Group frequencies about

The interactions of photons with molecules are described by molecular cross-sections. For IR spectroscopy the cross-section is some two orders of magnitude smaller with respect to UV or fluorescence spectroscopy but about 10 orders of magnitude bigger than for Raman scattering. The peaks in IR spectra represent the excitation of vibrational modes of the molecules in the sample and thus are associated with the various chemical bonds and functional groups present in the molecules. The frequencies of the characteristic absorption bands lie within a relatively narrow range, almost independent of the composition of the rest of the molecule. The relative constancy of these group frequencies allows determination of the characteristic... 

FIGURE 3.21. Acetophenone. A. Overtone of C=0 stretch —3350 cm-1 frequency about twice that of C=0 stretch. B. The C=0 stretch, 1685 cm-1, lower frequency than observed in Figure 3.20 because of the conjugation with the phenyl group. 

The carbonyl groups of amides absorb at particularly low IR frequencies about 1640 to 1680 cm-1 (Figure 12-13). The dipolar resonance structure (shown next) places part of the pi bond between carbon and nitrogen, leaving less than a full C=0 double bond. 

The F-chemical shift was measured in advance for each member of the library. These reference experiments were carried out in D -DMSO solutions with trifluo-roethanol (TFE) as an internal standard. The 400 ligands were divided into groups of about 10-30 compounds, resulting in 15 mixtures. The combinations were chosen such as to avoid reactions and overlap of F-chemical shifts. For the composition of these mixtures, stock solutions in D -DMSO were set up and added to buffer solutions (50 mM KH2PO4, 10 mM EDTA, 10 mM Na2S03, pH 7.1). To test for binding, a portion of the stock solution was added to a solution of RiSy (0.1 mM) in the same buffer. For both samples ID F-spectra were acquired at 14.09 T (564 MHz F-frequency). 

Certain functional groups in a molecule (e.g., hydroxyl, carbonyl, and amine) absorb IR radiation and exhibit absorption bands at characteristic frequencies regions regardless of the structure of the rest of the molecule. These bands are termed group frequencies. They are predictable and allow the analyst to deduce important structural information about an unknown molecule. An IR spectrum can be rapidly recorded for any phase, i.e., solid, liquid, or vapor. By coupling IR spectroscopy with other analytical techniques such as nuclear magnetic resonance (NMR)... 

Now we will discuss the characteristics of the vibrational and Ji-electronic subsystems of the chain of coupled C=C bonds [35]. An isolated C=C group undergoes high-frequency valence (intramolecular) vibration, (2 = 2n x 5 x 1013 s 1. The groups in the polyene chain are interconnected by a bonds (the vibrational frequency of the carbon atoms in the isolated C—C group equals about 2ji x 2 x 1013 s-1), hence the virtual vibrational quantum interchange is substantial. By such a strong bond the spectmm of intramolecular vibrations of... 

By the same token, some of the best known group frequency vibrations of molecular spectroscopy, like the strong carbonyl stretch at about 1700 cm" in the infrared, are almost invisible in neutron spectroscopy. However, many of the techniques of optical spectroscopy retain much of their significance and indeed the technique of isotopic substitution can be dramatically exploited in neutron spectroscopy. 

Characteristic group-frequencies are recognized as being most reliable when the associated vibration is well localized, as in X—H and C=0 stretching vibrations they lie above about 1350 cm.. Below that, vibration frequencies are very liable to shift, because many of the vibrations concerned are not localized in the particular bond of interest but involve other parts of the molecule. Even above 1350 cm. , some groups are difficult to identify. One such group is C=N—, because the intensity of its absorption is so variable. Several types of derivatives that normally contain this group are of interest in carbohydrate chemistry, where they may exist... 

I iiuill). the characteristic group frequency for a C—Cl stretching ibration is shown at about 800 cm in Figure 17-5d,... 

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