The uncatalyzed, thermal N-inversion reactions were studied of pyrimidin-4(3H)-imine (PMI), pyridin-2(1H)-imine (PYI), and 1H-purine-6(9H)-imine (PUI). Relevant regions of the potential energy surfaces were explored with second-order Møller-Plesset perturbation theory (MP2(full)/ 6-31G(d)) and with coupled cluster theory (CCSD/6-31G(d), CCSD/6-31+G(d)). The thermochemistry of stationary structures was evaluated at the MP2 level and their energies also were computed at the levels CCSD(T)/6-311+G(d,p) and CCSD(T)/6-311+G(2df,2p) and with structures optimized at lower CCSD levels. The best estimates for the (E)-preference free enthalpies ΔG298(Z vs. E) are 2.6 (PMI), 2.3 (PYI), and 6.0 (PUI) kcal/mol and for the free enthalpies of activation ΔG298(Z → E) they are 21.6 (PMI), 21.1 (PYI) and 19.7 (PUI) kcal/mol. Nonplanar N-inversion transition state (ITS) structures occur along enantiomeric reaction paths and stationary structures for in-plane N-inversion correspond to second-order saddle points (SOSP) on the potential energy surface. The deformation energy ΔEdef = E(SOSP) - E(ITS) is less than 0.5 kcal/mol for PMI and PUI, but it is as high as ΔEdef ≈ 2 kcal/mol for PYI. The detailed study of structures and electronic structures along the entire N-inversion path of the isomerization (Z)-PMI ⇆ (E)-PMI revealed a remarkable stabilization due to asymmetry in the ascent region from the (E)-isomer to ITS. Structures in this region of the potential energy surface allow best for additional bonding overlaps in the HOMO, and this amidine effect predicts lower N-inversion barriers in analogous imines with (Z)-preference energies. The discussion of the halogen-bonded aggregate PMI·ClCH3 exemplifies that the asymmetry in N-inversion paths is retained and perhaps even enhanced in chlorinated solvents of low polarity.
- Best Estimates,
- CCSD,
- Chemical Equations,
- Chlorinated Solvent,
- Coupled-Cluster Theory,
- Deformation Energy,
- Free Enthalpy,
- In-Plane,
- MP2 Levels,
- N-Inversion Barriers,
- Plesset Perturbation Theory,
- Reaction Paths,
- Saddle Point,
- Second Orders,
- Transition State,
- Electronic Structure,
- Enthalpy,
- Intelligent Vehicle Highway Systems,
- Isomers,
- Perturbation Techniques,
- Potential Energy,
- Potential Energy Surfaces,
- Quantum Chemistry,
- Surface Phenomena,
- Thermochemistry,
- Nitrogen Compounds,
- 1h Purin 6(9h) Imine,
- Imine,
- Polycyclic Aromatic Hydrocarbon,
- Purine Derivative,
- Pyridin 2(1h) Imine,
- Pyridine Derivative,
- Pyrimidin 4(3h) Imine,
- Pyrimidine Derivative,
- Unclassified Drug,
- Chemical Reaction,
- Chemical Structure,
- Enantiomer,
- Isomerization,
- Molecular Model,
- Computer Simulation,
- Imines,
- Isomerism,
- Models,
- Chemical,
- Molecular Structure,
- Purines,
- Pyrimidines,
- Quantum Theory,
- Stereoisomerism,
- Thermodynamics
Available at: http://works.bepress.com/rainer-glaser/33/