Proton distances

O- F distance proton is transferred towards fluoride and the hydrofluoric acid is formed. The O F distance for the proton transfer for variously substituted phenols correlate nicely with the Hammett constants o-p, as shown in Fig. 22. 

Interpretation of the above dependence is as follows. The stronger electron-attracting power of the substituent is (more positive value of o-p), the lesser is the electron density at the oxygen atom in -OH group, thus the weaker attraction of proton by the oxygen atom, and hence at a longer O- F distance proton may be transferred from the hydroxyl group to the fluoride anion. 

Figure 11 presents the dependencies between the H- H intermolecular distance proton acceptor (acceptor is the negatively charged hydrogen atom) and the interaction energy components. 

The C-4 and C-5 carbons do not show remarkable change in the spin-lattice relaxation times on the mixing with adenine. The relaxation of these carbon nuclei is governed by the long distance protons including the intermolecular interaction. Thus the change in the relaxation times can not be simply explained by the change in the molecular radius. 

At the stage of vinylgold or arylgold intermediates, most catalytic cycles show the proto-deauration as the final step. In many of these reactions, rather than a direct proton transfer from the nucleophile which was added, a long-distance proton transfer is necessary. A detailed investigation revealed that gold catalysis not only tolerates water but also utilizes clusters of four or more water molecules as proton shuttles [53]. 

Scheme 17.6. Ion-neutral complexes that allow long distance proton transfer along the steroid skeleton to yield two pairs of complementary product ions (a) thermal process, (b) charge driven process. (Adapted with permission from Ref. 48.)...

The formation of ion-dipole complexes from bifunctional compounds can yield suppression (or attenuation) of stereochemical effects. Indeed, from the cis- and trans-l-alkoxy-l-methyl-4-(2-aIkoxy-2-propyl)cyclohexanes, the fact that regioselective methanol loss induced by a 1,2-hydride transfer is not stereospecific suggests a possible isomerization of the protonated precursor ion into a common ion-neutral complex (Scheme 17.11). From this complex, long-distance proton transfer can be considered to be independent of the initial cis/trans stereochemistry. Such isomerization is especially favored from long lifetime precursor ions such as those that exist during ion storage in an ion trap instrument. ... 

The atomic unit (AU) of dipole moment is that of a proton and electron separated by a distance equal to the first Bohr orbit, oq. Similarly, the au of polarizability is Oq [125]. Express and o for NH3 using both the cgs/esu and SI approach. 

Figure A3.8.3 Quantum activation free energy curves calculated for the model A-H-A proton transfer reaction described 45. The frill line is for the classical limit of the proton transfer solute in isolation, while the other curves are for different fully quantized cases. The rigid curves were calculated by keeping the A-A distance fixed. An important feature here is the direct effect of the solvent activation process on both the solvated rigid and flexible solute curves. Another feature is the effect of a fluctuating A-A distance which both lowers the activation free energy and reduces the influence of the solvent. The latter feature enliances the rate by a factor of 20 over the rigid case.

Fig. 3. Time evolution of the distance between the Zr atom and each of the three hydrogen atoms belonging to the methyl group (the original methyl group bonded to the Zr) in the zirconocene-ethylene complex. The time-evolution of one of the hydrogen atoms depicted by the dotted curve shows the development of an a-agostic interaction. Later on in the simulation (after about 450 fs) one of the other protons (broken curve) takes over the agostic interaction (which is then a 7-agostic interaction).

In Eq. (16 i denotes an atom up to lour non-rotatable bonds away from the proton and is the total number of those atoms. A bond is deRned as non-rotatable if it belongs to a ring, to a. T-system, or to an amide functional group q- is the partial atomic charge of the atom i, and is the 3D distance between the proton j and the atom i. Figure 10.2-5 shows an example of a proton RDF descriptor. 

In the plot gii(rj against r. each 3D distance contributes to a peak, and the contribution is proportional to q-.. Values ofg (r) at fixed points arc used as descriptors of the proton. 

Here i is now a double bond up to the seventh sphere (D,7 ) of non-rotatablc bonds centered on the proton, Td is the distance of the proton from the center of the double bond, and is the angle, in radians, between the plane defined by the bond and the distance q, (Figure 10.2-6a). 

Shielding and unshiclding by. single bonds were encoded using Eq, (IS), where i is a single bond up to the seventh sphere (S,7j) of non-rotatable bonds centered on the proton, and and are distance and angle, respectively (Figure 1().2-6b). 

Figure 10.2.6. Special distance measures for the characterization of proton environments a) distance r and angle a, to double bonds b) distance and angle Oc, to single bonds c) dihedral angle a, to the third bond from the hydrogen atom.

For the atomic orbital = e where r is the radial distance between the proton and the electron, show that [Eq. (6-21)]... 

Structure determines properties and the properties of atoms depend on atomic struc ture All of an element s protons are m its nucleus but the element s electrons are dis tributed among orbitals of varying energy and distance from the nucleus More than any thing else we look at its electron configuration when we wish to understand how an element behaves The next section illustrates this with a brief review of ionic bonding... 

Use Learning By Modeling to compare the C—O bond distances in 1 2 epoxypropane and Its protonated form... 

How do the bond distances of 1 2 epoxypropane change on protonation of the nng oxygen" Assum ing that the longer C—O bond is the weaker of the two do the bond distances in the protonated form correlate with the regioselectivity of acid catalyzed ring opening ... 

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