Valence-State Atoms in Molecules. 7. Influence of Polarization and Bond-Charge on Spectroscopic Constants of Diatomic Molecules (original) (raw)

2004, Journal of Physical Chemistry A

The polarizable valence-state-atoms-in-molecules (pVSAM) model describes the electron-pair bond in A-B molecules by superposing core-polarized A + B -, A -B + , and A:B structures, whose weights are determined by electronegativity equalization. The polarizable valence state potential energy curve (pVS-PEC) is derived through the systematic improvement of the valence state potential energy curve (VS-PEC) von Szentpály, L. J. Phys. Chem. A 1999, 103, 9313] and is given as U(R) ) -[(K 1 /R) + (K 2 /R 4 ) + (K 3 /R 7 )] + (T/R) exp(-λR). The first bracketed term contains the Coulomb, charge-induced dipole, and induced dipole-induced dipole terms, derived from weighted ionic and covalent bond-charge contributions. The potential is tested on a broad variety of homonuclear diatoms and heteronuclear halides and hydrides (a total of 52 molecules). The accuracies of the dimensionless vibration-rotation coupling constant (F) and the anharmonicity constant (G) for the halides of the alkali and coinage metals are significantly better than those of the Morse, Rydberg, simple bond-charge, and Rittner potentials. Adding core polarization to the VS-PEC reduces the average unsigned errors in the spectroscopic constants of 47 diatomic molecules from 17.1% to 7.5% in F and 18.9% to 7.8% in G, whereas those of the Morse potential amount to 32.6% and 31.4%, respectively. hydrides, has been tentatively rationalized 4,5 by the increasing importance of core-polarization and core-valence intershell correlation, both of which may be accounted for by a corepolarization potential (CPP). 17,18 To prove the point, we extend the VS-PEC into a form that takes core polarization into consideration. The polarizable valence-state-atoms-in-molecules (pVSAM) model links the ionic and covalent descriptions of the bond, using the concept of configuration mixing among contributing ionic and covalent (bond-charge) structures within the framework of the valencestate-atoms-in-molecules (VSAM) model. By incorporating polarization terms, a greater degree of accuracy is expected, and the advantage of physical interpretation is maintained, illustrated, and exploited.