Thermochemistry Of Compounds Research Articles

Can DFT and ab initio methods describe all aspects of the potential energy surface of cycloreversion reactions?

We introduce a representative benchmark database of 20 cycloreversion reaction energies obtained by means of the high-level W1 thermochemical protocol. We use these benchmark values to assess the performance of a variety of contemporary DFT, double-hybrid DFT (DHDFT), standard ab initio, and compound thermochemistry methods. We show that this set of reaction energies provides an extremely challenging test for nearly all of the considered DFT and DHDFT methods. For example, about 80% of the considered functionals result in root-mean-square deviations (RMSDs) above 10 kJ mol−1. The best DFT and DHDFT procedures are ωB97X and DSD-PBEP86-D3, with RMSDs of 4.7 and 7.9 kJ mol−1, respectively. Coupled with the fact that the barrier heights for these reactions also pose a significant challenge for many DFT methods, this work shows that only a handful of functionals can quantitatively describe all aspects of the potential energy surface of this important class of reactions. In addition, this work shows that London dispersion effects are particularly large for this class of reactions. For example, empirical D3 dispersion corrections reduce the RMSDs for the DFT and DHDFT procedures by amounts ranging from 3.5 (PBE and B2K-PLYP) to 22.0 (BLYP) kJ mol−1.

Atomization energies of the carbon clusters C n (n = 2−10) revisited by means of W4 theory as well as density functional, Gn, and CBS methods

The thermochemistry of the carbon clusters C n (n = 2−10) has been revisited by means of W4 theory and W3.2lite theory. Particularly, the larger clusters exhibit very pronounced post-CCSD(T) correlation effects. Despite this, our best calculated total atomization energies agree surprisingly well with the 1991 estimates obtained from scaled CCD(ST)/6-31G* data. Accurately reproducing the small singlet–triplet splitting in C2 requires inclusion of connected quintuple and sextuple excitations. Post-CCSD(T) correlation effects in C4 stabilize the linear form. Linear/cyclic equilibria in C6, C8, and C10 are not strongly affected by connected quadruples, but they are affected by higher-order triples, which favor polyacetylenic rings but disfavor cumulenic rings. Near the CCSD(T) basis set limit, C10 does undergo bond angle alternation in the bottom-of-the-well structure, although it is expected to be absent in the vibrationally averaged structure. The thermochemistry of these systems, and particularly the longer linear chains, is a particularly difficult test for density functional methods. Particularly for the smaller chains and the rings, double-hybrid functionals clearly outperform conventional DFT functionals for these systems. Among compound thermochemistry schemes, G4 clearly outperforms the other members of the Gn family. Our best estimates for total atomization energies at 0 K are: C2() 144.07, C2(3Πu) 142.39, C3() 315.83, C4() 429.16, C4(1Ag) 430.09, C5() 596.64, C6() 717.19, C6() 729.68, C7() 877.45, C8() 1001.86, C8(1A g ) 1014.97, C9() 1159.21, C10() 1288.22, and C10() 1355.54 kcal mol−1.

Thermochemistry of Compounds Formed during Fast Pyrolysis of Lignocellulosic Biomass

This paper deals with the gas-phase thermodynamic properties of organic compounds formed during biomass thermochemical energetic conversion. The standard enthalpies of formation at 298.15 K are det...

Thermochemistry of Organolithium Compounds

AbstractFor Abstract see ChemInform Abstract in Full Text.

Accuracy of enthalpy and entropy determination using the kinetic method: are we approaching a consensus?

There is an emerging consensus regarding the applicability of the kinetic method. All parties acknowledge that it is an approximate quantitative technique, capable of yielding not only enthalpy, but also entropy values. Opinions differ mainly on the accuracy of the results but it is agreed that the energy (effective temperature) dependence of kinetic method plots needs to be checked in all but the simplest of cases. When the 'apparent basicity' is found to depend on collision energy (and hence effective temperature), the extended kinetic method must be used. We have performed a large-scale modeling study, involving thousands of randomly selected molecular systems and a variety of experimental conditions, using exact calculations and realistic data sets. The results show that when the measured entropy difference between the two competing reaction channels is less than approximately 35 J mol(-1) K(-1), overall errors (standard deviations) of DeltaH(298) determined by the kinetic method are +/-5 kJ mol(-1); those of DeltaS(298) are +/-10 J mol(-1) K(-1). These include not only inherent errors of the kinetic method, but also errors in ion abundance measurement (5%) and inaccurate knowledge of reference compound thermochemistry (+/-2 kJ mol(-1), on average). We recommend, in general, that these errors be reported in kinetic method studies. When the measured entropy difference between the two competing fragmentation channels is large (>35 J mol(-1) K(-1)), it is likely to be significantly underestimated and errors of the kinetic method increase significantly.

Thermochemistry of Compounds of the MII(PUO6)2·nH2O Series

The standard enthalpies of formation of uranophosphates MII(PUO6)2·nH2O (MII = Mn, Fe, Co, Ni, Cu, Zn, Cd) are determined. The standard enthalpies of the synthesis and dehydration of these compounds are calculated and analyzed.

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: δH1 298(HCCH) = 54.48 kcal mol−1, δHf 298(HCCH) = 25.15 kcal mol, δHf 298(FCCF) = 1.38 kcal mol−1, δHf 298(HCCC1) = 54.83 kcal mol−1, δHf 298(CICCC1) = 56.21 kcal mol−1, and δHf 298(FCCC1) = 28.47 kcal mo1−1. Enthalpies of hydrogenation and destabilization energies relative to acetylene were obtained at the WI level of theory. So doing we find the following destabilization order for acetylenes: FCCF > ClCCF > HCCF > ClCCCl > HCCCI > HCCH. By a combination of WI theory and isodesmic reactions. we show that the generally accepted heat of formation of 1,2-dichloroethane should be revised to −31.8 ± 0.6 kcal mol−1, 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 analysed.

Benchmark ab Initio Energy Profiles for the Gas-Phase SN2 Reactions Y- + CH3X → CH3Y + X- (X,Y = F,Cl,Br). Validation of Hybrid DFT Methods

The energetics of the gas-phase SN2 reactions Y- + CH3X- → CH3Y + X- (where X,Y = F,Cl,Br), were studied using (variants on) 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. Our computational predictions are in excellent agreement with experimental data where these have small error bars; in a number of other instances reexamination of the experimental data may be in order. Our computed benchmark data (including cases for which experimental data are unavailable altogether) are used to assess the quality of a number of compound thermochemistry schemes such as G2 theory, G3 theory, and CBS-QB3, as well as a variety of density functional theory methods. Upon applying some modifications to the level of theory used for the reference geometry (adding diffuse functions, replacing B3LYP by the very recently proposed mPW1K functional [Lynch, B. J.; Fast, P. L.; Harris, M.; Truhlar, D. G. J. Phys. Chem. A 2000, 104, 4811]), the compound methods appear to perform well. Only the “half-and-half” functionals BH&HLYP and mPWH&HPW91, and the empirical mPW1K functional, consistently find all required stationary points; the other functionals fail to find a transition state in the F/Br case. BH&HLYP and mPWH&HPW91 somewhat overcorrect for the tendency of B3LYP (and, to a somewhat lesser extent, mPW1PW91) to underestimate barrier heights. The Becke97 and Becke97-1 functionals perform similarly to B3LYP for the problem under study, while the HCTH and HCTH-120 functionals both appear to underestimate central barriers. HCTH underestimates complexation energies; this problem is resolved in HCTH-120. mPW1K appears to exhibit the best performance of the functionals considered, although its energetics are still inferior to the compound thermochemistry methods. mPW1K, however, appears to be very suitable for generating reference geometries for more elaborate thermochemical methods in kinetics applications.

Thermochemistry of some aqueous ions

Abstract

Comments on cyclopentadienyl compound thermochemistry

Comments on cyclopentadienyl compound thermochemistry

Thermochemistry of compounds and aqueous ions of copper

We have gathered, evaluated, and tabulated thermodynamic and electrochemical quantities for compounds and aqueous ions of copper, along with equilibrium constants for chemical reactions. We have been explicit about the sources of data and our treatments of data so that interested readers can check the steps leading to tabulated thermodynamic quantities and thus form their own opinions about reliability and accurary.