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

The ab initio optimized (MP2/cc-pVDZ and PBE0/cc-pVDZ) structures of the complexes of 1-nitro-1-(4-nitrophenyl)ethane with 1,5,7-triazabicyclo-[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 (−15.7 and −14.2 kcal mol−1), ion-pair (−7.5 and −5.1 kcal mol−1), relaxed ion-pair (−12.7 and −11.3 kcal mol−1), and free ions (+88.1 and +86.9 kcal mol−1), respectively. Inclusion of the polar-solvent (acetonitrile) effects changes substantially the appropriate energy levels for the hydrogen bonded complex (−8.3 and −9.6 kcal mol−1), ion-pair (−9.5 and −8.0 kcal mol−1), relaxed ion-pair (−11.4 and −11.2 kcal mol−1) and free ions (+5.8 and +10 kcal mol−1) 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 Å (9.9 kcal mol−1) and 1.5 Å (11.9 kcal mol−1) in vacuum, and at 1.3 Å (6.8 kcal mol−1) and 1.4 Å (8.9 kcal mol−1) in acetonitrile. The MP2 method at the PBE0 optimized geometries gives higher transition-state energies values at 1.45 Å (14.2 kcal mol−1) and at 1.5 Å (14.4 kcal mol−1) in vacuum, and at 1.35 Å (10.3 kcal mol−1) and at 1.4 Å (10.5 kcal mol−1) in acetonitrile, relative to the energy of the hydrogen-bonded complex at 1.1 Å (0 kcal mol−1). The width of the barriers is larger by at least ∼1 Å 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-(4-nitrophenyl)alkanes with the TBD and MTBD bases in aprotic solvents is approached.