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 D3h-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 D3h-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 D3h-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.
ASJC Scopus subject areas
- Colloid and Surface Chemistry