Relationships between DFT/RRKM Branching Ratios of the Complementary Fragment Ions [C5H5O]+ and [M 鈥?C5H5O]+ and Relative Abundances i

详细信息    查看全文

• 作者：Wilmer E. Vallejo Narv谩ez ; Paula V. Bacca Villota ; Eduardo A. Solano Espinoza
• 刊名：The Journal of Physical Chemistry A
• 出版年：2012
• 出版时间：December 13, 2012
• 年：2012
• 卷：116
• 期：49
• 页码：12136-12147
• 全文大小：670K
• 年卷期：v.116,no.49(December 13, 2012)
• ISSN：1520-5215

文摘

The energy-dependent branching ratios of the complementary fragment ions [C₅H₅O]⁺ and [HC₆H₄NH]⁺ ([M 鈥?C₅H₅O]⁺), originating from the N-(2-furylmethyl)aniline molecular ion, [HC₆H₄NH鈥揅₅H₅O]^{+鈥?/sup>, were obtained from Rice鈥揜amsperger鈥揔assel鈥揗arcus (RRKM) rate calculations based on density functional theory (DFT) energy profiles. The UB3LYP/6-311G+(3df,2p)//UB3LYP/6-31G(d) level of theory was used to model the competitive reaction mechanisms by which the molecular ion can be fragmented. Initially, eight pairs of products were taken into account, corresponding to the combination of two isomeric structures for each fragment ion and the concomitant radicals, which can be formed by direct dissociations or through some isomerization-fragmentation pathways. A great deal of the obtained pathways was discarded by looking over the kinetic barrier heights and the individual RRKM rate coefficients calculated for all the steps. This way, the potential energy profiles were simplified to only three reaction channels, two pathways to [C₅H₅O]+ and one to [M 鈥?C₅H₅O]+. The pre-equilibrium and steady-state approximations were then applied to different regions of the remaining potential energy profiles, allowing the branching ratios of the complementary fragment ions to be easily calculated and discriminated among the three rival processes. According to these results, the major fragment ion in the ion source is [C₅H₅O]+, which is produced as a mixture of two structures, the furfuryl and pyrylium cations, one formed by a direct C鈥揘 bond cleavage and the other through an isomerization-fragmentation channel. In turn, the direct fragmentation is the only mechanism to produce [M 鈥?C₅H₅O]+. To confront these results with the available experimental information, the model was broadened out to the 4-substituted analogues [4-R鈥揅₆H₄NH鈥揅₅H₅O]+鈥?/sup> in which R = F, Br, Cl, CH₃, and OCH₃, finding excellent correlations of the calculated branching ratios and the relative abundances in the electron ionization mass spectra.}