Equilibrium Structures of the Phosphorus Trihalides PF3 and PCl3, and the Phosphoranes PH3F2, PF5, PCl3F2, and PCl5.

Among the title species, a reliable and accurate equilibrium geometry ( re structure) is available only for PF3, which has been determined experimentally more than 20 years ago. Here, we report accurate re structures for all title molecules, which were obtained using a composite computational approach based on explicitly correlated coupled-cluster theory (CCSD(T)-F12b) in conjunction with a large correlation-consistent basis set (cc-pCVQZ-F12) to take core-valence electron correlation into account. Additional terms were included to correct for the effects of iterative triple excitations (CCSDT), noniterative quadruple excitations (CCSDT(Q)), and scalar relativistic contributions (DKH2-CCSD(T)). The performance of this computational procedure was established through test calculations on selected small molecules (PH, PF, PCl, PH2, PF2, and PH3). For PF3, PCl3, PH3F2, and PF5 sufficiently accurate experimental ground-state rotational constants from the literature were used to determine semiexperimental re structures, which were found to be in excellent agreement with the corresponding best estimates from the current composite approach. The recommended equilibrium structural parameters are for PCl3, re(PCl) = 203.94 pm and θe(ClPCl) = 100.18°; for PH3F2, re(PHeq) = 138.38 pm and re(PFax) = 164.15 pm; for PF5, re(PFeq) = 153.10 pm and re(PFax) = 157.14 pm; for PCl3F2, re(PCleq) = 200.21 pm and re(PFax) = 159.37 pm; and for PCl5, re(PCleq) = 201.29 pm and re(PClax) = 211.83 pm. The associated uncertainties are estimated to be ±0.10 pm and ±0.10°, respectively.

Correlated Ab Initio and Density Functional Studies on H2 Activation by FeO(.).

The reaction FeO(+) + H2 → Fe(+) + H2O is a simple model for hydrogen abstraction processes in biologically important heme systems. The geometries of all relevant stationary points on the lowest sextet and quartet surfaces were optimized using several density functionals as well as the CASSCF method. The corresponding energy profiles were computed at the following levels: density functional theory using gradient-corrected, hybrid, meta, hybrid-meta, and perturbatively corrected double hybrid functionals; single-reference coupled cluster theory including up to single, double, triple, and perturbative quadruple excitations [CCSDT(Q)]; correlated multireference ab initio methods (MRCI, MRAQCC, SORCI, SORCP, MRMP2, NEVPT2, and CASPT2). The calculated energies were corrected for scalar relativistic effects, zero-point vibrational energies, and core-valence correlation effects. MRCI and SORCI energies were corrected for size-consistency errors using an a posteriori Davidson correction (+Q) leading to MRCI+Q and SORCI+Q. Comparison with the available experimental data shows that CCSDT(Q) is most accurate and can thus serve as benchmark method for this electronically challenging reaction. Among the density functionals, B3LYP performs best. In the correlated ab initio calculations with a full-valence active space, SORCI+Q yields the lowest deviations from the CCSDT(Q) reference results, with qualitatively similar energy profiles being obtained from MRCI+Q and MRAQCC. SORCI+Q benefits from the quality of the approximate average natural orbitals used in the final step of the SORCI procedure. Many of the tested methods show surprisingly large errors. The present results validate the common use of B3LYP in computational studies of heme systems and offer guidance on which correlated ab initio methods are most suitable for such studies.

(Hexafluorosilicato-κ(2)F,F')bis(1,10-phenanthroline-κ(2)N,N')zinc(II) methanol monosolvate.

The title compound, [Zn(SiF6)(C12H8N2)2]·CH3OH, contains a neutral heteroleptic tris-chelate Zn(II) complex, viz. [Zn(SiF6)(phen)2] (phen is 1,10-phenanthroline), exhibiting approximate molecular C2 point-group symmetry. The Zn(II) cation adopts a severely distorted octahedral coordination. As far as can be ascertained, the title complex represents the first structurally characterized example of a Zn(II) complex bearing a bidentate-bound hexafluorosilicate ligand. A density functional theory study of the isolated [Zn(SiF6)(phen)2] complex was undertaken to reveal the influence of crystal packing on the molecular structure of the complex. In the crystal structure, the methanol solvent molecule forms a hydrogen bond to one F atom of the hexafluorosilicate ligand. The hydrogen-bonded assemblies so formed are tightly packed in the crystal, as indicated by a high packing coefficient (74.1%).

Thermochemistry of the fluoroformyloxyl radical: a computational study based on coupled cluster theory.

The standard enthalpy of formation of FCO(2) (X (2)B(2)) was determined by a computational approach based on coupled cluster theory [CCSD(T)] with energies extrapolated to the basis-set limit, with additional corrections accounting for core-valence correlation, scalar relativity, spin-orbit coupling, and zero-point vibrational motions. Utilizing a variety of independent reaction schemes, our best estimate is Delta(f)H(o)(0)(FCO(2)) = -86.0 +/- 0.6 kcal mol(-1) [Delta(f)H(o)(298) )(FCO(2)) = -86.7 +/- 0.6 kcal mol(-1)], which is shown to be more accurate than previous theoretical and experimental values. The chosen computational procedure was also applied to HCO (X (2)A'), where we find excellent agreement with experiment, and to FCO (X (2)A'), where we recommend an improved value of Delta(f)H(o)(0)(FCO) = -42.1 +/- 0.5 kcal mol(-1) [ Delta(f)H(o)(298)(FCO) = -42.0 +/- 0.5 kcal mol(-1)]. Further theoretical results concern the C-F bond dissociation energy, electron affinity, ionization energy, first and second excitation energies in FCO(2), fluoride ion affinity of CO(2), and equilibrium geometries of the molecules treated presently. For FCO (X (2)A') we propose an improved equilibrium structure: r(e)(CF) = 132.5(2) pm, r(e)(CO) = 116.7(2) pm, and theta(e)(FCO) = 127.8(2)(o).

A systematic ab initio study of the equilibrium geometry and vibrational wave numbers of bismuthine.

The equilibrium structure and the harmonic and anharmonic force fields of BiH(3) are determined by high-level ab initio calculations using a variety of correlation treatments, basis sets, and pseudopotentials, partly in combination with core polarization potentials. Spin-orbit effects are included by a configuration interaction treatment. This systematic study serves to establish a reliable computational protocol for such calculations and, in particular, to minimize basis set superposition errors through an improved new basis set and/or counterpoise corrections. Using the recommended procedures, the best ab initio results for the equilibrium geometry and the fundamental vibrational wave numbers are in good agreement with the available experimental data, which further supports the recent spectroscopic identification of BiH(3). The ground-state total atomization energy of BiH(3) is predicted to be 153.1 kcal/mol.