Weak hydrogen bonds theoretical calculation

It is well accepted that the high diastereospecificify of aldehyde allylboration reactions is a consequence of the compact cyclic transition structure. Theoretical calculations have shown that the chairlike transition structure shown in Scheme 1 and Fig. 1 is the lowest in energy relative to other possibilities such as the twist-boat conformation. With boronate reagents, it has also been suggested that a weak hydrogen bond between the axial boronate oxygen and the hydrogen of the polarized formyl unit contribntes to the preference for the transition structme with the aldehyde substituent in the psendo-eqnatorial position. ... 

The first reported gas phase electronic spectra of DNA base pairs described hydrogen bond frequencies of GC clusters, measured by REMPI [26], On the one hand these frequencies agreed quite well with theoretical predictions. On the other hand, the six hydrogen bonding modes between two molecules of a given mass are only very weakly dependent on cluster structure and can therefore neither serve as a structural tool nor as a good benchmark for theory. Moreover, REMPI only measures excited state vibrations, while the best calculations apply to the ground state. 

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