Dimers and trimers of formamidine and its mono-halogenated analogues HN[dbnd]CHNHX, (X = H, Cl, Br, or I): A comparative study of resonance-assisted hydrogen and halogen bonds

Abstract

The MP2 ab initio method, and the M052X and the B3LYP density functional theory methods were used to investigate the geometries and energetics of dimers and trimers of formamidine and its mono-halogenated analogues. The primary purpose of this study is to examine the strength of the resonance-assisted NX⋯N interactions for X=H (hydrogen bond), relative to that for X=Cl, Br, or I (halogen bond). It is found that, for the dimers and trimers studied here, the hydrogen bond interaction is stronger than the halogen bond interactions when the halogen is either Cl or Br. For dimers, the NH⋯N interaction is actually of comparable strength to that of the NI⋯N interaction. For example, at the MP2 level the NH⋯N interaction energy is −7.47kcal/mol, and the corresponding NI⋯N interaction energy is found to be −7.13kcal/mol. Trimerization produces a small decrease in the hydrogen bond strength, with a hydrogen bond energy of −6.90kcal/mol at the MP2 level. The opposite is true for the halogen bonds which actually get stronger upon trimer formation, not all to the same extent though. At the MP2 level, for instance, the magnitudes of the interaction energies increase by factors of 1.28 and 1.98 for NCl⋯N and NBr⋯N respectively. A much more substantial increase factor of 3.36 is found in the NI⋯N interaction energy upon trimer formation. The strengthening of the NI⋯N interaction is so dramatic that the iodine is found midway between the two nitrogen atoms, and the interaction in the trimer is better described as a symmetric N⋯I⋯N interaction.