Pujarini Banerjee

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 β-cyclohexanedione: combined infrared spectroscopic and electronic structure calculation study

Molecular association and keto-enol tautomerization of β-cyclohexanedione (β-CHD) have been investigated in argon matrix and also in a thin solid film prepared by depositing pure β-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 β-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.

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.

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.

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.

Matrix isolation infrared spectra of O-H ⋯ π Hydrogen bonded complexes of Acetic acid and Trifluoroacetic acid with Benzene

AbstractMid infrared spectra of two O–H ⋯π hydrogen-bonded binary complexes of acetic acid (AA) and trifluoroacetic acid (F3AA) with benzene (Bz) have been measured by isolating the complexes in an argon matrix at ∼8 K. In a matrix isolation condition, the O–H stretching fundamentals (νO−H) of the carboxylic acid groups of the two molecules are observed to have almost the same value. However, the spectral red-shifts of νO−H bands of the two acids on complexation with Bz are largely different, 90 and 150 cm−1 for AA and F3AA, respectively. Thus, the O–H bond weakening of the two acids upon binding with Bz in a non-interacting environment follows the sequence of their ionic dissociation tendencies (p Ka) in aqueous media. Furthermore, ΔνO−H of the latter complex is the largest among the known π-hydrogen bonded binary complexes of prototypical O–H donors reported so far with respect to Bz as acceptor. It is also observed that the spectral shifts (ΔνO−H) of phenol-Bz and carboxylic acid-Bz complexes show similar dependence on the acidity factor (p Ka). Electronic structure theory has been used to suggest suitable geometries of the complexes that are consistent with the measured IR spectral changes. Calculation at MP2 /6-311 ++G (d, p) level predicts a T-shaped geometry for both AA-Bz and F3AA-Bz complexes, and the corresponding binding energies are 3.0 and 4.5 kcal /mol, respectively. Natural Bond Orbital (NBO) analysis has been performed to correlate the observed spectral behavior of the complexes with the electronic structure parameters. Graphical AbstractThe spectral red-shifts of the probe νO-H bands of carboxylic acid-benzene π-hydrogen bonded complexes in an argon matrix were found to correlate with their respective aqueous phase acidities (pKa), and are explained in terms of local charge transfer effects.

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.

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.

Matrix isolation infrared spectroscopy and structures of weak (O-H•••π) and strongly bound (O-H•••O) binary hydrogen bonded complexes

Phononic structures (composite materials) in which a periodic distribution of elastic parameters facilitates control of the propagation of phonons, hold the promise to enable transformative material technologies in areas ranging from acoustic and thermal cloaking to thermoelectric devices. This requires strategies to deliberately engineer the phononic band structure of materials in the frequency range of interest. Phononics, the acoustic equivalents of the photonics are controlled by a larger number of material parameters, as phonon cannot propagate in vacuum. The study of hypersonic phononics (hPnC) imposes substantial demand on fabrication and characterization techniques. Colloid and polymer science offer methods to create novel materials that possess periodic variations of density and elastic properties at length scales commensurate with the wave length of hypersonic phonons and hence visible photons. The key quantity is the dispersion ω(q) of high frequency (GHz) acoustic excitations with wave vector q which is measured by the noninvasive high resolution Brillouin light scattering. The approach involves the exploitation of Bragg-type band gaps (BGs) that result from the destructive interference of waves in periodic media. However, the sensitivity of BG formation to structural disorder limits the application of self-assembly methods that are susceptible to defect formation. Hybridization gaps (HG), originating from the anticrossing between local resonant and propagating modes, are robust to structural disorder and occur at wavelengths much larger than the size of the resonant unit. Here, examples based on hierarchical structures will be highlighted: 1D-hPnC to acquire comprehensive understanding, while the incorporation of defects holds a wealth of opportunities to engineer ω(q); in colloid based phononics, ω(q) has revealed both types of band gabs; particle brush materials with controlled architecture of the grafted chains enable a new strategy to realize HG’s and; hierarchically nanostructured matter can involve unprecedented phonon phono propagation mechanisms. Phononic crystals, the acoustic equivalents of the photonic crystals, are controlled by a larger number of material parameters. The study of hypersonic crystals imposes substantial demand on fabrication and characterization techniques. Colloid and polymer science offer methods to create novel materials that possess periodic variations of density and elastic properties at mesoscopic length scales commensurate with the wave length of hypersonic phonons and hence photons of the visible light. Polymerand colloid-based phononics is an emerging new field at the interface of soft materials science and condensed matter physics with rich perspectives ahead. The key quantity is the dispersion of high frequency (GHz) acoustic excitations which is nowadays at best measured by high resolution spontaneous Brillouin light scattering. Depending on the components of the nanostructured composite materials, the resolved vibration eigenmodes of the individual particles sensitively depend on the particle architecture and their thermo-mechanical properties [T. Still et al., Nano Lett. 10, 3194 (2008)]. In periodic structures of polymer based colloids, the dispersion relation ω(k) between the frequency and the phonon wave vector k has revealed hypersonic phononic band gaps of different nature. Colloid and polymer science allows the engineering of acoustic and optical material functionalities of hierarchical structures on various length scales commensurate with and well below the characteristic length scales of phonons and photons. Periodic structures act as both hypersonic phononic and visible light photonic crystals (phoxonics). We recently extended the decade-old field to hypersonic phononics. Many important questions in this young field are just being raised and require new conceptual and technical approaches to address them. Powerful synthesis and assembly methods are able to create novel structures to host unconventional properties of flexibility and multi-functionality, locally resonant hypersonic soft metamaterials and topological phononic insulators. To complement our best world-wide Brillouin spectroscopy for retrieving the dispersion relations in transparent structures, two new experimental techniques based on laser-induced high frequency phonons and tapered fiber optomechanics will be implemented to engineer strong wavematter interactions. Band structure calculations will be used as tools to model and predict the acoustic wave propagation in composite structures of varying symmetry, architecture and topology of the building components. Our novel approach, together with intricate methods of processing such materials at a large scale, shows the outline of the emerging field of polymer-and colloid-based phononics. Promising applications range from tunable responsive filters and one way phonon waveguides to compact acousto-optic devices and sensors and from hypersonic imaging to materials and devices, which allow for directed heat flow and recovery. To access such fundamental concepts a detailed understanding of phonon propagation in nanostructured media is a precondition. This proposal ensures that we will hear much more about currently unknown and unexpected properties and functions of soft phononics and will open up many new lines of research.Z is one of promising solid acid catalysts for the conversion of renewable biomass-derived alcohols into fuels and chemicals. Dehydration of alcohols to alkenes is a well-known prototypical acid catalyzed reaction, where confinement and entropic effects impact the rates of these reactions. For such conversions, HZSM-5 zeolite is commonly used as a platform for acid catalyzed reactions due to its strong acidity and enhancement of reaction rates due to confinement in pores. In this talk, we present the structure and thermochemistry of ethanol adsorption on the Brønsted acid site of the HZMS-5 by means of ab inito molecular dynamics (AIMD) simulations directly compared with in situ IR spectroscopy and thermochemical measurements on the same material. Simulations were performed using two different ethanol loadings (with/without deuterium substitution) at different temperatures (100 ≤ T ≤ 700). This enables us to take into account enthalpic and entropic effects caused by the dynamics of the motion of the reaction intermediates. AIMD simulations show that hydrogen transfer from the zeolite scaffold to ethanol occurs as temperature increases. In the simulations with higher ethanol loading, proton transfer occurs via relay between H-bonded ethanol molecules. Calculated projected vibrational density of states (VDOS) obtained from velocity autocorrelation function show a broad peak around 1600 cm-1 related to H-O-H bending mode which is also observed experimentally. We estimated entropy and enthalpy of adsorption using the computed VDSO along with a quasi-harmonic approximation, which shows good agreement with experimental measurement conversely, the more commonly employed harmonic vibrations lead to free energy estimates that deviate from experiment substantially. Overall, this study exemplifies how enharmonic effects, as capture by AIMD, are critical for the quantitative modeling of the free energetics of zeolitecatalyzed processes.

Kinetics and mechanism of the interactions of imidazole and benzimidazole withcis-diaquo(nitrilotriacetato)cobalt(III)

The kinetics of the anation ofcis-diaquo(nitrilotriacetato)cobalt(III) with imidazole (imz) and benzimidazole (bimz) have been studied in the pH range 6.10–7.10 and 5.60–6.85 respectively at μ = 0.1 M and in the temperature range 40–60°C. Both reactions produce the bisubstituted product but the entry of the first ligand is rate determining in each case. The pH dependence of rate shows that the neutral ligand reacts with both the acid and the base forms of the complex, the reactivity of the base form being slightly higher in each case. The pseudo first-order rate constants show a non-linear dependency on the concentration of incoming ligand at a particular pH. A pureD (dissociative) mechanism rather thanId(dissociative interchange) is operative which is supported by previously reported results of imidazole substitution reactions.