Relationships between NMR shifts and interaction energies in biphenyls, alkanes, aza‐alkanes, and oxa‐alkanes with X─H…H─Y and X─H…Z (X, Y = C or N; Z = N or O) hydrogen bonding

Hydrogen–hydrogen C─H…H─C bonding between the bay‐area hydrogens in biphenyls, and more generally in congested alkanes, very strained polycyclic alkanes, and cis‐2‐butene, has been investigated by calculation of proton nuclear magnetic resonance (NMR) shifts and atom–atom interaction energies. Computed NMR shifts for all protons in the biphenyl derivatives correlate very well with experimental data, with zero intercept, unit slope, and a root mean square deviation of 0.06 ppm. For some congested alkanes, there is generally good agreement between computed values for a selected conformer and the experimental data, when it is available. In both cases, the shift of a given proton or pair of protons tends to increase with the corresponding interaction energy. Computed NMR shift differences for methylene protons in polycyclic alkanes, where one is involved in a very short contact (“in”) and the other is not (“out”), show a rough correlation with the corresponding C─H…H─C exchange energies. The “in” and “in,in” isomers of selected aza‐ and diaza‐cycloalkanes, respectively, are X─H…H─N hydrogen bonded, whereas the “out” and “in,out” isomers display X─H…N hydrogen bonds (X = C or N). Oxa‐alkanes and the “in” isomers of aza–oxa‐alkanes are X─H…O hydrogen bonded. There is a very good general correlation, including both N─H…H─Y (Y = C or N) and N─H…Z (Z = N or O) interactions, for NH proton shifts against the exchange energy. For “in” CH protons, the data for the different C─H…H─Y and C─H…Z interactions are much more dispersed and the overall shift/exchange energy correlation is less satisfactory. Computed NMR shifts for the bay‐area protons in biphenyls, for protons in congested octanes and di‐tert‐butylmethane and for protons in mainly hypothetical polycyclic alkanes with very short H…H distances, tend to increase with the absolute magnitude of the exchange component of the interaction energy for the protons in question. For the polycyclic alkanes and their heteroatom analogues, data for XH…HY and XH…Z hydrogen bonding can be compiled into two correlations, for X = N and X = C.