Ab Initio Molecular Orbital Theory on Intramolecular Charge Polarization:  Effect of Hydrogen Abstraction on the Charge Sensitivity of Aromatic and Nonaromatic Species

Abstract

We performed ab initio molecular orbital (MO) calculations of the response kernel (∂Q a /∂V b ), which represents the response of the intramolecular charge polarization by external electrostatic field, on the basis of the coupled perturbed Hartree−Fock equation. The response kernels of some organic molecules including pyrazine, pyrazinyl radical, acetone, and 2-hydroxypropyl radical were calculated along the present formulation. The results revealed that the hydrogen abstraction of pyrazine causes the product radical to be remarkably deformable in the partial charge distribution, while the hydrogen abstraction of acetone does not induce such enhancement of the charge sensitivity. The augmented sensitivity does not appear in the usual polarizability for a uniform field but emerges for a local fluctuated field. To elucidate the remarkable difference, we performed the normal mode analysis and decomposition based on the intrinsic soft MO pairs or localized orbitals. As a result, the enhancement in the aromatic species is attributed to the softest normal mode due to the π−σ mixing that facilitates the deformation of the π-electron orbitals. In the nonaromatic species, on the other hand, this effect is not dominant and is canceled by the breakdown of hyperconjugation. We suggest that the particular sensitivity of aromatic radicals is the origin of anomalously slow diffusion in solution.

Free Energies of Chemical Reactions in Solution and in Enzymes with Ab Initio Quantum Mechanics/Molecular Mechanics Methods

“…The QM reference charges are determined at a given QM geometry and with a given external MM electrostatic potential at the position of the QM atoms. After the perturbation of the QM geometry and the external MM electrostatic potential, the polarized charges are determined through two response kernels, namely [81,82], (7) and [81] (8) The second kernel can now be computed analytically with a recently introduced ESP fitting method [83]. Just as in the polarization effects introduced in the term , we also expand the term E 1 (r QM , r MM ) in a similar manner [81,62].…”

Determining polarizable force fields with electrostatic potentials from quantum mechanical linear response theory

“…(21). The denominator of the factor represents the sum of weights of all grid points belonging to molecule i.…”

“…Within the atomic charge framework, Morita and Kato developed the charge response kernel model in which atomic charges linearly respond to the external potential. 21,22 Piquemal et al developed the SIBFA method which includes non-classical effects such as exchange-polarization. [23][24][25][26][27][28][29][30][31] In the induced dipole model, the induced point dipole at atom site i is proportional to the electric field at the same site (µ i = α i E i , where α i is the atomic polarizability of atom i).…”

Molecular Dynamics Simulation with the Charge Response Kernel: Vibrational Spectra of Liquid Water and N-Methylacetamide in Aqueous Solution

“…This type of modeling is widely used in simulation studies to demonstrate that the explicit inclusion of many-body polarization effects could improve the description in various aspects, e.g., the diffusion coefficient of liquid water, [12][13][14][15] solvation structure around ion, [16][17][18] and solvation free energy. 19,20 Recently, we have proposed the charge response kernel (CRK) model 23 as an alternative method to express the electronic polarization effect. Molecular dynamics (MD) calculations with the CRK have been performed for evaluating the anomalously slow diffusion of pyrazinyl radical in methanol solution 24 and for investigating fast vibrational relaxation of azide ion in water.…”

“…Such nonlocal response would be crucial especially to molecules which have delocalized π electrons. 23 Regardless of the above merits, the CRK is as easily and straightforwardly implemented in the actual simulations as both the point-polarizability and FQ models.…”