Szabó István, Czakó Gábor
Laboratory of Molecular Structure and Dynamics, Institute of Chemistry, Eötvös University, P.O. Box 32, H-1518 Budapest 112, Hungary.
J Phys Chem A. 2015 Mar 26;119(12):3134-40. doi: 10.1021/acs.jpca.5b00988. Epub 2015 Mar 13.
The double-inversion and front-side attack transition states as well as the proton-abstraction channels of the X(-) + CH3Y [X,Y = F, Cl, Br, I] reactions are characterized by the explicitly correlated CCSD(T)-F12b/aug-cc-pVTZ(-PP) level of theory using small-core relativistic effective core potentials and the corresponding aug-cc-pVTZ-PP bases for Br and I. In the X = F case the double-inversion classical(adiabatic) barrier heights are 28.7(25.6), 15.8(13.4), 13.2(11.0), and 8.6(6.6) kcal mol(-1) for Y = F, Cl, Br, and I, respectively, whereas the barrier heights are in the 40-90 kcal mol(-1) range for the other 12 reactions. The abstraction channels are always above the double-inversion saddle points. For X = F, the front-side attack classical(adiabatic) barrier heights, 45.8(44.8), 31.0(30.3), 24.7(24.2), and 19.5(19.3) kcal mol(-1) for Y = F, Cl, Br, and I, respectively, are higher than the corresponding double-inversion ones, whereas for the other systems the front-side attack saddle points are in the 35-70 kcal mol(-1) range. The double-inversion transition states have XH···CH2Y(-) structures with Cs point-group symmetry, and the front-side attack saddle points have either Cs (X = F or X = Y) or C1 symmetry with XCY angles in the 78-88° range. On the basis of the previous reaction dynamics simulations and the minimum energy path computations along the inversion coordinate of selected XH···CH2Y(-) systems, we suggest that the double inversion may be a general mechanism for SN2 reactions.
利用小核相对论有效核势以及针对Br和I的相应aug-cc-pVTZ-PP基组，在显式相关的CCSD(T)-F12b/aug-cc-pVTZ(-PP)理论水平下，对X(-)+CH3Y [X,Y = F, Cl, Br, I]反应的双反转和前侧攻击过渡态以及质子抽取通道进行了表征。在X = F的情况下，对于Y = F、Cl、Br和I，双反转经典（绝热）势垒高度分别为28.7(25.6)、15.8(13.4)、13.2(11.0)和8.6(6.6) kcal mol(-1)，而其他12个反应的势垒高度在40 - 90 kcal mol(-1)范围内。抽取通道总是高于双反转鞍点。对于X = F，对于Y = F、Cl、Br和I，前侧攻击经典（绝热）势垒高度分别为45.8(44.8)、31.0(30.3)、24.7(24.2)和19.5(19.3) kcal mol(-1)，高于相应的双反转势垒高度，而对于其他体系，前侧攻击鞍点在35 - 70 kcal mol(-1)范围内。双反转过渡态具有Cs点群对称性的XH···CH2Y(-)结构，前侧攻击鞍点具有Cs（X = F或X = Y）或C1对称性，XCY角在78 - 88°范围内。基于先前的反应动力学模拟以及沿选定XH···CH2Y(-)体系反转坐标的最小能量路径计算，我们认为双反转可能是SN2反应的一种普遍机制。
