Donor and acceptor strengths in C–H···O hydrogen bonds quantified from crystallographic data of small solvent molecules

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Abstract
The influence of donor and acceptor strengths on mean distances in C–H···O hydrogen bonds is quantified in a crystallographic database study. Small solvent molecules are used as donors, which are cocrystallized in organic and organometallic crystals. This system provides chemically homogeneous sets of C–H groups, allowing better defined statistical analysis compared to studies based on molecular fragments. Eight kinds of O-atom acceptors are considered, ranging from the very strong acceptor PO to the weak acceptor M–CO (M=transition metal atom). For the strongest donor considered, chloroform, mean hydrogen bond distances vary by ca. 0.5 Å as the acceptor strength is varied from PO to M–CO. For the weak donor acetone, this variation is only ca. 0.1 Å. It is confirmed that even weakly polar C–H groups can donate, and that the O-atoms of M–CO groups can accept C–H···O hydrogen bonds.

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