A-H stretch

Figure 2.2 Potential of the A-H stretching mode, Ulnstrating the enhanced anharmonicity cansed by formation of an A-H- -B bond.

IR and Raman studies of the A—H stretching mode (it is important to note that frequency, band width, and intensity are all useful criteria). 

Isotope effects in linear transition states Let us now analyze the kinetic isotope effect in a simple system, a transfer of hydrogen from AH to B through a linear transition state (Equation 2.74).53 We assume that A and B are polyatomic fragments. In the reactants we have to consider the A—H stretching... 

Fig. 8 Ab initio ground and excited state potential curves for a dAdT Watson/Crick nucleoside pair along the N6(A)-H stretching coordinate. At each point along the curve, the ground-state geometry was optimized constraining the sugars to their positions in a B-DNA chain.

The infrared spectrum of isatin shows two strong bands at 1740 and 1620 cm 1 corresponding to the carbonyl stretching vibrations. A broad band occurs at 3190 cm 1 due to the A-H stretching, and it is accompanied by many sub-bands, all of which are moved to a lower frequency on deuteration, which also affects several bands in the region of 1400-1100 cm, associated with A-H in-plane bending . Although the vC=0 values are not modified... 

Vibrational frequencies involving the A-H stretch are not the only modes that are directly related to the strength of the H-bond. The acceptor molecule, too, is influenced in ways that mirror the interaction. For example, calculations [61] have illustrated that the shift in the C=0 stretching frequency of a carbonyl acceptor is linearly related to the interaction energy in such a way that each 1 kcal/mol increase in the binding energy results in a 2 cm-1 red shift. This sort of relationship is confirmed by experimental measurements [62]. 

Spectroscopic methods are useful for characterization. The A H stretching frequency in the IR spectrum typically falls on hydrogen bond formation and the AH proton resonance also shifts to high field. Where A or B has a nuclear spin (e.g. F), the coupling constant between A or B and H can be used as long as exchange is not too rapid. For example, in the gas phase, HF has an H,F coupling constant near 600 Hz, but in amine H-F adducts, this value falls to about 450 Hz. ... 

Vibrational Spectral Behavior. Possibly the most unique manifestation of the H bond is the perturbation of the A—H stretching mode, Vi, Each of the three changes that occur—frequency shift, increase in band width, and enhancement of intensity—is unusual and characteristic. Since these effects are so unusual they must contain interesting information about the nature of the H bond. Despite a great deal of study, this information has not yet been extracted. 

Frequency Shift. The frequency shift of v, reveals the change brought about in the potential function for the A—H stretching mode when it participates in a H bond. Despite theoretical interpretation from several points of view, the frequency shifts have not provided definitive support for any particular model of the H bond. For electrostatic model estimates of Av, see Section 8.2.1. For quantum mechanical calculations of Av, see 407, 1915-1919, 465. 

The classic technique used to study the hydrogen bond in condensed phases, both in solution and the solid state. The hydrogen bond is manifested by a decrease in the value of the A-H stretching (Aj ah) with respect to the free A-H value, together with a broadening of this vibrational band. 

In some cases, the so-called blue-shifted hydrogen bond, the shift of the A-H stretching frequency is toward higher wave numbers. The reasons for this exceptional behavior have been now properly rationalized [39]. 

---