Unimolecular precursor

SCHEME 13.1 Important potential processes available to a geminal radical pair in a polymer cage. A unimolecular precursor, A-B, is assumed here for purposes of illustrating step [1]. 

Molybdenum disulfide, M0S2, can be prepared by chemical vapor deposition using M0CI5 [116, 117], MoFf, [117] or Mo(CO)6 (9) [118] as the molybdenum source. The sulfur source in each case is hydrogen sulfide. The molybdenum tetrakis(thiolate) compound, Mo(S-f-Bu)4, is a unimolecular precursor to M0S2 and decomposes at temperatures as low as 100°C [111]. 

Tungsten nitride shows extreme hardness, high chemical stability and high electrical conductivity [172]. The compound is a promising candidate for diffusion barriers and gate electrodes in electronic devices [173-175]. This material has been prepared by the CVD of a mixture of tungsten hexachloride, NH3 and H2 at 500-900 °C [172], from a mixture of WF6/NH3/H2 at 450-650°C [176] and from the unimolecular precursor (t-BuN)2 V(NH-/-Bu)2 at 450-650°C ]177]. 

In the FFR of the sector mass spectrometer, the unimolecular decomposition fragments, and B, of tire mass selected metastable ion AB will, by the conservation of energy and momentum, have lower translational kinetic energy, T, than their precursor ... 

Correlated or geminate radical pairs are produced in unimolecular decomposition processes (e.g. peroxide decomposition) or bimolecular reactions of reactive precursors (e.g., carbene abstraction reactions). Radical pairs formed by the random encounter of freely diffusing radicals are referred to as uncorrelated or encounter (P) pairs. Once formed, the radical pairs can either collapse, to give combination or disproportionation products, or diffuse apart into free radicals (doublet states). The free radicals escaping may then either form new radical pairs with other radicals or react with some diamagnetic scavenger... 

Newkome, G.R., Moorefield, C.N., Keith, J.M., Baker, G.R., and Escamilla, G.H. (1994) Chemistry of micelles. 37. Internal chemical transformations in a precursor of a unimolecular micelle boron supercluster via site-specific addition of BioH14 to cascade molecules. Angew. Chem., Int. Ed. Engl. 33, 666-668. 

Precursor Ion Ion that reacts to form particular product ions. The reaction can be unimolecular dissociation, ion/molecule reaction, isomerization, or change in charge state. The term parent ion is deprecated (but still very much in use). 

The Lindemann kinetics for unimolecular reactions [185] can be formally recovered if one subsumes the formation of a collision complex with the precursor complex steps R1-R2 <—> APC) into one corresponding to the excited reactant Rl. The excited... 

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.

This mechanism clearly implicated alkane complexes as precursors to C-H activation but the IR absorptions of [Cp Rh(CO)Kr] and [Cp Rh(CO)(C6Hi2)] were not resolved and were presumed to be coincident. The temperature dependent data gave values of AH = 18 (or 22) kj mol for the unimolecular C-H (or C-D) activation step representing a normal kinetic isotope effect, kn/fco 10- However, an inverse equilibrium isotope effect (K /Kq 0.1) was found for the slightly exothermic pre-equilibrium displacement of Kr by CoHn/C Dn implying that C6Dj2 binds more strongly to the rhodium center than does C Hn-... 

The tin hydride method suffers from one major disadvantage, namely the efficiency of the reagent as a hydrogen atom donor. For successful synthesis, alkenes have to be reactive enough, otherwise direct reduction of the starting precursor becomes a considerable side reaction. In practice, the yields are increased by slow addition of a solution of tin hydride and a radical initiator into the reaction mixture containing an excess of alkene. However, a delicate balance must be maintained. If a large excess of olefin is used, polymerization can compete. 2,2-Azobisisobutyronitrile is the most commonly employed initiator, with a half-life time for unimolecular scission of 1 h at 80°C. 

Diels-Alder reaction, for example, is much better suited as a unimolecular reaction than the bimolecular cycloaddition because the former allows better control of precursors, in which the structural properties are well defined (96JA8755). Fragmentation of a molecule may be initiated by various methods depending on how the required energy is supplied. 

It is generally much easier to study unimolecular than bimolecular processes. It should be noted in this connection that some reactions that are bimolecular in solution are unimolecular in crystals. For example, the coupling or disproportionation of two radicals generated in a crystal cage from a single precursor molecule is a first-order reaction of a radical pair, not a second-order reaction of independent radicals. 

In competition with these oxidation reactions are two unimolecular elimination-reactions which eventually lead to 64 and 65 (Table V, reactions 128 to 133). Product 65 may also have the radical at C-3, namely, 66, as a precursor (reaction 134). As expected, the precursor radicals 62 and 66 are rapidly oxidized in the presence of Fe3+ ions, and the formation of 65 and 65 is suppressed (see Table V). 

Substitutions by the SRn 1 mechanism (substitution, radical-nucleophilic, unimolecular) are a well-studied group of reactions which involve SET steps and radical anion intermediates (see Scheme 10.4). They have been elucidated for a range of precursors which include aryl, vinyl and bridgehead halides (i.e. halides which cannot undergo SN1 or SN2 mechanisms), and substituted nitro compounds. Studies of aryl halide reactions are discussed in Chapter 2. The methods used to determine the mechanisms of these reactions include inhibition and trapping studies, ESR spectroscopy, variation of the functional group and nucleophile reactivity coupled with product analysis, and the effect of solvent. We exemplify SRN1 mechanistic studies with the reactions of o -substituted nitroalkanes (Scheme 10.29) [23,24]. 

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