Quinolinium Dichromate Oxidation of Diols: A Kinetic Study

Quinolinium dichromate (QDC) in water oxidizes vicinal and nonvicinal diols to the corresponding alpha-hydroxy carbonyl compound. The rate is proportional to the concentrations of diol, oxidant and hydrogen ions, and the reactions proceed almost twice as fast in D(2)O than in H(2)O. These data can be correlated with a mechanistic pathway involving the intermediate formation of a chromate ester, which undergoes decomposition to give the product.     

Kinetics and mechanism of the oxidation of some diols by benzyltrimethylammoniumtribromide

The kinetics of oxidation of five vicinal and four non-vicinal diols, and two of their monoethers by benzyltrimethylammonium tribromide (BTMAB) have been studied in 3:7 (v/v) acetic acid-water mixture. The vicinal diols yield the carbonyl compounds arising out of the glycol bond fission while the other diols give the hydroxycarbonyl compounds. The reaction is first-order with respect to BTMAB. Michaelis-Menten type kinetics is observed with respect to diol. Addition of benzyltrimethylammonium chloride does not affect the rate. Tribromide ion is postulated to be the reactive oxidizing species. Oxidation of [1,1,2,2-²H₄] ethanediol shows the absence of a kinetic isotope effect. The reaction exhibits substantial solvent isotope effect. A mechanism involving a glycol-bond fission has been proposed for the oxidation of the vicinal diols. The other diols are oxidized by a hydride ion transfer to the oxidant, as are the monohydric alcohols.     

Ab initio conformational studies on diols and binary diol-water systems using DFT methods. Intramolecular hydrogen bonding and 1:1 complex formation with water

Studies on the conformational equilibrium for the following diols, ethane-1,2-diol (12EG, CAS 107-21-1), 2R-D-(-)-propane-1,2-diol (12PG, CAS 4254-14-2), (2S,3S)-L-(+)-butane-2,3-diol (L23BD, CAS 19132-06-0), and (2S,3R)-meso-butane-2,3-diol (m23BD, CAS 5341-95-7), are described using Gaussian ab initio calculations involving density functional theory (DFT) methods. We also report in this article results on the stability and conformation for the 1:1 water-diol complex formed by ethane-1,2-diol, propane-1,2-diol, and L- and meso-butane-2,3-diol. The relative stability of the intramolecular (internal) hydrogen bond in a range of diols (n = 2 to 6), based on ab initio geometry optimization and determination of the -O...H- distance, dOH, and -O-H...O- angle, theta, increases through the sequence 1,2 approximately equals 2,3 < 1,3 < 1,4 approximately equals 1,5 approximately equals 1,6, as judged from the bond linearity and -O...H- separation. Quantum mechanical and topological analysis of possible intramolecular hydrogen bonding in this complete series of diols provides convincing evidence for this in diols in which the hydroxyl groups are separated by three or more carbon atoms, that is, in (n, n+m) diols for m > or = 2, but not for ethane-1,2-diol or other vicinal diols, which do not satisfy Popelier's topological and electron density criteria based on the AIM theory of Bader. Based on these criteria it is unlikely that vicinal diols are in fact capable of forming an intramolecular hydrogen bond, in spite of geometric and spectroscopic data in the literature suggesting otherwise.     

Revisiting the reaction of hydroxyl radicals with vicinal diols in water

The carbonyl products of the reactions of hydroxyl radicals with three vicinal diols (ethane-1,2-diol, propane-1,2-diol and butane-2,3-diol) have been identified and quantified. Hydroxyl radicals were produced by γ-radiolysis of N(2)O-saturated aqueous solutions. The reactions result in the formation of alkoxyl radicals (~15%) followed by β-fragmentation, and α-hydroxyl alkyl radicals that undergo H(2)O elimination. The latter process is part of a radical chain reaction at higher diol concentrations.     

The first observation of nitrogen-carbonyl bonding: self-assembly of N-oxalyl 2,4-dinitroanilide assisted by a weak N...O=C interaction

A new type of weak bond, i.e., the N...O=C interaction, that determines the crystal packing of N-oxalyl 2,4-dinitroanilide (1) in cooperation with C-H...O hydrogen bonds, has been found and is rationalized by ab initio calculations as being the result of electrostatic interactions.     

Modeling of hydrogen bonds in monohydrated 2,4-dithiothymine: an ab initio and AIM study

Twelve tautomers of 2,4-dithiothymine are calculated at the MP2/6-31+G(d) level, and the most stable one is referred to the di-keto form (P12). Then four H-bonded complexes between P12 and water are optimized at the MP2/6-31+G(d) level of theory. The calculation of vibrational frequencies and natural bond orbital analysis are also carried out at the same level to investigate the hydrogen bonds involved in all the systems. Within all the four complexes, three types of hydrogen bonds are formed, in which the O-H...S and N-H...O bonds are the normal bonds with the X-H bond elongation and red shift of the corresponding stretch frequencies, while the C-H...O interaction is an improper, blue-shifting hydrogen bond accompanied with the contraction of the C-H bond and a blue shift of the C-H stretch frequency. The topological properties are investigated with the atoms-in-molecules (AIM) theory. The NMR chemical shielding for the isolated and the four monohydrated 2,4-dithiothymine are calculated using the "gauge-including atomic orbital" (GIAO) method. The 1H chemical shifts are influenced by the formation of hydrogen bonds.     

Nature of the water/aromatic parallel alignment interactions

The water/aromatic parallel alignment interactions are interactions where the water molecule or one of its O? H bonds is parallel to the aromatic ring plane. The calculated energies of the interactions are significant, up to ΔE_{CCSD(T)(limit)} = ?2.45 kcal mol^?1 at large horizontal displacement, out of benzene ring and CH bond region. These interactions are stronger than CH···O water/benzene interactions, but weaker than OH···π interactions. To investigate the nature of water/aromatic parallel alignment interactions, energy decomposition methods, symmetry‐adapted perturbation theory, and extended transition state‐natural orbitals for chemical valence (NOCV), were used. The calculations have shown that, for the complexes at large horizontal displacements, major contribution to interaction energy comes from electrostatic interactions between monomers, and for the complexes at small horizontal displacements, dispersion interactions are dominant binding force. The NOCV‐based analysis has shown that in structures with strong interaction energies charge transfer of the type π → σ*(O? H) between the monomers also exists. © 2014 Wiley Periodicals, Inc.     

Complexes of atmospheric alpha-dicarbonyls with water: FTIR matrix isolation and theoretical study

The complexes of glyoxal (Gly), methylglyoxal (MGly), and diacetyl (DAc) with water have been studied using Fourier transform infrared (FTIR) matrix isolation spectroscopy and MP2 calculations with 6-311++G(2d,2p) basis set. The analysis of the experimental spectra of the Gly(MGly,DAc)/H2O/Ar matrixes indicates formation of one Gly...H2O complex, three MGly...H2O complexes, and two DAc...H2O ones. All the complexes are stabilized by the O-H...O(C) hydrogen bond between the water molecule and carbonyl oxygen as evidenced by the strong perturbation of the O-H, C=O stretching vibrations. The blue shift of the CH stretching vibration in the Gly...H2O complex and in two MGly...H2O ones suggests that these complexes are additionally stabilized by the improper C-H...O(H2) hydrogen bonding. The theoretical calculations confirm the experimental findings. They evidence the stability of three hydrogen-bonded Gly...H2O and DAc...H2O complexes and six MGly...H2O ones stabilized by the O-H...O(C) hydrogen bond. The calculated vibrational frequencies and geometrical parameters indicate that one DAc..H2O complexes, two Gly...H2O, and three MGly...H2O ones are additionally stabilized by the improper hydrogen bonding between the C-H group and water oxygen. The comparison of the theoretical frequencies with the experimental ones allowed us to attribute the calculated structures to the complexes present in the matrixes.     

From weak interactions to covalent bonds: a continuum in the complexes of 1,8-bis(dimethylamino)naphthalene

Experimental charge density distributions in a series of ionic complexes of 1,8-bis(dimethylamino)naphthalene (DMAN) with four different acids: 1,2,4,5-benzenetetracarboxylic acid (pyromellitic acid), 4,5-dichlorophthalic acid, dicyanoimidazole, and o-benzoic sulfimide dihydrate (saccharin) have been analyzed. Variation of charge density properties and derived local energy densities are investigated, over all inter- and intramolecular interactions present in altogether five complexes of DMAN. All the interactions studied [[O...H...O](-), C[bond]H...O, [N[bond]H...N](+), O[bond]H...O, C[bond]H...N, C pi...N pi, C pi...C pi, C[bond]H...Cl, N[bond]H(+)] follow exponential dependences of the electron density, local kinetic and potential energies at the bond critical points on the length of the interaction line. The local potential energy density at the bond critical points has a near-linear relationship to the electron density. There is also a Morse-like dependence of the laplacian of rho on the length of interaction line, which allows a differentiation of ionic and covalent bond characters. The strength of the interactions studied varies systematically with the relative penetration of the critical points into the van der Waals spheres of the donor and acceptor atoms, as well as on the interpenetration of the van der Waals spheres themselves. The strong, charge supported hydrogen bond in the DMANH(+) cation in each complex has a multicenter character involving a [[Me(2)N[bond]H....NMe(2)](+)....X(delta-)] assembly, where X is the nearest electronegative atom in the crystal lattice.     

Reaction of tin porphyrins with vicinal diols

Reactions of tin porphyrins with vicinal diols were investigated. Treatment of (TTP)Sn(CCPh)(2) or (TTP)Sn(NHtolyl)(2) with pinacol and 2,3-diphenylbutane-2,3-diol afforded diolato complexes (TTP)Sn[OC(Me)(2)C(Me)(2)O] (1) and (TTP)Sn[OC(Ph)(Me)C(Ph)(Me)O] (2), respectively. Both complexes underwent C-C cleavage reactions to give (TTP)Sn(II) and ketones. Reaction of (TTP)Sn(CCPh)(2) with 1 equivalent of o-catechol generated (TTP)Sn(CCPh)(OC(6)H(4)OH) (3), which subsequently transformed into (TTP)Sn(OC(6)H(4)O) (4). With excess catechol, disubstituted (TTP)Sn(OC(6)H(4)OH)(2) (5) was obtained. (TTP)Sn(CCPh)(OCHRCHROH) (R = H, 6; R = Ph, 8) and (TTP)Sn(OCHRCHROH)(2) (R = H, 7; R = Ph, 9) were obtained analogously by treatment of (TTP)Sn(CCPh)(2) with appropriate diols. In the presence of dioxygen, tin porphyrin complexes were found to promote the oxidative cleavage of vicinal diols and the oxidation of alpha-ketols to alpha-diketones. Possible reaction mechanisms involving diolato or enediolato intermediates are discussed. The molecular structure of (TTP)Sn(CCPh)(OC(6)H(4)OH) (3) was determined by X-ray crystallography.     

Effects of functional group interactions on the bimolecular and dissociation reactions of diols

The influence of functional group interactions on the bimolecular and dissociation reactions of diols were examined in a quadrupole ion trap mass spectrometer. Reactions of dimethyl ether ions with diols resulted in formation of (M + H)⁺ ions and (M + 13)⁺ ions (by net methyne addition). The product distribution depended on the relative separation of the hydroxyl groups within each diol, with the more proximate diols producing the greatest abundance of (M + 13)⁺ ions compared to (M + H)⁺ ions. The enhancement of the formation of (M + 13)⁺ ions is attributed to the capability for electrostatic interactions between the hydroxyl groups and the electropositive methylene group of the methoxymethylene reagent ion. The enhancement is most significant for diols that can adopt five- or to a lesser extent six-membered ring transition states (i.e, any 1,2 or 1,3 diol). Collision-activated dissociation (CAD) techniques, including both sequential activation experiments (MS ⁿ ) and comparison of CAD spectra for model compounds, suggest that the (M + 13)⁺ ions are protonated cyclic diethers.     

Thermodynamic and structural properties of aqueous linear diol solutions

Thermodynamic characteristics of aqueous linear diol solutions are calculated. These data are used to identify regularities in the variations of the structural properties of the mixtures being studied. The correlation between the entropy and enthalpy characteristics of water-diol systems with excess packing coefficients is evidence that the structural and energy properties of aqueous linear diol solutions are determined by universal interactions. The form of the concentration dependences of the solvation enthalpies and entropies of noble gases in water-linear diols mixtures is determined by the reorganization component and is attributed to the destruction of the H bond network of water, which results in the formation of the most densely packed solutions in the medium range of compositions.     

pi-H...O hydrogen bonds: multicenter covalent pi-H interaction acts as the proton-donating system

The MP2 method and the Pople-style basis sets 6-311++G(d,p), 6-311++G(2df,2pd), and 6-311++G(3df,3pd) were used to perform calculations on H3O+...C2H2 and C2H3+...C2H2 complexes and related species. Hydrogen bonds existing for the analyzed complexes were investigated as well as related pi-H...O --> pi...H-O and pi-H...pi --> pi...H-pi proton-transfer processes. For some of the complexes analyzed the multicenter pi-H interaction possessing the properties of a covalent bond acts as a proton donor; more generally it is classified as the Lewis acid. The quantum theory of "atoms in molecules" (QTAIM) was also applied to deepen the nature of these interactions in terms of characteristics of bond critical points. The pi-H...O, O-H...pi, and pi-H...pi interactions analyzed here may be classified as hydrogen bonds since their characteristics are the same as or at least similar to those of typical hydrogen bonds. H...pi interactions are common in crystal structures of organic and organometallic compounds. The analyses performed here show a continuum of such interactions since there are H...pi contacts possessing the characteristics of weak intermolecular interactions on the one hand and pi-H multicenter covalent bonds on the other. Ab initio and QTAIM results support the latter statements.     

Theoretical study of binding interactions and vibrational Raman spectra of water in hydrogen-bonded anionic complexes: (H2O)n- (n = 2 and 3), H2O...X- (X = F, Cl, Br, and I), and H2O...M- (M = Cu, Ag, and Au)

Binding interactions and Raman spectra of water in hydrogen-bonded anionic complexes have been studied by using the hybrid density functional theory method (B3LYP) and ab initio (MP2) method. In order to explore the influence of hydrogen bond interactions and the anionic effect on the Raman intensities of water, model complexes, such as the negatively charged water clusters ((H2O)n-, n = 2 and 3), the water...halide anions (H2O...X-, X = F, Cl, Br, and I), and the water-metal atom anionic complexes (H2O...M-, M = Cu, Ag, and Au), have been employed in the present calculations. These model complexes contained different types of hydrogen bonds, such as O-H...X-, O-H...M-, O-H...O, and O-H...e-. In particular, the last one is a dipole-bound electron involved in the anionic water clusters. Our results showed that there exists a large enhancement in the off-resonance Raman intensities of both the H-O-H bending mode and the hydrogen-bonded O-H stretching mode, and the enhancement factor is more significant for the former than for the latter. The reasons for these spectral properties can be attributed to the strong polarization effect of the proton acceptors (X-, M-, O, and e-) in these hydrogen-bonded complexes. We proposed that the strong Raman signal of the H-O-H bending mode may be used as a fingerprint to address the local microstructures of water molecules in the chemical and biological systems.     

The orientation of N-H...O=C and N-H...N hydrogen bonds in biological systems: How good is a point charge as a model for a hydrogen bonding atom?

In order to design new ligands for protein-binding sites of unknownstructure, it would be useful to predict the likely sites of hydrogenbonding of an unknown protein fragment to a known molecule. The positions ofmaxima and minima in the electrostatic potential at appropriate distancesfrom the van der Waals surface were calculated for various small molecules,nucleic acid bases, peptide units and amino acid side chains containinggroups which can form the biologically important N-H...O=C andN-H...N hydrogen bonds. Their ability to predict the positions of H andO/N in hydrogen bonded complexes, as predicted by optimising theelectrostatic interactions of pairs of such molecules constrained by themolecular shapes, was assessed. It is shown that extrema in theelectrostatic potential around the isolated molecules give worthwhilepredictions for the locations of hydrogen bonding partners. For moleculesbound by a single N-H...O=C hydrogen bond, the electrostatic maximumassociated with the H is usually less than 1 Å from an acceptor atom,while a C=O electrostatic minimum is generally less than 1.5 Å fromthe hydrogen bond proton. However, a significant number of hydrogen bondsform to the opposite lone pair from the electrostatic minimum, in which casethe separation is up to 3.3 Å. This reflects the broad electrostaticpotential well around a carbonyl oxygen between the lone pair directions.The model predicts when neighbouring atoms drastically change the hydrogenbonding characteristics of an N-H or C=O group. Although the geometries ofhydrogen bonded complexes are influenced by the other van der Waals contactsbetween the molecules, particularly multiple hydrogen bonds, theseinfluences are constant when considering hydrogen bonding to a specificuncharacterised binding site. Hence, the consideration of stericallyaccessible electrostatic extrema will be useful in the design of newligands.     

Association of symmetrical alkane diols with pyridine: DFT/GIAO calculation of 1H NMR chemical shifts

Proton nuclear magnetic resonance (NMR) shifts of the free diol and of its 1 : 1 and 1 : 2 hydrogen‐bonded complexes with pyridine have been computed for five symmetrical alkane diols on the basis of density functional theory, by applying the gauge‐including atomic orbital method to geometry‐optimized conformers. For certain conformers, intramolecular OH ···OH interactions, evidenced by high NMR OH proton shifts, are further enhanced on going from the free diol to the corresponding 1 : 1 diol/pyridine complex. This is confirmed by atoms‐in‐molecules and non‐covalent interaction plots. The computed OH and CH proton shifts for the diol and the two complexes correlate well with values obtained by analysing data from the NMR titration of the diols in benzene against pyridine. Shift values for the diols in neat pyridine are calculated by weighting the shifts of the various protons in the three forms (free diol, 1 : 1 and 1 : 2 diol/pyridine complexes) according to the experimentally determined association constants. The results are in good agreement with those observed, and after empirical scaling, the root mean square difference is 0.18 ppm. Copyright © 2016 John Wiley & Sons, Ltd.     

The role of chalcogen-chalcogen interactions in the intrinsic basicity acidity of beta-chalcogenovinyl(thio)aldehydes HC([double bond]X)[bond]CH[double bond]CH[bond]CYH

The intrinsic acidity and basicity of a series of beta-chalcogenovinyl(thio)aldehydes HC([double bond]X)[bond]CH[double bond]CH[bond]CYH (X=O, S; Y=Se, Te) were investigated by B3LYP/6-311+G(3df,2p) density functional and G2(MP2) calculations on geometries optimized at the B3LYP/6-31G(d) level for neutral molecules and at the B3LYP/6-31+G(d) level for anions. The results showed that selenovinylaldehyde and selenovinylthioaldehyde should behave as Se bases in the gas phase, because the most stable neutral conformer is stabilized by an X[bond]H...Se (X=O, S) intramolecular hydrogen bond (IHB). In contrast the Te-containing analogues behave as oxygen or sulfur bases, because the most stable conformer is stabilized by typical X...Y[bond]H chalcogen-chalcogen interactions. These compounds have a lower basicity than expected because either chalcogen-chalcogen interactions or IHBs become weaker upon protonation. Similarly, they are also weaker acids than expected because deprotonation results in a significantly destabilized anion. Loss of the proton from the X[bond]H or Y[bond]H groups is a much more favorable than from the C[bond]H groups. Therefore, for Se compounds the deprotonation process results in loss of the X[bond]H...Se (X=O, S) IHBs present in the most stable neutral conformer, while for Te-containing compounds the stabilizing X...Y[bond]H chalcogen-chalcogen interaction present in the most stable neutral conformer becomes repulsive in the corresponding anion.     

Computational study on the characteristics of the interaction in naphthalene...(H2X)n=1,2 (X = O,S) clusters

The characteristics of the interaction between the pi cloud of naphthalene and up to two H2O or H2S molecules were studied. Calculations show that clusters formed by naphthalene and one H2O or H2S molecule have similar geometric features, and also present similar interaction energies. Our best estimates for the interaction energy amount to -2.95 and -2.92 kcal/mol for H2O and H2S, respectively, as obtained with the CCSD(T) method. Calculations at the MP2 level employing large basis sets should be avoided because they produce highly overestimated interaction energies, especially for hydrogen sulfide complexes. The MPWB1K functional, however, provides values pretty similar to those obtained with the CCSD(T) method. Although the magnitude of the interaction is similar with both H2X molecules, its nature is somewhat different: the H2O complex presents electrostatic and dispersion contributions of similar magnitude, whereas for H2S the interaction is dominated by dispersion. In clusters containing two H2X molecules several minima were characterized. In water clusters, the total interaction energy is dominated by the presence of a O-H...O hydrogen bond and, as a consequence, structures where this contact is present are the most stable. However, clusters containing H2S show structures with no interaction between H2S moieties which are as stable as the hydrogen bonded ones, because they allow an optimal H2S...naphthalene interaction, which is stronger than the S-H...S contact. Therefore it is possible that in larger polycycles hydrogen sulfide molecules will be spread onto the surface maximizing S-H...pi interactions rather than aggregated, forming H2S clusters.     

QTAIM study of strong H-bonds with the O-H...a fragment (A=O, N) in three-dimensional periodical crystals

The relationship between the d(H...A) distance (A=O, N) and the topological properties at the H...A bond critical point of 37 strong (short) hydrogen bonds occurring in 26 molecular crystals are analyzed using the quantum theory of atoms in molecules (QTAIM). Ground-state wave functions of the three-dimensional periodical structures representing the accurate experimental geometries calculated at the B3LYP/6-31G** level of approximation were used to obtain the QTAIM electron density characteristics. The use of an electron-correlated method allowed us to reach the quantitatively correct values of electron density rhob at the H...A bond critical point. However, quite significant differences can appear for small absolute values of the Laplacian (<0.5 au). The difference between the H...O and H...N interactions is described using the rhob versus d(H...A) dependence. It is demonstrated that the values of parameters in this dependence are defined by the nature of the heavy atom forming the H...A bond. An intermediate (or transit) region separating the shared and closed-shell interactions is observed for the H-bonded crystals in which the bridging proton can move from one heavy atom to another. The crystalline environment changes the location of the bridging proton in strong H-bonded systems; however, the d(O-H)/d(H...O) ratio is approximately the same for both the gas-phase complexes and molecular crystals with a linear or near-linear O-H...O bond.     

Study on the nature of interaction of furan with various hydrides

The nature of interactions of furan with various hydrides (Y) (Y=HF,HCl,H2O,H2S,NH3,PH3) is investigated using ab initio calculations. The contribution of attractive (electrostatic, inductive, and dispersive) and repulsive (exchange) components to the interactions energy is analyzed. HF, H2O, and NH3 favor sigma o-type H bonding, while HCl, H2S, and PH3 favor pi-type H bonding. Interaction energy decomposition reveals that sigma o-type complexes interactions are predominantly electrostatic in nature, while the dispersion and electrostatic interactions dominate the pi-type complexes.