Gas-phase acidities of some neutral Bronsted superacids: A DFT and ab initio study
Gas-phase acidities of some neutral Bronsted superacids: A DFT and ab initio study
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DOI：
10.1021/ja0000753
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发表时间：
2000-05-31
影响因子：
15
通讯作者：
Mishima, M
中科院分区：
文献类型：
--
作者：
Koppel, IA;Burk, P;Mishima, M
首次使用 G2 或 G2(MP2) 计算或两者来计算经典强无机酸 HClO4、CF3SO3H、FSO3H、H2SO4、HBF4、HPO3 和 HNO3 的酸度和去质子化焓。此外，还使用 DFT B3LYP 6-311+G** 方法计算了 39 种中性强或超强布朗斯台德酸、布朗斯台德-刘易斯共轭酸和一些模拟沸石酸性簇的化合物的固有酸度和气相去质子化焓。采用6-31+G*基础上的DFT B3LYP方法计算碳硼烷阴离子CB11H12-及其单-的共轭酸的固有布朗斯台德酸度。六氟化和十二氟化类似物。 G2 和 G2(MP2) 理论比 DFT B3LYP//6-311+G** 更好地描述了不同化合物的酸度。然而，DFT 结果也可用于估计 G2 或 G2(MP2) 理论无法达到的化合物的酸度。通过这种方式获得的估计的 Delta G（酸）值可以用作无法获得的实验值的替代值，特别是对于那些（相当多）的 Delta G（酸）实验测定非常困难的化合物。在几乎所有考虑的化合物族的情况下，都可以达到极高的酸度（低Delta G（酸）值）。如果化合物以 KF 作为母体酸开始，则通过与 SO3 分子连续络合形成布朗斯台德-刘易斯共轭酸，可以达到估计的 Delta G（酸）低至 249.0 kcal/mol（对于 F(OSO2)(4)H）。通过 HF 与 SbF5 的络合，还可以达到非常低的 Delta G（酸）值（Delta G（酸）(HSbF6) = 255.5）。在将高电负性、正确取向的可极化偶极电子接受取代基逐渐引入酸性位点的情况下，至少可以达到与最强的布朗斯台德-刘易斯超强酸一样高的固有酸度。事实上，在环戊二烯中引入五个 CN 基团预计会导致酸度 Delta G(酸) = 250.1 kcal/mol，甚至低于六氟锑酸的相应量 (Delta G(酸) = 255.5 kcal/mol)。然而，迄今为止，十二氟取代碳硼烷酸 CB11F12H 的最强固有布朗斯台德酸度（Delta G（酸） = 209 kcal/mol）预计将超过硫酸的固有酸度约 90 kcal/mol 或几乎 10 的 70 次方，而半经验 PM3 计算表明，十二三氟甲基单碳硼烷阴离子的共轭酸CB11(CF3)(12) 可能是第一个中性布朗斯台德超强酸，其酸度（去质子化能）预计低于里程碑式的 200 kcal/mol 水平。发现强和超强布朗斯台德酸的计算气相酸度与相应纯酸的相应哈米特酸度函数之间存在近似线性关系。中性和阳离子布朗斯特酸气相酸度范围上广泛重叠区域的同时存在有力地证明了中性布朗斯特酸和碱之间自发质子转移平衡的可行性。
For the first time G2 or G2(MP2) calculations or both have been performed to calculate the acidity and deprotonation enthalpy of classical strong mineral acids HClO4, CF3SO3H, FSO3H, H2SO4, HBF4, HPO3, and HNO3. Also, the intrinsic acidities and gas-phase deprotonation enthalpies for 39 neutral strong or superstrong Bronsted acids, Bronsted-Lewis conjugate acids, and some compounds modeling the acidic clusters of zeolites were calculated using the DFT B3LYP 6-311+G** approach. DFT B3LYP method at 6-31+G* basis was used for the calculation of the intrinsic Bronsted acidities of the conjugate acids of the carborane anion CB11H12- and its mono-. hexa-, and dodecafluorinated analogues. G2 and G2(MP2) theories describe the acidities of different compounds better than DFT B3LYP//6-311+G**. However, the DFT results could also be used for the estimation of the acidity of compounds which are out of reach of G2 or G2(MP2) theory. The estimated Delta G(acid) values obtained this way can be used as the substitutes for the unavailable experimental values, especially for those (rather numerous) compounds for which the experimental determination of Delta G(acid) is very difficult. In the case of practically all considered families of compounds extremely high acidities (low Delta G(acid) values) could be reached. if the compounds were started from KF as the parent acid, then the estimated Delta G(acid) as low as 249.0 kcal/mol (for F(OSO2)(4)H) could be reached by formation of Bronsted-Lewis conjugate acids by consecutive complexation with SO3 molecules. Also very low Delta G(acid) value (Delta G(acid)(HSbF6) = 255.5) could be reached by complexation of HF with SbF5. At least as high intrinsic acidities as in case of the strongest Bronsted-Lewis superacids could be reached in the case of progressive introduction of highly electronegative, correctly oriented polarizable dipolar electron-accepting substituents into the acidity site. Indeed, the introduction of five CN groups into cyclopentadiene is expected to lead to the acidity Delta G(acid) = 250.1 kcal/mol which is lower than the corresponding quantity even for hexafluoroantimonic acid (Delta G(acid) = 255.5 kcal/mol). However, by far the strongest intrinsic Bronsted acidity (Delta G(acid) = 209 kcal/mol) for dodecafluorosubstituted carborane acid CB11F12H is predicted to exceed the intrinsic acidity of sulfuric acid by about 90 kcal/mol or by almost 70 powers of ten, whereas semi-empirical PM3 calculations suggest that the conjugate acid of the dodecatrifluoromethylmonocarborane anion CB11(CF3)(12) could be the first neutral Bronsted superacid whose acidity (deprotonation energy) is expected to be below the landmark 200 kcal/mol level. An approximate linear relationship is found to hold between the calculated gas-phase acidities of strong and superstrong Bronsted acids and the corresponding Hammett acidity functions of the corresponding neat acids. The simultaneous existence of the widely overlapping areas on the gas-phase acidity scale of neutral and cationic Bronstrd acids evidences strongly for the feasibility of the spontaneous proton-transfer equilibria between neutral Bronsted acids and bases.