σ- and π-hole interactions of iodotrifluoroethylene (C2F3I) with tertiary amines characterized by matrix isolation IR spectroscopy.

Abstract

Under matrix isolation conditions, weakly bound complexes can be stabilized and even higher energy species can be kinetically trapped and subsequently characterized spectroscopically. Halogen bonded and π-hole complexes of iodotrifluoroethylene and different tertiary amines are identified in pH2 matrices. For ITFE-trimethylamine, the CC stretching mode of ITFE is found to show a characteristic red-shift associated with σ-hole (halogen bonding) interactions with either N or p-systems as acceptors. In contrast, a blue-shift is characteristic of a π-hole interaction with either donor moiety. We demonstrate the transferability of these findings to other bulkier amines by studying complexes of ITFE with α,N,N-trimethyl benzyl amine (TMBA) and N-methyl-N,N-bis(α-phenylethyl)amine (MePE2A), for which only C-I⋯N and C-I⋯π complexes could be identified. The origin of the characteristic shifts of the CC stretching mode is discussed. Furthermore, it is shown that the matrix environment plays a key role in the complex formation process, that is, it blocks π-surfaces for secondary interactions after formation of stronger σ-hole contacts.