The C–H… O Hydrogen Bond in Crystals

A Practical Guide to the Design of Molecular Crystals

“…As far as the first question is concerned, the classification of C–H···O contacts as hydrogen bonds has been suggested and argued against since the 1960s. ,, Several comprehensive statistical and computational studies have since then established that such energetically weak interactions play a significant role in the stabilization of crystal structures. ,− Taylor recently conducted a statistical analysis of intermolecular contacts of structures deposited in the CSD to establish the significance of various types of close atom–atom contacts in the context of crystal packing, whereby a scale was devised by comparing the ratio ( R F ) of observed frequencies of occurrence of a specific atom···atom contact to its frequency expected at random. , These studies clearly established that molecular crystals exhibit significantly more C–H···X contacts than one would expect to observe if such contacts occurred randomly. Although computational studies of a smaller set of C–H···X interactions suggest that C–H···O and C–H···N interaction in structures of small molecules can be relevant in the stabilization of crystal lattices, it is now accepted that such interactions are of “secondary importance in directing the supramolecular assembly” in molecular crystals, and these were previously described as structure-directing only in the absence of stronger hydrogen and halogen bonds that are regularly used in crystal engineering exercises. , As to the second question, a related CSD study by Taylor demonstrated that O–H···F and N–H···F hydrogen bonds are indeed favorable interactions, but lack the strength to be competitive in the presence of other hydrogen-bond donors and acceptors …”

Analysis of Hydrogen Bonds in Crystals

“…One of the first debates on hydrogen bonds was based on the nature of interactions in crystal structures [11,12]. It was contested that for HB, the hydrogen atom has to be located only between electronegative centers; the C-H .…”

“…. O interactions were classified as HBs [11]. Several years later, Taylor and Kennard applied subtle statistical methods to analyze numerous crystal structures and they proved that the C-H .…”

“…It was contested that for HB, the hydrogen atom has to be located only between electronegative centers; the C-H…O interactions were classified as HBs [11]. Several years later, Taylor and Kennard applied subtle statistical methods to analyze numerous crystal structures and they proved that the C-H…Y (Y designates the Lewis base center) interactions possess characteristics of typical hydrogen bonds [13].…”

Exchange of ammine‐ and fluorido‐ligands in complex salts: The series [Cr(NH3)6][AlF6], [Cr(NH3)5F][SiF6] and K2[Cr(NH3)4F2][Si(NH3)0.5F5.5]2

“…This knowledge of hydrogen atom positions enables us to analyze the hydrogen bonding network occurring between the ammine ligands as donors and the fluorido ligands 4) pm indicate rather moderate strength of the hydrogen bonds, as the donor-acceptor distances are only slightly shorter than the sum of the van-der-Waals radii with 302 pm. [9,10] Additionally, the angles at the bridging hydrogen atoms with 145°-161°significantly deviate from 180°. Still, this compound proofed to be stable in air for at least several days without any apparent change in color or appearance, which might be associated with some effective stabilization by weak hydrogen bonding.…”