CH⋯O H-bond mediated tautomerization of 2-methyl-1,3-cyclohexanedione: A combined IR spectroscopic and theoretical study.

Molecular association and its impact on the keto-enol tautomerization of 2-methyl-1,3-cyclohexanedione (MCHD) have been investigated in low temperature argon matrix and thin solid film. The system exists exclusively in diketo tautomeric form in argon matrix. The CH⋯O H-bonded homodimers of the diketo tautomer are produced by annealing the matrix at 28 K. No trace of the keto-enol tautomer is observed in matrix isolated homodimers in the temperature range of 8-28 K. However, tautomeric conversion initiates in a thin film of pure diketo tautomer when the temperature of the film is raised to ~170 K. Transition state calculations on the monomeric and dimeric MCHD demonstrate that CH⋯O H-bond formations between diketo tautomers play a vital role in lowering the tautomerization barrier. However, the extent of CH⋯O H-bonded dimer formation in matrix isolation, as well as extent of tautomerization in the neat sample are found to be smaller than that for the previously reported 1,3-cyclohexanedione (CHD) under similar experimental conditions (J. Phys. Chem. A 2012, 116, 3836-3845). Electronic structure calculations suggest that formation of the CH⋯O H-bonded dimer is less feasible in presence of the bulky 2-methyl groups of MCHD, as compared to CHD. Additionally, the transition state geometry of the dimeric keto-enol product of MCHD, as compared to the same for CHD, is more strained and offers a weaker CH---O H-bond that contributes to lesser tautomeric conversion in the former.

Direct matrix isolation IR spectroscopic evidence of halogen bonding from a comparative study of complexes of CBr4 and CCl4with acetone and formic acid.

Binary complexes of acetone and formic acid with tetrahalomethanes CBr4 and CCl4 have been isolated in argon matrix. Spectral shifts in the characteristic νC=O 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 CBr4/CCl4 as donor, and the carbonyl oxygen of acetone as acceptor. The higher magnitude of shifts of νC=O and the fingerprint vibrations for the CBr4 complex, as compared to the CCl4 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 CBr4/CCl4 as donors is also being reported for the first time. In this case too, distinct shifts are obtained in the νC=O as well as νC-O regions of formic acid, which again are significantly larger in magnitude for the CBr4 complex, as compared to the CCl4 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 CBr4 as halogen bond donor, as compared to CCl4.

Modulations of νO-H and νC═O Stretching Frequencies of Difluoroacetic Acid with Internal Rotation of CHF2 Rotor: A Combined Vapor Phase and Matrix Isolation Infrared Spectroscopy Study.

Mid-infrared spectra of difluoroacetic acid (DFAA) have been measured by isolating the molecule in argon and nitrogen matrices at 8 K and also in the vapor phase at room temperature. In argon matrix, the O-H stretching fundamental (νO-H) of -COOH group appears as a doublet with band maxima at 3554 and 3558 cm-1, and a similar doublet for C═O stretching fundamental appears at 1800 and 1810 cm-1. In the vapor phase, the νO-H transition is featured with multiple peaks, and the observed band shape has been deconvoluted as superposition of two transitions both having A-type rotational band contours. We have attributed these transitions to the two internal rotational isomers corresponding to the two distinct minima along -CHF2 torsional coordinate of the molecule. Natural bond orbital (NBO) analysis reveals that these torsional minima are the manifestations of different second order interactions involving bonding and antibonding orbitals corresponding to the rotor -CHF2 and COOH groups of the molecule. By use of the theoretically predicted rotational constants of the rotamers, the band profile for νO-H has been simulated satisfactorily by means of the PGOPHER method, and this has allowed estimating accurately the energy difference between the two rotamers as 0.54 kcal/mol. The predicted energy barrier for interconversion between the rotamers is very small, ∼0.5 kcal/mol from rotamer II to rotamer I, which implies that the molecule could hop almost freely between the two rotameric forms at room temperature. As a result, the frequencies of the key stretching vibrational modes, like νO-H, νC═O, and νC-H, undergo modulation with internal rotation of the rotor -CHF2 group. Such modulation of high frequency modes could be an efficient mechanism for acceleration of rotor-induced IVR (intramolecular vibrational redistribution) well documented in the literature. Furthermore, the spectra measured in matrix isolated environment show signatures for an energetically higher third rotamer, where -OH and -C═O groups are in anti orientation. It has also been shown that DFAA can easily form weak hydrogen bonded dimeric complexes with molecular nitrogen (N2), which causes νO-H to undergo a red shift of ∼30 cm-1 in argon matrix for all three DFAA monomers.

Stereo-preference of camphor for H-bonding with phenol, methanol and chloroform: A combined matrix isolation IR spectroscopic and quantum chemical investigation.

Camphor is known to be held in the substrate pocket of cytochrome P450cam enzyme via H-bond with a tyrosine residue of the enzyme in a unique orientation. This structural exclusivity results in regio- and stereo-specific hydroxylation of camphor by the enzyme. We have carried out a combined IR spectroscopic and quantum chemical investigation to shed light on the factors influencing the conformational exclusivity of 1R-(+)-camphor in the substrate pocket of Cytochrome P450cam, and to determine whether the selectivity is an inherent property of the substrate itself, or is imposed by the enzyme. For this purpose, complexes of camphor have been studied with three H-bond donors namely phenol, methanol and chloroform. Each of the three donors was found to form stable complexes with two distinct conformers; the one mimicking the conformation in enzyme substrate pocket was found to be more stable of the two, for all three donors. Experimentally, both conformers of the H-bonded complexes were identified separately for phenol and methanol in an argon matrix at 8 K, but not for chloroform due to very small energy barrier for interconversion of the two conformers. In room temperature solution phase spectra of camphor with all three donors, the differences in spectral attributes between the two isomeric H-bonded complexes were lost due to thermal motions.

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.

Cooperative effect on phenolic νO-H frequencies in 1:1 hydrogen bonded complexes of o-fluorophenols with water: A matrix isolation infrared spectroscopic study.

Matrix isolation infrared spectra of 1:1 complexes of two ortho-fluorophenols, 2-fluorophenol (2-FPh) and 2,6-difluorophenol (2,6-DFPh), with water and benzene have been analyzed in combination with electronic structure calculations to investigate cooperative effect in O-H···O-H···F hydrogen bonded linkage, which manifests as large spectral shifts of the phenolic O-H stretching fundamental. Calculation predicts that a nearly planar cyclic geometry is preferred by the binary water complexes of the syn conformer of 2-FPh as well as 2,6-DFPh, and the observed spectral shifts are in good agreement with the predicted shifts for such conformers. On the other hand, for other possible isomeric structures, the molecular plane of water moiety is oriented perpendicular to that of the fluorophenols, and the observed as well as predicted shifts are smaller than those of the ortho substituted fluorophenols, although the total binding energies are predicted to be larger for the former. The observed spectral shifts are however consistent with local interaction energy parameters, like hyperconjugative charge transfer and accumulation of electron density (ρ) along the O-H···O hydrogen bond path. For the binary O-H···π hydrogen bonded benzene complexes of the fluorophenols, where cooperative interaction is not possible, the observed shifts are consistent with the conformers preferred according to total binding energies as well as local charge transfer effects of the complexes.

Matrix Isolation Infrared Spectroscopy of an O-H···π Hydrogen-Bonded Complex between Formic Acid and Benzene.

Mid-infrared spectra of an O-H···π hydrogen-bonded 1:1 complex between formic acid and benzene were measured by isolating the complex in an argon matrix at a temperature of 8 K. The O-H stretching fundamental of formic acid (νO-H) undergoes a red shift of 120 cm(-1), which is the largest among the known π-hydrogen bonded complexes of an O-H donor with respect to benzene as acceptor. Electronic structure theory methods were used extensively to suggest a suitable geometry of the complex that is consistent with a recent study performed at CCSD(T)/CBS level by Zhao et al. (J. Chem. Theory Comput. 2009, 5, 2726-2733), as well as with the measured IR spectral shifts of the present study. It has been determined that density functional theory (DFT) D functionals as well as parametrized DFT functionals like M06-2X, in conjunction with modestly sized basis sets like 6-31G (d, p), are sufficient for correct predictions of the spectral shifts observed in our measurement and also for reproducing the value of the binding energy reported by Zhao et al. We also verified that these low-cost methods are sufficient in predicting the νO-H spectral shifts of an analogous O-H···π hydrogen-bonded complex between phenol and benzene. However, some inconsistencies with respect to shifts of νO-H arise when diffuse functions are included in the basis sets, and the origin of this anomaly is shown to lie in the predicted geometry of the complex. Natural bond orbital (NBO) and atoms-in-molecule (AIM) analyses were performed to correlate the spectral behavior of the complex with its geometric parameters.

On the origin of donor O-H bond weakening in phenol-water complexes.

Matrix isolation infrared spectroscopy has been used to investigate intermolecular interactions in a series of binary O-H⋯O hydrogen bonded phenol-water complexes where water is the common acceptor. The interaction at the binding site has been tuned by incorporating multiple fluorine substitutions at different aromatic ring sites of the phenol moiety. The spectral effects for the aforesaid chemical changes are manifested in the infrared spectra of the complexes as systematic increase in spectral shift of the phenolic O-H stretching fundamental (ΔνO-H). While νO-H bands of the monomers of all the fluorophenols appear within a very narrow frequency range, the increase in ΔνO-H of the complexes from phenol to pentafluorophenol is very large, nearly 90%. The observed values of ΔνO-H do not show a linear correlation with the total binding energies (ΔEb) of the complexes, expected according to Badger-Bauer rule. However, in the same ΔνO-H vs ΔEb plot, nice linear correlations are revealed if the complexes of ortho-fluorophenols are treated separately from their meta/para-substituted analogues. The observations imply that in spite of having the same binding site (O-H⋯O) and the same chemical identities (phenolic), the complexes of ortho and non-ortho fluorophenols do not belong, from the viewpoint of detailed molecular interactions, to a homologous series. Linear correlations of ΔνO-H are, however, observed with respect to the electrostatic component of ΔEb as well as the quantum mechanical charge transfer interaction energy (ECT). From quantitative viewpoint, the latter correlation along with the associated electronic structure parameters appears more satisfactory. It has also been noted that the observed ΔνO-H values of the complexes display a linear relationship with the aqueous phase pKa values of the respective phenol derivatives.

Correlation of ν(OH) spectral shifts of phenol-benzene O-H···π hydrogen-bonded complexes with donor's acidity: a combined matrix isolation, infrared spectroscopy, and quantum chemistry study.

O-H stretching infrared fundamentals (νOH) of phenol and a series of fluorophenol monomers and their 1:1 complexes with benzene have been measured under a matrix isolation condition (8 K). Spectral analysis reveals that ring fluorine substitutions have little effect on phenolic νO-H as long as the molecules in the matrix are fully dispersed as monomers. The substitution effects are pronouncedly manifested only when the phenols are complexed with benzene, and the measured shift in phenolic νOH from the monomer value varies from ∼78 cm(-1) in phenol to ∼98 cm(-1) in 3,4,5-trifluorophenol. The spectral shifts are found to display a linear correlation with the aqueous phase acid dissociation constants (pKa) of the phenols. The spectral changes predicted by electronic structure calculations at several levels of theory are found to be consistent with the observations. Such correlations are also found to exist with respect to different energetic, geometric, and other electronic structure parameters of the complexes. Atoms in Molecules (AIM) analysis shows a distinct bond critical point due to accumulation of electron density at the hydrogen-bonding site. The variation of electron densities both on the hydrogen bond as well the donor O-H group is in accordance with the experimentally observed νO-H of the various fluorophenol-benzene complexes. Partitioning of binding energies into components following the Morokuma-Kitaura scheme shows that the π-hydrogen-bonded complexes are stabilized predominantly by dispersion interactions, although electrostatics, polarization, and charge-transfer terms have appreciable contribution to overall binding energies. NBO analysis reveals that hyperconjugative charge-transfers from the filled π-orbitals of the hydrogen bond acceptor (benzene) to the antibonding σ*(O-H) orbital of the donors (phenols) display correlations which are fully consistent with the observed variations of spectral shifts. The analysis also shows that the O-H bond dipole moments of all the phenolic species are nearly the same, implying that local electrostatics has only a little effect at the site of hydrogen bonding.

CH···O interaction lowers hydrogen transfer barrier to keto-enol tautomerization of ß-cyclohexanedione: combined infrared spectroscopic and electronic structure calculation study.

Molecular association and keto-enol tautomerization of ß-cyclohexanedione (ß-CHD) have been investigated in argon matrix and also in a thin solid film prepared by depositing pure ß-CHD vapor on a cold (8 K) KBr window. Infrared spectra reveal that, in low-pressure vapor and argon matrix, the molecules are exclusively in diketo tautomeric form. The CH···O hydrogen bonded dimers of the diketo tautomer are produced by annealing the matrix at 28 K. No indication is found for keto-enol tautomerization of ß-CHD in dimeric complexes in argon matrix within the temperature range of 8-28 K. On the other hand, in thin film of pure diketo tautomer, the conversion initiates only when the film is heated at temperatures above 165 K. The observed threshold appears to be associated with excitation of the intermolecular modes, and the IR spectra recorded at high temperatures display narrowing of vibrational bandwidths, which has been associated with reorientations of the molecules in the film. The nonoccurrence of tautomerization of the matrix isolated dimer is consistent with the barrier predicted by electronic structure calculations at B3LYP/6-311++G** and MP2/6-311++G** levels of theory. The transition state calculation predicts that CH···O interaction has a dramatic effect on lowering of the tautomerization barrier, from more than 60 kcal/mol for the bare molecule to ~35-45 kcal/mol for dimers.