Hydrogen bridged isomers

The first experimental example of a doubly-hydrogen bridged structure, i.e. fraws-[RSn(yU,-H)]2 (R = 2,6-Tip2CsH3, Tip = 2,4,6-triisopropylphenyl), was reported recently303. This compound is stable in the solid state at room temperature, but in hydrocarbon solvents (e.g. hexane, benzene, toluene) and diethyl ether it dissociates to RHSn monomers. 

Miriam Karni, Yitzhak Apeloig, JUrgen Kapp and Paul von R. Schleyer 

TABLE 28. Geometries and relative energies (AE) of various isomers of M2H4 and M2F4 and their dissociation energies (AE )a 

I iit i for the trans-isomer (48). The ns isomers are 2-3 kcal mol-1 less stable and their structural parameters are almost identical to those of the /ram-isomers. 

JFor 46, M = Si. k t. which is a minimum at B3LYP although with a very weak Si Si bond2843. k Values in parentheses are at B3LYP/DZP++ from Reference 284b. 

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Analogously, shieldings of antiperiplanar carbons by nitrogen lone pairs have been reported. It was concluded that a considerable part of the 10 ppm difference of the C-2 chemical shifts in the isomers of 8-methyl-8-azabicyclo[3.2.1]octane must be due to lone-pair interaction, since a difference of only 6 ppm is expected from a y-ganche effect of the A-methyl group73. This is supported by the fact that the difference is reduced if the solvent is methanol in this case the lone pairs are involved in hydrogen bridging. 

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