glycol. Evidence has been found that these and other complex species, including organic ones, form on icy dust grains that act as catalytic sites for molecule formation. It is here where particles 'accrete, meet, and greet' (i.e. freeze out, diffuse and react) upon energetic and non-energetic processing, such as irradiation by vacuum UV light, interaction with impacting particles (atoms, electrons and cosmic rays) or heating. This review paper summarises the state-of-the-art in laboratory based interstellar ice chemistry. The focus is on atom addition reactions, illustrating how water, carbon dioxide and methanol can form in the solid state at astronomically relevant temperatures, and also the formation of more complex species such as hydroxylamine, an important prebiotic molecule, and glycolaldehyde, the smallest sugar, is discussed. These reactions are particularly relevant during the 'dark' ages of star and planet formation, i.e. when the role of UV light is restricted. A quantitative characterization of such processes is only possible through dedicated laboratory studies, i.e. under full control of a large set of parameters such as temperature, atom-flux, and ice morphology. The resulting numbers, physical and chemical constants, e.g. barrier heights, reaction rates and branching ratios, provide information on the molecular processes at work and are needed as input for astrochemical models, in order to bridge the timescales typical for a laboratory setting to those needed to understand the evolutionary stages of the ISM. Details of the experiments as well as the astrochemical impact of the results are discussed.
The understanding of molecular structure and function is at the very heart of the chemical and molecular sciences. Experiments that allow for the creation of structurally pure samples and the investigation of their molecular dynamics and chemical function have developed tremendeously over the last few decades, although 'there's plenty of room at the bottom' for better control as well as further applications. Here, we describe the use of inhomogeneous electric fields for the manipulation of neutral molecules in the gas-phase, i.e. for the separation of complex molecules according to size, structural isomer, and quantum state. For these complex molecules, all quantum states are strong-field seeking, requiring dynamic fields for their confinement. Current applications of these controlled samples are summarised and interesting future applications discussed.
Electronic excitation energy transfer is ubiquitous in a variety of multichromophoric systems and has been a subject of numerous investigations in the last century. Recently, sophisticated experimental and theoretical studies of excited state dynamics have been developed with the purpose of attaining a more detailed picture of the coherent and incoherent quantum dynamics relevant to energy transfer processes in a variety of molecular aggregates. In particular, great efforts have been made towards finding experimental signatures of coherent superpositions of electronic states in some light-harvesting antenna complexes and to understand their practical implications. This review intends to provide some foundations, and perhaps inspirations, of new directions of research. In particular, we emphasise current opinions of several effects that go beyond normal Förster theory and highlight open problems in the description of energy transfer beyond standard approximations as well as the need of new approaches to characterise the 'quantumness' of excited states and energy transfer dynamics in multichromophoric systems.
The past few years have seen a particularly rich period in the development of the explicitly correlated R12 theories of electron correlation. These theories bypass the slow convergence of conventional methods, by augmenting the traditional orbital expansions with a small number of terms that depend explicitly on the interelectronic distance r 12 . Amongst the very numerous discoveries and developments that we will review here, two stand out as being of particular interest. First, the fundamental numerical approximations of the R12 methods withstand the closest scrutiny: Kutzelnigg's use of the resolution of the identity and the generalized Brillouin condition to avoid many-electronic integrals remains sound. Second, it transpires that great gains in accuracy can be made by changing the dependence on the interelectronic coordinate from linear (r 12 ) to some suitably chosen short-range form (e.g., exp(−αr 12 )). Modern R12 (or F12) methods can deliver MP2 energies (and beyond) that are converged to chemical accuracy (1 kcal/mol) in triple- or even double-zeta basis sets. Using a range of approximations, applications to large molecules become possible. Here, the major developments in the field are reviewed, and recommendations for future directions are presented. By comparing with commonly used extrapolation techniques, it is shown that modern R12 methods can deliver high accuracy dramatically faster than by using conventional methods. Contents PAGE 1. Introduction 429 1.1. The origin of the problem 429 1.2. Two-electron systems 430 1.3. Explicitly correlated MP2 methods 430 1.4. Gaussian geminals 431 1.5. Exponentially correlated Gaussians 432 1.6. The transcorrelated method 433 2. R12 wavefunctions 433 2.1. Definition 434 2.2. Correlation factors 435 2.3. Projection operators 437 2.4. Levels of theory 439 2.5. Methods for open shells 440 3. Approximations of many-electron integrals 441 3.1. Exact evaluation 442 3.2. Approximations: GBC, EBC and 443 3.3. Resolution of the identity 445 3.4. Numerical quadrature 447 3.5. Density fitting 449 3.6. DF combined with RI 451 4. Examples from second-order perturbation theory 452 4.1. Technical details 453 4.2. R12 results in comparison with extrapolated values 454 4.3. Comparison between R12 and F12 results 458 5. Perspectives 461 5.1. Higher level methods 461 5.2. Local approximations 461 5.3. Conclusions 462 5.3.1. Correlation factor 462 5.3.2. Projection operator 462 5.3.3. Formulation of intermediate B 463 5.3.4. Approximating integrals 463 5.3.5. Efficiency improvements 463 Acknowledgements 463 References 464
Sum frequency generation vibrational spectroscopy (SFG-VS) has been proven to be a uniquely effective spectroscopic technique in the investigation of molecular structure and conformations, as well as the dynamics of molecular interfaces. However, the ability to apply SFG-VS to complex molecular interfaces has been limited by the ability to abstract quantitative information from SFG-VS experiments. In this review, we try to make assessments of the limitations, issues and techniques as well as methodologies in quantitative orientational and spectral analysis with SFG-VS. Based on these assessments, we also try to summarize recent developments in methodologies on quantitative orientational and spectral analysis in SFG-VS, and their applications to detailed analysis of SFG-VS data of various vapour/neat liquid interfaces. A rigorous formulation of the polarization null angle (PNA) method is given for accurate determination of the orientational parameter D = ⟨cos θ ⟩/⟨cos 3 θ⟩, and comparison between the PNA method with the commonly used polarization intensity ratio (PIR) method is discussed. The polarization and incident angle dependencies of the SFG-VS intensity are also reviewed, in the light of how experimental arrangements can be optimized to effectively abstract crucial information from the SFG-VS experiments. The values and models of the local field factors in the molecular layers are discussed. In order to examine the validity and limitations of the bond polarizability derivative model, the general expressions for molecular hyperpolarizability tensors and their expression with the bond polarizability derivative model for C 3 v , C 2 v and C ∞ v molecular groups are given in the two appendixes. We show that the bond polarizability derivative model can quantitatively describe many aspects of the intensities observed in the SFG-VS spectrum of the vapour/neat liquid interfaces in different polarizations. Using the polarization analysis in SFG-VS, polarization selection rules or guidelines are developed for assignment of the SFG-VS spectrum. Using the selection rules, SFG-VS spectra of vapour/diol, and vapour/n-normal alcohol (n∼ 1-8) interfaces are assigned, and some of the ambiguity and confusion, as well as their implications in previous IR and Raman assignment, are duly discussed. The ability to assign a SFG-VS spectrum using the polarization selection rules makes SFG-VS not only an effective and useful vibrational spectroscopy technique for interface studies, but also a complementary vibrational spectroscopy method in general condensed phase studies. These developments will put quantitative orientational and spectral analysis in SFG-VS on a more solid foundation. The formulations, concepts and issues discussed in this review are expected to find broad applications for investigations on molecular interfaces in the future.