Single bond vibrations

The region below 1500 cm"1 is the fingerprint portion of the IR spectrum. A large number of absorptions due to a variety of C-C, C-O, C-N, and C-X single-bond vibrations occur here. 

The region of C-O single bond vibrations at 1300— 1100 wavenumbers with a significant selectivity for different groups when combined with the simultaneously occurring C=0 vibrations. 

The region below 1500 cm-1 is where the single bond vibrations occur. Here our hope that individual bonds may vibrate independently of the rest of the molecule is usually doomed to disappointment. The atoms C, N, and O all have about the same atomic weight and C-C, C-N, and C-0 single bonds all have about the same strength. 

This chart presents some correlations between structure and single-bond vibrations for a number of classes of compounds having absorption between 1500-650 cm . 

The transition state structures of enzyme-catalyzed reactions provide fundamental information about the interactions used to catalyze biological reactions. All chemical transformations pass through the transition states, which are proposed to have lifetimes near 10 s (equivalent to the time for a single bond vibration). Enzymes typically increase the rate over the noncatalyzed reaction by factors of 10 °-10 l The enzyme-substrate complexes often have dissociation constants in the range of 10 -10 M and it has been proposed that transition state complexes are bound with dissociation constants in the range of (Schramm, 1998). 

Singlet-state ntt reactions that occur in the gas phase and in solution may not be observed in crystals because the acyl-alkyl radical pair (E) is subject to the perfect cage effect . Radical-radical recombination in the singlet-state acyl-alkyl biradical (from E =>A) may occur within the time scale of a single bond vibration. However, because many decarbonylation reactions have been documented in crystals, conditions may be satisfied for a-cleavage to compete with excited-state decay and for decarbonylation to compete with recombination of the acyl-alkyl radical pair. In this chapter, we review various aspects of the a-cleavage and decarbonylation steps in the reaction, describe recent theoretical advances, and conclude with examples that reflect our current understanding of the reaction in the solid state. 

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