Marom Hili, Biedermann P Ulrich, Agranat Israel
Department of Organic Chemistry, The Hebrew University of Jerusalem, Jerusalem 91904, Israel.
Chirality. 2007 Jul;19(7):559-69. doi: 10.1002/chir.20417.
The pyramidal inversion mechanism of simple sulfoxides was studied, employing ab initio and DFT methods. The convergence of the geometrical and energetic parameters of H2SO and DMSO with respect to the Hamiltonian and basis set was analyzed in order to determine a computational level suitable for methyl phenyl sulfoxide (3), methyl 4-cyanophenyl sulfoxide (4), diphenyl sulfoxide (5), 4,4'-dicyanodiphenyl sulfoxide (6), benzyl methyl sulfoxide (7) and benzyl phenyl sulfoxide (8). The DFT B3LYP/6-311G(d,p) level was chosen for further calculations of larger sulfoxides. The barriers DeltaE calculated for the pyramidal inversion mechanism of sulfoxides 3-8 are in the range of 38.7-47.1 kcal/mol. These values are in good agreement with the experimental barriers for racemization via the pyramidal inversion mechanism. A resonance effect of a phenyl ring selectively stabilizes the transition state conformations, decreasing the energy barrier for pyramidal inversion by about 3 kcal/mol, compared to a similar molecule without a phenyl substituent. Introducing electron withdrawing groups (cyano) at the para positions of the phenyl ring(s) causes a further decrease of the energy barrier.
采用从头算和密度泛函理论（DFT）方法研究了简单亚砜的锥形反转机理。分析了H₂SO和二甲基亚砜（DMSO）的几何和能量参数相对于哈密顿量和基组的收敛性，以确定适用于甲基苯基亚砜（3）、甲基4-氰基苯基亚砜（4）、二苯基亚砜（5）、4,4'-二氰基二苯基亚砜（6）、苄基甲基亚砜（7）和苄基苯基亚砜（8）的计算水平。选择DFT B3LYP/6-311G(d,p)水平对更大的亚砜进行进一步计算。计算得到的亚砜3-8的锥形反转机理的能垒ΔE在38.7-47.1 kcal/mol范围内。这些值与通过锥形反转机理进行外消旋化的实验能垒高度吻合。与没有苯基取代基的类似分子相比，苯环的共振效应选择性地稳定了过渡态构象，使锥形反转的能垒降低了约3 kcal/mol。在苯环的对位引入吸电子基团（氰基）会导致能垒进一步降低。
