Structure and Aromaticity of Si3–Si7 Analogues of Fully Conjugated C3–C7 Aromatic Carbocycles

The persilylated Si3–Si7 analogues of the C3–C7 aromatic molecules and ions with all hydrogen or all fluorine atoms at silicon have been calculated at high levels of theory, up to MP2/aug-cc-pVTZ for all species and CCSD/6-311++G** for Si3 and Si4 species, both in the gas phase and in a polar solvent (water). The aromaticity of the calculated species was estimated using structural, energetic, and NMR criteria. (SiF)3+ cations are more aromatic than (SiH)3+ by the NICS (nuclear-independent chemoical shift) but less aromatic by the ASE (aromatic stabilization energy) criterion. Dications (SiX)42+ are planar (X = H) or slightly puckered (X = F); the ASE decreases by 4–5 kcal/mol upon going from gas to solution, or from X = H to X = F. Dianions (SiX)42−are nonplanar and antiaromatic. The ASE for the slightly distorted-from-planarity anion Si5H5− is ~53 kcal/mol, vs. 85 kcal/mol for its carbon analogue. The structure of Si6X6 molecules strongly depends on the level of calculations. The NICS and ASE values have been calculated for planar Si6H6 and (SiH)7+ but not for strongly distorted Si6F6 and (SiF)7+ species.