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

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 accepters are considered, ranging from the very strong acceptor P=O 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 Angstrom as the acceptor strength is varied from P=O to M-CO. For the weak donor acetone, this variation is only ca. 0.1 Angstrom. 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.