The proton affinity of the superbase 1,8-bis(tetramethylguanidino)naphthalene (TMGN) and some related compounds: a theoretical study.
The proton affinity of the superbase 1,8-bis(tetramethylguanidino)naphthalene (TMGN) and some related compounds: a theoretical study.
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超碱 1,8-双（四甲基胍）萘 (TMGN) 和一些相关化合物的质子亲和力：理论研究。
作者：
B. Kovačević;Z. Maksić
采用从头计算方法研究了强中性有机碱双（四甲基胍）萘（TMGN）、4,5-双（四甲基胍）芴（TMGF）及其相关化合物的空间和电子结构。它们对质子的亲和力在气相和乙腈溶液中都被仔细检查。在最基本的中心（亚胺氮）质子化产生不对称的和相对强的分子内氢键（IHB）。研究发现，TMGN中角应变效应和位阻斥力几乎消失，表明其高的绝对质子亲和（APA）源于胍片段的固有碱度和[TMGN]H（+）中较强的IHB。TMGF中的非键排斥比TMGN中的高，这与相应共轭酸中稍强的IHB结合使其更具碱性：APA(TMGF)>APA（TMGN）。在TMGN和TMGF中都观察到一个有趣的新现象：质子不仅在直接键合的胍片段中触发共振稳定，而且在距离质子较远的其他胍片段中也会触发共振稳定，尽管这种共振稳定的方式不那么明显。后一种特征称为部分质子化。这支持了氢键并有助于IHB的稳定。令人信服的证据表明，乙腈中的溶剂效应是由两个拮抗因素决定的：1)固有的（气相）质子亲和和2)分子阳离子（共轭酸）正电荷与分子表面大小之比的大小效应。得到的pK(a)值是由这些因素的相互作用给出的。
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.