Kovacević Borislav, Maksić Zvonimir B
Quantum Chemistry Group, Rudjer Bosković Institute Bijenicka c. 54, 10 000 Zagreb-HR, Croatia.
Chemistry. 2002 Apr 2;8(7):1694-702. doi: 10.1002/1521-3765(20020402)8:7<1694::aid-chem1694>3.0.co;2-d.
The spatial and electronic structure of the very strong neutral organic bases bis(tetramethylguanidino)naphthalene (TMGN), 4,5-bis(tetramethylguanidino)fluorene (TMGF) and some related compounds are explored by ab initio computational methods. Their affinity towards the proton is scrutinized both in the gas phase and in solution in acetonitrile. The protonation at the most basic center (the imine nitrogen) yields asymmetric and relatively strong intramolecular hydrogen bonds (IHB). It is found that the angular strain effect and steric repulsion practically vanish in TMGN which implies that its high absolute proton affinity (APA) has its origin in the inherent basicity of the guanidine fragment and a relatively strong IHB in [TMGN]H(+). The nonbonded repulsions in TMGF are higher than in TMGN, which in conjunction with a slightly stronger IHB in the corresponding conjugate acid makes it more basic: APA(TMGF)>APA(TMGN). An interesting new phenomenon is observed in both TMGN and TMGF: the proton triggers the resonance stabilization not only in the directly bonded guanidine moiety, but also in the other guanidine fragment which is more distant from the proton, albeit in a less pronounced manner. The latter feature is termed a partial protonation. This supports the hydrogen bonding and contributes to the IHB stabilization. Convincing evidence is presented that the solvent effect in acetonitrile is determined by two antagonistic factors: 1) the intrinsic (gas phase) proton affinity and 2) the size effect which is given by the ratio between the positive charge in molecular cation (conjugate acid) and the magnitude of the molecular surface. The resulting pK(a) values are given by an interplay of these factors.
采用从头算计算方法研究了极强的中性有机碱双（四甲基胍基）萘（TMGN）、4,5-双（四甲基胍基）芴（TMGF）及一些相关化合物的空间和电子结构。在气相和乙腈溶液中考察了它们对质子的亲和力。在最碱性中心（亚胺氮）处质子化产生不对称且相对较强的分子内氢键（IHB）。研究发现，TMGN中的角应变效应和空间排斥实际上消失了，这意味着其高的绝对质子亲和力（APA）源于胍片段的固有碱性以及[TMGN]H⁺中相对较强的IHB。TMGF中的非键排斥高于TMGN，这与相应共轭酸中稍强的IHB相结合，使其碱性更强：APA（TMGF）＞APA（TMGN）。在TMGN和TMGF中均观察到一个有趣的新现象：质子不仅能使直接相连的胍部分发生共振稳定，还能使离质子较远的另一个胍片段发生共振稳定，尽管程度较弱。后一特征被称为部分质子化。这支持了氢键作用并有助于IHB的稳定。有令人信服的证据表明，乙腈中的溶剂效应由两个相互拮抗的因素决定：1）固有（气相）质子亲和力；2）尺寸效应，其由分子阳离子（共轭酸）中的正电荷与分子表面积大小之比给出。由此产生的pKa值由这些因素的相互作用决定。
