Antagonistic Interplay Between an Intermolecular CH···O and an Intramolecular OH···O Hydrogen Bond in a 1:1 Complex Between 1,2-Cyclohexanedione and Chloroform: A Combined Matrix Isolation Infrared and Quantum Chemistry Study

Matrix isolation infrared spectra of a weak C–H···O hydrogen-bonded complex between the keto–enol form of 1,2-cyclohexanedione (HCHD) and chloroform have been measured. The spectra reveal that the intramolecular O–H···O H-bond of HCHD is weakened as a result of complex formation, manifesting in prominent blue shift (∼23 cm–1) of the νO–H band and red shifts (∼7 cm–1) of νC═O bands of the acceptor (HCHD). The νC–H band of donor CHCl3 undergoes a large red shift of ∼33 cm–1. Very similar spectral effects are also observed for formation of the complex in CCl4 solution at room temperature. Our analysis reveals that out of several possible iso-energetic conformational forms of the complex, the one involving antagonistic interplay between the two hydrogen bonds (intermolecular C–H···O and intramolecular O–H···O) is preferred. The combined experimental and calculated data presented here suggest that in condensed media, conformational preferences are guided by directional hyperconjugative charge transfer interactions at the C–H···O hydrogen bonding site of the complex.