C–X···X–C vs C–H···X–C, Which One Is the More Dominant Interaction in Crystal Packing (X = Halogen)?

Three different approaches of statistical analyses, after surface area corrections, on the C–X···X–C (X = halogen) and C–H···X–C interactions have been performed. The analyses suggest that with respect to the accessible surface area on H and X, the C–H···X–C interactions are preferable over C–X···X–C interactions for all the halogens in the organic crystals. This is in contrast to the previous statistical analysis which stated that lighter halogens prefer C–H···X–C interactions whereas heavier halogens prefer C–X···X–C interactions. We have shown here the origin of the difference from the previous analysis. We also have shown here why it gives the impression that lighter halogens generally do not go for C–X···X–C interactions, but when they are replaced with the heavier halogens in the same molecule, the probability of formation of the C–X···X–C contacts increases. The preferred geometry of the C–X···X–C interactions with respect to the two ∠CXX angles and X···X distance have been shown. This study suggests that the high frequency of the C–X···X–C interactions for the heavier halogens is mainly due to their large surface area rather than their interaction strength.