Search

O-type variations on C-H bond dissociation energies were studied.

Equilibrium acidities in DMSO for phenylhydrazine, five p-substituted derivatives, 1,2-diphenylhydrazine and 1,1,2-triphenylhydrazine were measured. The bond dissociation enthalpies (BDEs) of their acidic N-H bonds were estimated. The alpha-N-H bonds in CHsub3CONHNHsub2, PhCONHNHsub2, NHsub2NHCOsub2Et and PhSOsub2NHNHsub2 hydrazides were more acidic than the alpha-N-H bonds in corresponding amides, and the BDEs were weaker.

The delta G(sub hydride) (R+) data for some stable carbocations and less stable aromatic carbocations derived from substituted toluenes, fluorenes and 9-methylanthracenes are reported. The computation of hydride affinities of organic cations was based on two equations derived from thermochemical cycles. Linear correlations were observed between hydride affinities and solution electron affinities of hydride acceptors. Correlations between delta G(sub hydride)(R+) and pK(sub R+) of corresponding carbinols provide convenient estimates of each other as long as the other is known.

A thermodynamic study of the cleavage of carbon-sulfur bonds involves the analysis of the enthalpies of reactions between six thiophenoxides and fiveresonance-stabilized carbenium ions. C-S bond cleavage is of interest in the environmentally-critical desulfurization of coal. A battery of calorimetric andvoltammetric tests yield correlated data on the heats of heterolysis and homolysis, redox potentials and heats of entropies of the various processes involved in the reactions. These are presented and discussed in detail.

The first-ever Y-nitric oxide (NO) bond energy scale, where Y is the atom to which NO is attached, is established by direct calorimetric measurements and electrochemical data for three types of N-nitroso compounds. These bond scales establish a technique for determining the dissociation energy of Y-NO bonds in solution and pave the way for developing a complete data set of Y-NO bond energies for a wider range of organic and inorganic compounds.

Studies the hydride affinities of carbenium ions in acetonitrile and dimethyl sulfoxide solution. Use of equations derived from thermochemical cycles; Errors associated with the determination of the reversible electrode potentials; Stable carbenium ion in dimethyl sulfoxide solution; Cancellation of errors due to kinetic shifts of the experimental potentials from the reversible values.

Deoxygenation of phosphine oxides is of great significance to synthesis of phosphorus ligands and relevant catalysts, as well as to the sustainability of phosphorus chemistry. However, the thermodynamic inertness of P═O bonds poses a severe challenge to their reduction. Previous approaches in this regard rely primarily on a type of P═O bond activation with either Lewis/Brønsted acids or stoichiometric halogenating reagents under harsh conditions. Here, we wish to report a novel catalytic strategy for facile and efficient deoxygenation of phosphine oxides via successive isodesmic reactions, whose thermodynamic driving force for breaking the strong P═O bond was compensated by a synchronous formation of another P═O bond. The reaction was enabled by P III /P═O redox sequences with the cyclic organophosphorus catalyst and terminal reductant PhSiH 3 . This catalytic reaction avoids the use of the stoichiometric activator as in other cases and features a broad substrate scope, excellent reactivities, and mild reaction conditions. Preliminary thermodynamic and mechanistic investigations disclosed a dual synergistic role of the catalyst.

The heterolytic and homolytic N-NO bond dissociation energies of seven substituted N-methyl-N-nitrosobenzenesulfonamides (abbreviated as G-MNBS, G = p-OCH(3), p-CH(3), p-H, p-Cl, p-Br, 2,5-2Cl, m-NO(2)) in acetonitrile solution were evaluated for the first time by using titration calorimetry and relative thermodynamic cycles according to Hess' law. The results show that the energetic scales of the heterolytic and homolytic N-NO bond dissociation energies of G-MNBS in acetonitrile solution cover the ranges from 44.3 to 49.5 and from 33.0 to 34.9 kcal/mol for the neutral G-MNBS, respectively, which indicates that N-methyl-N-nitrosobenzenesulfonamides are much easier to release a NO radical (NO(*)) than to release a NO cation (NO(+)). The estimation of the heterolytic and homolytic (N-NO)(-)(*) bond dissociation energies of the seven G-MNBS radical anions in acetonitrile solution gives the energetic ranges of -15.8 to -12.9 and -3.1 to 1.8 kcal/mol for the (N-NO)(-)(*) bond homolysis and heterolysis, respectively, which means that G-MNBS radical anions are very unstable at room temperature and able to spontaneously or easily release a NO radical or NO anion (NO(-)), but releasing a NO radical is easier than releasing NO anion. These determined N-NO bond dissociation energies of G-MNBS and their radical anions have been successfully used in the mechanism analyses of NO transfer from G-MNBS to 3,6-dibromocarbazole and the reactions of NO with the substituted N-methyl-benzenesulfonamide nitranions (G-MBSN(-)) in acetonitrile solution.

The heterolytic and homolytic C4-H bond dissociation energies of NADH and its radical cation (NADH*+) in aqueous solution were estimated according to the reaction of NADH with N,N,N',N'-tetramethyl-p-phenylenediamine radical cation perchlorate (TMPA*+) in aqueous solution. The results show that the values of the heterolytic and homolytic C4-H bond dissociation energies of NADH in aqueous solution are 53.6 and 79.3 kcal/mol, respectively; the values of the heterolytic and homolytic C4-H bond dissociation energies of NADH*+*+ in aqueous solution are 5.1 and 36.3 kcal/mol, respectively, which, to our knowledge, is first reported. This energetic information disclosed in the present work should be believed to furnish hints to the understanding of the mechanisms for the redox interconversions of coenzyme couple NADH/NAD+ in vivo.

The first two series of Co-NO bond dissociation enthalpies in benzonitrile solution were determined for 12 cobalt(II) nitrosyl porphyrins and for 12 cobalt(III) nitrosyl porphyrins by titration calorimetry with suitable thermodynamic cycles. The results display that the energy scales of the heterolytic Co(III)-NO bond dissociation, the homolytic Co(III)-NO bond dissociation, and the homolytic Co(II)-NO bond dissociation are 14.7-23.2, 15.1-17.5, and 20.8-24.6 kcal/mol in benzonitrile solution, respectively, which not only indicates that the thermodynamic stability of cobalt(II) nitrosyl porphyrins is larger than that of the corresponding cobalt(III) nitrosyl porphyrins for homolysis in benzonitrile solution but also suggests that both cobalt(III) nitrosyl porphyrins and cobalt(II) nitrosyl porphyrins are excellent NO donors, and in addition, cobalt(III) nitrosyl porphyrins are also excellent NO(+) contributors. Hammett-type linear free energy analyses suggest that the nitrosyl group carries negative charges of 0.49 +/- 0.06 and 0.27 +/- 0.04 in T(G)PPCo(II)NO and in T(G)PPCo(III)NO, respectively, which indicates that nitric oxide is an electron-withdrawing group both in T(G)PPCo(II)NO and in T(G)PPCo(III)NO, behaving in a manner similar to Lewis acids rather than to Lewis bases. The energetic and structural information disclosed in the present work is believed to furnish hints to the understanding of cobalt nitrosyl porphyrins' biological functions in vivo.