Core-valence Corrections Research Articles

Accurate theoretical study of PSq (q = 0,+1,−1) in the gas phase

Highly correlated ab initio methods were used in order to generate the potential energy curves and spin-orbit couplings of electronic ground and excited states of PS and PS(+). We also computed those of the bound parts of the electronic states of the PS(-) anion. We used standard coupled cluster CCSD(T) level with augmented correlation-consistent basis sets, internally contacted multi-reference configuration interaction, and the newly developed CCSD(T)-F12 methods in connection with the explicitly correlated basis sets. Core-valence correction and scalar relativistic effects were examined. Our data consist of a set of spectroscopic parameters (equilibrium geometries, harmonic vibrational frequencies, rotational constants, spin-orbit, and spin-spin constants), adiabatic ionization energies, and electron affinities. For the low laying electronic states, our calculations are consistent with previous works whereas the high excited states present rather different shapes. Based on these new computations, the earlier ultraviolet bands of PS and PS(+) were reassigned. For PS(-) and in addition to the already known anionic three bound electronic states (i.e., X(3)Σ(-), (1)Δ, and 1(1)Σ(+)), our calculations show that the (1)Σ(-), (3)Σ(+), and the (3)Δ states are energetically below their quartet parent neutral state (a(4)Π). The depletion of the J = 3 component of PS(-)((3)Δ) will mainly occur via weak interactions with the electron continuum wave.

Multiconfiguration Dirac-Fock calculations on multi-valence-electron systems: Benchmarks on Ga-like ions

High-accuracy calculations of term energies and wavelengths of resonance lines in Ga-like ions have been performed as benchmarks in the request for accurate treatments of relativity, electron correction, and QED effects in multi-valence-electron systems. The calculated energy levels are in excellent agreement with the experimental results and the experimentally compiled energy values of the National Institute for Standards and Technology wherever available. The calculated values including core-valence correction are found to be in good agreement with other theoretical and experimental values for low- to medium-$Z$ ions. On the basis of our calculations, some theoretical wavelengths for Ga-like Rb vii to Mo xii are also given. For higher-$Z$ ions, computed wavelengths are compared well with the experimental results [E. Tr\abert, J. Clementson, P. Beiersdorfer, J. A. Santana, and Y. Ishikawa, Phys. Rev. A 82, 062519 (2010), I. N Dragani\ifmmode \acute{c}\else \'{c}\fi{} et al., J. Phys. B 44, 025001 (2011)].

A vacuum-ultraviolet laser pulsed field ionization-photoelectron study of sulfur monoxide (SO) and its cation (SO+)

Vacuum ultraviolet (VUV) laser pulsed field ionization-photoelectron (PFI-PE) spectroscopy has been applied to the study of the sulfur monoxide radical (SO) prepared by using a supersonically cooled radical beam source based on the 193 nm excimer laser photodissociation of SO(2). The vibronic VUV-PFI-PE bands for the photoionization transitions SO(+)(X(2)Π(1∕2); v(+) = 0) ← SO(X(3)Σ(-); v = 0); and SO(+)((2)Π(3∕2); v(+) = 0) ← SO(X(3)Σ(-); v = 0) have been recorded. On the basis of the semiempirical simulation of rotational branch contours observed in these PFI-PE bands, we have obtained highly precise ionization energies (IEs) of 83,034.2 ± 1.7 cm(-1) (10.2949 ± 0.0002 eV) and 83,400.4 ± 1.7 cm(-1) (10.3403 ± 0.0002 eV) for the formation of SO(+)(X(2)Π(1∕2); v(+) = 0) and SO(+)((2)Π(3∕2); v(+) = 0), respectively. The present VUV-PFI-PE measurement has enabled the direct determination of the spin-orbit coupling constant (A(0)) for SO(+)(X(2)Π(1∕2,3∕2)) to be 365.36 ± 0.12 cm(-1). We have also performed high-level ab initio quantum chemical calculations at the coupled-cluster level up to full quadruple excitations and complete basis set (CBS) extrapolation. The zero-point vibrational energy correction, the core-valence electronic correction, the spin-orbit coupling, and the high-level correction are included in the calculation. The IE[SO(+)(X(2)Π(1∕2,3∕2))] and A(0) predictions thus obtained are found to be in remarkable agreement with the experimental determinations.

Computational Study of the Reactions of Methanol with the Hydroperoxyl and Methyl Radicals. 1. Accurate Thermochemistry and Barrier Heights

The reactions of CH(3)OH with the HO(2) and CH(3) radicals are important in the combustion of methanol and are prototypes for reactions of heavier alcohols in biofuels. The reaction energies and barrier heights for these reaction systems are computed with CCSD(T) theory extrapolated to the complete basis set limit using correlation-consistent basis sets, both augmented and unaugmented, and further refined by including a fully coupled treatment of the connected triple excitations, a second-order perturbative treatment of quadruple excitations (by CCSDT(2)(Q)), core-valence corrections, and scalar relativistic effects. It is shown that the M08-HX and M08-SO hybrid meta-GGA density functionals can achieve sub-kcal mol(-1) agreement with the high-level ab initio results, identifying these functionals as important potential candidates for direct dynamics studies on the rates of these and homologous reaction systems.

High-Level ab Initio Predictions for the Ionization Energies and Heats of Formation of Five-Membered-Ring Molecules: Thiophene, Furan, Pyrrole, 1,3-Cyclopentadiene, and Borole, C4H4X/C4H4X+ (X = S, O, NH, CH2, and BH)

The ionization energies (IEs) and heats of formation (ΔH°(f0)/ΔH°(f298)) for thiophene (C(4)H(4)S), furan (C(4)H(4)O), pyrrole (C(4)H(4)NH), 1,3-cyclopentadiene (C(4)H(4)CH(2)), and borole (C(4)H(4)BH) have been calculated by the wave function-based ab initio CCSD(T)/CBS approach, which involves the approximation to the complete basis set (CBS) limit at the coupled-cluster level with single and double excitations plus a quasi-perturbative triple excitation [CCSD(T)]. Where appropriate, the zero-point vibrational energy correction (ZPVE), the core-valence electronic correction (CV), and the scalar relativistic effect (SR) are included in these calculations. The respective CCSD(T)/CBS predictions for C(4)H(4)S, C(4)H(4)O, C(4)H(4)NH, and C(4)H(4)CH(2), being 8.888, 8.897, 8.222, and 8.582 eV, are in excellent agreement with the experimental values obtained from previous photoelectron and photoion measurements. The ΔH°(f0)/ΔH°(f298) values for the aforementioned molecules and their corresponding cations have also been predicted by the CCSD(T)/CBS method, and the results are compared with the available experimental data. The comparisons between the CCSD(T)/CBS predictions and the experimental values for C(4)H(4)S, C(4)H(4)O, C(4)H(4)NH, and C(4)H(4)CH(2) suggest that the CCSD(T)/CBS procedure is capable of predicting reliable IE values for five-membered-ring molecules with an uncertainty of ±13 meV. In view of the excellent agreements between the CCSD(T)/CBS predictions and the experimental values for C(4)H(4)S, C(4)H(4)O, C(4)H(4)NH, and C(4)H(4)CH(2), the similar CCSD(T)/CBS IE and ΔH°(f0)/ΔH°(f298) predictions for C(4)H(4)BH, whose thermochemical data are not readily available due to its reactive nature, should constitute a reliable data set. The CCSD(T)/CBS IE(C(4)H(4)BH) value is 8.868 eV, and ΔH°(f0)/ΔH°(f298) values for C(4)H(4)BH and C(4)H(4)BH(+) are 269.5/258.6 and 1125.1/1114.6 kJ/mol, respectively. The highest occupied molecular orbitals (HOMO) of C(4)H(4)S, C(4)H(4)O, C(4)H(4)NH, C(4)H(4)CH(2), and C(4)H(4)BH have also been studied by the natural bond orbital (NBO) method, and the extent of π-electron delocalization in these five-membered rings are discussed in correlation with their molecular structures and orbitals.

High-Level ab Initio Predictions for the Ionization Energy, Bond Dissociation Energies, and Heats of Formations of Iron Carbide (FeC) and Its Cation (FeC+)

The ionization energy (IE) of FeC and the 0 K bond dissociation energies (D(0)) and the heats of formation at 0 K (DeltaH(o)(f0)) and 298 K (DeltaH(o)(f298)) for FeC and FeC(+) are predicted by the single-reference wave function based CCSDTQ(Full)/CBS approach, which involves the approximation to the complete basis set (CBS) limit at the coupled cluster level up to full quadruple excitations. The zero-point vibrational energy (ZPVE) correction, the core-valence electronic corrections (up to CCSDT level), spin-orbit couplings, and relativistic effects (up to CCSDTQ level) are included in the calculations. The present calculations provide the correct symmetry predictions for the ground states of FeC and FeC(+) to be (3)Delta and (2)Delta, respectively. We have also examined the theoretical harmonic vibrational frequencies of FeC/FeC(+) at the ROHF-UCCSD(T) and UHF-UCCSD(T) levels. While the UHF-UCCSD(T) harmonic frequencies are in good agreement with the experimental measurements, the ROHF-UCCSD(T) yields significantly higher harmonic frequency predictions for FeC/FeC(+). The CCSDTQ(Full)/CBS IE(FeC) = 7.565 eV is found to compare favorably with the experimental IE value of 7.59318 +/- 0.00006 eV, suggesting that the single-reference-based coupled cluster theory is capable of providing reliable IE prediction for FeC, despite its multireference character. The CCSDTQ(Full)/CBS D(0)(Fe(+)-C) and D(0)(Fe-C) give the prediction of D(0)(Fe(+)-C) - D(0)(Fe-C) = 0.334 eV, which is consistent with the experimental determination of 0.3094 +/- 0.0001 eV. The D(0) calculations also support the experimental D(0)(Fe(+)-C) = 4.1 +/- 0.3 eV and D(0)(Fe-C) = 3.8 +/- 0.3 eV determined by the previous ion photodissociation study. The present calculations also provide the DeltaH(o)(f0)(DeltaH(o)(f298)) predictions for FeC/FeC(+). The analysis of the correction terms in these calculations shows that the core-valence and valence-valence electronic correlations beyond CCSD(T) wave function and the relativistic effects make significant contributions to the calculated thermochemical properties of FeC/FeC(+). For the experimental D(0) and DeltaH(o)(f0) values of FeC/FeC(+), which are not known to high precision, we recommend the CCSDTQ(Full)/CBS predictions [D(0)(Fe-C) = 3.778 eV, D(0)(Fe(+)-C) = 4.112 eV, DeltaH(o)(f0)(FeC) = 760.8 kJ/mol and DeltaH(o)(f0)(FeC(+)) = 1490.6 kJ/mol] based on the ZPVE corrections using the experimental vibrational frequencies of FeC and FeC(+).

Determining the CH3SO2→CH3+SO2 barrier from methylsulfonyl chloride photodissociation at 193 nm using velocity map imaging

These imaging experiments study the formation of the methylsulfonyl radical, CH(3)SO(2), from the photodissociation of CH(3)SO(2)Cl at 193 nm and determine the energetic barrier for the radical's subsequent dissociation to CH(3) + SO(2). We first state-selectively detect the angular and recoil velocity distributions of the Cl((2)P(3/2)) and Cl((2)P(1/2)) atoms to further refine the distribution of internal energy partitioned to the momentum-matched CH(3)SO(2) radicals. The internal energy distribution of the radicals is bimodal, indicating that CH(3)SO(2) is formed in both the ground state and low-lying excited electronic states. All electronically excited CH(3)SO(2) radicals dissociate, while those formed in the ground electronic state have an internal energy distribution which spans the dissociation barrier to CH(3) + SO(2). We detect the recoil velocities of the energetically stable methylsulfonyl radicals with 118 nm photoionization. Comparison of the total recoil translational energy distribution for all radicals to the distribution obtained from the detection of stable radicals yields an onset for dissociation at a translational energy of 70+/-2 kcal/mol. This onset allows us to derive a CH(3)SO(2) --> CH(3) + SO(2) barrier height of 14+/-2 kcal/mol; this determination relies on the S-Cl bond dissociation energy, taken here as the CCSD(T) predicted energy of 65.6 kcal/mol. With 118 nm photoionization, we also detect the velocity distribution of the CH(3) radicals produced in this experiment. Using the velocity distributions of the SO(2) products from the dissociation of CH(3)SO(2) to CH(3) + SO(2) presented in the following paper, we show that our fastest detected methyl radicals are not from these radical dissociation channels, but rather from a primary S-CH(3) bond photofission channel in CH(3)SO(2)Cl. We also present critical points on the ground state potential energy surface of CH(3)SO(2) at the //CCSD(T)/aug-cc-pV(Q + d)ZCCSD(T)/6-311++G(2df,p) level. We include harmonic zero-point vibrational corrections as well as core-valence and scalar-relativistic corrections. The CCSD(T) predicted barrier of 14.6 kcal/mol for CH(3)SO(2) --> CH(3) + SO(2) agrees well with our experimental measurement. These results allow us to predict the unimolecular dissociation kinetics of CH(3)SO(2) radicals and critique the analysis of prior time-resolved photoionization studies on this system.

Ab Initio Wavenumber Accurate Spectroscopy: 1CH2 and HCN Vibrational Levels on Automatically Generated IMLS Potential Energy Surfaces

We report here calculated J = 0 vibrational frequencies for (1)CH(2) and HCN with root-mean-square error relative to available measurements of 2.0 cm(-1) and 3.2 cm(-1), respectively. These results are obtained with DVR calculations with a dense grid on ab initio potential energy surfaces (PESs). The ab initio electronic structure calculations employed are Davidson-corrected MRCI calculations with double-, triple-, and quadruple-zeta basis sets extrapolated to the complete basis set (CBS) limit. In the (1)CH(2) case, Full CI tests of the Davidson correction at small basis set levels lead to a scaling of the correction with the bend angle that can be profitably applied at the CBS limit. Core-valence corrections are added derived from CCSD(T) calculations with and without frozen cores. Relativistic and non-Born-Oppenheimer corrections are available for HCN and were applied. CBS limit CCSD(T) and CASPT2 calculations with the same basis sets were also tried for HCN. The CCSD(T) results are noticeably less accurate than the MRCI results while the CASPT2 results are much poorer. The PESs were generated automatically using the local interpolative moving least-squares method (L-IMLS). A general triatomic code is described where the L-IMLS method is interfaced with several common electronic structure packages. All PESs were computed with this code running in parallel on eight processors. The L-IMLS method provides global and local fitting error measures important in automatically growing the PES from initial ab initio seed points. The reliability of this approach was tested for (1)CH(2) by comparing DVR-calculated vibrational levels on an L-IMLS ab initio surface with levels generated by an explicit ab initio calculation at each DVR grid point. For all levels ( approximately 200) below 20 000 cm(-1), the mean unsigned difference between the levels of these two calculations was 0.1 cm(-1), consistent with the L-IMLS estimated mean unsigned fitting error of 0.3 cm(-1). All L-IMLS PESs used in this work have comparable mean unsigned fitting errors, implying that fitting errors have a negligible role in the final errors of the computed vibrational levels with experiment. Less than 500 ab initio calculations of the energy and gradients are required to achieve this level of accuracy.

Halogenated silanes, radicals, and cations: Theoretical predictions on ionization energies, structures and potential energy surfaces of cations, proton affinities, and enthalpies of formation

Quantum chemistry study has been carried out on the structure and energetics of halogenated silanes, radicals, and cations (SiH x X y 0,+1, X = F, Cl, Br; x + y = 1–4). The geometries are optimized at B3LYP/6-31+G(2df,p) level. The adiabatic ionization energiess (IE as), relative energetics of cations, proton affinities (PAs) of silanes, and the enthalpies of formation are predicted using G3(CC) model chemistry. Non-classical ion complex structures are found for hydrogenated cations and transition states connecting classical and non-classical structures are also located. The most stable cations for silylene and silyl radicals have their classical divalent and trivalent structures, and those for silanes have non-classical structures except for SiH 3Br + and SiH 2Br 2 +. The non-classical structures for halosilane cations imply difficulty in experimentally measurement of the adiabatic ionization energies using photoionization or photoelectron studies. For SiH 3X, SiH 2X 2, and SiHX 3, the G3(CC) adiabatic IE as to classical ionic structures closest to their neutrals agree better with the photoelectron spectroscopic measurements. The transition states between classical and non-classical structures also hamper the photoionization determination of the appearance energies for silylene cations from silanes. The G3(CC) results for SiH x 0,+1 agree excellently with the photoionization mass spectrometric study, and the results for fluorinated and chlorinated species also agree with the previous theoretical predictions at correlation levels from BAC-MP4 to CCSD(T)/CBS. The predicted enthalpy differences between SiH 2Cl +, SiHCl 2 +, and SiCl 3 + are also in accordance with previous kinetics study. The G3(CC) results show large discrepancies to the collision-induced charge transfer and/or dissociation reactions involving SiF x + and SiCl x + ions, for which the G3(CC) enthalpies of formation are also significantly differed from the previous theoretical predictions, especially on SiF x + ( x = 2–4). The G3(CC) IE a and PA of SiF 4 are significantly different from previous experimental and theoretical studies; however, they are supported by current benchmark calculations at level of CCSD(T)/CBS + core-valence correction.

Benchmark calculations on the adiabatic ionization potentials of M–NH3 (M=Na,Al,Ga,In,Cu,Ag)

The ground states of the M-NH(3) (M=Na,Al,Ga,In,Cu,Ag) complexes and their cations have been studied with density functional theory and coupled cluster [CCSD(T)] methods. The adiabatic ionization potentials (AIPs) of these complexes are calculated, and these are compared to results from high-resolution zero-electron kinetic energy photoelectron spectroscopy. By extrapolating the CCSD(T) energies to the complete basis set (CBS) limit and including the core-valence, scalar relativistic, spin-orbit, and zero-point corrections, the CCSD(T) method is shown to be able to predict the AIPs of these complexes to better than 6 meV or 0.15 kcal/mol. 27 exchange-correlation functionals, including one in the local density approximation, 13 in the generalized gradient approximation (GGA), and 13 with hybrid GGAs, were benchmarked in the calculations of the AIPs. The B1B95, mPW1PW91, B98, B97-1, PBE1PBE, O3LYP, TPSSh, and HCTH93 functionals give an average error of 0.1 eV for all the complexes studied, with the B98 functional alone yielding a maximum error of 0.1 eV. In addition, the calculated metal-ammonia harmonic stretching frequencies with the CCSD(T) method are in excellent agreement with their experimental values, whereas the B3LYP method tends to underestimate these stretching frequencies. The metal-ammonia binding energies were also calculated at the CCSD(T)/CBS level, and are in excellent agreement with the available experimental values considering the error limits, except for Ag-NH(3) and Ag(+)-NH(3), where the calculations predict stronger bond energies than measured by about 4 kcal/mol, just outside the experimental error bars of +/-3 kcal/mol.

Theoretical prediction of the ionization energies of the C4H7 radicals: 1-Methylallyl, 2-methylallyl, cyclopropylmethyl, and cyclobutyl radicals

The ionization energies (IEs) for the 1-methylallyl, 2-methylallyl, cyclopropylmethyl, and cyclobutyl radicals have been calculated by the wave function based ab initio CCSD(T)/CBS approach, which involves the approximation to the complete basis set (CBS) limit at the coupled cluster level with single and double excitations plus quasiperturbative triple excitation [CCSD(T)]. The zero-point vibrational energy correction, the core-valence electronic correction, and the scalar relativistic effect correction are included in these calculations. The present CCSD(T)/CBS results are then compared with the IEs determined in the photoelectron experiment by Schultz et al. [J. Am. Chem. Soc. 106, 7336 (1984)] The predicted IE value (7.881 eV) of 2-methylallyl radical is found to compare very favorably with the experimental value of 7.90+/-0.02 eV. Two ionization transitions for cis-1-methylallyl and trans-1-methylallyl radicals have been considered here. The comparison between the predicted IE values and the previous measurements shows that the photoelectron peak observed by Schultz et al. likely corresponds to the adiabatic ionization transition for the trans-1-methylallyl radical to form trans-1-methylallyl cation. Although a precise IE value for the cyclopropylmethyl radical has not been directly determined, the experimental value deduced indirectly using other known energetic data is found to be in good accord with the present CCSD(T)/CBS prediction. We expect that the Franck-Condon factor for ionization transition of c-C4H7-->bicyclobutonium is much less favorable than that for ionization transition of c-C4H7-->planar-C4H7+, and the observed IE in the previous photoelectron experiment is likely due to the ionization transition for c-C4H7-->planar-C4H7+. Based on our CCSD(T)/CBS prediction, the ionization transition of c-C4H7-->bicyclobutonium with an IE value around 6.92 eV should be taken as the adiabatic ionization transition for the cyclobutyl radical. The present study provides support for the conclusion that the CCSD(T)/CBS approach with high-level energetic corrections can be used to provide reliable IE predictions for C4 hydrocarbon radicals with an uncertainty of +/-22 meV. The CCSD(T)/CBS predictions to the heats of formation for the aforementioned radicals and cations are also presented.

Heats of Formation of Diphosphene, Phosphinophosphinidene, Diphosphine, and Their Methyl Derivatives, and Mechanism of the Borane-Assisted Hydrogen Release

The heats of formation of diphosphene (cis- and trans-P2H2), phopshinophosphinidene (singlet and triplet H2PP) and diphosphine (P2H4), as well as those of the P2H and P2H3 radicals resulting from PH bond cleavages, have been calculated by using high-level ab initio electronic structure theory. Energies were calculated using coupled-cluster theory with a perturbative treatment for triple excitations (CCSD(T)) and employing augmented correlation consistent basis sets with additional tight d-functions on P (aug-cc-pV(n+d)Z) up to quadruple- or quintuple-zeta, to perform a complete basis set extrapolation for the energy. Geometries and vibrational frequencies were determined with the CCSD(T) method. Core-valence and scalar relativistic corrections were included, as well as scaled zero-point energies. We find the following heats of formation (kcal/mol) at 298 [0] K: DeltaH(degree)(f)(P2H) = 53.4 [54.4]; DeltaH(degree)(f)(cis-P2H2) = 32.0 [33.9]; DeltaH(degree)(f)(trans-P2H2) = 28.7 [30.6]; DeltaH(degree)(f)(H2PP) = 53.7 [55.6]; DeltaH(degree)(f)(3H2PP) = 56.5 [58.3]; DeltaH(degree)(f)(P2H3) = 32.3 [34.8]; DeltaH(degree)(f)(P2H4) = 5.7 [9.1] (expt, 5.0 +/- 1.0 at 298 K); and DeltaH(degree)(f)(CH3PH2) = -5.0 [-1.4]. We estimate these values to have an accuracy of +/-1.0 kcal/mol. In contrast to earlier results, we found a singlet ground state for phosphinophosphinidene (H2PP) with a singlet-triplet energy gap of 2.8 kcal/mol. We calculated the heats of formation of the methylated derivatives CH3PPH, CH3HPPH2, CH3PPCH3, CH3HPP, (CH3)2PP, (CH3)2PPH2, and CH3HPPHCH3 by using isodesmic reactions at the MP2/CBS level. The calculated results for the hydrogenation reactions RPPR + H2 --> RHPPHR and R2PP + H2 --> R2PPH2 show that substitution of an organic substituent for H improves the energetics, suggesting that secondary diphosphines and diphosphenes are potential candidates for use in a chemical hydrogen storage system. A comparison with the nitrogen analogues is given. The mechanism for H2-generation from diphosphine without and with BH3 as a catalyst was examined. Including tunneling corrections, the rate constant for the catalyzed reaction is 4.5 x 1015 times faster than the uncatalyzed result starting from separated catalyst and PH2PH2.

The Heats of Formation of Diazene, Hydrazine, N2H3+, N2H5+, N2H, and N2H3 and the Methyl Derivatives CH3NNH, CH3NNCH3, and CH3HNNHCH3

The heats of formation of N(2)H, diazene (cis- and trans-N(2)H(2)), N(2)H(3), and hydrazine (N(2)H(4)), as well as their protonated species (diazenium, N(2)H(3)(+), and hydrazinium, N(2)H(5)(+)), have been calculated by using high level electronic structure theory. Energies were calculated by using coupled cluster theory with a perturbative treatment of the triple excitations (CCSD(T)) and employing augmented correlation consistent basis sets (aug-cc-pVnZ) up to quintuple-zeta, to perform a complete basis set extrapolation for the energy. Geometries were optimized at the CCSD(T) level with the aug-cc-pVDZ and aug-cc-pVTZ basis sets. Core-valence and scalar relativistic corrections were included, as well as scaled zero point energies. We find the following heats of formation (kcal/mol) at 0 (298) K: DeltaH(f)(N(2)H) = 60.8 (60.1); DeltaH(f)(cis-N(2)H(2)) = 54.9 (53.2); DeltaH(f)(trans-N(2)H(2)) = 49.9 (48.1) versus >/=48.8 +/- 0.5 (exptl, 0 K); DeltaH(f)(N(2)H(4)) = 26.6 (23.1) versus 22.8 +/- 0.2 (exptl, 298 K); DeltaH(f)(N(2)H(3)) = 56.2 (53.6); DeltaH(f)(N(2)H(3)(+)) = 231.6 (228.9); and DeltaH(f)(N(2)H(5)(+)) = 187.1 (182.7). In addition, we calculated the heats of formation of CH(3)NH(2), CH(3)NNH, and CH(3)HNNHCH(3) by using isodesmic reactions and at the G3(MP2) level. The calculated results for the hydrogenation reaction RNNR + H(2) --> RHNNHR show that substitution of an organic substituent for H improved the energetics, suggesting that these types of compounds may be possible to use in a chemical hydrogen storage system.

Sources of error in electronic structure calculations on small chemical systems

The sources of error in electronic structure calculations arising from the truncation of the one-particle and n-particle expansions are examined with very large correlation consistent basis sets, in some cases up through valence 10-zeta quality, and coupled cluster methods, up through connected quadruple excitations. A limited number of full configuration interaction corrections are also considered. For cases where full configuration interaction calculations were unavailable or prohibitively expensive, a continued fraction approximation was used. In addition, errors arising from corevalence and relativistic corrections are also probed for a number of small chemical systems. The accuracies of several formulas for estimating total energies and atomization energies in the complete basis set limit are compared in light of the present large basis set findings. In agreement with previous work, the CCSD(T) method is found to provide results that are closer to the CCSDTQ and full configuration-interaction results than the less approximate CCSDT method.

Accurateab initioPredictions of Ionization Energies and Heats of Formation for Cyclopropenylidene, Propargylene and Propadienylidene

The ionization energies (IEs) of cyclopropenylidene (c-C3H2), propargylene (HCCCH) and propadienylidene (H2CCC) have been computed using the CCSD(T)/CBS method, which involves the approximation to the complete basis set (CBS) limit at the coupled cluster level with single and double excitations plus quasi-perturbative triple excitation effect (CCSD(T)). The zero-point vibrational energy correction, the core-valence electronic correction, the scalar relativistic effect and the high level correction beyond the CCSD(T) excitations have also been made in these calculations. The CCSD(T)/CBS values for the IE(c-C3H2) and IE(HCCCH) of 9.164, 8.987 eV are in good agreement with the experimental values of (9.15 0.03) and (8.96 0.04) eV. The CCSD(T)/CBS calculations yield the IE values of 10.477 and 10.388 eV for the ionization transitions H2CCC H2CCC+(2A1, C2v) and H2CCC H2CCC+(2A0, Cs), respectively. On the basis of the FranckCondon factor consideration, the IE of (10.43 0.02) eV determined in the previous single-photon ionization experiment most likely corresponds to the ionization threshold for the H2CCC H2CCC+(2A1, C2v) transition. Although the precision of the experimental IE measurements for c-C3H2, HCCCH, and H2CCC is insufficient to pin down the accuracy of the theoretical calculations to better than 30 meV, the excellent agreement between the experimental and theoretical IE values observed in the present study indicates that the CCSD(T)/CBS calculations together with high-order correlation corrections are capable of yielding reliable IE predictions for simple hydrocarbon carbenes and bi-radicals. We have also reported the heats of formation at 0 K (Hof0) and 298 K (Hof298) for c-C3H2/c-C3H2+, HCCCH/HCCCH+, and H2CCC/H2CCC+. The available experimental Hf0o and Hof298 values for c-C3H2/c-C3H2+, HCCCH/HCCCH+ are found to be in good accord with the CCSD(T)/CBS predictions after taking into account the experimental uncertainties.

Accurate ab initio predictions of ionization energies and heats of formation for the 2-propyl, phenyl, and benzyl radicals

The ionization energies (IEs) for the 2-propyl (2-C(3)H(7)), phenyl (C(6)H(5)), and benzyl (C(6)H(5)CH(2)) radicals have been calculated by the wave-function-based ab initio CCSD(T)/CBS approach, which involves the approximation to the complete basis set (CBS) limit at the coupled cluster level with single and double excitations plus quasiperturbative triple excitation [CCSD(T)]. The zero-point vibrational energy correction, the core-valence electronic correction, and the scalar relativistic effect correction have been also made in these calculations. Although a precise IE value for the 2-C(3)H(7) radical has not been directly determined before due to the poor Franck-Condon factor for the photoionization transition at the ionization threshold, the experimental value deduced indirectly using other known energetic data is found to be in good accord with the present CCSD(T)/CBS prediction. The comparison between the predicted value through the focal-point analysis and the highly precise experimental value for the IE(C(6)H(5)CH(2)) determined in the previous pulsed field ionization photoelectron (PFI-PE) study shows that the CCSD(T)/CBS method is capable of providing an accurate IE prediction for C(6)H(5)CH(2), achieving an error limit of 35 meV. The benchmarking of the CCSD(T)/CBS IE(C(6)H(5)CH(2)) prediction suggests that the CCSD(T)/CBS IE(C(6)H(5)) prediction obtained here has a similar accuracy of 35 meV. Taking into account this error limit for the CCSD(T)/CBS prediction and the experimental uncertainty, the CCSD(T)/CBS IE(C(6)H(5)) value is also consistent with the IE(C(6)H(5)) reported in the previous HeI photoelectron measurement. Furthermore, the present study provides support for the conclusion that the CCSD(T)/CBS approach with high-level energy corrections can be used to provide reliable IE predictions for C(3)-C(7) hydrocarbon radicals with an uncertainty of +/-35 meV. Employing the atomization scheme, we have also computed the 0 K (298 K) heats of formation in kJ/mol at the CCSD(T)/CBS level for 2-C(3)H(7)/2-C(3)H(7) (+) ,C(6)H(5)/C(6)H(5) (+), and C(6)H(5)CH(2)/C(6)H(5)CH(2) (+) to be 105.2/822.7 (90.0/806.4), 351.4/1148.5 (340.4/1138.8), and 226.2/929.0 (210.3/912.7), respectively. Comparing these values with the available experimental values, we find that the discrepancies for the 0 and 298 K heats of formation values are < or =2.6 kJ/mol for 2-C(3)H(7)/2-C(3)H(7) (+),< or =4.1 kJ/mol for C(6)H(5)/C(6)H(5) (+), and < or =3.2 kJ//mol for C(6)H(5)CH(2)C(6)H(5)CH(2) (+).

Structural and thermochemical properties of the hydroxymethyl (CH2OH) radical: A high precision ab initio study

The variational solution of the CH2OH nuclear problem was performed by treatment of CH2OH as a planar molecule with highly anharmonic vibrations. A sophisticated dynamic model to describe the structural flexibility of CH2OH was applied. The detailed analysis of the rovibrational Hamiltonian is shown. The electron problem solution was obtained by use of the coupled cluster electron correlation technique and the augmented correlation-consistent polarized basis sets of triple- and quadruple-ζ qualities. The geometry and energy parameters were extrapolated to the complete basis set limit. The partition functions of CH2OH were calculated by the explicit summation of vibrational and rotational levels. The heat of formation of CH2OH, ΔfH∘(298.15 K)=−17.0±0.7 kJ/mol, was calculated by a study of the CH2OH→CH2O+H reaction using the augmented basis set of quintuple-ζ quality and the relativistic and core-valence corrections of the total energies. Some structural and thermodynamic properties of CH2O are reported.

An L-shaped equilibrium geometry for germanium dicarbide (GeC2)? Interesting effects of zero-point vibration, scalar relativity, and core–valence correlation

The ground state potential energy surface of the GeC2 molecule has been investigated at highly correlated coupled cluster levels of theory. Large basis sets including diffuse functions and functions to describe core correlation effects were employed in order to predict the true equilibrium geometry for GeC2. Like the much-studied valence isoelectronic SiC2, the linear (1∑+), L-shaped (1A′), and T-shaped structures (1A1) must be investigated. The L-shaped Cs geometry is found to have real harmonic vibrational frequencies along every internal coordinate, and the linear stationary point has an imaginary vibrational frequency along the bending mode at every level of theory employed. The T-shaped geometry is found to have an imaginary vibrational frequency along the asymmetric stretching mode. At the coupled cluster with single and double excitations and perturbative triple excitations [CCSD(T)]/correlation consistent polarized valence quadrupole-ζ (cc-pVQZ) level, the nonrelativistic classical relative energies of the T-shaped and linear structures with respect to the L-shaped minimum are 0.1 and 2.8 kcal/mol, respectively. Including zero-point vibrational energy, scalar relativistic, and core-valence corrections, the T-L energy separation is shifted to 0.4 kcal/mol and the relative energy between the L-shaped and linear structures is still 2.8 kcal/mol. All nonrelativistic and relativistic computations predict that the L-shaped (1A′) structure is most favored for the ground state. The linear structure is predicted to be a transition state, as the case of SiC2.

Computational chemistry and process design

The use of computational chemistry to address issues relative to process design is discussed. The need for efficient software for massively parallel architectures is described. Methods to predict the electronic structure of molecules are described for the molecular orbital and density functional theory approaches. The capabilities of the computational chemistry software NWChem and Ecce developed in the Environmental Molecular Sciences Laboratory are discussed. Two examples of electronic structure calculations are given. The first shows that one can now make extremely accurate predictions of the thermochemistry of small molecules if one carefully considers all of the details such as zero point energies, core-valence corrections, and relativistic corrections. The second example shows how more approximate computational methods, still based on high level electronic structure calculations, can be used to address a complex waste processing problem at a Department of Energy nuclear production facility.

A correlated ab initio study of the [formula omitted] transition in MgCCH

The A ̃ 2Π ← X ̃ 2Σ + transition in MgCCH was studied with correlation consistent basis sets and single- and multireference correlation methods. The à 2Π excited state was characterized in detail; the X ̃ 2Σ + ground state has been described elsewhere recently. The estimated complete basis set (CBS) limits for valence correlation, including zero-point energy corrections, are 22668, 23191, and 22795 cm −1 for the RCCSD(T), MRCI, and MRCI + Q methods, respectively. A core-valence correction of +162 cm −1 shifts the RCCSD(T) value to 22830 cm −1, in good agreement with the experimental result of 22807 cm −1.