Study of Nucleophilicity in the Gas Phase

(1)用FT-ICR测定三甲基硅基阳离子转移平衡测定了取代苯胺对三甲基硅基阳离子的气相碱性，发现取代基对三甲基硅基阳离子碱度的影响与相应取代基对质子碱度的影响相似，表明Si-N键具有较高的共价性。这一结果与苯乙酮系列的观察结果一致。另一方面，对Me_3Si~(++)的取代基效应表现为一个较小的Rho值，但共振需求降低，表明Me_3Si~(++)与Se_3Si~(2+)的加合离子的结构不是开放的，而是非经典的桥联结构。用LArSR方程成功地描述了取代基对这些体系电子亲合势的影响。结果表明，体系的共振需求参数(R)变化很大，从硝基苯的0.62到苯甲醛的1.11，再到苯甲酸甲酯系列的1.29。母体自由基阴离子(环取代基=H)的稳定性以相反的顺序下降。这种共振需求的趋势与苯酚、苯胺和甲苯的酸性相一致，表明自由基阴离子自组织轨道中的附加电子以与偶电子阴离子体系HOMO中的pi电子相同的方式离域到芳环中。

(1) Gas phase basicities of substituted anilines toward trimethylsilyl cation have been determined by measuring trimethylsilyl cation transfer equilibria using a FT-ICR-The substituent effect on trimethylsilyl cation basicity of aniline is found to be similar to the corresponding substituent effect on proton basicity, suggesting that the Si-N bond has high covalent character. This result is consistent with that observed for acetophenone series. On the other hand, the substituent effect for Me_3Si^+basicities of stirenes was characterized by a small rho value with reduced resonance demand, suggesting that the structure of Me_3Si^+ adduct ion of stirenes with is not a open structure but a non-classical bridged one.Relative electron affinities have been determined for a series of benzene derivatives (benzaldehydes, acetophenones, methyl benzoates, and some nitrobenzenes) by measurement of electron-transfer equilibrium in the gas phase. Substituent effects on electron affinities of these systems have successfully been described in terms of the LArSR equation. It has been shown that resonance demand parameter (r) varies significantly with the system from 0.62 for nitrobenzene, via 1.11 for benzaldehyde, to 1.29 for methyl benzoate series. The stability of a parent radical anion (ring-substituent=H) decreases in the opposite order. This trend of resonance demand is consistent with that for acidities of phenols, nilines, and toluenes, suggesting that the added electron in the SOMO of radical anions is delocalized into the aromatic ring in the same manner as that of pi-electron in the HOMO of an even-electron anion system.