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1.
J Phys Chem A ; 119(12): 2998-3007, 2015 Mar 26.
Artículo en Inglés | MEDLINE | ID: mdl-25734572

RESUMEN

The gas-phase enthalpy of formation of cubane (603.4 ± 4 kJ mol(-1)) was calculated using an explicitly correlated composite method (W1-F12). The result obtained for cubane, together with the experimental value for the enthalpy of sublimation, 54.8 ± 2.0 kJ mol(-1), led to 548.6 ± 4.5 kJ mol(-1) for the solid-phase enthalpy of formation. This value is only 6.8 kJ mol(-1) higher than the 50-year-old original calorimetric result. The carbon-hydrogen bond dissociation enthalpy (C-H BDE) of cubane (438.4 ± 4 kJ mol(-1)), together with properties relevant for its experimental determination using gas-phase ion thermochemistry, namely the cubane gas-phase acidity (1704.6 ± 4 kJ mol(-1)), cubyl radical electron affinity (45.8 ± 4 kJ mol(-1)), cubane ionization energy (1435.1 ± 4 kJ mol(-1)), cubyl radical cation proton affinity (918.8 ± 4 kJ mol(-1)), cubane cation appearance energy (1099.6 ± 4 kJ mol(-1)), and cubyl ionization energy (661.2 ± 4 kJ mol(-1)), were also determined. These values were compared with those calculated for unstrained hydrocarbons (viz., methane, ethane, and isobutane). The strain energy of cubane (667.2 kJ mol(-1)) and cubyl radical (689.4 kJ mol(-1)) were independently estimated via quasihomodesmotic reactions. These values were related via a simple model to the C-H BDE in cubane. Taking into account the accuracy of the computational method, the comparison with high-precision experimental results, and the data consistency afforded by the relevant thermodynamic cycles, we claim an uncertainty better than ±4 kJ mol(-1) for the new enthalpy of formation values presented.


Asunto(s)
Ciclooctanos/análogos & derivados , Ciclooctanos/química , Ciclooctanos/síntesis química , Teoría Cuántica , Termodinámica
2.
J Phys Chem A ; 117(13): 2873-8, 2013 Apr 04.
Artículo en Inglés | MEDLINE | ID: mdl-23441731

RESUMEN

The gas-phase enthalpies of formation for a set of ortho-substituted alkylbenzenes were obtained from CCSD(T*)-F12 and W1-F12 calculations. Most values are in keeping with available experimental data. The gas-phase enthalpies of formation of 1-ethyl-2-propylbenzene, 1-ethyl-2-isopropylbenzene, 1,2-diisopropylbenzene, 1,2,4-triethylbenzene, and 1,2,4,5-tetraethylbenzene, for which no experimental data are available, were determined as -46.0, -46.8, -68.7, -76.9, and -116.8 kJ mol(-1), respectively (estimated error bar ±4 kJ mol(-1)). The whole set of experimental and theoretical values are in good agreement with the estimates obtained using the extended Laidler bond additivity (ELBA) method. This agreement supports the approximation used in ELBA that interactions between ortho alkyl groups (other than tertiary alkyl groups) have roughly the same magnitude as a methyl-methyl interaction.

3.
J Phys Chem A ; 113(23): 6524-30, 2009 Jun 11.
Artículo en Inglés | MEDLINE | ID: mdl-19449831

RESUMEN

The energetics of tert-butoxyl radical addition reaction to norbornadiene was investigated by time-resolved photoacoustic calorimetry (TR-PAC). The result, together with the C-O bond dissociation enthalpy (BDE) in the addition product, allowed us to calculate the pi-bond dissociation enthalpy in norbornadiene. Quantum chemistry (QC) methods were also used to obtain several enthalpies of reaction of the addition of oxygen-centered radicals to alkenes. The pi-bond dissociation enthalpies in these molecules were calculated by a procedure similar to that used in the case of norbornadiene and were compared with the pi-BDE values obtained by the method proposed by Benson. These two different approaches yield similar values for the pi-BDEs in alkenes, indicating that the addition method proposed in the present study is a valid way to derive that quantity. The influence of strain in the pi-BDEs of cyclic alkenes was investigated and allowed us to justify the difference between the pi-BDE in norbornene and norbornadiene. Finally, the thermochemistry of the addition and abstraction reactions involving these two molecules and tert-butoxyl radical was analyzed.


Asunto(s)
Carbono/química , Norbornanos/química , Óxidos/química , Alquenos/química , Radicales Libres/química , Oxígeno/química , Termodinámica
4.
Org Lett ; 10(8): 1613-6, 2008 Apr 17.
Artículo en Inglés | MEDLINE | ID: mdl-18348570

RESUMEN

Gas-phase C-H bond dissociation enthalpies (BDEs) in norbornane were determined by quantum chemistry calculations and the C2-H BDE was experimentally obtained for the first time by time-resolved photoacoustic calorimetry. CBS-Q and CBS-QB3 methods were used to derive the values DH degrees (C1-H) = 449 kJ mol-1, DH degrees (C7-H) = 439 kJ mol-1, and DH degrees (C2-H) = 413 kJ mol-1. The experimental result DH degrees (C2-H) = 414.6 +/- 5.4 kJ mol-1 is in excellent agreement with the theoretical value. The trend DH degrees (C1-H) > DH degrees (C7-H) > DH degrees (C2-H) is discussed.


Asunto(s)
Carbono/química , Hidrógeno/química , Norbornanos/química , Termodinámica
5.
J Org Chem ; 73(16): 6213-23, 2008 Aug 15.
Artículo en Inglés | MEDLINE | ID: mdl-18651776

RESUMEN

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.


Asunto(s)
Ciclohexanos/química , Ciclopentanos/química , Ciclohexenos/química , Radicales Libres/química , Fotoquímica , Termodinámica
6.
J Agric Food Chem ; 54(2): 342-8, 2006 Jan 25.
Artículo en Inglés | MEDLINE | ID: mdl-16417289

RESUMEN

The demand for novel effective antioxidant-based drugs has led to the synthesis and evaluation of the antioxidant potential in several molecules derived from natural compounds. In this work the in vitro antioxidant activity of an abietic acid-derived catechol (methyl 11,12-dihydroxyabietate-8,11,13-trien-18-oate, MDTO) was evaluated. This substance, possessing important biological properties, is similar to carnosic acid, a natural antioxidant from rosemary or sage leaves. Aiming to understand the antioxidant activity of MDTO, the energetics of its O-H bond, using time-resolved photoacoustic calorimetry (TR-PAC), was investigated. On energetic grounds it is predicted that MDTO is a good free radical scavenger, although its activity is lower than that of quercetin, a very effective antioxidant, which was used for comparison. In agreement with these predictions, the DPPH(*) and ABTS(*+) radical scavenging activities are lower than those of quercetin. In addition, MDTO also reacts with HOCl, a powerful proinflammatory oxidant produced by activated neutrophils, and protects liposomes against iron-ascorbate-induced oxidation. The discussion of these results foresees potential applications of MDTO as an antioxidant.


Asunto(s)
Abietanos/química , Antioxidantes/farmacología , Catecoles/química , Catecoles/farmacología , Fenantrenos/química , Benzotiazoles , Compuestos de Bifenilo , Calorimetría/métodos , Depuradores de Radicales Libres/química , Peroxidación de Lípido/efectos de los fármacos , Liposomas/química , Picratos , Ácidos Sulfónicos , Termodinámica
7.
J Phys Chem A ; 110(15): 5130-4, 2006 Apr 20.
Artículo en Inglés | MEDLINE | ID: mdl-16610835

RESUMEN

The gas-phase C-H bond dissociation enthalpy (BDE) in 1,3-cyclopentadiene has been determined by time-resolved photoacoustic calorimetry (TR-PAC) as 358 +/- 7 kJ mol(-1). Theoretical results from ab initio complete basis-set approaches, including the composite CBS-Q and CBS-QB3 procedures, and basis-set extrapolated coupled-cluster calculations (CCSD(T)) are reported. The CCSD(T) prediction for the C-H BDE of 1,3-cyclopentadiene (353.3 kJ mol(-1)) is in good agreement with the TR-PAC result. On the basis of the experimental and the theoretical values obtained, we recommend 355 +/- 8 kJ mol(-1) for the C-H BDE of 1,3-cyclopentadiene and 271 +/- 8 kJ mol(-1) for the enthalpy of formation of cyclopentadienyl radical.


Asunto(s)
Ciclopentanos/química , Termodinámica , Calorimetría/instrumentación , Calorimetría/métodos , Ciclopentanos/síntesis química , Radicales Libres/química , Gases/química , Fotólisis , Teoría Cuántica
8.
J Phys Chem A ; 109(42): 9700-8, 2005 Oct 27.
Artículo en Inglés | MEDLINE | ID: mdl-16866422

RESUMEN

The energetics of the phenolic O-H bond in the three hydroxybenzoic acid isomers and of the intramolecular hydrogen O-H- - -O-C bond in 2-hydroxybenzoic acid, 2-OHBA, were investigated by using a combination of experimental and theoretical methods. The standard molar enthalpies of formation of monoclinic 3- and 4-hydroxybenzoic acids, at 298.15 K, were determined as Delta(f)(3-OHBA, cr) = -593.9 +/- 2.0 kJ x mol(-1) and Delta(f)(4-OHBA, cr) = -597.2 +/- 1.4 kJ x mol(-1), by combustion calorimetry. Calvet drop-sublimation calorimetric measurements on monoclinic samples of 2-, 3-, and 4-OHBA, led to the following enthalpy of sublimation values at 298.15 K: Delta(sub)(2-OHBA) = 94.4 +/- 0.4 kJ x mol(-1), Delta(sub)(3-OHBA) = 118.3 +/- 1.1 kJ x mol(-1), and Delta(sub)(4-OHBA) = 117.0 +/- 0.5 kJ x mol(-1). From the obtained Delta(f)(cr) and Delta(sub) values and the previously reported enthalpy of formation of monoclinic 2-OHBA (-591.7 +/- 1.3 kJ x mol(-1)), it was possible to derive Delta(f)(2-OHBA, g) = -497.3 +/- 1.4 kJ x mol(-1), Delta(f)(3-OHBA, g) = -475.6 +/- 2.3 kJ x mol(-1), and Delta(f)(4-OHBA, cr) = -480.2 +/- 1.5 kJ x mol(-1). These values, together with the enthalpies of isodesmic and isogyric gas-phase reactions predicted by density functional theory (B3PW91/aug-cc-pVDZ, MPW1PW91/aug-cc-pVDZ, and MPW1PW91/aug-cc-pVTZ) and the CBS-QMPW1 methods, were used to derive the enthalpies of formation of the gaseous 2-, 3-, and 4-carboxyphenoxyl radicals as (2-HOOCC(6)H(4)O(*), g) = -322.5 +/- 3.0 kJ.mol(-1) Delta(f)(3-HOOCC(6)H(4)O(*), g) = -310.0 +/- 3.0 kJ x mol(-1), and Delta(f)(4-HOOCC(6)H(4)O(*), g) = -318.2 +/- 3.0 kJ x mol(-1). The O-H bond dissociation enthalpies in 2-OHBA, 3-OHBA, and 4-OHBA were 392.8 +/- 3.3, 383.6 +/- 3.8, and 380.0 +/- 3.4 kJ x mol(-1), respectively. Finally, by using the ortho-para method, it was found that the H- - -O intramolecular hydrogen bond in the 2-carboxyphenoxyl radical is 25.7 kJ x mol(-1), which is ca. 6-9 kJ x mol(-1) above the one estimated in its parent (2-OHBA), viz. 20.2 kJ x mol(-1) (theoretical) or 17.1 +/- 2.1 kJ x mol(-1) (experimental).


Asunto(s)
Fenoles/química , Ácido Salicílico/química , Cristalografía por Rayos X , Enlace de Hidrógeno , Modelos Químicos , Modelos Moleculares , Estructura Molecular , Teoría Cuántica
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