Dissociative protonation sites: reactive centers in protonated molecules leading to fragmentation in mass spectrometry.
Dissociative protonation sites: reactive centers in protonated molecules leading to fragmentation in mass spectrometry.
复制标题
DOI：
10.1021/jo060439v
复制
发表时间：
2006-06
期刊：
影响因子：
--
通讯作者：
Yaping Tu
中科院分区：
文献类型：
--
作者：
Yaping Tu
在质谱学中经常发现，当一个分子在热力学上最有利的位置质子化时，没有发生碎裂，但当质子迁移到不同的位置时，可以观察到主要的反应。对于在氧上优先质子化的二苯甲酮、苯乙酮和二苯甲醚，质子化分子在碰撞诱导解离过程中发生了脱酰化或脱烷基反应。对于对位取代的二苯甲酮，吸电子取代基有利于RC6H4CO+(R=取代基)的形成，而电子释放基团有利于竞争反应生成C6H5CO+。Ln[(RC6H4CO+)/(C6H5CO+)]值与Sigmap+取代基常数有很好的相关性。在质子化的苯乙酮的裂解过程中，脱乙酰化得到中间的质子结合的二聚体酮和苯的络合物。结果表明，产物离子的分布取决于烯酮和取代苯的质子亲和力，并用动力学方法对反应中间体进行了鉴定。质子化的二苄基醚在脱烷基化时通过苄氧甲基阳离子和中性苯的离子中性络合物失去甲醛。这些气相逆Friedel-Craft反应是由于质子对芳环上的碳原子的攻击造成的，该碳原子上的羰基或亚甲基被描述为解离质子化中心。
It is often found in mass spectrometry that when a molecule is protonated at the thermodynamically most favorable site, no fragmentation occurs, but a major reaction is observed when the proton migrates to a different position. For benzophenones, acetophenones, and dibenzyl ether, which are all preferentially protonated at the oxygen, deacylation or dealkylation was observed in the collision-induced dissociation of the protonated molecules. For para-monosubstituted benzophenones, electron-withdrawing substituents favor the formation of RC6H4CO+ (R = substituent), whereas electron-releasing groups favor the competing reaction leading to C6H5CO+. The ln[(RC6H4CO+)/(C6H5CO+)] values are well-correlated with the sigmap+ substituent constants. In the fragmentation of protonated acetophenones, deacetylation proceeds to give an intermediate proton-bound dimeric complex of ketene and benzene. The distribution of the product ions was found to depend on the proton affinities of ketene and substituted benzenes, and the kinetic method was applied in identifying the reaction intermediate. Protonated dibenzyl ether loses formaldehyde upon dealkylation, via an ion-neutral complex of the benzyloxymethyl cation and neutral benzene. These gas-phase retro-Friedel-Crafts reactions occurred as a result of the attack of the proton at the carbon atom to which the carbonyl or the methylene group is attached on the aromatic ring, which is described as the dissociative protonation site.