Rei Okuda

Theoretical study of [Na(NH3)n]− (n=1–4)

In connection with the recent photoelectron spectroscopy of negatively charged Na atom in ammonia clusters, the geometries, electronic state, vertical detachment energies, and harmonic frequencies of [Na(NH3)n]− (n=1–4) have been studied by the ab initio MO method. Structures having as many Na–N bonds as possible becomes more stable than other isomers as n grows. The Na 3s electrons are widely spread and delocalized in space outside the [Na(NH3)n]+ core for n⩾2. The dramatic redshifts of the photoelectron band for the 32P-type transition with increasing n reflect the electronic change from an atomic state to one-center Rydberg-type states in the neutrals. The frequencies of the combined vibrations of the NH3 ν1 mode are nearly degenerate and are expected to coalesce into only one strong IR band in the NH stretch region irrespective of n.

Solvation process of Nam in small ammonia clusters: photoelectron spectroscopy of Nam−(NH3)n (m≤3)

Abstract Photoelectron spectra (PESs) of Nam−(NH3)n (m≤3) are investigated to explore the solvation of sodium atom and its aggregates in small ammonia clusters. For Na−(NH3)n, we examine the PESs with an improved resolution and confirm the spontaneous ionization of the Na atom in small clusters. As for Na2−(NH3)n (n≤8), vertical detachment energy (VDE) of the transition from the anion to the neutral ground state is found to shift to the red with respect to that of Na2(X1∑g+), while that of the first excited state derived from Na2(a3∑u+) increases gradually for n⩾4. In addition, the transitions to the higher-excited states derived from those correlated to the 32S+32P asymptote are found to be shifted rapidly to the red and become almost degenerate with the a3∑u+-type transition. The extensive spectral change is ascribed to the asymmetrical ammoniation and spontaneous ionization of Na2 in small clusters. We also find a drastic change in the PESs of Na3−(NH3)n; the neutral ground (2∑u+-type) and first excited (2∑g+-type) states of these clusters become degenerate with each other as the number of NH3 molecules increases. With the aid of the theoretical calculations, this spectral change is ascribed to the dissociation of the Na3− core in clusters. This may be the first observation of the dissolution of metal aggregates in small clusters.

Geometries and electronic structures of the ground and low-lying excited states of FeCO: An ab initio study

FeCO is a molecule of astrophysical interest. We report here theoretical calculations of its geometrical parameters, electronic structures, and molecular constants (such as dipole moment and spin-orbit coupling constant) in the electronic ground state X(3)Σ(-) and the low-lying triplet and quintet excited states. The calculations were made at the MR-SDCI+Q_DK3/[5ZP ANO-RCC (Fe, C, O)] and MR-AQCC_DK3/[5ZP ANO-RCC (Fe, C, O)] levels of theory. A multi-reference calculation was required to describe correctly the wavefunctions of all states studied. For all triplet states, the σ-donation through the 10σ molecular orbital (MO) as well as the π-back-donation through the 4π MO are observed, and the dipole moment vector points from O toward Fe as expected. However, in the excited quintet states (5)Π, (5)Φ, and (5)Δ, the almost negligible contribution of Fe 4s to the 10σ MO makes the dipole moment vector point from Fe toward O, i.e., in the same direction as in CO. In the X(3)Σ(-) state, the electron provided by the σ-donation through the 10σ MO is shared between the Fe atom and the C end of the CO residue to form a coordinate-covalent Fe-C bond. In the ã(5)Σ(-) state (the high-spin counterpart of X(3)Σ(-)), the σ-donation through the 10σ MO is not significant and so the Fe-C bond is rather ionic. The π-back-donation through the 4π MO is found to be of comparable importance in the two electronic states; it has a slightly larger magnitude in the X(3)Σ(-) state. The difference in the molecular properties of the low-spin X(3)Σ(-) and the high-spin ã(5)Σ(-) states can be understood in terms of the dynamical electron correlation effects.