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Naturaleza no adiabática de los efectos de sustitución en azobenceno

Jacob Jan van der Wal¹, Roman Yu Peshkov¹, Jorn D Steen¹

¹Department of Chemistry - Ångström Laboratory, Uppsala University, Uppsala, Sweden.

Angewandte Chemie (International ed. in English)

|February 20, 2026

Resumen

La isomerización térmica Z → E en azobencenos sigue un mecanismo de rotación no adiabática, no un cambio de vía. Esto explica los efectos de sustitución en las velocidades de reacción, aclarando un debate centenario.

Área de la Ciencia:

Química Orgánica
Cinética Química
Fotoquímica

Sus antecedentes:

Se debate el mecanismo de la isomerización térmica Z → E en azobencenos.
Los mecanismos propuestos incluyen inversión, rotación y vías no adiabáticas.
Los efectos de los sustituyentes en las velocidades de reacción muestran un comportamiento no lineal.

Objetivo del estudio:

Evaluar experimentalmente el origen de la dependencia no lineal del sustituyente en la isomerización de azobenceno.
Determinar el mecanismo operativo para la isomerización térmica Z → E.
Aclarar el debate centenario sobre la isomerización de azobenceno.

Principales métodos:

Análisis cinético sistemático.
Evaluaciones de Eyring y isocinéticas dependientes de la temperatura.
Análisis de azobencenos para-sustituidos.
Estudios computacionales utilizando métodos de referencia múltiple y de referencia única.

Principales resultados:

Se observaron entropías de activación uniformemente negativas en todos los sustituyentes.
La correlación de Hammett en "V" surge de la insuficiencia de la escala "σp".
El parámetro radicalario Creary "σ·" restaura la linealidad, indicando la estabilización de especies diradicaloides.
Se confirma la vía no adiabática mediante datos experimentales y estudios computacionales.
La teoría de funcionales de la densidad (DFT) no reproduce las tendencias experimentales.

Conclusiones:

Un único mecanismo de rotación no adiabática está operativo para la isomerización térmica Z → E en azobencenos.
Los sustituyentes estabilizan las especies diradicaloides, aumentando la velocidad de reacción.
El parámetro Creary "σ·" describe con precisión los efectos de los sustituyentes en esta reacción.

Palabras clave:

Azobenceno Isomerización Relaciones lineales de energía libre Mecanismos de reacción Termoquímica

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