ALBERT

Notes: By the accummulation of branching factors in diffusion quantum Monte Carlo (DQMC) and their use as statistical weights, instead of the standard deletion and replication of configurations, we can estimate the averages of (nondifferential) operators taken over the exact electron distribution. This requires only a trivial modification of existing DQMC codes. We illustrate our algorithm by computing ground-state properties for H2 and LiH which are related to the interelectron distance. We also estimate the dipole moment of LiH.

Notes: The heat of formation of NCO has been determined rigorously by state-of-the-art ab initio electronic structure methods, including Møller–Plesset perturbation theory from second through fifth order (MP2–MP5) and coupled-cluster and Brueckner methods incorporating various degrees of excitation [CCSD, CCSD(T), BD, BD(T), and BD(TQ)]. Five independent reactions were investigated to establish a consistent value for ΔHf,0(open circle)(NCO): (a) HNCO(X˜ 1A')→H(2S)+NCO(2Π), (b) HNCO(X˜ 1A')→H++NCO−, (c) N(4S)+CO→NCO(2Π), (d) HCN+O(3P)→H(2S)+NCO(2Π), and (e) NH(3Σ−)+CO→H(2S)+NCO(2Π). The one-particle basis sets employed in the study were comprised of as many as 377 contracted Gaussian functions and ranged in quality from [4s2p1d] to [14s9p6d4f] on the (C,N,O) atoms and from [2s1p] to [8s6p4d] on hydrogen. After the addition of bond additivity corrections evaluated from related reactions of precisely known thermochemistry, all five approaches were found to converge on the value ΔHf,0(open circle)(NCO)=31.4(5) kcal mol−1. Appurtenant refinements were obtained for the heat of formation of isocyanic acid, ΔHf,0(open circle)(HNCO)=−27.5(5) kcal mol−1, and hydrogen cyanide, ΔHf,0(open circle)(HCN)=31.9(5) kcal mol−1. The final proposals for ΔHf,0(open circle)(NCO) and ΔHf,0(open circle)(HNCO) resolve outstanding discrepancies with experiment and provide updates for thermochemical cycles of relevance to combustion chemistry.

Notes: The [FHCl]− molecular anion has been investigated in detail by means of state-of-the-art ab initio electronic structure methods, including restricted Hartree–Fock (RHF), Møller–Plesset perturbation theory (MP2–MP4), and coupled-cluster and Brueckner methods incorporating various degrees of excitation [CCSD, CCSD(T), BD, BD(T), and BD(TQ)]. The one-particle Gaussian basis sets ranged in quality from F[6s4p2d], Cl[10s7p2d], and H[4s2p] to F[18s13p6d4f], Cl[20s14p7d5f], and H[8s3p2d1f]. The first phase of the investigation focused on the prediction of thermochemical, spectroscopic, and bonding properties of [FHCl]− and the chemical interpretation thereof.The final proposals for the geometric structure and binding energy of the complex are re(H–F)=0.963±0.003 A(ring), Re(H–Cl)=1.925±0.015 A(ring), and D0(HF+Cl−)=21.8±0.4 kcal mol−1. A Morokuma decomposition of the ion-molecule bonding gave the following electrostatic (ES), polarization (PL), exchange repulsion (EX), dispersion (DISP), and charge-transfer plus higher-order mixing (CT+MIX) components of the vibrationless complexation energy: −27.3 (ES), −5.2 (PL), +18.3 (EX), −4.5 (DISP), and −5.0 (CT+MIX) kcal mol−1. The second phase of the work involved the construction of a CCSD global surface from 208 and 228 energy points for linear and bent conformations, respectively, these being fit to rms errors of only 3.9 and 9.3 cm−1, respectively, below 8000 cm−1. The surface was represented by a flexible analytic form which reproduces the quartic force field at equilibrium, exhibits the proper asymptotic properties, and is generally applicable to ion-molecule systems. The final phase of the study entailed the determination of converged J=0 and J=1 variational eigenstates of the [FHCl]− surface to near the HF+Cl− dissociation threshold by employing Jacobi coordinates and vibrational configuration interaction expansions in terms of natural modals.The fundamental vibrational frequencies given by the analysis were ν1=247, ν2=876, and ν3=2884 cm−1. The complete vibrational eigenspectrum was then analyzed in terms of several contemporary dynamical issues, including vibrational adiabaticity, anharmonic resonances, densities of high-lying states, and signatures of quantum ergodicity.

Notes: Characteristics of the ground electronic state of HNCO have been investigated theoretically in a series of eight ab initio analyses involving qualitative features of the electronic structure, the barrier to linearity, the NH(3Σ−)+CO fragmentation energy, the H–NCO bond dissociation energy, heats of formation of isomers of HNCO, fundamental vibrational frequencies and anharmonic force fields, the rovibrational spectrum of DNCO, and the precise Re structure of isocyanic acid. Sundry state-of-the-art electronic structure methods were employed in the study, including restricted and unrestricted Hartree–Fock (RHF and UHF), complete-active-space self-consistent-field (CASSCF), configuration interaction singles and doubles (CISD), Møller–Plesset perturbation theory through fourth and occasionally fifth order (MP2–MP5), coupled-cluster singles and doubles (CCSD), and CCSD augmented by a perturbative contribution from connected triple excitations [CCSD(T)]. The one-particle basis sets ranged in quality from (9s5p1d/4s2p1d) to (13s8p3d2f/6s5p3d2f ) on the heavy atoms and from (4s1p/2s1p) to (6s2p1d/4s2p1d) on hydrogen. Several revisions of thermochemical data are proposed, in particular, a larger barrier to linearity of 5.7(3) kcal mol−1, an enhanced bond energy of 85.4(10) kcal mol−1 for D0(NH–CO), and more reliable relative energies for the isomers of HNCO, viz., γe(HOCN)=25.5(10), γe(HCNO)=70(2), and γe(HONC)=84.5(15) kcal mol−1. In addition, the experimental value D0(H–NCO)=113.0(2) kcal mol−1 is confirmed. These results lead to several new proposals for heats of formation (ΔH°f,0, kcal mol−1): HNCO(−26.1), HOCN(−0.7), HCNO(+43.0), HONC (+57.6), and NCO(+35.3). A complete quartic force field has been constructed for HNCO by combining RHF third- and fourth-derivative predictions with CCSD quadratic force constants subjected to the scaled quantum mechanical (SQM) optimization scheme.This force field yields a set of ωi and χij vibrational constants which gives the following fundamental frequencies (with total anharmonicities in parentheses): ν1=3534(−186), ν2=2268(−45), ν3=1330(−9), ν4=778(−50), ν5=576(+9), and ν6=657(+21) cm−1, thus reproducing the observed band origins to 4 cm−1 or less. For DNCO the theoretical force field reveals misassignments of the low-frequency bending vibrations and predicts ν4(a')=727, ν5(a')=458, and ν6(a‘)=633 cm−1. Finally, the theoretical vibration–rotation interaction constants (αi) for five isotopic species of HNCO have been used in conjunction with empirical rotational constants and the Kraitchman equations to determine re(N–H)=1.0030(20) A(ring), re(N–C)=1.2145(6) A(ring), re(C–O)=1.1634(4) A(ring), θe(H–N–C)=123.34(20)°, and θe(N–C–O)=172.22(20)°.

Notes: A comprehensive anharmonic vibrational analysis of isotopic ketenes has been performed on the basis of a complete ab initio quartic force field constructed by means of second-order Møller–Plesset perturbation theory (MP2) and the coupled-cluster singles and doubles (CCSD) approach, augmented for structural optimizations by a contribution for connected triple excitations [CCSD(T)]. The atomic-orbital basis sets of the study entailed C,O(10s6p/5s4p) and H(6s/4s) spaces multiply polarized in the valence region to give QZ(2d,2p) and QZ(2d1f,2p1d) sets. An iterative anharmonic vibrational refinement of a limited set of quadratic scaling parameters on 27 fundamentals of H2CCO, HDCCO, D2CCO, and H2C13CO generates a final quartic force field which reproduces the empirical νi data with an average absolute error of only 1.1 cm−1. This force field yields a complete and self-consistent set of Coriolis (ζij), vibrational anharmonic (χij), vibration–rotation interaction (αi), and quartic and sextic centrifugal distortion constants, providing a critical assessment of the assorted spectroscopic constants determined over many years and also facilitating future computations of vibrational state densities for detailed tests of unimolecular dissociation theories.The harmonic frequencies ascertained for H2CCO (in cm−1), with associated anharmonicities in parentheses, are ω1(a1)=3202.2(−129.2), ω2(a1)=2197.2(−44.4), ω3(a1)=1415.2(−25.9), ω4(a1)=1146.0(−29.7), ω5(b1)=581.9(+7.1), ω6(b1)=502.6(+26.3), ω7(b2)=3308.2(−141.3), ω8(b2)=996.0(−17.9), and ω9(b2)=433.6(+5.0). The large positive anharmonicity for the ν6(b1) C=C=O bending mode, which is principally a Coriolis effect, warrants continued investigation. Explicit first-order treatments of the strong Fermi interactions within the (ν4,2ν5,ν5+ν6,2ν6) manifold reveal resonance shifts for ν4(H2CCO, HDCCO, D2CCO) of (−12.1, −10.0, +12.2) cm−1, in order. The experimental assignments for this Fermi tetrad are confirmed to be problematic. From high-precision empirical rotational constants of six isotopomers and the theoretical anharmonic force field, the equilibrium structure of ketene is derived: re(C=O)=1.160 30(29) A(ring), re(C=C)=1.312 12(30) A(ring), re(C–H)=1.075 76(7) A(ring), and θe(H–C–H)=121.781(12)°. A natural bond orbital (NBO) analysis shows that the unusually large methylene angle is attributable to extensive in-plane π delocalization. © 1995 American Institute of Physics.

Notes: Diverse aspects of the potential surface for the proton-transfer reaction CH3OH+F−→CH3O−+HF have been investigated by means of high-level ab initio electronic structure methods based on single-reference wave functions, namely, Møller–Plesset perturbation theory from second through fourth order (MP2–MP4), the configuration interaction and coupled-cluster singles and doubles methods (CISD and CCSD), and CCSD theory augmented by a perturbative correction for connected triple excitations [CCSD(T)]. The one-particle Gaussian basis sets for (C,O,F;H) ranged in quality from [4s2p1d;2s1p] to [14s9p6d4f;9s6p4d], including as many as 482 atomic orbitals for the CH3OHF− system. The ion–molecule complex on the proton-transfer surface is a tight, hydrogen-bonded structure of CH3OH⋅F− character, exhibiting a nearly linear -OHF− framework, an elongated O–H distance of 1.07(1) A(ring), and a small interfragment separation, r(H–F)=1.32(1) A(ring). Improved structural data for F−⋅H2O are obtained for calibration purposes. A large fluoride affinity is found for the CH3OHF− adduct, D0=30.4±1 kcal mol−1, and a bonding analysis via the Morokuma decomposition scheme reveals considerable covalent character. The harmonic stretching frequencies within the -OHF− moiety are predicted to be 421 and 2006 cm−1, the latter protonic vibration being downshifted 1857 cm−1 relative to ω1(O–H) of free methanol.A systematic thermochemical analysis of the reactants and products on the CH3OHF− surface yields a proton-transfer energy of 10.6 kcal mol−1, a gas-phase acidity for methanol of 381.7±1 kcal mol−1, and D0(CH3O–H)=104.1±1 kcal mol−1, facilitating the resolution of previous inconsistencies in associated thermochemical cycles. A minimum-energy path in geometric configuration space is mapped out and parametrized on the basis of constrained structural optimizations for fixed values of an aptly chosen reaction variable. The evaluation of numerous energy points along this path establishes the nonexistence of either a proton-transfer barrier, an inflection region, or a secondary minimum of CH3O−⋅HF type. The mathematical considerations for a classical multipole analysis of reaction path asymptotes are outlined for ion–dipole systems and applied to the CH3OHF− surface with due concern for bifurcations in the exit channel for the proton-transfer process. A global analytic surface for vibrational stretching motion in the -OHF− moiety of the CH3OHF− system is constructed, and a suitable dynamical model is tested which involves an effective, triatomic hydrogen pseudobihalide anion, [-OHF]−. Converged variational eigenstates of [-OHF]− to one-half its dissociation limit are determined using vibrational configuration interaction expansions in terms of self-consistent-field modals. The fundamental stretching frequencies of the CH3OHF− complex predicted by the [-OHF]− model are 504 (+84) and 1456 (−549) cm−1, the corresponding anharmonicities appearing in parentheses.

Notes: State-of-the-art ab initio quantum chemical techniques have been employed in the determination of the reaction path and attendant energetics for the singlet dissociation of CH2CO. Variational RRKM calculations implementing these results provide first principles predictions for the dissociation kinetics which are in quantitative agreement with the corresponding experimental data.

Notes: Computations have been performed for the singlet and triplet electronic states of varying orientations of naphthalene dimer. The dependence of exciton splitting upon orientation and intermonomer distance was explored. Splittings of triplet states are seen to be nontrivial at typical bonding distances, commensurate with the splittings of weakly allowed singlet states. Charge-transfer interaction with the excimer states is seen to be most significant in face-to-face orientations which can allow closer approach of the two monomers. Predictions of the prominent features of the singlet–singlet and triplet–triplet absorption spectra agree well with experimental findings. A spin-orbit channel-counting scheme is introduced to account for observed radiative and nonradiative decay of the T1 triplet state of the monomer, and then applied to the dimer. The mechanism has been found for the observed more rapid phosphorescence of the T1 state of the dimer when placed in orientations lacking inversion symmetry. © 2000 American Institute of Physics.

Notes: State-of-the-art ab initio quantum chemical techniques have been employed to ascertain the reaction path and associated energetics for the dissociation of CH2CO into 1CH2+CO and thereby to investigate the kinetics of this dissociation via variational Rice–Ramsperger–Kassel–Marcus (RRKM) theory. The quantum chemical computations focused on the determination of geometric structures, energies, and force fields for four constrained C–C distances (2.2, 2.5, 2.8, and 3.1 A(ring)) spanning the inner transition-state region. Optimized structures were obtained with the coupled-cluster singles and doubles method including a perturbative triples term [CCSD(T)], as implemented with a contracted [C/O, H] basis set of [5s4p2d1f, 4s2p1d] quality. The resulting energetics were corrected for basis set incompleteness and higher-order electron correlation with the aid of second-order Møller–Plesset perturbation theory (MP2) predictions given by an immense [13s8p6d4f, 8s6p4d] basis combined with 6–31G* Brueckner doubles results augmented with perturbative contributions from both connected triple and quadruple excitations. Quadratic force fields along the reaction path were determined at the CCSD/[5s4p2d, 4s2p] level of theory. Anharmonic effects in the enumeration of accessible states for the transition state were accounted for by a direct statistics approach involving repeated MP2/6-31G* energy evaluations. Two separate reaction coordinates defined by the C–C bond length or alternatively the center-of-mass separation between the 1CH2 and CO fragments were explicitly considered in these direct statistical analyses. A spectroscopic quality quartic force field for ketene derived in a companion ab initio study was employed in the evaluation of the anharmonic reactant density of states. The final statistical predictions for the energy dependence of the dissociation rate constant are found to be in quantitative agreement with experiment (i.e., generally within 30%), thereby providing strong evidence for the quantitative validity of variational RRKM theory. © 1996 American Institute of Physics.