Ab initio prediction of the structure and energetics of the complexes of 1-nitro-1-(4-nitrophenyl)ethane and TBD and MTBD bases

✍ Scribed by Arnold Jarczewski; Jacek Koput; Iwona Nowak

• Publisher: Elsevier Science
• Year: 2006
• Tongue: English
• Weight: 160 KB
• Volume: 788
• Category: Article
• ISSN: 0022-2860
• DOI: 10.1016/j.molstruc.2005.11.029

✦ Synopsis

The ab initio optimized (MP2/cc-pVDZ and PBE0/cc-pVDZ) structures of the complexes of 1-nitro-1-(4-nitrophenyl)ethane with 1,5,7triazabicyclo-[4.4.0]dec-5-ene (TBD) and 7-methyl-1,5,7-triazabicyclo[4.4.0]dec-5-ene (MTBD) formed along the way of proton abstraction are presented. The structures of the transition states and the products are discussed. The relative energies in the gas phase for each individual entity along the proton abstraction with TBD and MTBD are predicted to be: hydrogen bonded complex (K15.7 and K14.2 kcal mol K1 ), ion-pair (K7.5 and K5.1 kcal mol K1 ), relaxed ion-pair (K12.7 and K11.3 kcal mol K1 ), and free ions (C88.1 and C86.9 kcal mol K1 ), respectively. Inclusion of the polar-solvent (acetonitrile) effects changes substantially the appropriate energy levels for the hydrogen bonded complex (K8.3 and K9.6 kcal mol K1 ), ion-pair (K9.5 and K8.0 kcal mol K1 ), relaxed ion-pair (K11.4 and K11.2 kcal mol K1 ) and free ions (C5.8 and C10 kcal mol K1 ) for TBD and MTBD, respectively. The energy changes along the C-H internuclear distance computed by the PBE0 method for the TBD and MTBD bases show the maximum energy for the transition-state complex at 1.4 A ˚(9.9 kcal mol K1 ) and 1.5 A ˚(11.9 kcal mol K1 ) in vacuum, and at 1.3 A ˚(6.8 kcal mol K1 ) and 1.4 A ˚(8.9 kcal mol K1 ) in acetonitrile. The MP2 method at the PBE0 optimized geometries gives higher transition-state energies values at 1.45 A ˚(14.2 kcal mol K1 ) and at 1.5 A ˚(14.4 kcal mol K1 ) in vacuum, and at 1.35 A ˚(10.3 kcal mol K1 ) and at 1.4 A ˚(10.5 kcal mol K1 ) in acetonitrile, relative to the energy of the hydrogen-bonded complex at 1.1 A ˚(0 kcal mol K1 ). The width of the barriers is larger by at least w1 A ˚for MTBD than that for the TBD base. The shape of the energy profiles indicates exo-endoergic course of reaction in MeCN and vacuum, respectively. On virtue of these values, the mechanism of proton abstraction from 1-nitro-1-(4nitrophenyl)alkanes with the TBD and MTBD bases in aprotic solvents is approached.