EVIDENCE FOR RESONANCE-ASSISTED HYDROGEN-BONDING FROM CRYSTAL-STRUCTURE CORRELATIONS ON THE ENOL FORM OF THE BETA-DIKETONE FRAGMENT
EVIDENCE FOR RESONANCE-ASSISTED HYDROGEN-BONDING FROM CRYSTAL-STRUCTURE CORRELATIONS ON THE ENOL FORM OF THE BETA-DIKETONE FRAGMENT
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DOI：
10.1021/ja00185a035
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发表时间：
1989-02-01
影响因子：
15
通讯作者：
BERTOLASI, V
中科院分区：
文献类型：
--
作者：
GILLI, G;BELLUCCI, F;BERTOLASI, V
用晶体结构关联方法研究了25个含acacH（HOCR= CR-CR= 0）的烯醇式乙酰丙酮及其它相关的O-二酮和O-酮酯分子的晶体结构。acacH既可以形成一个封闭六元环的分子内HB，也可以形成以无限线性阵列首尾相连的分子间HB。实验数据表明，在所形成的HB的强度（通过d 0. 0和0o_H值）和共轭双键的离域。该效应被定性解释的共振和HB形成的协同相互作用的机制，这是所谓的共振辅助氢键（RAHB），它表明，所提出的模型是在协议与所有目前的NMR和IR数据和最新的从头量子力学计算报告的丙二醛。对于更复杂的分子，已经开发了一个半经验模型来评估形成的HB的能量（£ j）和质子转移的势垒高度（E2）;关于水，其中£ j= 20且E2= 40 kj摩尔1，E…和E2在乙酰丙酮中计算为53.4和35 kJ mol-1，在六甲基乙酰丙酮中计算为82.5和27.5 kJ mol-1，这表明具有中心氢位置的非常强的HB受到相关空间位阻取代基的青睐。这些结论被推广到考虑RAHB在各种杂共轭体系中可能发挥的作用。用X射线或中子衍射测定分子晶体结构的方法越来越多，有助于系统地比较不同晶体结构中分子碎片的几何形状，以提取新的、有价值的化学信息。已经开发了一些通常被称为结构相关方法的特定技术。1在本文中，这样的方法被应用到0-二酮片段在其烯醇形式1（以下称为acacH从乙酰丙酮）的目的是了解发生了什么片段的几何形状时，它是由分子内，2，或分子间，3，氢键扰动。的
The geometry variations caused by inter-and intramolecular hydrogen bond (HB) formation on the molecular fragment acacH (HOCR= CR—CR= 0) contained in the enol form of acetylacetone and other related 0-diketones and 0-keto esters have been studied by crystal structure correlation methods on 25 X-ray or neutron crystallographically determined molecules containing such a fragment. acacH can form either a single intramolecularHB closing a 6-membered ring or intermolecular HBs connecting the groups head-to-tail in infinite linear arrays. Experimental data show that there is a strong correlation between the strength of the HB formed (measured by both the d0... 0 and 0o_H values) and the delocalizationof thesystem of conjugated double bonds. The effect is qualitatively interpreted in terms of a mechanism of synergistic interplay of resonance and HB formation which is called resonance-assisted hydrogen bonding (RAHB), and it is shown that the proposed model is in agreement with all present NMR and IR data and with the most recent ab initio quantum mechanical calculations reported for malonaldehyde. For more complex molecules a semiempirical model has been developed to evaluate both the energy of the HB formed (£,) and the heightof the barrier for proton transfer (E2); with reference to water, where£ j= 20 and E2= 40 kj mol'1, E¡ and E2 are calculated to be 53.4 and 35 kj mol" 1 in acetylacetone and 82.5 and 27.5 kJ mol" 1 in hexamethylacetylacetone, showing that very strong HB’s with central hydrogen position are favored by substituents of relevant steric hindrance. These conclusions are generalized to take into account the possiblerole played by RAHB in a variety of heteroconjugated systems.The ever growing number of structure determinations of mo-lecular crystals by X-ray or neutron diffraction has contributed to the systematic comparison of the geometries of a molecular fragment in different crystal structures with the aim of extracting new, valuable chemicalinformation, and in this connection, some specific techniques have been developed which are usually referred to as structure correlation methods. 1 In this paper such methods are applied to the 0-diketone fragment in its enol form 1 (hereafter named acacHfrom acetylacetone) with the aim of understanding what happens to the fragment geometry when it is perturbed by intramolecular, 2, or intermolecular, 3, hydrogen bonding. The