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Desentrañar la dinámica fotoquímica ultrarrápida del nitrobenzeno en solución acuosa

Nicholas A Lau¹, Deborin Ghosh², Susannah Bourne-Worster¹

¹Department of Chemistry, University College London, 20 Gordon Street, London WC1H 0AJ, U.K.

Journal of the American Chemical Society

|April 4, 2024

Resumen

Los compuestos nitroaromáticos como el nitrobenzeno son componentes clave de los aerosoles de carbono marrón. En el agua, sus reacciones fotoquímicas se ralentizan por la unión de hidrógeno y la transferencia de energía, evitando la formación de fotoproducto.

Área de la Ciencia:

Química de la atmósfera
La fotoquímica
Espectroscopia

Sus antecedentes:

Los aerosoles de carbono marrón, incluidos los compuestos nitroaromáticos, tienen un impacto significativo en el clima de la Tierra al absorber la radiación solar.
La fotoquímica acuosa en las gotas de nubes y niebla es una fuente secundaria crucial de carbono marrón.
El nitrobenzeno sirve como un compuesto modelo para comprender el comportamiento fotoquímico de las moléculas nitroaromáticas más grandes en la atmósfera.

Objetivo del estudio:

Investigar la dinámica fotoquímica ultrarrápida del nitrobenzeno en solución acuosa después de la fotoexcitación casi ultravioleta.
Para aclarar el papel del entorno disolvente circundante (agua, metanol, acetonitrilo, ciclohexano) en los procesos fotoquímicos del nitrobenzeno.
Para comparar la fotoquímica acuosa con el comportamiento en fase gaseosa.

Principales métodos:

Espectroscopia de absorción transitorio en femtosegundos
Espectroscopia infrarroja con resolución de tiempo
Cálculos de la química cuántica
Estudios comparativos con disolventes (agua, metanol, acetonitrilo y ciclohexano)

Principales resultados:

La transferencia de energía vibratoria al medio acuoso apaga efectivamente la excitación interna del nitrobenzeno.
A diferencia de la fase gaseosa, no se observaron fotoprodutos en escalas de tiempo de hasta 500 ns en solución acuosa.
Se encontró que el enlace de hidrógeno entre el nitrobenzeno y las moléculas de agua desaceleraba el proceso de conversión interna S₁/S_0.

Conclusiones:

El entorno acuoso altera significativamente la vía fotoquímica del nitrobenzeno en comparación con la fase gaseosa.
El apagado por transferencia de energía vibratoria y la conversión interna más lenta debido al enlace de hidrógeno inhiben la formación de fotoproductos en el agua.
Estos hallazgos son críticos para modelar con precisión el destino atmosférico y el impacto climático de los compuestos nitroaromáticos.

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