Non-adiabatic Reactions. Rotation about the Double Bond<sup>*</sup>

Magee, John L.; Shand, W. A.; Eyring, Henry

doi:10.1021/ja01848a012

Cited by 101 publications

(62 citation statements)

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“…These authors proposed their well-known "singlet" and "triplet" mechanisms according to whether the reaction proceeds entirely via the singlet-state PES or involves a switch to the triplet state at the torsion angle less than 90". They obtained a conclusion that the isomerization of ethylene-d, should proceed by the singlet mechanism in the absence of effective collisional perturbations, since the soc connecting the s and T states is very small [51]. The ab initio soc calculations [54] supported this result.…”

Section: Paramagnetic Exchange Spin-catalysismentioning

confidence: 91%

“…The rare earth ions accelerate the rate of decarboxylation of malonic acid derivatives [ll]. An example of particular relevance is the catalysis of &-trans isomerization reactions by paramagnetic gases and ions in solution [51]. The simplest example of this type of spin-catalysis is represented by the ethylene-d, gas-phase thermal cis-trans isomerization reaction, catalyzed by NO and 0, paramagnetic molecules.…”

Section: Paramagnetic Exchange Spin-catalysismentioning

confidence: 99%

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Spin-catalysis phenomena

Минаев

Ågren

1996

Int. J. Quantum Chem.

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DOn the grounds of results from quantum chemical calculations, we introduce spin-catalysis as a new concept, and classify different types of chemical reactions according to this concept. We propose a general definition of spin-catalysis as phenomena in which chemical reactions are promoted by substances which assist in overcoming spinprohibition or in which the activation barrier is lowered through spin uncoupling induced by a paramagnetic catalyst. A number of known phenomena fall into this definition and can be classified according to two main categories: (1) spin-orbit coupling induced and (2) paramagnetic-exchange induced spin-catalysis. Other types of spincatalysis are also discussed: (3) processes with participation of substances which assist in (a) photochemical and Cb) thermal or electrochemical generation of active particles by energy and-by electron transfer [radicals, diradicals, O,(' La,1, etc.], reacting further without spin-prohibition: (4) processes induced by an external magnetic field. Processes (3) and (4) are quite general and well known; their inclusion in the spin-catalysis classification does not introduce any new findings for the chemical kinetics, but the general features, which unite them with the important catalytic processes of the first two types, serve as a useful guide in catalysis theory. Few models of spin-catalytic processes have been simulated for the purpose of illuminating the principles of spin-catalysis; the cis-trans isomerization of ethylene catalyzed by molecular oxygen, the external heavy atom effect in ethylene photochemistry, and some others.

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Section: Paramagnetic Exchange Spin-catalysismentioning

confidence: 91%

Section: Paramagnetic Exchange Spin-catalysismentioning

confidence: 99%

Spin-catalysis phenomena

Минаев

Ågren

1996

Int. J. Quantum Chem.

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“…Una teoría que intenta explicar el mecanismo cinético de esta clase de reacciones, 21 clasifica a éstas, en isomerizaciones de alta (E ≈ 45 kcal mol ) energía de activación. La primera procede por un estado electrónico singulete a través de todo el proceso, sin desacoplamiento de los espines de los electrones del enlace π; mientras que la segunda, a aproximadamente Φ 1 = 90°, involucra una transición a un estado triplete (biradical), pero la baja probabilidad de éste evento (isomerización no adiabática) reduce la velocidad de reacción a aproximadamente el mismo valor que el primer mecanismo.…”

Section: Figura 3 Diagrama De Contorno De La Pes Relajada Del áCido unclassified

Estudio teórico de la isomerización del ácido maleico en ácido fumárico: un enfoque basado en el concepto de superficie de energía potencial

Ugarte¹,

Salgado²,

Martínez³

et al. 2010

Quím. Nova

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POTENTIAL ENERGY SURFACE. A comparative study based on potential energy surfaces (PES) of 2-butanedioic and hypothetic 2-butanedioic/HCl acids is useful for understanding the maleic acid isomerization. The PES enables locating conformers of minimum energy, intermediates of reactions and transition states. From contour diagrams, a set of possible reaction paths are depicted interconnecting the proposed structures. The study was carried out in absentia and in the presence of the catalyst (HCl), using an solvatation model provided by the Gaussian software package. Clearly, the effect of HCl is given by new reaction paths with lower energetic barriers in relation to the reaction without catalyzing.Keywords: potencial energy surface; isomerization; maleic acid. INTRODUCCIÓNUna clase particular de isómeros geométricos (cis-trans) son aquellos compuestos que difieren en el ordenamiento espacial de los grupos ligados a un enlace doble. En estos isómeros, los dos grupos unidos al átomo de carbono de un enlace doble son diferentes entre sí; lo mismo es válido para el otro átomo de carbono; sin esta condición, los dos isómeros serían indistinguibles. Normalmente, existirá una rotación libre y una restringida alrededor de un enlace simple y uno doble, respectivamente. Así, los grupos que se unen a átomos de carbono de un enlace doble permanecerán fijos a uno u otro lado del enlace. Al presentar propiedades diferentes, los isómeros cis-trans pueden ser distinguidos uno del otro.La proporción en que coexisten en equilibrio, a una determinada temperatura, las diferentes poblaciones de isómeros depende de sus diferencias en energías libres, ΔGº. No obstante, el equilibrio de interconversión se producirá siempre y cuando la barrera rotacional, representada por la energía libre de activación (ΔG ‡ ), no sea demasiado alta. Un ejemplo clásico de isomería cis-trans es la representada por el ácido 2-butenodioico, que en condiciones normales es un sólido. En este compuesto el isómero cis-2-butenodioico recibe el nombre de ácido maleico, mientras que el trans-2-butenodioico se denomina ácido fumárico.El ácido fumárico se distribuye ampliamente en la naturaleza. Se encuentra, por ejemplo, en plantas del género fumaria (Fumaria officinalis), en el liquén Cetraria islandica y en hongos de las especies Boletus y Agaricus.1 La forma ionizada del ácido fumárico (fumarato) es un intermediario clave en el ciclo de Krebs. El ácido maleico es un producto de síntesis obtenido a partir de la hidrólisis del anhídrido maleico; este último, se produce a partir de la oxidación catalítica (V 2 O 5 , aire, 400-500 ºC) del benceno.De acuerdo a los valores del calor de formación de ambos isóme-ros en fase sólida ( Tabla 1) 2 se deduce que el isómero cis es ≈ 4.9 kcal mol -1 menos estable que el isómero trans. Esta diferencia se explica sí consideramos los posibles puentes de hidrógeno intermoleculares en ambos isómeros (dos para la forma trans y uno para la forma cis) y la proximidad de los dos grupos carbóxilo en el isómero cis; si bien la repulsión est...

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“…Examples of nonadiabatic reactions are the cis-trans isomerizations of certain substituted ethylenes. Figure 1 shows a schematic potential-energy profile (22) for such a system, the energy being plotted against the angle of twist about the double bond. There is a triplet excited state having a minimum corresponding to a twist of 90°so that there are two alternative reaction paths.…”

Section: "Adiabatic" As Used In Thermodynamicsmentioning

confidence: 99%

The meaning of "adiabatic"

Laidler¹

1994

Can. J. Chem.

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This paper is dedicated to Professor John C. Polanyi or1 the occasiorl ofhis 65th birtlldcryKEITH J. LAIDLER Can. J. Chem. 72, 936 (1 994). In chemical kinetics the word "adiabatic" has come to refer to a process in which there is no change of quantum state, a meaning that has no connection with the thermodynamic meaning or with the etymology of the word. This usage is traced to early discussions of reversible adiabatic processes in classical and quantum mechanics, processes that came to be referred to simply as adiabatic.KEITH J. LAIDLER Can. J. Chem. 72,936 (1994). En cinktique chimique, le mot adiabatiques se refkre aujourd'hui a un processus dans lequel il n'y a pas de changement d'ktat quantique, une signification qui n'a aucune relation avec le sens thermodynamique ou avec l'itymologie du mot. On peut retracer I'origine de ce nouvel usage dans les premieres discussions des processus adiabatiques re'versibles de la micanique classique ou quantique auxquels on a fini parse rifkrer simplement adiabatiques.[Traduit par la ridaction] Introduction In chemical kinetics the word "adiabatic" has long been used to refer to a chemical reaction in which the products remain in the same quantum state as the reactants. This usage has created some puzzlement as it does not seem to have, and indeed does not in fact have, any connection with the etymology of the word adiabatic, or with the appropriate use of the word in thermodynamics. The word adiabatic means "not passing through," and in thermodynamics refers to a condition imposed on a system, a condition that prevents any passage of heat into or out of the system. In its kinetic usage the word relates not to a condition imposed on the system, but rather to an inherent property of a chemical process, namely its tendency under certain conditions to occur without any change in quantum states.A consideration of how the kinetic meaning of adiabatic developed is of some interest in the history of science, and is also instructive in leading to a better understanding of some of the fundamental principles of statistical mechanics and quantum mechanics. The trail begins with a paper (1) published in 1866 by Ludwig Boltzmann, in which he mistakenly claimed to have derived the second law of thermodynamics on the basis of classical mechanics. Within a few years, thanks to his study of Clerk Maxwell's papers, he had realized that his argument was invalid, and that the second law must be explained along statistical lines. In his 1868 paper, however, he had considered a reversible adiabatic process, and it is from that argument that the winding and sometimes indistinguishable trail eventually led, through several false turns, to the inappropriate use of the word adiabatic in modern kinetics.

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Non-adiabatic Reactions. Rotation about the Double Bond^*

Cited by 101 publications

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Spin-catalysis phenomena

Spin-catalysis phenomena

Estudio teórico de la isomerización del ácido maleico en ácido fumárico: un enfoque basado en el concepto de superficie de energía potencial

The meaning of "adiabatic"

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Non-adiabatic Reactions. Rotation about the Double Bond*

Cited by 101 publications

References 0 publications

Spin-catalysis phenomena

Spin-catalysis phenomena

Estudio teórico de la isomerización del ácido maleico en ácido fumárico: un enfoque basado en el concepto de superficie de energía potencial

The meaning of "adiabatic"

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Non-adiabatic Reactions. Rotation about the Double Bond^*