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The ring closure reaction of C4H3 with acetylene to give phenyl radical is one proposed mechanism for the formation of the first aromatic ring in hydrocarbon combustion. There are two low-lying isomers of C4H3; 1-dehydro-buta-1-ene-3-yne (n-C4H3) and 2-dehydro-buta-1-ene-3-yne (iso-C4H3). It has been proposed that only n-C4H3 reacts with acetylene to give phenyl radical, and since iso-C4H3 is more stable than n-C4H3, formation of phenyl radical by this mechanism is unlikely. We report restricted Hartree–Fock (RHF) plus singles and doubles configuration interaction calculations with a Davidson’s correction (RHF+1+2+Q) using the Dunning correlation consistent polarized valence double zeta basis set (cc-pVDZ) for stationary point structures along the reaction pathway for the reactions of n-C4H3 and iso-C4H3 with acetylene. n-C4H3 plus acetylene (9.4) has a small entrance channel barrier (17.7) (all energetics in parentheses are in kcal/mol with respect to iso-C4H3 plus acetylene) and the subsequent closure steps leading to phenyl radical (-91.9) are downhill with respect to the entrance channel barrier. Iso-C4H3 plus acetylene also has an entrance channel barrier (14.9) and there is a downhill pathway to 1-dehydro-fulvene (-55.0). 1-dehydro-fulvene can rearrange to 6-dehydro-fulvene (-60.3) by a 1,3-hydrogen shift over a barrier (4.0), which is still below the entrance channel barrier, from which rearrangement to phenyl radical can occur by a downhill pathway. Thus, both n-C4H3 and iso-C4H3 can react with acetylene to give phenyl radical with small barriers. © 1995 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The reactions of CH(2Π) and singlet methylene (1CH2) with acetylene lead to intermediates which may be important in soot formation. CH(2Π)+acetylene leads to CHCHCH (C3H3), CHCCH, CH2CC, and propargyl (CH2CCH). 1CH2+acetylene leads to cyclopropene, allene, and propyne. Except for CHCCH, which involves a barrier of 7.5 kcal/mol with respect to reactants, all of these reaction products are formed with no barrier. Miller and Melius have previously discussed the dimerization of propargyl to give benzene. C3H3 and CHCCH are shown to dimerize with no barrier to give benzene and para-benzyne, respectively. C3H3, CHCCH, and CH2CC can also add to smaller polycyclic aromatic hydrocarbons (PAH), and may be important species in forming larger PAH or fullerenes. © 1995 American Institute of Physics. [ABSTRACT FROM AUTHOR]

We report calculations of the minimum energy pathways connecting 1CH2+N2 to diazomethane and diazirine, for the rearrangement of diazirine to diazomethane, for the dissociation of diazirine to HCN2+H, and of diazomethane to CH2N+N. The calculations use complete active space self-consistent field (CASSCF) derivative methods to characterize the stationary points and internally contracted configuration interaction (ICCI) to determine the energetics. The calculations suggest a potential new source of prompt NO from the reaction of 1CH2 with N2 to give diazirine, and subsequent reaction of diazirine with hydrogen abstracters to form doublet HCN2, which leads to HCN+N(4S) on the previously studied CH+N2 surface. The calculations also predict accurate 0 K heats of formation of 77.7 kcal/mol and 68.0 kcal/mol for diazirine and diazomethane, respectively. © 1995 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The reaction of vinylidene (CH2C) with acetylene may be an initiating reaction in soot formation. We report minimum energy paths and accurate energetics for a pathway leading to vinylacetylene and for a number of isomers of C4H4. The calculations use complete active space self-consistent field (CASSCF) derivative methods to characterize the stationary points and internally contacted configuration interaction (ICCI) and/or coupled cluster singles and doubles with a perturbational estimate of triple excitations [CCSD(T)] to determine the energetics. We find an entrance channel barrier of about 5 kcal/mol for the addition of vinylidene to acetylene, but no barriers above reactants for the reaction pathway leading to vinylacetylene. © 1995 American Institute of Physics. [ABSTRACT FROM AUTHOR]

We report a new calculation of the N+O2 potential energy surface using complete active space self-consistent field internally contracted configuration interaction with the Dunning correlation consistent basis sets. The peroxy isomer of NO2 is found to be a very shallow minimum separated from NO+O by a barrier of only 0.3 kcal/mol (excluding zero-point effects). The entrance channel barrier height is estimated to be 8.6 kcal/mol for ICCI+Q calculations correlating all but the O1s and N1s electrons with a cc-pVQZ basis set. © 1995 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Two-dimensional potential energy surfaces for the CH+N2→HCN+N(4S) reaction are obtained which describe (i) the C2v HCN2 region of the doublet potential energy surface; (ii) the region of the quartet potential energy surface including the C2v minimum and the dissociation pathway to HCN+N(4S); and (iii) the region of crossing of the doublet and quartet surfaces. These surfaces are fit using simple, physically motivated functional forms. In the first and third regions, the active coordinates are the C–N2 [center-of-mass (c.m.)] distance and the N–N distance. In the second region, the active coordinates are the C–N’ separation, where N’ denotes the departing N atom, and the angle H–C–N. In the following paper, this potential is used to study the dynamics of the title reaction and to compute Boltzmann rate constants. [ABSTRACT FROM AUTHOR]

The potential energy surface for NH2+NO has been characterized using complete active space self-consistent-field (CASSCF)/derivative calculations to determine the stationary point geometries and frequencies followed by internally contracted configuration interaction (ICCI) calculations to determine the energetics. Production of N2+H2O is found to involve a complex mechanism. The initially formed NH2NO undergoes a 1,3-hydrogen shift to give an HNNOH isomer (with the substituents trans about the NN bond and cis about the NO bond) which undergoes subsequent cis–trans isomerizations about the NN and NO bonds before decomposing to N2+H2O. The saddle point for production of N2+H2O has an approximately rectangular arrangement of one H atom, the two N atoms, and the O atom. This process does not involve a barrier with respect to NH2+NO. Formation of HN2+OH can occur from any of the isomers of HNNOH with no barrier, but the overall process is endothermic by 0.7 kcal/mol [based on the computed Δ Hf0 (0 K) of HN2]. The results obtained in this work are qualitatively the same as previous work, but both the stationary point geometries and energies should be more reliable due to the use of larger basis sets and more extensive inclusion of electron correlation effects. [ABSTRACT FROM AUTHOR]

The potential energy surface for NH2+O has been characterized using complete active space self-consistent field (CASSCF)/derivative calculations to determine stationary point geometries and frequencies followed by internally contracted configuration interaction (ICCI) calculations to determine the energetics. The calculations predict a NO bond strength of 85.8 kcal/mol for NH2O. The barrier for isomerization of NH2O to trans-HNOH is predicted to be 48.0 kcal/mol and the barriers for H+HNO forming NH2O and NHOH are predicted to be 2.1 and 8.3 kcal/mol, respectively (all corrected for zero-point energy). The computed heats of formation for NH2O and cis- and trans-HNOH reported by Soto, Page, and McKee are in good agreement with the present results. Our results also agree with those obtained by Melius and Binkley to within their stated ±3 kcal/mol error limits. The barrier for H+HNO→H2+NO is computed to be ≊0.3 kcal/mol, which is about 0.7 kcal/mol lower than the value computed by Soto and Page (without correction for zero-point energy). [ABSTRACT FROM AUTHOR]

We describe an analytic representation of the ArH2 potential energy surface which well reproduces the results of extensive ab initio electronic structure calculations. We also give an empirical modification of the function designed to improve agreement with experimental estimates of the van der Waals minimum. The analytic representation smoothly interpolates between the H+H and strong bonding H2 limits. In the fitting process, an accurate reproduction of regions of the potential expected to be important for high temperature (∼3000 K) collision processes is emphasized. Overall, the analytic representation well reproduces the anisotropy and H2 bond length dependence of the input data. [ABSTRACT FROM AUTHOR]

The potential energy surface (PES) for the CH3OH system has been characterized for the 1CH2+H2O, H2+HCOH, and H2+H2CO product channels using complete-active-space self-consistent-field (CASSCF) gradient calculations to determine the stationary point geometries and frequencies followed by CASSCF/internally contracted configuration-interaction (CCI) calculations to refine the energetics. The 1CH2+H2O channel is found to have no barrier. The long range interaction is dominated by the dipole–dipole term, which orients the respective dipole moments parallel to each other but pointing in opposite directions. At shorter separations there is a dative bond structure in which a water lone pair donates into the empty a‘ orbital of CH2. Subsequent insertion of CH2 into an OH bond of water involves a non-least-motion pathway. The H2+HCOH, and H2+H2CO pathways have barriers located at -5.2 and 1.7 kcal/mol, respectively, with respect to CH3+OH. From comparison of the computed energetics of the reactants and products to known thermochemical data it is estimated that the computed PES is accurate to ±2 kcal/mol. [ABSTRACT FROM AUTHOR]

The potential energy surface (PES) for NH+NO has been characterized using complete active space self-consistent field (CASSCF) gradient calculations to determine the stationary point geometries and frequencies followed by CASSCF/internally contracted configuration interaction (CCI) calculations to refine the energetics. The present results are in qualitative accord with the BAC-MP4 calculations of Melius and Binkley, but there are differences as large as 8 kcal/mol in the detailed energetics. Addition of NH to NO on a 2A’ surface, which correlates with N2+OH or H+N2O products, involves barriers of 3.2 kcal/mol (trans) and 6.3 kcal/mol (cis). Experimental evidence for these barriers is found in the work of Böhmer et al. The 2A‘ surface has no barrier to addition, but does not correlate with ground state products. Surface crossings between the barrierless 2A‘ surface and the 2A’ surface may be important. Production of N2+OH products is predicted to occur via a planar saddle point of 2A’ symmetry. This is in accord with the preferential formation of Π(A’) Λ doublet levels of OH in the experiments of Patel-Misra and Dagdigian. Addition of NH 1Δ to NO is found to occur on an excited state surface and is predicted to lead to N2O product as observed by Yamasaki et al. [ABSTRACT FROM AUTHOR]

In a previous paper in this series [J. Chem. Phys. 93, 2384 (1990)] a global potential energy surface (PES) was presented for H+N2→HN2. In this paper, we report additional calculations which characterize PES’s for the excited 2A‘ state, for a bound 2 2A’ state, for HN+2, and for the Rydberg states associated with HN+2. It is anticipated that these excited state PES’s will be important in interpreting and designing experiments to characterize the ground state HN2 species via neutralized ion beam techniques. [ABSTRACT FROM AUTHOR]

Recent ab initio calculations have focused on the minimum energy path region of this surface [J. Chem. Phys. 88, 6273 (1988), Paper I] and on the saddle point region for H atom exchange via a T-shaped HO2 complex (J. Chem. Phys. 91, 2373 (1989), Paper II). In this paper, the results of additional calculations are discussed that, combined with the previously reported results, provide a global representation of the potential energy surface for this reaction. Complete active space SCF/externally contracted configuration interaction calculations (CASSCF/CCI) were carried out using the same wave function employed in II. The new calculations presented here characterize the potential energy surface for a variety of H atom approach angles, ranging from perpendicular to collinear (measured with respect to the O2 bond) and for a variety of H atom to O2 center-of-mass distances. Additionally, a new collinear exchange saddle point is reported. Using the ab initio results from these new calculations, optimal geometries, and harmonic frequencies based on local polynomial representations of the potential energy surface are reported along the constrained energy minimum path, while anharmonic frequencies are calculated at both the O2 and OH asymptotes and at the HO2 intermediate. [ABSTRACT FROM AUTHOR]

We have performed extensive ab initio calculations on the ground state potential energy surface of H2+H2O, using a large contracted Gaussian basis set and a high level of correlation treatment. An analytical representation of the potential energy surface was then obtained which reproduces the calculated energies with an overall root-mean-square error of only 0.64 mEh. The analytic representation explicitly includes all nine internal degrees of freedom and is also well behaved as the H2 dissociates; it thus can be used to study collision-induced dissociation or recombination of H2. The strategy used to minimize the number of energy calculations is discussed as well as other advantages of the present method for determining the analytical representation. [ABSTRACT FROM AUTHOR]

The reactant, product, and saddle point regions of the 5Π and 3Π potential energy surfaces for the reaction NH+O→N+OH have been characterized using complete active space self consistent field/externally contracted configuration interaction (CASSCF/CCI) calculations with large atomic natural orbital (ANO) basis sets. The computed barrier heights are 5.6 and 11.7 kcal/mol on the 5Π and 3Π surfaces, respectively. Transition state theory with an Eckart tunneling correction is used to estimate the rate constant on the 5Π surface. [ABSTRACT FROM AUTHOR]

A previous calculation for H+N2 [Walch, Duchovic, and Rohlfing, J. Chem. Phys. 90, 3230 (1989)] focused on the minimum energy path (MEP) region of the potential energy surface and on estimates of the lifetime of the HN2 species. In this paper, we report energies computed at geometries selected to permit a global representation of the potential energy surface (PES). As in the previous work, the calculations were performed using the complete active space self-consistent field/externally contracted configuration interaction (CASSCF/CCI) method. The surface was characterized using the same basis set as in the previous paper except that an improved contraction of the H s basis is used. Calculations with a larger basis set were carried out along an approximate MEP obtained with the smaller basis set. The new PES exhibits a sharp curvature, which was not present in the previous calculations, and has a slightly narrower and smaller barrier to dissociation. Saddle points for H atom exchange via collinear and T-shaped HN2 complexes are also reported. [ABSTRACT FROM AUTHOR]

The results of complete active space SCF/multireference contracted CI(CASSCF/CCI) calculations with large atomic natural orbital (ANO) basis sets are presented for the H + NO region of the lowest three potential surfaces of HNO (1A’, 3A‘, and 1A‘). The calculations focus on the minimum energy path for H atom addition to the N end of NO and on the equilibrium geometry region of HNO and HON. [ABSTRACT FROM AUTHOR]

The results of CASSCF/multireference contracted CI calculations with large atomic natural orbital (ANO) basis sets are presented for selected regions of the N2H2 potential surface. The calculations focused on the addition of a H atom to HN2. This process involves four channels. Approach of the H atom along the HN bond leads to N2+H2 with no barrier (H abstraction), while approach of the H atom to the other three positions leads to trans-HNNH, cis-HNNH, and NNH2, which are stable minima. These calculations provide the most accurate energetics for these three stable species obtained to date. Combining the results of these calculations with previous results for HN2 provides accurate thermochemical data on these important molecules. [ABSTRACT FROM AUTHOR]

The minimum energy path (MEP) for the addition of a hydrogen atom to N2 is characterized in CASSCF/externally contracted CI calculations using a [4s3p2d1f/3s2p1d] ANO basis set, with additional single point calculations at the stationary points of the potential energy surface using a [5s4p3d2f/4s3p2s] ANO basis set. These calculations represent the most extensive set of ab initio calculations on HN2 completed to date, yielding a zero-point corrected barrier for HN2 dissociation of ≊8.5 kcal mol-1. The lifetime of the HN2 species is estimated from the calculated geometries and energetics using both conventional transition state theory and a method which utilizes an Eckart barrier to compute one-dimensional quantum mechanical tunneling effects. This study concludes that the lifetime of the HN2 species is very short, greatly limiting its role in combustion processes. [ABSTRACT FROM AUTHOR]

Several dynamics studies have indicated that the product isotopic distributions and vibrational energy distributions in the reaction O(1D)+H2(HD) are very sensitive to the long range interaction of the reactants. In this paper we report an improved calculation of the reactant region of the potential energy surface. In agreement with previous work we find no barrier to edge-on insertion (1A’ ground state water surface), but find a smaller (<0.2 kcal/mol) barrier to collinear addition (1Σ+ surface) than in previous work. The long range potential obtained in the present work most closely resembles the SL3 surface. [ABSTRACT FROM AUTHOR]

The potential energy surface for the reaction H+O2→HO[ATOTHER]@B|[/ATOTHER]2 →HO+O has been characterized in the vicinity of the minimum energy path using CASSCF/contracted CI calculations with a basis set which is triple zeta valence quality plus three sets of polarization functions. CASSCF/CI calculations were carried out along the CCI minimum energy path. The latter calculation shows essentially no barrier for addition of an H atom to O2, in agreement with predictions made in earlier studies. The potential surface for recombination of OH and O is complicated by a crossing, at rOO approx. 5.5a0, between the surface for electrostatic (OH dipole–O quadrupole) interaction and that for the formation of an O–O chemical bond. This surface crossing results in a small (approx. 0.5 kcal/mol) barrier. [ABSTRACT FROM AUTHOR]

The F+H2→HF+H potential energy hypersurface has been studied in the saddle-point and entrance channel regions. Using a large [5s 5p 3d 2f 1g/4s 3p 2d] atomic natural orbital basis set, we obtain a classical barrier height of 1.86 kcal/mol at the CASSCF/multireference CI level (MRCI) after correcting for basis set superposition error and including a Davidson correction (+Q) for higher excitations. Based upon an analysis of the computed results, the true classical barrier is estimated to be about 1.4 kcal/mol. We also compute the location of the bottleneck on the lowest vibrationally adiabatic potential curve, and determine the translational energy threshold from a one-dimensional tunneling calculation. Using the difference between the calculated and experimental threshold to adjust the classical barrier height on the computed surface yields a classical barrier in the range of 1.0–1.5 kcal/mol. Combining the results of our direct estimates of the classical barrier height with the empirical values obtained from our approximate calculations of the dynamical threshold, we predict that the true classical barrier height is 1.4±0.4 kcal/mol. Arguments are presented in favor of including the relatively large (approx. 1 kcal/mol)+Q correction obtained when nine electrons are correlated at the CASSCF/MRCI level. [ABSTRACT FROM AUTHOR]

Complete active space SCF/contracted CI (CASSCF/CCI) calculations, using large Gaussian basis sets, are presented for selected portions of the potential surfaces for the reactions in the Zeldovich mechanism for the conversion of N2 to NO. The N+O2 reaction is exoergic by 32 kcal/mol and is computed to have an early barrier of 10.2 kcal/mol for the 2A’ surface and 18.0 kcal/mol for the 4A’ surface. The O+N2 reaction is endoergic by 75 kcal/mol. The 3A‘ surface is calculated to have a late barrier of 0.5 kcal/mol, while the 3A’ surface is calculated to have a late barrier of 14.4 kcal/mol relative to NO+N. [ABSTRACT FROM AUTHOR]

The convergence of the barrier height for the O+H2→OH+H reaction has been studied as a function of the size of the active space and basis set completeness. The barrier height is rapidly convergent with respect to expansion of the active space. Addition of 2p→2p’ correlation terms to the active space lowers the barrier to the O+H2 reaction by about 2.0 kcal/mol, but addition of 3d and other terms has little additional effect. Multireference singles and doubles contracted CI plus Davidson’s correction calculations using a [5s5p3d2f1g/4s3p2d1f] basis set with a 5σ2π active space lead to a barrier height of 12.7 kcal/mol. Including an estimate of the CI contraction error and basis set superposition error leads to 12.4 kcal/mol as the best estimate of the barrier height. [ABSTRACT FROM AUTHOR]

SCF/CCI calculations are presented for selected portions of the potential surfaces for six low-lying states of Ni3. The calculations use the effective core potentials developed by Hay and Wadt. For near equilateral triangle geometries, four states were studied, all of which arise from three Ni atoms in the 4s13d9 state. The 4s electron configuration here is 4sa’214se1, which leads to Jahn–Teller distortion analogous to Cu3. All of these states are within 0.04 eV of each other. For linear geometries two states were studied. The first linear state, which arises from three Ni atoms in the 4s13d9 state, is 0.16 eV higher than the corresponding near equilateral triangle state. The second linear state, which has one 4s23d8 center atom and the other two atoms in the 4s13d9 state, is estimated to be nearly degenerate (within 0.01 eV) with the near equilateral triangle structures. [ABSTRACT FROM AUTHOR]

Selected portions of the ground state potential energy surfaces of the Cu3, Ag3, AgCu2, and AuCu2 trimers are studied at the single-reference singles plus doubles configuration interaction and coupled pair functional levels correlating 33 electrons. The calculations use the effective core potentials (ECP) of Hay and Wadt to replace the deep core levels. The Cu3 and Ag3 molecules are found to have 2B2 obtuse-angled ground states, with low-lying 2A1 acute-angled excited states. The AgCu2 and AuCu2 molecules have 2A1 acute-angled ground states. The Cu3 molecule is found to have a smaller 3d population than Ag3, which is probably a consequence of the larger (n+1)s1nd10 → (n+1)s2nd9 excitation energy for Ag as compared to Cu. We also find that Cu3 has a smaller s electron density in the open-shell orbital than does Ag3, in agreement with recent ESR experiments. Calculations are also carried out for Ag2, AgCu, and Au2. The Ag2 calculations indicate that the estimated re value for Ag2, which is based on comparison to Cu2 and Au2, is too large. [ABSTRACT FROM AUTHOR]

The results of an SCF/SDCI treatment are presented for selected portions of the ground state potential energy surface for the Cu3 molecule. For equilateral triangle geometries (D3h) the lowest state is 2E’ arising from 4sa’214se’1. The 2E’ state exhibits strong Jahn–Teller distortion, leading to 2A1 (acute angle) and 2B2 (obtuse angle) minima in C2v symmetry. Here the 2B2 minimum is a true minimum on the surface while the 2A1 minimum is a saddle point or very shallow secondary minimum connecting adjacent 2B2 minima. This strong Jahn–Teller distortion is consistent with the observed ground state vibrational levels in the gas phase spectroscopic studies of Morse, Hopkins, Langridge-Smith, and Smalley and in the matrix Raman studies of Moskovits. The 2B2 minimum is also consistent with the observed ESR spectrum of Cu3 in a matrix, which has been interpreted as an obtuse angle structure with most of the spin density on the end Cu atoms. A linear 2Σ+u state is found to be 0.26 eV higher. Two possible candidates have been found for the upper state in the spectrum of Morse et al. (i) a 3s Rydberg state of 2A1 symmetry and (ii) a 3d→4s state of 2E‘ symmetry. Both of these states are consistent with the observed selection rules. The 2E‘ state would be expected to be weakly Jahn–Teller distorted in agreement with the fit to the upper state levels by Morse et al. and by Thompson, Truhlar, and Mead. [ABSTRACT FROM AUTHOR]

CASSCF/CCI calculations are presented for the low-lying states of Sc3 and Sc+3 and SCF/CI calculations are presented for the 1A’1 state of Ca3 arising from three ground state (4s2) Ca atoms, all for equilateral triangle geometries. The calculations use effective core potentials, developed by Hay and Wadt, which replace the Ne core but include the 3s and 3p core levels along with the valence electrons in the calculations. The bonding in Ca3 arises by 4s→4p promotion and leads to a well depth of about 0.5 eV for R(Ca–Ca)=7.5a0. For Sc3 the 4s bonding is similar to that in Ca3, but the 3d electrons are also strongly bonding leading to a 2A’2 ground state with a well depth of about 1.0 eV and R(Sc–Sc)=5.75a0. The good 3d bonding orbitals (bonding between all three atoms) are 3da″2 derived from atomic 3dπ″ and 3da1 derived from atomic 3dσ, while 3dπ’ atomic orbitals lead to 3de’ orbitals which are bonding between pairs of atoms, and the 3dδ’ and 3dδ″ derived levels are nonbonding. (Here the atomic symmetry is given with respect to an axis connecting the atom to the center of the molecule.) Based on the Sc3 calculations and preliminary calculations on Ti+3 , the bonding in V+3 and Cr+3 is also discussed. [ABSTRACT FROM AUTHOR]

The results of CASSCF/multireference contracted CI calculations with large ANO basis sets are presented for the exchange region of the HO2 potential energy surface. The saddle point for H atom exchange is ≊13 kcal/mol below the energy of H+O2; therefore, this region of the surface will be accessible during H+O2 recombination and metathesis reactions. [ABSTRACT FROM AUTHOR]

Focuses on a theoretical study of the exited states of silver. Methodology; Potential surface parameters for silver; Details on the Rydberg orbitals.

We have studied the electron affinities and ionization potentials for models of DNA. We conclude that removing an electron is most favorable on the PO4− groups; whereas, addition of an electron is most favorable on a nucleic acid. The computed electron affinities are 1.9, 4.8, 5.7, and 7.9 kcal/mol for adenine, cytosine, guanine, and thymine, respectively. Our best estimate of the IP for hydrated DNA is 4.5 eV and for hydrated DNA plus Na+ counter ions is 5.6 eV. [Copyright &y& Elsevier]

We have studied the reaction of N and O atoms with a (9,0) carbon nanotube using clusters and the ONIOM method with the B3LYP/6-31G* level of theory. For both N and O atoms in their ground state, addition to the CNT requires a surface crossing and presumably involves a barrier, while we show that addition of the excited state of N(2D) has no barrier, and we believe this also to be true for O(1D). Both N and O atoms bond in a bridged structure and have bond strengths of 25.0 and 67.9 kcal/mol (estimated), respectively, with respect to CNT plus the ground state of the N or O atom. [Copyright &y& Elsevier]

We have studied the bonding of N2 to a (9, 0) carbon nanotube using the MP2 and ONIOM methods with extended basis sets. We find a weak dispersion type bond with the N2 ∼3.3 A˚ away from the surface. Our best estimate of the binding energy, including the effect of expanding the high accuracy piece from ethylene to cluster A, is 1.88 kcal/mol for bonding in a twofold site and 1.97 kcal/mol for bonding in a sixfold site. We find N2 is bound more strongly to a graphitic surface by ∼0.3 kcal/mol. [Copyright &y& Elsevier]

The barrier heights (including zero-point effects) for H+CH2O→CH3O and CH3O→CH2OH have been computed using complete active space self-consistent field (CASSCF)/gradient calculations to define the stationary point geometries and harmonic frequencies and internally contracted configuration-interaction (CCI) to refine the energetics. The computed barrier heights are 5.6 and 30.1 kcal/mol, respectively. The former barrier height compares favorably to an experimental activation energy of 5.2 kcal/mol. [ABSTRACT FROM AUTHOR]

A calculated dipole moment of 2.39 D is reported for the X 2Δ state of NiH (based on extensive CASSCF/CI calculations). The computed dipole moment is found to be a sensitive function of the 3d population of the wave function; thus, the excellent agreement with the experimental dipole moment, 2.4±0.1 D, provides strong confirmation of the mixed state character of NiH (computed 3d population of 8.66 electrons). [ABSTRACT FROM AUTHOR]

Reports that in the article 'Theoretical characterization of the potential energy surface for H+O[sub2]...HO[sub2]...OH+O. IIII. Computed points to define a global potential energy surface,' from the Journal of Chemical Physics volume 94, p. 7068, the CASSCF/CCI calculations described in the paper were incorrectly identified as [4s3p2d1f/3s*2p] instead of [4s3p2d1f/3s*2p1d].

Presents a correction to the article 'Theoretical Characterization of Selected Regions of the Ground State Potential Surface of N[sub2]H[sub2].

Presents a correction to the 13 of the energies quoted in Table VII of an article dealing with theoretical characterization of the minimum energy path.

We present global ab initio potential energy surfaces for the three lowest energy 1A’, 3A‘, and 1A‘ surfaces of HNO. These surfaces are the lowest three states of the HNO and HON molecules, and they correlate to the ground electronic states of H+NO and O+NH. In addition, the 3A‘ surface correlates to the ground state of N+OH. The surfaces are based on approximately 800 ab initio calculations that were done using an internally contracted multireference configuration interaction calculation with a large basis set. The ab initio points were fit to a combination of Morse and spline functions in each of the three possible Jacobi coordinates, and the resulting splines were smoothly switched together, and combined with other functions to yield globally defined potentials. Properties of the HNO and HON minima and dissociation energies on these potentials are in good agreement with previous high quality calculations. The N+OH and O+NH reactions are found to have no barriers to formation of HON or HNO, respectively. Isomerization of HON to HNO involves barriers that are higher than the HON dissociation barrier on the singlet surfaces but not on the triplet surface. © 1995 American Institute of Physics. [ABSTRACT FROM AUTHOR]

We present a quasiclassical trajectory study of the collisions which occur on the 1A’, 3A‘, and 1A‘ potential surfaces of HNO using recently developed global potential surfaces that were derived from ab initio calculations. Each of these surfaces was assumed to be uncoupled from the other surfaces for the purposes of the calculations, and the appropriate statistical average of electronic states for each process was calculated. For N+OH and O+NH, we specifically studied reactive collisions which give H+NO as products, and we also studied the production of N+OH from O+NH. Overall rate constants calculated for either N+OH or O+NH are in good agreement with most experiments, and in addition, the product NO vibrational distribution from the N+OH reaction is in good correspondence with recent measurements, revealing modest excitation that is close to what would be expected from a statistical distribution. For O+NH, the calculated NO vibrational distributions are much hotter than statistical, in apparent disagreement with recent measurements. However, a careful analysis of limitations on the measurements due to spectral interference and to collisional relaxation indicates that it is not possible to say if theory and experiment are at odds for this reaction. We find a significant cross section for O+NH→N+OH on the 3A‘ surface (roughly 5% of the total reactive cross section, independent of energy), and this leads to rate constants at low temperatures that are orders of magnitude higher at low temperature than estimates made earlier based on H atom abstraction on the 5Π surface.The mechanism of this reaction is found to involve three steps: addition to form HNO, isomerization to HON, and dissociation to produce N+OH. We have also studied nonreactive vibrational and rotational excitation in H+NO collisions, and we obtain distributions that are somewhat closer to experiment than obtained in previous theoretical studies, although... [ABSTRACT FROM AUTHOR]

The energies for the interaction of a hydrogen atom with a nitrogen molecule have been calculated for large separation distances using a complete-active-space self-consistent-field/externally contracted configuration interaction method. H–N2 transport cross sections and collision integrals have been calculated using sudden approximations and a semiclassical description of the scattering. The values of these quantities are found to be close to the corresponding values determined from the average (isotropic) potential energy. The collision integrals are applied to determine diffusion and viscosity coefficients; the theoretical diffusion agrees well with the measured data available from experiments at low temperatures. [ABSTRACT FROM AUTHOR]

We present a theoretical study of the bond dissociation energies (D0) for H2O and CH3OH. The C–H and O–H bond energies are computed accurately with the modified coupled-pair functional (MCPF) method using a large basis set. For these bonds, an accuracy of ±2 kcal/mol is achieved, which is consistent with the C–H and C–C single bond energies of other molecules. The C–O bond is much more difficult to compute accurately because it requires higher levels of correlation treatment and more extensive one-particle basis sets. [ABSTRACT FROM AUTHOR]

In this paper values of the unimolecular decay lifetimes of several vibrational states of HN2 based on an accurate coupled channel dynamics study using a global analytical potential surface that was derived from previously reported ab initio calculations are reported. The surface was developed by fitting the N–N stretch part to a Morse function, with parameters that are represented by a two-dimensional spline function in terms of the H to N2 center of mass distance and approach angle. This surface reproduces the ab initio points with a root mean square error of 0.08 kcal/mol for energies below 20 kcal/mol. Modifications to the potential that describe the effect of improving the basis set in the ab initio calculations are also provided. Converged coupled channel calculations have been done for the ground rotational state of HN2 to determine lifetimes of the lowest ten vibrational states. We find that only the ground vibrational state (000) and first excited bend (001) have lifetimes longer than 1 ps. The best estimates of the lifetimes of these states are 3×10-9 and 2×10-10 s, respectively. Variation of these results with quality of the ab initio calculations is less than or equal to a factor of 5. [ABSTRACT FROM AUTHOR]

An ab initio study that includes relativistic effects via first-order perturbation theory and correlation effects using the coupled pair functional formalism is presented for the ground state surface of Cu3. An analogous calculation is presented for the 1Σ+g ground state of Cu2 for calibration. The ground state of Cu3 is found to be a 2B2 state corresponding to a Jahn–Teller distortion (R=4.396 a0, θ=64.5°) of a 2E’ equilateral triangle geometry. This structure is found to lie 59 cm-1 below the 2A1C2v geometry and 280 cm-1 below the D3h equilateral geometry in good agreement with the pseudorotation barrier and Jahn–Teller stabilization energy deduced by Truhlar and Thompson from analysis of the fluorescence spectrum of Rohlfing and Valentini. [ABSTRACT FROM AUTHOR]

We present the results of an ab initio quantum-chemical investigation of the structure of the anion O[sub 4][sup -] and potential energy surfaces for several electronic states. In addition to ground-state vibrational frequencies, which are in good agreement with other calculations and with matrix isolation experiments, we have investigated both photodissociation of O[sub 4][sup -] into various states of O[sub 2] and O[sub 2][sup -] and dissociative photodetachment leading to a free electron and various states of two O[sub 2] molecules. Our surfaces lead to predictions for both photodissociation and photodetachment processes that are in excellent agreement with experiment. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]

We present a quasiclassical trajectory study of the NH+NO reaction using a global potential energy surface that is capable of describing branching to the H+N2O and OH+N2 products after initial formation of a HNNO intermediate complex. The surface is based on a many-body expansion wherein fragment potentials for the species N2H, HNO, and N2O are incorporated, using either previously developed potentials, or in the case of N2O, a newly developed potential. The three-body parts of these fragment potentials are damped in the four-body region to provide a zeroth order four-body surface, and then additional four-body terms and mapping transformations are applied to make the final four-body potential match the results of ab initio calculations for eight important HNNO stationary points (minima and saddle points) and for several reaction paths. In addition to this ‘‘best fit’’ surface (surface I), a second surface (surface II) is developed in which the ordering of the saddle points leading to formation of H+N2O and OH+N2 is reversed, and the energy release during 1,3 hydrogen migration is modified so that the N–N stretch experiences smaller distortions from N2 equilibrium during the reaction leading to OH+N2. Quasiclassical trajectory results on surface I show generally good correspondence with experiment, with a branching fraction of 13±3% for the formation of OH+N2 at 300 K, and relatively low OH and N2 vibration/rotation excitation. The results on surface II are similar with respect to both branching and energy partitioning, indicating relatively weak sensitivity of the results of key features of the surface. © 1995 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The spectroscopic parameters and separations between the three low-lying X 3Πu, A 3Σ-g, and a 1Σ+g states of Al2 are studied as a function of both the one-particle and n-particle basis sets. Approximate correlation treatments are calibrated against full CI calculations correlating the six valence electrons in a double-zeta plus two d-function basis set. Since the CASSCF/MRCI 3Πu–3Σ-g separation is in excellent agreement with the FCI value, the MRCI calculations were carried out in an extended (20s13p6d4f)/[6s5p3d2f] Gaussian basis. Including a small correction for relativistic effects, our best estimate is that the 3Σ-g state lies 174 cm-1 above the 3Πu ground state. The 1Σ+g state lies at least 2000 cm-1 higher in energy. At the CPF level, inclusion of 2s and 2p correlation has little effect on De, reduces Te by only 26 cm-1, and shortens the bond lengths by about 0.02 a0. Further strong support for a 3Πu ground state comes from the experimental absorption spectra, since both observed transitions can be convincingly assigned as 3Πu→3Πg. The (2)3Πg state is observed to be sensitive to the level of correlation treatment, and to have its minimum shifted to shorter r values, such that the strongest experimental absorption peak probably corresponds to the 0→2 transition. [ABSTRACT FROM AUTHOR]

Theoretical studies of selected diatomic and triatomic molecules containing copper and the simple metals Li, Na, K, Be, and Al are presented, with emphasis on elucidating the nature of the bonding in mixed transition metal–simple metal systems. Large Gaussian basis sets are used in the diatomic calculations, and are used to calibrate the triatomic calculations, in which somewhat smaller Gaussian basis are employed. Electron correlation is incorporated using both the single-reference singles plus doubles configuration interaction and coupled pair functional methods. We find that alkali atoms form very polar σ bonds with copper, and that the ionicity increases with the inclusion of higher excitations, because they improve the electron affinity of copper, which in turn allows a larger negative charge on copper. Aluminum is found to form stronger bonds than beryllium, since it does not have to undergo sp hybridization. Some of the trimers bond by forming three-center three-electron bonds. These multicenter bonds are quite strong even when compared to the two-electron bonds in the dimers or to other bonding mechanisms in the trimers. [ABSTRACT FROM AUTHOR]

Spectroscopic parameters (De, re, μ) are determined for the first-row transition metal hydrides using better than DZP basis sets at the modified coupled pair functional (MCPF) level. Extensive comparisons between MCPF and complete-active space self-consistent field (CASSCF)/MRCI calculations with natural orbital iterations, and studies with more extensive basis sets, show this level of treatment to supply an accurate and cost-effective treatment of these systems. For the transition metal hydrides, the bonding can arise from either the 3dn4s2 or 3dn+14s1 atomic asymptotes, or a mixture of both. Since the dipole moment arising from these two bonding mechanisms is very different, the dipole moment is found to be directly related to the 3d population. Thus, the magnitude of the dipole moments provide a sensitive test of the wave function, and gives insight into the nature of the bonding. [ABSTRACT FROM AUTHOR]