Respective Potential Energy Surfaces Research Articles

Theoretical study of the water activation by a cobalt cation: Ab initio multireference theory versus density functional theory

The reaction mechanism of Co+ (5F,3F) with H2O has been studied by the ab initio multireference-based theory (MR–SDCI and MC–QDPT) and the density functional theory (B3LYP and BLYP). In the energetics derived by the MR–SDCI(+Q) plus the B3LYP zero-point vibrational energy, the ion–dipole complex, CoOH2+, is initially formed with the binding energy of 38.2 (triplet) and 34.1 (quintet) kcal/mol, which is the most stable complex in the respective potential energy surfaces. Then, Co+ activates one O–H bond of H2O, leading to the insertion complex, HCoOH+. There are three possible dissociation channels from HCoOH+, i.e., →CoOH++H, →CoH++OH, and →CoO++H2. The third dissociation is expected to occur through the transition state of a four-centered structure, with the activation barrier of 61.6 (triplet) and 49.2 (quintet) kcal/mol, although this dissociation has not been detected in the experiment. The ground state of CoO+ is predicted to be Δ,5 and the lowest triplet state is Γ3 with the energy level of 20.8 kcal/mol above. The B3LYP provides the energetics qualitatively similar to the MR–SDCI(+Q) ones through the reactions, with the maximum deviation of 13 kcal/mol. The calculated results are consistent with experimental observations.

Ketoenol, imineenamine, and nitroaci-nitro tautomerism and their interrelationship in substituted nitroethylenes. Keto, imine, nitro, and vinyl substituent effects and the importance of H-bonding

Tautomeric isomers and conformers of 2-nitrovinyl alcohol (1), 2-nitrovinylamine (2), and 1-nitro-propene (3) are reported at the MP2 and B3LYP levels of theory, using the 6-31G* basis set, with energy evaluation at B3LYP/6-311+G** and G2MP2. The nitroalkenes are the global minima on their respective potential energy surfaces. The barriers for the concerted 1,5-H transfer to the corresponding nitronic acids amount to only 5.0 kcal/mol for 1, 13.2 kcal/mol for 2, and a sizable 37.8 kcal/mol for 3. Whereas the aci-nitro tautomer of 2-nitrovinyl alcohol is easily accessible, beta-iminonitronic acid has little kinetic stability. H-bonding is a strong stabilizing factor in these nitroalkenes, estimated at 7.0 and 3.7 kcal/mol for the OH and NH2 derivatives, respectively, while its stabilization in their nitronic acids amounts to as much as 13 kcal/mol. The H-bonds are evident from the very short O...H and N...H distances and are characterized by bond critical points. The NO2 substituent effect of about 11.4 kcal/mol at G2MP2 on both the classical keto <==> enol and imine <==> enamine tautomeric processes stabilizes the nitroethylene derivatives. The keto, imine, and vinyl substituent effects at G2MP2 on the nitro <==> aci-nitro tautomeric process are also determined as are their pi-resonance components. The substituents have a large influence on the ionization energies of the nitroethylene derivatives.

Theoretical study of ammonia activation by M+ (M=Sc, Ni, Cu)

The reactions of the first-row transition metal cations, Sc+ (3D,1D), Ni+ (2D), Cu+ (1S), with NH3 have been studied by the multiconfigurational and multireference-based theories, to clarify the similarities and differences in the reactivity of early (Sc+) and late (Ni+, Cu+) transition metal cations. In all the cases, the ion–dipole complex, MNH3+, is initially formed with a C3v symmetry structure, which is the most stable complex in the respective potential energy surfaces except for Sc+ (1D). The M+–NH3 binding energy was evaluated as 42.4, 37.8, 50.9, and 48.1 kcal/mol for Sc+ (3D), Sc+ (1D), Ni+, and Cu+, respectively. In the second step, M+ is expected to activate one N–H bond of NH3, leading to the insertion complex, HMNH2+. In Sc+ (3D,1D), three different stationary points of HScNH2+, i.e., Cs (in-plane), Cs (out-of-plane), and C2v structures, were located, which correspond to a minimum point, a first-order saddle point, and a second-order saddle point, respectively. In these complexes, the singlet state originating from Sc+ (1D) is largely stabilized compared to the triplets. The singlet HScNH2+ (in-plane) is calculated to be the most stable compound. There are three dissociation channels from HScNH2+, i.e., →ScNH2++H, →ScH++NH2, and →ScNH++H2. The third dissociation occurs through the transition state of a four-centered structure, with a small activation barrier of 23 kcal/mol, in both singlet and triplet surfaces. As to the late transition metal cations Ni+ and Cu+, there is no intermediate complex of HMNH2+, thus, all the dissociations occur through highly vibrational excitations of MNH3+. The calculated results are consistent with experimental observations.

Ab Initio Molecular Orbital Study of XO2+(X = F, Cl, Br, I) Systems

The depletion of stratospheric ozone has resulted in an increasing interest in the study of the possible reaction mechanisms responsible for its depletion. The structures and relative stabilities of the cationic forms of the halogen dioxides have been studied by means of ab initio molecular orbital calculations. For fluorine- and chlorine-containing compounds the geometries and the harmonic vibrational frequencies of all possible isomers were calculated at the QCISD/6-311+G(2d) level of theory. For bromine- and iodine-containing compounds the effective core-potential basis sets of Hay and Wadt, modified to include a set of diffuse functions and two sets of polarization functions, were employed. For all systems the final energies were obtained at the QCISD(T)/6-311+G(3df) level of theory. In addition, multiconfiguration-based methods have also been used. The relative stabilities of structures XOO{sup +} and OXO{sup +} are greatly reduced relative to those observed for the corresponding neutral species. In fact, for Cl and I derivatives, the lowest energy isomer corresponds to the symmetric OXO{sup +} open-chain species. The corresponding cyclic structures arise as local minima on the respective potential energy surfaces, but they lie much higher in energy than the OXO{sup +} open-chain form or the XOO{sup +} isomer. There are significant differencesmore » in bonding between XOO{sup +} and OXO{sup +}, the X-O interaction in OXO{sup +} being more covalent than in XOO{sup +}. There are also trends along the series that reflect the pronounced disparity between the electron affinity of F{sup +} and those of the heavier atoms of the group. FOO{sup +} species can be viewed as F({sup 2}P)-O{sub 2}{sup +} complexes, whereas XOO{sup +}(X = Br, I) species can be regarded as X{sup +}({sup 3}P)-O{sub 2} complexes. The OXO{sup +} open-chain species have an electron charge distribution similar to that of the ozone molecule, reflecting the same number of valence electrons in each case.« less

Electronic Structure and Low-Lying Electronic States of Al3O and Al3O-: Photoelectron Spectrum of Al3O-

We have calculated the equilibrium geometries of the ground electronic state of Al3O and Al3O- using Hartree−Fock, density functional, and coupled-cluster doubles methods. These molecules are nominally equilateral triangles of Al atoms with an oxygen in the center which distort due to the Jahn−Teller effect. The calculated global minima had C2v symmetry with (b2)1 2B2 and (b2)2 1A1 configurations, respectively. The global minimum of the lowest triplet state of Al3O- was found to have D3h symmetry with an (e‘)2 3A2‘ electron configuration. While this was the lowest energy state of the molecule among D3h symmetry points, this triplet minimum had slightly higher energy than the 1A1 state at its C2v global minimum. The 1A1 (a1)2 and the 1B2 (b2)1(a1)1 configurations of Al3O- and the 2A1 (a1)1 configuration of Al3O lead to transition states of C2v symmetry on the respective potential energy surfaces for pseudorotation. Using the CCD geometries of (b2)2 1A1 and (e‘)2 3A2‘ Al3O-, configuration interaction calculations have been performed to determine the low-lying vertical excited states of Al3O, and those results have been utilized to interpret the recently reported experimental photoelectron spectrum of Al3O-. On the basis of the present CI results, new assignments have been made for some of the peaks.

Conformational Analysis (ab Initio HF/3-21G*) and Optical Properties of Symmetrically Disubstituted Terthiophenes

We report a conformational analysis of several substituted terthiophenes using ab initio calculations performed at the HF/3-21G* level. Geometries of terthiophenes having methoxy substituents in 3,3‘‘ positions (DMOTT), methyl groups in the same positions (DMTT), and ethyl substituents in 3‘,4‘ positions (DETT) are compared with that of the unsubstituted molecule (TT). For all these symmetrical molecules, it is observed that the two dihedral angles are independent of each other. The most stable conformation of TT is found for dihedral angles θ = φ = 147.2°, whereas three maxima are located at 0°, 90°, and 180°. The insertion of methoxy groups in 3,3‘‘ positions favors a more planar conformation with a higher rotational barrier at 90°. This behavior is explained by the electron donor properties of the methoxy groups. By contrast, the addition of two methyl groups at the same positions induces a twisting in the molecule which is caused by the steric hindrance between the methyl substituents and the sulfur atom. The presence of two ethyl groups in 3‘,4‘ positions creates an even stronger steric effect, giving rise to a more twisted conformation for DETT compared to that of DMTT. Absorption and fluorescence spectra of each terthiophene derivative are also reported and are correlated with their respective potential energy surfaces. The more planar molecule (DMOTT) shows a red-shifted absorption band with a higher vibrational resolution and a smaller bandwidth. For more twisted molecules, the blue shift and the bandwidth of the absorption bands increase with twisting while the absorption coefficient decreases. The fluorescence bands, in all molecules, show a better vibrational resolution with a smaller bandwidth compared to their absorption counterparts, while their maximum wavelengths are practically the same, showing that in the first excited singlet state, all molecules relax to a more planar conformation.

Protonated High Energy Density Materials: N4 Tetrahedron and N8 Octahedron

Previous theoretical studies have demonstrated that tetrahedral N4 should be an extraordinarily effective high energy density material. But this species has thus far resisted laboratory efforts directed to its synthesis. Ab initio electronic structure methods have been used to examine the Td N4 and Oh N8 nitrogen clusters, including their protonated forms. We have optimized geometries using DZP, TZ2P, and TZ2P(f,d) basis sets with the Hartree−Fock self-consistent-field (SCF) method, second-order Moller−Plesset perturbation theory (MP2), single and double excitation configuration interaction (CISD), coupled-cluster (CCSD and CCSD(T)) methods, and three DFT/Hartree−Fock hybrid (B3LYP, B3P86, BHLYP) methods. Harmonic vibrational frequencies and infrared intensities have been obtained at the SCF, MP2, B3LYP, B3P86, and BHLYP levels of theory. The vertex protonated Oh N8 and Td N4 structures are found to represent minima on their respective potential energy surfaces. The bond protonated Td N4 molecule was dete...

On the existence of SH3, SeH3, and TeH3: Discrepancies between all‐electron and pseudopotential calculations

AbstractAb initio calculations using both pseudopotential and double and triple‐ζ all‐electron basis sets, with and without electron correlation (MP2, QCISD), have been performed on the λ4‐sulfanyl (SH3), λ4‐selanyl (SeH3), and λ4‐tellanyl (TeH3) radicals. All‐electron basis sets of double‐ζ quality predict that SH3 and SeH3 correspond to transition states on their respective potential energy surfaces. In contrast, the pseudopotentials of Hay and Wadt predict that SH3 and SeH3 correspond to local minima at the QCISD level of theory while the pseudopotentials of Christiansen and Stevens predict transition states. By comparison, TeH3 proved to be a local minimum at all levels of theory. Interestingly, when a very large (triple‐ζ) all‐electron basis set was used, SH3 proved to be a transition state; however, in this instance the potential energy surface was found to be much flatter than in the case for which a double‐ζ basis set was used, suggesting that further improvements in the basis set may lead to a local minimum. Further improvements in the all‐electron selenium basis also led to a local minimum for SeH3 at the QCISD level of theory. © 1995 by John Wiley &amp; Sons, Inc.

A photoionization and molecular orbital study of cyclobutanol and cyclobutylamine radical cations

Dissociative ionization of cyclobutanol, 1 and cyclobutylamine, 2 to produce [CH 2CHX] ·+ + C 2H 4 (X  OH or NH 2) has been studied by photoionization and ab initio molecular orbital calculations. The latter reveal that ionized cyclobutanol and cyclobutylamine are not minimia in their respective potential energy surfaces. It is further concluded that ionization of 1 and 2 leads to the corresponding distonic species [CH 2CH 2CH 2CHX] ·+. From the measurement of the first dissociation onset, an accurate determination of the heats of formation of [CH 2CHX] ·+ ions is made, i.e. Δ H° f,300[CH 2CHOH] &,middot;+ = 768 ± 5kJ mol −1 and Δ H° f,300[CH 2CHNH 2] &,middot;+ = 838 ± 5kJmol −1, after correction for the thermal energy of the precursor molecules. By combining calculations (at the MP2/6-311G **//UHF/6-31G * + ZPE level) and experiments, heats of formation of the distonic ions have been estimated: Δ H° f,300[CH 2CH 2CH 2CHOH] ·+ = 733kJmol −1 and Δ H° f300[CH 2CH 2CH 2CHNH 2] ·+ = 792kJmol −1, with a probable error of ±12kJmol −1.

An Empirical Potential Study of the Interaction of L—;Lysine—;L—;Alanine—;L—;Alanine Tripeptide with Four models of B—;DNA with different Compositions

The empirical potential including the intra- and intermolecular energy terms was used to study the interaction of L-Lysine-L-Alanine-L-Alanine Tripeptide with four models of B-DNA with different compositions. On the basis of a detailed search of the respective potential energy surface, it was found that the peptide is preferentially bounded to the AT-rich sequences. Analysis of the different energy contributions indicated that the electrostatic term is responsible for this preference. The results agree with the experimental data on the selectivity of some DNA--binding proteins and polypeptides to AT-rich DNA.

Stability and energetics of metastable molecules: tetraazatetrahedrane (N4), hexaazabenzene (N6), and octaazacubane (N8)

By use of ab initio self-consistent field (SCF), coupled-cluster, and many-body perturbation theory (MBPT) methods, the potential nitrogen molecules, tetrahedral N 4 and octahedral N 8 , are found to be metastable, corresponding to local minima on their respective potential energy surfaces. Barriers to unimolecular dissociation appear to be large enough that the unknown molecules could be formed. Three additional N, structures (C s , C 2v , and D 2h ) are also found

A quasiclassical trajectory study of the collisional dissociation of H2 by H atoms

AbstractThe collision induced dissociation of H2 by H atoms was studied by quasiclassical trajectories using the Liu‐Siegbahn‐Truhlar‐Horowitz potential energy surface. Dissociation cross sections were obtained for five highly internally excited initial states of H2 for translational energies up to 100 kcal mol−1. Rate constants for dissociation out of these states were calculated for temperatures of 1000 to 10,000 K. Initial internal energy strongly enhances the probability of collisional dissociation, vibrational energy being more effective than rotational. The results are compared to those from a similar study of the H2−He system, and are discussed in relation tothe respective potential energy surfaces. The implications for the kineticsof thermal dissociation are also considered.

The relative stability of some C4H4F+ and C4H4Cl+ isomers investigated by ab initio quantum chemical methods. Are cyclic halogenophenium ions aromatic?

Molecules with the general formulae C4H4F+, C4H4Cl+, and C4H4O have been investigated by applying ab initio quantum chemical methods. The relative stability and the geometries of a number of isomers were obtained from the calculations. Of special interest are the cyclic fluorophenium and chlorophenium ions. These two structures represent local but not global minima of their respective potential energy surfaces. Populational analysis and the structural parameters show that these cyclic ions are not aromatic. These findings are discussed in connection with previous results from solution chemistry.

The N4 molecule and the N3 + ion

The possibility of the nitrogen molecule forming a stable dimer is investigated. Through ab initio calculations in an extended contracted-gaussian basis, it is demonstrated that N4 in tetrahedral geometry and N3 + in D 3h geometry are local minimum points on their respective internuclear potential energy surfaces. The N4 molecule is considerably higher in energy than two ground-state N2 molecules and some study of the barrier for N4 decomposition has been made. The bond lengths and force constants for the two systems are shown to be similar to their hydrocarbon counterparts, C4H4 and C3H3 +, tetrahedrane and cyclopropenyl cation. Other related systems are discussed as well.