Transferability in alkyl monoethers. II. Methyl and methylene fragments

Abstract

The transferability of the atomic and bond properties of the methyl and methylene fragments in linear unbranched alkyl monoethers was studied using the Theory of Atoms in Molecules (AIM). This theory has been applied to the analysis of the HF/6-31++G**//HF/6-31G* electron charge distributions of a series of 33 dialkyl ethers, CH3(CH2)mO(CH2)nCH3, [n=0,1(n⩽m⩽9), n=2,3(n⩽m⩽8), n=4(n⩽m⩽5)]. The results obtained indicate that the methyl and methylene fragments situated in α, β, γ, or δ positions with respect to the oxygen atom are different to those of an n-alkane. Nevertheless, CH3 and CH2 at more distant positions can be considered as standard units, whose nonenergetic properties coincide with those of the corresponding fragment in an n-alkane. On the contrary, the energetic properties of the fragments maintain a differential value with respect to the n-alkane in all of the positions studied in the series. The properties of the methyl or methylene fragments in α to the oxygen depend on the size (methyl or larger) of the other alkyl chain bonded to the oxygen. The properties of methylenes are also different when they are α to the terminal CH3. Thus, the CH2 and CH3 fragments of dialkyl ethers can be classified into 9 CH3 and 12 CH2 groups. All of the groups proposed verify the transferability of bond properties, charge, and volume throughout all the fragments that it includes. Though the energy of the fragments depend on the size of the molecule, fragments included in the same group display a common dependence. This dependence does not impede the appearance of excellent linear relationships between the total molecular energy and the number of CH2 groups. Nevertheless, the AIM computed energies for the oxygen atom are always more negative than those obtained from the fittings of total electron energies to the number of CH2 groups in the molecule. This stabilization is produced at the cost of destabilizing the CH2 or CH3 groups in the α position. Whereas, if the CH2 groups bonded to a methyl group are excluded, the remaining CH2 and CH3 groups are slightly stabilized (in a magnitude that depends on the size of the molecule and which oscillates slightly in its position with respect to the oxygen atom).