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4: Determinación de estructura parte l

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  • Viendo adelante, Capitulo 5 será dedicado al estudio de la resonancia magnética nuclear (RMN), donde usamos imanes ultra-fuertes y la radiación de radiofrecuencia para aprender sobre el entorno electrónico de los átomos individuales en una molécula. Para la mayoría de químicos orgánicos, RMN es la herramienta analítica más poderosa disponible en términos de la cantidad de información detallada que puede proporcionar acerca de la estructure de una molécula. Es lo más cercano que tenemos a una ‘camara molecular’. 

    En resumen, las técnicas analíticas que estudiaremos en este capitulo y el siguiente principalmente tratan de responder a las siguientes preguntas sobre una molecular orgánica:

    • Espectroscopia infrarroja (IR): ¿Qué los grupos funcionales tiene la molécula?
    • Espectroscopia (UV-VIS) ultravioleta-visible: ¿En qué medida la molécula contiene un sistema de enlaces pi conjugados?
    • Espectrometría de masas (MS): ¿Cuánto pesa la molécula y cuales son sus fragmentos comunes?
    • Espectroscopia de resonancia magnética nuclear (RMN): ¿Cuál es la conexión total de la molécula?

    In the first three chapters of this text, we have focused our efforts on learning about the structure of organic compounds.  Now that we know what organic molecules look like, we can start to address the question of how chemists are able to elucidate organic structures. The individual atoms and functional groups in organic compounds are far too small to be directly observed or photographed, even with the best electron microscope.   How, then, are chemists able to draw with confidence the bonding arrangements in organic molecules, even simple ones such as acetone or ethanol?

    The answer lies, for the most part, in a field of chemistry called molecular spectroscopy. Spectroscopy is the study of how electromagnetic radiation, across a spectrum of different wavelengths, interacts with molecules - and how these interactions can be quantified, analyzed, and ultimately interpreted to gain information about molecular structure.

    After first reviewing some basic information about the properties of light and introducing the basic ideas behind spectroscopy, we will move to a discussion of infrared (IR) spectroscopy, a technique which is used in organic chemistry to detect the presence or absence of common functional groups.  Next, we will look at ultraviolet-visible (UV-vis) spectroscopy, in which light of a shorter wavelength is employed to provide information about organic molecules containing conjugated p-bonding systems.  

    In the final section of this chapter, we will change tack slightly and consider another analytical technique called mass spectrometry (MS).   Here, we learn about the structure of a molecule by, in a sense, taking a hammer to it and smashing it into small pieces, then measuring the mass of each piece.  Although this metaphorical description makes mass spectrometry sound somewhat crude, it is in fact an extremely powerful and sensitive technique, one which has in recent years become central to the study of life at the molecular level.