Thermochemistry Calculations Research Articles

Dopant Sources Choice for Formation of p-Type ZnO: Phosphorus Compound Sources

Fabrication of p-type ZnO has proven difficult and usually inconsistent despite numerous worldwide efforts. In this theoretical study we explored the problem of p-type ZnO formation using both DFT and thermochemistry calculations. Our DFT models predicted that Zn3P2 is a good dopant because it can lead to a shallow acceptor level at 0 K. Additional thermochemistry calculations demonstrated that this shallow acceptor can be further stabilized in a real fabrication process. Our models explain well the observed trends for both n- and p-type phosphorus-doped ZnO and especially the observed inconsistent behaviors of P2O5 dopant. A new strategy of fabricating p-type ZnO was proposed thereafter. This research may also help to address the problem of dopant selection in the fabrication of other wide-gap semiconductors.

Selection of crucible oxides in molten titanium and titanium aluminum alloys by thermo-chemistry calculations

Titanium and its alloys interstitially dissolve a large amount of impurities such as oxygen and nitrogen, which degrade the mechanical and physical properties of alloys. On the other hand crucible oxides based on CaO, ZrO2 Y2O3, etc., and their spinels (combination of two or more oxides) can be used for melting titanium and its alloys. However, the thermodynamic behavior of calcium, zirconium, yttrium on the one side, and oxygen on the other side, in molten Ti and Ti-Al alloys have not been made clear and because of that, it is very interesting for research. Owing of literature data, as well as these crucibles are cheaper than standard crucibles for melting titanium and titanium alloys, in this paper will be presented the results of selection of thermo-chemistry analysis with the aim to determine the crucible oxide stability in contact with molten titanium and titanium-aluminum alloys.

Advanced software for the calculation of thermochemistry, kinetics, and dynamics

Recent results are presented for the efficient computation of matrix eigenvalue and eigenvector equations, the fitting of discrete sets of energy points using interpolating moving least squares methods, and time-dependent and time-independent calculations of molecular dynamics and cumulative reaction probabilities.

New correlation energy functionals with explicit dependence on the number of electrons

Based upon the idea of effective number of electrons, we develop simple but accurate correlation energy functionals to be used for density functional theory calculations. We derive both a spin-independent and a spin-dependent functional. The spin-dependent one, used in conjunction with Becke’s exchange functional [A. D. Becke, Phys. Rev. A 38, 3098 (1988)], yields excellent results for thermochemistry calculations, giving an average absolute error of 2.9 kcal/mol for a test set comprised of the enthalpies of formation of the 148 molecules in the extended G2 set [L. A. Curtiss, K. Raghavachari, P. C. Redfern, and J. A. Pople, J. Chem. Phys. 106, 1063 (1997); L. A. Curtiss, P. C. Redfern, K. Raghavachari, and J. A. Pople, ibid. 109, 42 (1998)] plus the total energies of the atoms H through Ar. We also discuss the problem of fractional occupation number, and we show that the corresponding principle of integer preference can be fulfilled by the procedure that we propose to build correlation energy functionals.

Etching effects during the chemical vapor deposition of (100) diamond

Current theories of CVD growth on (100) diamond are unable to account for the numerous experimental observations of slow-growing, locally smooth (100)(2×1) films. In this paper we use quantum mechanical calculations of diamond surface thermochemistry and atomic-scale kinetic Monte Carlo simulations of deposition to investigate the efficacy of preferential etching as a mechanism that can help to reconcile this discrepancy. This etching mechanism allows for the removal of undercoordinated carbon atoms from the diamond surface. In the absence of etching, simulated growth on the (100)(2×1) surface is faster than growth on the (110) and (111) surfaces, and the (100) surface is atomically rough. When etching is included in the simulations, the (100) growth rates decrease to values near those observed experimentally, while the rates of growth on the other surfaces remain largely unaffected and similar to those observed experimentally. In addition, the etching mechanism promotes the growth of smooth (100) surface regions in agreement with numerous scanning probe studies.

Erratum: “An assessment of theoretical procedures for the calculation of reliable free radical thermochemistry: A recommended new procedure” [J. Chem. Phys. 108, 604 (1998)

First Page

An assessment of theoretical procedures for the calculation of reliable free radical thermochemistry: A recommended new procedure

The ability to predict reliable thermochemical properties of molecules and ions has led to an ever increasing application of ab initio molecular orbital theory. Methods such as G2 theory have been shown to generally give accurate heats of formation (ΔfH) for closed-shell molecules and ions. Open-shell systems have been less thoroughly examined to date and the present paper attempts to redress this situation through a detailed assessment of the performance of a variety of levels of theory in calculating ΔfH values for free radicals. Representatives of three families of theoretical procedures have been studied: the infinite basis set extrapolation techniques of Martin, the CBS procedures of Petersson et al., and the G2 methods of Pople et al. Among the specific influences investigated are choice of geometry, zero-point vibrational energy, high level electron correlation treatment and basis set size. We recommend a new procedure called CBS-RAD for the treatment of free radicals. CBS-RAD is a modification of the CBS-Q method in which the geometry and zero-point energies are obtained at the QCISD/6-31G(d) level, and coupled-cluster theory is used in place of quadratic configuration interaction in single-point energy calculations. We find that for free radicals with low spin contamination G2 theory also performs adequately, but as 〈S2〉 increases the results of G2 calculations can become increasingly unsatisfactory. The recommended CBS-RAD procedure should yield more reliable results over a broader range of free radicals.

Chemical ionization of the nitrate ester explosives EGDN and PETN by trimethylsilyl cation and comparison of the reactivity of nitrate ester and nitro explosives toward trimethylsilyl cation

Fourier transform ion cyclotron resonance mass spectrometry has been used to examine the reactions of Si(CH 3) 3) + with EGDN and PETN. In addition, the reactions of Si(CH 3) 3 + with EGDN and PETN was examined in a magnetic sector mass spectrometer. No adduct formation was observed with either EGDN or PETN in the Fourier transform ion cyclotron resonance mass spectrometer, but characteristic fragment ions are seen. Both EGDN and PETN form adducts with Si(CH 3) 3 + in the sector mass spectrometer. The bimolecular rate constant for the reaction of Si(CH 3) 3 + with EGDN is measured to be 0.91 × 10 −10 cm 3 s −1 molecule −1, and the bimolecular rate constant for the reaction of Si(CH 3) 3 + with PETN is estimated to be 8 × 10 −10cm 3 s −1 molecule −. Collision-induced dissociation experiments were performed on the major fragment ion products to characterize the observed fragmentation patterns. The fragment ions observed in both cases are characteristic of each explosive and could be useful in the analytical detection and identification of EGDN and especially PETN. Reaction coordinate diagrams for the reactions of EGDN and PETN with Si(CH 3) 3 + are derived (from known thermochemistry and semi-empirical calculations on the involved species). The reactivity of nitro and nitrate ester explosives with Si(CH 3) 3 + is compared and reasons for their different chemical behavior are discussed.

Chemical Ionization of TNT and RDX with Trimethylsilyl Cation

Fourier transform ion cyclotron resonance mass spectrometry has been used to examine the reactions of Si(CH3)3+ with nitrobenzene, TNT, and RDX. With nitrobenzene, the only reaction observed is adduct formation which generates the C6H5NO2Si(CH3)3+ ion. The bimolecular rate constant for the reaction of Si(CH3)3+ with nitrobenzene is measured to be 1.8 × 10-9 cm3 s-1 molecule-1. With TNT, fragmentation and adduct formation were observed. The bimolecular rate constant for the reaction of Si(CH3)3+ with TNT is measured to be 0.85 × 10-9 cm3 s-1 molecule-1. With RDX, the dominant reaction observed is adduct formation, but some fragmentation is seen as a minor reaction pathway. The bimolecular rate constant for the reaction of Si(CH3)3+ with RDX is estimated to be similar to that observed with TNT (∼0.7 × 10-9 cm3 s-1 molecule-1). Collision-induced dissociation experiments performed on both the TNT−Si(CH3)3+ and the RDX−Si(CH3)3+ adducts using off-resonance collisional activation show the same fragmentation pattern that is observed during adduct formation. This fragmentation pattern appears to be a “fingerprint” for both adducts. These reactions appear to be driven by the high affinity of Si for oxygen and the attraction of the Si(CH3)3+ ion to the formal negative charge of oxygen in a nitro group. A reaction coordinate diagram for reactions of RDX with Si(CH3)3+ is derived (from known thermochemistry and ab initio calculations on the reactive intermediates) and its implications are discussed. Reactions of this type could be useful as a detection scheme for common explosives.

Theoretical Calculation of Thermochemistry for Molecules in the Si−P−H System

Ab initio molecular orbital calculations have been performed on species belonging to the Si-P-H system. These computations have been coupled to a bond additivity correction procedure to obtain heats of formation for 27 species. The Si-P single bond energy was found to be nominally about 300 kJ/mol, which is somewhat weaker than a Si-Si single bond. Multiple bond character in Si-P was found to be relatively weak. 11 refs., 7 tabs.

Theoretical Calculation of Thermochemistry, Energetics, and Kinetics of High-Temperature SixHyOz Reactions

Theoretical Calculation of Thermochemistry, Energetics, and Kinetics of High-Temperature SixHyOz Reactions

Dissociative attachment in silane

Cross sections for production of SiH−n from silane by electron impact from 6–12 eV have been measured with Fourier transform mass spectrometry techniques. The peak cross sections decrease and thresholds increase for the more extensively dissociated silicon hydride anions: σa(SiH−3)=1.9×10−18 cm2; σa(SiH−2)=0.8×10−18 cm2; σa (SiH−)=0.2×10−18 cm2. The cross sections for production of SiH−4 and H− are less than 8×10−21 cm2 over the 6–12 eV energy range. Ab initio calculations of the anion thermochemistry show that if the neutral products include molecular hydrogen substantial excitation of the anion or neutral fragments is required. The rates of anion production calculated from dissociative attachment cross sections show that attachment of electrons to radicals or excited species are the most important source of negative ions in silane plasmas.