Gaussian-2 Theory Research Articles

Simultaneous Adjustment of Experimentally Based Enthalpies of Formation of CF3X, X = nil, H, Cl, Br, I, CF3, CN, and a Probe of G3 Theory

The enthalpies of formation of the title compounds have been determined by simultaneous least-squares solution of a thermochemical network containing 28 experimental measurements extracted from the literature. The new enthalpies differ considerably from the older tabulated values and now show a high degree of internal consistency achieved through excellent agreement with newer experiments, as well as older calorimetric determinations. The fitted values are: ΔHf298°(CF3) = −111.3 ± 0.5 kcal/mol (−110.6 ± 0.5 kcal/mol at 0 K), ΔHf298°(CF3H) = −165.6 ± 0.5 kcal/mol (−163.9 ± 0.5 kcal/mol at 0 K), ΔHf298°(CF3Cl) = −169.5 ± 0.7 kcal/mol (−168.3 ± 0.7 kcal/mol at 0 K), ΔHf298°(CF3Br) = −155.3 ± 0.5 kcal/mol (−152.4 ± 0.5 kcal/mol at 0 K), ΔHf298°(CF3I) = −140.1 ± 0.5 kcal/mol (−138.7 ± 0.5 kcal/mol at 0 K), ΔHf298°(C2F6) = −321.3 ± 0.8 kcal/mol (−319.3 ± 0.8 kcal/mol at 0 K), and ΔHf298°(CF3CN) = −118.4 ± 0.4 kcal/mol (−117.5 ± 0.4 kcal/mol at 0 K). Theoretical calculations at the Gaussian-3 (G3) level of theo...

Gaussian-3 (G3) theory for molecules containing first and second-row atoms

Gaussian-3 theory (G3 theory) for the calculation of molecular energies of compounds containing first (Li–F) and second row (Na–Cl) atoms is presented. This new theoretical procedure, which is based on ab initio molecular-orbital theory, modifies G2 theory [J. Chem. Phys. 94, 7221 (1991)] in several ways including a new sequence of single point energy calculations using different basis sets, a new formulation of the higher level correction, a spin–orbit correction for atoms, and a correction for core correlation. G3 theory is assessed using 299 energies from the G2/97 test set including enthalpies of formation, ionization potentials, electron affinities, and proton affinities. This new procedure corrects many of the deficiencies of G2 theory. There is a large improvement for nonhydrogen systems such as SiF4 and CF4, substituted hydrocarbons, and unsaturated cyclic species. Core-related correlation is found to be a significant factor, especially for species with unsaturated rings. The average absolute deviation from experiment for the 148 calculated enthalpies of formation is reduced to under one kcal/mol, from 1.56 kcal/mol for G2 theory to 0.94 kcal/mol for G3 theory. Significant improvement is also found for ionization potentials and electron affinities. The overall average absolute deviation of G3 theory from experiment for the 299 energies is 1.02 kcal/mol compared to 1.48 kcal/mol for G2 theory.

Gaussian-2 (G2) theory for third-row elements: A systematic study of the effect of the 3d orbitals

The importance of the inclusion of the 3d orbitals on third-row atoms in the correlation space in G2 theory has been systematically examined through calculations on the third-row G2 test set. Compared with standard G2, this G2(d) approach gives better agreement with experiment for the evaluation of ionization energies, a slightly poorer agreement for atomization energies, and much the same agreement for the very small sub-set of electron affinities and proton affinities. Overall, there is only slightly better agreement with experiment. However, when mixing of the 3d orbitals of the third-row atom with valence orbitals on the adjacent atoms is strong, inclusion of the 3d orbitals in the correlation space becomes a prerequisite to obtaining reliable results. Standard G2 theory is unsuitable in these circumstances. Similar conclusions pertain for the more economical G2(MP2)(d) method and for the full G2(QCI)(d) method. Inclusion of the 3d orbitals in the correlation space greatly increases the computer time required for a G2 calculation so some simple additive corrections to the G2 energy to approximate the effect of this inclusion have been investigated. These additivity methods generally underestimate the effect of the 3d orbitals but give reasonable agreement with the full G2(d) calculations in most cases. They cannot be used, however, in situations where the 3d orbital mixing is strong.

Assessment of Gaussian-2 and density functional theories for the computation of ionization potentials and electron affinities

A set of 146 well-established ionization potentials and electron affinities is presented. This set, referred to as the G2 ion test set, includes the 63 atoms and molecules whose ionization potentials and electron affinities were used to test Gaussian-2 (G2) theory [J. Chem. Phys. 94, 7221 (1991)] and 83 new atoms and molecules. It is hoped that this new test set combined with the recently published test set of enthalpies of neutral molecules [J. Chem. Phys. 106, 1063 (1997)] will provide a means for assessing and improving theoretical models. From an assessment of G2 and density functional theories on this test set, it is found that G2 theory is the most reliable method. It has an average absolute deviation of 0.06 eV for both ionization potentials and electron affinities. The two modified versions of G2 theory, G2(MP2,SVP) and G2(MP2) theory, have average absolute deviations of 0.08–0.09 eV for both ionization potentials and electron affinities. The hybrid B3LYP density functional method has the smallest average absolute deviation (0.18 eV) of the seven density functional methods tested for ionization potentials. The largest deviation for the density functional methods is for the ionization potential of CN (>1 eV). The BLYP density functional method has the smallest average absolute deviation (0.11 eV) of the seven density functional methods tested for electron affinities, while the BPW91, B3LYP, and B3PW91 methods also do quite well.

Experimental and Computational Investigations of the Reaction of OH with CF3I and the Enthalpy of Formation of HOI

The kinetics of the reaction of hydroxyl radicals with trifluoroiodomethane were investigated by the flash photolysis−resonance fluorescence technique. A rate constant of k = 5.8 × 10-12 exp((−11.3 kJ mol-1)/RT) cm3 molecule-1 s-1 was measured over the temperature range 280−450 K with accuracy limits of 20% (450 K) to 30% (280 K). Different product channels were investigated by ab initio methods, and the dominant products are CF3 + HOI. The enthalpy of formation of hypoiodous acid was analyzed with Gaussian-2 theory, in conjunction with G2 energies for INO, ICN, ClCN, and other species. The transition state and reaction coordinate for OH + CF3I was characterized at the G2(MP2) level, and the results suggest a negligible barrier to the reverse reaction of CF3 + HOI, so that the measured forward activation energy can be used to derive ΔfH298(HOI) = −69.6 ± 5.4 kJ mol-1. The implications of the kinetics and thermochemistry for iodine chemistry in flames and the atmosphere are discussed, and for the range 280−2000 K a proposed rate expression is k = 2.9 × 10-16(T/K)1.5 exp(−(960 K)/T) cm3 molecule-1 s-1.

Dissociation dynamics of the HCl dimer ion

The dissociation pathways of (HCl) 2 + are studied by a molecular beam-femtosecond multiphoton ionization technique, and via ab initio calculations at the MP2/6-31G(d,p) level and using Gaussian-2 theory. Results indicate, besides the commonly accepted proton-transferred HClH +Cl structure, a hydrogen-peroxide-like structure, (HCl–ClH) +, also exists and appears to be more stable. Their interconversion involves a barrier of 0.24 eV. A new dissociation product, Cl 2 +, is observed and shown to come from an intracluster reaction of the HCl dimer cation. Dissociations of the (HCl) 2 + into H 2Cl + and HCl + channels are preferred at low internal excitations, whereas dissociations into Cl 2 + and HCl 2 + require higher excitation energies.

An electronic structure investigation of the BNO-BON-NBO system

The potential energy hypersurface of the ground triplet states of the BNO-BON-NBO system has been investigated using traditional ab initio electronic structure theory. The molecules studied have the molecular formula BON and include three linear and three angular species, and two transition states for the isomerization of an angular NBO to an angular BON and a linear BNO, respectively. All stationary points on the BNO-BON-NBO isomerization potential energy surface have been characterized employing UMP2, UMP4, and Gaussian-2 (G2) theory with the 6-311 G(d), 6-311 G(2d), and TZ2P basis sets. The isomerization for an angular NBO to the linear BNO has a lower energy barrier than that of the former to an angular BON. Energetics are presented with G2 energies. Two sets of resonance structures for both bent BNO (boron nitrosyl) and BON (boron isonitrosyl) were proposed and the bonding in the two species was analyzed. For the purpose of comparison, the density functional theory based hybrid methods B3LYP/6-311G(d) and B3LYP/TZ2P have also been applied to both geometry optimization and single-point calculations. It is found that the B3LYP prediction of the nature of the linear BO is contradictory to that made by all MPn ( n = 2 and 4) calculations. The cause for this contradiction is discussed.

Theoretical study of reaction of trifluoromethyl radical with hydroxyl and hydrogen radicals

Ab initio molecular orbital theory and density functional theory calculations have been carried out on the reactions of the trifluoromethyl radical with the hydroxyl and the hydrogen radicals. These reactions are key reactions that underlie a new fire extinguishing mechanism of non-bromine-containing halon replacements. The activation energies calculated by the MP2 and QCISD methods are in good agreement with the experimental values. The B3LYP, as well as MP2 and QCISD, give good results for the calculations of the heats of reactions. The GAUSSIAN-1 and GAUSSIAN-2 theory calculations present the most acxcurate results on both the activation energies and the heats of reactions. The effects of the scaling factors on the activation energies and the heats of reactions are also evaluated. © 1998 John Wiley & Sons, Inc. J Comput Chem 19: 277–289, 1998

Theoretical study of reaction of trifluoromethyl radical with hydroxyl and hydrogen radicals

Ab initio molecular orbital theory and density functional theory calculations have been carried out on the reactions of the trifluoromethyl radical with the hydroxyl and the hydrogen radicals. These reactions are key reactions that underlie a new fire extinguishing mechanism of non-bromine-containing halon replacements. The activation energies calculated by the MP2 and QCISD methods are in good agreement with the experimental values. The B3LYP, as well as MP2 and QCISD, give good results for the calculations of the heats of reactions. The GAUSSIAN-1 and GAUSSIAN-2 theory calculations present the most acxcurate results on both the activation energies and the heats of reactions. The effects of the scaling factors on the activation energies and the heats of reactions are also evaluated. © 1998 John Wiley & Sons, Inc. J Comput Chem 19: 277–289, 1998

Gaussian-2 theory, Na +, and HF: a cautionary tale

The Gaussian-2 (G2) theoretical procedure delivers a sodium cation affinity (SCA) for HF of −119.1 kJ mol −1. In contrast, SCA values for NH 3, H 2O, PH 3, H 2S, and HCl are all positive according to G2 theory. Here we show that the negative SCA(HF) value delivered by G2 indicates a shortcoming in the electron correlation for metal atoms in frozen-core calculations on highly polar cations. Using a modified technique, denoted `G2(thaw)', we obtain a value of E 0(NaFH +)=−262.03453 hartree, which yields the quantities Δ H f,0 o(NaFH +)=255.2 kJ mol −1, SCA(HF)=66.8 kJ mol −1, and PA(NaF)=975.2 kJ mol −1.

Assessment of Modified Gaussian-2 (G2) and Density Functional Theories for Molecules Containing Third-Row Atoms Ga−Kr

The performance of G2(MP2) and G2(MP2,SVP) theories for molecules containing third-row nontransition elements Ga−Kr is assessed. The average absolute deviation from experiment for 40 test energies is 1.92 kcal/mol for both methods compared to 1.37 kcal/mol for G2 theory. Four density functional theories (BPW91, BLYP, B3PW91, and B3LYP) are also assessed for the 40 test energies and found to have average absolute deviations of 3.58, 4.75, 2.03, and 2.62 kcal/mol, respectively. The B3PW91 density functional theory gives the best agreement with experiment in contrast to first- and second-row systems, where B3LYP does better than B3PW91. Of the four density functional methods, the B3PW91 method gave the best agreement with experiment for geometries and is about as accurate as second-order perturbation theory.

Molecular decompositions of acetaldehyde and formamide: theoretical studies using Hartree-Fock, Moller-Plesset and density functional theories

Optimized geometries and vibrational frequencies for all reactants, transition states and products of the unimolecular decompositions of acetaldehyde (CH3CHO) to CO+CH4 and to H2 + CH2CO, and of formamide (NH2CHO to CO + NH3 and to H2+ HNCO were investigated at the HF, MP2 and B3LYP levels of theory with the 6-31G(d,p) basis set. Total energies for all species were calculated with the 6-311G(d,p) basis set at the MP4 level at both the HF and MP2 optimized geometries, with B3LYP at the B3LYP optimized geometries, and with Gaussian-2 theory. The effects of basis sets and correlation corrections on barrier heights, reaction enthalpies, and molecular structures are discussed. Most notable are the similarities in barrier heights for any level of theory which incorporates electron correlation. Inclusion of electron correlation has significant effects on energetics, and for some geometrical parameters, including a change in point group symmetry for the transition state in the decomposition of NH2CHO to CO + NH3. Very good agreement with experiment, both for structures and energetics, is given by B3LYP theory.

Ab Initio Energetic Calculations of Elementary Reactions Relevant to Low-Temperature Silane Oxidation by Gaussian-2 Theory

Ab initio G-2 calculations have been carried out for a number of elementary reactions relevant to low-temperature silane oxidation. The calculations have been made to identify the chain-branching process which enables the spontaneous ignition of silane at room temperature. Extensive calculations have also been made for the reactions of major intermediates with hydrogen atoms and water vapor.

Application of Gaussian-2 theory for the energetics of XO/XO+/XO- and XOH/XOH+ (X = F, Cl, Br, I)

The Gaussian-2 (G2) procedure, with spin-orbit coupling correction estimated by the Breit-Pauli approach, is applied to study the energetics of XO/XO+/XO- and XOH/XOH+(X = F Cl Br, I). Accurate thermochemical data for these environmentally important species, in particular for the anions and iodine containing species, have not been accurately determined experimentally. For example, for IO the reported experimental heat of formation varies from -109 kJ mol-1 to 177 kJ mol-1 Using the G2 results it is now possible to recommend a set of self-consistent thermochemical data of heats of formation, ionization energies, electron affinities, proton affinities and bond dissociation energies for XO and related species. In general, among the disparate experimental data, the more recently reported heats of formation for FO-, BrO+, BrO-, BrOH, BrOH+, IO, IO+, IO-, IOH, IOH+ are found to be in good agreement with the G2 results.

Application of Gaussian-2 theory for the energetics of XO/XO+ /XO- and XOH/XOH+ (X = F, Cl, Br, I)

The Gaussian-2 (G2) procedure, with spin-orbit coupling correction estimated by the Breit-Pauli approach, is applied to study the energetics of XO/XO+/XO- and XOH/XOH+(X = F Cl Br, I). Accurate thermochemical data for these environmentally important species, in particular for the anions and iodine containing species, have not been accurately determined experimentally. For example, for IO the reported experimental heat of formation varies from -109 kJ mol-1 to 177 kJ mol-1 Using the G2 results it is now possible to recommend a set of self-consistent thermochemical data of heats of formation, ionization energies, electron affinities, proton affinities and bond dissociation energies for XO and related species. In general, among the disparate experimental data, the more recently reported heats of formation for FO-, BrO+, BrO-, BrOH, BrOH+, IO, IO+, IO-, IOH, IOH+ are found to be in good agreement with the G2 results.

Binding energies of germanium clusters, Ge n ( n = 2−5)

Gaussian-2 (G2) theory for third-row non-transition elements is used to calculate energies of germanium clusters, Ge n ( n = 2−5). The G2 energies are used to derive accurate binding energies for the clusters. The results for Ge 2 and Ge 3 are in agreement with experiment while there is some disagreement for Ge 4 and Ge 5. The binding energies are also calculated using the B3LYP density functional method with the 6–311 + G(3df,2p) basis set and compared with the G2 results and experiment.

Accurate thermochemistry for larger molecules: Gaussian-2 theory with bond separation energies

Gaussian-2 (G2) theory is combined with isodesmic bond separation reaction energies to yield accurate thermochemistry for larger molecules. For a test set of 40 molecules composed of H, C, O, and N, our method yields enthalpies of formation, ΔHf0(298 K), with a mean absolute deviation from experiment of only 0.5 kcal/mol. This is an improvement of a factor of three over the deviation of 1.5 kcal/mol seen in standard G2 theory.

Photofragmentation of Acetyl Cyanide at 193 nm

The photodissociation of gaseous acetyl cyanide has been examined following excitation at 193 nm. CN X2Σ+ photofragments were probed via laser fluorescence excitation to determine their rotational, vibrational, and translational energy distributions. CN was produced in v‘‘ = 0 and 1 with mean rotational energy (13.5 ± 2) kJ mol-1, and v‘‘ = 2 with mean rotational energy (10 ± 4) kJ mol-1. Mean translational energies of the CN fragments were (32 ± 10) kJ mol-1. Ab initio electronic structure theory has been used to characterize the heat of formation for acetyl cyanide along with its geometries and vibrational frequencies. The acetyl cyanide heat of formation, ΔH0f,0, is predicted to be (−0.4 ± 8) kJ mol-1 using Gaussian-2 theory (G2). The theoretical results are used to compute bond dissociation energies of acetyl cyanide for further interpretation of the experimental photodissociation data. Evidence is presented that the majority of CN fragments are produced via dissociation of the parent acetyl cyanide t...

Assessment of Gaussian-2 and density functional theories for the computation of enthalpies of formation

A set of 148 molecules having well-established enthalpies of formation at 298 K is presented. This set, referred to as the G2 neutral test set, includes the 55 molecules whose atomization energies were used to test Gaussian-2 (G2) theory [J. Chem. Phys. 94, 7221 (1991)] and 93 new molecules. The G2 test set includes 29 radicals, 35 nonhydrogen systems, 22 hydrocarbons, 47 substituted hydrocarbons, and 15 inorganic hydrides. It is hoped that this new test set will provide a means for assessing and improving new theoretical models. From an assessment of G2 and density functional theories (DFT) on this test set it is found that G2 theory is the most reliable method both in terms of average absolute deviation (1.58 kcal/mol) and maximum deviation (8.2 kcal/mol). The largest deviations between experiment and G2 theory occur for molecules having multiple halogens. Inclusion of spin–orbit effects reduces the average absolute deviation to 1.47 kcal/mol and significantly improves the results for the chlorine substituted molecules, but little overall improvement is seen for the fluorine substituted molecules. Of the two modified versions of G2 theory examined in this study, G2(MP2,SVP) theory (average absolute deviation=1.93 kcal/mol) performs better than G2(MP2) theory (2.04 kcal/mol). The G2(MP2,SVP) theory is found to perform very well for hydrocarbons, radicals, and inorganic hydrides. Of the seven DFT methods investigated, the B3LYP method has the smallest average absolute deviation (3.11 kcal/mol). It also has a significantly larger distribution of error than the G2 methods with a maximum deviation of 20.1 kcal/mol.

Gaussian-2 (G2) theory: Reduced basis set requirements

Two variations of G2(MP2) theory which employ smaller basis sets in evaluating the quadratic configuration interaction [QCISD(T)] component of the energy are presented. The first, G2(MP2,SVP), uses the split-valence plus polarization (SVP) 6-31G(d) basis, while the second, G2(MP2,SV), uses the split-valence (SV) 6-31G basis. The methods are evaluated on the basis of results for the set of 125 systems used for testing G2 theory. The mean absolute deviation of G2(MP2,SVP) results from experimental values is 1.63 kcal mol−1 compared with 1.58 and 1.21 kcal mol−1 for G2(MP2) and G2, respectively. The G2(MP2,SVP) method thus provides results which are generally very similar in quality to those obtained from G2(MP2) but at considerably reduced computational expense. On the other hand, the mean absolute deviation of G2(MP2,SV) results from experiment is substantially larger (2.13 kcal mol−1). The G2(MP2,SV) method exceeds the 2 kcal mol−1 target accuracy of G2 theory for an unacceptably large number of comparisons.