Atomic Charges in Highly Ionic Diatomic Molecules

Abstract

Atomic charges were investigated as functions of detectable atomic and molecular constants at equilibrium structures. It was found based upon the variation idea that atomic charges in highly ionic molecules can be expressed as a function of molecular dipole moments, polarizabilities of free cations, and polarizabilities of free neutral atoms of the corresponding anions. The function can be given in the form of classical Rittner’s relationship ( J. Chem. Phys . 1951 , 19 , 1030). For the ground states of alkali halide molecules, the predicted atomic charges are close to an elementary charge e and the predicted dipole moments are in good agreement with the observed values; for spin-restricted high-ionic systems such as the lowest 9 Σ electronic states of BN, AlN, GaN, BP, AlP, GaP, BAs, AlAs, and GaAs molecules, the predicted atomic charges are also near 1 e and in good agreement with the results of natural population analysis at MRCI/cc-pvqz and HF/6-311+G(3df) levels. Polarizabilities for the lowest quintet states of B – , Al – , Ga – , N + , P + , and As + ions were also obtained based upon high-level ab initio computations. Atomic charges from other related methods are also investigated for comparison. The results demonstrate that high-quality atomic charges can be obtained with detectable variables, such as molecular dipole moment, vibrational frequency, as well as polarizabilities of the related free atoms and ions.

Universal scales of electronegativity and ionicity from electron scattering factors

“…The electronegativity (w A , w B ) and its related asymmetric and symmetric functions together with our ionicity scales calculated by eqn ( 30)- (32), as well as the ionicity scales proposed by Philips and Pauling, 33 are listed in Tables 5 and 6 for comparison. As emphasized by Philips, 33 a reasonable scale of ionicity should be able to distinguish the coordination of crystalline A N B 8ÀN compounds.…”

Energy Electronegativity and Chemical Bonding

“…Owing to the flexibility of the electron clouds around each nucleus, the effective charge center is not at the nucleus, and the accurate dipole moments should be equal to μ = qR c , where R c is the separation between these charges [ 56 ]. Authors of [ 57 ] indicated that atomic charges in a highly ionic molecule can be computed only if the molecular dipole moment, the polarizabilities of free cations and neutral atoms of the anions, are all known. In [ 58 ], a new approach to calculating the local dipole moments and charges of atoms based on Bader’s AIM theory has been presented.…”

Subverting covalency gives rise to ionic bonding in high spin states of heterodiatomics

“…14 The exhaustive studies on spin ladder based PECs reveal a unique finding which shows that after the decimation of covalency is achieved in the purely dissociative high spin sigma state it is followed by an even higher spin state which has purely ionic bonding. 16 The persistent presence of such an ionic bonded state at a certain juncture of finding shows that in 2 nd row hetero-diatomics the negation of covalency paves the path for ionic bonding. The unique periodic trend may in turn suggest that covalency is indeed negated at a certain high spin sigma state.…”

“…For further analysis we computed the natural bond orbital (NBO) charge analysis and we got a high negative charge density on boron and a high positive charge density on nitrogen. 16 Thus, at this extreme high state regime where all the conventional covalent bonds are broken, the polarity of the molecule is reversed with respect to the electronegativity of these atoms. Similar results are obtained when we plotted the PECs of 7 Σ + and 9 Σ + states of BeO and 5 Σ + , 7 Σ + and 9 Σ + of LiF (see Figure 4(b) and 4(c)).…”

“…12b If one attempts to predict the number of bonds from computed bond orders for a heterodiatomic species like BeO the answer varies from one method to another substantially (see Table 1). 16,17 This certainly suggests that to address the question of number of bonds one needs to develop an approach which will rely upon a more fundamental feature of bonding in molecules, at the least for diatomics. The definitive signature for the presence of a chemical bond between two atoms is the marked presence of a distinct minimum on the potential energy curve.…”