Energies Of Low-lying Electronic States Research Articles

Molecular structure and spectra of ytterbium dihalides from first principles

The molecular structure, vibrational frequencies, infrared intensities and electric dipole moments of ytterbium dihalides YbX2 (X = F, Cl, Br, I) in their ground electronic state are studied at the coupled-cluster singles, doubles and perturbative triples CCSD(T) level using a series of large all-electron basis sets together with the complete basis set (CBS) extrapolation procedure, and with both scalar-relativistic and spin–orbit coupling effects taken into account. Excitation energies of low-lying electronic states are calculated at the multireference configuration interaction, equation-of-motion CCSD and single-reference CCSD(T) levels of theory. For all of the molecules under study, the CCSD(T)/CBS equilibrium geometries are found to be non-linear (C2v symmetry). There is a rapid decrease in heights of the barriers to linearity on passing through the YbX2 molecular series from fluoride to iodide: 1587 → 261 → 110 → 7 (all in cm−1) for YbF2 → YbCl2 → YbBr2 → YbI2, respectively. The uncertainties in the molecular structure data published to date are discussed and resolved in the light of the present results.

Structure and spectra of the molecules of manganese, iron, and cobalt trifluorides: A CCSD(T) study in the complete basis set

The coupled-cluster singles and doubles with perturbative triples (CCSD(T)) method in triple-, quadruple-, and quintuple-zeta basis sets with extrapolation to the complete basis set limit is used to analyze the properties of MnF3, FeF3, and CoF3 molecules. The relative energies of low-lying electronic states are determined. The Jahn-Teller effect is investigated in the ground electronic state 5 E′ of the MnF3 molecule and the first excited electronic state 5 E″ of the CoF3 molecule. Geometric parameters, atomization enthalpies, vibrational frequencies, intensities in the infrared and Raman spectra are found with high accuracy. The assignment of the bands observed in the low-frequency region of the IR and Raman spectra of MnF3 and CoF3 molecules are revised.

Ab initio study of scandium fluoride molecules: ScF, ScF2, AND ScF3

Equilibrium geometric parameters, normal mode frequencies and intensities in IR spectra, atomization enthalpy, and relative energies of low-lying electronic states of scandium fluoride molecules (ScF, ScF2, and ScF3) are calculated by the coupled-cluster method (CCSD(T)) in triple-, quadruple, and quintuple-zeta basis sets with the subsequent extrapolation of the calculation results to the complete basis set limit. The ScF molecule is also studied by the CCSDT technique. The error in the approximate calculation of triple excitations in the CCSD(T) method does not exceed 0.002 A for the equilibrium internuclear distance Re, 4 cm−1 for the vibrational frequency, and 0.2 kcal/mol for the dissociation energy of the molecule. In the ground electronic state \(\tilde X^2 \)A1(C2ν) of ScF2 molecules, Re(Sc-F) = 1.827 A and αe(F-Sc-F) = 124.2°; the energy barrier to bending (linearization) h = Emin(Dg8h) − Emin(C2ν) = 1652 cm−1. The relative energies of A2Δg and \(\tilde B^2 \)Πg electronic states are 3522 cm−1 and 14633 cm−1 respectively. The bond distance in the ScF3 molecule (\(\tilde X^1 \)A′1, D3h) is refined: Re(Sc-F) = 1.842 A. The atomization enthalpies ΔatH2980 of ScFk molecules are 139.9 kcal/mol, 289.0 kcal/mol, and 444.8 kcal/mol for k = 1, 2, 3 respectively.

Theoretical Studies of the Complex [(BPMEN)Fe(II)(NCCH3)2]2+, Precursor of Non-Heme Iron Catalysts for Olefin Epoxidation and Cis-Dihydroxylation

The density functional theory (DFT) was applied to elucidate the electronic structure of the BPMEN [N,N'-bis(6-R-2-pyridylmethyl)-1,2-diaminoethane] iron complex, [(BPMEN)Fe(NCCH3)2](2+), a precursor of catalysts that catalyze the stereospecific olefin oxidation. The low-lying high and low spin states of complex 1 (R=H) are nearly degenerate with a slight preference for the high spin state. For complexes with substituents (R) at the 6-positions of two pyridine rings of BPMEN, the ground state is the high spin (HS) state, with the low spin (LS) state higher in energy by 9.5, 5.3, 8.5, 6.3, and 5.1 kcal/mol for complexes 2 (R=CH3), 3 (R=SiH3), 4 (R=OH), 5 (R=F), and 6 (R=CN), respectively, with the B3LYP method. Our findings for complexes 1 and 2 are in good agreement with the experimental observations. The calculated LS-HS difference of 17.9 kcal/mol for complex 7 (R=t-Bu) clearly demonstrates that the steric effect causes the Fe-N bonds to stretch and favors the high spin state. Comparison of the B3LYP and B3PW91 results with experiments and ab initio MP2 and CCSD(T) as well as pure DFT results shows that hybrid DFT methods provide a qualitatively correct description of the relative energies of low-lying electronic states of a model of the [(BPMEN)Fe(NCCH3)2](2+) complex, while pure DFT methods underestimate the stability of the HS state.

Ab Initio Investigation of Structures and Energies of Low-Lying Electronic States of AlN3, Al3N, and Al2N2

The structures and energies of low-lying electronic states of AlN3, Al3N, and Al2N2 have been evaluated at the HF, MP2, QCISD(T), CCSD, and CCSD(T) levels of theory, using the several basis sets of 6-31G* (for HF), cc-pVDZ (for MP2 and QCISD(T)), and cc-pVTZ (for CCSD and CCSD(T)). The ground state of AlN3 is predicted to be a 1Σ+ state with a linear Al−N−N−N structure. The most stable species of Al3N is found, however, to have D3h symmetry and 1A‘1 ground state. For Al2N2, various isomers are found to be energetically favorable. A rhombic isomer with the nitrogen atoms along the short diagonal and with a 1Ag electronic state is the lowest in energy at the MP2/cc-pVDZ, QCISD(T)/cc-pVDZ, CCSD/cc-pVTZ, and CCSD(T)/cc-pVTZ levels. A linear structure Al−N−N−Al with a 3 electronic state is the second lowest. The third stable isomer with the aluminum atoms bonded directly to the N2 π orbital seems to be one of the model species for the sake of the nitrogen fixation. Our results suggest that the formation of a variety of the configuration of Al2N2 is energetically plausible under the reaction conditions employed since the energy differences in the Al2N2 species are relatively small.

Ab initio study of the electronic structures of lithium containing diatomic molecules and ions

Ab initio calculations are used to provide bond lengths, harmonic frequencies, and dissociation energies of low-lying electronic states for LiX, LiX+, and LiX− (with X=Li through F and Na through Cl). Most of these species represent hitherto experimentally unknown molecules or ions, which provides the focus of the work presented here. All of these species are stable to dissociation and the anions are stable to loss of an electron. Differences among the electronic structures of the valence isoelectronic LiX; and HX, LiX+, and HX+; and LiX− and HX− species are analyzed. Optimized geometries, dissociation energies, ionization potentials, and electron affinities were calculated for the following ground states of the respective species: 1Σ+ for Li2(1Σ+g) LiNa, LiBe+, LiBe−, LiMg+, LiMg−, LiF, LiAl, LiS−, and LiCl; 2Σ+ for Li+2(2Σ+g), Li−2(2Σ+u) LiBe, LiB+, LiF−, LiNa+, LiNa−, LiMg, LiAl+, and LiCl−; 2Πr for LiB−, LiAl−; 2Πi for LiO, LiF+, LiS, and LiCl+; 3Πr for LiB, LiC+, and LiSi+; 3Σ− for LiN, LiO+, LiSi−, LiP, and LiS+; 4Σ− for LiC, LiN+, LiN−, LiSi, LiP+, and LiP−; and 5Σ− for LiC−.

Theoretical calculation of transition energies of low-lying electronic states of the CuCl molecule

Ab initio multiconfiguration self-consistent field followed by multireference configuration interaction calculations using localised molecular orbitals were made to estimate adiabatic purely electronic transition energies of the six lowest excited states (3 Sigma +, 1 Sigma +, 3 Pi , 1 Pi , 3 Delta , and 1 Delta ) to the X 1 Sigma + ground state of the CuCl molecule. Results show the very strong ionic character of the wavefunctions for all the studied states, di-ionic species having a less important role in the excited states than in CuF, this being especially true for both Pi states. The unobserved X 1 Sigma + from 1 Delta transition is predicted to be found around 26 500 cm-1. Spectroscopic constants are reported and found to be in good agreement with experimental data for most of the states studied. A comparative analysis of ionicity with CuF is performed through valence, total and orbital electronic densities and a correlated first-order Moller-Plesset Mulliken population analysis for the ground state of both molecules.

The structure of CCH+

Molecular orbital theory is used to examine relative energies of low-lying electronic states of the ethynyl cation CCH+. A triplet 3ru is found to be the ground state at the highest level of theory with another triplet 3𝒥− only 2.4 kcal/mole higher in energy. The closed-shell 1𝒥+ state lies 59 kcal/mole above the ground state.