Peculiar properties of «spherical» pentanes 1H NMR Spectra.

We propose the concept of the spherical or «ball-shaped») compounds. In this communication we are considered NMR ¹H spectra of 53 «spherical pentanes» - saturated hydrocarbons with a maximum length of the chain of five carbon atoms. Depending on the alkane sphericity these include substances from simplest n-pentane to most complicated tetra(tert-butyl)methane. We introduce the conception of the spherical alkane structure, which includes: a) a central carbon atom of the whole molecule (С³), b) two, three or four carboncontaining branches attached to it. These include monocarbonic methyl (Me) and dicarbonic (longest chain) Et, Prⁱ, Bu^t- fragments. Besides of carboncontaining branches one or two branches may be the hydrogen atoms.

The central atom (С³) is surrounded by three layers, called: a) inner - α-layer, b) intermediate - β-layer, and c) the outer - γ-layer, which is often referred to - ω-layer. The inner and intermediate layers may contain both carbon and hydrogen atoms, but the outer layer contain only hydrogen atoms. We consider several types of alkanes sphericity: «quazi-spherical», «minimally-spherical», and «maximally-spherical», as well as linear (n-pentane) and «quazi-linear».

We consider the values of proton chemical shifts in all 53 investigated spherical alkanes depending on the location of the studied hydrogen atoms in a certain layer.
It has been shown that the number of hydrogen atoms in the outer ω-layer (denoted by the symbol H_ω) and the uniformity of their distribution in the volume of this outer layer (which depends on the type of alkane sphericity) have a significant influence on the studied values of proton chemical shifts in spherical alkanes.

It has been shown that when parameter H_ω = 12 (or 15) we observe the maximal upfield signals shift of all studied protons, except the methyne hydrogens of isopropyl group. Then, with the growth of H_ω parameter we observe an increase of the chemical shifts (downfield shift). It is shown that when the magnitude H_ω is equal to 12 (or 15) we observe for the methyl groups of ethyl fragments in the maximally-spherical compounds the highest upfield shift relative to the accepted standard (the δ^Н_СН3 value of terminal methyl groups of linear long chain alkanes). The same also applies to methylene groups of ethyl fragments.

To explain the observed results, we propose the hypothesis, which is associated with the assumption of interaction to each other of three (or especially four) carboncontaining branches, radiating from the central carbon atom С³ in spherical molecules. This interaction, in our opinion, is similar to the interaction (which we propose earlier) of two (in CH₂XY) or three (in CHXYZ) heteroatoms in the case of «anomeric effect» in compounds with gem-location of heteroatoms X, Y, Z. Namely this interaction, in our opinion, causes the observable upfield shift of the methylene or methyne proton’s signals.