Biman Bandyopadhyay

Blue Shifting C-H ··· O Hydrogen Bonded Complexes between Chloroform and Small Cyclic Ketones: Ring-Size Effects on Stability and Spectral Shifts

Blue-shifting C-H...O hydrogen bonded complexes between chloroform and three small cyclic ketones (cyclohexanone, cyclopentanone, and cyclobutanone) have been identified by use of FTIR spectroscopy in CCl(4) solution at room temperature. The shifts of the C-H stretching fundamental of chloroform (nu(C-H)) in the said three complexes are +1, +2, and +5 cm(-1), respectively, and the complexation results in enhancement of the nu(C-H) transition intensity in all three cases. The 1:1 stoichiometry of the complexes is suggested by identifying distinct isosbestic points between the carbonyl stretching (nu(C=O)) fundamentals of the monomers and corresponding complexes for spectra measured with different chloroform to ketone concentrations. The nu(C=O) bands in the three complexes are red-shifted by 8, 19, and 6 cm(-1), and apparently have no correlation with the respective blue shifts of the nu(C-H) bands. Spectral analysis reveals that the complex with cyclohexanone is most stable, and the stability decreases with the ring size of the cyclic ketones. A qualitative explanation of the relative stabilities of the complexes is presented by correlating the hydrogen bond acceptor abilities of the carbonyl groups with the ring size of the cyclic ketones. Quantum mechanical calculations at the DFT/B3LYP/6-311++G(d,p) and MP2/6-31+G(d) levels were performed for predictions of the shapes of the complexes, electronic structure parameters of C-H (donor) and C=O (acceptor) groups, intermolecular interaction energies, spectral shifts, and evolution of those properties when the hydrogen bond distance between the donor-acceptor moieties is scanned. The results show that the binding energies of the complexes are correlated with the dipole moments, proton affinity, and n(O) --> sigma*(C-H) hyperconjugative charge transfer abilities of the three ketones. NBO analysis reveals that the blue shifting of the nu(C-H) transition in a complex is the net effect of hyperconjugation and repolarization/rehybridization of the bond under the influence of the electric field of carbonyl oxygen.

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

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.

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.

Grindstone chemistry: a highly efficient and green method for synthesis of 3,4-dihydropyrimidin-2-(1H)-ones by L-tyrosine as an organocatalyst: a combined experimental and DFT study

A single step, mild, environmentally friendly green method has been developed for the synthesis of physiologically active 3,4-dihydropyrimidin-2-(1H)-ones employing L-tyrosine as catalyst under solvent-free conditions at room temperature via grinding. The procedure is efficient, time saving and gives high-yields. The structures and purity of these compounds were confirmed by FT-IR, NMR (1H and 13C) and HRMS spectral analysis. DFT calculations have been used to show the effectiveness of L-tyrosine as a suitable catalyst for the above reaction.

Ammonia Catalyzed Formation of Sulfuric Acid in Troposphere: The Curious Case of a Base Promoting Acid Rain

Electronic structure calculations have been performed to investigate the role of ammonia in catalyzing the formation of sulfuric acid through hydrolysis of SO3 in Earths atmosphere. The uncatalyzed process involves a high activation barrier and, until date, is mainly known to occur in Earths atmosphere only when catalyzed by water and acids. Here we show that hydrolysis of SO3 can be very efficiently catalyzed by ammonia, the most abundant basic component in Earths atmosphere. It was found, based on magnitude of relative potential energies as well as rate coefficients, that ammonia is the best among all the catalysts studied until now (water and acids) and could be a considerable factor in formation of sulfuric acid in troposphere. The calculated rate coefficient (at 298 K) of ammonia catalyzed reaction has been found to be ∼105-107 times greater than that for water catalyzed ones. It was found, based on relative rates of ammonia and water catalyzed processes, that in troposphere ammonia, together with water, could be the key factor in determining the rate of formation of sulfuric acid. In fact, ammonia could surpass water in catalyzing sulfuric acid formation via hydrolysis of SO3 at various altitudes in troposphere depending upon their relative concentrations.

Effect of Ammonia and Formic Acid on the OH• + HCl Reaction in the Troposphere: Competition between Single and Double Hydrogen Atom Transfer Pathways

Quantum chemical calculations at QCISD and CCSD(T) levels of theory have been performed to investigate the effect of NH3 and HCO2H on the reaction between OH• and HCl. Potential energy profiles indicate that both NH3 and HCO2H catalyzed reactions could proceed through two different channels, namely, single and double hydrogen atom transfer. Theoretically calculated rate constants for both the catalysts show that both NH3 and HCO2H catalyzed channels prefer a single hydrogen atom transfer path. Besides, both NH3 and HCO2H catalyzed paths have higher rate constant values as compared to that of the water catalyzed path.

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.

External control over tautomeric distribution and inter-conversion: new insights into the realm of catalyzed tautomerization

This study shows that relative stabilities of two tautomeric forms for four isomeric amide–imidic acid pairs can be extensively altered by use of a simple carboxylic acid and an amine base. Further, the acid and base act as highly efficient catalysts by drastically reducing the barriers for tautomeric transformations. It is also shown that both the relative stabilities and barrier heights can be precisely modulated by changing the carboxylic acids and amine bases. The extents of both selective stabilization and barrier lowering are found to show linear dependence on the strength of the catalysts.

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.