Nascent reaction products

The spectroscopic methods are based on time-resolved pump-probe schemes where the collision-free regime is usually attained by using low pressure conditions. Application of various linear and non-linear laser techniques, such as LIF (laser-induced fluorescence), REMPI (resonant-enhanced multiphoton ionization) and CARS (coherent antistokes Raman spectroscopy) have provided detailed information on the internal states of nascent reaction products [58]. Obviously, an essential prerequisite for the application of these techniques is the knowledge of the spectroscopic properties of the products. 

Chemical lasers are complex nonequilibrium molecular systems governed by an intricate interplay between a variety of chemical, radiative, and collisional relaxation processes. Many of their kinetic properties are reflected by the temporal, spectral, and power characteristics of the out-coupled laser radiation. For example, threshold time measurements and other gain experiments have provided detailed information on vibrational distributions of nascent reaction products. Another, more qualitative, example Single-line and simultaneous multiline operation indicate, respectively, whether the lasing molecules are rotationally equilibrated or not. Besides their practical applications, chemical lasers are widely used as means of selective excitation in state-to-state kinetic studies. On the other hand, many experimental and theoretical studies have been motivated by the wish to understand and improve the mechanism of chemical laser operation. 

When a reaction is studied in the bulk gas phase, the nascent products soon collide with other molecules, energy is transferred upon collision (thus becoming effectively partitioned among all molecules), and the overall reaction exoergicity is finally liberated in its most degraded form, i.e., heat. In macroscopic terms, the reaction is exothermic, i.e., A/f < 0. The microscopic approach of molecular dynamics, however, is concerned with the outcome of the individual reactive collisions. The experimental challenge, as discussed in Section 1.2.5, is to arrest the collisional relaxation of the nascent reaction products and to probe them as they exit from the reactive collision. In this sense, it is customary to speak about the nascent or newborn reaction products. 

Whenever sulphur dioxide, water and nascent sulphur meet, for example in the action of water on sulphur chloride, of mineral acids on a thiosulphate, or of hydrogen sulphide on aqueous sulphur dioxide solution, formation of polythionic acids is likely to occur. Dalton 1 in 1812 demonstrated that the last-named reagents gave rise to an acid liquid, a result which was confirmed later by Thomson 2 in 1846 Wackenroder 3 proved the presence of pentathionic acid in the liquid, since which date the aqueous reaction product has been known as Wackenroder s Solution. ... 

A strong motivation for the Rice et al. [99] simulations was to try to interpret the Zhao et al. [33] observations that the HONO elimination channel dominates while the N-N bond rupture reaction does not occur. A possible explanation is that the nascent CH2NN02 product of the RDX ring fission reaction is highly excited and has a nonstatistical distribution of energy. Sewell and Thompson [35] estimated that it may be formed with 55 to 65 kcal/mol of energy, which is well in excess of the predicted energy... 

A very fundamental difference between reactions in condensed matter and isolated molecular processes in the gas phase is the cage effect when a reaction or excitation process occurs in a cluster or in a condensed phase, the surrounding solvent molecules may prevent the separation of the reaction products or excited interacting species or delay such separation, confining the nascent species to the initial cage for an extended period of time. As in the work reviewed in subsection 1.3.2, this involves the interplay of dissociation and energy transfer. There, the emphasis was... 

The arsine evolved by nascent hydrogen is absorbed in a pyridine solution of silver diethyidithiocarbamate. The pyridine-soluble product of reaction between AsHa and Ag-DDTC is intensely violet, whereas the pyridine Ag-DDTC solution is pale yellow [35-38]. The molar absorptivity of the reaction product e=1.410 (a = 0.19) at A ax = 535 nm, whereas the reagent absorbs at <500 nm. 

This mechanistic proposal has extensive experimental support Imine 16 has been trapped and localized, and enzyme-catalyzed hydrogen/deuterium (H/D) exchange at C3 and C4 of DXP as well as carbonyl oxygen exchange has been detected. Acyl transfer to the C4 hydroxyl (18 to 19) as well as transfer of oxygen from DXP to nascent ThiS-COOH (21 to 22) has been demonstrated. Intermediate 22 has been trapped and detected by mass spectrometric analysis and the reaction product 14 has been fully characterized. Thiazole synthase complexed with ThiS-COOH has been structurally characterized and the DXP imine 16 has been modeled into the active site revealing key catalytic residues. ... 

Fig. 2. The nascent vibrotational product distribution in the F-l-H2- HF(c, J) + H reaction (full lines). The curves within each c represent X,(J v). The areas under these curves are proportional to X (v). The dashed lines describe thermal rotational populations Xg(/lv). The doubly peaked (dotted and dash-dotted) curves correspond to partly relaxed distributions.

Thermal decomposition of some metal compounds whose anions are reducing agents gives the metal, which may have a very high catalytic activity. The active metal may then react with air or with gaseous reaction products. If air is not allowed to penetrate (the reaction is conducted in an inert liquid), and the nascent gaseous products are quickly removed (use of high vacuum or a stream of inert gas—see use of Hg, p. 1616), it is sometimes possible to obtain the metal in its active form. 

An infrared laser-based method for investigating the nascent state distribution of the reaction products HF from the reaction F - - H2 HF -I- H in crossed molecular beams under single collision conditions has been presented by the group of D.J. Nesbit [1061]. The experimental setup (Fig. 8.24) consisted of a pulsed supersonic discharge source of F atoms, which collide with H2 molecules in a second pulsed jet source. The product HF(v, J) is probed in the intersection region by the absorption of a single-mode tunable IR laser, where full vibrationn-otation resolution can be achieved (Fig. 8.25). 

J = 10, which is appreciably populated by the reaction, is only 10 of that in J = 1. The ratio of the v = 0, 1, and 2, populations at 300 K is 1 3x10 2x10 . Therefore, the nascent rotational and vibrational population distribution of the HD reaction product is very far from equilibrium, and this distribution will begin to relax toward the equilibrium distribution after but a single collision. This makes the detected HD products vary short-lived. 

Nowadays, femtosecond laser "pump-and-probe" techniques - as pioneered by Ahmed Zewail and his group at the California Institute of Technology [11] -can provide us with fascinating series of snap shots of how reactive systems behave while passing over the transition state [12] and nanosecond laser "pump-and-probe" techniques allow to measure asymptotic scalar and vectorial quantities of the reactive collision e.g. nascent quantum state distributions of reaction products [13], absolute reaction cross sections [14], state-specific rates [15], and stereodynamical correlations [16], respectively. 

If the formation of molecular hydrogen is suppressed, nascent atomic hydrogen may diffuse into the interstices of the metal instead of being harmlessly evolved as a gaseous reaction product There are many chemical species which poison this recombination (e.g., cyanides, arsenic, antimony, or selenium compounds). However, the most commonly encountered species is hydrogen sulfide (H S), which is formed in many natural decompositions, and in many petrochemical processes [21]. 

Figure 1.2 Two distributions, P v), of vibrational states of HCl, drawn on a log scale vs. the vibrational quantum number v. Left the distribution measured for the nascent HCl product of the Cl + HI reaction (adapted from the observed chemiluminescence (in the infrared) of the vibrationally excited HCI(v) by D. H. Maylotte, J. C. Polanyi, and K. B. Woodall, J. Chem. Phys. 57, 1547 (1972). See also Polanyi (1987)). The observed distribution of HCI(v4 immediately after the reaction is qualitatively different from the distribution at thermal equilibrium, which is shown on the right. The distribution at thermal equilibrium is exponentially decreasing with the excess vibrational energy of HCl. Not so for the distribution on the left, which is loosely described as showing a population inversion.

Figure 1.3 shows a complete vibrational and rotational state population distribution for the nascent HD product of the H + D2 DH -1- D reaction studied by the pump and probe technique. 

Perfluoroisobutylene and potassium sulfide give 2,4-bis[hexafluoroisopro-pylidene]-l,3-dithietane and a small amount of the bis(perfluoro-tcrr-butyl)trisul-fide. The trisulfide is the result of a reaction of the nascent perfluoro-rerr-butyl carbanion with sulfur. The same products in different yields are obtained with sulfur and cesium fluoride [i] (equation 2). 

In the chemical process industry molybdenum has found use as washers and bolts to patch glass-lined vessels used in sulphuric acid and acid environments where nascent hydrogen is produced. Molybdenum thermocouples and valves have also been used in sulphuric acid applications, and molybdenum alloys have been used as reactor linings in plant used for the production of n-butyl chloride by reactions involving hydrochloric and sulphuric acids at temperatures in excess of 170°C. Miscellaneous applications where molybdenum has been used include the liquid phase Zircex hydrochlorination process, the Van Arkel Iodide process for zirconium production and the Metal Hydrides process for the production of super-pure thorium from thorium iodide. 

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