Dissociation, of molecules

As a result of many observations on the energetics of the formation and dissociation of molecules it has been found possible to give typical bond energies and bond lengths to a number of bonds. Some of these are given in Table 5.2. 

Ionization The dissociation of molecules into charged particles. 

In a recent paper [11] this approach has been generalized to deal with reactions at surfaces, notably dissociation of molecules. A lattice gas model is employed for homonuclear molecules with both atoms and molecules present on the surface, also accounting for lateral interactions between all species. In a series of model calculations equilibrium properties, such as heats of adsorption, are discussed, and the role of dissociation disequilibrium on the time evolution of an adsorbate during temperature-programmed desorption is examined. This approach is adaptable to more complicated systems, provided the individual species remain in local equilibrium, allowing of course for dissociation and reaction disequilibria. 

Several elements react with the N atoms in active nitrogen to form nitrides. The excited Ni molecules are also highly reactive and can cause the dissociation of molecules that are normally stable to attack either by ordinary N2 or even N atoms, e.g. ... 

Of course, the converse situation, in which the entropy of the transition state is lower than that of the ground state of the reactant, can also occur (Fig. 3.11). In this case, one speaks of a tight transition state tight, because rotations, vibration or motion of the activated complex are more restricted than in the ground state of the reactant. The dissociation of molecules on a surface provides an example that we shall discuss in the next section. 

Explain how electronic promoters assist the dissociation of molecules such as N2. 

In a number of cases, the temperature of the filament and thermodynamic parameters allow one to calculate [9] the flux intensity of free atoms produced in dissociation of molecules. Specifically, in the case of dissociation of hydrogen, oxygen, and nitrogen molecules on hot metal filaments under pressures of molecular gases higher than lO" Torr, the flux intensity of atoms A originating from A2 molecules is given by... 

Substantial abundance anomalies occur among the heavy oxygen isotopes 170 and 180, which are underabundant by up to about 4 per cent relative to 160 in oxide grains of certain of the CAIs, compared with the bulk composition in which the isotope ratios are closer to a terrestrial standard. The intriguing feature of these anomalous ratios is that, in common with some other meteorites, but in contrast to terrestrial and lunar samples, the relative deviations of the two heavy isotopes are equal most normal fractionation processes would cause 180 to have twice the anomaly of 170, as indeed is observed in terrestrial samples and more differentiated meteorites, where the anomalies are also usually much smaller. While there has been speculation that there might be a substantial admixture of pure 160 from a supernova, there are fractionation mechanisms that may be able to account for the effect, e.g. photo-dissociation of molecules affected by selfshielding (R. Clayton 2002). In this case, it is possible that the terrestrial standard is enriched in the heavy O-isotopes while the inclusions have more nearly the true solar ratio. 

When an excited molecule also consists of a definite vibrational level as shown in Fig. 5.1(a), there shall be no direct dissociation of molecule and a fine structure in the electronic band spectra of the molecule will be observed. The excess energy, in usual course may be dissipated as heat or may give rise to fluorescence. But the molecule may retain its energy until it has not reacted with another molecule or transfer its energy to another molecule, e.g. decomposition of NOC1 as follows ... 

The dissociation of molecules and excitation of atoms usually occur at a specific temperature. 

The value for k will normally decrease as the concentration of the solution decreases but the value for A will increase because of the increased dissociation of molecules in dilute solutions. A value for the molar conductance at infinite dilution (A,)) can be determined by plotting the calculated values for A against the molar concentration of the solution used and determining the plateau value for A. From such investigations it is possible to determine the ionic mobilities of ions (Table 4.3) and calculate the molar conductance of an... 

Woodin, R.L. Bomse, D.S. Beauchamp, J.L. Multiphoton Dissociation of Molecules With Low Power Continuous Wave Infrared Laser Radiation. J. Am. Chem. Soc. 1978,100, 3248-3250. 

Both UPS and XPS may be used to detect the conditions under which complete dissociation of molecules takes place. The UPS spectra for CO adsorbed on Fe(lOO) for various thermal treatments are shown in Fig. 7 (41). 

Photodissociation of molecules may also be achieved by depositing energy directly in the vibrational degrees of freedom. With hi -power pulsed CO2 lasers dissociation of molecules which absorb CO2 -laser radiation has been observed to proceed at an initial rate that far exceeds the measured thermal rate 169 ). The appearance of luminescence spectra of dissociation products preceding the occurrence of gas breakdown 169b) indicates that a considerable degree of dissociation exists for some time before breakdown. 

Adsorption from the gas-phase Desorption to the gas-phase Dissociation of molecules at the surface Reactions between adsorbed molecules Reactions between gas and adsorbed molecules. 

H2, N2, Og, ci2 or He, Ne, A Kr. The luminosity was described as very feeble in H3 or He. The expln of 0.4ml of a 3 1 mixture of TNM-toluene in argon gave a flash lasting under 3 micro secs with a brilliancy of 2.5—10 million candles. The effect was attributed to heating of the gas by adiabatic compression by the expln wave and to the dissociation of molecules and... 

These systems represent bonding to surfaces where the adsorbate atoms have unpaired electrons available for covalent interaction with unsaturated electronic states on the metal surface. We denote this bonding mechanism as radical adsorption where the open-shell electrons on the adsorbate atom can form electron pairs with the metal atoms at the surface. These radical atoms have in most cases been obtained through the dissociation of molecules on the surface. Let us make a simple picture of the electronic structure when a simple atomic adsorbate interacts with a transition metal, denoted the d-band model [31,32]. A similar description can also be found in Chapter 4. 

Translation to lattice energy transfer is the dominant aspect of atomic and molecular adsorption, scattering and desorption from surfaces. Dissipation of incident translational energy (principally into the lattice) allows adsorption, i.e., bond formation with the surface, and thermal excitation from the lattice to the translational coordiantes causes desorption and diffusion i.e., bond breaking with the surface. This is also the key ingredient in trapping, the first step in precursor-mediated dissociation of molecules at surfaces. For direct molecular dissociation processes, the implications of Z,X,Y [Pg.158]

PES is far less well understood (and generally ignored). It may, however, be quite important for direct dissociations of molecules heavier than H2 or D2. 

Using (1 + 1) REMPI, the quadrupole alignment Aq2) (E, v, J, Ts = 925 ) has also been measured for D2 associative desorption from Cu(lll) [229] and some of the results are given in Figure 3.24. Aq2) favors helicoptering alignment, in agreement with a PES that favors dissociation of molecules preferentially parallel to the surface (and detailed balance). It increases substantially with J but decreases with Ef. The... 

Several years later the process analyzed in Ya.B. s article became the basis for the most powerful method for experimental study of physico-chemical kinetics in gases at high temperatures—shock tubes. For two decades practically all measurements of the probabilities of excitation of molecular oscillations and of dissociation of molecules, i.e., everything that was needed for calculations of the motion of space vehicles through the atmosphere, were performed in shock tubes by recording current parameters in the relaxation layer. We may note that even in the experimental study of ignition in a shock tube, the first work was done by Ya.B. with Ya. T. Gershanik and A. I. Rozlovskii [19],... 

The necessary starting point for any study of the chemistry of a planetary atmosphere is the dissociation of molecules, which results from the absorption of solar ultraviolet radiation. This atmospheric chemistry must take into account not only the general characteristics of the atmosphere (constitution), but also its particular chemical constituents (composition). The absorption of solar radiation can be attributed to carbon dioxide (C02) for Mars and Venus, to molecular oxygen (02) for the Earth, and to methane (CH4) and ammonia (NH3) for Jupiter and the outer planets. 

However, resolution of detailed rotational and vibrational lines in simple polyatomic molecules is possible only in the vapor state. The normal liquid and solid-state investigations of photochromic dissociations do not benefit from such analyses. Although detailed analysis of the times involved for the dissociation of molecules discussed in this section have not been made, they probably occur in less than 10-8 sec. after excitation. 

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