Ab initio investigations of the excited electronic states of caoca

Chemical bonding in alkaline earth hypermetalic oxides is of fundamental interest. Previous Ab initio studies of CaOCa predicted a centrosymmetric linear geometry for both the $^{1}\Sigma_{g}^+$ ground state and the low lying triplet $^{3}\Sigma_{u}^+$ state\footnote{B. Ostojic{\'i}, P.R. Bunker, P. Schwerdtfeger, Artur Gertych, and Per Jensen, Journal of Molecular Structure 1023 (2012) 101–107.}. However, there have been no reports concerning the higher energy singlet and triplet states. The present work is focused on characterization of the potential energy surface (PES) of the excited $^{1}\Sigma_{u}^+$ state (assuming a centrosymmetric linear geometry) and obtaining predictions for the $^{1}\Sigma_{u}^+\leftarrow^{1}\Sigma_{g}^+$ vibronic transitions. We employed the multireference configuration interaction (MRCISD) method with state-averaged, full-valence complete active space self-consistent field (SA-FV-CASSCF) wavefunctions. In these calculations, the active space consisted of ten valence electrons in twelve orbitals, where all the valence electrons were correlated. Contributions of higher excitation and relativistic effects were taken into account using the Davidson correction and the Douglas-Kroll (DK) Hamiltonian, respectively. The correlation-consistent polarized weighed core-valence quadruple zeta basis set (cc-pwCVQZ-DK) was used for all three atoms. The full level of theory is abbreviated as SA-FV-CASSCF (10,12)-MRCISD-Q/cc-pwCVQZ-DK. The calculations were carried out using the MOLPRO2012 suite of programs. For the centrosymmetric linear geometry in all states, initial investigations of one-dimensional radial cuts provided equilibrium bond distances of 2.034 {\AA}, 2.034 {\AA}, and 1.999 {\AA} for the $^{1}\Sigma_{g}^+$ , $^{3}\Sigma_{u}^+$ , and $^{1}\Sigma_{u}^+$ states, respectively. The vertical excitation frequency of the $^{1}\Sigma_{u}^+\leftarrow^{1}\Sigma_{g}^+$ optical transition was calculated to occur at 14801 \wn . These predictions were followed by spectroscopic searches by Heaven et al. Indeed, rotationally resolved vibronic progressions were recorded in the vicinity of the predicted electronic band origin. Calculation of the three-dimensional PES showed that the potential minimum in the $^{1}\Sigma_{u}^+$ corresponds to a bent equilibrium geometry with a bond angle of $120^{\circ}$ (C$_{2v}$ point group, where the electronic symmetry is $^{1}A_{1}$). This result suggests that the Ca-O bonds in CaOCa possess covalent character in the $^{1}A_{1}$ excited state and ionic character in the $^{1}\Sigma_{g}^+$ ground state. The current results, as well as those from ongoing studies will be presented

Ab initio investigations of the excited electronic states of caoca

Chemical bonding in alkaline earth hypermetalic oxides is of fundamental interest. Previous Ab initio studies of CaOCa predicted a centrosymmetric linear geometry for both the $^{1}\Sigma_{g}^+$ ground state and the low lying triplet $^{3}\Sigma_{u}^+$ state\footnote{B. Ostojic{\'i}, P.R. Bunker, P. Schwerdtfeger, Artur Gertych, and Per Jensen, Journal of Molecular Structure 1023 (2012) 101–107.}. However, there have been no reports concerning the higher energy singlet and triplet states. The present work is focused on characterization of the potential energy surface (PES) of the excited $^{1}\Sigma_{u}^+$ state (assuming a centrosymmetric linear geometry) and obtaining predictions for the $^{1}\Sigma_{u}^+\leftarrow^{1}\Sigma_{g}^+$ vibronic transitions. We employed the multireference configuration interaction (MRCISD) method with state-averaged, full-valence complete active space self-consistent field (SA-FV-CASSCF) wavefunctions. In these calculations, the active space consisted of ten valence electrons in twelve orbitals, where all the valence electrons were correlated. Contributions of higher excitation and relativistic effects were taken into account using the Davidson correction and the Douglas-Kroll (DK) Hamiltonian, respectively. The correlation-consistent polarized weighed core-valence quadruple zeta basis set (cc-pwCVQZ-DK) was used for all three atoms. The full level of theory is abbreviated as SA-FV-CASSCF (10,12)-MRCISD-Q/cc-pwCVQZ-DK. The calculations were carried out using the MOLPRO2012 suite of programs. For the centrosymmetric linear geometry in all states, initial investigations of one-dimensional radial cuts provided equilibrium bond distances of 2.034 {\AA}, 2.034 {\AA}, and 1.999 {\AA} for the $^{1}\Sigma_{g}^+$ , $^{3}\Sigma_{u}^+$ , and $^{1}\Sigma_{u}^+$ states, respectively. The vertical excitation frequency of the $^{1}\Sigma_{u}^+\leftarrow^{1}\Sigma_{g}^+$ optical transition was calculated to occur at 14801 \wn . These predictions were followed by spectroscopic searches by Heaven et al. Indeed, rotationally resolved vibronic progressions were recorded in the vicinity of the predicted electronic band origin. Calculation of the three-dimensional PES showed that the potential minimum in the $^{1}\Sigma_{u}^+$ corresponds to a bent equilibrium geometry with a bond angle of $120^{\circ}$ (C$_{2v}$ point group, where the electronic symmetry is $^{1}A_{1}$). This result suggests that the Ca-O bonds in CaOCa possess covalent character in the $^{1}A_{1}$ excited state and ionic character in the $^{1}\Sigma_{g}^+$ ground state. The current results, as well as those from ongoing studies will be presented

CORRELATED AB INITIO STUDY OF THE GROUND ELECTRONIC STATE OF THE H2_2{--}O2−_2^{-} COMPLEX

Wafaa M. Fawzy, in preparation for publicationAuthor Institution: Department of Chemistry, Murray State University, Murray, KY 42071The H$_2${--}O$_2^{-} (X$^2$\Pi$_g$) complex has been examined using the coupled-cluster theory at the CCSD(T)/aug-cc-pVDZ and CCSD(T)/aug-cc-pVTZ levels. Electronic structure calculations show that the global minimum energy structure corresponds to a planar bent geometry with a well depth of 1550 cm$^{-1}$. For this geometry, the distance between centers of masses of moieties of the complex is 2.57{\AA}, angstroms and the angles between the internuclear axes of the superoxide radical and the hydrogen molecule with respect to the axis that connects their centers of masses are$104^{irc}$and$165^{irc}$, respectively. These results indicate that the hydrogen molecule and the superoxide radical are held together by a strong hydrogen bond within the complex. Results of the current work will be discussed and compared to results of our recent ab initio study of the H$_2${--}O$_2$(X$^3$\Sigma$_g^{-}$) complex

Ab initio exploration of the potential energy surface of the O2-SO2 open-shell complex.

The O$_2$-SO$_2$ complex is believed to be a precursor to acid rain. The previously observed FTMW spectrum suggested internal motions within the complex, but their nature was not identified. Development of an effective Hamiltonian for an open-shell molecule with tunneling requires knowledge of the potential energy surface (PES) and the intrinsic reaction coordinates (IRC) for the paths between minima. A recent ab initio study reported two different nonplanar minima in the ground electronic state of O$_2$-SO$_2$. These predictions were based on geometry optimization calculations at the MP2/aug-cc-pVnZ level of theory, with n = 2 and 3. The current work is focused on a highly correlated ab initio investigation of the global PES (a 9-D problem) in the ground triplet electronic state of O$_2$-SO$_2$. Because of the high dimensionality in the complex, the PES calculations are partitioned into several two-dimensional cuts through the PES. We have so far explored only a 3-D part of the global PES to look for stable planar configurations. These calculations included geometry optimization, frequency, and single point energy calculations. Calculations were performed using UCCSD(T)/aug-cc-pV(n+D)Z,where n = 2 and 3, level of theory. We used an axis system that defines the radial and the angular van der Waals coordinates for a planar complex as R$_{vW}$, $\theta_1$, and $\theta_2$. The bond length (R$_{vW}$) is the distance between the center of mass of the O$_2$ unit and the S atom. $\theta_1$ and $\theta_2$ are the angles between the van der Waals bond and the O$_2$ internuclear axis or one of the SO bonds in the SO$_2$ moiety, respectively. Full geometry optimization calculations predicted a minimum of C$_s$ symmetry in which both the O$_2$ and SO$_2$ units are tilted with respect to the van der Waals bond, and R$_{vW}$ = 3.63 {\AA}. 3-D PES surface calculations, which involve the R$_{vW}$, $\theta_1$, and $\theta_2$ vdW coordinates, showed that the optimized structure is the global minimum. In addition, a local minimum at R$_{vW}$ = 3.9 {\AA}, which represents a different chemical isomer, was identified. If the four oxygen atoms are labeled, each isomer is a part of four equivalent minima, and three distinguishable transition states between these various minima are identified. These results suggest that PES calculations should consider at least five dimensions. Our progress in exploring possible non-planar coordinates and IRC paths will also be presented

INTERMOLECULAR INTERACTIONS BETWEEN URACIL AND REACTIVE SPECIES

Author Institution: Department of Chemistry, Murray State University, Murray, KY 42071We have investigated intermolecular interactions between uracil (U) and each of the fluoride ion, superoxide anion, and the hydroxyl radical. Computational study of these new systems presents several challenges, most importantly is the choice of the proper level of theory and the appropriate size of the basis set. Our investigations on the U-F$^{-}$ complex showed that the MP2 and density functional method (DFT) with aug-cc-pVDZ and aug-cc-pVTZ basis sets provide results that are consistent with those obtained with highly correlated methods for H${_2}$O-F$^{-}$. This suggests that these levels of calculations are suitable for exploring the structures and the potential energy surfaces of the U-O${_2}$$^{-}$ and U-OH complexes. Our preliminary results show that each of the F$^{-}$ and the superoxide ions forms a very strong hydrogen bond with a specific hydrogen atom in the uracil ring. Also, results suggest that proton transfer occurs between the bonding site in uracil and each of the F$^{-}$ and O${_2}$$^{-}$ ions. On the other hand, preliminary results show that the OH radical chemically reacts with the uracil molecule. Discussion of details of calculations and results will be presented

AB INITIO INVESTIGATION OF THE ELECTRONIC GROUND STATE OF THE NH−-N2_2 COMPLEX

Author Institution: Department of Chemistry, East Tennessee State University, Johnson City, TN 37504; Department of Chemistry, Emory University, Atlanta, GA 30322The NH$-$N$_2$ van der Waals complex has been examined at the CCSD(T) level of theory using the aug-cc-pVDZ basis set. The full basis set superposition error correction was applied. Two minimum energy structures were located for the electronic ground state. The global minimum corresponds to a linear geometry of the complex (NH$-$N-N), with D$_e$=199 \wn and R$_{cm}$=4.3 \AA. The secondary minimum corresponds to a T-shaped geometry of C$_{2v}$ symmetry, where the nitrogen atom of the H-N moiety points toward the center of mass of the N$_2$ unit, aligned with the $a$-inertial axis of the complex. The binding energy and R$_{cm}$ value for the secondary minimum were 117 \wn and 3.7 \AA, respectively. Results of the current work on the NH$-$N$_2$ complex will be discussed and compared to results of our previous work on the HN$-$H$_2$ complex} {\textbf{122}}, 144318, (2005).}

RADICAL VAN DER WAALS MOLECULES

Author Institution: Department of Chemistry, Emory UniversityComplexes consisting of a rare gas atom bound to a neutral free-radical provide prototype systems for the study of weak bonding interactions. In particular, when the radical possesses a permanent dipole moment, the relative importance of electrostatic interaction (dispersion forces) and chemical bonding (significant orbital overlap) can be qualitatively assessed. Spectroscopic data $(0, 1 cm^{-1}$ resolution) for the vinoxy -Ar $(C_{2}H_{3}O-Ar)$ complex has been partially analyzed to obtain structural information. The Ar is quite tightly bound, but the geometry is consistent with the dominance of electrostatic interactions. Studies of this complex at higher resolution, and attempts to observe the $C_{2}H_{3}O-Ne$ complex are currently in progress. We are also examining the possibility of obtaining information about the weak bonding interaction by observing their effects on the spin-orbit coupling in triatomic adducts. The van der Waals complexes of SH and OH are useful model systems for this purpose, and we are searching for the electronic spectra of these adducts. A summary of the progress achieved in these studies will be presented