Leslie A. Barnes

Theoretical studies of the first- and second-row transition-metal methyls and their positive ions

The metal–carbon bond dissociation energies (D0) and geometries for the first‐ and second‐row transition‐metal methyl neutrals and positive ions are determined. The computed D0 values for the positive ions compare favorably with experiment, except for RuCH+3, RhCH+3, and PdCH+3 where the experimental values are 10–15 kcal/mol larger. The computed D0 values for the hydride and methyl positive ions are similar for all metals in both transition rows except for Cu and Ag. However, for the neutral systems the D0 values for the methyls are smaller, especially on the right‐hand side of both transition rows where the differences approach 15 kcal/mol. In general, the dissociation energies do not follow simple trends, as the individual D0 values are significantly affected by the relative spacings between the atomic states of the metal. The study of all of the methyl neutral and ions of both transition rows presented here provides a consistent set of data for the dissociation energies, thereby allowing a critical as...

Theoretical studies of the first- and second-row transition-metal mono- and dicarbonyl positive ions

Ab initio calculations have been carried out on the first‐ and second‐row transition‐metal mono‐ and dicarbonyl positive ions. The bonding in these systems is discussed in detail. Trends in the series of mono‐ and dicarbonyl ions and between the first‐ and second‐row transition metals are explained in terms of a dominantly electrostatic bonding interaction and differences in metal ion state separations, ionization potentials, and s and d orbital sizes. Dissociation energies are presented and a detailed comparison is made with experimental data. Where reliable experimental data exists, agreement with the theoretical results is generally good. An exception is Mo(CO)+1,2, where the computed binding energies are much smaller than the experimental values.

An ab initio study of Fe(CO)n, n = 1,5, and Cr(CO)6

Ab initio calculations have been performed for Cr(CO)6 and Fe(CO)n, n=1,5. Basis sets of better than double zeta quality are used and correlation is included using the modified coupled‐pair functional method. The computed geometries and force constants are in reasonable agreement with experiment. The sequential bond dissociation energies of CO from Fe(CO)5 are estimated to be: 39, 31, 25, 22, and ≳5 kcal/mol. We note that the first bond dissociation energy is relative to the singlet ground state of Fe(CO)5 and the lowest singlet state of Fe(CO)4, whereas the second is relative to the ground triplet states of Fe(CO)4 and Fe(CO)3. In addition, the binding energy for Fe–CO would be modified to 18 kcal/mol if dissociation occurred to the Fe(5F) excited state asymptote. The CO binding energies for Fe and Cr are found to be in poorer agreement with experiment than those found in a previous study on Ni(CO)4. The origins of this difference are discussed.

Symmetry breaking in O+4: an application of the Brueckner coupled-cluster method

Abstract A recent calculation of the antisymmetric stretch frequency for the rectangular structure of quartet O + 4 using the QCISD(T) method gave a value of 3710 cm −1 . This anomalous frequency is shown to be a consequence of symmetry-breaking effects, which occur even though the QCISD(T) solution derived from a delocalized SCF reference function lies energetically well below the two localized (symmetry-broken) solutions at the equilibrium geometry. The symmetry breaking is almost eliminated at the CCSD level of theory, but the small remaining symmetry-breaking effects are magnified at the CCSD(T) level of theory so that the antisymmetric stretch frequency is still significantly in error. The Brueckner coupled-cluster method, however, leads to a symmetrical solution which is free of symmetry-breaking effects, with an antisymmetric stretch frequency of 1322 cm −1 , in good agreement with our earlier calculations using the CASSCF/CASSI method.

Structure and energetics of Cr(CO)6 and Cr(CO)5

The geometric structure of Cr(CO)6 is optimized at the modified coupled-pair functional (MCPF), single and double excitation coupled-cluster (CCSD), and CCSD(T) levels of theory (including a pertur ...

The fraternal twins of quartet O+4

Eleven stationary geometries of quartet O4+ have been studied by ab initio methods. The geometries were optimized at the complete active space self-consistent field (CASSCF) level of theory and the energies were calculated by the multiconfigurational second order pertubation method (CASPT2), using double-zeta plus polarization (DZP), triple-zeta plus double polarization (TZ2P), average atomic natural orbital (ANO) [5s4p2d] and average ANO [6s5p3d2f] basis sets. The rectangular and trans-planar structures are found to be the most stable, with an energy barrier to conversion between the two at the threshold of dissociation. Both have a delocalized hole and are stable relative to separated O2 and O2+ by 11.0 and 11. 5 kcal/mol for the rectangular and the trans-planar structure, respectively, compared with the experimentally deduced energy in the range of 9.2 to 10.8 kcal/mol. The adiabatic ionization potentials of O4 and O2 are computed to be 11.67 and 12.21 eV, while experimental values are 11.66 and 12.07 eV, respectively. The vibrational frequencies have been computed for all degrees of freedom at the CASSCF level of theory. Symmetry breaking is found to be a particular problem in the computation of the antisymmetric stretch frequency for the delocalized structures at the CASSCF level of theory. Attempts to rectify these problems using the restricted active space self-consistent field (RASSCF) method leads to additional difficulties, but further analysis yields insight into the symmetry breaking and problems with earlier calculations. Finally, a nonorthogonal configuration interaction (CI) calculation based on the interaction of localized CASSCF wave functions using the complete active space state interation (CASSI) method leads to a balanced treatment of the antisymmetric stretch which is free from symmetry breaking. The study explains the four most prominent absorption frequencies observed in the partially unassigned IR spectrum of 04+ isolated in solid neon as the antisymmetric OO stretch, and the combination band of the symmetric and antisymmetric OO stretch of both the rectangular and trans-planar structures.

Theoretical studies of the transition metal―carbonyl systems MCO and M(CO)2, M=Ti, Sc, and V

Ab initio calculations on the transition metal–carbonyl systems MCO and M(CO)2, M=Ti, Sc, and V, have been carried out using large Gaussian basis sets and an extensive treatment of electron correlation. The dissociation energies (De) and geometries of these molecules are given, and the bonding mechanisms are discussed. High‐spin ground states are favored for the monocarbonyl molecules, whereas for the dicarbonyl molecules there is a competition between high‐, intermediate‐, and low‐spin states, which are found to be very close in energy. The computed De(Ti–CO) is 0.62 eV whereas for Ti(CO)2 it is 1.02 eV, relative to the ground state Ti atomic asymptote and CO(1Σ+). This suggests that the recent experiment giving a value of ≊1.75 eV for De[Ti–(CO)x] should be interpreted as giving the De for Ti(CO)x, x≥2. For the three metal atoms the binding energy per carbonyl is found to be significantly lower for the dicarbonyl than the monocarbonyl molecules. This is in contrast to the Ni(CO)x molecules, where each C...

Bonding in zerovalent Ni compounds: NiN2 and Ni(N2)4 compared with NiCO and Ni(CO)4

Abstract Ab initio calculations have been carried out on the 1 Σ + , 3 Δ, 3 Σ + and 3 Π states of NiN 2 using a large Gaussian basis set and an MCPF treatment of electron correlation. In addition, we have considered the 2 Σ + states of NiN 2 − , NiCO − and NiN 2 + , the low-lying 2 Δ and 2 Π states of NiN 2 + , and the 1 A 1 states of Ni(CO) 4 and Ni(N 2 ) 4 . The bonding in the Ni(N 2 ) x systems is found to be systematically weaker than in the comparable Ni(CO) x systems, primarily because the N 2 ligands do not accept π back-donation from the metal as readily as the CO ligands. For example, the calculated 1 Σ + metal-ligand bond dissociation energy, D e , of 15.9 kcal/mole for NiN 2 is about half that obtained for NiCO. The 2 Σ + state of NiN 2 + has a calculated D e of 23.7 kcal/mole, which is about 9 kcal/mole lower than the 2 Σ + state of NiCO + Unlike NiCO − , which is a stable molecule, NiN 2 − is predicted to be unbound with respect to Ni − and N 2 . The dissociation energy of the 1 A 1 state of Ni(N 2 ) 4 is found to be about 45 kcal/mole, or about one third the value of the 1 A 1 state of Ni(CO) 4 . We present simple arguments, and additional qualitative calculations, which clearly explain the differences in binding energies for these systems.

Bond length, dipole moment, and harmonic frequency of CO

A detailed comparison of some properties of CO is given, at the modified coupled‐pair functional, single and double excitation coupled‐cluster (CCSD), and CCSD(T) levels of theory (including a perturbational estimate for connected triple excitations), using a variety of basis sets. With very large one‐particle basis sets, the CCSD(T) method gives excellent results for the bond distance, dipole moment, and harmonic frequency of CO. In a [6s 5p 4d 3f 2g 1h]+(1s 1p 1d) basis set, the bond distance is about 0.005a0 too large, the dipole moment about 0.005 a.u. too small, and the frequency about 6 cm−1 too small, when compared with experimental results.

On the interpretation of the photoelectron spectrum of NiCO

Abstract Ab initio calculations have been carried out for the X 1Σ+, a 3Δ, b 3Σ+ and c 3Π states of NiCO, using a large Gaussian basis set and an MCPF treatment of electron correlation. The calculated dissociation energies, De (kcal/mol), are X1Σ+ (30.1), a 3Δ (11.5), b 3Σ (10.1) and c 3Π(4.3). An analysis of the relative cross sections for photodetachment indicates that the lower energy feature in the photoelectron spectrum of NiCO- results primarily from bound-free transitions to the c 3Π state, with some contribution from bound-bound transitions to both the b 3Σ+ and c 3Π states.