Ab Initio Counterparts Research Articles

Ab initio counterpart of infrared atomic charges. Comparison with charges obtained from electrostatic potentials

Atomic charges obtained from infrared intensities have already been shown to be adequately represented by Mulliken net charges and even better by a version of Mulliken charges which makes them fit the molecular dipole moment. It is shown in this paper that the atomic charges derived from electrostatic potentials give the same picture of the charge distribution within molecules as that obtained from the corrected Mulliken charges.

Ab initio counterpart of infrared atomic charges

Comparison is made between experimental atomic charges on hydrogens ( q H 0, derived from infrared intensities) and ab initio 6-31G** and 4-31G results. The simultaneous application of both the Mulliken charge and the overlap term allows a better classification of the hydrogen atoms according to their chemical environment. Although in general the overlap term is relatively small with respect to the Mulliken charge for both basis sets, Mulliken charges corrected by addition of the overlap term show a better linear correlation with the experimental atomic charges. This suggests taking the corrected charges as the ab initio counterpart of the infrared atomic charges. Moreover, the corrected Mulliken charges correctly reproduce the experimental molecular dipole moment.

Charge distributions and chemical effects. XXII. On the partitioning of molecular energies and the relationships between energy components

The exact quantum mechanical formulation of atomic and molecular energies and the postulate that ’’chemical bonds’’ exist combine to show that the nuclear–electronic and nuclear–nuclear potential energy V (k,mol) involving the kth atom in a molecule is for its largest part determined by ’’local effects’’ related to the number and type of bonds formed by k. These local effects are measured by the derivatives ∂εkj/∂Zk of the bond energies εkj involving k, with respect to its nuclear charge Zk. To a good approximation, neglecting the small nonbonded contributions, V (k,mol) =Vne (free atom k)−Zkj𝒥∂εkj/∂Zk. Applications to saturated hydrocarbons at the level of experimental accuracy indicate that the Emol/(Vne+2Vnn) ratios derived in this manner are close to their ab initio counterparts. These ratios are averages KmolAv=𝒥KmolkV (k,mol)/𝒥V (k,mol) of the ratios Kmolk=Ek(mol)/V (k,mol) defining the individual total energies Ek(mol) of atoms being part of a molecule, whereby Emol=𝒥Ek(mol). The Kmolk’s, in turn, are shown to be constants in saturated hydrocarbons, i.e., 1/2.3329 for C and 1/2 for H, and are instrumental in accurate calculations of molecular energies, Emol=𝒥Kmolk V (k,mol), and for deriving energy differences, Ek(free atom)−Ek(mol) =ΔEk =kmolkZk𝒥∂εkj/∂Zk +(Katomk−Kmolk) Vne(free atom k), between free and bonded atoms. Positive ΔEk values indicate that both the carbon and the hydrogen atoms are more stable in their bonded states than the free atoms, which is now understood in terms of dominating local binding properties, rather than in terms of local electron populations which play only a minor, though chemically significant, part on the scale of molecular energies.