Structure Determination in Conjugated Systems Ultraviolet Spectroscopy

Structure Determination in Conjugated Systems Ultraviolet Spectroscopy 

Mass spcctrometiy, infrared spectroscopy and nuclear inagnetic resonance spectroscopy are techniques of structure determination applicable to all organic molecules. In addition to these three generally useful methods, thcre s a fourth—ultraviolet (UV) spectroscopy—that is ajipHcable only to conuigated systems. UV is less commonly used than the other three spectroscopic techniques because of the specialized information it gives, so we ll mentior it only briefly. 

Molecular size and formula Functional groups present Ca rbon-hy drogen f ra me wo rk Nature of conjugated v electron system 

The amount of UV light absorbed is expressed as the sample s molar absorptivity (e), defined by the equation 

Molar absorptivity is a physical constant, characteristic of the particular substance being observed and thus characteristic of the particular -jt electron system in the molecule. Typical values for conjugated dienes are n the range e = 10,000 to 25,000. Note that the units are usually dropped. 

Ultraviolet spectroscopy Nature of conjugated rt electron system 

Ultraviolet spectroscopy is less commonly used than the other three spectroscopic techniques because of the specialized information it gives. We ll therefore study it only briefly. 

Problem 14.11 Calculate the energy range of electromagnetic radiation in the UV region of th spectrum from 200 to 400 nm, Recall the equation 

Mass spectrometry Molecular size and formula IR spectroscopy Functional groups present NMR spectroscopy Carbon-hydrogen framework UV spectroscopy Nature of conjugated tt electron system 

Mass spectrometry Infrared spectroscopy NMR spectroscopy Ultraviolet spectroscopy 

1 X rays I Vacuum ultraviolet Visible Near infrared Infrared i ) V / 1 

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Structure Determination in Conjugated Systems Ultraviolet Spectroscopy 501... 

Because of the interaction between the double bonds, systems containing conjugated double bonds tend to be more stable than similar systems with isolated double bonds. In this chapter, we consider the unique properties of conjugated systems, the theoretical reasons for this extra stability, and some of the characteristic reactions of molecules containing conjugated double bonds. We also study ultraviolet spectroscopy, a tool for determining the structures of conjugated systems. 

The bulk composition of the SAN copolymer can be determined by ultraviolet spectroscopy. Absorbances consistent with conjugated systems such as Figures 13.1 and 13.2 have been observed. Studies usually compare the UV spectra of model systems with the actual absorbances seen in SAN copolymers. The models represent chemically reasonable species based upon the starting monomers, known reactivity ratios, and oxidation and rearrangement chemistry [2]. The data are self-consistent with these criteria however, identification of all chromophores responsible for color formation in SAN copolymers is still work in progress. One source of species with extended conjugation is the cyclization of acrylonitrile triads to form heteroaromatic structures (Figure... 

We saw in Chapter 10 that mass spectrometry gives a molecule s formula, infrared spectroscopy identifies a molecule s functional groups, and ultraviolet spectroscopy identifies a molecule s conjugated tt electron system. Nuclear magnetic resonance spectroscopy complements these other techniques by mapping a molecule s carbon-hydrogen framework. Taken together, mass spectrometry, IR, UV, and NMR make it possible to determine the structures of even very complex molecules. 

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