AN AB INITIO MOLECULAR ORBITAL STUDY OF ELECTRONIC EXCITED STATES OF FeC

$^{a}$W. J. Balfour, J. Cao, C. V. V. Prasad, and C. X. W. Qian J. Chem. Phys. 103, 4046, (1995) $^{b}$D. J. Brugh and M. D. Morse J. Chem. Phys. 107, 9772, (1997) $^{c}$K. Aiuchi, K. Tsuji, and K. Shibuya Chem. Phys. Lett. 309, 229, (1999)Author Institution: Ochanomizu University; Department of Chemistry, Faculty of Science, Ochanomizu University; Department of Chemistry, Faculty of Science, National Institute for Advanced Interdisciplinary ResearchThere has been a continuing effort to investigate the electronic states of iron carbide, FeC, by several research groups in the gas phase spectroscopy, but only electronic ground states have been established as $^{3}\Delta_{i}$ state arising from the configuration $\ldots 8\sigma^{2}3\pi^{4}1\delta^{3}9\sigma^{1}$. In this study, we have focused on characterizing the electronic excited states observed by $LIF^{a}$, $R2PI^{b}$ and $DF^{c}$ spectra. Spectroscopic constants and energy levels of the excited states have been calculated from the ab initio multireference singles and doubles configuration interaction (MR-SDCI) molecular orbital method using all-electron large basis sets. Both relativistic and spin-orbit coupling effects were taken into account. Many electronic states which are of complicated nature were found to populate densely in the low energy region. The lowest excited states for each spin and spatial symmetry, except the $^{1}\Delta$ state, were approximately described as the excited states derived by the promotion of one electron from the ground state. The excited states higher than these were found to be of completely multiconfigurational character. Each electronic state has been discussed in terms of electronic configuration, dipole moment, charge distribution, spin-orbit coupling constant, and spectroscopic constants

THEORETICAL PREDICTION OF THE SPECTROSCOPIC CONSTANTS OF FeS: AN AB INITIO MOLECULAR ORBITAL STUDY

Author Institution: Department of Chemistry Faculty of Science, Ochanomizu University; Department of Chemistry Faculty of Science, National Institute for Advanced Interdisciplinary ResearchEnergy levels of nearly degenerated $^{5} \Delta$ and $^{5}\Sigma$ states have been studied by the MR-SDCI + Q/Roos-ANO (or various combinations of other basis sets) method with Breit-Pauli Hamiltonian for relativistic effects and spin-orbit coupling interaction corrections. The $^{5}\Sigma$ state has been predicted to be situated between $^{5}\Delta_{2}$ and $^{5}\Delta_{1}$ substates, and hence the ground state is $^{5}\Delta_{i}$. Spectroscopic constants for $^{5}\Delta$ state (and those for $^{5}\Sigma$ state) are predicted as follows: $r_{e}=2.2047$ (1.9963) \AA; $B_{0}=6043.2 (6217.1)$ MHz; $D_{0}=3.80 (3.64)$ kHz; $\nu=510.4 (543.9) cm^{-1}$; $\mu_{e}=5.92 (4.88)D$

MR-SDCI + Q AB INITIO MOLECULAR ORBITAL CALCULATIONS OF FeCO: IMPORTANCE OF WELL-CONTRIVED SA-MCSCF WAVE FUNCTIONS AND 8−10σ8-10\sigma ELECTRON CORRELATIONS

$^{a}$A. Ricca and C. W. Bauschlicher, Theor Chem. Acc. 106, 314, (2001). $^{b}$T. Noro, M. Sekiya, T. Koga, and H. Matsuyama, Theor Chem. Acc. 104, 146, (2000). $^{c}$P. W. Villalta and D. G. Leopold, J. Chem. Phys. 98, 7730, (1993). $^{d}$K. Tanaka, S. Sakaguchi, and T. Tanaka, J. Chem. Phys. 106, 2118, (1997).Author Institution: Department of Chemistry, Faculty of Science, Ochanomizu University; Tsukuba Advanced Computing Center, National Institute of Advanced Industrial Science and Technology; Division of Chemistry, Graduate School of Science, Hokkaido UniversityFeCO has been used as a benchmark molecule to evaluate the basis functions for $Fe^{a,b}$ and the methods of calculation since it is known to be difficult to reproduce the energy difference between the $\tilde{a}^{5}\Sigma^{-}$ and the $\tilde{X}^{3}\Sigma^{-}$ states as well as the experimentally observed bond lengths of the $\tilde{X}^{3}\Sigma^{-}$ state by ab initio molecular orbital calculations. We carried out the MR-SDCI + Q and MR-ACPF calculations, based on the state-averaged MCSCF orbitals, taking into account the electron-correlation of $8-10\sigma$ electrons with the active space consisting of Fe 3d, 4s orbitals and $CO \pi, \pi^{\ast}$ orbitals. Our predicted term value of the $\tilde{a}^{5}\Sigma^{-}$ state, bond lengths $r_{e}(Fe-C)$ and $r_{e}(CO)$ of the $\tilde{X}^{3}\Sigma^{-}$ state are 0.87 kcal $mol^{-1}, 1.720 {\AA}$, and $1.159 {\AA}$ with relativistic energy corrections, which are to be compared with the corresponding experimental values of 3.24 kcal $mol^{-1},{^{c}} 1.7270 {\AA} [r_{s}(Fe-C)],^{d}$ and $1.1586 {\AA} [r_{s}(CO)],^{d}$ respectively. Similar results have also been obtained by the MR-ACPF methods

AN AB INITIO MOLECULAR ORBITAL STUDY ON THE STRUCTURE AND SPECTROSCOPIC PROPERTIES OF MAGNESIUM DICARBIDE

Author Institution: Department of Chemistry, Faculty of Science, Ochanomizu University; National Institute of Materials and Chemical Research, 1-1 Higashi; Hiroshima City University, 3-4-1 Ozukahigashi; Nobeyama Radio Obverservatory, MinamimakiA magnesium-bearing molecule $MgC_{2}$ is one of the candidate to be found in the envelope of a carbon star. Element magnesium may be rich in astrophysical objects because the elemental cosmic abundance of magnesium and silicon is almost the same, while silicon containing molecules such as $SiO, SiS, SiC, SiC_{2}, SiC_{4}, SiH_{4}, SiN$, have been found in interstellar space. Silicon dicarbide $SiC_{2}$ was discovered in IRC+10216. Now, magnesium dicarbide, $MgC_{2}$, is strongly expected to be observed. Since none of the experimental spectroscopic data on $MgC_{2}$ has been reported in any frequency region, ab initio molecular orbital predicition has been requested for its identification. The MR-SDCI+Q calculations with augmented cc-pVQZ basis sets have predicted that the ground state $MgC_{2} (^{1}A_{1})$ has T-shaped structure of $C_{2v}$ symmetry consisting of $Mg^{-}$ cation and $CC^{-}$ moiety with the dipole moment of 7.9 Debye. The CC and MgC distances have been found to be 1.275 and 2.012 \AA in its equilibrium geometry. The MR-SDCI+Q three-dimensional potential energy surface consisting of 497 points were analysed by the 2nd-order perturbation theory, predicting the rotational constants $A_{0}, B_{0}$, and $C_{0}$ to be $51794.0, 11493.9$ and 9378.7 MHz, and the centrifugal distortion constants $\Delta_{J}, \Delta_{JK}, \Delta_{K}, \delta_{J}, \delta_{K}$ to be $0.014, 0.21, -0.023, 0.0027, 0.14 MHz$, respectively. The $\nu_{1} (CC stretching), \nu_{2}(Mg-C_{2} streching)$, and $_{3}$(bending) vibrational frequencies have been estimated to be 1704.2, 594.8 and $455.8 cm^{-1}$, respectively. These results indicate that $MgC_{2}$ molecule is a rigid molecule unlike the analogue $SiC_{2}$, which is known as a molecule with large-amplitude motion. Toward the laser induced fluorescent spectroscopy, the vertical excitation energies for $\bar{A}^{1}A_{1}\leftarrow \tilde{X}{^{1}}A_{1}$ and $\bar{B}{^{1}}B_{2}$ transitions have also been calculated to be 8334 and $13034 cm^{-1}$, respectively, at the CAS-SCF level of theory

AN AB INITIO MOLECULAR ORBITAL STUDY OF LOW LYING ELECTRONIC EXCITED STATES OF FeC

$^{a}$K. Aiuchi, K. Tsuji and K. Shibuya Chem. Phys. Lett. 309, 229, (1999).Author Institution: Department of Chemistry, Faculty of Science, Ochanomizu University,; Department of Chemistry, Faculty of Science, National Institute for Advanced Interdisciplinary Research; Department of Chemistry, Graduate School of Science and Engineering, Tokyo Institute of TechnologySpectroscopic constants and energy levels of the ground and low-lying excited states of iron carbide, FeC, have been calculated from potential energy functions obtained by the ab initio MR-SDCI molecular orbital approach. Investigated states are $^{1}\Delta$ and $^{5}\Pi$, both of which are proposed by DF $spectra^{a}$ as a candidate for the new $\Omega = 2$ electronic state observed above the $^{3}\Delta_{2}$ state by $3460 cm^{-1}$. The character of each electronic state has been discussed theoretically. Contrary to the previous tentative assignment to $^{5}\Pi$, the MR-SDCI results predict that the observed $\Omega = 2$ state should be the $^{1}\Delta$ state and be located at $3252 cm^{-1}$ above $^{3}\Delta_{2}$. The spin-orbit coupling constant for $^{3}\Delta$ state has also been calculated