Direct matrix isolation IR spectroscopic evidence of halogen bonding from a comparative study of complexes of CBr and CClwith acetone and formic acid.

Binary complexes of acetone and formic acid with tetrahalomethanes CBr and CCl have been isolated in argon matrix. Spectral shifts in the characteristic ν region of acetone, as well as in the fingerprint regions, are unambiguously assigned to the formation of halogen bond involving one of the halogen atoms on CBr/CCl as donor, and the carbonyl oxygen of acetone as acceptor. The higher magnitude of shifts of ν and the fingerprint vibrations for the CBr complex, as compared to the CCl complex, is consistent with theoretical predictions of higher value of positive electrostatic potential in the "σ-hole" region of the former, and hence its higher susceptibility to halogen bonding. The formation of halogen bonded complexes involving formic acid as acceptor and CBr/CCl as donors is also being reported for the first time. In this case too, distinct shifts are obtained in the ν as well as ν regions of formic acid, which again are significantly larger in magnitude for the CBr complex, as compared to the CCl complex. Electronic structure calculations have been carried out using different theoretical methods to identify the various possible structural isomers of the halogen bonded complexes, and to obtain relevant information regarding their energies and intermolecular geometrical parameters. In addition, NBO and AIM analysis have been carried out to understand the role of local interactions at the halogen bonded interface. Such predicted data are found to be consistent with experimental observations, and re-assert the stronger nature of CBr as halogen bond donor, as compared to CCl.