On the Angular Independence of Sets of Atomic Orbitals

The conditions under which a set of atomic orbitals becomes angularly independent are investigated for both atomic and molecular systems. How these results can be applied to various molecules is considered

Enthalpy of formation of carbocycles : a precise theoretical determination of experimentally imprecise measurements

Despite being one of the best-known families of organic compounds, the fact that hydrocarbons exhibit a rich variety of structures owing to branching, cyclization, and the presence of multiple bonds, means that many of their properties are yet to be determined accurately, or even at all. Among cyclic hydrocarbons, those with three- membered rings are particularly interesting because of their strain energy. In this paper, we report accurate calculations of the enthalpy of formation of three-membered carbocycles, whose experimental values have not been obtained by direct measurement of the heat of combustion. For this purpose, we used several accurate composite methods to obtain the gas-phase enthalpies of atomization and derived from them the gas-phase enthalpies of formation, using experimentally determined accurate values for the atoms. Moreover, to mini- mize the inaccuracy that can possibly arise in this procedure, we also used homodesmotic reactions designed to balance systematic errors in the geometric and electronic structure of some of the species. A careful analysis of the results shows that some of the indirectly derived values reported in the literature are far from the most accurate theoretical outcomes, and we suggest that these new ones should be adopted

Estimating Solid–Liquid Phase Change Enthalpies and Entropies

This article discusses estimating solid-liquid phase change enthalpies and entropies

Estimating Solid-Liquid Phase Change Enthalpies and Entropies

This article discusses estimating solid-liquid phase change enthalpies and entropies

The heats of formation of the haloacetylenes XCCY [X, Y = H, F, Cl]: basis set limit ab initio results and thermochemical analysis

The heats of formation of haloacetylenes are evaluated using the recent W1 and W2 ab initio computational thermochemistry methods. These calculations involve CCSD and CCSD(T) coupled cluster methods, basis sets of up to spdfgh quality, extrapolations to the one-particle basis set limit, and contributions of inner-shell correlation, scalar relativistic effects, and (where relevant) first-order spin-orbit coupling. The heats of formation determined using W2 theory are: \hof(HCCH) = 54.48 kcal/mol, \hof(HCCF) = 25.15 kcal/mol, \hof(FCCF) = 1.38 kcal/mol, \hof(HCCCl) = 54.83 kcal/mol, \hof(ClCCCl) = 56.21 kcal/mol, and \hof(FCCCl) = 28.47 kcal/mol. Enthalpies of hydrogenation and destabilization energies relative to acetylene were obtained at the W1 level of theory. So doing we find the following destabilization order for acetylenes: FCCF $>$ ClCCF $>$ HCCF $>$ ClCCCl $>$ HCCCl $>$ HCCH. By a combination of W1 theory and isodesmic reactions, we show that the generally accepted heat of formation of 1,2-dichloroethane should be revised to -31.8$\pm$0.6 kcal/mol, in excellent agreement with a very recent critically evaluated review. The performance of compound thermochemistry schemes such as G2, G3, G3X and CBS-QB3 theories has been analyzed.Comment: Mol. Phys., in press (E. R. Davidson issue

The Energetics of Halogenated Ethylenes (Ethynes) and 1,3-Butadienes (Butadiynes): A Computational and Conceptual Study of Substituent Effects and “Dimerization”

The energetics of ethylenes and 1,3-butadienes may be interrelated by the reaction: RHC=CH2 + H2C=CHR\u27 → RHC=CH−CH=CHR\u27 + H2. Shown earlier to be nearly enthalpically thermoneutral for a variety of hydrocarbon cases, we are now interested in the related energetics of halogenated alkenes and alkynes. Using quantum chemical calculations, we have studied this as recast as the isodesmic reactions: 2(H2C=CHX) + H2C=CH−CH=CH2 → p,q-di-X-1,3-butadiene + 2H2C=CH2 2(HC≡CX) + HC≡C−C≡CH → di-X-butadiyne + 2HC≡CH. Here p,q- = 1,3-; 1,4- and 2,3- with X = F, Cl, Br, and I. The halogen and location-dependent deviations from near enthalpic thermoneutrality are discussed

Experimental and theoretical study of the structures and enthalpies of formation of the synthetic reagents l,3-thiazolidine-2-thione and l,3-oxazolidine-2-thione

This paper reports an experimental and a theoretical study of the structures and standard (po = 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.M.T. would like to thank MEC/SEUI, FPU AP2002-0603, Spain, for financial support. A.V.D. thanks the National Science Foundation (CHE-0547566) and the American Heart Association (0855743G) for financial support of this research. The support of the Spanish Ministerio de Educación y Ciencia under Projects CTQ2007-60895/BQU and CTQ2006-10178/BQU is gratefully acknowledged

Unique thermodynamic relationships for ΔfHo and ΔfGo for crystalline inorganic salts. I, Predicting the possible existence and synthesis of Na2SO2 and Na2SeO2

The concept that equates oxidation and pressure has been successfully utilized in explaining the structural changes observed in the M2S subnets of M2SOx (x = 3, 4) compounds (M = Na, K) when compared with the structures (room- and high-pressure phases) of their parent M2S 'alloy' [Martinez-Cruz et al. (1994), J. Solid State Chem. 110, 397-398; Vegas (2000), Crystallogr. Rev. 7, 189-286; Vegas et al. (2002), Solid State Sci. 4, 1077-1081]. These structural changes suggest that if M2SO2 would exist, its cation array might well have an anti-CaF2 structure. On the other hand, in an analysis of the existing thermodynamic data for M2S, M2SO3 and M2SO4 we have identified, and report, a series of unique linear relationships between the known Delta H-f(o) and Delta(f)G(o) values of the alkali metal (M) sulfide (x = 0) and their oxyanion salts M2SOx (x = 3 and 4), and the similarly between M2S2 disulfide (x = 0) and disulfur oxyanion salts M2S2Ox (x = 3, 4, 5, 6 and 7) and the number of O atoms in their anions x. These linear relationships appear to be unique to sulfur compounds and their inherent simplicity permits us to interpolate thermochemical data (Delta H-f(o)) for as yet unprepared compounds, M2SO (x = 1) and M2SO2 (x = 2). The excellent linearity indicates the reliability of the interpolated data. Making use of the volume-based thermodynamics, VBT [Jenkins et al. (1999), Inorg. Chem. 38, 3609-3620], the values of the absolute entropies were estimated and from them, the standard Delta S-f(o) values, and then the Delta(f)G(o) values of the salts. A tentative proposal is made for the synthesis of Na2SO2 which involves bubbling SO2 through a solution of sodium in liquid ammonia. For this attractive thermodynamic route, we estimate Delta G(o) to be approximately -500 kJ mol(-1). However, examination of the stability of Na2SO2 raises doubts and Na2SeO2 emerges as a more attractive target material. Its synthesis is likely to be easier and it is stable to disproportionation into Na2S and Na2SeO4. Like Na2SO2, this compound is predicted to have an anti-CaF2 Na2Se subnet

Enthalpies of formation of N-substituted pyrazoles and imidazoles

Accurate experimental enthalpies of formation measured using static bomb combustion calorimetry, the “vacuum sublimation” drop calorimetry method, and the Knudsen-effusion method are reported for the first time for four azoles:  1-methylimidazole (1MeIMI), 1-methylpyrazole (1MePYR), 1-benzylimidazole (1BnIMI), and 1-benzylpyrazole (1BnPYR). These values and those corresponding to imidazole (1HIMI), pyrazole (1HPYR), 1-ethylimidazole (1EtIMI), 1-ethylpyrazole (1EtPYR), 1-phenylimidazole (1PhIMI), and 1-phenylpyrazole (1PhPYR) are compared with theoretical values using the G2(MP2) and the B3LYP/6-311*G(3df,2p)//6-31G(d) approaches. In general, there is a very good agreement between calculated and experimental values for the series of N-substituted imidazoles, while the agreement is less good for the series of the N-substituted pyrazoles. Experimentally, the gap between the enthalpies of formation of imidazoles and pyrazoles decreases significantly upon N-substitution, while the theoretical estimates indicate that this decrease is smaller.This work has been partially supported by the DGES Projects PB 96-0001-C03-03, PB96-0067, and PB96-0927-C02-01. A generous allocation of computational time at the Centro de Computacio´n Cientı´fica de la Facultad de Ciencias (CCCFC) de la UAM is also gratefully acknowledged. Thanks are due to Junta National de Investigac¸a˜o Cientı´fica e Tecnolo´gica (JNICT), Lisbon, Portugal and Consejo Superior de Investigaciones Cientı´ficas (CSIC), Madrid, Spain, for a joint research project CSIC/JNICT. Financial support from the Praxis XXI, Project 2/2.1/qui/54/94, is acknowledged. L.M.P.F.A. thanks Fundac¸aˆo para a Cieˆncia e Tecnologia, Lisbon, Portugal for the award of a postdoctoral fellowship (Praxis XXI/BDP/16319/98). J.F.L. acknowledges funding from “Dow Chemical Company” for partial support of his thermochemical studies