Studies of hydrogen bonding: Part XXIX. An experimental and theoretical approach to the structure of nitriles and of hydrogen-bonded complexes with phenol

Dipole moments, μ_a, of four nitriles, have been determined experimentally and calculated by the CNDO/2 method. The results obtained can be accounted for in terms of a planar trimer parallel-antiparallel model held together in clusters. Experimental and CNDO/2 calculated dipole moments of hydrogen-bonded complexes between nitriles and phenol indicate strongly that the O-H proton in phenol is located perpendicular to the triple bond.     

Ligand effects in the substitution chemistry of cis-bis(piperidine)tetracarbonylmolybdenum(O). A molybdenum-95 NMR study

Molybdenum-95 NMR chemical shifts are reported for a series of Mo(O) compounds of the type Mo(CO)₄(pip)_2−nL_n(n = 1,2; L = substituted pyridine ligands). The σ(⁹⁵Mo) values correlate well with the pK_a values for the substituted pyridines; for the n = 1 series, σ (⁹⁵Mo) ranges from − 1053 ppm (pK_a = 1.86 for 4-CN) to − 1120 ppm (pK_a = 9.61 for 4-NMe₂). The effects of solvent polarity and some in situ reactivity studies are described and the nature of the MoL bond compared to that with piperidine and some other ligands is discussed.     

Spectroscopic studies of multiple charge transfer complexes of p-toluidine with π-acceptor picric acid in different polar solvents

The charge transfer complexes of the donor p-toluidine with π-acceptor picric acid have been studied spectrophotometrically in various solvents such as acetone, ethanol, and methanol at room temperature using absorption spectrophotometer. The results indicate that formation of CTC in less polar solvent is high. The stoichiometry of the complex was found to be 1: 1 ratio by straight line method between donor and acceptor with maximum absorption bands. The data are discussed in terms of formation constant (K _CT), molar extinction coefficient (?_CT), standard free energy (ΔG°), oscillator strength (f), transition dipole moment (μ_EN), resonance energy (R _N) and ionization potential (I _D). The results indicate that the formation constant (K _CT) for the complex were shown to be dependent upon the nature of electron acceptor, donor and polarity of solvents which were used.     

Spectrophotometric and spectroscopic studies of charge transfer complexes of p-toluidine as an electron donor with picric acid as an electron acceptor in different solvents

The charge transfer complexes of the donor p-toluidine with π-acceptor picric acid have been studied spectrophotometrically in various solvents such as carbon tetrachloride, chloroform, dichloromethane acetone, ethanol, and methanol at room temperature using absorption spectrophotometer. The results indicate that formation of CTC in non-polar solvent is high. The stoichiometry of the complex was found to be 1:1 ratio by straight-line method between donor and acceptor with maximum absorption bands. The data are discussed in terms of formation constant (K_CT), molar extinction coefficient (?_CT), standard free energy (ΔG^o), oscillator strength (f), transition dipole moment (μ_EN), resonance energy (R_N) and ionization potential (I_D). The results indicate that the formation constant (K_CT) for the complex was shown to be dependent upon the nature of electron acceptor, donor and polarity of solvents that were used.     

Infrared spectrometric study of the interaction between 2-dimethylamino-3,3-dimethyl-1 azirine and some phenol derivatives

The H-bonded complexes between 2-dimethylamino-3,3-dimethyl-1-azirine (TMAAZ) and some phenol derivatives have been studied by IR spectrometry in carbon tetrachloride. The equilibrium constants at 300, 315 and 328 K and the ? ΔH°, ? Δν_OH and Δν_CN values have been determined. The comparison with previously studied OH … N complexes shows that TMAAZ forms stronger complexes than would be expected from the pK_a value. These facts are discussed in terms of steric, hybridization, solvation and charge transfer effects. From a pK_a of 8, the IR spectrum shows the appearance of ion pairs N⁺-H …^?O. A predominance of protonated species can be calculated for a pK_a of 6. This can explain why 1,2-ring cleavage whose first step is probably the formation of an azirinium cation, occurs for acids characterized by pK_a values lower than 5 but not for enolizable ketones having pK_a values higher than 9.     

Examining the impact of ancillary ligand basicity on copper(I)–ethylene binding interactions: a DFT study

A theoretical investigation at the density functional theory level (B3LYP) has been conducted to elucidate the impact of ligand basicity on the binding interactions between ethylene and copper(I) ions in [Cu(?? ²-C₂H₄)]⁺ and a series of [Cu(L)(?? ²-C₂H₄)]⁺ complexes, where L?=?substituted 1,10-phenanthroline ligands. Molecular orbital analysis shows that binding in [Cu(?? ²-C₂H₄)]⁺ primarily involves interaction between the filled ethylene ??-bonding orbital and the empty Cu(4s) and Cu(4p) orbitals, with less interaction observed between the low energy Cu(3d) orbitals and the empty ethylene ??*-orbital. The presence of electron-donating ligands in the [Cu(L)(?? ²-C₂H₄)]⁺ complexes destabilizes the predominantly Cu(3d)-character filled frontier orbital of the [Cu(L)]⁺ fragment, promoting better overlap with the vacant ethylene ??*-orbital and increasing Cu????ethylene ??-backbonding. Moreover, the energy of the filled [Cu(L)]⁺ frontier orbital and mixing with the ethylene ??*-orbital increase with increasing pK _a of the 1,10-phenanthroline ligand. Natural bond orbital analysis reveals an increase in Cu????ethylene electron donation with addition of ligands to [Cu(?? ²-C₂H₄)]⁺ and an increase in backbonding with increasing ligand pK _a in the [Cu(L)(?? ²-C₂H₄)]⁺ complexes. Energy decomposition analysis (ALMO-EDA) calculations show that, while Cu????ethylene charge transfer (CT) increases with more basic ligands, ethylene????Cu CT and non-CT frozen density and polarization effects become less favorable, yielding little change in copper(I)?Cethylene binding energy with ligand pK _a. ALMO-EDA calculations on related [Cu(L)(NCCH₃)]⁺ complexes and calculated free energy changes for the displacement of acetonitrile by ethylene reveal a direct correlation between increasing ligand pK _a and the favorability of ethylene binding, consistent with experimental observations.     

Displacement of neutral nitrogen donors by chloride in [Pt(SNS)(R-py)](ClO4)2 (SNS = 2,6-bis(methylthiomethyl)pyridine; R-py = pyridines): A reactivity comparison between meta- and para-substituted pyridines

The kinetics of the process [Pt(SNS)(R-py)]²⁺ + Cl⁻ → [Pt(SNS)Cl]⁺ + R-py {SNS = 2,6-bis(methylsulfanylmethyl)pyridine; R-py = meta- or para-substituted pyridines covering a wide range of basicity} were studied in methanol at 25 °C. The reactions obey the usual two-term rate law observed in the substitution reactions of square-planar d⁸ complexes. The plots of log k₂ {k₂ = second-order rate constants} against the pK_a of the heterocycles conjugate acids highlighted a different sensitivity of the two groups of N-donors to changes in basicity, thepara-substituted pyridines (4R-py) showing a weaker dependence on pK_a than the meta-substituted (3R-py). The results have been explained on the basis of a π-acidity difference between 3R-py and 4R-py which influences the reaction ground state.     

Charge-transfer properties of the hydrogen bond: Part 6. Charge-transfer theory and dipole moments of hydrogen-bonded complexes

The enhancement of the dipole moments of hydrogen-bonded complexes are discussed using Mulliken's charge-transfer theory.A linear relation is found between the ratio a/b and the ionization potential of the donor, I^V_D. This behaviour is similar to that previously found for halogen charge-transfer complexes [6].     

1H NMR studies of solvent effects on hydrogen bonding in some pyridine trifluoroacetates

The chemical shifts of hydrogen bonded protons in complexes of 11 substituted pyridines with trifluoroacetic acid were examined, in five dry solvents of different activity, with respect to proton transfer and aggregation effects. The results were correlated with ΔpK_a, the Kirkwood function and E _T parameters. The solvent effect on the intersection point obtained from the plot of the chemical shift of the hydrogen bonded protons against ΔpK_a can be used, similar to an isotopic effect, to differentiate strong hydrogen bonds. The aggregation of acid–base complexes can lead to downfield or upfield shifts; the variation of chemical shift with aggregation depends on the position of the proton in the hydrogen bridge.     

Generation and stabilization of free selenium-containing radicals by complex formation

The reactions of complex formation of some p-XC₆H₄SeSeC₆H₄p-X-MX₃-benzene (or cyclohexane) systems, where X = H, CH₃, F, Cl, Br and MX₃ = GaCl₃ AlBr₃, have been studied by calorimetry, cryoscopy, dielectric measurements, GLC, and ESR spectroscopy. Diaryldiselenides react with gallium trichloride in solution forming 1:1 complexes. The enthalpies of formation and the dipole moments of the complexes have been determined. Formation of diaryldiselenides complexes with aluminium bromide, which is a stronger acceptor, involves cleavage of the SeSe and CSe bonds in the diselenide molecule and yields stable selenium-centred radical complexes of the type ArSeSe^? · AlBr₃ (g-value 2.073) and ArSe^? · AlBr₃ (g value 2.033). In addition, the ESR spectrum reveals a singlet (g value 2.0025) assigned to a hydrocarbon radical. Corresponding di- and mono-selenides as well as biphenyls have been identified by GLC in the products of interaction of free radicals displaced from the mixture of radical complexes by an electron donor (diethyl ether, water) stronger than the radical.     

Microwave spectrum,structure, dipole moment and internal rotation of trans-ethylmethylether

Microwave spectra of ethylmethylether and its eleven isotopically substituted species were measured. The r_s structure of the trans isomer was determined from the observed moments of inertia. Structural parameters of this isomer were roughly equal to those of the reported r_s structure for dimethylether and diethylether. The CH₂-O bond length was definitely shorter by about 0.01 Å than the CH₃-O bond length and the C-C bond length was nearly equal to those of ethylchloride and bromide. The OCH₃ group tilted by about 2° 13' towards lone pair electrons of the oxygen atom while no significant tilt angle was found for the CH₃C group.Dipole moments of the trans isomer for the normal and two deuterated species were determined by Stark-effect measurements. For the normal species, the dipole moment was μ_a = 0.146 ± 0.022 D,μ_b = 1.165 ± 0.020 D and μ_total=1.174 ± 0.022 D making an angle of 7° 5' ± 32' with the b inertial axis. Direction of the dipole moment in the molecule was discussed.From splittings of the observed spectra, barriers to internal rotations of two CH₃ groups were obtained in the one-top approximation. They were 2702 ± 7 and 3300 ± 25 cal mol^?1 for the OCH₃ and CH₃C groups, respectively, from the analysis of splittings in the first excited CH₃ torsional states. The coupling effects among two tops and the skeletal torsion were briefly discussed.     

The effect of complexation on the rates of electron transfer of iron porphyrins

A study has been made of the heterogeneous rate constants of iron porphyrins with pyridines and other nitrogenous base ligands. A direct relationship between the pK_a of the base and the rate constant has been found with a few exceptions. The measurements were by cyclic voltammetry. The larger the pK_a the stronger the complex between iron porphyrins and axial ligands. With strong ligands no ligand exchange or spin state change is expected. Both of these actions show the rate in agreement with the rates found in this work.     

Correlations of 95Mo NMR chemical shifts with ligand basicity in substituted pyridine pentacarbonylmolybdenum(O) complexes

Molybdenum-95 NMR chemical shifts are reported for a series of Mo(O) compounds of the type Mo(CO)₅L (L = pyridine derivatives). A good correlation is found between the δ(⁹⁵Mo) values and the Hammett sigma constant of the pyridine substituent or the pK_a of the substituted pyridine. The chemical shift values, which range from −1366 ppm (3-CN, σ = 0.62, pK_a = 1.35) to −1433 ppm (4-NMe₂, σ = −0.83, pK_a = 9.61), directly reflect the electronic properties of the pyridine derivatives even though the substituent is four or five bonds away from the molybdenum atom.     

Solvent effects on electronic absorption spectra of nitrochlorobenzenes,nitrophenols and nitroanilines—lll. Excited state dipole moments and specific solute—solvent interaction energies employing Bakhshiev's approach

A step-wise procedure for employing Bakhshiev's expression for spectral frequency shifts to elucidate the excited state dipole moments (μ_e) and specific solute—solvent interaction energies (E_s) from electronic absorption spectra has been described. The μ_es and E_ss of several substituted benzenes deduced on the basis of this approach and using cavity radii estimated in two different ways are found to be closely comparable with similar results deduced on the basis of the more commonly used McRae's expression, and exhibit similar trends as in the latter approach.     

Polarizability effect in silatranes and related compounds

The dipole moments (μ) of the molecules, the dipole moments (μ_DA) and the length (d_DA) of the Si ← N bonds, and the electrochemical oxidation potentials (E_p) of Si-substituted silatranes, the dipole moments (μ^hs) of the molecules of Si-substituted 3-homosilatranes as well as the enthalpies of formation (ΔH⁰) of the intermolecular complexes of SiF₄ with aniline derivatives depend not only on the inductive and resonance effects, but also on the polarizability of substituents which can be characterized by the σ_α constants.     

Dielectric study of dipolar association in dilute solutions of nitrobenzene in benzene

The correlation factors R_p and R_s are determined for the dielectric polarization and the non-linear dielectric effect in dilute solutions of nitrobenzene in benzene. Assuming dipole association of the nitrobenzene molecules to be restricted to dimerization, we determined the concentration x₂ of dimers, their dipole moment μ_dim, and the equilibrium constant K_x and Gibbs energy ΔG of the dimerization process.     

Mononuclear and Dinuclear Copper Complexes of Tridentate Redox-active Ligands with Tunable H-bonding Donors: Structure,Spectroscopy and H+/e− Reactivity

In this research article, we describe the synthesis and characterization of mononuclear and dinuclear Cu complexes bound by a family of tridentate redox-active ligands with tunable H-bonding donors. The mononuclear Cu-anion complexes were oxidized to the corresponding “high-valent” intermediates by oxidation of the redox-active ligand. These species were capable of oxidizing phenols with weak O−H bonds via H-atom abstraction. Thermodynamic analysis of the H-atom abstractions, which included reduction potential measurements, pK_a determination and kinetic studies, revealed that modification of the anion coordinated to the Cu and changes in the H-bonding donor did not lead to major differences in the reactivity of the “high-valent” CuY complexes (Y: hydroxide, phenolate and acetate), which indicated that the tridentate ligand scaffold acts as the H⁺ and e⁻ acceptor.     

Spectroscopic studies of intermolecular hydrogen bonding and proton transfer complexes of chromotropic acid with some amines in methanol

The proton transfer reactions between chromotropic acid (CTA) and some amines including benzylamine (BA), triethylamine (TEA), pyrrolidine (PY) and 1,8-bis(dimethylamino) naphthalene (DMAN) have been investigated spectrophotometrically in methanol. A long wavelength band at 365 nm has been recorded due to the proton transfer (PT) complex formation. The proton transfer equilibrium constants K_PT were estimated utilizing the minimum–maximum absorbances method. It has been found that K_PT were not depend on the amine pKa values, but strongly depend on the formed structures of the PT complexes. Job^’s method of continuous variations and photometric titrations were applied to identify the compositions of the formed PT complexes where 1:1 complexes (proton donor: proton acceptor) were produced. Due to the rapidity and simplicity of the proton transfer reactions and the stability of the formed complexes, a rapid and accurate spectrophotometric method for the determination of CTA was proposed for the first time.     

EFFECT OF THE BROMINE ATOM UPON THE DISSOCIATION CONSTANTS OF BROMOPYRIDINES,BROMOQUINOLINES AND 4-BROMOISOQUINOLINE IN THEIR LOWEST EXCITED SINGLET AND TRIPLET STATES*

Abstract— The lowest excited singlet-state dissociation constants (pK^S_a) of bromosubstituted pyridines, quinolines, and isoquinolines were determined from the pH-dependent shifts in their electronic absorption spectra. The lowest excited triplet-state dissociation constants (pK^T_a) of bromosubstituted quinolines and 4-bromoisoquinoline were obtained from the shifts of the 0–0 phosphorescence bands measured in rigid aqueous solution at 77 K. The pK^S_a values indicate that the basicity of these brominated nitrogen heterocycles is increased in the lowest excited singlet state by 2 to 10 orders of magnitude as compared with the ground state. The pK^T_a values are found to be significantly different from the corresponding ground-state pK_a values, indicating that the basicity of bromoquinolines is increased in the lowest excited triplet state by 1.7 to 3.0 pK units. The enhancement of the excited singlet-and triplet-state basicity of brominated nitrogen heterocycle derivatives as compared with the unsuhstituted parent compounds is attributed to the increased electron-donor conjugative interactions of the bromine atom pπ orbitals with π orbitals in the lowest excited singlet and triplet state.     

Quantitative structure–property relationships in the pyridine series

By means of the MNDO, AM1, and PM3 methods, standard heats of formation, entropies, ionization potentials, and molecular dipole moments of the pyridine series compounds have been computed. The information on the experimental values of the above quantities has been systematized. Linear dependencies allowing a priori evaluation of thermodynamic and molecular characteristics of pyridines have been stated. Correlations of the 2,2′‐bipyridines pK_a values for aqueous solutions with the gaseous‐phase proton affinities have been found. © 2002 Wiley Periodicals, Inc. Heteroatom Chem 13:229–241, 2002; Published online in Wiley Interscience (www.interscience.wiley.com). DOI 10.1002/hc.10013