Translational excitation, selective

Comparison of the collision numbers given above for vibration-vibration transfer with those for vibration-translation, given in Section 4, shows that in many cases vibration-vibration transfer between two resonant or near-resonant modes is much more efficient than vibration-translation transfer from either. This applies equally to homomolecular and heteromolecular collisions, and carries the interesting consequence that the quickest route for vibrational excitation of upper levels from the ground level by homomolecular collisions is an initial vibration-translation excitation to the v = 1 level, followed by successive vibration-vibration transfers to higher levels. Because of the selection rule, Av = 1,... 

SELECTIVITY OF TRANSLATIONAL EXCITATION IN REACTIVE COLLISIONS. THE ALKALINE PLUS ALKYL HALIDE REACTION FAMILY... 

ABSTRACT. This paper represents recent results on the reaction dynamics of the M + RX MX + R (M = alkali, X = halogen and R = radical group) family obtained from crossed molecular beam studies. The selectivity of the translational excitation of the reactants as weU as of the chemical nature of the M, R and X group is outlined. A comparison of these reactive processes with photofragmentation and electron attachment studies of the same RX molecule is also presented revealing important similarities. This common behaviour seems to indicate ttie same overall selectivity as a result of the similar main dynamics associated with the same R-X bond rupture. 

One of the main goals of the present paper is to review and discuss the most recent data and new findings on the family of reactions of the title. Emphasis will be made on the selectivity of the translational excitation of the reagents, as well as of the chemical nature of the M, X and R groups. A comparison of these reactive processes with those of photofragmentation and electron attachment of the same alkyl halides is also presented. 

TABLE 1. Family trends and comparisons in the M + RX -> MX + R system. Selectivity and translational excitation effects. 

As with the quadmpole ion trap, ions with a particular m/z ratio can be selected and stored in tlie FT-ICR cell by the resonant ejection of all other ions. Once isolated, the ions can be stored for variable periods of time (even hours) and allowed to react with neutral reagents that are introduced into the trapping cell. In this maimer, the products of bi-molecular reactions can be monitored and, if done as a fiinction of trapping time, it is possible to derive rate constants for the reactions [47]. Collision-induced dissociation can also be perfomied in the FT-ICR cell by tlie isolation and subsequent excitation of the cyclotron frequency of the ions. The extra translational kinetic energy of the ion packet results in energetic collisions between the ions and background... 

Fig. 11.16. Representation of three tandem mass spectrometry (MS/MS) scan modes illustrated for a triple quadrupole instrument configuration. The top panel shows the attributes of the popular and prevalent product ion CID experiment. The first mass filter is held at a constant m/z value transmitting only ions of a single mlz value into the collision region. Conversion of a portion of translational energy into internal energy in the collision event results in excitation of the mass-selected ions, followed by unimolecular dissociation. The spectrum of product ions is recorded by scanning the second mass filter (commonly referred to as Q3 ). The center panel illustrates the precursor ion CID experiment. Ions of all mlz values are transmitted sequentially into the collision region as the first analyzer (Ql) is scanned. Only dissociation processes that generate product ions of a specific mlz ratio are transmitted by Q3 to the detector. The lower panel shows the constant neutral loss CID experiment. Both mass analyzers are scanned simultaneously, at the same rate, and at a constant mlz offset. The mlz offset is selected on the basis of known neutral elimination products (e.g., H20, NH3, CH3COOH, etc.) that may be particularly diagnostic of one or more compound classes that may be present in a sample mixture. The utility of the two compound class-specific scans (precursor ion and neutral loss) is illustrated in Fig. 11.17.

It is now possible to design the experiments using molecular beams and laser techniques such that the initial vibrational, rotational, translational or electronic states of the reagent are selected or final states of products are specified. In contrast to the measurement of overall rate constants in a bulk kinetics experiment, state-to-state differential and integral cross sections can be measured for different initial states of reactants and final states of products in these sophisticated experiments. Molecular beam studies have become more common, lasers have been used to excite the reagent molecules and it has become possible to detect the product molecules by laser-induced fluorescence . These experimental studies have put forward a dramatic change in experimental study of chemical reactions at the molecular level and has culminated in what is now called state-to-state chemistry. 

Brook and co-workers were able to excite the HC1 molecules to v = 1 state using a chemical laser and showed that HC1 (v = 1) was 100 times more reactive than HC1 (v = 0) on collision with K atom. In a velocity selected beam experiment it was also observed that for the same amount of energy reagent, vibration was ten times more effective than translational in bringing out the reaction... 

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