This research reports a search for peculiar monobridged structures of the E2H2 molecules (E = Be, Mg, Ca, Sr, Ba). For Be2H2 and Mg2H2, the monobridged geometry is not an equilibrium but rather a transition state between the vinylidene-like structure and the global minimum HE–EH linear geometry. However, for Ca2H2, Sr2H2, and Ba2H2, this situation changes significantly; the linear structure is no longer the global minimum but lies higher in energy than two other equilibria, the dibridged and monobridged structures. The planar dibridged structures of both Sr2H2 and Ba2H2 should be observable via IR spectroscopy. Although the remarkable monobridged structures lie 8.3 (Sr) and 7.6 kcal/mol (Ba) higher, the large IR intensities of the terminal E–H stretching frequencies may make the monobridged structures observable. The monobridged structures have sizable permanent dipole moments (3.07 and 3.06 D for Sr and Ba, respectively) and also should be observable via microwave spectroscopy.

Author ORCID Identifier

Alexander I. Boldyrev

Nikolay Tkachenko



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NSF, Division of Chemistry (CHE)


Utah State University

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NSF, Division of Chemistry (CHE) 1664379

Award Title

Deciphering Delocalized Bonding in Excited States, Solvated Species and Novel 0-, 1-, 2-, and 3-Dimensional Chemical Systems


The search for the global minimum of Be2H2, Mg2H2, and Ca2H2 at singlet states was performed using the Coalescence Kick program (10,000 trial structures for each stoichiometry) at the PBE0/3-21G level of theory. The lowest in energy isomers then were reoptimized at PBE0/6-311++G** and CCSD(T)/cc-pVQZ levels. The Gaussian-16 software was used for the geometry optimization and frequency calculations. In order to assess the multireference character of wavefunctions of investigated systems, the CASSCF/cc-pvqz (for Be, Mg, and Ca containing structures) and CASSCF/def2qzvp (for Sr and Ba containing structures) calculations were performed via the ORCA software. The active space was chosen in such a way, to account for all valence electrons and six lowest unoccupied molecular orbitals (6e, 9o). The PBE0/cc-qcvp wavefunctions were chosen as initial guesses for those calculations. The geometries obtained at the CCSD(T)/cc-pvqz level were used, the ZPE correction was taken into account using values calculated at the CCSD(T)/cc-pvqz level. Dynamical correlation is added through the second-order N-electron valence state perturbation theory (NEVPT2) method. The chemical bonding pattern was analyzed using the AdNDP 2.0 code at PBE0/6-311++G** level of theory.

Referenced by

Narendrapurapu, B. S., Bowman, M. C., Xie, Y., Schaefer, H. F., Tkachenko, N. V., Boldyrev, A. I., & Li, G. (2020). Dibridged, Monobridged, Vinylidene-Like, and Linear Structures for the Alkaline Earth Dihydrides Be 2 H 2 , Mg 2 H 2 , Ca 2 H 2 , Sr 2 H 2 , and Ba 2 H 2. Proposals for Observations. Inorganic Chemistry, 59(15), 10404–10408.



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Total 132 data files are included, zipped into their original directories. Chemical bonding analysis files, Supplementary information file.




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