H and H2 transfer

The tandem mass spectrometer study with partially deuterated propanes and butanes by Abramson and Futrell [275] gave finer details of the mechanism of H and H2 transfer reactions. The more important conclusions obtained in the study of the -butane ion + propene reaction are summarized as (a) there is no H2 elimination from one carbon atom, (b) there is no H2 elimination from 1—3 (2—4) and 1—4 positions, (c) the ratio of the probabilities of 1—2 (3—4) elimination and 2—3 elimination is 1/2, (d) H transfer occurs only from secondary carbons and (e) the intermolecular and intramolecular isotope effects are very small. 

In the beginning of this section, it was stated that the olefin molecule must have fewer carbon atoms than the reactant alkane (or cycloalkane) ion (R R ) in order to be an H or H2 acceptor. However, it should be mentioned that very slight neutral transfer reactions have been found to occur between C2D6 and C2H4 and between CsDs and (R = R ) 

The H2 transfer reaction between C3H6 and CsHg in the propane system, described in Section 6.1.3, constitutes a similar case in the complementary process. On the other hand, no H or H2 transfer occurs when cyclopropane ions are impacted on cyclohexane, benzene or cyclobutane. 

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As has been mentioned in Section 6.1.5, H" and H2 transfer processes are general reactions between a saturated hydrocarbon (R Hj) and an olefinic ion (R ) having fewer carbon atoms (R < R ). Among many such combinations [29, 293], the systems CjHg + C4H,o have been most extensively studied [29, 267, 269, 292—295] and through this Section we will make special reference to these. 

The reactions with both propylene and cyclohexane involve H-atom abstraction as a prominent process. Both H and H2 transfer from cyclohexane are observed. These reactions have also been detected in experiments in which parent ions from 1-butene were impacted on cyclohexane, although the ratio of H"/H2 transfer in the latter case (0.18) was quite different from that observed here (1.6). These differences apparently reflect in part a steric sensitivity of these reactions which can provide useful information regarding reactant ion structure. 

F. P. Abramson and J. H. Futrell, Mass spectrometric investigation of H and H2 transfer reactions of hydrocarbon ions, J. Phys. Chem. 71, 1233-37 (1967). 

Since the experiments only involve interchanging the ionic and neutral reactants, it is plausible to consider that a similar reaction complex is involved in the respective H, H and H2, H2 transfer reactions. 

When both reactants have comparable ionization potentials, there is a close relationship between the H2 and H2 transfer reactions. For instance, a low efficiency of the H. transfer reaction from the alkane to a neutral olefin molecule (at least in the case of cyclohexane) is paralleled by a low efficiency of the corresponding H2 transfer process. Such a relationship can be accounted for by resonance phenomena. Unfortunately, not enough information is available on those systems where the... 

In the case of hydrocarbons, following Ausloos and Lias (1967) we can look at the various ion-molecule reactions in terms of transfer of H and H2 from the molecule to the ion, transfer of H, H2, and H+ from the ion to the molecule, and condensation reactions. Some examples are now given. 

A wide variety of chemical reactions can occur following ionization or excitation of a molecule in both gaseous and condensed phases. These may be of uni-molecular or bi-molecular nature, initiated by electrons, ions or by the transformations of excited or ionized molecules. These reactions include, but are not limited to, dissociation, elimination of atoms and smaller molecules (H, H2, etc.), transfer of H, H2, H, and H2, fragmentation, ion-molecule reaction, luminescence and energy transfer, neutralization, chain reaction, condensation, and polymerization, etc. These reactions will not be reviewed in this chapter but may be found elsewhere in this book. A brief summary is also found in Chapters 4 and 5 of Ref. 2. In the next section, some features of yields and mechanisms following excitation and/ or ionization in the liquid phase are discussed with special reference to water. 

Extremely slow class (4) reactions were observed for scavenging of (a) cyclohexane hole by cyclopropane [60] and (b) cyclohexane and decalins holes by O2 [75]. H atom transfer from the hole to O2 and H2 transfer from cyclopropane to the hole were suggested as the possible reaction mechanisms. 

A slab separates two fluids of different temperatures as shown in Fig. 4. There are no heat sources or sinks in the slab. Heat will be transferred from the fluid of higher temperature to the slab, then conducted through the slab, and then transferred from the wall to the fluid of lower temperature. Under steady state conditions, the heat transfer will be the same on both surfaces and through the slab. If the heat transfer coefficients h and h2 at each side of the slab are constant, the following equations apply ... 

Recent kinetics studies on protonation of [Ni(SEt)((Ph2 PCH2CH2)2PPh)]+ (14) proposed that the proton interacts with both the nickel and sulfur sites,consistent with the proposal of intramolecular proton transfer between cysteinate sulfur and Ni atom in the Ni-based hydrogenases. Additionally, the mononuclear complex [Ni(psnet)]+ (15) of known structure and a mildly negative redox potential can stoichiometrically evolve H2 from protic sources. On the basis of kinetics analysis, the reaction paths considered most probable involve steps of protic oxidative addition to Ni(I) to generate Ni -H , and electron transfer to Ni(III) followed... 

Consider a flat plate of thickness S (Fig. 3.16), Let the temperature distribution in the plate be time dependent. The upward and downward heat transfer coefficients respectively are h and h2, and the Biot numbers based on these coefficients allow a transversally lumped formulation. 

L. W. Sieck and S. K. Searles, High-pressure photoionization mass spectrometry. II. A study of thermal H (H) and H2 (H2) transfer reactions occurring in alkane-olefin mixtures, J. Am. Chem. Soc. 92, 7627-34 (1969). 

Oxygen exists mostly in neutral atomic form in diffuse interstellar clouds. In this case, the ionization necessary to initiate the ion-molecule reactions is provided by cosmic rays which penetrate the clouds and ionize H and H2 to form H and Hj. H2 subsequently reacts very rapidly with H2 to form Hj". The rate at which atomic hydrogen is ionized is denoted by Since the ionization potential of O is accidentally close to that of H, the charge transfer reaction... 

Elementary steps in forming and destro5ung small hydrocarbons in the ISM include collisions of C+ ions with H and H2, proton transfer from H3 to C or collisions of CH+ with H atoms. In the last few years, many relevant studies have been performed using ion traps. Of fundamental importance is the radiative association reaction... 

The formation and destruction of N2H are important for understanding the chemistry of interstellar clouds, comets, and planetary atmospheres. It is generally assumed that the N2H ion in interstellar clouds arises mainly from the reactions H3-HN2- N2H +H2 and N2 + H2- N2H + H and that dissociative recombination N2H -he - N2H-H and proton transfer reactions with abundant interstellar species such as CO are the major loss processes see for example [11 to 15]. 

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