Properties of atoms in molecules: dipole moments and substituent effects in ethyl and carbonyl compounds

Abstract

The theory of atoms in molecules is used to analyze ab initio electron distributions for series of simple ethyl and formyl derivatives. The dipole moments of these molecules are decomposed into their unique charge-transfer and atomic polarization contributions, as defined by the theory. It is shown how atomic charges are related by quantum mechanics to the observable dipole moments of molecules. Substituent effects on atomic charges and hence on dipole moments are described. An analysis of the atomic populations shows that the calculated behavior of substituents on the electron density of the formyl and ethyl groups is in accord with the predictions of qualitative orbital models, employing a separation of and it substituent effects. This paper thus gives quantitative expression to those models, by using a theory with a firm quantum mechanical basis. It is suggested that substituent constants can be extracted from the theory of atoms in molecules in an empirical manner. Substituents affect the atomic charges in ethyl and formyl derivatives in contrasting ways, and a perturbation model of changes to the electron density is used to explain this different behavior.

Hirshfeld partitioning of the electron density: Atomic dipoles and their relation with functional group properties

“…These can also be identified within the framework of Bader's atoms in molecules. 24,25 Studies involving these Hirshfeld atomic dipole moments have been relatively scarce. De Proft et al have computed correlated atomic dipoles for a series of small molecules.…”

Atomic and bond properties in functionalized esters and amides

“…This relationship is based in the fact that s values can also be related to electronegativity because the bond critical point is closer to the more electronegative atoms. Slee et al 36 found a similar relationship for ethyl and formyl derivatives with X s F, G RANA AND MOSQUERA OH, NH , CN, HCHϭCH , and CH . According to Slee and coworkers, charge transfer in these bonds is governed by the electronegativity of the substituent rather than by its -electron-releasing or -withdrawing character, which has a weaker influence on the position of the bond critical point.…”

“…7᎐ 12 This theory has even enabled the distinction between CH groups in ␣ with respect to a CH group and 2 3 the other CH groups, 9 and distinction among the 2 carbonyl group in aldehydes, methylketones, and dialkylketones. This work aimed at expanding the description of carbonyl compounds based on the AIM theory, which was started in previous work and involves the analysis of transferability of atom groups in carbonyls bonded to hydrogen or alkyl groups, 5 the effect of an ␣ substituent on the CϭO unit, 15 variability in the properties of the carbonyl oxygen Ž . 16 with two ␣ substituents X, Y , the origin of the rotational barrier in amides and esters containing no additional heteroatom, 17 and the hydrolysis of formamide.…”

“…work were: a to establish quantitative relationships between the properties of the distributed Ž . carbonyl group X COY and those of -electron- 15,20 Ž . releasing groups ; b to study the influence of the different X COY units on the properties of the neighboring CH fragments as a function of their 2 relative position to the CO group and the nature of Ž .…”

Application of various population methods to some oxygenated compounds

“…Local polarizations must be considered in assessing distant electrostatic effects or dipole moments. 33 For many chemical applications, absolute values are frequently less important than relative values on changing structure. Listed in Table I are the relative oxygen populations compared to formaldehyde.…”