Relative cyanide cation (+CN) affinities of pyridines determined by the kinetic method using multiple-stage (MS3) mass spectrometry
作者:Sheng S. Yang、Olga Bortolini、Annick Steinmetz、R. Graham Cooks
DOI:10.1002/jms.1190300128
日期:1995.1
Ion–molecule reactions occurring in a pentaquadrupole mass spectrometer are used to generate and characterize ions in which one or two pyridine molecules are bound by a +CN cation. Cyanide cation binds strongly to the nitrogen atom of pyridine to generate a mono-adduct, which undergoes pyridine exchange reactions and from which one can generate the dipyridine adduct in low abundance. The dimeric ions have two structures, loosely bound and covalently bound, and both fragment to yield the constituent cyanide-bound monomers. In the case of dimers comprised of meta-substituted alkylpyridines, there is a quantitative correlation between relative cyanide cation affinity, as measured using the kinetic method, and literature values of relative proton affinities. These dimers fragment analogously to the corresponding H+- and Cl+- bound dimers, and on this basis are assigned analogous structures, viz. the loosely bound form Py1+CNPy2. Semi-empirical molecular orbital calculations show that both pyridines are bound to the carbon atom of the cyanide cation. Making the assumption that the effective temperatures of the activated cyanide-bound dimers are similar to those of the corresponding Cl+- and H+-bound dimers, relative +CN cation affinities are estimated to be 1.5 kcal mol−1 (3-MePy), 1.7 kcal mol−1 (4-MePy), 2.6 kcal mol−1 (3-EtPy), 3.5 kcal mol−1 (3-n-BuPy) and 3.6 kcal mol−1 (3,5-diMePy), all expressed relative to pyridine (1 kcal = 4.184 kJ). A linear relationship between the relative +CN affinity and relative proton affinity (PA) is derived as Δ +CN affinity (kcal mol−1) = 0.78 (ΔPA), with the assumption that the +CN dimer effective temperature is 600 K. The estimated uncertainty is 0.5 kcal mol−1. Relative +CN affinities of pairs of pyridines are smaller by ca. 1 kcal mol−1 than the corresponding Cl+ affinities. Dimers in which one of the pyridines is meta-chlorine- or para-alkyl-substituted have the covalently bound, ring-carbon-substituted structure, in which the +CN group is attached to the pyridine nitrogen and the second pyridine molecule is bound to a ring carbon. The fragmentation of these isomeric dimers yields the corresponding monomers, in addition to other minor ions, but the distribution of the cyanide cation between the two pyridines does not correlate with Cl+ affinity or proton affinity. In the special cases of the 3-methylpyridine–3-n-butylpyridine and the 4-methylpyridine–pyridine cyanide cation adducts, both the loosely bound dimer and the covalently bound adduct are generated and distinguished by their fragmentation behavior. Evidence for the formation of the covalently bound, ring-carbon-substituted structure was also obtained in semi empirical AMI calculations.
在五重四极杆质谱仪中发生的离子-分子反应被用来生成和表征其中一种或两种吡啶分子通过+CN阳离子结合的离子。氰化物阳离子与吡啶的氮原子强烈结合,生成单加合物,该加合物经历吡啶交换反应,并从中以低丰度生成双吡啶加合物。二聚离子具有两种结构,松散结合和共价结合,并且两者断裂都会产生构成氰化物结合的单体。对于由间位取代的烷基吡啶组成的二聚体,使用动力学方法测得的相对氰化物阳离子亲和力与文献中的相对质子亲和力值之间存在定量相关性。这些二聚体类似物与相应的H+-和Cl+-结合的二聚体断裂相似,基于此,它们被赋予相似的结构,即松散结合的形式Py1+CNPy2。半经验分子轨道计算表明,两个吡啶都与氰化物阳离子的碳原子结合。假设活化的氰化物结合二聚体的有效温度与相应的Cl+-和H+-结合二聚体相似,则相对+CN阳离子亲和力估计为1.5 kcal mol−1 (3-MePy),1.7 kcal mol–1 (4-MePy),2.6 kcal mol-1 (3-EtPy),3.5 kcal mol-1 (3-n-BuPy)和3.6 kcal mol–1 (3,5-diMePy),所有这些都是相对于吡啶表示的(1 kcal = 4.184 kJ)。在假设+CN二聚体有效温度为600 K的情况下,推导出相对+CN亲和力与相对质子亲和力(PA)之间的线性关系,即Δ +CN亲和力(kcal mol-1)= 0.78(ΔPA)。估计的不确定性为0.5 kcal mol-1。相对于相应的Cl+亲和力,吡啶对的相对+CN亲和力低约1 kcal mol-1。其中一个吡啶是间氯或对烷基取代的二聚体具有共价结合、环碳取代的结构,其中+CN基团连接到吡啶氮,第二个吡啶分子连接到环碳。这些异构二聚体的断裂会产生相应的单体以及其他次要离子,但氰化物阳离子在两个吡啶之间的分布与Cl+亲和力或质子亲和力没有相关性。在3-甲基吡啶-3-正丁基吡啶和4-甲基吡啶-吡啶氰化物阳离子加合物的特殊情况下,生成了松散结合的二聚体和共价结合的加合物,并通过它们的断裂行为进行区分。在半经验AMI计算中还获得了形成共价结合、环碳取代结构的证据。