P-Phenylene bridge induced dual twisting and metameric impacts on molecular structure and dipole Moments: A computational study on pyridinium phenolate mesomeric betaines using linear response theory

Abstract

In this contribution, molecular structures, and dipole moments (using self-consistent and linear response theories) were reported for two p-phenylene bridged mesomeric betaines, using AM1, HF, B3LYP, B3PW91, M06-2X, LC-ωPBE, CAM-B3LYP, MP2 and CASSCF methodologies. In this work (1) metameric influences on molecular conformations, and (2) molecular electric dipole moments, and then (3) impact of bridge by comparing with unbridged derivatives, are investigated. Metameric influence on conformational preference was observed and p-phenylene bridge was found to be acting like a facilitator. While most of the methodologies predicted only reasonable impacts of metamerism on molecular dipole moments; the CASSCF method predicted around 2-fold enhanced dipole moment for Reichardt’s type than Brooker’s type. p-phenylene bridge effectively enhanced the dipole moments (∼2-fold) than non-bridged zwitterions. Linear response theory-based (field-dependent) results were found to be similar to the static (field-independent) dipole moments. HF, MP2 and CASSCF exhibited similar behaviors, and DFT-based methodologies very different characteristics.