UV-Photoelectron Spectroscopy of BN Indoles: Experimental and Computational Electronic Structure Analysis
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- 作者：Anna Chrostowska ; Senmiao Xu ; Audrey Mazi猫re ; Katherine Boknevitz ; Bo Li ; Eric R. Abbey ; Alain Dargelos ; Alain Graciaa ; Shih-Yuan Liu
- 刊名：Journal of the American Chemical Society
- 出版年：2014
- 出版时间：August 20, 2014
- 年：2014
- 卷：136
- 期：33
- 页码：11813-11820
- 全文大小：517K
- ISSN：1520-5126
文摘
We present a comprehensive electronic structure analysis of two BN isosteres of indole using a combined UV-photoelectron spectroscopy (UV-PES)/computational chemistry approach. Gas-phase He I photoelectron spectra of external BN indole I and fused BN indole II have been recorded, assessed by density functional theory calculations, and compared with natural indole. The first ionization energies of these indoles are natural indole (7.9 eV), external BN indole I (7.9 eV), and fused BN indole II (8.05 eV). The computationally determined molecular dipole moments are in the order: natural indole (2.177 D) > fused BN indole II (1.512 D) > external BN indole I (0.543 D). The 位max in the UV鈥搗is absorption spectra are in the order: fused BN indole II (292 nm) > external BN indole I (282 nm) > natural indole (270 nm). The observed relative electrophilic aromatic substitution reactivity of the investigated indoles with dimethyliminium chloride as the electrophile is as follows: fused BN indole II > natural indole > external BN indole I, and this trend correlates with the 蟺-orbital coefficient at the 3-position. Nucleus-independent chemical shifts calculations show that the introduction of boron into an aromatic 6蟺-electron system leads to a reduction in aromaticity, presumably due to a stronger bond localization. Trends and conclusions from BN isosteres of simple monocyclic aromatic systems such as benzene and toluene are not necessarily translated to the bicyclic indole core. Thus, electronic structure consequences resulting from BN/CC isosterism will need to be evaluated individually from system to system.