Intermolecular H-Transfer

In poly(methacrylic acid), where this type of reaction is 50 times faster (von Sonntag et al. 1995 Ulanski et al. 1999a), even at room temperature subsequent depolymerization occurs as a very efficient process [reactions (12) and (13) Ulanski et al. 1999a,b], 

Because of a ready addition of the methacrylic acid released in reaction (13) to all radicals in this system, the situation of equilibrium polymerization is approached even at room temperature (Ulanski et al. 2000). 

In carbohydrates, P-fragmentation of radicals adjacent to the glycosidic linkage [e.g reactions (14) and (15)] will also lead to chain scission. 

These reactions are in competition with a fast hydrolysis of the glycosidic linkage [e.g. reaction (16)] which can be many orders of magnitude faster than that of the parent. 

These reactions are that fast that they occur at the time scale of pulse radiolysis (Deeble et al. 1990,1991 Ulanski and von Sonntag 2000). In DNA, such reactions will not lead immediately to a stand break, but they may well contribute to base release. 

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This intermolecular H transfer forms the (CH,),C ion which adds to another molecule of (CH,),C=CH2 continue the chain. A 3°H usually transfers to leave a 3° R. ... 

Two formyl transfer" reactions of isolated formyl complexes are shown in Eqs. (20) (37, 42) and (21) (37, 38, 42, 47, 66). Control experiments indicate that these reactions do not involve metal hydride intermediates (formed via decarbonylation). Straightforward intermolecular H transfer (rather than formyl ligand transfer) is believed to be taking place. 

According to Eigen s scheme of H-transfer [52] in solution the reaction partners have first to meet in order to react. For an intermolecular H-transfer from AH to B one can write... 

Table 6.5 Selected distances in systems with intra- and intermolecular H-transfers. 

Ho range - 3.0 + 6.8. The acidities do not change much with pretreatment temperature. Acid sites of strength Ho —3.0 — 6.8 appeared on pretreatment at 573 K, and remained unchanged up to 773 K, and slightly decreased at 873 K. The presence of Bransted acid sites is suggested by the incorporation of surface D atoms into the products as well as by the involvement of an intermolecular H transfer in butene isomerization. The acid sites are eliminated on reduction of in the binary oxide... 

Figure 14.35 Main types of proton tautomerizations. (a) Intramolecular H transfer, (b) Intermolecular H transfer, (c) Proton tau-tomerization coupled to intermolecular pro-...

Consequently, all the alkene 12 detected under such conditions was formed by the Cope-type elimination (b). On the other hand, alkene formed in the absence of thiophenol was generated by both paths. Recently, Marque et al showed that thermolysis of alkoxyamines in the presence of the scavenger PhSH induces chemically induced dynamic nuclear polarization (CIDNP). The CIDNP effect, detected by H-NMR, indicates that the intermolecular H-transfer occurs, in contrast to earlier reports,already in the spin correlated geminate (cage) radical pair, formed immediately after the cleavage of the NO C bond. 

Venkatesh P K, Dean A M, Cohen M H and Carr R W 1999 Master equation analysis of intermolecular energy transfer in multiple-well, multiple-channel unimolecular reactions. II. Numerical methods and application to the mechanism of the C. + O2 reaction J. Chem. Phys. Ill 8313... 

Further expansion of 13-vertex species or thermal metal transfer reactions leads to the 14-vertex cluster [(T -C H )Co]2C2B2qH22 [52649-56-6] and [52649-57-7] (199). Similar 14-vertex species have been obtained from tetracarbaboranes (203) and show unusual stmctures. The isomeric bimetallic cobaltacarborane complexes /(9j (9-(Tj -CpCo)2C2BgH2Q (cp = C H ) can be formed by either polyhedral expansion or contraction reactions. Six isomers of this cluster are formed in the thermally-induced intermolecular metal transfer and polyhedral expansion of the 11-vertex f/oj o-(ri -C H )CoC2BgH Q. 

Morise H, Shimomura O, Johnson FH, Winant J (1974) Intermolecular energy-transfer in bioluminescent system of Aequorea. Biochemistry 13 2656-2662... 

Table 5.3. The NBO descriptors of binary B HA H-bonded complexes (see Fig. 5.1), showing net intermolecular charge transfer A0b a, ctah bond ionicity / ah. and PNBO overlap integrals for attractive ri j ctah ( Vncr.) and repulsive iib-ctah (5no ) interactions...

Consistently with (5.30b)-(5.30e), the intermolecular charge transfer A0b a (Table 5.3, first column) clearly correlates with the strength of nB— oah interactions (Fig. 5.1), the A b h binding energy (Table 5.1, first column), and the (inverse) Rb...h distance (Table 5.1, second column). Furthermore, the transferred... 

Figure 5.14 Correlation of net H-bond energy (A2shb, squares) and principal n-a stabilization energy (AEn a,(2), circles) with intermolecular charge transfer (Qcr) cf. Table 5.15. (Approximate trend-lines are shown for each quantity to aid visualization.)...

In summary, the covalent/ionic-resonance picture can be used to describe the entire range of neutral and charged H-bonding phenomena. The NRT resonance weights (wcov and / , ) and bond orders (6a—h and 6b...h) are correlated in the expected manner with bond lengths, IR frequencies, intermolecular charge transfer, and other properties. 

The examples of Section 5.2 can be summarized by saying that H-bonds are an unusually strong form of n—a donor-acceptor interaction exhibiting the expected strong cooperative effects associated with intermolecular charge transfer. 

Laser flash photolysis of phenylchlorodiazirine was used to measure the absolute rate constants for intermolecular insertion of phenylchlorocarbene into CH bonds of a variety of co-reactants. Selective stabilization of the carbene ground state by r-complexation to benzene was proposed to explain the slower insertions observed in this solvent in comparison with those in pentane. Insertion into the secondary CH bond of cyclohexane showed a primary kinetic isotope effect k ikY) of 3.8. l-Hydroxymethyl-9-fluorenylidene (79), generated by photolysis of the corresponding diazo compound, gave aldehyde (80) in benzene or acetonitrile via intramolecular H-transfer. In methanol, the major product was the ether, formed by insertion of the carbene into the MeO-H bond, and the aldehyde (80) was formed in minor amounts through H-transfer from the triplet carbene to give a triplet diradical which can relax to the enol. 

Imahori H, Yamada H, Guldi DM et al (2002) Comparison of reorganization energies for intra- and intermolecular electron transfer. Angew Chem Int Ed 41 2344—2347... 

Probtem 6.38 Suggest a mechanism for alkane addition where the key step is an intermolecular hydride (H ) transfer. <... 

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