Properties of atoms in molecules: Dipole moments and transferability of properties

Abstract

This paper uses the theory of atoms in molecules to investigate the origin of molecular dipole moments. The dipole moment is given by a sum over the net charge and first moment of every atom in a molecule. The first term leads to a charge transfer contributionJlc' the second to an atomic polarization contribution Jla' It is shown that both terms are, in general, of equal importance in determining both the static molecular dipole moment and the moment induced by a nuclear displacement. Models which imploy only point charges and corresponding bond moments which follow rigidly the nuclear framework, i.e., models which approximate Jlc and ignoreJla' are shown to lead to results that are incompatible with the changes that are found to occur in a molecular charge distribution during a nuclear vibration. The dipole moment is shown to be another group property that is transferable between molecules in the normal hydrocarbons, This property, along with the net charge, the energy, the correlation energy (expressed as a functional of the charge density), and the atomic volumes are transferable for methyl and methylene groups because of a corresponding transferability of the distribution of charge over the basins of the atoms in these groups up to their surfaces of zero flux in the gradient vector of the charge density. Transferability of group properties is not a result of the transferability of individual orbitals or geminals, which necessarily have tails lying outside of the group in question. Atoms have no tails and their energy, as is known from experiment, can in certain instances be transferred between systems with changes of less than 1 kcal/mol. The properties of atoms in molecules can be determined experimentally and quantum mechanics predicts these properties, just as it predicts the properties of a total system.

Distributed Atomic Polarizabilities of Amino Acids and their Hydrogen-Bonded Aggregates

“…In fact, a given molecular property may especially originate from one particular group. In this respect, the transferability of atomic and functional group properties is a key concept which allows exporting quantities, calculated with high accuracy in small reference molecules, to atoms or functional groups belonging to complex systems, like macromolecules, polymers or crystals, [10][11][12] that would be too expensive to calculate ab initio.…”

Implementação computacional do modelo carga-fluxo de carga-fluxo de dipolo para cálculo e interpretação das intensidades do espectro infravermelho

“…Isso foi contornado através da inclusão no modelo de mais um termo da expansão multipolar do dipolo elétrico molecular p, que são os dipolos atômicos p (a) : 15 (1) σ = x, y ou z; q (a) = carga do a-ésimo átomo; N = número de átomos na molécula A inclusão dos dipolos atômicos na descrição do dipolo molecular leva a um novo esquema de decomposição das derivadas do dipolo molecular, onde, além das componentes de carga e fluxo de cargas atômicas, surge uma componente de fluxo de dipolo atômico. Este tipo de decomposição dos Tensores Polares Atômicos, conhecida pelo acrônimo em inglês CCFDF 16,17 (Charge-Charge Flux-Dipole Flux), possibilita uma nova interpretação das mudanças ocorridas na estrutura eletrônica durante os movimentos de vibração que é bastante atrativa porque utiliza somente conceitos físicos clássicos de simples interpretação. Na teoria CCFDF, a intensidade de uma transição fundamental tem origem em três processos fisicamente distintos: a presença de cargas atômicas estáticas na direção do movimento de vibração (termo de carga); a transferência intramolecular de carga ocorrida durante a vibração (fluxo de carga) e, a polarização da densidade eletrônica em virtude da deformação molecular decorrente do movimento vibratório, resultando em uma variação dos momentos dipolares atômicos (fluxo de dipolo).…”

“…Quando o operador gradiente atômico ∇ ξ (a) atua sobre este novo momento dipolar, um novo TPA é gerado, composto por três parcelas distintas: 16,17 o termo de carga atômica c x (a)…”

Relation between interatomic distances and sizes of ions in molecules and crystals

“…Corrections can be made to account for the polarizability of ions. According to Bader et al (1987), the net charge and the polarizability affect the value of the dipole moment, ", in an approximately equal measure. From the analysis made in x 3.1, particularly from the observed correlation between the values of the deformation dR m (= dR crm À 20), dissociation energy D 0 and net ionic charge e* (Table 1 and Fig.…”