Translational energy requirements

The importance of electron transfer was indicated by the threshold translational energy required to excite substrates with high electron affinities which was lower than, eg, that required for noble gas substrates (177). 

It can be shown that ifH ,exp(A+/AB) is obtained by linear extrapolation of the ionization efficiency curve [64], the products have only the translational energy required to conserve momentum, and the relationship between d/icxp(A+/AB) (or ArH ) andH b(A+/AB) is... 

Figure 2 The initial dissociative sticking probability for D2 on Cu(l 1 1) extracted from the state selected measurements of desorbing molecules for various vibrational (A) and rotational (B) states of the molecule [19]. Vibrational energy couples effectively to the reaction coordinate, lowering the translational energy requirement for dissociation. Rotational energy initially hinders and then promoted dissociation. Similar effects of rotational energy are predicted in the trajectory calculations shown in (C) for molecules constrained to rotate in a plane perpendicular to the surface [29].

Angular distribution measurements85 of AI from Ca, Sr and Ba with HI yield no information concerning the centre of mass differential cross sections due to kinematic constraint of AI to the centroid distribution. However, advantage was taken of this constraint to estimate the threshold translational energies required for reaction of 5,4 and 2-5 kcal mol-1, respectively. These values establish lower bounds to the AI bond dissociation energies. 

It is seen how the sticking takes off at relative low energies (substantially below the 48 kj mol or 0.5 eV mentioned above). This is because this sort of translational energy requires high temperatures where also the first vibrational state of the molecules will become populated. This is demonstrated by the back-seeded-beam where the translational energy is lowered for a fixed nozzle temperature. 

Details of dissociation dynamics can be probed using scattering experiments. Vibrational excitation of the molecular bond lowers the translational energy required for dissociation upon adsorption. Also, the relative orientation of the molecule with respect to the surface is important for dissociation. 

On the basis of the translational energy requirements of chemical reactions we can thus make the following rough correlation. 

The impact operator corrected in such a way still remains semiclassical though the requirements of detailed balance are satisfied. It is reasonable provided that the change of rotational energy is small on average, relative to translational energy ey — ej < ikT, where the overbar means averaging performed over the distribution of products after collision. 

In porous media, liquid-gas phase equilibrium depends upon the nature of the adsorbate and adsorbent, gas pressure and temperature [24]. Overlapping attractive potentials of the pore walls readily overcome the translational energy of the adsorbate, leading to enhanced adsorption of gas molecules at low pressures. In addition, condensation of gas in very small pores may occur at a lower pressure than that normally required on a plane surface, as expressed by the Kelvin equation, which relates the radius of a curved surface to the equilibrium vapor pressure [25],... 

In an ideal gas, molecules spend most of the time isolated from the other molecules in the system and therefore have well defined velocities. In a liquid, the molecules are in a constant state of collision. The derivation of the translational energy distribution from equation 6.3-5 (which requires obtaining g(e)) gives the distribution (expressed as dNIN, the fraction of molecules with energy between e and e + de) ... 

Like amino acid activation (see p. 248), protein biosynthesis (translation) takes piace in the cytopiasm. it is cataiyzed by compiex nucieoprotein particies, the ribosomes, and mainiy requires GTP to cover its energy requirements. 

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