Cooperative Strengthening of an Intramolecular O—H···O Hydrogen Bond by a Weak C—H···O Counterpart: Matrix-Isolation Infrared Spectroscopy and Quantum Chemical Studies on 3-Methyl-1,2-cyclohexanedione

Abstract

Matrix-isolation infrared spectra of 1,2-cyclohexanedione (CD) and 3-methyl-1,2-cyclohexanedione (3-MeCD) were measured in a nitrogen matrix at 8 K. The spectral features reveal that, in the matrix environment, both molecules exist exclusively in the monohydroxy tautomeric form, which is stabilized by an intramolecular O-H...O=C hydrogen bond (HB). The nu(O-H) band of the enol tautomer of 3-MeCD appears at a relatively lower frequency and displays a somewhat broader bandwidth compared to that of CD, and these spectral differences between the two molecules are interpreted as being due to the formation of an interconnected C-H...O HB, where the enolic oxygen is the HB acceptor and one of the C-H covalent bonds of the methyl group is the HB donor. Electronic structure calculations at the B3LYP/6-311++G**, MP2/6-311++G**, and MP2/cc-pVTZ levels predict that this tautomer (enol-2) is approximately 3.5 kcal/mol more stable than a second enolic form (enol-1) where such interconnected H-bonding is absent. Theoretical analysis with a series of molecules having similar functional groups reveals that part of the excess stability (approximately 1 kcal/mol) of enol-2 originates from a cooperative interaction between the interconnected C-H...O and O-H...O HBs. In the IR spectrum, a weak band at 3007 cm(-1) is assigned to nu(C-H) of the methyl C-H bond involved in the H-bonded network. The spectra predicted by both harmonic and anharmonic calculations reveal that this transition is largely blue-shifted compared to the fundamentals of the other two methyl C-H stretching frequencies that are not involved in H-bonding. The conclusions are corroborated further by natural bond orbital (NBO) analysis.

Structural significance of hypermodified nucleic acid base hydroxywybutine (OHyW) which occur at 37th position in the anticodon loop of yeast tRNAPhe

“…Additional stabilization is also expected from interactions between O(23)· · ·HC (15) and O(22)· · ·HC (16). Such C O· · ·HC interactions have also been found important in the conformational study of wybutine [21] as well as in case of other molecules [49][50][51][52][53][54][55][56]. The bifurcated hydrogen bonding interactions between HO(16)· · ·O(6)· · ·HC(15) (Fig.…”

A theoretical investigation of the interaction between substituted carbonyl derivatives and water: Open or cyclic complexes?

“…Let us notice the fact that the cyclic structures are more stable than the open ones that have been outlined in other bifunctional water complexes where the CH···O w interaction stabilizes the structure. [57–60] The cooperativity of CH···O and OH···O is still a matter of discussion. In some systems, the CH···O bonds are much less cooperative than the OH···O ones[13] and in other ones, the OH···O bonds seem to be less cooperative than the CH···O ones.…”

Microsolvation of anions by molecules forming CH··X− hydrogen bonds

“…Indeed the first direct evidence of aromatic CH••O HBs in gas phase clusters did not arise until 2005 [56] wherein 1,2,4,5-tetrafluorobenzene served as proton donor to a variety of bases. In terms of calculations, most work has been limited to only small clusters [57][58][59][60][61][62]. For example, homooligomers of CH 3 CN up to tetramer were examined [63] so as to measure the degree of cooperativity, as were clusters of H 2 CO up to the pentamer level [64].…”