Cyclic dimers of formamidine with its N-halogenated formamidine analogues: Structure, energetics, and proton-halonium transfer

Abstract

Ab initio calculations were carried out to investigate cyclic dimers of formamidine with its N-halogenated analogues, HN = CHNHX (X = Cl, Br, or I). Geometry optimizations and frequency calculations were completed with the MP2 method and both the aug-cc-pVDZ and the aug-cc-pVTZ basis sets. BSSE-corrected interaction energies were calculated at the MP2, QCISD, and CCSD(T) methods. Electron density at relevant critical points, and charge transfers were examined through AIM and NBO analyses respectively. Both the electron density at each of the critical points and the amount of charge transfer correlate with the strength of the dimer interactions. The N-X stretching frequencies and the 15N NMR chemical shifts were also found to correlate with the dimer interaction strengths. Proton-halonium transfer for each of the dimers was also investigated. For any given X, the potential energy profile along the intrinsic reaction coordinate demonstrates that the proton-halonium transfer occurs in one step, without any intermediates. Similarly, the profile of the corresponding force constant (the second derivative of the potential energy) in the transition region indicates that the proton-halonium transfer is a synchronous process. In all cases, the energy barrier for proton-halonium transfer was found to decrease with increasing size of the halogen. Solvent effects were examined using the SMD model with two solvents of differing polarity: tetrahydrofuran and water. Although the proton-halonium transfer remains concerted and synchronous, the energy barrier to proton-halonium transfer increases in solution to the extent that the barrier becomes higher for X = I than it is for X = Br.