Showing papers in "International Reviews in Physical Chemistry in 2015"

Quantum dynamics simulations using Gaussian wavepackets: the vMCG method

Abstract: Gaussian wavepacket methods are an attractive way to solve the time-dependent Schrodinger 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.

The physical chemistry of Criegee intermediates in the gas phase

Abstract: 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 r...

Cryogenic ion trap vibrational spectroscopy of hydrogen-bonded clusters relevant to atmospheric chemistry

Abstract: 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.

Atom addition reactions in interstellar ice analogues

Abstract: 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 molec...

Spatially-controlled complex molecules and their applications

Abstract: 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.

Metal extraction to ionic liquids: the relationship between structure, mechanism and application

Abstract: Ionic liquids (ILs) have recently emerged as an extremely promising medium for the extraction of metals from aqueous phases. ILs are salts that are liquid at room temperature, and have physical and chemical properties that make them potentially valuable replacements for organic molecular solvents. However, that same ionic character gives rise to extraction mechanisms that have no analogue in molecular liquids. While metals generally exist in hydrophobic neutral complexes in molecular liquids, charged complexes incorporating hydrophobic ligands may be highly soluble in IL phases. Further, the ionic character of ILs permits ion exchange mechanisms involving component ions of the IL, raising the possibility that the extraction process may degrade the IL and contaminate the aqueous phase. The purpose of this review is to provide a broad overview of metal extraction from aqueous to IL phase, using the extraction mechanism as a common basis for analysis of extraction phenomena that might otherwise appear quite ...

Nature and strength of sulfur-centred hydrogen bonds: laser spectroscopic investigations in the gas phase and quantum-chemical calculations

Abstract: The importance of Sulfur centred hydrogen bonds (SCHBs) cannot be underestimated given the current day knowledge of its non-covalent interactions prevalent in many biopolymers as well as in organic systems. Based on the distance/angle constraints available from the structural database, these interactions have been interchangeably termed as van der Waals/hydrogen bonded complexes. There is a lack of sufficient spectroscopic evidence that can unequivocally term these interactions as hydrogen bonding interactions. In this review we present laser spectroscopic investigations of isolated binary complexes of H-bond donor-acceptor molecules containing Sulfur atom. The complexes were formed using supersonic jet expansion method and the IR/UV spectroscopic investigations were carried out on mass selected binary complexes. The pertinent questions regarding SCHBs addressed herein are (1) Is electronegativity the controlling factor to be a potent H-bond donor/acceptor? (2) How do SCHBs compare with their oxygen count...

Reaction dynamics of oxygen atoms with unsaturated hydrocarbons from crossed molecular beam studies: primary products, branching ratios and role of intersystem crossing

Abstract: We review the progress made in the understanding of the dynamics of the reactions of ground state oxygen atoms, O(3P), with unsaturated hydrocarbons (acetylene, ethylene, allene, propyne and propene) which are of great relevance, besides from a fundamental point of view, in combustion chemistry and of interest also in atmosphere- and astro-chemistry. Advances in this area have been made possible by an improved crossed molecular beams (CMBs) instrument with rotating mass spectrometric detection and time-of-flight analysis which features product detection by low-energy electron soft-ionisation for increased sensitivity and universal detection power. This apparatus offers the capability of identifying virtually all primary reaction channels, characterising the dynamics, and determining the branching ratios (BRs) for these polyatomic multichannel nonadiabatic reactions. The reactive scattering results are rationalised with the assistance of theoretical information from other laboratories on the stationary poi...

Ultrafast electronic dynamics in helium nanodroplets

Abstract: Helium nanodroplets have emerged as a test bed for the study of isolated quantum liquids and as an ideal matrix for trapping atoms and molecules in a weakly interacting, cryogenic environment Their high transparency at visible and infrared wavelengths facilitates the study of dissolved species with traditional spectroscopy techniques At photon energies above ~21 eV, however, the droplets themselves begin to absorb to form complex excited states that have proven a challenge for both experiment and theory A variety of frequency- and time-domain methods have been used to characterise electronically excited droplet states and their relaxation channels This review focuses on a recent series of time-domain experimental studies that have revealed several phenomena such as interband relaxation dynamics within the droplet environment, and provided deeper insight into previously detected relaxation channels, including the ejection of Rydberg atoms (He*) and molecules (), the dynamics of highly excited droplet s

Formation of resonantly stabilised free radicals via the reactions of atomic carbon, dicarbon, and tricarbon with unsaturated hydrocarbons: theory and crossed molecular beams experiments

Abstract: Resonance stabilised free radicals (RSFRs) play an important role in the growth of polycyclic aromatic hydrocarbons and ultimately in the production of soot and carbonaceous particles in combustion flames, in the interstellar medium, and in planetary atmospheres. This article reviews extensive experimental crossed molecular beams and theoretical ab initio/Rice–Ramsperger–Kassel–Marcus studies in the last two decades of the reactions of atomic carbon, C(3P), dicarbon, C2(X1Σg+/a3Πu), and tricarbon, C3(X1Σg+), with unsaturated hydrocarbons, from acetylene to benzene, showing that the reactions form various types of RSFR via Cn(n = 1–3)-for-H, Cn-for-CH3, and Cn-for-CxHy exchange mechanisms. The RSFRs produced in these reactions include CxH (x = 1–8), propargyl (C3H3) and its substituted analogues, 2,4-pentadiynyl-1 (i-C5H3) and 1,4-pentadiynyl-3 (n-C5H3) together with their methyl substituted counterparts, butatrienyl (i-C4H3) and its substituted analogues, and hexenediynyl, i-C6H3, as well as cyclic five-,...

Modelling the assembly of nanoporous silica materials

Abstract: We present a perspective on the molecular modelling of nanoporous silica material synthesis. We focus on two classes of materials: microporous zeolite materials in their all-silica forms, and ordered mesoporous silica materials. Several approaches have provided insight into the synthesis processes. These approaches range from quantum chemistry modelling of silica polymerisation to molecular simulations of ordered mesoporous silica assembly, and consider physical and chemical phenomena over several lengths and time scales. Our article focuses on models of porous silica material formation based on the assembly of corner-sharing tetrahedra, which we illustrate with applications to silica polymerisation, the formation of microporous crystals and the formation of ordered mesoporous materials. This is a research area where theoretical developments must closely align with experimentation. For this reason, we also devote a significant component of the present review to a survey of key developments in the experime...

Computational methods to study the formation of small molecules by radiative association

Abstract: To form a stable molecule by association of two colliding fragments, energy must be removed or else the fragments will eventually dissociate again. Energy can be removed by a third body and by emission of a photon, where the latter process is termed radiative association. Radiative association is a ubiquitous process for forming molecules, albeit not so well known as on Earth it is normally outcompeted by three body collisions. In interstellar space however, particularly in regions with little dust (few grains), it can be important. There are only few experimental studies of radiative association as the process is improbable and therefore hard to measure. We will briefly mention the experimental work but our main focus is on theoretical approaches to calculate radiative association cross sections and thermal rate constants. We limit the descriptions to the formation of diatomic molecules. We begin with an introduction to and overview of radiative association. This is followed by a brief section on how cro...

Structure and dynamic role of conical intersections in the πσ*-mediated photodissociation reactions

Abstract: Conical intersection as a dynamic funnel in nonadiabatic transition dictates many important chemical reaction outputs such as reaction rates, yields, and energy disposals especially for chemical reactions taking place on electronically excited states. Therefore, the energetics and topology of conical intersections have been subjected to intensive theoretical and experimental studies for decades as these things are the keys to understanding and controlling nonadiabatic transitions which are ubiquitous in nature. In this article, we focus on πσ*-mediated photodissociation reactions of thiophenols and thioanisoles. Interestingly, for these chemical systems, the nonadiabatic transition probability can be precisely measured as a function of the excitation energy, giving a great opportunity for spectroscopic characterization of the multi-dimensional conical intersection seam that governs the nonadiabatic transition dynamics of polyatomic molecules. The passage of the reactive flux in the proximity of the conica...

Excitation energy transfer in artificial antennas: from photoactive materials to molecular assemblies

Abstract: The development of nanometrically templated artificial light harvesting antennas and energy transfer devices is a highly active area with outstanding challenges. The herein presented review deals with the design of photoactive nanomaterials and multichromophoric arrays looking towards the development of artificial antenna systems. In particular we have focused in the conditions which rule the excitation energy transfer processes in each case. To this aim, a wide variety of luminescent fluorophores encapsulated into either inorganic or organic hosts, as well as molecular systems based on scaffolding of suitable laser dyes have been deeply studied. The main goal is to design systems which harvest the light over a broad spectral region (in particular the ultraviolet-visible section of the electromagnetic spectrum) and transfer it to the target place and with a desired energy (especially in the red edge of the visible) via successive energy transfer hops. To this purpose, three different approaches have been ...

Ultrafast 2D-IR spectroscopy of haemoproteins

Abstract: Applications of ultrafast two-dimensional infrared (2D-IR) spectroscopy to study the structural dynamics of haem-containing proteins are reviewed. The 2D-IR experiments discussed exploit diatomic ligands bound to the haem as reporters on the dynamic protein environment in the electronic ground-state. This is possible because fluctuations of the protein give rise to inhomogeneous broadening of the ligand stretching vibrational mode that is manifest as spectral diffusion in a time-resolved 2D-IR measurement. Methods for measuring and quantifying spectral diffusion data are introduced, prior to a discussion of recent results focussing on the influence of protein structure, water ingress into the haem pocket and substrate binding on the measured dynamics. Particular emphasis will be placed on proteins featuring the ferric oxidation state of the haem ligated by a nitric oxide molecule, though comparisons with other haem systems will be drawn throughout.

Diagrammatic theories of anharmonic molecular vibrations

Abstract: In this review, we summarise recent developments in our laboratory in the field of many-body quantum-mechanical calculations of the anharmonic vibrational structure of molecules Our size-extensive vibrational self-consistent field (XVSCF) and size-extensive second-order many-body perturbation (XVMP2) methods are, unlike their parent methods (VSCF and VMP2), defined in diagrammatic formulations of the energies and Dyson self-energies, leading to manifestly size-consistent expressions for zero-point energies and anharmonic vibrational frequencies calculable with much greater efficiency The effective one-mode potentials of XVSCF are quadratic and hence the Schrodinger equation for each mode can be solved analytically, unlike VSCF, where a basis-set expansion of wave functions on more complex one-mode potentials need to be performed; VSCF potentials and their minima (anharmonic geometry) are shown to reduce to the quadratic potentials and their minima (also given analytically) of XVSCF in the thermodynamic