Molecular orientations during collisions

The answer lies in the molecular orientations during collisions. We can illustrate this effect by using the reaction between two BrNO molecules, as shown in Fig. 15.13. Some collision orientations can lead to reaction, and others cannot. Therefore, we must include a correction factor to allow for collisions with nonproductive molecular orientations. 

As the density of a gas increases, free rotation of the molecules is gradually transformed into rotational diffusion of the molecular orientation. After unfreezing , rotational motion in molecular crystals also transforms into rotational diffusion. Although a phenomenological description of rotational diffusion with the Debye theory [1] is universal, the gas-like and solid-like mechanisms are different in essence. In a dense gas the change of molecular orientation results from a sequence of short free rotations interrupted by collisions [2], In contrast, reorientation in solids results from jumps between various directions defined by a crystal structure, and in these orientational sites libration occurs during intervals between jumps. We consider these mechanisms to be competing models of molecular rotation in liquids. The only way to discriminate between them is to compare the theory with experiment, which is mainly spectroscopic. 

The first possibility is an increase in the pre-exponential factor, A, which represents the probability of molecular impacts. The collision efficiency can be effectively influenced by mutual orientation of polar molecules involved in the reaction. Because this factor depends on the frequency of vibration of the atoms at the reaction interface, it could be postulated that the microwave field might affect this. Binner et al. [21] explained the increased reaction rates observed during the microwave synthesis of titanium carbide in this way ... 

Thus it seems clear that no direct transitions between essentially repulsive covalent potential surfaces Na + BC and Na + BC are possible. This view is also supported by calculations.68 Under such circumstances an additional ionic potential surface has been postulated,69-70 namely, Na+ + BC, which was supposed to be strongly attractive and to couple with the covalent surfaces. All potentials depend on the molecular distance RM, on the atom-molecule distance Rc during the collision, and on the molecular orientation relative to Rc measured by the angle y. A two-dimensional cut through these surfaces along Rc is shown schematically in Fig. 3 for the... 

It should be noted that result given by equation (91) coincides with that calculated under the assumption that the orientation of atomic orbitals remains fixed during collision ( fixed-atom approximation ) [84, 87]. This is to be expected, because the strong Coriolis mixing of molecular functions in the rotating frame prevents rotation of orbitals in a space fixed frame. 

Molecular orientations (kinetics) orientations of molecules during collisions, some of which can lead to a reaction and some of which cannot. (15.8)... 

The orientation of linear rotators in space is defined by a single vector directed along a molecular axis. The orientation of this vector and the angular momentum may be specified within the limits set by the uncertainty relation. In a rarefied gas angular momentum is well conserved at least during the free path. In a dense liquid it is a molecule s orientation that is kept fixed to a first approximation. Since collisions in dense gas and liquid change the direction and rate of rotation too often, the rotation turns into a process of small random walks of the molecular axis. Consequently, reorientation of molecules in a liquid may be considered as diffusion of the symmetry axis in angular space, as was first done by Debye [1],... 

Here, pathway 1 (reaction 1) is the coordinated addition of ozone (1) to ethylene (2), which proceeds through the formation of a weakly-boimd complex that transforms into primary ethylene ozonide (PO) or 1.2.3-trioxolene upon passing through the symmetrical transient state (TSl). Pathway 2 (reaction 2, the DeMore mechanism [15]) involves the collision during spontaneous orientation of the reagents (3) and the rotational transition to the biradical transient state (TS2) (4) followed by the formation of the same PO. Proceeding from the above-said, we supplement this pathway with the reaction of detachment of molecular oxygen and the formation of intermediate biradical (5) the latter may either decompose with the formation of formaldehyde (6) and carbene (7) or transform into acetaldehyde (8) or epoxide (9). Finally, pathway 3 involves the transition of ozone into the triplet state (10). This pathway is similar to reaction 2. Here, the same biradical (5) is formed it transforms into the... 

The selective population of electronic fine structure states is observed in many other molecular processes, like chemical reactions, inelastic collisions and surface scattering. The basic origin for the selectivity is the same in all these cases. It requires some orientation of the unpaired lobe in the product and a well defined rotational motion during the break up of the complex. If too many different initial rotations are present, the A-doublet selectivity is smeared out and a statistical population is obtained. In most cases, the A-doublet selectivity is indeed considerably lower than expected from the degree of electron alignment. This suggests, in the view of these results, that still considerable out-of-plane motion is present in the excited complex. 

Sometimes, even if you hit the ball hard enough, it doesn t go into the pocket because you didn t hit it in the right spot. The same is true during a molecular collision. The molecules must collide in the right orientation, or hit at the right spot, in order for the reaction to occur. 

Compared to state 111 >, state 12> represents a reorientation of both the p orbital as well as the orbital angular momentum. This reorientation is accomplished by rotation of L and C in an opposite sense, so that the total projection quantum number (M=mj+m 2) remains 0. Physically the mixing of states 111 > and 12> implies that as soon as R becomes small enough that the I and H electrostatic potentials begin to differ significantly, the initially prepared orientation of the p-orbital will be scrambled. There is another implication in many semiclassical treatments of atomic and molecular collisions both the orientation and the magnitude of C are assumed to remain fixed during the collision [55,56]. The discussion in... 

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