Electron–vibration entanglement in the Born–Oppenheimer description of chemical reactions and spectroscopy

McKemmish, Laura K.; McKenzie, Ross H.; Hush, Noel S.; Reimers, Jeffrey R.

doi:10.1039/c5cp02239h

Cited by 33 publications

(46 citation statements)

References 88 publications

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“…It also forms the basis for understanding of primary charge separation in natural and artificial solar-energy harvesting systems used in plants, bacteria, organic photovoltaics and artificial photosynthesis, as well as the reverse process, light emission in organic light-emitting diodes. A critical feature of the approach is that it leads to very many analytical or numerically exactly-solvable relationships linking system properties, as recently summarized in a number of contexts [28,[43][44][45][46]. Exemplary in Hush's description of intervalence compounds is that he showed how from measurement of the visible absorption spectrum of the dye Prussian blue, it was possible to deduce kinetics parameters critical to the understanding of the charge conductivity of the material [5].…”

Section: Related Contentmentioning

confidence: 99%

Diabatic models with transferrable parameters for generalized chemical reactions

Reimers

McKemmish

McKenzie

et al. 2017

J. Phys.: Conf. Ser.

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Abstract. Diabatic models applied to adiabatic electron-transfer theory yield many equations involving just a few parameters that connect ground-state geometries and vibration frequencies to excited-state transition energies and vibration frequencies to the rate constants for electrontransfer reactions, utilizing properties of the conical-intersection seam linking the ground and excited states through the Pseudo Jahn-Teller effect. We review how such simplicity in basic understanding can also be obtained for general chemical reactions. The key feature that must be recognized is that electron-transfer (or hole transfer) processes typically involve one electron (hole) moving between two orbitals, whereas general reactions typically involve two electrons or even four electrons for processes in aromatic molecules. Each additional moving electron leads to new high-energy but interrelated conical-intersection seams that distort the shape of the critical lowest-energy seam. Recognizing this feature shows how conicalintersection descriptors can be transferred between systems, and how general chemical reactions can be compared using the same set of simple parameters. Mathematical relationships are presented depicting how different conical-intersection seams relate to each other, showing that complex problems can be reduced into an effective interaction between the ground-state and a critical excited state to provide the first semi-quantitative implementation of Shaik's "twin state" concept. Applications are made (i) demonstrating why the chemistry of the firstrow elements is qualitatively so different to that of the second and later rows, (ii) deducing the bond-length alternation in hypothetical cyclohexatriene from the observed UV spectroscopy of benzene, (iii) demonstrating that commonly used procedures for modelling surface hopping based on inclusion of only the first-derivative correction to the Born-Oppenheimer approximation are valid in no region of the chemical parameter space, and (iv), demonstrating the types of chemical reactions that may be suitable for exploitation as a chemical qubit in some quantum information processor. IntroductionThough chemical reactions involve complicated restructuring of the electronic and nuclear configuration and dynamics, it can be useful to use the simplification of envisaging reactions as occurring along a single reaction coordinate. This basic concept forms the conceptual framework of Transition-State Theory and much of modern chemistry. Modern electronic-structure calculation methods can predict wide ranges of properties to within the accuracy of experimental measurements, capturing all chemical and spectroscopic features of the system, but such approaches rarely provide insight into why processes occur and have to be repeated for each small system variation. Diabatic models can be used to interpret these calculations, however, revealing the chemical features controlling the process. Basically, simple forms, called diabatic surfaces, are used to describe the reactants and pro...

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Section: Related Contentmentioning

confidence: 99%

Diabatic models with transferrable parameters for generalized chemical reactions

Reimers

McKemmish

McKenzie

et al. 2017

J. Phys.: Conf. Ser.

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“…This led to an understanding of the effects of the Born-Oppenheimer approximation (i.e. the effects of the nuclear momentum on the electronic wave functions) across the whole range of feasible chemical processes, [35] an understanding of how quantum entanglement can be used to gauge the magnitude of Born-Oppenheimer breakdown, [64] and an understanding of what types of chemical reactions are likely to be of practical use in the construction of chemical qubits in a quantum information processor. [65] Such analytical models usually focus on just the primary nuclear motion involved in the chemical process, the reaction coordinate.…”

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The Importance of Motions that Accompany Those Occurring Along the Reaction Coordinate

Reimers

2015

Aust. J. Chem.

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The reaction coordinate is a well known quantity used to define the motions critical to chemical reactions, but many other motions always accompany it. These other motions are typically ignored but this is not always possible. Sometimes it is not even clear as to which motions comprise the reaction coordinate: spectral measurements that one may assume are dominated by the reaction coordinate could instead be dominated by the accompanying modes. Examples of different scenarios are considered. The assignment of the visible absorption spectrum of chlorophyll-a was debated for 50 years, with profound consequences for the understanding of how light energy is transported and harvested in natural and artificial solar-energy devices. We recently introduced a new, comprehensive, assignment, the centrepiece of which was determination of the reaction coordinate for an unrecognized photochemical process. The notion that spectroscopy and reactivity are so closely connected comes directly from Hush's adiabatic theory of electron-transfer reactions. Its basic ideas are reviewed, similarities to traditional chemical theories drawn, key analytical results described, and the importance of the accompanying modes stressed. Also highlighted are recent advances that allow this theory to be applied to general transformations including isomerization processes, hybridization, aromaticity, hydrogen bonding, and understanding why the properties of first-row molecules such as NH 3 (bond angle 1088) are so different to those of PH 3 -BiH 3 (bond angles 90-938). Historically, the question of what is the reaction coordinate and what is just an accompanying motion has not commonly been at the forefront of attention. In our new approach in which all chemical processes are described using the same core theory, this question becomes thrust forward as always being the most important qualitative feature to determine. Chemists mostly consider reactions using a single nuclear coordinate, the reaction coordinate.[1] A simple example of this is apparent in the London-Eyring-Polanyi-Sato [2] (LEPS) potentialenergy surface for the chlorine atom-exchange reaction [3] Clthat is shown in Fig. 1a as a function of the bond lengths R ab and R bc . The reaction coordinate indicates the lowest-energy path that connects reactants to products and is shown in blue in the figure. When a reaction occurs, molecular geometries change holistically, and this coordinate in general is therefore complex, embodying what happens to all atoms including atoms in any solvent or surrounding medium. All other motions are thought to smoothly and perhaps even instantaneously adjust to motion along the reaction coordinate. Fig. 1b displays the energy profile [3] for the chlorine atom-exchange reaction as a function of the reaction coordinate, showing how the energy increases from that at the floor of the reactant valley as the approaching atom gets closer to the molecule, going through a maximum at the reaction's transition state (TS). However, Fig. 1c shows how the distance R ac bet...

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“…For symmetric systems (E 0 ¼ 0) this parameter alone controls this localization or delocalization [205][206][207] and hence the class of a mixed-valence system, [208] but general expressions are also known. [184] For the molecules considered it ranges from 0.029 for the non-adiabatic electron transfer charge recombination process in FcPC 60 to 0.8 in CT and ammonia to 3.3 in benzene. The parameter…”

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confidence: 99%

“…This work was done in conjunction with Ross McKenzie (Physics, University of Queensland) who brought his interest in valencebond processes and quantum coherence, as well as his vast experience in studying coupled quantum nuclear-electronic problems in solid-state physics. [135,[180][181][182][183][184][185][186] Many key advances in solid-state physics [187][188][189] were made at around the same time as Hush's theory of adiabatic electron transfer was developed, and much was to be gained by merging results from the two disciplines. Also, much of the work was performed by our student Laura McKemmish as undergraduate research projects and during her honours year.…”

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confidence: 99%

“…Also, much of the work was performed by our student Laura McKemmish as undergraduate research projects and during her honours year. [135,[180][181][182][183][184][185][186]190] Previously, diabatic models had been very successful only for electron-transfer theory. For it, a simple model with just two or three parameters could qualitatively describe a wide range of spectroscopic and kinetics properties.…”

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Putting David Craig’s Legacy to Work in Nanotechnology and Biotechnology

Reimers¹

2016

Aust. J. Chem.

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left us with a lasting legacy concerning basic understanding of chemical spectroscopy and bonding. This is expressed in terms of some of the recent achievements of my own research career, with a focus on integration of Craig's theories with those of Noel Hush to solve fundamental problems in photosynthesis, molecular electronics (particularly in regard to the molecules synthesized by Maxwell Crossley), and self-assembled monolayer structure and function. Reviewed in particular is the relation of Craig's legacy to: the 50-year struggle to assign the visible absorption spectrum of arguably the world's most significant chromophore, chlorophyll; general theories for chemical bonding and structure extending Hush's adiabatic theory of electron-transfer processes; inelastic electron-tunnelling spectroscopy (IETS); chemical quantum entanglement and the Penrose-Hameroff model for quantum consciousness; synthetic design strategies for NMR quantum computing; Gibbs free-energy measurements and calculations for formation and polymorphism of organic self-assembled monolayers on graphite surfaces from organic solution; and understanding the basic chemical processes involved in the formation of gold surfaces and nanoparticles protected by sulfur-bound ligands, ligands whose form is that of Au 0 -thiyl rather than its commonly believed Au I -thiolate tautomer.

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Electron–vibration entanglement in the Born–Oppenheimer description of chemical reactions and spectroscopy

Cited by 33 publications

References 88 publications

Diabatic models with transferrable parameters for generalized chemical reactions

Diabatic models with transferrable parameters for generalized chemical reactions

The Importance of Motions that Accompany Those Occurring Along the Reaction Coordinate

Putting David Craig’s Legacy to Work in Nanotechnology and Biotechnology

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