Enthalpies Of Adduct Formation Research Articles

Enthalpies of Adduct Formation between Boron Trifluoride and Selected Organic Bases in Solution: Toward an Accurate Theoretical Entry to Lewis Basicity.

The Lewis basicity of selected organic bases, modeled by the enthalpies of adduct formation between gaseous BF3 and bases in dichloromethane (DCM) solution, is critically examined. Although experimental enthalpies for a large number of molecules have been reported in the literature, it may be desirable to estimate missing or uncertain data for important Lewis bases. We decided to use high-level ab initio procedures, combined with a polarized continuum solvation model, in which the solvated species were the clusters formed by specific hydrogen bonding of DCM with the Lewis base and the Lewis base/BF3 adduct. This mode of interaction with DCM corresponds to a specific solvation model (SSM). The results essentially showed that the enthalpy of BF3 adduct formation in DCM solution was clearly influenced by specific interactions, with DCM acting as hydrogen-bonding donor (HBD) molecule in two ways: base/DCM and adduct/DCM, confirming that specific solvation is an important contribution to experimentally determined Lewis basicity scales. This analysis allowed us to conclude that there are reasons to suspect some gas-phase values to be in error by more than the stated experimental uncertainty. Some experimental values in DCM solution that were uncertain for identified reasons could be complemented by the computed values.

Theoretical study on the enthalpies of adduct formation between alkyl iodides and chlorine atoms

The equilibrium geometries for the ground state of the adducts formed between alkyl iodides (RI) and chlorine (Cl) atoms were optimized, and the enthalpies of adduct formations (EAFs) were obtained. The EAFs decreased when the number of alkyl substituents on the C atom bonded to I atom was increased. This trend suggests that the amount of charge transferred from the electron-donating alkyl group to ICl increases in accordance with the number of substituents, which results in the stabilization of the adduct. Charge density distribution between the Cl and H atoms was an additional mechanism to stabilize the adducts.

Thermochemistry of adducts of some bivalent transition metal bromides with quinoline

The compounds [MBr 2(quin) n ] (where M is Mn(II), Co(II), Ni(II), Cu(II) or Zn(II); quin, quinoline; n = 1 or 2) were synthesized and characterized by melting points, elemental analysis, thermal analysis and electronic and IR spectroscopy. The enthalpies of dissolution of the adducts, metal(II) bromides and quinoline in methanol or 1.2 M aqueous HCl were measured and by using thermochemical cycles, the following parameters for the adducts have been determined: the standard enthalpies for the Lewis acid/base reactions (Δ r H°), the standard enthalpies of formation (Δ f H°), the standard enthalpies of decomposition (Δ D H°), the lattice standard enthalpies (Δ M H°) and the standard enthalpies of the Lewis acid/base reactions in the gaseous phase (Δ r H°(g)). The mean bond dissociation enthalpies of the M(II)–nitrogen bonds ( D ¯ ( M – N ) ) have been estimated as well as the enthalpies of adduct formation in the gaseous phase.

Thermochemistry of adducts of some bivalent transition metal bromides with 4-chloroaniline

The compounds [MBr 2( p-clan) 2] (where M is Mn(II), Fe(II), Co(II), Ni(II), Cu(II) or Zn(II); p-clan = 4-chloroaniline) were synthesized and characterized by melting points, elemental analysis, thermal analysis and electronic and IR spectroscopy. The enthalpies of solution of the adducts, metal(II) bromides and 4-chloroaniline in methanol, 1.2 M aqueous HCl or 25% (v/v) 1.2 M aqueous HCl in methanol were measures and by using thermochemical cycles, the following thermochemical parameters for the adducts have been determined: the standard enthalpies for the Lewis acid/base reactions (Δ r H°), the standard enthalpies of formation (Δ f H°), the standard enthalpies of decomposition (Δ D H°), the lattice standard enthalpies (Δ M H°) and the standard enthalpies of the Lewis acid/base reactions in the gaseous phase (Δ r H°(g)). The mean bond dissociation enthalpies of the metal(II)-nitrogen coordinated bonds ( D ¯ ( M – N ) ) and the enthalpies of adduct formation in the gaseous phase have been estimated.

Calculation of Vibrational Spectra of Linear Tetrapyrroles. 3. Hydrogen-Bonded Hexamethylpyrromethene Dimers

The structure and vibrational spectra of hexamethylpyrromethene (HMPM) have been investigated by X-ray crystallography, IR and Raman spectroscopies, and density functional theory calculations. HMPM crystallizes in the form of dimers, which are held together by bifurcated N-H(...N)(2) hydrogen bonds, involving one intramolecular and one intermolecular N-H...N interaction. The monomers are essentially planar, and the mean planes of the monomers lie approximately perpendicular to one another, so that the four N atoms in the dimer form a distorted tetrahedron. The structure of the HMPM dimer is well-reproduced by B3LYP/6-31G calculations. A comparison of the calculated geometry of the dimer with that of the monomer reveals only small changes in the N-H...N entity and the methine bridge angles upon dimerization. These are a result of weakening of the intramolecular N-H...N hydrogen bond and the formation of a more linear N-H...N intermolecular hydrogen bond. Using an empirical relation between the shift of the N-H stretching frequency of pyrrole and the enthalpy of adduct formation with bases [Nozari, M. S.; Drago, R. S. J. Am. Chem. Soc. 1970, 92, 7086-7090], estimates of the strength of the intra- and intermolecular hydrogen bonds are obtained. IR and Raman spectroscopies of HMPM and its isotopomers deuterated at the pyrrolic nitrogen atom and at the methine bridge reveal that the molecule is monomeric in nonpolar organic solvents but dimeric in a solid Ar matrix and in KBr pellets. The matrix IR spectra show a splitting of vibrational modes for the dimer, particularly those involving the N-H coordinates. Due to intrinsic deficiencies of the B3LYP/6-31G approximation, a satisfactory reproduction of these modes of the monomeric and dimeric HMPM requires specific adjustments of the NH scaling factors for the calculated force constants and, in the case of the NH out-of-plane modes of HMPM dimers, also of intra- and intermolecular coupling constants. This parametrization does not significantly affect the other calculated modes, which in general reveal a very good agreement with the experimental data.

Evidence of Acid-Base Interfacial Adducts in Various Polymer/Metal Systems by IRAS: Improvement of Adhesion

Abstract The physical interactions of polymers with inorganic substrates are determined by two major contributions: Van der Waals forces and acid-base interactions, taken in the most general “Lewis” electron acceptor-donor sense. The present work shows that the work of adhesion can be very appreciably increased by the creation of interfacial acid-base interactions. Practically, polymers such as poly(ethylene-co-vinyl acetate) (EVA), terpene-phenol resins (TPR), and their blends, were solution cast on basic and acidic substrates. The nature of the interfacial bonds and the enthalpy of adduct formation through electron exchange are evidenced by Fourier transform infrared reflection-absorption spectroscopy (IRAS). Moreover, it is shown that, on the one hand, modification of the electron donor ability of the polymer functionalities reveals the amphoteric character of the substrate and, on the other hand, modification of the electron donor ability of the substrate changes the nature of the species involved in ...

Inductive transfer and coordination of ligands in metal—metal bonded systems

Donor-acceptor complex formation between a range of neutral, dinuclear metal carboxylates and a range of Lewis bases has been monitored to obtain equilibrium constants and enthalpies for adduct formation. The acceptors studied are Rh 2(but) 4, Rh 2(pfb) 4, Mo 2(pfb) 4, Ru 2(but) 4Cl and Cr 2(tfa) 4(Et 2O) 2 (but = O 2C 4H 7 −, pfb = O 2C 4F 7 −, tfa = O 2CCF 3 −) and the donors include oxygen, sulphur, nitrogen and phosphorous donor atom bases. Spectrophotometric and calorimetric titrations were used to measure and apportion the experimental heats. The measured enthalpies of adduct bond formation thus correspond to stepwise complexation equilibria. Measurements with bases which varied in their covalent and electrostatic donor properties allowed definition of acid parameters to describe the dinuclear complexes by the E and C treatment. The spectral and calorimetric data point towards unusual stabilization of some of the donor-Rh dimer adduct bonds which was attributed to π-backbonding from populated Rh(II)Rh(II) π orbitals to empty donor orbitals of the proper symmetry. This provides one of the first quantitative estimates of π-backbond stabilization energies in transition metal systems. The π-backbonding effects were less pronounced for adduct formation with Ru 2(but) 4Cl (half-filled π orbitals) and not observed for Mo 2(pfb) 4 (unpopulated π orbitals). Correlation of the first and second enthalpies of adduct formation allowed determination of inductive transfer parameters to describe the inductive effects of coordination of the first base through the metal-metal bond to influence the coordination of the second base. The lowered acidity of the second metal centre upon complex formation at the first could be quantified by this treatment and the results correlated with the nature of the metal-metal bond. This correlation treatment was extended to IR data, ν co of (CO)Rh 2(pfb) 4 (B) (B = base) adducts, and EPR data, g of (TEMPO) Rh 2 (pfb) 4 (B) adducts to demonstrate the relative E and C contributions to these spectral parameters. Donor complexation was observed to have profound influences upon the electronic structures of the various complexes as followed by electronic spectral shifts. The Cr 2 (tfa) 4(B) 2 adducts provided additional means for monitoring changes in electronic structure. Magnetic susceptibilities of the Cr 2(tfa) 4(B) 2 adducts were found to increase with B donor strength suggesting a direct exchange pathway to account for observed antiferromagnetism. Solid state measurements on three of the adducts (B = Et 2O, Et 3PO 4, HMPA) gave -2 J ranging from 611 to 688 cm −1, values which are larger than those measured for the well-studied cupric carboxylates, Cu 2(O 2CR) 4 (B) 2.

Calorimetric titration studies of adduct formation between lanthanide β-diketonates and organic bases in benzene solution

The technique of titration calorimetry has been used to study the reactions of lanthanide shift reagents with organic substrates in benzene solution. Stability constants and enthalpies and entropies of formation have been determined for adducts of tris(2,2,6,6-tetramethylheptane-3,5-dionato)-europium(III) and -ytterbium(III) and tris(6,6,7,7,8,8,8-heptafluoro-2,2-dimethyloctane-3,5-dionato)-europium(III) and -praseodymium(III) with pyridine, 2-methyl-, 3-methyl-, and 4-methyl-pyridine, 2,4,6-trimethylpyridine, dimethyl sulphoxide, and pyridine N-oxide. Adduct-formation constants mostly lie in the range 2 < log K < 4 and enthalpies of adduct formation in the range 15 < –ΔH⊖ < 35 kJ mol–1 with some variations which can be related to steric effects. Unusual positive entropy changes associated with the formation of adducts of the fluorinated reagents can be correlated with their unusually high solubility.

Etude thermodynamique des complexes de transfert de charge du type n‐σ* en solution Complexes de la quinoléine et de quinoléines substituées avec I2, ICI et IBr

Thermodynamic study of charge transfert complexes (n‐σ*) in solutionCharge transfer complexes of quinoline and substituted quinolines (donors) with iodine, iodine chloride, and iodine bromide (acceptors) have been studied spectrophotometrically in CCl4. The 1:1 stoechiometry of these complexes was verified by means of the continous variations method and the appearance of isosbestic points. Simultaneous determination of the equilibrium constant and enthalpy of adduct formation was carried out by calorimetry.It was observed for every donor that the equilibrium constants of the complexes studied increase with the strength of the acceptors. A linear correlation between enthalpy of adduct formation and donor strength was obtained only for the complexes Donor ICI.

Thermodynamics of metalligand bond formation : XXVI. Adducts of tertiary phosphines with organotin halides

Thermodynamic data, obtained by calorimetric titration in benzene solution at 30° C, are reported for formation of 1 : 1 adducts of Ph 2SnCl 2, Me 2SnCl 2 and Bu 2SnCl 2 with the tertiary phosphines PBu 3, PhPBu 2, PhPPr 2, PhPMe 2, Ph 2PPr and Ph 2PMe. Enthalpies of formation and stabilities of adducts decrease in the order Ph 2SnCl 2 > Me 2SnCl 2 > Bu 2SnCl 2. There is no systematic relationship between the enthalpies of adduct formation and phosphine structure, but adduct stabilities decrease in the order PBu 3 > PhPAlk 2 ∼ Ph 2PAlk > PPh 3.

Lewis acidity of organosilicon acetates

Thermodynamic data, obtained calorimetrically, are reported for the formation of adducts of organosilicon acetates, R n Si(OCOME) 4- n (0 ⩽ n ⩽ 3) with a wide variety of Lewis bases in acetonitrile solution. Only 1 1 adducts are formed, in which the silicon is 5-coordinate and there is evidence that the coordination number cannot exceed five in these compounds. Enthalpies of adduct formation (−Δ H 0) increase regularly as R is replaced by OCOMe and become very large with Si(OCOMe) 4, exceeding 150 kJ mol −1 for addition of amines to this compound. Despite the large enthalpies of formation, adduct stabilities are not high and adduct formation constants, K, rarely exceed 200 l mol −1; stabilites are similar for 0 ⩽ n ⩽ 2 but a little greater for n = 3 and are thus little affected by variations in the enthalpy.

Thermodynamics of metal-ligand bond formation. XXIV. Reaction of mercury(II) halides with tertiary arsines and phosphine and arsine sulphides

Thermodynamic data have been obtained by calorimetric titration for the formation of 1 : 1 adducts of mercury(11) halides with tertiary arsines in benzene solution. In contrast to phosphines, only one molecule of arsine adds to HgX2 in dilute solution. Enthalpies of adduct formation, in the range - ΔH� = 50-80 kJ mol-1, are not much changed by replacement of phenyl by methyl, but adduct stabilities increase in the order Ph3As &lt; Ph2AsMe &lt; PhAsMe2 &lt; AsMe3 HgI2AsMe3 is dimeric in solution. The diarsines Ph2As(CH2),AsPh2 are unidentate, but Ph2P- (CH2),AsPh2 is bidentate. Data are also reported for addition to HgX2 of Ph3PS,Ph2PSCH2PSPh2 and corresponding arsine sulphides; the disulphides are bidentate. Factors affecting chelation in these compounds are discussed.

Thermodynamics of metal-ligand bond formation. XXI. Base adducts of nickel(II) glyoximato complexes

Bis(glyoximato)nicke1(11) complexes and their BF2-bridged derivatives react with Lewis bases in non-donor solvents forming low-spin 1 : 1 adducts. Thermodynamic data are reported for formation of adducts of glyoximato complexes with PBu3 and of BF2-bridged complexes with pyridine (py), 4-methylpyridine (mpy), piperidine (pip), tributylphosphine, triphenylphosphine, ethane-1,2-diyi- bis(diphenylphosphine) and tetrahydrothiophen. Adduct stability is increased by BF2-bridging; in the Pbu3 adducts this is due to entropy factors. Variation of alkyl groups in the glyoxinie has only a small effect, but replacement of alkyl by Ph or H increases adduct stability while lowering enthalpy of adduct formation. Average values of ΔH� and K (at 30�C) for adducts of BF2-bridged complexes and for PBu3 adducts of H-bridged complexes (last entry) are: PY mpy pip PPh3 PBu3 Pbu3 -ΔH� (kJ mol-l) 29 31 35 23 43 42 K (1.mol-1) 3 5 80 4 800 60

Stability and bond strength in adducts of mercury(II) halides with transition metal carbonyl derivatives

Thermodynamic data have been obtained calorimetrically for the reaction of a range of Group VI transition metal carbonyl derivatives with mercury(II) halides in 1,2-dichloroethane solution. The results show that the enthalpies of adduct formation are more negative when the transition metal atom has a lower electronegativity, due to large size or coordination of arenes or phosphines. Enthalpies of adduct formation are slightly more negative than those of adducts of mercury(II) halides with conventional Lewis bases, suggesting that the mercurymetal bonds formed are at least as strong as equivalent HgN or HgP bonds. The influence of entropy effects on adduct stabilities is shown by the high stabilities of chromium compounds, despite generally less negative enthalpies of formation.

Thermodynamics of metal-ligand bond formation. XI. Adducts of heterocyclic bases with copper(II) complexes of fluorinated β-diketones

Thermodynamic data have been obtained for the reaction in benzene solution of copper(11) complexes of hexafluoropentanedione and three trifluorinated α-diketones with several unidentate and bidentate heterocyclic bases and terpyridine. With pyridine the hexafluoro� complex forms very stable 1 : 1 and 1 : 2 adducts. The enthalpy of adduct formation (-ΔH) is about 39 kJ mol-1 for the addition of each molecule of pyridine, about 10 kJ mol-1 more than for the formation of the 1 : 1 adduct with the acetylacetone complex. The trifluoro complexes form only 1 : 1 adducts with enthalpy of formation about 31 kJ mol-1 ; these are much more stable than Cu(acac)2(py), but the extra stability is mainly due to entropy factors. With bipyridine and phenanthroline all four fluoro complexes form very stable 1 : 1 adducts with enthalpies of formation about twice those for addition of one molecule of pyridine; this shows that two Cu-N bonds are formed. 2,9-Dimethyl-1,l0-phenanthroline forms a similar adduct, but the enthalpy of formation is much less, due to steric interference. Terpyridine forms a 1 : 1 adduct of much lower stability, probably behaving as an ortho-substituted bidentate ligand. Infrared spectra of the adducts of Cu(tfaa)2 and Cu(hfaa)2 support molecular structures in which one of the Cu-O bonds linking each β-diketone is lengthened to allow the formation of a strong pair of Cu-N bonds.

Thermodynamics of metal-ligand bond formation. XV. Adducts of mercury(II) halides with bidentate bases

Thermodynamic data are reported for the reaction of mercury(11) halides with a range of potentially bidentate bases in benzene solution. The enthalpy data show that in 1 : 1 adducts with tetramethyl-alkanediamines and bis(diphenylphosphino)ethane both donor atoms are coordinated, the mercury being four-coordinate with two Hg-N or Hg-P bonds, each of comparable strength to those in 1 : 1 adducts with unidentate bases. N-Methylmorpholine and dioxan are almost certainly unidentate. 1,10-Phenanthroline, 2,2'-bipyridine and N,N'-dimethylpiperazine gave 1 : 1 adducts of higher stability than those of unidentate bases, but the enthalpies of adduct formation were similar or smaller; the most satisfactory explanation of this seems to be that both nitrogen atoms arecoordinated but that the two Hg-N bonds are unusually weak because of steric misfitting of the bidentate base with the mercury atom.

Thermodynamics of metal-ligand bond formation. XIV. Adducts of hetercyclic bases with nickel(II) alkylxanthates

Thermodynamic data have been obtained for the formation in benzene solution of adducts of nickel(11) O-alkylxanthate complexes with one molecule of 2,2'-bipyridine or two molecules of pyridine or 4-methylpyridine. The results show that changes in the free energy and enthalpy of adduct formation with variation of the alkyl group in the xanthate can be explained wholly by inductive effects; there is no evidence for steric effects even with branched alkyl chains.

The chemistry of uranium. Part VII. Thermodynamic aspects of uranium tetrachloride complexes

During previous work in this laboratory (1) in which the object was to assess the relative importance of size, structure and donor strength of neutral oxygen donor ligands upon stoichiometry, structure and stability of their uranium tetrachloride and tetrabromide complexes, it became clear that quantitative thermodynamic data is needed to illuminate the donor strength aspect. Since we were interested not only in thermodynamic data, but also its relationship towards stoichiometry (UCl~ ligand mole ratios) it was decided to use enThalpimetric %itrimetry, which will provide this information simultaneously. Enthalpies of adduct formation between monobasic Lewis acids and monoacid Lew~s bases were previously determined by Gutmann and others (2) through the reaction of excess ligand with a dilute solution of the Lewis acid in a non-polar (weak donor) solvent. This procedure could not be followed here since UCI 4 is very insoluble in such solvents, polar solvents were, theregore, used for the same purpose.

Enthalpy of Adduct Formation between Antimony Pentachloride and Water.

Enthalpy of Adduct Formation between Antimony Pentachloride and Water.