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Determinación de la simetría molecular en el naftaleno cristalino utilizando RMN en estado sólido

J C Facelli¹, D M Grant

¹Department of Chemistry, University of Utah, Salt Lake City 84112.

|September 23, 1993

Resumen

La resonancia magnética nuclear de estado sólido (RMN) revela sutiles distorsiones moleculares en el naftaleno invisibles para la difracción. Los cálculos de la mecánica cuántica confirman estas distorsiones, mejorando el análisis estructural más allá de los métodos tradicionales.

Área de la Ciencia:

Química del estado sólido.
La química cuántica es una química cuántica.
La cristalografía es una técnica de cristalografía.

Sus antecedentes:

Las técnicas de difracción revelan la simetría de la célula unitaria del naftaleno como Ci.
Estudios anteriores no pudieron detectar desviaciones de la estructura molecular de la simetría D2h.
Los desplazamientos químicos de la resonancia magnética nuclear (RMN) de estado sólido 13C sugieren una simetría de Ci.

Objetivo del estudio:

Para investigar si los cambios químicos del 13C-NMR reflejan verdaderas distorsiones estructurales moleculares en el naftaleno.
Para determinar en qué medida los efectos intermoleculares influyen en los cambios observados en la RMN.
Demostrar la capacidad de la RMN para revelar información estructural más allá de los límites de difracción.

Principales métodos:

Realización de cálculos mecánicos cuánticos ab initio de desplazamientos químicos del 13C para el naftaleno y sus derivados.
Analizando la contribución de los efectos intermoleculares a los cambios químicos observados.
Correlación de los desplazamientos químicos calculados con los parámetros estructurales.

Principales resultados:

Los efectos intermoleculares representan sólo ~30% de la desviación de la simetría D2h.
La desviación restante indica distorsiones estructurales genuinas dentro de la molécula de naftaleno.
Los cambios químicos de RMN muestran una alta sensibilidad a cambios estructurales menores.

Conclusiones:

La RMN en estado sólido, combinada con cálculos cuántico-químicos, puede detectar distorsiones estructurales moleculares que se pierden por la difracción.
Este enfoque ofrece un método poderoso para refinar los parámetros estructurales obtenidos a partir de estudios de difracción.
La RMN proporciona una herramienta complementaria para el análisis detallado de la estructura molecular en estado sólido.

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