Dissociation polyatomic molecules

Johnson B R, Kittrell C, Kelly P B and Kinsey J L 1996 Resonance Raman spectroscopy of dissociative polyatomic molecules J. Chem. Educ. 100 7743-64... 

Fast transient studies are largely focused on elementary kinetic processes in atoms and molecules, i.e., on unimolecular and bimolecular reactions with first and second order kinetics, respectively (although confonnational heterogeneity in macromolecules may lead to the observation of more complicated unimolecular kinetics). Examples of fast thennally activated unimolecular processes include dissociation reactions in molecules as simple as diatomics, and isomerization and tautomerization reactions in polyatomic molecules. A very rough estimate of the minimum time scale required for an elementary unimolecular reaction may be obtained from the Arrhenius expression for the reaction rate constant, k = A. The quantity /cg T//i from transition state theory provides... 

The energy required to break the bond between two covalently bonded atoms is called the bond dissociation energy . In polyatomic molecules this quantity varies with environment. For example, ammonia has three N—H bond dissociation energies ... 

Another area of research ia laser photochemistry is the dissociation of molecular species by absorption of many photons (105). The dissociation energy of many molecules is around 4.8 x 10 J (3 eV). If one uses an iafrared laser with a photon energy around 1.6 x 10 ° J (0.1 eV), about 30 photons would have to be absorbed to produce dissociation (Eig. 17). The curve shows the molecular binding energy for a polyatomic molecule as a function of interatomic distance. The horizontal lines iadicate bound excited states of the molecule. These are the vibrational states of the molecule. Eor... 

The dissociation problem is solved in the case of a full Cl wave function. As seen from eq. (4.19), the ionic term can be made to disappear by setting ai = —no- The full Cl wave function generates the lowest possible energy (within the limitations of the chosen basis set) at all distances, with the optimum weights of the HF and doubly excited determinants determined by the variational principle. In the general case of a polyatomic molecule and a large basis set, correct dissociation of all bonds can be achieved if the Cl wave function contains all determinants generated by a full Cl in the valence orbital space. The latter corresponds to a full Cl if a minimum basis is employed, but is much smaller than a full Cl if an extended basis is used. 

Polyatomic molecules provide a still richer environment for studying phase control, where coupling between different dissociation channels can occur. Indeed, one of the original motivations for studying coherent control was to develop a means for bond-selective chemistry [25]. The first example of bond-selective two-pathway interference is the dissociation of dimethyl-sulfide to yield either H or CH3 fragments [74]. The peak in Fig. 11 is indicative of a resonance embedded in an elastic continuum (case 4). 

Another polyatomic molecule provided an opportunity to study the effect of the Gouy phase discussed in Section III [62]. Figure 12 depicts a slice of the potential energy surfaces of vinyl chloride, where the vertical arrows correspond to 532 nm photons. The two pathways for dissociation correspond to CO3 versus 3ce>i, whereas those for ionization correspond to m3 + 2coi versus 5ce>i (i.e., I = 2, m = 1, n = 3). Figure 13 shows the phase lag for ionization... 

In addition to the previously mentioned disadvantages, all of these methods have another drawback in the large molecule photofragment velocity measurements. For example, in the studies of UV photon photodissociation of polyatomic molecules, like alkene and aromatic molecules, molecules excited by the UV photons quickly become highly vibrationally excited in the ground electronic state through fast internal conversion, and dissociation occurs in the ground electronic state. 

A//298 is called the bond dissociation enthalpy or simply the bond enthalpy. Bond enthalpies are often also called bond energies because the small difference between the two values (ca. 2.5 kJ mol-1) can be ignored in many cases, particularly for polyatomic molecules. 

For polyatomic molecules, the dissociation energy can be measured directly only for the weakest bond, and even then the value may only be approximate because the energies of the other bonds in the molecule generally change when one bond is broken. To obtain the energies of other bonds, some assumptions must be made. For molecules of the type AB with only one type of bond, the enthalpy of atomization, that is, the enthalpy change for the reaction... 

It is more difficult to assign lines in laser systems which use polyatomic molecules in pulsed gas discharges 355) since here it is not always easy to decide whether the observed lines belong to transitions of the parent molecule or of dissociation products. 

For diatomic molecules, of course, A = u = Ojj. In polyatomic molecules, consideration of a stepwise cleavage of all the bonds gives which does not mean that the individual dissociation... 

The terms mode-selective and bond-selective dissociation refer to the control of the dissociation products in VMP. The terms are usually used as synonyms although, strictly speaking, the former should refer to selective preexcitation of a vibrational mode and the latter to the resulting selective bond cleavage. Control of the dissociation products in VMP has been extensively reviewed [28-31] and our discussion will focus on molecules studied (or continued to be smdied) after the latest comprehensive review was published [31], An exception will be a short overview on the VMP of water isotopologues since it was the extensive theoretical and experimental investigations of these molecules, in particular H2O and HOD, that opened a new era of detailed smdies of state-to-state photodissociation out of specific rovibrationally excited states of polyatomic molecules. 

Clearly, one should not use tabulated bond energy data for polyatomic molecules without knowing whether they are mean bond energies or bond dissociation energies. In any event, the relationship with the heat of formation of CH4 can be illustrated by an enthalpy cycle ... 

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