Ab Initio Study of Destabilized π-Conjugated Systems with Large π-Overlap: Sulfur Trimethylene and Chlorine Trimethylene Cation. Electrostatics as a Dominant Factor in Multiple Bonding for Second-Row Elements

Ab initio calculations at the MP2/6-31G*//HF/6-31G*/ZPVE level of theory indicate that conjugative stabilization in 67r-electron Y-conjugated systems, X(CH₂)3 and X0₃ (X = P (anion), S (neutral), and Cl (cation)), is dominated by electrostatic interactions; that is, increasing the X-C overlap and decreasing X-C charge separation via change in electronegativity of X decreases stability of the D_3h-symmetric structures versus the structures with lower symmetry (less 7r-overlap). These energy patterns are reflected in properties of the electron densities (ellipticities at bond critical points, integated atomic populations, etc.) at the HF/6-31G* level for the D₃-and D_3h-symmetric structures; the more the atomic charges (relative electronegativities) approach or even exceed the formal charges of “no formal double bond” resonance structure, Xⁿ⁺(Z^-)₃, the greater is the preference for the geometries where π-conjugation is feasible. S(CH₂)₃ (Z)₃), which is most predisposed to 7r-overlap, shows signs of a very weak bonding; it possesses a “pseudoatom” (local maximum of the electron density) and two bond critical points along each SC bond axis. For the ylide S(CH₂)₃, a double-well potential energy surface (PES) is found with two D₃-symmetric local minima that are 9 kcal mol^-1 below the D_3h-symmetric transition structure for methylene rotation; 47 kcal mol^-1 below the _D3-symmetric structure, a very flat fragment of the PES, which corresponds to the methylene thirane structure, is located. Preparation of the Y-conjugated S(CH₂)₃(D₃) may be quite difficult but feasible in view of the fact that the barrier for its collapse to the global minimum is estimated as close to 6.0 kcal mol^-1 (MP3/6-31G*//HF/6-31G*/ZPVE); IR and Raman spectra calculated at the HF/6-31G* level may aid the experiment.