Why is Si₂H₂ Not Linear? An Intrinsic Quasi-Atomic Bonding Analysis

Area of Science:

• Computational Chemistry
• Quantum Chemistry
• Molecular Spectroscopy

Background:

• Acetylene (C2H2) and its silicon analog (Si2H2) are valence isoelectronic molecules.
• The relative stability of different geometric isomers (dibridged, trans-bent, linear) varies between C2H2 and Si2H2.
• Understanding energy contributions is key to explaining structural preferences.

Purpose of the Study:

• To analyze the intra-atomic and inter-atomic energy contributions to the molecular energy of C2H2 and Si2H2.
• To elucidate the differing energetic preferences of geometric isomers in Si2H2 compared to C2H2.
• To relate intra-atomic energy changes to heavy atom hybridization.

Main Methods:

• Computation of molecular energies for various geometric structures of Si2H2 and C2H2.
• Analysis of density matrices from multiconfiguration self-consistent field (MCSCF) wave functions.
• Expression of density matrices in terms of quasi-atomic orbitals to separate intra-atomic and inter-atomic contributions.

Main Results:

• Molecular energy increases in the order dibridged < trans-bent < linear for Si2H2, unlike C2H2 where linear is the global minimum.
• Intra-atomic contributions become less favorable, while inter-atomic bonding contributions become more favorable in the order dibridged → trans-bent → linear for both molecules.
• For Si2H2, unfavorable intra-atomic energy changes outweigh bonding interactions, while for C2H2, bonding interactions dominate over intra-atomic changes.

Conclusions:

• The differing stability of Si2H2 isomers is attributed to the prevalence of intra-atomic antibonding energy changes over interatomic bonding contributions.
• In contrast, C2H2's stability is governed by the dominance of interatomic bonding interactions over intra-atomic energy changes.
• Intra-atomic energy changes in these systems are analogous to the hybridization changes observed in methane (CH4).