Gaussian-2 Theory Research Articles

Extension of Gaussian-2 (G2) theory to molecules containing third-row atoms K and Ca

Gaussian-2 (G2) theory has been extended to molecules containing the third-row nontransition elements K and Ca. Basis sets compatible with those used in G2 theory for molecules containing first- and second-row atoms, as well as the third-row elements Ga–Kr, have been derived. As for Ga–Kr, first-order spin–orbit corrections are included explicitly in the G2 energies of K- and Ca-containing atoms and molecules. In contrast to the procedure for Ga–Kr, the 3s and 3p orbitals need to be included in the correlation space for K- and Ca-containing molecules. The geometries obtained with the new basis sets are in satisfactory agreement with experiment. Assessment of the thermochemical predictions is limited because of the sparsity of accurate experimental data for molecules containing K and Ca. Ionization energies are generally in good agreement with experimental values, as are the atomization energies for K-containing molecules. However, there are larger differences between theoretical and experimental atomization energies for Ca-containing molecules.

Extension of Gaussian-2 theory to molecules containing third-row atoms Ga–Kr

Gaussian-2 (G2) theory has been extended to molecules containing the third-row nontransition elements Ga–Kr. Basis sets compatible with those used in G2 theory for molecules containing first- and second-row atoms have been derived. Spin–orbit corrections for atoms and molecules having spatially degenerate states (2P, 3P for atoms and 2Π for molecules in this work) are included explicitly in the G2 energies. The average absolute deviation from experiment for 40 test reactions is 1.37 kcal/mol. In contrast to the situation for first- and second-row molecules, inclusion of spin–orbit effects is very important in attaining accurate energies for molecules containing third-row atoms. Without spin–orbit effects, the average absolute deviation is 2.36 kcal/mol.

Gaussian-2 theory: Use of higher level correlation methods, quadratic configuration interaction geometries, and second-order Mo/ller–Plesset zero-point energies

The performance of Gaussian-2 theory is investigated when higher level theoretical methods are included for correlation effects, geometries, and zero-point energies. A higher level of correlation treatment is examined using Brueckner doubles [BD(T)] and coupled cluster [CCSD(T)] methods rather than quadratic configuration interaction [QCISD(T)]. The use of geometries optimized at the QCISD level rather than the second-order Mo/ller–Plesset level (MP2) and the use of scaled MP2 zero-point energies rather than scaled Hartree–Fock (HF) zero-point energies have also been examined. The set of 125 energies used for validation of G2 theory [J. Chem. Phys. 94, 7221 (1991)] is used to test out these variations of G2 theory. Inclusion of higher levels of correlation treatment has little effect except in the cases of multiply-bonded systems. In these cases better agreement is obtained in some cases and poorer agreement in others so that there is no improvement in overall performance. The use of QCISD geometries yields significantly better agreement with experiment for several cases including the ionization potentials of CS and O2, electron affinity of CN, and dissociation energies of N2, O2, CN, and SO2. This leads to a slightly better agreement with experiment overall. The MP2 zero-point energies gives no overall improvement. These methods may be useful for specific systems.

Extension of Gaussian-2 (G2) theory to bromine- and iodine-containing molecules: Use of effective core potentials

Basis sets have been developed for carrying out G2 calculations on bromine- and iodine-containing molecules using all-electron (AE) calculations and quasirelativistic energy-adjusted spin–orbit-averaged seven-valence–electron effective core potentials (ECPs). Our recommended procedure for calculating G2[ECP] energies for such systems involves the standard G2 steps introduced by Pople and co-workers, together with the following modifications: (i) second-order Mo/ller–Plesset (MP2) geometry optimizations use polarized split-valence [31,31,1] basis sets for bromine and iodine together with 6-31G(d) for first- and second-row atoms; (ii) single-point higher-level energies are calculated for these geometries using our new supplemented bromine and iodine valence basis sets along with supplemented 6-311G and McLean–Chandler 6-311G bases for first- and second-row atoms, respectively; and (iii) first-order spin–orbit corrections are explicitly taken into account. An assessment of the results obtained using such a procedure is presented. The results are also compared with corresponding all-electron calculations. We find that the G2[ECP] calculations give results which are generally comparable in accuracy to those of the G2[AE] calculations but which involve considerably lower computational cost. They are therefore potentially useful for larger bromine- and iodine-containing molecules for which G2[AE] calculations would not be feasible.

Computational studies of the potential energy surface for O(3P)+H2S: Characterization of transition states and the enthalpy of formation of HSO and HOS

Structures and vibrational frequencies for minima and 11 transition states on the O(3P)+H2S potential energy surface have been characterized at the MP2=FULL/6-31G(d) level. GAUSSIAN-2 theory was employed to calculate ΔHf,298 for HSO and HOS of −19.9 and −5.5 kJ mol−1, respectively. The kinetics of HSO=HOS isomerization are analyzed by Rice–Ramsperger–Kassel–Marcus theory. Transition state theory analysis for O+H2S suggests OH+HS is the dominant product channel, with a rate constant given by 1.24×10−16 (T/K)1.746 exp(−1457 K/T) cm3 molecule−1 s−1. Kinetic isotope effects and the branching ratio for H+HSO production are also analyzed. The other possible products H2+SO and H2O+S do not appear to be formed in single elementary steps, but low-barrier pathways to these species via secondary reactions are identified. No bound adducts of O+H2S were found, but results for weakly bound triplet HOSH are presented. The likely kinetics for the reactions OH+SH→S(3P)+H2O, OH+SH→cis and trans 3HOSH, cis 3HOSH→HOS+H, and HSO and HOS+H→H2+3SO are discussed.

Heat of formation determination of the ground and excited state of cyanomethylene (HCCN) radical

Ab initio electronic structure theory has been used to characterize the structure of the ground triplet and lowest singlet excited states of cyanomethylene. The geometries, vibrational frequencies, and heats of formation have been determined using second-order Moller-Plesset perturbation, single and double excitation configuration interaction, and quadratic configuration interaction theory. The heat of formation is predicted with isodesmic reaction and Gaussian-2 theory (G2) for the ground triplet and first excited singlet states of cyanomethylene. For the ground state Δ H f,0 0 is 114.8 ± 2 kcal mol −1 while for the excited single state it is 126.5 ± 2kcal mol −1.

Determination of the heats of formation of CCCN and HCCCN

The electronic structure, vibrational spectra, and thermodynamic stability of CCCN and HCCCN have been determined using ab initio molecular orbital theory. The heat of formation for HCCCN was determined using Gaussian-2 theory (G2) and was found to be 91.5±2 kcal mol−1. There is good agreement between the G2 value and that determined using an isodesmic reaction (i.e., one in which both the number and types of bonds are conserved), which yields 90.7±2 kcal mol−1. The heat of formation for the CCCN radical is 178.2±2 kcal mol−1. These data are used to compute the bond dissociation energies in HCCCN, D0(C–C) and D0(C–H), which are important in assessing the chemistry in Titan’s atmosphere, as well as in other interstellar media.

Theoretical studies of the RSOO, ROSO, RSO 2 and HOOS (R=H, CH 3) radicals

The geometries of the radicals HSOO, HOSO, HSO 2 and HOOS have been optimized at the MP2=FULL/6-31G * level, and energies obtained with Gaussian-2 theory. Internal rotations and vibrational frequencies are analyzed. The results yield values of Δ H f,298 for the four doublet radicals of 111.5, −241.4, −141.4 and 58.9 kJ mol −1, respectively. Implications for reactions of interest in combustion and atmospheric chemistry are discussed. The results are employed to derive Δ H f,0 for CH 3SOO, CH 3OSO and CH 3SO 2 of 91.8, −222.6 and −199.4 kJ mol −1, respectively. The calculated CH 3SOO bond strength is in excellent accord with a recent measurement.

An ab Initio and Photoelectron Spectroscopic Study of the Trichloromethyl Radical and Cation

We report the 10.49 eV laser photoelectron spectrum of pyrolytically produced CCl[sub 3] and extract an adiabatic ionization potential of IP[sub ad] = 8.06 [+-] 0.02 eV by a deconvolution of the partially resolved spectrum using ab initio potential functions. The heat of formation for the trichloromethyl cation derived from the ionization potential is [Delta]H[sup 0][sub f,0][CCl[sub 3][sup +]] = 202.6 [+-] 0.8 kcal/mol, which is compared to a Gaussian-2 (G2) theory prediction of [Delta]H[sup 0][sub f,0][CCl[sub 3][sup +]] = 200.73 kcal/mol. We find the G2 prediction of [Delta]H[sup 0][sub f,0][CCl[sub 3]] = 17.30 kcal/mol to be in excellent agreement with the most recent experimental determination for the radical; the G2 prediction for the cation is slightly low but within the claimed accuracy of the method. 26 refs., 3 figs., 5 tabs.

Experimental evidence for gaseous diaminocarbene H 2NCNH 2 and its radical cation

Dissociative electron impact ionization of aminoguanidine, HNC(NH 2)NHNH 2 cleanly produces diaminocarbene radical cations H 2NCNH + 2. Their reduction by single electron transfer in neutralization—reionization mass spectrometric experiments yields the prototype nucleophilic carbene H 2NC̈NH 2 as a stable species in the gas phase, distinct from its isomer formamidine, H 2NC(H)NH. Deuterium labelling and multiple collision experiments confirm the identities of the ions and neutrals. The experimental findings are in excellent agreement with the results of ab initio calculations. Computations using GAUSSIAN-2 (G2) theory lead to Δ H 0 f(H 2N-C̈-NH 2)=39.4 kcal/mol and Δ H 0 f(H 2NCNH + 2) = 211 kcal/mol.

Ideal Gas Thermodynamic Properties of HOBr

On the basis of the Gaussian-2 theory of molecular energies, supplemented with empirical data, we have determined the ideal gas heat of formation of HOBr, [Delta]H[sub f][degrees] (300), to be [minus]14.2 [+-] 1.6 kcal mol[sup [minus]1]. This is in contrast to the widely used estimate of [minus]19 kcal mol[sup [minus]1] and the recently suggested value of [minus]9 kcal mol[sup [minus]1]. Thermodynamic data for HOBr are derived in the 0-500 K range and used to compute reaction energies of importance to atmospheric chemistry. 29 refs., 3 tabs.

Higher Level Theoretical Binding Energies of Methyldiazonium Ion. Is an Experimental Reinvestigation Warranted?

Several experimental values were reported for the binding energy of methyldiazonium ion, and they differ by 13 kcal/mol. Previous theoretical investigations also were associated with considerable uncertainty. The methyl cation affinity (MCA) of N 2 plays a crucial role as the anchor point for the absolute gas phase methyl cation affinity scale, and an accurate value is thus especially pertinent. To resolve this longstanding controversy, the reaction energy for the process CH 3 + +N 2 →CH 3 N 2 + has been determined using full fourth-order MOller-Plesset perturbation theory, CI theory (CID, CISD), quadratic CI theory (QCISD, QCISD(T)), Gaussian-1 (G1) and Gaussian-2 (G2) theory, and coupled cluster methods (CCD, CCSD, ST4CCD, CCSD(T)) with large basis sets

Comparison of molecular energy predictions for the neutral and ionic (C, H n, O; n=0–4) system by ab initio Gaussian-2 and density functional methods

The ionization energies (IEs), electron affinities (EAs), total atomization energies (Σ D 0), and bond dissociation energies at 0 K (BDEs) for the neutral and ionic (C, H n , O, n=0–4) system are calculated at the Gaussian-2 (G2), and two levels of density functional (DF) theories. On comparing the calculated values with known experimental data, both the G2 and DF theories are capable of predicting IEs and EAs quite accurately. However, the G2 theory is far more superior to the DF theories in predicting Σ D 0 and BDEs. Both G2 and DF theoretical calculations indicate that CO − is unstable with respect to electron detachment by approximately 1.6 eV, contrary to the experimental EA(CO)=1.4 eV based on the endothermic ion-molecule reaction study by Refaey and Franklin [Intern. J. Mass Spectrom. Ion Phys. 20 (1976) 19]. The large deviation (3 eV) between experimental and theoretical results suggests that the experimental EA(CO) should be re-examined.

Theoretical study of the silicon–oxygen hydrides SiOHn (n=0–4) and SiOH+n (n=0–5): Dissociation energies, ionization energies, enthalpies of formation, and proton affinities

The energies of silicon–oxygen hydrides SiOHn (n=0–4) and their cations SiOHn+ (n=0–5) have been studied using Gaussian-2 (G2) theory. Atomization energies, enthalpies of formation, bond dissociation energies, and ionization energies have been evaluated. There is little experimental thermochemical data available for these species, although many of them have been detected. In silanol, the O–H bond dissociation energy is 119.3 kcal/mol and the first Si–H bond dissociation energy is 92.6 kcal/mol. The O–H bond is significantly stronger than the same bond in methanol (105.0 kcal/mol) and the S–H bond in methylmercapton (86.0 kcal/mol). The silicon–oxygen systems tend to show a preference for isomers containing O–H bonds as opposed to H–Si bonds because of the strong O–H bond. The G2 proton affinity of SiO (191.1 kcal/mol at 298 K) is in good agreement with a recently measured value (189.3±2.6 kcal/mol at 295 K). The G2 ionization energy of SiO is 11.58 eV, in agreement with the observed value (11.61 eV) from the vacuum ultraviolet photoelectron spectrum.

The accurate determination of enthalpies of formation

The importance of the quadratic configuration interaction (QCISD(T)) step in Gaussian-2 (G2) theory is investigated for 125 test cases. The average deviation with experiment increases from 1.21 to 1.91 kcal/mol when this step is not included in G2 theory.

Transition state structures and energetics using Gaussian-2 theory

The availability of the easily implemented Gaussian-2 (G2) methodology has made it possible for the nonspecialist to calculate accurate heats of formation for many molecules on workstations. In order to quantify its performance for transition state structures, we have used G2 and a modified G2 on several transition states whose structures and energies have been well characterized either by experiment or multireference configuration interaction studies. The G2 method performs well in predicting energies of transition states (even for nonisogyric reactions), with an absolute average deviation of 1.5 kcal/mole in the classical barrier height for the cases studied, while it is less successful in predicting geometries and frequencies. We investigated modifying the G2 method for use with transition states by using QCISD/6-311G(d,p) geometries and frequencies instead of MP2/6-31G(d) geometries and scaled HF/6-31G(d) frequencies. The QCISD geometries and frequencies agree well with values from the literature, and this modified G2 procedure offers improved performance in predicting transition state energies.

Theoretical study of the proton affinity of N2O

The proton affinity of N2O has been calculated with the combination of theoretical methods called Gaussian-1 theory (567·7 kJ mol-1) and Gaussian-2 theory (570·4 kJ mol-1). The site of protonation at the highest levels of theory (G1 and G2 theory) is predicted to be oxygen; however, this prediction is very sensitive to the level of electron correlation. At MP3, MP4SDQ, and QCISD(T) levels of electron correlation, O protonation is preferred, whereas at MP2 and MP4SDTQ levels, N protonation is preferred. At the MP4 level, the effect of triple excitations on relative energies is overestimated and strongly favors the HNNO+ isomer over the HONN+ isomer. At the QCISD level, the effect of triple excitations also favors the HNNO+ isomer, but it is not sufficiently large to cause the N protonated isomer to become more stable. Evidence is presented indicating that biradical character in HNNO+ leads to a nonlinear NNO bond angle.

Gaussian-2 theory using reduced Mo/ller–Plesset orders

Two variations of Gaussian-2 (G2) theory are presented. In the first, referred to as G2 (MP2) theory, the basis-set-extension energy corrections are obtained at the 2nd order Mo/ller–Plesset (MP2) level and in the second, referred to as G2(MP3) theory, they are obtained at the MP3 level. The methods are tested out on the set of 125 systems used for validation of G2 theory [J. Chem Phys. 94, 7221 (1991)]. The average absolute deviation of the G2(MP2) and G2(MP3) theories from experiment are 1.58 and 1.52 kcal/mol, respectively, compared to 1.21 kcal/mol for G2 theory. The new methods provide significant savings in computational time and disk storage.

Predictions of adiabatic ionization energies and electron affinities for CH3S and CH2SH by density functional method

Using density functional method, we have obtained predictions of ionization energies and electron affinities for CH3S and CH2SH. These predictions are in good accord with experimental values and theoretical values based on the Gaussian-2 theory.

The dissociation energy of the SiN radical determined from a crossed molecular beam study of the Si+N 2O→SiN+NO reaction

The Si( 3P J )+N 2O(X 1Σ +→SiN(X 2Σ +)+NO(X 2Π r) reaction is studied using pulsed, crossed, supersonic molecular The SiN(X 2Σ +) product is probed by laser-induced fluorescence. The reaction exhibits a translational energy threshold between 0.2 and 0.3 eV. The determination of the SiN population limit just above the threshold leads to a reaction endoergicity: 0.12⩽Δε 0⩽0.38 eV, which combined with D 0(N−NO)=4.93±0.01 eV yields: 4.54⩽ D 0(SiN)⩽4.82 eV. This value is in agreemen with a recent ab initio result from Curtiss, Raghavachari, Trucks and Pople (4.58 eV) derived from the so-called Gaussian-2 theory. The validity of the experimental approach is also discussed.