Anti-hydrogen

Figure 16. Optimized structures of the endo (lOa-anf and exo (IQb-anti) r -silyll allyl complexes with 4-(dimethylamino)-styrene as a substrate. Highlighted in (a) is the steric contact between the hydrogen of the benzylic carbon of the substrate and a hydrogen of the mesityl substituent 12 63 A) for the endo-anti intermediate, lOa-on/i. Highlighted in (b) is the steric contact between on hydrogen of the methyl group of the substrate and a hydrogen of the mesityl suhstituent 12 47 A) for the exo-anti silyl-allyl intermediate, Wb-anti. Hydrogen atoms are not shown except those whose interactions are highlighted.

To belabour the point on the occurrence of unphysical solutions made above, we will briefly refer to the issue of the Lamb shift in anti hydrogen, i.e. 

Quint, W., Kaiser, R., Hall, D.S. and Gabrielse, G. (1993). (Anti)hydrogen recombination studies in a nested Penning trap. Hyperfine Interactions 76 181-188. 

Walraven, J.T.M. (1993). Trapping and cooling of (anti)hydrogen. Hyperfine Interactions 76 205-220. 

There are three planes of symmetry that pass through any pair of anti-hydrogens. Ethane in this conformation is achiral. 

In (39), we have an example of a stereospecific formation of a pyrazo-line. When pyrazolines are photolyzed, the elimination of nitrogen, if concerted, should be syn. This stereospecific product has been observed (McGreer and Wu, 1967). The pyrolysis of pyrazolines, if concerted, should be anti. It is not clear why this process turns out to be complex in (39), one alkene is formed stereospecifically by an anti hydrogen migration, but two isomeric cyclopropanes are formed. Crawford and Ali (1967), find rather different product patterns in the pyrolysis of 3-methyl and 3,4-dimethylpyrazolines, and give evidence for diradical intermediates. The general problem of unstable intermediates in a step-wise process will be discussed in a later section. 

Hobza P, Spirko V, Havlas Z, Buchhold K, Reimann B, Barth H-D, Brutschy B (1999) Anti-hydrogen bond between chloroform and fluorobenzene. Chem Phys Lett 299 180-186... 

Cubero E, Orozco M, Hobza P, Luque FJ (1999) Hydrogen bond versus anti-hydrogen bond A comparative analysis based on the electron density topology. J Phys Chem A 103 6394-6401... 

The difference in the deprotonation rate of anti and syn oximes, the much higher kinetic acidity of the primary or secondary syn hydrogens than the primary anti hydrogens, and the fact that the anti dianion, in spite of the favorable deprotonation... 

P. Hobza, V. Spirko, H. L. Selzle, and E. W. Schlag, Anti-hydrogen bond in the benzene dimer and other carbon proton donor complexes, J. Phys. Chem. A 102, 2501-2504 (1998). 

E. Cubero, M. Orozco, and F. J. Luque, Electron density topological analysis of the C—H O anti-hydrogen bond in the fluoroform-oxirane complex, Chem. Phys. Lett. 310, 445-450 (1990). 

Hobza, P., and Spirko, V., Anti-hydrogen bond in the benzene dimer and other carbon proton... 

The magnitude of the pre-exponential factor provides further confirmation that methyl inversion is essentially a classical process, as expected for such a massive group. However, because lljj and are expected to have roughly the same inversion barriers, the failure to resolve the syn and anti hydrogen atoms in lljj and even in llj, is a strong indication that QMTdominates both H and D inversions. 

Resonance structure b is commonly used to describe delocalization of tt electrons in butadiene. Resonance structures c and d describe hyperconjugation. The structures represent electron transfer between anti hydrogens by ct-ct interactions. Note that the hydrogen atoms act as both electron donors and acceptors. As the two hydrogens are in very similar chemical environments, we expect littie net transfer of charge, but the delocalization affects such properties as NMR spin coupling constants. We will see in Topic 1.1 that this kind of delocalization is also important for hydrogens bonded to sp carbon atoms. 

The similar exchange reaction with t-indanechromium tricarbonyl, 77, led to the exchange of only the two anti hydrogen atoms. 18, indicating the great stereoselectivity of this reaction ... 

It seems likely that -H elimination occurs syn to Pd. However, Takacs et al. have made an interesting observation. When the cyclohexenyl carbonate 504, which has both syn and anti hydrogens, was subjected to the elimination, the cyclohexadiene 507 was obtained as a sole product, which should be formed by elimination of H anti to Pd in 505. The diene by syn elimination to give 506 was not formed [192]. 

Figure 12.66 (a) terms (in Hz) for cis- and trans-t-butylcyclohexanol. Both experimental and calculated (in parentheses) are given, (b) Comparison of j y terms for the syn and anti hydrogens in formaldehyde oxime. 

---