Highly Accurate ab Initio π-Electron Hamiltonians for Small Protonated Schiff Bases

Abstract

The effective valence shell Hamiltonian (ℋν) method is used to compute the ab initio π-electron Hamiltonians for the two small protonated schiff bases propeniminium (C3H4NH2+) and pentaniminium (C5H6NH2+), which are small analogs of the photoreceptor chromophore retinal. Because these small polyenes lack low lying, ionic excited states, the ℋν calculations perform remarkably well even though they employ constrained, semiempirical-like valence spaces. In contrast to studies on other polyenes, even the second-order ℋν performs extremely well, while the third-order ℋν calculations yield an effective π-electron Hamiltonian that resembles the familiar semiempirical Pariser−Parr−Pople (PPP) Hamiltonian but which also retains ab initio accuracy for the vertical excitation energies. The ab initio π-electron Hamiltonian need only retain the PPP-type αi, βi,j, and γi,j effective integrals to produce excitation energies that compare well with previous multireference single and double configuration interaction (MRSDCI) calculations. Since the ab initio π-electron effective integrals arise from first principles, they do not require any empirical corrections. However, they do lack the simple transferability generally assumed in semiempirical models for these short-chain polyenes.