Recent theoretical studies of [m + n], m = 2, 3, and 4, cycloaddition reactions involving acetylene derivatives, n = 2, have been recompiled in this review. They include Diels-Alder and [3 + 2] cycloaddition reactions. The role of polar solvents and Lewis acid catalysts in these [m + 2] cycloaddition reactions are analysed. The studies carefully selected for this review attempt to provide an overview of the main approaches used to study the reactivity of C - C triple bonds as the n = 2 component in cycloaddition reactions, and a special focus has been given to the studies carried out within the Molecular Electron Density Theory as it provides insight into molecular mechanisms and reactivity in organic chemistry.
Recent advances in our knowledge of heavy Rydberg and ion-pair states are critically reviewed, with emphasis placed on the close kinship between the two. Heavy Rydberg states are long-range vibrational states, reaching far beyond >> 10(2) angstrom for higher levels. Enhanced chemical reactivity and efficient energy transfer are frequently encountered. Unusual physical properties result from the large dipole moments, including laser-induced reactions and amplified spontaneous emission, and are discussed in the context of the underlying electronic structure. Heavy Rydberg states have a rich spectroscopy which is amenable to quantum defect analysis, as illustrated for a wide range of UV and VUV spectra previously analyzed in terms of Dunham coefficients. The lifetimes of heavy Rydberg states can be long, enabling them to be isolated in cryogenic matrices or as high angular momentum states in the gas phase. Heavy Rydberg and electronic Rydberg states often occupy the same energy region and this, together with the high density of heavy Rydberg vibrational levels, leads to vibronic mixing and numerous perturbations that are a fertile field for analysis by multichannel quantum defect theory and reactive scattering calculations.
Three-body recombination, or ternary association, is a termolecular reaction in which three particles collide, forming a bound state between two, whereas the third escapes freely. Three-body recombination reactions play a significant role in many systems relevant to physics and chemistry. In particular, they are relevant in cold and ultracold chemistry, quantum gases, astrochemistry, atmospheric physics, physical chemistry, and plasma physics. As a result, three-body recombination has been the subject of extensive work during the last 50 years, although primarily from an experimental perspective. Indeed, a general theory for three-body recombination remains elusive despite the available experimental information. Our group recently developed a direct approach based on classical trajectory calculations in hyperspherical coordinates for three-body recombination to amend this situation, leading to a first principle explanation of ion-atom-atom and atom-atom-atom three-body recombination processes. This review aims to summarise our findings on three-body recombination reactions and identify the remaining challenges in the field.
Graphyne (GYs) is a class of 2D carbon allotropes with highly p-conjugated structure consisting of sp- and sp(2)-hybridized car -bon atoms, leading to unique molecular configuration and electronic structure, showing excellent electrical, mechanical, photo-electric and semiconducting properties, and having great potentials in gas-separation, chemical-reaction catalysis, energy-storage, and sensor applications. GYs can be classified into several struc-tural forms, including graphdiyne (GDY) and graphtriyne (GTY). Structural characterisation is crucial for understanding the relation-ship between their structure and properties. At present, quite a few experimental methods, including scanning electron microscopy, transmission electron microscopy, X-ray diffraction, X-ray photoelectron spectroscopy, nuclear magnetic resonance, infrared and Raman spectroscopies, have been used to characterise the structure of GYs. This review focuses on the structural and vibrational characterisations of GYs using Infrared (IR) and Raman spectroscopies. The vibrational signature, including linear and nonlinear IR characteristics of the periodically appearing C =C bond, will be reviewed. The intensity enhanced C=C stretching mode as an IR marker in monitoring vibrational energy redistribution and transfer in GYs will be discussed. This review will shed light on the understanding of the structures and structural distributions, and vibrational energy-transfer pathways of the GY systems, which are important for their design, fabrication and applications.
This review aims to highlight the most recent and remarkable advances in our laboratory in designing efficient and long-lasting tunable dye lasers from the visible green region to the far-red-NIR edge. In recent years, we have synthesised, characterised, and applied a set of organic molecules covering this spectral region. The well-known BODIPY dye was selected as the photoactive scaffold owing to its rich and versatile chemistry. This modern dye allows deep and selective functionalization, which in turn modulates its photophysical properties. A deep understanding of the interplay between the molecular structure and photonic performance, as well as the unravelling of the key underlying photophysical mechanisms, is essential for designing photoactive dyes endowed with improved laser performance, outperforming the corresponding commercially available dyes in each spectral region. The design was focused on the chemical modification of the boron-dipyrrin core, as well as on the combination of dissimilar BODIPYs into a single molecular structure. Indeed, these complex and challenging multichromophoric assemblies exemplify a new generation of laser dyes with enhanced photonic performance. Following that, we provide an overview of the main structural and photophysical guidelines governing laser performance.
Photoinduced hydrogen-atom-transfer processes presented in this review commonly occur in heterocyclic oxo and thione compounds isolated in low-temperature solid-noble-gas or solid-nitrogen environment. Analogous phototransformations were found for simple amides, thioamides and selenoamides. Such photoinduced hydrogen-atom-transfer processes, concerning compounds with no intramolecular hydrogen bond, are fundamentally different from the well-known Excited State Intramolecular Proton Transfer (ESIPT) phenomena. Attempts aimed at finding a mechanism, explaining phototautomerism in compounds where high bariers separate minima of the tautomers in S-0 and S-1, gave rise to introduction of the hydrogen-atom detachment on the surface of the repulsive pi sigma* states as a new paradigm of photochemical behaviour of heterocyclic compounds. UV-induced hydrogen-atom-transfer processes allowed photoproduction of new molecular structures never generated using other methods. Transfer of two or three hydrogen atoms was also discovered for some compounds. In most cases, hydrogen atoms were transferred from N-H groups of UV-excited molecules to such heteroatoms as O, S, Se or other N. For some species, hydrogen atoms from other groups, such as OH, SH or NH2, underwent UV-induced transfer to other atom of the molecule. This is the basis of photoreversibility observed for some oxo <-> hydroxy, thione <-> thiol or amino <-> imino phototransformations.
Although the reaction He+ + H-2 -> He + H-2 (+) is highly exothermic and the reaction He + H-2 (+) -> HeH+ + H is endothermic, the latter reaction occurs more readily than the former because of orbital symmetry considerations. For the same reasons, the dissociative charge transfer process He+ + H-2 -> He + H + H+ is more likely to occur. Availability of highly accurate ab initio potential-energy surfaces for the ground electronic state of (He, H-2 (+)) has enabled dynamical studies, classical as well as quantum mechanical. While the experimentally observed vibrational enhancement of the exchange reaction and the collision-induced dissociation process has been well accounted for by theory, the narrow sharp reactive-scattering resonances reported in quantum mechanical scattering studies have eluded experimental verification. The isotope effect in (He, HD+) collisions seems to be a sensitive probe of the interaction potential. Although the possible role of the first excited electronic state of (He, H-2 (+)) in the collision-induced dissociation process has been discussed in the literature, the role of nonadiabatic coupling terms between different electronic states in influencing the dynamics in the system remains to be investigated fully.
In this review, we give a comprehensive comparison of the most widely used coherent state (CS) based methods to solve the time-dependent Schrodinger equation (TDSE). Starting from the fully variational coherent states (VCS) method, after a first approximation, the coupled coherent states (CCS) method can be derived, whereas an additional approximation leads to the semiclassical Herman-Kluk (HK) method. We numerically compare the different methods with another one, based on a static rectangular grid of coherent states (SCS), by applying all of them to the revival dynamics in a 1D Morse oscillator, with a special focus on the number of basis states (for the CCS and HK methods the number of classical trajectories) needed for convergence and the related issue of tight frames, which in principle allow the usage of CSs as if they were orthogonal. Different discretisation strategies for the occurring phase space integrals for systems with more degrees of freedom are also discussed and the apoptosis procedure that allows to circumvent the linear dependency problem in the VCS method is reviewed. The Holstein molecular crystal model serves to further illustrate the latter point.
We present a broad-brush picture of the covalent and electrostatic interactions controlling the structures and stabilities of cluster anions and discuss how one should think about chemical bonding in these species. Accordingly, the review emphasises the broad general trends, which stem from the aggregate nature of clusters rather than from the individual chemistry of the compounds comprising the specific systems considered. The offered perspective relies on a coupled-monomers approach, which assumes first-order separability of the inter- and intra-monomer interactions. It effectively treats the cluster components as interlocking but self-contained building blocks. A Hückel-style formalism, adapted specifically to a mixed network of covalent and solvation interactions in cluster anions, offers general insight into the cooperation and competition between the multitudes of interactions implicated in solvated environments.
In recent years, combined experimental and theoretical efforts have brought valuable information on the kinetics of reactive collisions between molecular hydrogen and an electronically excited atom X (where X = C (D-1) , N (D-2) , O (D-1) or S (D-1)). These four reactions have been comparatively studied together in numerous occasions in the past due to the similar importance of complex-forming mechanisms found in their overall dynamics. In this work, we compile the most updated information on these investigations making a special emphasis from the theoretical side on statistically based techniques, in an attempt to test the possible insertion nature of the overall dynamics. Besides a description of the experimental details of the kinetics investigation, a comparison of the measured rate constants over a temperature range between 50 and 300 K with the most recent theoretical calculations is presented.
Traditionally, over the last century, approaches used to elucidate the 'static' and the 'dynamic' nature of chemical bonding have been fundamentally different. The 'static' nature of chemical bonding has been explored using either valence bond or molecular orbital theory with the time-independent atomic or molecular orbitals. The 'dynamic' nature of chemical bonding, on the other hand, has been explored under the name 'chemical dynamics' through the notion of a transition state (rearrangement of nuclei). Understanding of the 'dynamic' nature of chemical bonding could, however, be developed through a time-dependent change of atomic and molecular orbitals (or broadly the time-dependent electron density). In the present review article, we have presented our state-of-the-art understanding of attosecond dynamics of chemical bonding from a general chemical point of view. We have demonstrated our viewpoints on dynamics of covalent and noncovalent bonds using both time-dependent natural bond orbital and canonical molecular orbitals. Finally, we have demonstrated the efficacy of high harmonic generation spectroscopic investigation to decipher attosecond charge migration through noncovalent bonds. Several chemically important systems, in which attosecond dynamics can play an important role, are discussed.
This review summarizes the state-of-the-art knowledge of heme ligation in the gas phase. The unique aspect of the gas phase approach is to allow a step-by-step ligation of heme and thus enables the analysis of the properties of -four, -five and -six coordinate hemes in vacuo, under conditions directly comparable with quantum calculations. This approach also allows the characterization of situations uncommon in Nature, completing the coordination spectrum of hemes: four coordinate heme and protonated heme, an intermediate between ferrous and ferric heme. Therefore, a complete set of systems is described for the ferrous and ferric cases and there is no discontinuity between the two oxidation states of iron, so that the same mechanisms are at work, donation and back donation of different strengths depending upon the ligand. The similarity of ligation properties in ferrous and ferric hemes is consistent with calculations of the electron density at the Fe atom level, rather independent of the formal oxidation state in contrast with the porphyrin cycle. Hemes spin states have been reviewed, for they identify the electronic distribution of the metal. In ligated ferrous and ferric hemes, we find that binding energy measurements combined with spectroscopy describe their properties most effectively.
The interaction of electronic and nuclear motion - broadly categorised as 'vibronic coupling' - plays a number of roles in areas that range from molecular dynamics to electronic spectroscopy. Additionally, these phenomena pose significant challenges to both computational electronic spectroscopy and quantum chemistry, as the usual approximations (Franck-Condon and Born-Oppenheimer) are often rendered unsatisfactory. After beginning with a broad overview of vibronic coupling effects and some computational strategies for characterising them, the review discusses how these effects are manifested in various types of spectra. Particular emphasis is given to fine-structure effects in Jahn-Teller systems that arise from couplings involving rotational, orbital and spin angular momenta. Unlike overall vibronic level structure, which has been quite well studied both theoretically and experimentally, these more subtle effects are seen only at high (rotationally-resolved) resolution, and are less well understood. The review gives a detailed description of the quantum-mechanical origin of these splittings and provides some computational strategies for predicting them. A broad overview is given of families of Jahn-Teller active molecules that have been investigated experimentally and theoretically. Detailed discussion is given for two JT-active radicals where theory and experiment are compared at both low and high resolution: cyclopentadienyl (C5H5) and methoxy (CH3O).
Dust grains play a central role in the physics and chemistry of cosmic environments. They influence the optical and thermal properties of the medium due to their interaction with stellar radiation; provide surfaces for the chemical reactions that are responsible for the synthesis of a significant fraction of key astronomical molecules; and they are building blocks of pebbles, comets, asteroids, planetesimals, and planets. In this paper, we review experimental studies of physical and chemical processes, such as adsorption, desorption, diffusion and reactions forming molecules, on the surface of reliable cosmic dust grain analogues as related to processes in diffuse, translucent, and dense interstellar clouds, protostellar envelopes, planet-forming disks, and planetary atmospheres. The information that such experiments reveal should be flexible enough to be used in many different environments. In addition, we provide a forward look discussing new ideas, experimental approaches, and research directions.
It is remarkable that time delay is an experimentally measurable quantity, but time itself is not. Time delay in quantum collisions and in photoionisation/photodetachment of atomic and molecular systems is reviewed in this paper. Wigner-Eisenbud formalism of time delay in quantum collision of a wavepacket with a target is discussed. Its equivalence with Smith's formalism of time delay, based on an independent basis for time delay in terms of excess particle density in the collision zone, is demonstrated. Similarity and difference between quantum collision of an electron with a positive atomic/molecular ion and photoionisation/photodetachment of a neutral atom/molecule are discussed, and the underlying quantum dynamics involving the time-reversal symmetry between solutions with outgoing and ingoing wave boundary conditions is pointed out to interpret photoionisation/photodetachment as half-scattering. This relationship is subsequently taken advantage to extend the formalism of Wigner-Eisenbud-Smith time delay in photoionisation/photodetachment. The measurability of time delay is accounted for in terms of a self-adjoint quantum operator that characterises it, even if there is no such operator for time itself. A few illustrative examples of theoretical and experimental studies of time delay are given to indicate outstanding advances made in this field in the last two decades.
While existing ion mobility calculators are capable of feats as impressive as calculating collision cross sections (CCS) within a few per cent and within a very reasonable time, the simplifications assumed in their estimations precludes them from being more precise, potentially overreaching with respect to the interpretation of existing calculations. With ion mobility instrumentation progressively reaching resolutions of several hundreds to thousands (accuracy in the range of ∼0.1%), a more accurate theoretical description of gas-phase ion mobility becomes necessary to correctly interpret experimental state-of-the-art separations. This manuscript entails an effort to consolidate the most relevant theoretical work pertaining to ion mobility within the ‘free molecular’ regime, describing in detail the rationale for approximations up to the two-temperature theory, using both a momentum transfer approach as well as the solution to the moments of the Boltzmann equation for the ion. With knowledge of the existing deficiencies in the numerical methods, the manuscript provides a series of necessary additions in order to better simulate some of the separations observed experimentally due to second-order effects, namely, high field effects, dipole alignment, angular velocities and moments of inertia, potential interactions and inelastic collisions among others.
