Theoretical Investigation of the M + –RG 2 (M = Alkaline Earth Metal; RG = Rare Gas) Complexes Adrian M. Gardner, Richard J. Plowright, Jack Graneek, Timothy G. Wright and W. H. Breckenridge 67 th International Symposium on Molecular Spectroscopy Ohio State University 22 nd June 2012

M + –RG Complexes: A Model of Solvation The M + –RG complexes are prototypical systems for the investigation of solvation. Experimental studies have focused on the electronic spectroscopy of complexes containing alkaline earth metal cations. 1 The complexes of the closed shell alkali metal cations have been studied intensely using high level ab intio techniques. 2 Trends in equilibrium bond length and dissociation energy of the closed shell complexes are easy to rationalize. Whereas trends in the open shell M + –RG complexes were not! 1.See for example, M. A. Duncan, Annu. Rev. Phys. Chem., 48, 69, (1997). 2.See for example, Breckenridge et al., Chem. Phys., 333, 77 (2007).

M + –RG Complexes: A Model of Solvation Gardner et al., J. Phys. Chem. A., 114, 7631, (2010).

M + –RG Complexes: A Model of Solvation The M + –RG complexes are prototypical systems for the investigation of solvation. Experimental studies have focused on the electronic spectroscopy of complexes containing alkaline earth metal cations. 1 The complexes of the closed shell alkali metal cations have been studied intensely using high level ab intio techniques. 2 Trends in equilibrium bond length and dissociation energy of the closed shell complexes are easy to rationalize. Whereas trends in the open shell M + –RG complexes were not! In the present investigation aim to investigate increasing levels of solvation of Li + and Be + cations. 1.See for example, M. A. Duncan, Annu. Rev. Phys. Chem., 48, 69, (1997). 2.See for example, Breckenridge et al., Chem. Phys., 333, 77 (2007).

Computational Methodology Standard aug-cc-pVTZ basis sets were employed for He, Ne and Ar. For Kr, and Xe the ECP10MDF and ECP28MDF effective core potentials along with the aug-cc-pwCVTZ-PP basis sets were utilized. For the metals, Li + and Be +, aug-cc-pwCVTZ basis sets were employed. All calculations were carried out at the MP2 level of theory. Geometry optimizations and dissociation energies have been calculated using QZ and 5Z versions of the basis sets described above, but are not discussed herein. MOLPRO was used for all geometry optimization and energy calculations.

“Prototypical” Systems: Li + –He 2 A global minimum is found with a bond angle of 180 o. The Li + –He bond length and dissociation energy calculated for the Li + –He 2 complex is almost identical to that of the Li + –He dimer complex. A stationary point is found with a linear Li + –He–He conformation, with the He–He separation shorter than calculated for the He 2 cluster.

“Prototypical” Systems: Li + –He 2 A very shallow minimum is observed at bond angle of ≈115 o. This conformation has a slightly longer He–He bond length than in the He 2 cluster.

“Prototypical” Systems: Li + –Xe 2 A global minimum is found with a Xe–Li + –Xe bond angle of 180 o. A second minimum is observed with a linear Li + –Xe–Xe conformation, with a Xe–Xe separation that is shorter than in the Xe 2 cluster.

Open Shell Complexes: Be + –He 2 The calculated He-He bond length in the helium dimer is 3.06 Å.

Open Shell Complexes: Be + –Ar 2 The calculated Ar-Ar bond length in the argon dimer is 3.77 Å The D e calculated for the Be + –Ar complex is 4050 cm -1.

Open Shell Complexes: Be + –Ar 2 Δ occupancy Charge Be s p x p y p z 0.22 Ar s p x p y p z Natural Population Analysis

Open Shell Complexes: Be + –Ar 2 E int = -379 cm -1 E int = +200 cm -1

Open Shell Complexes: Be + –Ar 2 E int = -379 cm -1 E int = +200 cm -1

Open Shell Complexes: Be + –Ar 2 E int = -379 cm -1 E int = +200 cm -1 Δ occupancy Charge Be s0.00 2p x p y p z 0.12 Ar s p x p y p z Ar s0.00 3p x p y p z Δ occupancy Charge Be s0.05 2p x p y p z 0.06 Ar s p x p y p z Ar s p x p y p z Δ occupancy Charge Be s p x p y p z 0.13 Ar s p x p y p z E int = cm -1

Conclusions Slices through the Li + –RG 2 and Be + –RG 2 potential energy surfaces have been presented and discussed. Even for the expectedly simple, closed shell Li + –RG 2 complexes, multiple minima were observed. The linear RG–Li + –RG conformations were determined to be the most stable for all Li + –RG 2 complexes. The Be + –RG 2 surfaces were considerably more complicated. Bent RG–Be + –RG conformations were determined to be the most stable for all Be + –RG 2 complexes. This was determined to be an effect of synergic interactions within the complex; sp 2 hybridization of the Be + orbital, and charge transfer from the RG 2 dimer to Be +.

Prof. Timothy Wright Prof. Bill Breckenridge Dr. Richard Plowright Jack Graneek Acknowledgements