Gaussian-2 Theory Research Articles

Thermochemistry of disputed soot formation intermediates C4H3 and C4H5

Accurate isomeric energy differences and standard enthalpies of formation for disputed intermediates in soot formation, C(4)H(3) and C(4)H(5), have been determined through systematic extrapolations of ab initio energies. Electron correlation has been included through second-order Z-averaged perturbation theory (ZAPT2), and spin-restricted, open-shell coupled-cluster methods through triple excitations [ROCCSD, ROCCSD(T), and ROCCSDT] utilizing the correlation-consistent hierarchy of basis sets, cc-pVXZ (X = D, T, Q, 5, and 6), followed by extrapolations to the complete basis set limit via the focal point method of Allen and co-workers. Reference geometries were fully optimized at the ROCCSD(T) level with a TZ(2d1f,2p1d) basis set. Our analysis finds that the resonance-stabilized i-C(4)H(3) and i-C(4)H(5) isomers lie 11.8 and 10.7 kcal mol(-1) below E-n-C(4)H(3) and E-n-C(4)H(5), respectively, several kcal mol(-1) (more, less) than reported in recent (diffusion Monte Carlo, B3LYP density-functional) studies. Moreover, in these systems Gaussian-3 (G3) theory suffers from large spin contamination in electronic wave functions, poor reference geometries, and anomalous vibrational frequencies, but fortuitous cancellation of these sizable errors leads to isomerization energies apparently accurate to 1 kcal mol(-1). Using focal-point extrapolations for isodesmic reactions, we determine the enthalpies of formation (delta(f)H(0) (composite function)) for i-C(4)H(3), Z-n-C(4)H(3), E-n-C(4)H(3), i-C(4)H(5), Z-n-C(4)H(5), and E-n-C(4)H(5) to be 119.0, 130.8, 130.8, 78.4, 89.7, and 89.1 kcal mol(-1), respectively. These definitive values remove any remaining uncertainty surrounding the thermochemistry of these isomers in combustion models, allowing for better assessment of whether even-carbon pathways contribute to soot formation.

FN+Cl Ions from Ionized F2NCl: a Computational Investigation on the Structure and Reactivity toward H2O

AbstractWe have used various ab initio methods and basis sets to ascertain that the FN+Cl cation has a singlet ground state, 1A′, which is more stable than the triplet state 3A″ by ca. 30 kcal mol−1. We have subsequently used the Gaussian‐3 (G3) theory to explore the potential‐energy profile for the reaction between singlet FN+Cl and H2O. The process commences by the effortless formation of a FN+Cl/H2O complex, which, in principle, can undergo several alternative processes, including isomerization to N‐protonated FN(Cl)OH, 1,2‐elimination of HX (X=F or Cl), and 1,1‐loss of H2. However, the energy barriers of all these processes are invariably larger than the energy (+18.1 kcal mol−1) required for the formation of FN+Cl/H2O from FN+Cl and H2O, thus suggesting that, under gas‐phase thermal conditions, FN+Cl should be essentially unreactive toward H2O. Comparing these theoretical findings with those concerning the reaction between FN+H, ClN+H, F2N+, and H2O, the reactivity order FN+H>F2N+>ClN+H>FN+Cl, was derived, which parallels the trend we recently found by G2MS calculations concerning the Lewis acidity of these ions. This suggests the conceivable occurrence of correlations between the reactivity and thermochemical properties of these simple halonitrenium ions.

Ab initio investigations of the equilibria between trichlorothioacetyl chloride, tetrachlorothiirane, and trichloroethenesulfenyl chloride, their reactants and decomposition products

Trichlorothioacetyl chloride (1), tetrachlorothiirane (2), and trichloroethenesulfenyl chloride (3) and the equilibria between them have been investigated by ab initio Hartree–Fock (HF), Møller–Plesset second order perturbation (MP2) calculations, and by Gaussian-3 theory, G3 (MP2). Transition states of the isomerization reactions have been identified. Also investigated were possible reactions leading to the isomers and their possible decomposition products. The results show that the unobserved isomerization reactions are feasible.

Thermochemistry of Silicon−Hydrogen Compounds Generalized from Quantum Chemical Calculations

Particulate contamination formed by homogeneous clustering reactions of silicon hydrides within silicon chemical vapor deposition processes is an important source of yield loss during semiconductor processing. On the other hand, intentional synthesis of silicon nanoparticles may be of great interest because of the unique optical and electronic properties of nanostructured silicon. Kinetic modeling can play an important role in developing a fundamental understanding of the particle clustering chemistry, and knowledge of the thermochemistry and reactivity of these silicon hydrides is necessary if a mechanistic kinetic model is to be constructed. Experimental measurements of thermochemical properties are usually expensive and difficult, and it is desirable to use computational quantum chemistry as an alternative. In this work, several theoretical methods were used to calculate thermochemical properties of silicon hydrides. Among the methods used, Gaussian-3 theory (G3) using the geometries from B3LYP density functional theory (B3LYP/6-31G(d)), referred to as G3//B3LYP, showed the most promising results with an average absolute deviation of 1.23 kcal/mol from experimental data for standard enthalpies of formation of small (<Si4) silicon hydrides. A series of calculations using G3//B3LYP was carried out on small to medium (<Si8) silicon hydrides to obtain thermochemical properties, and a bond additivity correction was incorporated to obtain more accurate thermochemical properties. A group additivity scheme was fit to these corrected values, allowing accurate estimation of the thermochemical properties of arbitrary silicon−hydrogen clusters. This generalization of the results is essential, since the many thousands of possible isomers of these molecules cannot be treated quantum chemically at even the least expensive levels of theory.

Modified Gaussian-2 level investigation of the identity ion-pair SN2 reactions of lithium halide and methyl halide with inversion and retention mechanisms.

Identity ion-pair S(N)2 reactions LiX + CH(3)X --> XCH(3) + LiX (X = F, Cl, Br, and I) have been investigated in the gas phase and in solution at the level of the modified Gaussian-2 theory. Two possible reaction mechanisms, inversion and retention, are discussed. The reaction barriers relative to the complexes for the inversion mechanism [DeltaH(cent) ( not equal )(inv)] are found to be much higher than the corresponding values for the gas phase anionic S(N)2 reactions, decreasing in the following order: F (263.6 kJ mol(-1)) > Cl (203.3 kJ mol(-1)) > Br (174.7 kJ mol(-1)) > I (150.7 kJ mol(-1)). The barrier gaps between the two mechanisms [DeltaH(cent) ( not equal ) (ret) - DeltaH(cent) ( not equal ) (inv)] increase in the order F (-62.7 kJ mol(-1)) < Cl (4.4 kJ mol(-1)) < Br (24.9 kJ mol(-1)) < I (45.1 kJ mol(-1)). Thus, the retention mechanism is energetically favorable for fluorine and the inversion mechanism is favored for other halogens, in contrast to the anionic S(N)2 reactions at carbon where the inversion reaction channel is much more favorable for all of the halogens. The stabilization energies for the dipole-dipole complexes CH(3)X. LiX (DeltaH(comp)) are found to be similar for the entire set of systems with X = F, Cl, Br, and I, ranging from 53.4 kJ mol(-1) for I up to 58.9 kJ mol(-1) for F. The polarizable continuum model (PCM) has been used to evaluate the direct solvent effects on the energetics of the anionic and ion-pair S(N)2 reactions. The energetic profiles are found to be still double-well shaped for most of the ion-pair S(N)2 reactions in the solution, but the potential profile for reaction LiI + CH(3)I is predicted to be unimodal in the protic solvent. Good correlations between central barriers [DeltaH(cent) ( not equal ) (inv)] with the geometric looseness of the inversion transition state %C-X( not equal ), the dissociation energies of the C-X bond (D(C-X)) and Li-X bond (D(Li-X)) are observed, respectively.

Novel Hückel stabilised azole ring-based lithium salts studied by ab initio Gaussian-3 theory

Gaussian-3 theory calculations have been performed for a new family of lithium salts with heterocyclic anions–Li+[N5C2n]− (0 ≤ n ≤ 5). Eight different anions and the most stable 1∶1 lithium ion pairs have been studied for each anion. The lithium ion affinity of the anions decreases with the gradual CN-substitution and is thus lowest for the [N5C10]− anion. The stability vs. oxidation, inferred from the HOMO values, is large for all anions. In addition, the volume and aromaticity of the anions have been evaluated.

Ab initio investigations of the C 2F 4S isomers and of their interconversions

The transition states and the activation energies for the unobserved isomerization reactions between the three possible C 2F 4S isomers with divalent sulfur, trifluorothioacetyl fluoride 1, tetrafluorothiirane 2, and trifluoroethenesulfenyl fluoride 3, have been determined by ab initio Hartree-Fock (HF), Møller-Plesset second-order perturbation (MP2) calculations and by Gaussian-3 theory. The results show that the unobserved isomerization reactions are feasible. Furthermore, all three isomers should exist as stable species, but the unknown isomer, 3, is considerably less stable than the known isomers, 1 and 2.

A comparative coupled cluster and density functional theory study of the HOCI → HCIO transition state

The structural features of the HOCl → HClO isomerization mechanism, including all stationary points, and one saddle point, were examined by use of coupled cluster and the B3LYP density functional theory methodology. To improve the results a very large 6–311++G (3df, 3pd) Gaussian-type basis set was employed in the presented calculations. In addition, Gaussian-3 theory was tested against our coupled cluster (with single, double and triple excitations) results, and they were found to correlate closely with one another by around 1–2 kcal mol−1. The energy change for this isomerization reaction is predicted to be 54.5 kcalmol−1 and 52.5 kcalmol−1 with the B3LYP and CCSD (T) methods, respectively, and the activation barrier is 76.1 kcal mol−1 and 70.1 kcalmol−1 with the same methods.

Gaussian-3 and related methods for accurate thermochemistry

In this paper, we present an overview of Gaussian-3 (G3) theory, a composite technique that employs a sequence of ab initio molecular orbital calculations to derive a total energy of a given molecular species. This method provides accurate energies of molecular systems for the calculation of enthalpies of formation, ionization potentials, electron affinities, proton affinities, etc. Also covered in this review are several variants of G3 theory including one based on scale factors (G3S) and an extended version (G3X) that uses improved geometries and larger Hartree-Fock basis sets. Finally, the G3/99 test set of accurate experimental data that is used for critical assessment is described. Overall, G3 theory has a mean absolute deviation from experiment of 1.07 kcal mol−1 for the G3/99 test set and G3S theory has a similar accuracy of 1.08 kcal mol−1. G3X theory is significantly more accurate with the mean absolute deviation from experiment decreasing from 1.07 kcal mol−1 (G3) to 0.95 kcal mol−1 (G3X). The scaled version of G3X theory shows a similar improvement.

A theoretical study of high electron affinity sulfur oxyfluorides: SO 3F, SO 2F 3, and SOF 5

Gaussian-2 theory was used to study a series of sulfur oxyfluorides that have exceptionally high electron affinities (EAs). Optimized geometries were computed for SO 3F, SO 2F 3, SOF 5, and their corresponding anions at the MP2(full)/6-311+G(d) level of theory. Both SO 3F and SOF 5 are found to be stable with respect to dissociation, while SO 2F 3 is metastable with respect to SO 2F 2+F. In all cases, electron attachment results in a shortening of one long SO bond within the molecule and a corresponding increase in the remaining SF and SO bonds; a significant portion of the excess charge is accommodated in an SO bonding orbital. Harmonic vibrational frequencies with IR and Raman intensities were computed for all species, and the calculated vibrational spectra of SOF 5 − and SO 2F 3 − are compared to the reported experimental spectra. The EAs of SO 3F, SO 2F 3, and SOF 5 are found to be 5.53, 5.50, and 5.14 eV, respectively, all well exceeding the threshold of 3.6 eV that defines a superhalogen species. The corresponding anion vertical detachment energies and neutral vertical attachment energies are also reported here. The calculated EA of SO 3F is discussed in relation to the gas-phase acidity of FSO 3H. The homolytic bond dissociation energy of FSO 3H is found to differ significantly from the previous estimate, suggesting the previous experimental estimate for the EA of SO 3F [J. Am. Chem. Soc. 114 (1992) 4299] be revised upward from 4.8 to 5.2 eV.

Assessment of parametrized core correlation schemes in Gaussian-3 theory

The use of parametrized core correlation equations in Gaussian-3 (G3) theory is assessed on the G2/97 test set. Two parametrized equations for estimating core correlation effects in molecules, one based on bond order and the other on atomic number, are considered in this study. The parameters are obtained by fitting to actual calculated core correlation contributions to atomization energies. Both equations give mean absolute deviations for the G2/97 test set similar to that in original G3 theory. It is also found that use of a fit based on more accurate core correlation values from the CCSD(T) method does not improve the accuracy of G3 theory.

Nonexistent electron affinity of OCS and the stabilization of carbonyl sulfide anions by gas phase hydration

We report the formation of heterogeneous OCS–water cluster anions [(OCS)n(H2O)k]− (n⩾1,n+k⩾2), of which OCS−⋅H2O is the most interesting species in view of the near absence of unhydrated OCS− in the same ion source. The presence of OCS−⋅H2O indicates that the intra-cluster formation of OCS− does occur as part of the [(OCS)n(H2O)k]− formation mechanism. In this light, the near absence of unhydrated OCS− anions points towards their metastable nature, while the abundance of the hydrated anions is attributed to the stabilizing effect of hydration. These conclusions are supported by the results of an extensive theoretical investigation of the adiabatic electron affinity (EA) of OCS. We conclude that the EA of OCS is either negative or essentially zero. The best estimate based on the Gaussian-3 theory calculation puts the EA at −0.059±0.061 eV. A study of the structure and energetics of OCS−⋅H2O predicts the existence of four structural isomers. Using the coupled-cluster theory, we find that the most stable structure is stabilized by 0.543 eV relative to the separated OCS−+H2O limit.

Theoretical calculation of heat of formation and heat of combustion for several flammable gases

Heats of formation have been calculated by the Gaussian-2 (G2) and/or G2MP2 method for a number of flammable gases. As a result, it has been found that the calculated heat of formation for compounds containing, such atoms as fluorine and chlorine tends to deviate from the observed values more than calculations for other molecules do. A simple atom additivity correction (AAC) has been found effective to improve the quality of the heat of formation calculation from the G2 and G2MP2 theories for these molecules. The values of heat of formation thus obtained have been used to calculate the heat of combustion and related constants for evaluating the combustion hazard of flammable gases.

Formaldehyde oxime nitrosomethane tautomerism.

Formaldehyde oxime <--> nitrosomethane tautomerism, isomeric nitrone, and their common cations and anions are studied with Gaussian-2 theory using MP2(full)/6-31G geometries and with density functional theory using B3LYP/6-311+G**. Geometrical parameters, harmonic vibrational frequencies, relative stabilities, conformational stabilities, and ionization energies are compared with experimental gas-phase data when available. The formaldehyde oxime <--> nitrosomethane tautomerism is compared with the amide <--> imidol, imine <--> enamine, keto <--> enol, and nitro <--> aci-nitro tautomeric processes. Solvent effects are estimated by the self-consistent isodensity polarizable continuum model (SCIPCM). The influence of hydrogen bonding interactions with the solvent is addressed by including two water molecules. In the final evaluation, formaldehyde oxime is 15.8 kcal/mol more stable than nitrosomethane when the aqueous solvation correction of 3.8 kcal/mol is applied to the G2 energies. Unsolvated formaldehyde oxime is estimated to be 11.1 kcal/mol more stable than nitrone. The estimated gas-phase ionization energies (G2) are 362.5 kcal/mol for formaldehyde oxime, 350.6 kcal/mol for nitrosomethane, and 351.4 kcal/mol for nitrone.

Electron affinities of germanium anion clusters, Ge n− ( n=2–5)

Gaussian-2 (G2) theory for third-row non-transition elements is used to calculate accurate electron affinities of germanium clusters, Ge n ( n=2–5). The results for n=2–4 are in agreement with experiment while there is some disagreement for Ge 5. The electron affinities are also calculated using G2 theory modified by adding a diffuse function to the basis set for MP2 geometry optimizations and using the B3LYP density functional method with the 6-311+G(3df,2p) basis set.

Structural and conformational properties of 2-propynylphosphine (propargylphosphine) as studied by microwave spectroscopy supplemented by quantum chemical calculations.

The microwave spectrum of 2-propynylphosphine (propargylphosphine), H-C triple bond C-CH2-PH2, has been investigated in the 18-26.5 and 32-48 GHz spectral regions at about -50 degrees C. Two conformers with different orientation of the phosphino group, denoted conformer I and conformer II, respectively, were assigned. Conformer I has a symmetry plane (Cs symmetry) with both hydrogen atoms of the phosphino group pointing toward the triple bond (C-C-P-H dihedral angles approximately 47 degrees from syn-periplanar (0 degrees )). The C-C-P-H dihedral angles are 73 and 167 degrees, respectively, from syn-periplanar in conformer II. Only one of the hydrogen atoms of the phosphino group points toward the triple bond in this rotamer. Conformer I is 1.5(20) kJ/mol more stable than II. The dipole moment of II was determined to be (in units of 10(-30) C m) mu(a) = 0 (assumed), mu(b) = 3.05(7), mu(c) = 1.60(9), and mu(tot) = 3.44(9) [mu(tot) = 1.03(3) D]. Two vibrationally excited states were assigned for each of the two rotamers I and II. Their frequencies were determined by relative intensity measurements. Many of the transitions of conformer II were split into two components presumably because of tunneling of the phosphino group. The tunneling frequency was determined to be 0.814(42) MHz for the ground vibrational state and 11.49(18) MHz for the first excited state of the C-P torsional vibration. Quantum chemical calculations at the B3LYP and MP2 levels of theory using the 6-311++G(3df,2pd) basis set reproduced experimental rotational constants, quartic centrifugal distortion constants, and dipole moment components within a few percent. The energy difference between the two conformers was calculated using the Gaussian-2 theory, and conformer I was found to be more stable than conformer II by 2.1 kJ/mol.

Extension of Gaussian-3 theory to molecules containing third-row atoms K, Ca, Ga–Kr

Gaussian-3 (G3) theory is extended to molecules containing the third-row nontransition elements K, Ca, Ga–Kr. Basis sets compatible with those used in G3 theory for molecules containing first- and second-row atoms have been derived. The G3 average absolute deviation from experiment for a set of 47 test reactions containing these elements is 0.94 kcal/mol. This is a substantial improvement over Gaussian-2 theory, which has an average absolute deviation of 1.43 kcal/mol for the same set. Variations of G3 theory are also presented that are based on reduced orders of perturbation theory. These variations also show similar improvement over the corresponding G2 methods. The use of scaling parameters in G3 theory for the third row was investigated and found to perform nearly as well as use of the higher level correction. In addition, these methods are evaluated on a set of molecules containing K and Ca for which the experimental data are not accurate enough for them to be included in the test set. Results for this set indicate that G3 theory performs significantly better than G2 for molecules containing Ca. When the 47 third-row systems are added to the G3/99 database the complete G3 average absolute deviation becomes 1.06 kcal/mol for 423 energies.

Ab Initio Studies of the Glyoxal Unimolecular Dissociation Pathways

Glyoxal (C2H2O2) unimolecular dissociation has been the subject of many experimental and theoretical studies, and some questions remain regarding the predominant pathway for dissociation. Molecular beam photodissociation studies identify the major pathway as that which leads to the formation of formaldehyde (CH2O) and carbon monoxide (CO). Thermal decomposition experiments and theoretical studies, on the other hand, predict the triple whammy pathway that leads directly to H2 + 2CO to be predominant. The objective of this study is to elucidate this discrepancy by performing a thorough ab initio exploration of the free energy surface for glyoxal dissociation. Gaussian-3 (G3) theory was shown to be an appropriate method for studying glyoxal unimolecular dissociation since the calculated heats of reaction agree well with reliable experimental values. On the other hand, other standard model chemistries, including the popular hybrid-density functional theory methods, predict poor heats of reaction for glyoxal dissociation. Various pathways for ground-state glyoxal unimolecular dissociation were then explored with G3 theory. We identified nonplanar transition state structures that lie lower in energy than the previously reported planar stationary points for both the triple whammy and formaldehyde channels. Three pathways, i.e., the formaldehyde, triple whammy and hydroxymethylene channels, have activation free energies that fall below typical experimental glyoxal photoexcitation energies and are thus possible under the experimental conditions. The formaldehyde channel was shown to be predominant at low temperatures, in agreement with molecular beam experiments and in contrast to previous theoretical predictions. As temperature increases, however, the ordering of the activation barriers is reversed and the predominant channel becomes the triple whammy channel, as observed in high-temperature thermal experiments. The temperature dependence of the activation barriers is attributed to the vibrational structure of the transition states relative to that of glyoxal, which significantly affects the entropic contributions to the activation barriers. The present study ultimately reconciles various sets of experimental findings and theory.

Theoretical investigations on (CO) n(CO 2) m cyclic cooligomers

Ab initio calculations have been performed on the (CO) n (CO 2) m cyclic copolymers, where n, m=1,2 with m≥ n. The cyclic molecules were studied at the HF/6-311G, HF/6-311+G(df), MP2/6-311+G(df) levels and Gaussian-2 theory with reduced Møller–Plesset order, G2MP2. The structures of the three cyclic oligomers investigated, C 2O 3, C 3O 5 and C 4O 6 are found to have a minimum and the geometries are planar. The energy releases of the dissociation reactions indicate that these molecules are thermodynamically unstable with respect to the parent monomers CO and CO 2. In addition, the heats of formation for the cyclic molecules were evaluated at the G2MP2 level using an isodesmic approach and bond separation energies.

The enthalpy of formation of dibenzofuran and some related oxygen-containing heterocycles in the gas phase. A GAUSSIAN-3 theoretical study

The discrepancies between the three experimental determinations of the enthalpy of formation of dibenzofuran have prompted us to carry out a theoretical study. Standard ab initio molecular orbital calculations were performed using the recently proposed GAUSSIAN-3 theory. Owing to the size of the dibenzofuran molecule, calculations were carried out at the G3(MP2)//B3LYP level. ΔfHm° values of dibenzofuran and a series of related oxygen-containing heterocycles have been calculated through atomization reactions, bond separation isodesmic reactions and other isodesmic processes. The averaged calculated ΔfHm° value, 12.4 ± 1.6 kcal mol−1, is inconclusive, and two of the three experimental values (R. Sabbah and I. Antipine, Bull. Soc. Chim. Fr., 1987, 392; R. Sabbah, Bull. Soc. Chim. Fr, 1991, 350 and R. D. Chirico, B. E. Gamman, S. E. Knipmeyer, A. Nguyen, M. M. Strube, C. Tsanopoulos and W. B. Steele, J. Chem. Thermodyn., 1990, 22, 1075), are equally acceptable.