Chemical bond stretching

A given atom participates in all vibrational modes. Even if any vibrational mode makes all atoms move, some atoms move more than others. It may happen that a particular mode changes mainly the length of one of the chemical bonds (stretching mode), another mode moves another bond, and yet another changes a particular bond angle bending mode), etc. 

Table 7.1. Characteristic frequencies (wave numbers, in cm ) typical for some chemical bonds (stretching vibrations) and bond angles (bending vibrations).

Each chemical bond stretches or bends at a specific wavelength, and thus it is possible to use IR spectroscopy to readily identify the functional groups within a molecule. The abbreviated correlation chart given in Table 10.4 lists ranges for the major organic functional groups. Expanded correlation charts are found on the internet and in many organic texts. 

Most of the molecules we shall be interested in are polyatomic. In polyatomic molecules, each atom is held in place by one or more chemical bonds. Each chemical bond may be modeled as a harmonic oscillator in a space defined by its potential energy as a function of the degree of stretching or compression of the bond along its axis (Fig. 4-3). The potential energy function V = kx j2 from Eq. (4-8), or W = ki/2) ri — riof in temis of internal coordinates, is a parabola open upward in the V vs. r plane, where r replaces x as the extension of the rth chemical bond. The force constant ki and the equilibrium bond distance riQ, unique to each chemical bond, are typical force field parameters. Because there are many bonds, the potential energy-bond axis space is a many-dimensional space. 

Chemical processes, such as bond stretching or reactions, can be divided into adiabatic and diabatic processes. Adiabatic processes are those in which the system does not change state throughout the process. Diabatic, or nonadiabatic, processes are those in which a change in the electronic state is part of the process. Diabatic processes usually follow the lowest energy path, changing state as necessary. 

This finding may be rationalized for the following reasons. The total length of the Me pendant moiety from the cylinder axis is approximately 15 A when the dye moiety is stretched out from the polymer main chain. Since the dye moiety is linked to the main chain via a flexible chemical bond, it may be able to reside at any distance between 3 and 15 A from the cylinder axis. Thus, on average the Me residues would experience the polyanion microenvironment at a distance of about 10 A. 

Both of the above approaches rely in most cases on classical ideas that picture the atoms and molecules in the system interacting via ordinary electrical and steric forces. These interactions between the species are expressed in terms of force fields, i.e., sets of mathematical equations that describe the attractions and repulsions between the atomic charges, the forces needed to stretch or compress the chemical bonds, repulsions between the atoms due to then-excluded volumes, etc. A variety of different force fields have been developed by different workers to represent the forces present in chemical systems, and although these differ in their details, they generally tend to include the same aspects of the molecular interactions. Some are directed more specifically at the forces important for, say, protein structure, while others focus more on features important in liquids. With time more and more sophisticated force fields are continually being introduced to include additional aspects of the interatomic interactions, e.g., polarizations of the atomic charge clouds and more subtle effects associated with quantum chemical effects. Naturally, inclusion of these additional features requires greater computational effort, so that a compromise between sophistication and practicality is required. 

If the pulse from laser 1 is ultrafast, the bond-energizing step occurs in a veiy short time, about 10 fs (lfs = 10 S). As the bond stretches through the specific length at which it absorbs photons from laser 2, the molecule can absorb a photon from the second laser beam. This absorption causes the transmitted intensity of laser 2 to fall rapidly as the bond stretches. When the bond breaks, photons from laser 2 are no longer absorbed and the transmitted intensity returns to its original value. By measuring the time it takes for this to occur, chemists have determined that it takes about 200 fs for a chemical bond to break. 

It had been assnmed in the past that the main reason for development of an activated transition state with enhanced energy is a stretching of chemical bonds. Thus, in the model of Horinti and Polanyi it was assnmed that stretching of H+-H2O bonds... 

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