We are proposing a new computational thermochemistry protocol denoted W3 theory, as a successor to W1 and W2 theory proposed earlier [Martin and De Oliveira, J. Chem. Phys. 111, 1843 (1999)]. The new method is both more accurate overall (error statistics for total atomization energies approximately cut in half) and more robust (particularly towards systems exhibiting significant nondynamical correlation) than W2 theory. The cardinal improvement rests in an approximate account for post-CCSD(T) correlation effects. Iterative T3 (connected triple excitations) effects exhibit a basis set convergence behavior similar to the T3 contribution overall. They almost universally decrease molecular binding energies. Their inclusion in isolation yields less accurate results than CCSD(T) nearly across the board: It is only when T4 (connected quadruple excitations) effects are included that superior performance is achieved. T4 effects systematically increase molecular binding energies. Their basis set convergence is quite rapid, and even CCSDTQ/cc-pVDZ scaled by an empirical factor of 1.2532 will yield a quite passable quadruples contribution. The effect of still higher-order excitations was gauged for a subset of molecules (notably the eight-valence electron systems): T5 (connected quintuple excitations) contributions reach 0.3 kcal/mol for the pathologically multireference X 1Sigmag+ state of C2 but are quite small for other systems. A variety of avenues for achieving accuracy beyond that of W3 theory were explored, to no significant avail. W3 thus appears to represent a good compromise between accuracy and computational cost for those seeking a robust method for computational thermochemistry in the kJ/mol accuracy range on small systems.

To reduce remaining basis-set errors in the determination of molecular equilibrium geometries, a basis-set extrapolation (BSE) scheme is suggested for the forces used in geometry optimizations. The proposed BSE scheme is based on separating the Hartree-Fock and electron-correlation contributions and uses expressions obtained by straightforward differentiation of well established extrapolation formulas for energies when using basis sets from Dunning's hierarchy of correlation-consistent basis sets. Comparison with reference data obtained at the R12 coupled-cluster level [CCSD(T)-R12] demonstrates that BSE significantly accelerates the convergence to the basis-set limit, thus leading to improvements comparable to or even better than those obtained by increasing the cardinal number in the used basis set by one. However, BSE alone is insufficient to improve agreement with experiment, even after additional consideration of inner-shell correlation and quadruple-excitation effects (mean error and standard deviation with extrapolation are -0.014 and 0.047 pm in comparison with mean error and standard deviation of -0.002 and 0.036 pm without extrapolation). Improvement is obtained only when other contributions of similar magnitude as the BSE contributions (e.g., pentuple-excitation effects and relativistic effects) are also considered. A rather large discrepancy (of the order of a few tenths of a picometer) is observed for the F(2) molecule indicating an enhanced basis-set requirement for the various contributions in this case.

No abstract

No abstract

Quantum mechanical investigations at the MP4(SDTQ)/6-31 lG(2df,2pd)//MP2/6-31G(d,p) + ZPE level of theory show that helium is capable of forming strong bonds with carbon in cations and that even a neutral molecule containing He (HeBeO) can be thermodynamically stable in its ground state. The electronic state of a binding partner is crucially important for the bond strength and bond length of the He bond. He2C2+ has a rather long (1.605 A) He-C atomic distance in its 'A, ground state, but a much shorter bond (1.170 A) is found in the 3B! excited state. The shortest He-C bonds (1.080-1.085 A) are found in the 2+(4-rr) states of HeCC2+, HeCCHe2+, and HeCC+. The bond dissociation energies of the dications in these electronic states yielding neutral He and a cationic fragment are predicted to be as high as 89.9 kcal/mol for HeCC2+. Helium compounds are best understood as donor-acceptor molecules consisting of He as electron donor and the respective acceptor fragment. Strong helium bonds are formed when a binding partner (acceptor) provides low-lying empty a orbitals (tr-holes). Electronegative elements such as fluorine or oxygen are not suitable for binding He due to their highly filled valence shells. More promising candidates should provide empty orbitals which are still capable of attracting the low-lying Is electrons of the poor electron donor He. The stability of HeBeO is confirmed by CASSCF calculations with a 6-31G(d,p) basis set and an active space of all 14 electrons in 11 orbitals. The structures and energies of the helium compounds are rationalized by molecular orbital arguments and by analysis of the electron density and its associated Laplace field. The strongly bound helium ions are characterized by covalent semipolar He-C bonds, whereas the weaker bonds in some structures are caused by electrostatic interactions between closed-shell systems. The impact of our study on experiment, especially interstellar chemistry, is discussed.

V vs. N H E (turnover frequency-lo3 mol of C O produced per mol of nickel complex, in 1 h) and selective, even in a purely aqueous medium. The stability of the complex used makes it a promising electrocatalyst (I O4 electrocatalytic cycles on Ni cy-clam2+ without degradation). In the present study, we have detected a nickel(1) carbonyl complex which may participate in the catalytic cycle. Other important reaction parameters have been investigated. The efficiency of Ni cyclam2+ in the electroreduction of C02 may be due to the size of the cyclam ring (1 4 atoms), which greatly stabilizes nickel complexes. A reason for the selectivity of the electrocatalyst may be difunctional activation of C02. The acidic character of the N-H protons of cyclam could favor C 0 2 fixation by hydrogen bonding (N-H-0) in addition to the carbon to nickel(1) link.

Cram presented the first evidence for the intermediacy of phenonium ions in the solvolysis of 8-arylalkyl halides in 1949.'~~

Benchmark, frozen-core CCSD(T) equilibrium harmonic vibrational frequencies of 12 closed-shell and five open-shell molecules are computed to within 1 cm-1 of the basis set limit using the explicitly correlated CCSD(T)-R12 method. The convergence of the standard CCSD(T) method with the one-particle basis sets of Dunning and co-workers is examined and found to be slow, with mean and maximum absolute errors of 1.3 and 3.5 cm-1 remaining at the cc-pV6Z level. Finite basis set effects do not appear to introduce systematic errors in equilibrium harmonic frequencies, and mean absolute errors reduce by a factor of 2 for each basis set cardinal number increment. The convergence of individual equilibrium harmonic frequencies is not guaranteed to be monotonic due to the associated shift in the equilibrium structure. The inclusion of computed scalar relativistic effects and previously available corrections for core-valence correlation and higher-order excitations in the cluster operator results in an agreement with experimentally derived harmonic frequencies of 0.1, 0.3, and -0.4 cm-1 for HF, N2, and CO, respectively. F2 continues to present a challenge to computational chemistry with an error of 3.2 cm-1, primarily resulting from the high basis set dependence of the quadruples contribution.