José A. Martinho Simões

Energetics of the O–H Bond in Phenol and Substituted Phenols: A Critical Evaluation of Literature Data

This review presents a critical assessment of the available experimental information (contained in ∼90 literature references) on the thermochemistry of the O–H bond in phenol and substituted phenols. The analysis led to a set of recommended values for the O–H bond dissociation enthalpies, which in turn allowed us to discuss several empirical and theoretical methodologies used to estimate these data.

Energetics of Organic Free Radicals

Preface. Series Preface. Contributors. Free Radical Reactions C. Walling. Heats of Formation of Organic Free Radicals by Kinetic Mehods Wing Tsang. Thermochemical Data for Free Radicals From Studies of Ions J.C Traeger, B.M. Kompe. Theoretical Studies of the Energetics of Free Radicals J.S. Francisco, J.A. Montgomery Jr. Photoacoustic Calorimetry of Radicals and Biradicals. J.L. Goodman. A Short and Illustrated Guide to Metal-Alkyl Bonding Energetics F.J.A. Martinho Simoes, M.E. Minas da Piedade. Resonance and 1,2 Rearrangements in Radicals: From Alkyl Radicals to Alkylcobalamins A. Greenberg, J.F. Liebman. Solvent Effects in the Reactions of Neutral Free Radicals J.M. Tanko, N. Kamrudin Suleman. Index.

Bonding and energetics of phosphorus (III) ligands in transition metal complexes

Abstract Thermochemical data for transition metal complexes containing phosphorus(III) ligands have been reviewed and used to derive and discuss metal-phosphine and metal-phosphite bond dissociation enthalpies. The survey is preceded by brief overviews of the nature of the M-P bond and of the steric and electronic properties of the phosphorus ligands. Also included is a critical survey of thermochemical data of phosphines and phosphites.

Substituent effects on the O-H bond dissociation enthalpies in phenolic compounds: agreements and controversies

The available experimental data on O-H bond dissociation enthalpies in phenolic compounds indicate that the ring substituent effects on the thermodynamic stability of that bond can be predicted by using a group additivity method. However, the reliability of the estimates is still affected by the uncertainties assigned to many of those experimental results and also by the scarce information on the solvation of phenoxy radicals.

Thermochemistry of M(η5-C5H5)2Ln complexes (M = Ti, Mo, W)

Abstract The available thermochemical data for the title compounds are summarized and discussed. Metal—ligand bond enthalpies are derived for a variety of ligands through a simple method that involves both the experimental standard enthalpies of formation and theoretical calculations. It is suggested that this method may be used to predict the energetics of new complexes with fair accuracy. In addition, the method provides estimates of meta—ligand stepwise bond dissociation enthalpies. Some of these estimates are compared with recent experimental results obtained for bis (pentamethylcyclopentadienyl) titanium complexes.

Estimation of standard enthalpies of formation of crystalline inorganic and organometallic complexes

Abstract Good linear correlations have been found between standard enthalpies of formation of crystalline inorganic and organometallic complexes MXnLm and enthalpies of formation of ligands L or LH in their standard reference state. These correlations may be used to estimate enthalpies of formation of new complexes.

The enthalpy of sublimation of diphenylacetylene from Knudsen effusion studies

The enthalpy of sublimation of diphenylacetylene at 298.15 K, ΔgcrH⊖m(C2(C6H5)2] = 95.1 ± 1.1 kJmol−1, was derived from vapour pressure-temperature data, obtained with two different Knudsen effusion apparatus, and from heat capacity measurements obtained by differential scanning calorimetry. The molybdenum-diphenylacetylene bond dissociation enthalpy in Mo(η5-C5H5)2[C2(C6H5)2] was reevaluated as 115 ± 26 kJ mol−1, on the basis of the new value for ΔcrgH⊖m[C2(C6H5)2].

Standard Enthalpies of Formation of 2,6-Di-tert-butyl4-methylphenol and 3,5-Di-tert-butylphenol and Their Phenoxy Radicals

The standard (po = 0.1 MPa) enthalpies of formation of 2,6-di-tert-butyl-4-methylphenol and 3,5-di-tert-butylphenol in the gaseous phase, −315.5 ± 4.4 kJ mol−1 and −312.7 ± 4.6 kJ mol−1, respectively, were derived from the standard enthalpies of combustion, in oxygen, at 298.15 K, measured by static bomb combustion calorimetry, and from the standard enthalpies of sublimation, at 298.15 K, measured by Calvet microcalorimetry. The O—H bond dissociation enthalpies in those compounds were determined in benzene by photoacoustic calorimetry, leading to the standard enthalpies of formation of the gaseous phenoxy radicals: −189 ± 8 kJ mol−1 and −154 ± 6 kJ mol−1, respectively. These results were used to calculate enthalpies of substituent redistribution reactions, which are proposed as a method to estimate new data for substituted phenols.

Energetic Differences between the Five-and Six-Membered Ring Hydrocarbons : Strain Energies in the Parent and Radical Molecules

The C-H bond dissociation enthalpies (BDEs) for the five- and six-membered ring alkanes, alkenes, and dienes were investigated and discussed in terms of conventional strain energies (SEs). New determinations are reported for cyclopentane and cyclohexane by time-resolved photoacoustic calorimetry and quantum chemistry methods. The C-H BDEs for the alkenes yielding the alkyl radicals cyclopenten-4-yl and cyclohexen-4-yl and the alpha-C-H BDE in cyclopentene were also calculated. The s-homodesmotic model was used to determine SEs for both the parent molecules and the radicals. When the appropriate s-homodesmotic model is chosen, the obtained SEs are in good agreement with the ones derived from group additivity schemes. The different BDEs in the title molecules are explained by the calculated SEs in the parent molecules and their radicals: (1) BDEs leading to alkyl radicals are ca. 10 kJ mol (-1) lower in cyclopentane and cyclopentene than in cyclohexane and cyclohexene, due to a smaller eclipsing strain in the five-membered radicals relative to the parent molecules (six-membered hydrocarbons and their radicals are essentially strain free). (2) C-H BDEs in cyclopentene and cyclohexene leading to the allyl radicals are similar because cyclopenten-3-yl has almost as much strain as its parent molecule, due to a synperiplanar configuration. (3) The C-H BDE in 1,3-cyclopentadiene is 27 kJ mol (-1) higher than in 1,4-cyclohexadiene due to the stabilizing effect of the conjugated double bond in 1,3-cyclopentadiene and not to a destabilization of the cyclopentadienyl radical. The chemical insight afforded by group additivity methods in choosing the correct model for SE estimation is highlighted.

O-H bond dissociation enthalpies in hydroxyphenols. A time-resolved photoacoustic calorimetry and quantum chemistry study

Time-resolved photoacoustic calorimetry (TR-PAC) was used to investigate the energetics of O–H bonds of phenol, catechol, pyrogallol, and phloroglucinol. Values of −27.1 ± 3.9, −44.1 ± 4.4 and −1.6 ± 3.8 kJ mol−1, respectively, were obtained for the solution-phase (acetonitrile) O–H bond dissociation enthalpies of the last three compounds relative to the O–H bond dissociation enthalpy in phenol, ΔDHosln(ArO–H) = DHosln(ArO–H) − DHosln(PhO–H). A value of 388.7 ± 3.7 kJ mol−1 was determined for the PhO–H bond dissociation enthalpy in acetonitrile. Density functional theory (MPW1PW91/aug-cc-pVDZ) calculations and complete basis set (CBS-4M) calculations were carried out to analyse intramolecular hydrogen bonding and to predict gas-phase O–H bond dissociation enthalpies, DHo(ArO–H). A microsolvation model, based on the DFT calculations, was used to study the differential solvation of the phenols and their radicals in acetonitrile and to bridge solution- and gas-phase data. The results strongly suggest that ΔDHosln(ArO–H) ≈ ΔDHo(ArO–H). Hence, to calculate absolute gas-phase O–H bond dissociation enthalpies in substituted phenols from the corresponding solution-phase values, the solvation enthalpies of the substituted phenols and their radicals are not required.