Infrared spectroscopic demonstration of cooperative and anti-cooperative effects in C-H--O hydrogen bonds

Summary

Matrix isolation infrared spectra of 1,2-cyclohexanedione (1,2-CHD) and 3-methyl-1,2-cyclohexanedione are measured in a nitrogen matrix at 8K temperature. The spectra reveal that in the matrix environment both the molecules exist exclusively in monohydroxy tautomeric forms with an intramolecular O–H⋯O=C hydrogen bonding. In the case of 3-MeCD, the fundamental of OH stretching ν_O−−H band appears more red-shifted with larger bandwidth indicating that the intramolecular O–H⋯O hydrogen bond of this molecule is somewhat stronger compared to that of 1,2-CD. Electronic structure calculations at B3LYP/6-311++G^∗∗ and MP2/cc-pVTZ levels predict that the monohydroxy tautomer of 1,2-CD is nearly 4.5 kcal/mol more stable than the corresponding diketo tautomer, but in the case of 3-MeCD, the stability difference between the diketo and preferred enol tautomer is more than 7.5 kcal/mol. Analysis of the geometric parameters reveals that the excess stabilization of the latter originates as a result of formation of an intramolecular O⋯H–O⋯H–C type interconnected hydrogen bonding network involving a methyl C-H bond, which interact in a cooperative fashion. The predicted infrared spectrum shows that the formation of such hydrogen bonding network causes large blue-shifting of the H-bonded methyl ν_C−−H transition, and this spectral prediction matches well with the features displayed in the measured spectrum.For intermolecular case, 1:1 complex between 1,2-cyclohexanedione and chloroform have been studied. Here two types of complex is possible, interconnected and bifurcated. In the interconnected complex a cooperative stabilizing effect and in the bifurcated complex an anti-cooperative destabilizing effect of the C–H⋯O hydrogen bond on the intramolecular O–H⋯O bond is observable. In the room temperature solution phase of FTIR spectra, the anti-cooperative complex is observable.