Spectroscopic evidence of special intermolecular interaction in iodomethane-ethanol mixtures: the cooperative effect of halogen bonding, hydrogen bonding, and solvent effect

Halogen bonding, an attractive interaction analogous to hydrogen bonding, has been widely investigated by computational methods. However, halogen bonding in solution is hard to study by spectroscopic technique since the intermolecular interaction often gives overlapped bands and may difficult to interpret. The traditional interpretation of iodomethane-ethanol mixtures considered only hydrogen bonding effect and the experimental investigation was limited. Here we employed the near-infrared (NIR) spectroscopy, Raman, density functional theory calculation, and the two-dimensional (2D) correlation analysis to find the evidence of the halogen bonding in iodomethane-ethanol mixtures. Our results suggest that the blue-shifting C-I stretching band is probably due to the cooperative effect from halogen bonding, hydrogen bonding, and the solvent effect, while the O-H band is the cumulative result of the dimer complex. The 2D correlation spectra further validate the hypothesis above and reveal the interaction evolution from the ethanol-rich region to the iodomethane region. These results indicate that the unique nature of iodomethane-ethanol mixture and the larger σ-hole strengthen the halogen bond, leading to the spectral results different from that of the other halogenated alkane.