QT-AIM analysis of neutral pterin and its anionic and cationic forms

Quantum theory of atoms in molecules (QT-AIM) allows detailed insight into the electronic structure of molecules by analysis of the gradient vector field of the electronic density distribution function. First results of a QT-AIM analysis of neutral pterin as well as its anionic and cationic forms in aqueous solution are reported based on density functional theory using B3LYP/6-311+G(2d,p)//6-31G(d) level of theory. Besides reporting QT-AIM results of the atomic partial charges and bond orders of the molecules, their electron density functions, Laplacians and electrostatic potential functions are also shown. The results demonstrate the rather extensive delocalization of both negative and positive extra charges predominantly over the pyrimidine moiety of the pterin ring system and allow a precise quantitation of the effects of addition or elimination of a proton on the bonding structure. Quantum chemical techniques in combination with QT-AIM procedures are thus well-suited to describing the bonding structure as well as the major atomic and bonding features of neutral pterin and its cationic and anionic forms. Moreover, the data demonstrate that the chosen level of theory yields chemically reasonable results while not being computationally too expensive, thus providing a sound basis for further theoretical chemical work on pteridines.