Colin J. Marsden

Theoretical studies of the actinides: method calibration for the UO22+ and PuO22+ ions

Abstract As part of method assessment for the theoretical study of actinide systems, we have performed 1-component relativistic pseudopotential calculations for the uranyl and plutonyl ions. The calculated spectroscopic constants compare well with fully relativistic 4-component results at both the Hartree–Fock and correlated levels of theory. We deduce that second-order spin–orbit effects in these systems are minor and show that the 1-component method chosen (B3LYP in particular) gives reliable results at low computational cost. The ground state of the plutonyl ion has been determined as 3 H g .

Theoretical characterization of the lowest triplet excited states of the tris-(1,4,5,8-tetraazaphenanthrene) ruthenium dication complex.

We present a theoretical study of the ground and the lowest triplet excited states of the tris-(1,4,5,8-tetraazaphenanthrene) ruthenium complex [Ru(tap)3]2+. Density functional theory (DFT) was used to obtain the relaxed geometries and emission energies (Delta-SCF), whereas time-dependent DFT (TD-DFT) was used to compute the absorption spectrum. Our calculations have revealed the presence of three low-lying excited-state minima, which may be relevant in the photophysical/photochemical properties of this complex. Two minima with similar energies correspond to the MLCT 3A2 and MLCT 3B metal-to-ligand charge-transfer states, the first one corresponding to a D3 structure, whereas the second is a slightly localized C2 species. The third and lowest one corresponds to the metal-centered MC 3A state and displays a pronounced C2 distortion. We have examined for the first time the localized character of the excitation in the computed MLCT states. In particular, we have evaluated the pseudorotation barrier between the Jahn-Teller C2 MLCT 3B minima in the moat around the D3 conical intersection. We have shown that the complex should be viewed as a delocalized [Ru3+(tap(-1/3))3]2+ complex in the lowest MLCT states, in agreement with subpicosecond interligand electron transfer observed by femtosecond transient absorption anisotropy study. Upper-bound estimates of the MLCT-->MC (3 kcal/mol) and MC-->MLCT (10 kcal/mol) activation energy barriers obtained from potential energy profiles in vacuum corroborate the high photoinstability of the MLCT states of the [Ru(tap)3]2+complex.

Can density functional methods be used for open-shell actinide molecules? Comparison with multiconfigurational spin-orbit studies

The geometries, electronic structures, and vibrational frequencies of two isoelectronic compounds PuO(2)(2+) and PuN(2) have been studied in detail at the density functional theory (DFT) and multiconfigurational ab initio levels of theory. Dynamic correlation was taken into account using second-order perturbation theory (CASPT2) and the variational difference-dedicated configuration interaction method for comparison with the results of the DFT study. Spin-orbit effects were included within the framework of an effective uncontracted spin-orbit configuration-interaction method which considers electron correlation effects and spin-orbit coupling on equal footing. The twelve lowest f-f electronic transitions are reported. The electronic ground state of both systems is found to be the Omega=4 component of (3)H(g). We thus disagree with an earlier assignment of the ground state of PuN(2) [E. F. Archibong and A. K. Ray, J. Mol. Struct: THEOCHEM 530, 165 (2000)]. Spin-orbit effects are small on both the geometry and vibrational frequencies of the ground states of PuO(2)(2+) and PuN(2), but they completely change the distribution of electronically excited states. A comparison of results obtained with the two classes of methods allows us to demonstrate that an unambiguous assignment of the electronic ground state and electronic spectra requires the use of multireference methods including spin-orbit coupling. Single-reference methods such as DFT provide a reasonable description of the electronic properties of ground states of these open-shell systems, and therefore also of their structural and vibrational properties. The experimental antisymmetric stretching frequency of matrix-isolated PuN(2) is reproduced well by both CASPT2 and DFT calculations; generalized gradient approximation formulations of DFT are more successful than hybrid versions in this respect. Ground-state properties of UO(2) (2+), UN(2), UO(2), PuO(2) (2+), and PuN(2) are compared and discussed.

The ammonia dimer potential energy surface : resolution of the apparent discrepancy between theory and experiment ?

Abstract Seven different structures of the ammonia dimer have been investigated by ab initio MO techniques. Geometries have been optimized and vibrational frequencies calculated using SCF MP2 and QCISD methods, with basis sets as large as 6–311 +G(3d′, 2p). Final energies were obtained using QCISD(T) theory. Several stationary points have been vibrationally characterized; the only minimum with large, balanced bases possesses an almost linear H bond, and is qualitatively different from the asymmetric cyclic structure deduced from spectroscopic data. Our best value for De is 13.27 kJ mol−1 (11.60 kJ mol−1 after BSSE correction). We could not find a stationary point resembling the spectroscopic model. The overall potential surface is extraordinarily flat. The ZPE for a rocking motion of the two NH3 units is comparable to the barrier separating two equivalent forms of the dimer. Spectroscopic ground-state rotational constants are thus grossly contaminated by vibrational averaging, and cannot be directly compared with the computed equilibrium values.

Characterization of the bifurcated structure of the water dimer

The bifurcated structure of the water dimer has been the subject of considerable uncertainty with respect to its vibrational characterization. We have considered this question at the self‐consistent‐field (SCF) level of theory using finite basis sets that allow a close approach to the Hartree–Fock limit. As one approaches the Hartree–Fock limit, the bifurcated structure is predicted to be a true transition state, with one imaginary vibrational frequency, ω12(B2)∼200i cm−1.

The geometric structures of the disulphur difluoride isomers: an experimental and ab initio study

The molecular structures of FSSF and SSF2 have been studied by gas-phase electron diffraction, in combination with literature data on rotational constants from microwave spectroscopy. The following rav parameters were obtained, with 3σ uncertainties in parameters: for FSSF, r(SS) = 1.890(2) A, r(SF) = 1.635(2) A, ∠ SSF = 108.3(2)° and τFSSF = 87.7(4)°; for SSF2, r(SS) = 1.856(2) A, r(SF) = 1.608(2) A, s SSF = 108.1(2)° and < FSF = 91.7(3)°. These results are consistent with, and more precise than, those originally reported from microwave spectroscopy (R.L. Kuczkowski, J. Am. Chem. Soc., 86 (1964) 3617), and also consistent with, but slightly less precise than, very recent microwave results for SSF2 only (R.W. Davis, J. Mol. Spectrosc., 116 (1986) 371). Ab initio SCF calculations predict the structure of SSF2 satisfactorily, but do not account completely for the extraordinarily short SS bond in FSSF, which is shown to have a substantial (p-p)π component. Correlated calculations at the MP2 level with a DZP basis provide reasonably accurate geometrical predictions for FSSF. At the SCF level, FSSF is more stable than SSF2, but this error is removed with the inclusion of MP2 energies. Both cis and trans barriers to internal roation in FSSF are predicted to be very high, at 135 and 117 kJ mol−1, consistent with the observed torsional frequency. The SS distance in planar cis and trans forms of FSSF is much longer than in the equilibrium skew form, due to the elimination of π overlap.

Examining the Performance of DFT Methods in Uranium Chemistry : Does Core Size Matter for a Pseudopotential?

We have investigated the performance of DFT in U(VI) chemistry. A large, representative selection of functionals has been tested, in combination with two ECPs developed in Stuttgart that have different-sized cores (60 and 78 electrons for U). In addition, several tests were undertaken with another 14 electron pseudopotential, which was developed in Los Alamos. The experimental database contained vibrational wavenumbers, thermochemical data, and (19)F chemical shifts for molecules of the type UF(6-n)Cl(n). For the prediction of vibrational wavenumbers, the large-core RECP (14 electrons) gives results that are at least as good as those obtained with the small-core RECP (32 electrons). GGA functionals are as successful as hybrid GGA for vibrational spectroscopy; typical errors are only a few percent with the Stuttgart pseudopotentials. For thermochemistry, hybrid versions of DFT are more successful than GGA, LDA, or meta-GGA. Marginally better results are obtained with a 32 electron ECP than with 14; since the experimental uncertainties are at least 25 kJ/mol for each reaction, the best functionals give results that are essentially indistinguishable from experiment. However, large-basis CCSD(T) results match experiment better than any DFT that we examined. Our findings for NMR spectroscopy are rather disappointing; no combination of pseudopotential, functional, and basis yields even a qualitatively correct prediction of trends in the (19)F chemical shifts of UF(6-n)Cl(n) species. Results yielded by the large-core RECP are, in general, slightly less bad than those obtained with the small core. We conclude that DFT cannot be recommended for predictions of NMR spectra in this series of compounds, though this conclusion should not be generalized. Our most important result concerns the good performance of the large-core Stuttgart pseudopotential. Given its computational efficiency, we recommend that it be used with DFT methods for the prediction of molecular geometries, vibrational frequencies, and thermochemistry of a given oxidation state. The hybrid GGA functionals MPW1PW91 and PBE0 give the best results overall.

The trans influence of CF3: gas phase structure of CF3SF5

Abstract The molecular structure of CF 3 SF 5 has been determined by gas electron diffraction. The microwave spectrum was recorded in the frequency range 18–32 GHz. Superimposed on the essentially symmetric top transitions are found perturbations due to the large amplitude torsional motion of the CF 3 group. Therefore, a joint analysis of the electron diffraction intensities and rotational constant was not attempted. An approximate valence force field has been derived and used to calculate vibrational amplitudes. The structure is based on a slightly distorted octahedron with the following zkeletal parameters (r a values, 2σ in parentheses): SF a = 1.562(7) A, SF e = 1.572(2) A, SC = 1.887(8) A and CSF e = 90.5(2)°. The axial SF bond ( trans to CF 3 ) in shorter by 1.010(7) A than the equational bonds and the mean SF distances are longer by 0.008(2) A than the bonds in SF 0 . The SF bond lengths are discussed together with analogous bond lengths in other XSF 5 derivatives on the basis of the VSEPR model and “ trans influence” concept. The variation of the SCF 3 distance with the sulfur oxidation state is analyzed.

Abinitio correlated potential energy surfaces for monomeric sodium and potassium cyanides

Ab initio molecular orbital calculations using large Gaussian basis sets have been performed for NaCN and KCN with full geometry optimization. Potential energy surface obtained are sensitive to the quality of polarization functions used on C and N. A bent structure, as found experimentally for KCN, is the lowest energy geometry for both NaCN and KCN, with the isocyanide form only 5–6 kJ mol−1 higher at the SCF level. Inclusion of electron correlation using third‐order perturbation theory leads to a substantial relative destabilization of the isocyanide form for the both NaCN and KCN, and improves agreement between observed and theoretical geometries for KCN.

Reactions of Uranium Atoms with Ammonia : Infrared Spectra and Quasi-Relativistic Calculations of the U:NH3, H2N-UH, and HN=UH2 Complexes

Ammonia molecules interact with U atoms, and the resulting U:NH3 complex rearranges upon visible irradiation to form the H2N--UH and HN==UH2 molecules in excess argon. These products are identified by functional group frequencies, 15NH3 and ND3 isotopic shifts, and comparison to frequencies calculated by using density functional theory. The N==U pi bond in HN==UH2 is enhanced by partial triple-bond character through N(2p) to U(5f) conjugation, which is comparable to that found in the analogous HN==ThH2 molecule. These products also form complexes with additional ammonia molecules in the matrix. The interesting higher-energy N[triple chemical bond]UH3 complex is not formed.