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¿Dónde está la carga? Los sitios de protonación en los iones gaseosos cambian con la hidratación
Terrence M Chang1, James S Prell, Erika R Warrick
1Department of Chemistry, University of California, Berkeley, California 94720-1460, United States.
Journal of the American Chemical Society
|September 8, 2012
Resumen
Las moléculas de agua influyen en los sitios de protonación en iones gaseosos como el ácido p-aminobenzoico (PABA) y su éster metílico (PABAOMe). Los niveles específicos de hidratación cambian la protonación del oxígeno carbonilo al nitrógeno amino, imitando el comportamiento de la solución.
Área de la Ciencia:
- La Química Física es la química física.
- Física Química Física Química es la física de la química.
- La espectroscopia es una técnica de espectroscopia.
Sus antecedentes:
- Los sitios de protonación en los iones gaseosos pueden diferir de los de la solución.
- Comprender los efectos de solvación es crucial para interpretar los datos de la espectrometría de masas.
Objetivo del estudio:
- Investigar el papel del agua en la estabilización de los sitios de protonación del ácido p-aminobenzoico (PABA) y el éster metílico del ácido p-aminobenzoico (PABAOMe) en la fase gaseosa.
- Determinar el número de moléculas de agua requeridas para inducir un cambio en el sitio de protonación del oxígeno carbonilo al nitrógeno amino.
Principales métodos:
- La ionización por electrospray (ESI) junto con la espectroscopia de fotodisociación por infrarrojos (IRPD).
- Química computacional, incluyendo cálculos DFT con varios funcionales (B3LYP, MP2, ωB97X-D) y conjuntos de bases (6-311++G**).
Principales resultados:
- La protonación se produce en el oxígeno carbonilo para PABA y PABAOMe con un mínimo de agua (0-1 molécula).
- La protonación cambia al nitrógeno amínico para PABAOMeH (((+) con 3 moléculas de agua y para PABAH (((+) con ≥6 moléculas de agua.
- El método computacional ωB97X-D/6-311++G** mostró el mejor acuerdo con los hallazgos experimentales.
Conclusiones:
- La unión de hidrógeno y la deslocalización de resonancia influyen significativamente en la estabilidad del sitio de protonación.
- El número de moléculas de agua necesarias para estabilizar el sitio de protonación de la fase de solución es menor para PABAOMeH (((+) que para PABAH (((+)).
- Los hallazgos ofrecen información sobre la estabilización del sitio de carga y las transiciones estructurales durante la desolución por ionización por electrospray.
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