Bond specific energies

The relatively small bond specific energy for heavy nuclei makes the process of the fission of these nuclei into fragments energetically favourable. The most energetically favourable and thus the most widespread process is decay with a-particle emission X — 1VX + jHe... 

This is connected with the fact that the 2He nucleus has the biggest bond specific energy among the lightest nuclei (7.1 MeV). Bond specific energies for the other light nuclei 2H, 3He, and 6Li are 1.1, 2.6, and 5.3MeV, respectively. 

As discussed earlier in Section lOC.l, ultraviolet, visible and infrared absorption bands result from the absorption of electromagnetic radiation by specific valence electrons or bonds. The energy at which the absorption occurs, as well as the intensity of the absorption, is determined by the chemical environment of the absorbing moiety. Eor example, benzene has several ultraviolet absorption bands due to 7t —> 71 transitions. The position and intensity of two of these bands, 203.5 nm (8 = 7400) and 254 nm (8 = 204), are very sensitive to substitution. Eor benzoic acid, in which a carboxylic acid group replaces one of the aromatic hydrogens, the... 

The amount of energy a molecule contains is not continuously variable but is quantized. That is, a molecule can stretch or bend only at specific frequencies corresponding to specific energy levels. Take bond-stretching, for example. Although we usually7 speak of bond lengths as if they were fixed, the numbers... 

For such situations we have developed a different approach. The parameters calculated by our methods are taken as coordinates in a space, the reactivity space, A bond of a molecule is represented in such a space as a specific point, having characteristic values for the parameters taken as coordinates. Figure 6 shows a three-dimensional reactivity space spanned by bond polarity, bond dissociation energy, and the value for the resonance effect as coordinates. 

It turns to our advantage to consider eh and Fj i jointly. The idea is best explained by an example. Suppose that the CC and CH bond of ethane were selected as reference bonds, with energies ecc and ecH. respectively, and F =0. New references are required for olefins, specifically tailored for C(sp )—C(sp ) and C(sp )—H bonds the reference for C(sp )-C(sp ) bonds is deduced from that representing C(sp )—C(sp ) while C(sp )—H is derived from C(ip )—H by incorporating the appropriate parts of F into the new reference energies ... 

Genera/. In the minds of many, spectroscopy involves the use of intensity-wavelength curves to determine the wavelength at which maxima occur in the absoiption of the incident light These maxima indicate the unique value of wavelength (or frequency) at which a specific chemical bond absorbs energy. Thus, absoiption spectroscopy enables the researcher to identify bonds present in the system undo- examination. Observation of evidence for a characteristic combination of bonds enables the experimenter to determine the presence of a certain compound. 

A very interesting beginning has been made in experimental determinations of the behavior of fatty acids in a water-montmorillonite system at one atmosphere pressure (Johns and Shimoyama, 1972). The basis of the study is the transformation of a long chain molecule to smaller units by breaking specific carbon-carbon bonds. The energy necessary to do this is estimated to be about 46 Kcal/mole in a montmorillonite system, whereas calculations put the value at 56-58 Kcal in a uniquely hydrocarbon... 

Thus, we have detailed how to construct a molecular PES as a sum of energies from chemically intuitive functional forms that depend on internal coordinates and on atomic (and possibly bond-specific) properties. However, we have not paid much attention to the individual parameters appearing in those functional forms (force constants, equilibrium coordinate values, phase angles, etc.) other than pointing out the relationship of many of them to certain spectroscopically measurable quantities. Let us now look more closely at the Art and Science of the parameterization process. 

The G2 and G3 methods go beyond extrapolation to include small and entirely general empirical corrections associated with the total numbers of paired and unpaired electrons. When sufficient experimental data are available to permit more constrained parameterizations, such empirical corrections can be associated with more specific properties, e.g., with individual bonds. Such bond-specific corrections are employed by the BAG method described in Section 7.7.3. Note that this approach is different from those above insofar as the fundamentally modified quantity is not Feiec, but rather A/7. That is, the goal of the method is to predict improved heats of formation, not to compute more accurate electronic energies, per se. Irikura (2002) has expanded upon this idea by proposing correction schemes that depend not only on types of bonds, but also on their lengths and their electron densities at their midpoints. Such detailed correction schemes can offer very high accuracy, but require extensive sets of high quality experimental data for their formulation. 

Reaction dynamics on the femtosecond time scale are now studied in all phases of matter, including physical, chemical, and biological systems (see Fig. 1). Perhaps the most important concepts to have emerged from studies over the past 20 years are the five we summarize in Fig. 2. These concepts are fundamental to the elementary processes of chemistry—bond breaking and bond making—and are central to the nature of the dynamics of the chemical bond, specifically intramolecular vibrational-energy redistribution, reaction rates, and transition states. 

The approach taken in our laboratory combines both of these trends. Specifically, we have developed a new experiment that allows us to study, for the first time, the photodissociation spectroscopy and dynamics of an important class of molecules reactive free radicals. This work is motivated in part by the desire to obtain accurate bond dissociation energies for radicals, in order to better determine their possible role in complex chemical mechanisms such as typically occur in combustion or atmospheric chemistry. Moreover, since radicals are open-shell species, one expects many more low-lying electronic states than in closed-shell molecules of similar size and composition. Thus, the spectroscopy and dissociation dynamics of these excited states should, in many cases, be qualitatively different from that of closed-shell species. 

When atoms or molecules are excited conventionally by elevated temperatures or pressure, they can follow several reaction paths yielding a variety of byproducts in addition to the desired substance. Since the basis of a chemical reaction is to weaken or break or make specific chemical bonds tu yield the final product, energy ideally should be selectively introduced at the particular level necessary to accomplish this. The high energy and monochromaticity of laser output arc ideal for imposition of the specific energy changes that induce or catalyze chemical changes. 

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