Ioannis S. K. Kerkines

On the ground states of CaC and ZnC: A multireference Brillouin–Wigner coupled cluster study

We test the recently developed state-specific multireference Brillouin–Wigner coupled cluster (MRBWCCSD) method against the single reference CCSD method by examining theoretically the competing X 3Σ− and 5Σ− states of the (experimentally unknown) isovalent calcium and zinc carbide diatomics (CaC, ZnC). At the CCSD level, CaC is “incorrectly” predicted to have a ground 5Σ− state; however, the MRBWCCSD treatment restores the correct state ordering, and improves significantly the energetics for both molecules. Further comparison with various single- and multireference treatments shows that the latter are absolutely necessary for obtaining meaningful results for the ground states in both molecules.

Excited-State Intramolecular Proton Transfer in Hydroxyoxime-Based Chemical Sensors

The electronic structure of a series of β-hydroxy-oximes, with different aromatic cores (naphthalene, pyrene, coumarin, pyridine) between the oxime and the hydroxyl groups, has been investigated by time-dependent density functional theory (TDDFT) and of the naphthalene-based oxime, in addition, by resolution-of-identity second-order perturbative coupled cluster (RICC2) calculations with basis sets up to augmented triple-ζ quality. The particular systems have been proposed as fluorescent sensors of organophosphorus (OP) nerve agents, with enhancement of fluorescence accompanying the sensing of OP agents. It is found that the experimentally observed fluorescence quenching of the oxime sensors in their initial form can be attributed to intramolecular proton transfer upon excitation from the β-hydroxyl group to the nitrogen atom, thus forming a weakly emitting hydroxylaminoquinoid.

Low-lying absorption and emission spectra of pyrene, 1,6-dithiapyrene, and tetrathiafulvalene: A comparison between ab initio and time-dependent density functional methods

The gas-phase and in-solvent absorption and emission spectra of pyrene, 1,6-dithiapyrene, and tetrathiafulvalene are studied theoretically in the visible spectral region with the complete active space self-consistent field method, the complete active space second order perturbation theory method, and the resolution-of-identity second order perturbative corrected coupled cluster doubles (RICC2) method, with basis sets up to augmented polarized triple-zeta quality. The time-dependent density functional theory (TDDFT) formalism is also used employing a series of functionals. The nature of the excited states is discussed. With respect to literature theoretical values of the absorption and emission wavelengths of these three molecules, substantial improvements are achieved and comparison with experiment is favorable. Moreover, theoretical absorption and emission spectra of 1,6-dithiapyrene are presented for the first time. It is also exhibited that in most cases, a TDDFT treatment with hybrid functionals combined with a modest basis set (6-31G( *)) appears to be capable of providing reliable estimates for absorption and emission in all three molecules with relatively low computational cost. Furthermore, the RICC2 method (standalone or in conjunction with TDDFT) provides a satisfactory ab initio alternative, providing a good compromise between accuracy and computational effort.

Theoretical investigation of the X 2Σ+, A 2Π, and B 2Σ+ states of LiAr and LiKr

The X 2Σ+, A 2Π, and B 2Σ+ states of the LiAr and LiKr molecules have been examined theoretically employing the coupled cluster method combined with augmented correlation consistent basis sets of double through sextuple zeta quality. After constructing basis set superposition error-free potential energy curves for the above states, dissociation energies (De), bond distances (re), and common spectroscopic parameters are extracted through the numerical solution of the one-dimensional rovibrational Schrodinger equation. For the “bound” A 2Π states of LiAr and LiKr, the De values can be considered in harmony with experimental values: De=890.4 (957±30) and 1220.0 (1200) cm−1 (experimental values in parentheses), respectively. Corresponding bond lengths, re=2.545 (2.50±0.08) and 2.673 (3.27) A indicate that the experimental bond distance of the LiKr A 2Π state is rather too large.

On the electron affinity of SiN and spectroscopic constants of SiN(

Accurate spectroscopic constants and energetics were calculated for the two lowest-lying states of SiN and SiN- employing the coupled cluster methodology and very large basis sets (up to doubly augmented sextuple-zeta quality) accounting also for core/valence correlation, one-electron Douglas-Kroll-Hess relativistic effects, and atomic spin-orbit couplings. Our best estimate for the adiabatic electron affinity of SiN is 3.002 eV, in very good agreement with the recent, experimentally determined value of 2.949(8) eV. However, the calculated bond length of the SiN- X 1sigma+ state at the same level, r(e) = 1.5904 angstroms, is smaller than the indirectly extracted experimental value of 1.604(5) angstroms, pointing out that the latter value is either a bit overestimated or not as accurate as the +/- 0.005 angstroms error bar indicates. For the neutral SiN, all calculated data are in excellent agreement with previous accurate experimental results.

Electronic structure of vanadium and chromium carbide cations, VC + and CrC + . Ground and low-lying states

The ground and low-lying states of the monopositive vanadium and chromium carbides, VC+ and CrC+ have been studied by multireference methods and quantitative basis sets. Potential energy curves for 17 (VC+) and 19 (CrC+) states have been fully calculated. A variety of binding modes is revealed in the low-lying spectrum of the two molecular cations, often accompanied with an electronic charge transfer from the metal cation towards carbon. Two states compete for the ground state identity in both systems. One state comprises two π and ½σ bonds (similarly to ScC+ and TiC+), while the other state forms a genuine triple bond. After a rather intricate analysis including core electron effects, scalar relativity and curve shifts, the formal ground states of VC+ and CrC+ are found to be of 3Δ and 2Δ symmetry, with estimated energy differences from the competing 1Σ+ and 4Σ− states of 1–3 and 3–7 kcal/mol, respectively. At the highest level of theory including core/valence correlation and one-electron relativistic effects, the calculated ground-state binding energies are in satisfactory agreement with available experimental values.

Designing non-classical non-transition-metal hydrogen complexes: Theoretical prediction of Si2F3(μ2-H2)

Following theoretical analysis and systematic computational results that were published from this institute in the early 1990s concerning the formation and stability of non-classical hydrogen complexes (NCHC) for light, non-transitionmetal compounds, we predict the existence of a silicon-containing NCHC, namely Si2F3(μ2-H2). High-level correlated coupled cluster calculations using basis sets up to quadruple-ζ quality pinpoint that Si2F3(μ2-H2) is stable towards dissociation to Si2F3 + H2 by 3.7 kcal/mol. An energy barrier of comparable size is found to separate the NCHC from the more stable dihydride isomer, an oxidation addition product.