Cold reactant molecules

Figure 13.1 (b) shows the change of enthalpy for a reaction in which heat is taken in (an endothermic reaction). The container holding the reaction mixture will be cool or cold to the touch. In this case, the reactant molecules are at a lower level of enthalpy than the product molecules. Therefore, for endothermic reactions AH is positive. 

The molecular beams in our experiments are also formed by supersonic expansion. In addition to enhanced intensity and narrow velocity distributions obtained in this way, the molecules in such beams can generally be considered to be in their lowest vibrational and rotational states. This "freezing out" of internal degrees of freedom is important in further defining the initial experimental conditions. In understanding the mechanics of collisions, one can obtain simplifications in interpretation by knowing that reactant molecules are rotationally and vibra-tionally "cold". This point will be discussed further in later sections of this paper. 

There are significant differences between tliese two types of reactions as far as how they are treated experimentally and theoretically. Photodissociation typically involves excitation to an excited electronic state, whereas bimolecular reactions often occur on the ground-state potential energy surface for a reaction. In addition, the initial conditions are very different. In bimolecular collisions one has no control over the reactant orbital angular momentum (impact parameter), whereas m photodissociation one can start with cold molecules with total angular momentum 0. Nonetheless, many theoretical constructs and experimental methods can be applied to both types of reactions, and from the point of view of this chapter their similarities are more important than their differences. 

However, the MEP may be a convenient measure of the progress of a molecule in a reaction, because in general a molecule will move, on average, along the MEP in a well-defined valley, and it is a good approximation of the motion of vibrationally cold systems (e.g., for photochemical reactions in which the excited state reactant has a small/controlled amount of vibrational excess energy). 

In 2002 extensive kinetic and product studies on the reactions of gaseous Hg with molecular and atomic halogens (X/X2 where X = Cl, Br) were performed at atmospheric pressure (750 1 Torr) and room temperature (298 1 K) in air and N2 [24]. Kinetics of fhe reactions with X/X2 were studied using both relative and absolute techniques. Cold vapour atomic absorption spectroscopy (CVAAS) and gas chromatography with mass spectroscopic detection (GC-MS) were the analytical methods applied. The measured rate constants for the reactions of Hg with CI2, Cl, Br2, and Br were (2.6 0.2) x IQ-i , (1.0 0.2) x 10" , < (0.9 0.2) x and (3.2 0.3) x 10 cm molecule s , respectively. Thus CI2 and Br2 are not important reactants in the troposphere for the CI2 and Br2 concentrations reported in literature [24]. 

The purpose of our work was to examine whether the compressed hot cluster provides an effective medium for reactions with high barriers. In the simplest case of cluster impact experiments, the reactants are embedded inside a rare gas cluster, and this cold droplet is incident on an inert surface at various velocities. Two classes of reactions were examined in detail using standard molecular dynamics simulations the dissociation of halogen molecules and four-center reactions. ... 

The suggestion that the molecular building blocks of life could be formed in space is intriguing since such regions would seem to be rather unlikely places for the development of chemistry. The ISM is cold (temperatures of 10-30 K) and "empty" with pressures of less than 10 2 torr such that the probability for a collision between two compounds is low and, at such low temperatures, the "reaction rate" would be expected to be very low (hence in most industrial chemistry the reactants are heated to increase their reactivity). Nevertheless the detection of such molecules within the ISM makes it clear that these are chemically active zones. The solution to this apparent paradox is that the chemistry in the ISM is somewhat different from the conventional chemistry we observe on Earth, much of it being induced by radiation. The ISM contains several different sources of radiation, namely ... 

Mobility of the reactants and reaction products of the oxidative kinetic chain reaction, of stabilisers and of the polymer molecules themselves affects the kinetics of the radical reactions. Morphology of a polymer material and its physical state, e g. stress, strain and orientation, has an effect on the mobility and therefore on the process of oxidative degradation. Fibres or slit films of polyethylene or pol ropylene are cold-drawn in the production. The orientation of the cold-drawn polymer material produced here has a particularly strong repercussion on oxidation stability. 

In such cases, the values of —AEq and Kc are determined by the difference between the sums of the zero-point energies for the products and reactants. For reaction (1.18), (—A oAb) is equal to ca. 187 K. Although this is small relative to the same quantity for most chemical reactions, it is nevertheless much larger than the temperature in the cold regions of dense interstellar clouds where many molecules are found. Consequently, the observed fractions of many deuterated molecules are far larger than would be expected simply on the basis of the cosmic abundance of deuterium relative to hydrogen. 

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