Publications (18)60.59 Total impact
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ABSTRACT: Acylperoxy radicals [RC(O)OO·] play an important catalytic role in many atmospheric and combustion reactions. Accordingly, the prototypical formylperoxy radical [HC(O)OO·] is characterized here using highlevel ab initio coupledcluster theory. Important experiments have been carried out on this system, but have not comprehensively described the properties of even the ground electronic state. We report cis and trans geometries for the ground (X∼ 2A″) and first excited (Ã 2A′) state equilibrium conformers and the torsional saddle point on the ground state surface at the CCSD(T)/ANO2 level of theory. Relative energies of these ground and excitedstate stationary points were obtained using coupled cluster theory with up to perturbative quadruple excitations, extrapolated from the sextuple zeta basis set to the complete basis set limit. These methods predict conformational energy differences ΔE(transX∼ → cisX∼) = 2.35 kcal mol1 and ΔE(transÃ → cisÃ) = 2.95 kcal mol1. On the X∼ surface, the transition state for the conformational change lies 8.42 kcal mol1 above the trans ground state minima. The adiabatic electronic excitation energies from the ground state isomers are predicted to be 18.17 ± 0.10 (trans) and 13.03 ± 0.10 kcal mol1 (cis). The former is in excellent agreement with the 18.1 ± 1.4 kcal mol1 transition found by Lineberger and coworkers. Additionally, transition properties between the X∼ 2A″ and Ã 2A′ states are reported for the first time, using the equation of motion (EOM)CCSD method, which predicts lifetimes for transÃ 2A′ HC(O)OO· of 5.4 ms and cisÃ 2A′ HC(O)OO· of 20.5 ms. Secondorder vibrational perturbation theory was utilized to determine the fundamental frequencies at the CCSD(T)/ANO2 level of theory for the cis and trans conformers of the X∼ and Ã states and five ground state isotopologues of both conformers: H13C(O)OO·, HC(18O)OO·, HC(O)18O18O·, DC(O)OO·, and DC(O)18O18O·. This study provides high accuracy predictions of vibrational frequencies, helping to resolve large uncertainties and disagreements in the experimental values. Furthermore, we characterize experimentally unassigned vibrational frequencies and transition properties.  [Show abstract] [Hide abstract]
ABSTRACT: The ethyl radical has been isolated and spectroscopically characterized in (4)He nanodroplets. The band origins of the five CH stretch fundamentals are shifted by < 2 cm(1) from those reported for the gas phase species [S. Davis, D. Uy, and D. J. Nesbitt, J. Chem. Phys. 112, 1823 (2000); T. Häber, A. C. Blair, D. J. Nesbitt, and M. D. Schuder, J. Chem. Phys. 124, 054316 (2006)]. The symmetric CH2 stretching band (v1) is rotationally resolved, revealing nuclear spin statistical weights predicted by G12 permutationinversion group theory. A permanent electric dipole moment of 0.28 (2) D is obtained via the Stark spectrum of the v1 band. The four other CH stretch fundamental bands are significantly broadened in He droplets and lack rotational fine structure. This broadening is attributed to symmetry dependent vibrationtovibration relaxation facilitated by the He droplet environment. In addition to the five fundamentals, three a1' overtone∕combination bands are observed, and each of these have resolved rotational substructure. These are assigned to the 2v12, v4 + v6, and 2v6 bands through comparisons to anharmonic frequency computations at the CCSD(T)∕ccpVTZ level of theory.  [Show abstract] [Hide abstract]
ABSTRACT: The PSI4 program is a new approach to modern quantum chemistry, encompassing Hartree–Fock and densityfunctional theory to configuration interaction and coupled cluster. The program is written entirely in C++ and relies on a new infrastructure that has been designed to permit highefficiency computations of both standard and emerging electronic structure methods on conventional and highperformance parallel computer architectures. PSI4 offers flexible user input built on the Python scripting language that enables both new and experienced users to make full use of the program's capabilities, and even to implement new functionality with moderate effort. To maximize its impact and usefulness, PSI4 is available through an opensource license to the entire scientific community.  [Show abstract] [Hide abstract]
ABSTRACT: The lowestlying electronic singlet and triplet potential energy surfaces (PES) for the HNONOH system have been investigated employing high level ab initio quantum chemical methods. The reaction energies and barriers have been predicted for two isomerization and four dissociation reactions. Total energies are extrapolated to the complete basis set limit applying focal point analyses. Anharmonic zeropoint vibrational energies, diagonal BornOppenheimer corrections, relativistic effects, and core correlation corrections are also taken into account. On the singlet PES, the (1)HNO → (1)NOH endothermicity including all corrections is predicted to be 42.23 ± 0.2 kcal mol(1). For the barrierless decomposition of (1)HNO to H + NO, the dissociation energy is estimated to be 47.48 ± 0.2 kcal mol(1). For (1)NOH → H + NO, the reaction endothermicity and barrier are 5.25 ± 0.2 and 7.88 ± 0.2 kcal mol(1). On the triplet PES the reaction energy and barrier including all corrections are predicted to be 7.73 ± 0.2 and 39.31 ± 0.2 kcal mol(1) for the isomerization reaction (3)HNO → (3)NOH. For the triplet dissociation reaction (to H + NO) the corresponding results are 29.03 ± 0.2 and 32.41 ± 0.2 kcal mol(1). Analogous results are 21.30 ± 0.2 and 33.67 ± 0.2 kcal mol(1) for the dissociation reaction of (3)NOH (to H + NO). Unimolecular rate constants for the isomerization and dissociation reactions were obtained utilizing kinetic modeling methods. The tunneling and kinetic isotope effects are also investigated for these reactions. The adiabatic singlettriplet energy splittings are predicted to be 18.45 ± 0.2 and 16.05 ± 0.2 kcal mol(1) for HNO and NOH, respectively. Kinetic analyses based on solution of simultaneous firstorder ordinarydifferential rate equations demonstrate that the singlet NOH molecule will be difficult to prepare at room temperature, while the triplet NOH molecule is viable with respect to isomerization and dissociation reactions up to 400 K. Hence, our theoretical findings clearly explain why (1)NOH has not yet been observed experimentally.  [Show abstract] [Hide abstract]
ABSTRACT: Symmetryadapted perturbation theory (SAPT) provides a means of probing the fundamental nature of intermolecular interactions. Loworders of SAPT (here, SAPT0) are especially attractive since they provide qualitative (sometimes quantitative) results while remaining tractable for large systems. The application of density fitting and Laplace transformation techniques to SAPT0 can significantly reduce the expense associated with these computations and make even larger systems accessible. We present new factorizations of the SAPT0 equations with densityfitted twoelectron integrals and the first application of Laplace transformations of energy denominators to SAPT. The improved scalability of the DFSAPT0 implementation allows it to be applied to systems with more than 200 atoms and 2800 basis functions. The Laplacetransformed energy denominators are compared to analogous partial Cholesky decompositions of the energy denominator tensor. Application of our new DFSAPT0 program to the intercalation of DNA by proflavine has allowed us to determine the nature of the proflavineDNA interaction. Overall, the proflavineDNA interaction contains important contributions from both electrostatics and dispersion. The energetics of the intercalator interaction are are dominated by the stacking interactions (twothirds of the total), but contain important contributions from the intercalatorbackbone interactions. It is hypothesized that the geometry of the complex will be determined by the interactions of the intercalator with the backbone, because by shifting toward one side of the backbone, the intercalator can form two long hydrogenbonding type interactions. The longrange interactions between the intercalator and the nextnearest base pairs appear to be negligible, justifying the use of truncated DNA models in computational studies of intercalation interaction energies.  [Show abstract] [Hide abstract]
ABSTRACT: The fundamentally important combustion reaction of vinyl radical with hydrogen has been studied in the laboratory by at least five experimental groups. Herein, the reaction C2H3 + H2 → C2H4 + H has been examined using focalpoint analysis. Molecular energies were determined from extrapolations to the complete basisset limit using correlationconsistent basis sets (ccpVTZ, ccpVQZ, and ccpV5Z) and coupledcluster theory with single and double excitations (CCSD), perturbative triples [CCSD(T)], full triples [CCSDT], and perturbative quadruples [CCSDT(Q)]. Reference geometries were optimized at the allelectron CCSD(T)/ccpCVQZ level. Computed energies were also corrected for relativistic effects and the Born–Oppenheimer approximation. The activation energy for hydrogen abstraction is predicted to be 9.65 kcal mol–1, and the overall reaction is predicted to be exothermic by 5.65 kcal mol–1. Natural resonance theory (NRT) analysis was performed to verify the reaction pathway and describe bondbreaking and bondforming events along the reaction coordinate.  [Show abstract] [Hide abstract]
ABSTRACT: Using a Lagrangianbased approach, we present a more elegant derivation of the equations necessary for the variational optimization of the molecular orbitals (MOs) for the coupledcluster doubles (CCD) method and secondorder MøllerPlesset perturbation theory (MP2). These orbitaloptimized theories are referred to as OOCCD and OOMP2 (or simply "OD" and "OMP2" for short), respectively. We also present an improved algorithm for orbital optimization in these methods. Explicit equations for response density matrices, the MO gradient, and the MO Hessian are reported both in spinorbital and closedshell spinadapted forms. The NewtonRaphson algorithm is used for the optimization procedure using the MO gradient and Hessian. Further, orbital stability analyses are also carried out at correlated levels. The OD and OMP2 approaches are compared with the standard MP2, CCD, CCSD, and CCSD(T) methods. All these methods are applied to H(2)O, three diatomics, and the O(4)(+) molecule. Results demonstrate that the CCSD and OD methods give nearly identical results for H(2)O and diatomics; however, in symmetrybreaking problems as exemplified by O(4)(+), the OD method provides better results for vibrational frequencies. The OD method has further advantages over CCSD: its analytic gradients are easier to compute since there is no need to solve the coupledperturbed equations for the orbital response, the computation of oneelectron properties are easier because there is no response contribution to the particle density matrices, the variational optimized orbitals can be readily extended to allow inactive orbitals, it avoids spurious secondorder poles in its response function, and its transition dipole moments are gauge invariant. The OMP2 has these same advantages over canonical MP2, making it promising for excited state properties via linear response theory. The quadratically convergent orbitaloptimization procedure converges quickly for OMP2, and provides molecular properties that are somewhat different than those of MP2 for most of the test cases considered (although they are similar for H(2)O). Bond lengths are somewhat longer, and vibrational frequencies somewhat smaller, for OMP2 compared to MP2. In the difficult case of O(4)(+), results for several vibrational frequencies are significantly improved in going from MP2 to OMP2.  [Show abstract] [Hide abstract]
ABSTRACT: The C 1s and N 1s nearedge xray absorption fine structure (NEXAFS) spectra of three prototype tetraphenyl porphyrin (TPP) molecules are discussed in the framework of a combined experimental and theoretical study. We employ timedependent densityfunctional theory (TDDFT) to compute the NEXAFS spectra of the open and closedshell metalloporphyrins CoTPP and ZnTPP as well as the freebase 2HTPP in realistic nonplanar conformations. Using Becke's wellknown halfandhalf hybrid functional, the computed core excitation spectra are mostly in good agreement with the experimental data in the lowenergy region below the appropriate ionization threshold. To make these calculations feasible, we apply a new, simple scheme based on TDDFT using a modified singleparticle input spectrum. This scheme is very easy to implement in standard codes and allows one to compute core excitation spectra at a similar cost as ordinary UV/vis spectra even for larger molecules. We employ these calculations for a detailed assignment of the NEXAFS spectra including subtle shifts in certain peaks of the N 1s spectra, which depend on the central coordination of the TPP ligand. We furthermore assign the observed NEXAFS resonances to the individual molecular subunits of the investigated TPP molecules.  [Show abstract] [Hide abstract]
ABSTRACT: Although never spectroscopically identified in the laboratory, hydrogenated nitrogen (HN(2)) is thought to be an important species in combustion chemistry. The classical barrier height (10.6+/0.2 kcal mol(1)) and exothermicity (3.6+/0.2 kcal mol(1)) for the HN(2)>N(2)+H reaction are predicted by high level ab initio quantum mechanical methods [up to CCSDT(Q)]. Total energies are extrapolated to the complete basis set limit applying the focal point analysis. Zeropoint vibrational energies are computed using fundamental (anharmonic) frequencies obtained from a quartic force field. Relativistic and diagonal BornOppenheimer corrections are also taken into account. The quantum mechanical barrier with these corrections is predicted to be 6.4+/0.2 kcal mol(1) and the reaction exothermicity to be 8.8+/0.2 kcal mol(1). The importance of these parameters for the thermal NO(x) decomposition (DeNO(x)) process is discussed. The unimolecular rate constant for dissociation of the HN(2) molecule and its lifetime are estimated by canonical transitionstate theory and RiceRamspergerKasselMarcus theory. The lifetime of the HN(2) molecule is here estimated to be 2.8x10(10) s at room temperature. Our result is in marginal agreement with the latest experimental kinetic modeling studies (tau=1.5x10(8) s), albeit consistent with the very rough experimental upper limit (tau<0.5 mus). For the dissociation reaction, kinetic isotope effects are investigated. Our analysis demonstrates that the DN(2) molecule has a longer lifetime than the HN(2) molecule. Thus, DN(2) might be more readily identified experimentally. The ionization potential of the HN(2) molecule is determined by analogous high level ab initio methods and focal point analysis. The adiabatic IP of HN(2) is predicted to be 8.19+/0.05 eV, in only fair agreement with the experimental upper limit of 7.92 eV deduced from sychrothonradiationbased photoionization mass spectrometry.  [Show abstract] [Hide abstract]
ABSTRACT: Since the discovery of ozone depletion, the doublet electronic states of the ozone radical cation have received much attention in experimental and theoretical investigations, while the lowlying quartet states have not. In the present research, viable pathways to the quartet states from the lowest three triplet states of ozone, (3)A(2), (3)B(2), and (3)B(1), and excitations from the (2)A(1) and (2)B(2) states of the ozone radical cation have been studied in detail. The potential energy surfaces, structural optimizations, and vibrational frequencies for several states of ozone and its radical cation have been thoroughly investigated using the complete active space selfconsistent field, unrestricted coupled cluster theory from a restricted openshell HartreeFock reference including all single and double excitations (UCCSD), UCCSD method with the effects of connected triple excitations included perturbatively, and unrestricted coupled cluster including all single, double, and triple excitations with the effects of connected quadruple excitations included perturbatively. These methods used Dunning's correlationconsistent polarized corevalence basis sets, ccpCVXZ (X = D, T, Q, and 5). The most feasible pathways (symmetry and spin allowed transitions) to the quartet states are (4)A(1)<(3)A(2), (4)A(2)<(3)A(2), (4)A(1)<(3)B(2), (4)A(2)<(3)B(1), (4)B(2)<(3)B(1), (4)A(2)<(1)A(1), (4)B(2)<(1)A(1), and (4)A(1)<(1)A(1) with vertical ionization potentials of 12.46, 12.85, 12.82, 12.46, 12.65, 13.43, 13.93, and 14.90 eV, respectively. 
Article: Vibrational energy levels for the electronic ground state of the diazocarbene (CNN) molecule
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ABSTRACT: The vibrational energy levels of diazocarbene (diazomethylene) in its electronic ground state, CNN, have been predicted using the variational method. The potential energy surfaces of CNN were determined by employing ab initio single reference coupled cluster with single and double excitations (CCSD), CCSD with perturbative triple excitations [CCSD(T)], multireference complete active space selfconsistentfield (CASSCF), and internally contracted multireference configuration interaction (ICMRCI) methods. The correlationconsistent polarised valence quadruple zeta (ccpVQZ) basis set was used. Four sets of vibrational energy levels determined from the four distinct analytical potential functions have been compared with the experimental values from the laserinduced fluorescence measurements of Wurfel et al. obtained in 1992. The CCSD, CCSD(T), and CASSCF potentials have not provided satisfactory agreement with the experimental observations. In this light, the importance of both nondynamic (static) and dynamic correlation effects in describing the ground state of CNN is emphasised. Our best theoretical fundamental frequencies at the ccpVQZ ICMRCI level of theory, ν1 = 1230, ν2 = 394, and ν3 = 1420 cm− 1, are in excellent agreement with the experimental values of ν1 = 1235, ν2 = 396, and ν3 = 1419 cm− 1, and the mean absolute deviation between the 23 calculated and experimental vibrational energy levels is only 7.4 cm− 1. It is shown that the previously suggested observation of the ν3 frequency at about 2847 cm− 1 was in fact the first overtone 2ν3.  [Show abstract] [Hide abstract]
ABSTRACT: The triplet electronic ground state potential energy surface of lithium nitroxide has been systematically investigated using convergent quantum mechanical methods. Equilibrium structures and physical properties for five stationary points (three minima and two transition states) have been determined employing highly correlated coupled cluster theory with four correlationconsistent polarizedvalence (ccpVXZ and augccpVXZ, X = T and Q) and two core correlationconsistent polarizedvalence (ccpCVXZ, X = T and Q) basis sets. The global minimum, roughly Lshaped LiON, is predicted to lie 6.5 kcal mol<sup>1</sup> below the linear LiON minimum and 2.4 kcal mol<sup>1</sup> below the linear LiON minimum. The barrier to isomerization from the global minimum to LiON was found to be 7.4 kcal mol<sup>1</sup> and with regard to LiNO 6.9 kcal mol<sup>1</sup>. The dissociation energies, D <sub>0</sub>, with respect to Li + NO, have been predicted for all minima and for the global minimum was found to be 34.9 kcal mol<sup>1</sup>.  [Show abstract] [Hide abstract]
ABSTRACT: Hypercoordinate boron is most unusual, leading to considerable theoretical and experimental research on the parent BH 5 molecule. The deprotonation energies of BH 5 and the related molecules AlH 5 and GaH 5 have been of particular interest. Here the energy differences for XH→XH4+H(X=BandAl) are computed to be 332.4 and 326.3 kcal mol 1, respectively, with an augccpVQZ basis set at the CCSD(T) level of theory. Vibrational frequencies for BH4 and AlH4 are also reported as 1098, 1210, 2263, and 2284 cm 1 and 760, 779, 1658, and 1745 cm 1, respectively, again at the CCSD(T) augccpVQZ level of theory. Comparisons with the valence isoelectronic GaH 5 molecule are made.  [Show abstract] [Hide abstract]
ABSTRACT: The singlet ground ((approximate)X(1)Sigma1+) and excited (1Sigma,1Delta) states of HCP and HPC have been systematically investigated using ab initio molecular electronic structure theory. For the ground state, geometries of the two linear stationary points have been optimized and physical properties have been predicted utilizing restricted selfconsistent field theory, coupled cluster theory with single and double excitations (CCSD), CCSD with perturbative triple corrections [CCSD(T)], and CCSD with partial iterative triple excitations (CCSDT3 and CC3). Physical properties computed for the global minimum ((approximate)X(1)Sigma+HCP) include harmonic vibrational frequencies with the ccpV5Z CCSD(T) method of omega1=3344 cm(1), omega2=689 cm(1), and omega3=1298 cm(1). Linear HPC, a stationary point of Hessian index 2, is predicted to lie 75.2 kcal mol(1) above the global minimum HCP. The dissociation energy D0[HCP((approximate)X(1)Sigma+)>H(2S)+CP(X2Sigma+)] of HCP is predicted to be 119.0 kcal mol(1), which is very close to the experimental lower limit of 119.1 kcal mol(1). Eight singlet excited states were examined and their physical properties were determined employing three equationofmotion coupled cluster methods (EOMCCSD, EOMCCSDT3, and EOMCC3). Four stationary points were located on the lowestlying excited state potential energy surface, 1Sigma >1A", with excitation energies Te of 101.4 kcal mol(1) (1A"HCP), 104.6 kcal mol(1)(1SigmaHCP), 122.3 kcal mol(1)(1A" HPC), and 171.6 kcal mol(1)(1SigmaHPC) at the ccpVQZ EOMCCSDT3 level of theory. The physical properties of the 1A" state with a predicted bond angle of 129.5 degrees compare well with the experimentally reported first singlet state ((approximate)A1A"). The excitation energy predicted for this excitation is T0=99.4 kcal mol(1) (34 800 cm(1),4.31 eV), in essentially perfect agreement with the experimental value of T0=99.3 kcal mol(1)(34 746 cm(1),4.308 eV). For the second lowestlying excited singlet surface, 1Delta>1A', four stationary points were found with Te values of 111.2 kcal mol(1) (2(1)A' HCP), 112.4 kcal mol(1) (1Delta HPC), 125.6 kcal mol(1)(2(1)A' HCP), and 177.8 kcal mol(1)(1Delta HPC). The predicted CP bond length and frequencies of the 2(1)A' state with a bond angle of 89.8 degrees (1.707 A, 666 and 979 cm(1)) compare reasonably well with those for the experimentally reported (approximate)C(1)A' state (1.69 A, 615 and 969 cm(1)). However, the excitation energy and bond angle do not agree well: theoretical values of 108.7 kcal mol(1) and 89.8 degrees versus experimental values of 115.1 kcal mol(1) and 113 degrees. of 115.1 kcal mol(1) and 113 degrees.  [Show abstract] [Hide abstract]
ABSTRACT: The vibrational–rotational energy levels of aluminum monohydroxide in its electronic ground state, X∼1A′ AlOH, have been predicted using the variational method. The potential energy surface of the X∼1A′ ground state of AlOH was determined employing the ab initio coupled cluster method with single, double, and perturbative triple excitations [CCSD(T)] and the correlationconsistent polarized valence quadruple zeta (ccpVQZ) basis set. Lowlying J=0 and J=1 vibrational levels are reported. These are analyzed in terms of the quasilinearity of the molecule. Coriolis effects are shown to be significant. We hope that our predictions will be of value in the future when assigning rovibrational transitions in spectroscopic studies.  [Show abstract] [Hide abstract]
ABSTRACT: The absorption spectrum of the ν2 fundamental band of the cisconformer of the transient molecule HOPO, namely the terminal PO stretching mode, has been detected and measured using diode laser spectroscopy. The molecule was generated in a discharge flow system containing hydrogen and white phosphorus vapour (P4) and a trace of oxygen. The spectrum has the appearance of an atype band of a near prolate asymmetric top. Above Ka=5 the spectrum is perturbed and transitions terminating on these higher Ka levels were excluded from the fit. The vibrational frequency and rotational constants derived from the unperturbed parts of the spectrum are compatible with new high precision ab initio calculations reported here. A combined fit of the ν2 band and the ν4 band data, measured earlier, was carried out. The ν2 band origin was determined to be 1258.539525(32)cm−1, approximately 5.5cm−1 higher than the matrix value. 
Article: Does GaH5 exist?
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ABSTRACT: The existence or nonexistence of GaH(5) has been widely discussed [N. M. Mitzel, Angew. Chem. Int. Ed. 42, 3856 (2003)]. Seven possible structures for gallium pentahydride have been systematically investigated using ab initio electronic structure theory. Structures and vibrational frequencies have been determined employing selfconsistent field, coupled cluster including all single and double excitations (CCSD), and CCSD with perturbative triples levels of theory, with at least three correlationconsistent polarizedvalence(ccpVXZ and augccpVXZ) type basis sets. The X (1)A(') state for GaH(5) is predicted to be weakly bound complex 1 between gallane and molecular hydrogen, with C(s) symmetry. The dissociation energy corresponding to GaH(5)>GaH(3)+H(2) is predicted to be D(e)=2.05 kcal mol(1). The HH stretching fundamental is predicted to be v=4060 cm(1), compared to the tentatively assigned experimental feature of Wang and Andrews [J. Phys. Chem. A 107, 11371 (2003)] at 4087 cm(1). A second C(s) structure 2 with nearly equal energy is predicted to be a transition state, corresponding to a 90 degrees rotation of the H(2) bond. Thus the rotation of the hydrogen molecule is essentially free. However, hydrogen scrambling through the C(2v) structure 3 seems unlikely, as the activation barrier for scrambling is at least 30 kcal mol(1) higher in energy than that for the dissociation of GaH(5) to GaH(3) and H(2). Two additional structures consisting of GaH(3) with a dihydrogen bond perpendicular to gallane (C(3v) structure 4) and an inplane dihydrogen bond [C(s)(III) structure 5] were also examined. A C(3v) symmetry secondorder saddle point has nearly the same energy as the GaH(3)+H(2) dissociation limit, while the C(s)(III) structure 5 is a transition structure to the C(3v) structure. The C(4v) structure 6 and the D(3h) structure 7 are much higher in energy than GaH(3)+H(2) by 88 and 103 kcal mol(1), respectively.  [Show abstract] [Hide abstract]
ABSTRACT: The singlet electronic ground state isomers, X̃ 1Σ g+ (AlOAl D ∞h) and X̃ 1Σ + (AlAlO C ∞ν), of dialuminum monoxide have been systematically investigated using ab initio electronic structure theory. The equilibrium structures and physical properties for the two molecules have been predicted employing selfconsistent field (SCF) configuration interaction with single and double excitations (CISD), multireference CISD (MRCISD), coupled cluster with single and double excitations (CCSD), CCSD with perturbative triples [CCSD(T)], CCSD with iterative partial triple excitations (CCSDT3 and CC3), and full triples (CCSDT) coupled cluster methods. Four correlation consistent polarized valence (ccpVXZ) type basis sets were used. The AlAlO system is rather challenging theoretically. The two isomers are confirmed to have linear structures at all levels of theory. The symmetric isomer AlOAl is predicted to lie 81.9 kcal mol 1 below the asymmetric isomer AlAlO at the ccpV(Q+d)Z CCSD(T) level of theory. The predicted harmonic vibrational frequencies for the X̃ 1Σ g+ AlOAl molecule, ω 1=517 cm 1, ω 2=95 cm 1, and ω 3= 1014 cm 1, are in good agreement with experimental values. The harmonic vibrational frequencies for the X̃ 1Σ + AlAlO structure, ω 1=1042 cm 1, ω 2= 73 cm 1, and ω 3=253 cm 1, presently have no experimental values with which to be compared. With the same methods the barrier heights for the isomerization AlOAl→AlAlO and AlAlO→AlOAl reactions were predicted to be 84.3 and 2.4 kcal mol 1, respectively. The dissociation energies D 0 for AlOAl (X̃ 1Σ g+) and AlAlO (X̃ 1Σ +) →AlO (X 2Σ +)+Al ( 2P) were determined to be 130.8 and 48.9 kcal mol 1, respectively. Thus, both symmetric AlOAl (X̃ 1Σ g+) and asymmetric AlAlO (X̃ 1Σ +) isomers are expected to be thermodynamically stable with respect to the dissociation into AlO (X 2Σ +) + Al ( 2P) and kinetically stable for the isomerization reaction (AlAlO→AlOAl) at sufficiently low temperatures.
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20052015

University of Georgia
 Center for Computational Chemistry
Атина, Georgia, United States
