No abstract

The enthalpy for the reaction of di-ferr-butyl peroxide with phenol to give zerf-butyl alcohol and phenoxyl radical (i.e. í-BuOOBu-í + 2 PhOH -2 f-BuOH + 2 PhO*) has been determined in a number of solvents using photoacoustic calorimetry. The effect of the solvent on the thermochemistry of this process is remarkably large with the reaction being ca. 10 kcal mol-1 more exothermic in acetonitrile or ethyl acetate than in isooctane. The relationship between the observed enthalpy changes and the PhO-H bond energy is discussed in detail. It is shown that in order to extract the bond energy from the experimental (apparent) enthalpy change, it is necessary to account for a number of processes, viz., the volume change for the overall reaction, the solvent effect associated with the conversion of 1 mol of di-ferf-butyl peroxide to 2 mol of ferf-butyl alcohol, and the differences in solvation of phenol and the phenoxyl radical. The contributions from each of these processes to the observed reaction enthalpy were derived from a separate set of experiments with 1,4-cyclohexadiene instead of phenol or from data available in the literature. These data allow one to determine solution bond energies, i.e., the enthalpy of homolysis for which the standard state is the solvated reactant and products, and to quantify the solvent effect on these values. Thus, PhO-H bond energies in isooctane (88 kcal mol-1), benzene (89 kcal mol-1), carbon tetrachloride (90 kcal mol-1), ethyl acetate (95 kcal mol-1), and acetonitrile (95 kcal mol-1) have been obtained. Most of the differences between these values can be accounted for from the known hydrogen bonding equilibrium between the solvents and the phenol. A number of purported determinations of the PhO-H "gas-phase bond energy" which utilized electrochemical (EC) measurements and, of necessity, highly polar solvents are shown to be seriously in error. Similar errors must be present in many other EC "gas-phase" bond energies which also were determined in polar solvents.Bond dissociation energies (BDE) are of fundamental importance because they permit chemists to decide whether or not a particular reaction will be enthalpically favored. Such decisions are usually based on bond dissociation energy differences (ABDEs). For example, for the hydrogen atom abstraction X' + RH -XH + R"(1) the reaction enthalpy, AHr, is determined from the difference in BDEs for XH (eq 2) and RH (eq 3), ABDE = BDE(X-H) -BDE(R-H). Unfortunately, the vast majority of BDEs in the XH -X' + H* (2) RH -R' + H* (3) literature refer to gas-phase reactions, while most of the chemistry to which they are applied occurs in solution. However, even for homolytic reactions involving a neutral radical and substrate, it is not certain that the quantity (AflD£)gas will be equal to (ABDE)ml. In addition, many BDEs of interest

Vous avez des questions? Nous pouvons vous aider. Pour communiquer directement avec un auteur, consultez la première page de la revue dans laquelle son article a été publié afin de trouver ses coordonnées. Si vous n'arrivez pas à les repérer, communiquez avec nous à PublicationsArchive-ArchivesPublications@nrc-cnrc.gc.ca. Questions? Contact the NRC Publications Archive team atPublicationsArchive-ArchivesPublications@nrc-cnrc.gc.ca. If you wish to email the authors directly, please see the first page of the publication for their contact information. NRC Publications Archive Archives des publications du CNRCThis publication could be one of several versions: author's original, accepted manuscript or the publisher's version. / La version de cette publication peut être l'une des suivantes : la version prépublication de l'auteur, la version acceptée du manuscrit ou la version de l'éditeur. For the publisher's version, please access the DOI link below./ Pour consulter la version de l'éditeur, utilisez le lien DOI ci-dessous. NRC Publications Record / Notice d'Archives des publications de CNRC:http://nparc.cisti-icist.nrc-cnrc.gc.ca/eng/view/object/?id=61fe7128-8efa-4f51-847f-6a0f4fb214ed http://nparc.cisti-icist.nrc-cnrc.gc.ca/fra/voir/objet/?id=61fe7128-8efa-4f51-847f-6a0f4fb214ed Science, National Research Council of Canada, Ottawa, Ontario, Canada K1A OR6 ReceiVed: June 25, 2001; In Final Form: October 25, 2001 The results of density functional theory (DFT) energy calculations and geometry optimizations for selected planar and ruffled conformers of nickel octaethylporphyrin (NiOEP) are reported. Calculated geometric parameters show remarkably good agreement with experimental X-ray crystallography data. The tendency for ruffling of the porphyrin macrocycle to allow for a shorter Ni-N bond is accurately predicted by DFT calculations. Energy values indicate that ruffling of the macrocycle lowers the energy of the different conformers by about 0.2 kcal/mol.