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ABSTRACT: Treatment of V(N[<sup>t</sup>Bu]Ar)<sub>3</sub> = 1 (Ar = 3,5-Me<sub>2</sub>C<sub>6</sub>H<sub>3</sub>) with O<sub>2</sub> was shown by stopped flow kinetic studies to rapidly form (η<sup>1</sup>-O<sub>2<sub>)V(N[<sup>t</sup>Bu]Ar)<sub>3</sub> = 2 (Δ H <sup>‡</sup> = 3.3 ± 0.2 kcal/mol and Δ S <sup>‡</sup> = -22 ± 1 cal/mol·K) which subsequently isomerizes to (η<sup>2</sup>-O<sub>2<sub>)V(N[<sup>t</sup>Bu]Ar)<sub>3</sub> = 3 (Δ H <sup>‡</sup> = 10.3 ± 0.9 kcal/mol and Δ S <sup>‡</sup> = -6 ± 4 cal/mol·K). The enthalpy of binding of O<sub>2</sub> to form 3 is -75.0 ± 2.0 kcal/mol as measured by solution calorimetry. Reaction of 3 and 1 to form two moles of O≡V(N[<sup>t</sup>Bu]Ar)<sub>3</sub> = 4 occurs by initial isomerization of 3 to 2 . Computational studies of this rearrangement (Δ H = 4.2 kcal/mol; Δ H <sup>‡</sup> = 16 kcal/mol) are in accord with experimental data (Δ H = 4 ± 3 kcal/mol; Δ H <sup>‡</sup> = 14 ± 3 kcal/mol). With the aim of supressing formation of 4 , the reaction of O<sub>2</sub> with 1 in the presence of <sup>t</sup>BuCN was studied. At -45 °C the principal products of this reaction are 3 and <sup>t</sup>BuC(=O)N≡V(N[<sup>t</sup>Bu]Ar)<sub>3</sub> = 5 in which the bound nitrile has been oxidized. Crystal structures of 3 and 5 are reported.
Journal of the American Chemical Society 10/2012; · 9.91 Impact Factor
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ABSTRACT: The present study reports a DSC study of the thio-and dithiocarbamates: 3H-benzoxazole-2-thione (2-mercaptobenzoxazole), 3H-benzothiazole-2-thione (2-mercaptobenzothiazole), thiazolidine-2-thione (2-mercapto-2-thiazoline), oxazolidine-2-thione (2-mercapto-2-oxazoline)
and tetrahydro-1,3-oxazine-2-thione (5,6-dihydro-4H-1,3-oxazine-2-thiol) in the temperature interval T=268 K and the melting temperatures. Temperatures, enthalpies and entropies of fusion are reported. No solid-solid phase transitions
were observed for the compounds in the temperature interval studied. The heat capacity of the compounds as a function of temperature
was measured.
Journal of Thermal Analysis and Calorimetry 04/2012; 91(2):471-475. · 1.60 Impact Factor
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ABSTRACT: Very good linear correlations between experimental and calculated enthalpies of formation in the gas phase (G3(MP2)//B3LYP and G3) for 48 thiophene derivatives have been obtained. These correlations permit a correction of the calculated enthalpies of formation in order to estimate more reliable "experimental" values for the enthalpies of formation of substituted thiophenes, check the reliability of experimental measurements, and also predict the enthalpies of formation of new thiophenes that are not available in the literature. Moreover, the difference between the enthalpies of formation of isomeric thiophenes with the same substituent in positions 2 and 3 of the ring has been analyzed. Likewise, a comparison of the substituent effect in the thiophene and benzene rings has been established.
The Journal of Physical Chemistry A 04/2012; 116(17):4363-70. · 2.95 Impact Factor
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J. Am. Chem. Soc. 01/2012;
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ABSTRACT: In a continuation of our investigations of the thermochemistry of heterocyclic ring systems and sulfur-containing compounds, the standard molar enthalpy of formation of tetrahydro-2H-1,3-oxazine-2-thione (CAS no. 17374-18-4) at T = 298 K has been evaluated both experimentally and computationally. Combined enthalpies of combustion and sublimation, measured by rotary bomb combustion calorimetry and the Knudsen effusion technique, yielded a gas-phase enthalpy of formation of −(104.0 ± 5.5) kJ·mol–1. The G3 value of −104.6 kJ·mol–1 calculated using an isodesmic bond separation reaction is in excellent agreement with the experimental result. The variation in enthalpy of formation with ring size is discussed, and the G3 enthalpy of formation of tetrahydro-2H-1,3-thiazine-2-thione (72.4 kJ·mol–1) was also computed for this purpose.
09/2011;
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Xiaochen Cai,
Subhojit Majumdar,
George C Fortman,
Luis Manuel Frutos, Manuel Temprado,
Christopher R Clough,
Christopher C Cummins,
Meaghan E Germain,
Taryn Palluccio,
Elena V Rybak-Akimova,
Burjor Captain,
Carl D Hoff
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ABSTRACT: The enthalpies of oxygen atom transfer (OAT) from mesityl nitrile oxide (MesCNO) to Me(3)P, Cy(3)P, Ph(3)P, and the complex (Ar[(t)Bu]N)(3)MoP (Ar = 3,5-C(6)H(3)Me(2)) have been measured by solution calorimetry yielding the following P-O bond dissociation enthalpy estimates in toluene solution (±3 kcal mol(-1)): Me(3)PO [138.5], Cy(3)PO [137.6], Ph(3)PO [132.2], (Ar[(t)Bu]N)(3)MoPO [108.9]. The data for (Ar[(t)Bu]N)(3)MoPO yield an estimate of 60.2 kcal mol(-1) for dissociation of PO from (Ar[(t)Bu]N)(3)MoPO. The mechanism of OAT from MesCNO to R(3)P and (Ar[(t)Bu]N)(3)MoP has been investigated by UV-vis and FTIR kinetic studies as well as computationally. Reactivity of R(3)P and (Ar[(t)Bu]N)(3)MoP with MesCNO is proposed to occur by nucleophilic attack by the lone pair of electrons on the phosphine or phosphide to the electrophilic C atom of MesCNO forming an adduct rather than direct attack at the terminal O. This mechanism is supported by computational studies. In addition, reaction of the N-heterocyclic carbene SIPr (SIPr = 1,3-bis(diisopropyl)phenylimidazolin-2-ylidene) with MesCNO results in formation of a stable adduct in which the lone pair of the carbene attacks the C atom of MesCNO. The crystal structure of the blue SIPr·MesCNO adduct is reported, and resembles one of the computed structures for attack of the lone pair of electrons of Me(3)P on the C atom of MesCNO. Furthermore, this adduct in which the electrophilic C atom of MesCNO is blocked by coordination to the NHC does not undergo OAT with R(3)P. However, it does undergo rapid OAT with coordinatively unsaturated metal complexes such as (Ar[(t)Bu]N)(3)V since these proceed by attack of the unblocked terminal O site of the SIPr·MesCNO adduct rather than at the blocked C site. OAT from MesCNO to pyridine, tetrahydrothiophene, and (Ar[(t)Bu]N)(3)MoN was found not to proceed in spite of thermochemical favorability.
Inorganic Chemistry 08/2011; 50(19):9620-30. · 4.60 Impact Factor
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ABSTRACT: This paper reports an experimental and computational thermochemical study on 1,3-dimethylbarbituric acid. The value of the standard (p° = 0.1 MPa) molar enthalpy of formation in the gas phase at T = 298.15 K has been determined. The energy of combustion was measured by static bomb combustion calorimetry, and from the result obtained, the standard molar enthalpy of formation in the crystalline state at T = 298.15 K was calculated as -639.6 ± 1.9 kJ·mol(-1). The enthalpy of sublimation was determined using a transference (transpiration) method in a saturated N(2) stream and a value of the enthalpy of sublimation at T = 298.15 K was derived as 92.3 ± 0.6 kJ·mol(-1). From these results a value of -547.3 ± 2.0 kJ·mol(-1) for the gas-phase enthalpy of formation at T = 298.15 K was determined. Theoretical calculations at the G3 and G4 levels were performed, and a study on molecular and electronic structure of the compound has been carried out. Calculated enthalpies of formation are in very good agreement with the experimental value.
The Journal of Physical Chemistry A 03/2011; 115(14):3167-73. · 2.95 Impact Factor
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Xiaochen Cai,
Subhojit Majumdar,
George C Fortman,
Catherine S J Cazin,
Alexandra M Z Slawin,
Charles Lhermitte,
Rajeev Prabhakar,
Meaghan E Germain,
Taryn Palluccio,
Steven P Nolan,
Elena V Rybak-Akimova, Manuel Temprado,
Burjor Captain,
Carl D Hoff
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ABSTRACT: The reactivity of a number of two-coordinate [Pd(L)(L')] (L = N-heterocyclic carbene (NHC) and L' = NHC or PR(3)) complexes with O(2) has been examined. Stopped-flow kinetic studies show that O(2) binding to [Pd(IPr)(P(p-tolyl)(3))] to form cis-[Pd(IPr)(P(p-tolyl)(3))(η(2)-O(2))] occurs in a rapid, second-order process. The enthalpy of O(2) binding to the Pd(0) center has been determined by solution calorimetry to be -26.2(1.9) kcal/mol. Extension of this work to the bis-NHC complex [Pd(IPr)(2)], however, did not lead to the formation of the expected diamagnetic complex cis-[Pd(IPr)(2)(η(2)-O(2))] but to paramagnetic trans-[(Pd(IPr)(2)(η(1)-O(2))(2)], which has been fully characterized. Computational studies addressing the energetics of O(2) binding have been performed and provide insight into reactivity changes as steric pressure is increased.
Journal of the American Chemical Society 02/2011; 133(5):1290-3. · 9.91 Impact Factor
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ABSTRACT: The present study reports a differential scanning calorimetry (DSC) study of the barbituric acid derivatives: 1,3-dimethylbarbituric acid [CAS 769-42-6], 5,5-dimethylbarbituric acid [CAS 24448-94-0], 1,3-diethylbarbituric acid [CAS 32479-73-5], 1,3,5-trimethylbarbituric acid [CAS 7358-61-4], 1,5,5-trimethylbarbituric acid [CAS 702-47-6], and tetramethylbarbituric acid [CAS 13566-66-0] in the temperature interval from T = 268 K to their respective melting temperatures. Temperatures, enthalpies and entropies of fusion, and the heat capacities of the solid compounds as a function of temperature are reported.
12/2010;
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ABSTRACT: This paper reports an experimental and theoretical study of the structures and standard (p(o) = 0.1 MPa) molar enthalpies of formation of 3H-1,3-benzoxazole-2-thione and 3H-1,3-benzothiazole-2-thione. The enthalpies of combustion and sublimation were measured by rotary bomb combustion calorimetry and the Knudsen effusion technique, and gas-phase enthalpies of formation values at T = 298.15 K of (42.0 +/- 2.7) and (205.5 +/- 3.8) kJ x mol(-1) for 3H-1,3-benzoxazole-2-thione and 3H-1,3-benzothiazole-2-thione, respectively, were determined. G3-calculated enthalpies of formation are in excellent agreement with the experimental values. The present work discusses the question of tautomerism explicitly for both compounds and compares the energetics of all the related species. A comparison of the theoretical results with the structural data is also reported.
The Journal of Physical Chemistry A 06/2010; 114(21):6336-41. · 2.95 Impact Factor
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04/2010: pages 429 - 494; , ISBN: 9780470602577
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ABSTRACT: This paper reports an experimental and computational thermochemical study on 5,5-dimethylbarbituric acid and the solid-phase structure of the compound. The value of the standard (p(o) = 0.1 MPa) molar enthalpy of formation in the gas phase at T = 298.15 K has been determined. The energy of combustion was measured by static bomb combustion calorimetry, and from the result obtained, the standard molar enthalpy of formation in the crystalline state at T = 298.15 K was calculated as -(706.4 +/- 2.2) kJ x mol(-1). The enthalpy of sublimation was determined using a transference (transpiration) method in a saturated NB(2) stream, and a value of the enthalpy of sublimation at T = 298.15 K was derived as (115.8 +/- 0.5) kJ x mol(-1). From these results a value of -(590.6 +/- 2.3) kJ x mol(-1) for the gas-phase enthalpy of formation at T = 298.15 K was determined. Theoretical calculations at the G3 level were performed, and a study on molecular and electronic structure of the compound has been carried out. Calculated enthalpies of formation are in reasonable agreement with the experimental value. 5,5-Dimethylbarbituric acid was characterized by single crystal X-ray diffraction analysis. In the crystal structure, N-H...O=C hydrogen bonds lead to the formation of ribbons connected further by weak C-H...O=C hydrogen bonds into a three-dimensional network. The molecular and supramolecular structures observed in the solid state were also investigated in the gas phase by DFT calculations.
The Journal of Physical Chemistry A 02/2010; 114(10):3583-90. · 2.95 Impact Factor
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Meaghan E Germain, Manuel Temprado,
Annie Castonguay,
Olga P Kryatova,
Elena V Rybak-Akimova,
John J Curley,
Arjun Mendiratta,
Yi-Chou Tsai,
Christopher C Cummins,
Rajeev Prabhakar,
James E McDonough,
Carl D Hoff
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ABSTRACT: Variable temperature equilibrium studies were used to derive thermodynamic data for formation of eta(1) nitrile complexes with Mo(N[(t)Bu]Ar)(3), 1. (1-AdamantylCN = AdCN: DeltaH(degrees) = -6 +/- 2 kcal mol(-1), DeltaS(degrees) = -20 +/- 7 cal mol(-1) K(-1). C(6)H(5)CN = PhCN: DeltaH(degrees) = -14.5 +/- 1.5 kcal mol(-1), DeltaS(degrees) = -40 +/- 5 cal mol(-1) K(-1). 2,4,6-(H(3)C)(3)C(6)H(2)CN = MesCN: DeltaH(degrees) = -15.4 +/- 1.5 kcal mol(-1), DeltaS(degrees) = -52 +/- 5 cal mol(-1) K(-1).) Solution calorimetric studies show that the enthalpy of formation of 1-[eta(2)-NCNMe(2)] is more exothermic (DeltaH(degrees) = -22.0 +/- 1.0 kcal mol(-1)). Rate and activation parameters for eta(1) binding of nitriles were measured by stopped flow kinetic studies (AdCN: DeltaH(on)(++) = 5 +/- 1 kcal mol(-1), DeltaS(on)(++) = -28 +/- 5 cal mol(-1) K(-1); PhCN: DeltaH(on)(++) = 5.2 +/- 0.2 kcal mol(-1), DeltaS(on)(++) = -24 +/- 1 cal mol(-1) K(-1); MesCN: DeltaH(on)(++) = 5.0 +/- 0.3 kcal mol(-1), DeltaS(on)(++) = -26 +/- 1 cal mol(-1) K(-1)). Binding of Me(2)NCN was observed to proceed by reversible formation of an intermediate complex 1-[eta(1)-NCNMe(2)] which subsequently forms 1-[eta(2)-NCNMe(2)]: DeltaH(++)(k1) = 6.4 +/- 0.4 kcal mol(-1), DeltaS(++)(k1) = -18 +/- 2 cal mol(-1) K(-1), and DeltaH(++)(k2) = 11.1 +/- 0.2 kcal mol(-1), DeltaS(++)(k2) = -7.5 +/- 0.8 cal mol(-1) K(-1). The oxidative addition of PhSSPh to 1-[eta(1)-NCPh] is a rapid second-order process with activation parameters: DeltaH(++) = 6.7 +/- 0.6 kcal mol(-1), DeltaS(++) = -27 +/- 4 cal mol(-1) K(-1). The oxidative addition of PhSSPh to 1-[eta(2)-NCNMe(2)] also followed a second-order rate law but was much slower: DeltaH(++) = 12.2 +/- 1.5 kcal mol(-1) and DeltaS(++) = -25.4 +/- 5.0 cal mol(-1) K(-1). The crystal structure of 1-[eta(1)-NC(SPh)NMe(2)] is reported. Trapping of in situ generated 1-[eta(1)-NCNMe(2)] by PhSSPh was successful at low temperatures (-80 to -40 degrees C) as studied by stopped flow methods. If 1-[eta(1)-NCNMe(2)] is not intercepted before isomerization to 1-[eta(2)-NCNMe(2)] no oxidative addition occurs at low temperatures. The structures of key intermediates have been studied by density functional theory, confirming partial radical character of the carbon atom in eta(1)-bound nitriles. A complete reaction profile for reversible ligand binding, eta(1) to eta(2) isomerization, and oxidative addition of PhSSPh has been assembled and gives a clear picture of ligand reactivity as a function of hapticity in this system.
Journal of the American Chemical Society 10/2009; 131(42):15412-23. · 9.91 Impact Factor
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María Victoria Roux, Manuel Temprado,
Pilar Jiménez,
Concepción Foces-Foces,
Rafael Notario,
Archana R Parameswar,
Alexei V Demchenko,
James S Chickos,
Carol A Deakyne,
Alicia K Ludden,
Joel F Liebman
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ABSTRACT: This paper reports an experimental and a theoretical study of the structures and standard (p(o) = 0.1 MPa) molar enthalpies of formation of the synthetic reagents 1,3-thiazolidine-2-thione [CAS 96-53-7] and 1,3-oxazolidine-2-thione [CAS 5840-81-3]. The enthalpies of combustion and sublimation were measured by rotary bomb combustion calorimetry, and the Knudsen effusion technique and gas-phase enthalpies of formation values at T = 298.15 K of (97.1 +/- 4.0) and -(74.4 +/- 4.6) kJ.mol(-1) for 1,3-thiazolidine-2-thione and 1,3-oxazolidine-2-thione, respectively, were determined. G3-calculated enthalpies of formation are in reasonable agreement with the experimental values. In the solid state, 1,3-thiazolidine-2-thione exists in two polymorphic forms (monoclinic and triclinic) and 1,3-oxazolidine-2-thione exits in the triclinic form. The isostructural nature of these compounds and comparison of their molecular and crystal structures have been analyzed. The experimental X-ray powder diffractograms have been compared with the calculated patterns from their structures for identification of the polymorphic samples used in this study. A comparison of our results with literature thermochemical and structural data for related compounds is also reported.
The Journal of Physical Chemistry A 09/2009; 113(40):10772-8. · 2.95 Impact Factor
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ABSTRACT: The relative stabilities of 2,2'- and 3,3'-bithiophenes were evaluated by experimental thermochemistry and the results compared with data obtained from state of the art calculations, which were also extended to 2,3'-bithiophene. The standard (p degrees = 0.1 MPa) molar enthalpies of formation of crystalline 2,2'-bithiophene and 3,3'-bithiophene were calculated from the standard molar energies of combustion, in oxygen, to yield CO(2) (g) and H(2)SO(4) x 115 H(2)O, measured by rotating-bomb combustion calorimetry at T = 298.15 K. The vapor pressures of these two compounds were measured as a function of temperature by Knudsen effusion mass-loss technique. The standard molar enthalpies of sublimation, at T = 298.15 K, were derived from the Clausius-Clapeyron equation. The experimental values were used to calculate the standard (p(o) = 0.1 MPa) enthalpies of formation of the title compounds in the gaseous phase; the results were analyzed and interpreted in terms of enthalpic increments and molecular structure. Standard ab initio molecular orbital calculations at the G3(MP2)//B3LYP level were performed. Enthalpies of formation, using homodesmotic reactions, were calculated and compared with experimental data. The computational study was also extended to the isomeric compound 2,3'-bithiophene. Detailed inspections of the molecular and electronic structures of the compounds studied were carried out. Finally, bond dissociation enthalpies (BDE) and enthalpies of formation of thienyl radicals were also computed.
The Journal of Physical Chemistry A 09/2009; 113(41):11042-50. · 2.95 Impact Factor
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Patrick Achord,
Etsuko Fujita,
James T Muckerman,
Brian Scott,
George C Fortman, Manuel Temprado,
Xiaochen Cai,
Burjor Captain,
Derek Isrow,
John J Weir,
James Eric McDonough,
Carl D Hoff
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ABSTRACT: The enthalpies of binding of a number of N-donor ligands to the complex Mo(P(i)Pr(3))(2)(CO)(3) in toluene have been determined by solution calorimetry and equilibrium measurements. The measured binding enthalpies span a range of approximately 10 kcal mol(-1): DeltaH(binding) = -8.8 +/- 1.2 (N(2)-Mo(P(i)Pr(3))(2)(CO)(3)); -10.3 +/- 0.8 (N(2)); -11.2 +/- 0.4 (AdN(3) (Ad = 1-adamantyl)); -13.8 +/- 0.5 (N(2)CHSiMe(3)); -14.9 +/- 0.9 (pyrazine = pz); -14.8 +/- 0.6 (2,6-Me(2)pz); -15.5 +/- 1.8 (Me(2)NCN); -16.6 +/- 0.4 (CH(3)CN); -17.0 +/- 0.4 (pyridine); -17.5 +/- 0.8 ([4-CH(3)pz][PF(6)] (in tetrahydrofuran)); -17.6 +/- 0.4 (C(6)H(5)CN); -18.6 +/- 1.8 (N(2)CHC( horizontal lineO)OEt); and -19.3 +/- 2.5 kcal mol(-1) (pz)Mo(P(i)Pr(3))(2)(CO)(3)). The value for the isonitrile AdNC (-29.0 +/- 0.3) is 12.3 kcal mol(-1) more exothermic than that of the nitrile AdCN (-16.7 +/- 0.6 kcal mol(-1)). The enthalpies of binding of a range of arene nitrile ligands were also studied, and remarkably, most nitrile complexes were clustered within a 1 kcal mol(-1) range despite dramatic color changes and variation of nu(CN). Computed structural and spectroscopic parameters for the complexes Mo(P(i)Pr(3))(2)(CO)(3)L are in good agreement with experimental data. Computed binding enthalpies for Mo(P(i)Pr(3))(2)(CO)(3)L exhibit considerable scatter and are generally smaller compared to the experimental values, but relative agreement is reasonable. Computed enthalpies of binding using a larger basis set for Mo(PMe(3))(2)(CO)(3)L show a better fit to experimental data than that for Mo(P(i)Pr(3))(2)(CO)(3)L using a smaller basis set. Crystal structures of Mo(P(i)Pr(3))(2)(CO)(3)(AdCN), W(P(i)Pr(3))(2)(CO)(3)(Me(2)NCN), W(P(i)Pr(3))(2)(CO)(3)(2,6-F(2)C(6)H(3)CN), W(P(i)Pr(3))(2)(CO)(3)(2,4,6-Me(3)C(6)H(2)CN), W(P(i)Pr(3))(2)(CO)(3)(2,6-Me(2)pz), W(P(i)Pr(3))(2)(CO)(3)(AdCN), Mo(P(i)Pr(3))(2)(CO)(3)(AdNC), and W(P(i)Pr(3))(2)(CO)(3)(AdNC) are reported.
Inorganic Chemistry 08/2009; 48(16):7891-904. · 4.60 Impact Factor
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ABSTRACT: The enthalpies of formation in the condensed and gas states, Delta f H m degrees (cd) and Delta f H m degrees (g), of 2- and 3-thiopheneacetic acids were derived from their respective enthalpies of combustion in oxygen, measured by a rotating bomb calorimeter, and the variation of vapor pressure with temperature determined by the Knudsen effusion technique. Theoretical calculations at the G3 level were performed, and a study on molecular and electronic structure of the compounds has been carried out. Calculated Delta f H m degrees (g) values using atomization and isodesmic reactions are compared with the experimental data. Experimental and theoretical results show that the 3-thiopheneacetic acid is thermodynamically more stable than the 2-isomer.
The Journal of Physical Chemistry A 10/2008; 112(41):10378-85. · 2.95 Impact Factor
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ABSTRACT: Synthetic studies are reported that show that the reaction of either H2SnR2 (R = Ph, n-Bu) or HMo(CO)3(Cp) (1-H, Cp = eta(5)-C5H5) with Mo(N[t-Bu]Ar)3 (2, Ar = 3,5-C6H3Me2) produce HMo(N[t-Bu]Ar)3 (2-H). The benzonitrile adduct (PhCN)Mo(N[t-Bu]Ar)3 (2-NCPh) reacts rapidly with H2SnR2 or 1-H to produce the ketimide complex (Ph(H)C=N)Mo(N[t-Bu]Ar)3 (2-NC(H)Ph). The X-ray crystal structures of both 2-H and 2-NC(H)Ph are reported. The enthalpy of reaction of 1-H and 2 in toluene solution has been measured by solution calorimetry (DeltaH = -13.1 +/- 0.7 kcal mol(-1)) and used to estimate the Mo-H bond dissociation enthalpy (BDE) in 2-H as 62 kcal mol(-1). The enthalpy of reaction of 1-H and 2-NCPh in toluene solution was determined calorimetrically as DeltaH = -35.1 +/- 2.1 kcal mol(-1). This value combined with the enthalpy of hydrogenation of [Mo(CO)3(Cp)]2 (1(2)) gives an estimated value of 90 kcal mol(-1) for the BDE of the ketimide C-H of 2-NC(H)Ph. These data led to the prediction that formation of 2-NC(H)Ph via nitrile insertion into 2-H would be exothermic by approximately 36 kcal mol(-1), and this reaction was observed experimentally. Stopped flow kinetic studies of the rapid reaction of 1-H with 2-NCPh yielded DeltaH(double dagger) = 11.9 +/- 0.4 kcal mol(-1), DeltaS(double dagger) = -2.7 +/- 1.2 cal K(-1) mol(-1). Corresponding studies with DMo(CO)3(Cp) (1-D) showed a normal kinetic isotope effect with kH/kD approximately 1.6, DeltaH(double dagger) = 13.1 +/- 0.4 kcal mol(-1) and DeltaS(double dagger) = 1.1 +/- 1.6 cal K(-1) mol(-1). Spectroscopic studies of the much slower reaction of 1-H and 2 yielding 2-H and 1/2 1(2) showed generation of variable amounts of a complex proposed to be (Ar[t-Bu]N)3Mo-Mo(CO)3(Cp) (1-2). Complex 1-2 can also be formed in small equilibrium amounts by direct reaction of excess 2 and 1(2). The presence of 1-2 complicates the kinetic picture; however, in the presence of excess 2, the second-order rate constant for H atom transfer from 1-H has been measured: 0.09 +/- 0.01 M(-1) s(-1) at 1.3 degrees C and 0.26 +/- 0.04 M(-1) s(-1) at 17 degrees C. Study of the rate of reaction of 1-D yielded kH/kD = 1.00 +/- 0.05 consistent with an early transition state in which formation of the adduct (Ar[t-Bu]N)3Mo...HMo(CO)3(Cp) is rate limiting.
Inorganic Chemistry 10/2008; 47(20):9380-9. · 4.60 Impact Factor
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ABSTRACT: This paper reports the value of the standard (p(o) = 0.1 MPa) molar enthalpy of formation in the gas phase at T = 298.15 K for barbituric acid. The enthalpies of combustion and sublimation were measured by static bomb combustion calorimetry and transference (transpiration) method in a saturated N2 stream and a gas-phase enthalpy of formation value of -(534.3 +/- 1.7) kJ x mol(-1) was determined at T = 298.15 K. G3-calculated enthalpies of formation are in very good agreement with the experimental value. The behavior of the sample as a function of the temperature was studied by differential scanning calorimetry, and a new polymorph of barbituric acid at high temperature was found. In the solid state, two anhydrous forms are known displaying two out of the six hydrogen-bonding patterns observed in the alkyl/alkenyl derivatives retrieved from the Cambridge Crystallographic Database. The stability of these motifs has been analyzed by theoretical calculations. X-ray powder diffraction technique was used to establish to which polymorphic form corresponds to the commercial sample used in this study and to characterize the new form at high temperature.
The Journal of Physical Chemistry A 08/2008; 112(32):7455-65. · 2.95 Impact Factor
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ABSTRACT: The relative stabilities of 2- and 3-acetylthiophenes have been evaluated by experimental thermochemistry and the results compared to high-level ab initio calculations. The enthalpies of combustion, vaporization, and sublimation were measured by rotating-bomb combustion calorimetry, Calvet microcalorimetry, correlation gas chromatography, and Knudsen effusion techniques and the gas-phase enthalpies of formation, at T = 298.15 K, were determined. Standard ab initio molecular orbital calculations at the G2 and G3 levels were performed, and a theoretical study on the molecular and electronic structures of the compounds studied has been conducted. Calculated enthalpies of formation using atomization and isodesmic reactions are compared with the experimental data. Experimental and theoretical results show that 2-acetylthiophene is thermodynamically more stable than the 3-isomer. A comparison of the substituent effect of the acetyl group in benzene and thiophene rings has been carried out.
The Journal of Physical Chemistry A 12/2007; 111(43):11084-92. · 2.95 Impact Factor
