Levent Sari

3Σ− and 3Π states of GeC and GeSi: The problematic dissociation energy of GeC

The 3Σ− and 3Π states for the GeC and GeSi diatomics have been investigated at the highly correlated coupled-cluster levels of theory. Large basis sets [including TZ3P(2d,2f)+2diff, cc-pVQZ, and aug-cc-pVQZ] were employed in order to predict reliable values for the experimentally unknown spectroscopic properties. The ground states were confirmed to be the 3Π state for GeC and the 3Σ− state for GeSi. Advanced theoretical treatments such as inclusion of core-valence correlation, scalar relativity, and complete basis set extrapolations have been performed to determine accurate energetic properties. The dissociation energies (D0) of 91.4 kcal/mol and 72.9 kcal/mol have been predicted for the X 3Π state of GeC and X 3Σ− state of GeSi, respectively. It is observed that the theoretical value of 72.9 kcal/mol for GeSi is in very good agreement with the experimental value of 71±5 kcal/mol. However, the predicted dissociation energy for the X 3Π state of GeC is found to be 18 kcal/mol smaller than the mass spectrom...

An L-shaped equilibrium geometry for germanium dicarbide (GeC2)? Interesting effects of zero-point vibration, scalar relativity, and core–valence correlation

The ground state potential energy surface of the GeC2 molecule has been investigated at highly correlated coupled cluster levels of theory. Large basis sets including diffuse functions and functions to describe core correlation effects were employed in order to predict the true equilibrium geometry for GeC2. Like the much-studied valence isoelectronic SiC2, the linear (1∑+), L-shaped (1A′), and T-shaped structures (1A1) must be investigated. The L-shaped Cs geometry is found to have real harmonic vibrational frequencies along every internal coordinate, and the linear stationary point has an imaginary vibrational frequency along the bending mode at every level of theory employed. The T-shaped geometry is found to have an imaginary vibrational frequency along the asymmetric stretching mode. At the coupled cluster with single and double excitations and perturbative triple excitations [CCSD(T)]/correlation consistent polarized valence quadrupole-ζ (cc-pVQZ) level, the nonrelativistic classical relative energi...

The X̃ 2Π and à 2Σ+ electronic states of the HCSi radical: Characterization of the Renner–Teller effect in the ground state

The electronic structures of the ground and lowest lying excited state of the silicon methylidyne radical (HCSi) have been investigated at the self-consistent field, configuration interaction with single and double excitations, coupled cluster with single and double excitations (CCSD), and CCSD including a perturbative expansion for connected triples CCSD(T) levels of theory with a wide range of basis sets. The total energies and physical properties including equilibrium geometries, dipole moments, harmonic vibrational frequencies, and Renner–Teller splitting are reported. At our highest level of theory [CCSD(T)/cc-pVQZ], the ground electronic state (X 2Π) has a linear geometry with re(CH)=1.0781 A and re(CSi)=1.6956 A. This is in good agreement with the experimental values of r0(CH)=1.0677 A and r0(CSi)=1.6925 A, respectively. In the A 2Σ+ state, HCSi is also found to have a linear geometry with re(CH)=1.0737 A and re(CSi)=1.6130 A at the [CCSD(T)/cc-pVQZ] level, confirming experimental values of r0(CH)...

Coupled cluster study of the X̃ 2Π and à 2Σ+ electronic states of the HCGe radical: Renner–Teller splitting and the effects of relativistic corrections

The X 2Π and A 2Σ+ states of the germanium methylidyne radical (HCGe) have been investigated at the SCF, CISD, CCSD, and CCSD(T) levels of theory. The total energies, equilibrium geometries, dipole moments, harmonic vibrational frequencies, infrared intensities, and Renner–Teller splitting are reported. The relativistic one-electron Darwin and mass-velocity terms are calculated using first-order perturbation theory and the effects of these corrections on energetics, harmonic vibrational frequencies, and Renner–Teller splitting are discussed. At our highest level of theory [CCSD(T)/cc-pVQZ], the ground electronic state (X 2Π) has a linear geometry with re(CH)=1.079 A and re(CGe)=1.769 A, in good agreement with the experimental values of r0(CH)=1.067 A and r0(CGe)=1.776 A. In the electronically excited A 2Σ+ state, HCGe is also found to have a linear geometry with re(CH)=1.074 A and a much shorter re(CGe)=1.669 A at the [CCSD(T)/cc-pVQZ] level, in agreement with experimental values of r0(CH)=1.059 A, r0(C...

Theoretical characterization of the disilaethynyl anion (Si2H

The singlet-state potential energy surface of the disilaethynyl anion (Si2H−) has been investigated using ab initio self-consistent-field (SCF), configuration interaction with single and double excitations (CISD), coupled cluster with single and double excitations (CCSD), and CCSD with perturbative triple excitations [CCSD(T)] levels of theory with large basis sets. Four stationary points [cyclic (monobridged) 1A1 (C2v), linear 1Σ+ (C∞v), bent 1A′ (Cs), and quasilinear 1A′ (Cs) structures] were located with the correlated wave functions, while only two stationary points [cyclic (monobridged) 1A1 (C2v) and linear 1Σ+ (C∞v) structures] were found with the SCF method. The cyclic structure (C2v) is predicted to be the global minimum at all levels of theory. The linear structure (C∞v) is found to be a transition state between the two quasilinear structures (Cs) at the correlated levels of theory, while the SCF linear structure is predicted to be a transition state between the two cyclic structures. The quasili...

The singlet electronic ground state isomers of dialuminum monoxide: AlOAl, AlAlO, and the transition state connecting them

The singlet electronic ground state isomers, X (1)Sigma(g) (+) (AlOAl D(infinityh)) and X (1)Sigma(+) (AlAlO C(infinitynu)), of dialuminum monoxide have been systematically investigated using ab initio electronic structure theory. The equilibrium structures and physical properties for the two molecules have been predicted employing self-consistent field (SCF) configuration interaction with single and double excitations (CISD), multireference CISD (MRCISD), coupled cluster with single and double excitations (CCSD), CCSD with perturbative triples [CCSD(T)], CCSD with iterative partial triple excitations (CCSDT-3 and CC3), and full triples (CCSDT) coupled cluster methods. Four correlation consistent polarized valence (cc-pVXZ) type basis sets were used. The AlAlO system is rather challenging theoretically. The two isomers are confirmed to have linear structures at all levels of theory. The symmetric isomer AlOAl is predicted to lie 81.9 kcal mol(-1) below the asymmetric isomer AlAlO at the cc-pV(Q+d)Z CCSD(T) level of theory. The predicted harmonic vibrational frequencies for the X (1)Sigma(g) (+) AlOAl molecule, omega(1)=517 cm(-1), omega(2)=95 cm(-1), and omega(3)=1014 cm(-1), are in good agreement with experimental values. The harmonic vibrational frequencies for the X (1)Sigma(+) AlAlO structure, omega(1)=1042 cm(-1), omega(2)=73 cm(-1), and omega(3)=253 cm(-1), presently have no experimental values with which to be compared. With the same methods the barrier heights for the isomerization AlOAl-->AlAlO and AlAlO-->AlOAl reactions were predicted to be 84.3 and 2.4 kcal mol(-1), respectively. The dissociation energies D(0) for AlOAl (X (1)Sigma(g) (+)) and AlAlO (X (1)Sigma(+))-->AlO (X (2)Sigma(+))+Al ((2)P) were determined to be 130.8 and 48.9 kcal mol(-1), respectively. Thus, both symmetric AlOAl (X (1)Sigma(g) (+)) and asymmetric AlAlO (X (1)Sigma(+)) isomers are expected to be thermodynamically stable with respect to the dissociation into AlO (X (2)Sigma(+)) + Al ((2)P) and kinetically stable for the isomerization reaction (AlAlO-->AlOAl) at sufficiently low temperatures.

CHARACTERIZATION OF THE

The electronic ground state and first excited state (A2Σ+) of phosphaethyne cation (HCP+) have been systematically investigated using ab initio electronic structure theory. The total energies, geometries, rotational constants, dipole moments, harmonic vibrational frequencies, and parameters for Renner–Teller splittings were determined using self-consistent-field (SCF), configuration interaction with single and double excitations (CISD), coupled cluster (CC) with single and double excitations (CCSD), CCSD with perturbative triple excitations [CCSD(T)], CC with single, double, and iterative partial triple excitations (CCSDT-3), and CC with single, double, and full triple excitations (CCSDT) methods and eight different basis sets. Some of the largest full triples coupled cluster computations to date are reported. Degenerate bending frequencies for the A2Σ+ state were determined using the equation-of-motion (EOM)-CCSD technique. The two states have been confirmed to have linear equilibrium structures. At the full CCSDT level of theory with the correlation-consistent polarized valence quadruple zeta (cc-pVQZ) basis set, the classical splitting (Te value) is predicted to be 47.7 kcal/mol (2.07 eV, 16,700 cm-1) and the quantum mechanical splitting (T0 value) to be 48.1 kcal/mol (2.08 eV, 16,800 cm-1), which are in excellent agreement with the experimental values of Te = 47.77 kcal/mol (2.072 eV, 16,708 cm-1) and T0 = 47.94 kcal/mol (2.079 eV, 16,766 cm-1). The excitation energies predicted by the CCSDT-3 and CCSD(T) methods differ from the full triples CCSDT result by 0.38 and 0.45 kcal/mol, respectively. With the aug-cc-pVQZ CCSDT-3 method the Renner parameter and the averaged harmonic bending vibrational frequency are determined to be ∊= -0.0390 and for the ground state of HCP+, which are reasonably consistent with the experimental values of ∊=-0.0415 and . The predicted dipole moments are 1.30 Debye ( state, polarity-hydrogen atom positive) and 0.06 Debye (A2Σ+ state, polarity-phosphorus atom positive).

\tilde{X}\,^2 \Pi

The three lowest-lying electronic states, [Xtilde] 1Σ+, à 3II and à 1II, of the linear BBO molecule have been systematically investigated using ab initio electronic structure theory. The equilibrium structures and physical properties including dipole moments, vibrational frequencies and associated infrared intensities, Renner parameters and energetics for the three states of BBO have been determined employing SCF, CISD, CCSD and CCSD(T) levels of theory and a wide range of basis sets. The ground state of BBO presents a degenerate real bending frequency, while the à 3II and à 1II states show two distinct real bending frequencies due to the Renner-Teller interaction. The bending motion of the à 1II state was analysed using the equation-of-motion (EOM)-CCSD and EOM-CC3 techniques in order to avoid possible variational collapse to a lower-lying state. The [Xtilde] 1Σ+-à 3II separation was predicted to be T 0 = 16.6 kcal mol−1 (5800 cm−1, 0.719 eV) at the cc-pVQZ CCSD(T) level of theory. With the cc-pVQZ EOM-CC3 method the [Xtilde] 1Σ+-à 1II splitting was predicted to be T 0 = 48.0 kcal mol−1 (16 800 cm−1, 2.08 eV), which is in good agreement with the experimental value of T 0 = 46.6 kcal mol−1 (16 300 cm−1, 2.02 eV). The Renner parameters and averaged harmonic frequencies of the bending mode were determined to be ε = 0.184 and ω2 = 363 cm−1 for the à 3II state, and ε = 0.246 and ω2 = 383cm−1 for the à 1II state. The theoretical [Xtilde] 1Σ+ state harmonic B-B stretching frequency ω3 = 636 cm−1 is somewhat higher than the experimental estimate of 582 cm−1 and the predicted à 1II state harmonic B-B stretching frequency ω3 = 861 cm−1 is significantly higher than the experimental estimate of 440 cm−1