Hydrogen bonding to chloroform.

Chloroform, often in the deuterated form CDCl₃, is a very common solvent for NMR and other types of spectroscopy. Quantum mechanics is increasingly used to calculate such spectra to aid assignment and the solvent is here normally simulated as a continuum rather than by explicit inclusion of one or more chloroform molecules. But what are the features of the hydrogen bonds that form from chloroform to other acceptors? Here I do a quick search for the common characteristics of such interactions.

1. This first search (R < 0.05, no errors, no disorder) is for interactions from the CH… O, and is a plot of that distance against the angle subtended at the oxygen.

   Note that there are not that many crystalline examples. The “hotspot” is at a distance of ~2.3Å, but real examples down to 1.9Å exist. The angle subtended at the oxygen is close to 120° (the angle subtended at the hydrogen is always close to 180°). The plot below constrains the search to data collected below 140K to reduce the thermal noise in the measurements, with the hotspot shortening slightly to 2.2Å. 

2. The next search is for interactions to N rather than O (T < 140K). There are rather fewer hits, but again with similar features.
3. Finally, an attempt to see if there is a correlation between the C-H length and the H…O length. 

   This has odd characteristics, which suggests that in most cases the C-H distance is not measured from the diffraction data but simply “idealised” (and which therefore renders this plot meaningless). Unless its been added recently, it is not possible to specify in the search how the hydrogen positions have been refined, if at all and hence to restrict the search only to those structures where the C-H distance is meaningful.

In the last ten years or so, great progress has been made in assigning experimental spectra with the help of quantum calculations. This is true of chemical shifts in NMR, but especially so for chiroptical measurements such as ORP, ECD and VCD. Given that explicit hydrogen bonds can introduce anisotropy into the otherwise isotropic solvent continuum, it might be worth including perhaps one chloroform molecule into these calculations, especially if the  CH…O distance is <2Å (which suggests it is fairly strong). If nothing else, chloroform is rather big and might exert effects based on dispersion attractions or steric repulsions as well as the H-bonding.