Unravelling the Origin of the Linear Structures Distortions of the Acetylene Isoelectronic Compounds (Disilyne, Digermyne, and Distannyne)

Abstract

The role and contributions of the effective factors on the structural properties of acetylene (1), disilyne (2), digermyne (3), and distannyne (4) were examined by means of LC-ωPBE long-range corrected density functional method with TZP basis set on all atoms. Although the ground state structure of acetylene (1) is linear (D∞h-symmetry), the trans-bent (C2h, as a local minimum) and di-bridged structures (C2v, as a global minimum) were found for compounds 2–4. Plotted electronic energies of the ground and excited states versus the nuclear displacement (by considering the symmetries of the normal modes) revealed that the negative curvatures of the ground state electronic configurations and the positive curvatures of the excited states of the adiabatic potential energy surfaces (APESs) result from mixing of the ground $$^{1}\Sigma _{{\text{g}}}^{ + }$$ and excited 1Πg states, revealing the presence of the strong pseudo Jahn–Teller effect (i.e., PJT ( $$^{1}\Sigma _{{\text{g}}}^{ + }$$ + 1Πg) ⊗ πg problem). The weak PJTE in acetylene (1) does not distort its linear configurations. The distortions of the linear (D∞h) configurations of compounds 2–4 to their corresponding trans-bent structures (C2h) are due to the PJTE associated with the mixing of the ground $$^{1}\Sigma _{{\text{g}}}^{ + }$$ state and third excited Πg states through the mixing of their HOMOs(πu) with their corresponding LUMOs+2(σu) ([HOMO(πu)] + [LUMO+1(σu)]) in compound 2, the mixing of the ground $$^{1}\Sigma _{{\text{g}}}^{ + }$$ state and second excited Πg states ([HOMO(πu)] + [LUMO(σu)]) in compound 3 and the ground $$^{1}\Sigma _{{\text{g}}}^{ + }$$ state and second excited Πg states ([HOMO(πu)] – [LUMO+1(σu)]) in compound 4. The higher- and lower-lying $$^{1}\Sigma _{{\text{u}}}^{ - }$$ , $$^{1}\Sigma _{{\text{u}}}^{ - }$$ , 1Δu, and 1Πu states are not involved in the PJT interactions. The trans-bent local minimum structures could be converted to their corresponding global minimum structures (di-bridge, C2v) by passing from axial symmetrical (C2) transition state geometries. The energy gaps between reference states (∆) in the linear (D∞h) configurations decrease from disilyne (2) to distannyne (4) while the energy difference between the linear (D∞h) and trans-bent (C2h) configurations increases from compound 2 to 3 but decreases from compound to compound 4, revealing the determining impacts of their corresponding vibronic coupling constants (F) which may control their corresponding D∞h → C2h distortion PJT stabilization energies. Importantly, the variations of the M–M-bond length differences in the linear (D∞h) and trans-bent (C2h) structures of compounds 2–4, Δ(rM–M) D∞h–C2h, correlate well with their corresponding PJT-stabilization energies.