Besides oxygen, nitrogen, and fluorine, there is evidence that weaker hydrogen
bonding exists in other systems.44 Although many searches have been made for
hydrogen bonding where A is carbon,45 only three types of C–H bonds have
been found that are acidic enough to form weak hydrogen bonds.46 These are found
in terminal alkynes, RCCH,47 chloroform and some other halogenated alkanes,
and HCN. Sterically unhindered C–H groups (CHCl3, CH2Cl2, RCCH) form
short contact hydrogen bonds with carbonyl acceptors, where there is a significant
preference for coordination with the conventional carbonyl lone-pair direction.48