K. Balasubramanian

A comparative theoretical study of bonding in UO2++,UO2+,UO2,UO2−, OUCO, O2U(CO)2 and UO2CO3

Abstract Extensive ab initio calculations have been carried out to study the structure and bonding characteristics of UO2++,UO2,UO2+,UO2−,OUCO,O2U(CO)2, and UO2CO3 using various methods including complete active space multiconfiguration self consistent filed (CASMCSCF) and multireference singles plus doubles configuration (MRSDCI) techniques. The calculated vibrational frequencies of these species are compared with the matrix spectra of Andrews and coworkers. The nature of bonding in these species is discussed.

Infrared vibronic absorption spectrum and spin–orbit calculations of the upper spin–orbit component of the Au3 ground state

Laser-ablated gold co-deposited with excess neon at 3.5 K produces a new sharp 2025.5-cm−1 absorption. Annealing to 8 K increases this absorption ten-fold and produces six weaker associated vibronic bands with 172- and 118-cm−1 intervals. Relativistic configurations (RCI) computations are carried out for several electronic states lying below 4.4 eV. These calculations show that the spin–orbit effect stabilizes the D3h structure and quenches Jahn–Teller distortion. They also predict a 0.2-eV spin–orbit splitting of the Au3 ground state, which is in excellent agreement with the 2025.5-cm−1 neon matrix band origin. We have also confirmed the assignment of the A-X system at 1.8 eV observed by Bishea and Morse. The observed vibronic intervals are in accord with calculated a1′ and e′ stretching fundamentals and they confirm the D3h geometry for Au3. This work reports the first observation of the ground-state spin–orbit splitting of a heavy metal trimer.

Spectroscopic properties of mixed gallium arsenide tetramers: GaAs3±, GaAs3, Ga3As±, and Ga3As

Spectroscopic properties of the low-lying electronic states of Ga3As, As3Ga, and their anions and cations are computed by the complete active-space self-consistent field (CASSCF) followed by multireference singles+doubles configuration interaction (MRSDCI) calculations that included up to 4.4 million configurations. Whereas the ground state of Ga3As is found to be a symmetric triangular pyramidal (C3v) 1A1 structure, the ground state of As3Ga is predicted to undergo Jahn-Teller distortion to a Cs 1A′ state with a folded geometry. The ground state of the Ga3As− ion is found to be surprisingly a planar (C2v) structure but the ground state of the GaAs3− ion exhibits a Cs nonplanar Jahn-Teller distorted geometry. The energy separations of a number of excited electronic states have been computed to predict the spectra of these species. The equilibrium geometries, vibrational frequencies, atomization energies, adiabatic ionization potentials, electron affinities, and other properties for the electronic states o...

Geometries and spectroscopic properties of silicon clusters (Si5, Si5+, Si5−, Si6, Si6+, and Si6−)

Ground and excited electronic states of the neutral, cationic, and anionic silicon pentamer and hexamer (Si5 and Si6) are investigated. Different geometries such as trigonal bipyramid (TBP; D3h), distorted-TBP (C2v), and edge-capped tetrahedron (ECT; C2v) for Si5 and tetragonal bipyramid (TEBP; D4h, D2h), edge-capped trigonal bipyramid (ECTBP; C2v) for Si6 were studied. We have employed a number of techniques such as large scale complete active-space multiconfiguration self-consistent field (CAS-MCSCF), mutireference singles+doubles configuration interaction (MRSDCI) computations up to 12 million configurations, Moller–Plesset (MP2) and coupled cluster singles and doubles+triple excitation estimate [CCSD(T)] techniques to investigate the low-lying electronic states, their geometries and energy separations of neutral, cationic and anionic Si5 and Si6. A 1A1g TEBP structure (D4h symmetry) is computed as the ground state for Si6, in accord with the previously suggested experimental assignments, while the Si5...

New theoretical insight into the interactions and properties of formic acid: Development of a quantum-based pair potential for formic acid

We performed ab initio quantum-chemical studies for the development of intra- and intermolecular interaction potentials for formic acid for use in molecular-dynamics simulations of formic acid molecular crystal. The formic acid structures considered in the ab initio studies include both the cis and trans monomers which are the conformers that have been postulated as part of chains constituting liquid and crystal phases under extreme conditions. Although the cis to trans transformation is not energetically favored, the trans isomer was found as a component of stable gas-phase species. Our decomposition scheme for the interaction energy indicates that the hydrogen-bonded complexes are dominated by the Hartree-Fock forces while parallel clusters are stabilized by the electron correlation energy. The calculated three-body and higher interactions are found to be negligible, thus rationalizing the development of an atom-atom pair potential for formic acid based on high-level ab initio calculations of small formic acid clusters. Here we present an atom-atom pair potential that includes both intra- and inter molecular degrees of freedom for formic acid. The newly developed pair potential is used to examine formic acid in the condensed phase via molecular-dynamics simulations. The isothermal compression under hydrostatic pressure obtained from molecular-dynamics simulations is in good agreement with experiment. Further, the calculated equilibrium melting temperature is found to be in good agreement with experiment.

Theoretical study of aqueous uranyl carbonate (UO2CO3) and its hydrated complexes: UO2CO3.nH2O (n = 1-3)

Abstract Extensive ab initio calculations have been carried out to study the structure and bonding of hydrated UO 2 CO 3 complexes using state-of-the-art techniques. The structures of aqueous UO 2 CO 3 and its hydrated complexes have been further studied by considering the solvent as a polarizable continuum dielectric. The calculations have been carried out using polarization continuum, self-consistent isodensity polarizable continuum model, and conductor like screen models. The calculated uranyl frequencies of the hydrated UO 2 CO 3 have been compared with the available vibrational frequencies and the nature of water binding has been analyzed using energy decomposition techniques.

Electronic structure and spectroscopic properties of electronic states of VC2, VC2−, and VC2+

Theoretical studies on the electronic and thermodynamic properties of several electronic states are presented for the VC2 molecule, the VC2− anion, and the VC2+ cation employing state-of-the-art techniques that included up to 12 million configurations. The ground and the low-lying electronic states of these three species have been found to have C2v triangular structures. On the basis of our computed results, we have suggested an assignment of the observed anion photodetachment spectra of VC2− and predicted transitions that were not observed. Our computed electron affinity is in excellent agreement with experiment. The observed thermodynamic properties of reactions involving VC2, VC2−, and VC2+ are corrected using the computed gas phase properties of the molecule and the partition functions. The bent quartet states of VC2 exhibit large dipole moments (8.65−9.3u200aD).

Spectroscopic properties of lead trimer (Pb3 and Pb3+): Potential energy surfaces, spin–orbit and Jahn–Teller effects

Spectroscopic properties of the low-lying electronic states of neutral, cationic, and anionic lead trimer (Pb3) are investigated. We have obtained the bending potential energy surfaces of several electronic states of Pb3 and Pb3+ both with and without spin–orbit coupling. These computations were carried out using high level techniques that included electron correlation effects and spin–orbit coupling simultaneously using a multireference relativistic configuration interaction (RCI) scheme in the double group, subsequent to complete active-space–multiconfiguration self-consistent-field (CAS–MCSCF) computations. We have computed the equilibrium geometries, vibrational frequencies, excitation energies, atomization energies, ionization potentials, and adiabatic electron affinities. Our computations facilitated the assignment of the anion photodetachment spectra of Pb3− and explained the “closed-shell singlet like” structures in the observed photodetachment spectra. Our computations show that spin–orbit coupli...

Potential energy surfaces for the uranium hydriding reaction

We have computed the potential energy surfaces for the low-lying electronic states of uranium hydrides, UHn (n=1–3), which are important in the uranium hydriding reactions. We have employed a number of computational methods including the complete active space multiconfiguration self-consistent field followed by multireference relativistic configuration interaction computations with spin–orbit coupling that included up to 6 million configurations. We find that the activation barrier to insert uranium into H2 is reduced substantially by spin–orbit coupling, and the product species UH2 in its A1 spin–orbit ground state is substantially stable over U(5L)+H2 dissociated products. We have found two electronic states for UH to be quite close to each other, and depending on the level of theory the relative ordering of the 6Λ and 4I states changes, 4I state being the lowest at the highest second-order configuration interaction level. The UH2 species also exhibits a similar feature in that the triplet state is favo...

Spectroscopic properties of novel aromatic metal clusters: NaM4 (M=Al,Ga,In) and their cations and anions.

The ground- and several excited states of metal aromatic clusters, namely NaM(4) and NaM(4) (+/-) (M=Al,Ga,In) clusters have been investigated by employing complete active-space self-consistent-field followed by multireference singles and doubles configuration interaction computations that included up to 10 million configurations and other methods. The ground states NaM(4) (-) of aromatic anions are found to be symmetric C(4nu) ((1)A(1)) electronic states with ideal square pyramid geometries. While the ground state of NaIn(4) is also predicted to be a symmetric C(4nu) ((2)A(1)) square pyramid, the ground state of the NaAl(4) cluster is found to have a C(2nu) ((2)A(1)) pyramid with a rhombus base, and the ground state of NaGa(4) possesses a C(2nu) ((2)A(1)) pyramid with a rectangle base. In general, these structures exhibit two competing geometries, viz., an ideal C(4nu) structure and a distorted rhomboidal or rectangular pyramid structure (C(2nu)). All of the ground states of the NaM(4) (+) (M=Al,Ga,In) cations are computed to be C(2nu) ((3)A(2)) pyramids with rhombus bases. The equilibrium geometries, vibrational frequencies, dissociation energies, adiabatic ionization potentials, adiabatic electron affinities for the electronic states of NaM(4) (M=Al,Ga,In), and their ions are computed and compared with experimental results and other theoretical calculations. On the basis of our computed excited states energy separations, we have tentatively suggested assignments to the observed X and A states in the anion photoelectron spectra of Al(4)Na(-) reported by Li et al. [X. Li, A. E. Kuznetov, H. F. Zheng, A. I. Boldyrev, and L. S. Wang, Science 291, 859 (2001)]. The X state can be assigned to a C(2nu) ((2)A(1)) rhomboidal pyramid. The A state observed in the anion spectrum is assigned to the first excited state ((2)B(1)) of the neutral NaAl(4) with the C(4nu) symmetry. The assignments of the excited states are consistent with the experimental excitation energies and the previous Greens function-based methods for the vertical transition energy separations between the X and A bands.