PubMed · 13571747
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1958. [PENTAMETHOMIUM (dibromide)].. https://pubmed.ncbi.nlm.nih.gov/13571747/
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We study the electronic density charge topology of CH(5)(+) species 1 (C(s)()), 2 (C(s)()), and 3 (C(2)(v)) at ab initio level using the theory of atoms in molecules developed by Bader. Despite the reports of previous studies concerning carbocationic species, the methane molecule is protonated at the carbon atom, which clearly shows its pentacoordination. In addition to the fact that hydrogen atoms in the methonium molecule behave in a very fluxional fashion and that the energy difference among the species 1, 2, and 3 are very low, is important to point out that two different topological situations can be defined on the basis of our study of the topology of the electronic charge density. Then, the species 1 and 2 present a three-center-two-electron (3c-2e) bond of singular characteristics as compared with other carbocationic species, but in the species 3, the absence of a 3c-2e bond is noteworthy. This structure can be characterized through the three bond critical points found, corresponding to saddle points on the path bonds between the C-H(2,3,5) that lie in the same plane. These nuclei define a four-center interaction where the electronic delocalization produced among the sigma(C-H) bonds provide a stabilization of the three C-H bonds involved in this interaction (the remaining two C-H bonds are similar to those belonging to the nonprotonated species). Our results show that bonding situations with a higher number of atom arrays are possible in protonated hydrocarbons.
The boronium-carbonium continuum was extended to include hypercoordinated protonated methanes and their boron analogs. The 11B NMR chemical shifts of the hypercoordinated hydriodo boron compounds and the 13C NMR chemical shifts of the corresponding isoelectronic and isostructural carbocations were calculated by using the GIAO-MP2 method. The data show good linear correlation between 11B and 13C NMR chemical shifts, which indicates that the same factors that determine the chemical shifts of the boron nuclei also govern the chemical shifts of carbon nuclei of these hypercoordinated hydriodo compounds.
Interactions of divalent polyamines with double-helical DNA in aqueous solution are investigated by monitoring the decrease in 23Na NMR relaxation rates as NaDNA is titrated with H3N(+)-(CH2)m-+NH3, where m = 3, 4, 5, or 6. Analogous measurements are made for the same homologous series of methylated polyamines (methonium ions). The dependence of the 23Na relaxation rates on the amount of added divalent cation (M2+) is analyzed quantitatively in terms of a two-state model. The sodium ions are assumed to be in rapid exchange between a "bound" state, where they are close enough to DNA so that it affects their relaxation rate, and a "free" state in bulk solution, where their relaxation rate is the same as in solutions containing no DNA. The distribution of Na+ and M2+ between these states is described quantitatively in terms of an ion-exchange parameter: DM = (pMB)(1-pNaB)n/(pNaB)n(1-pMB), where pNaB and pMB are the fractions of Na+ and M2+ that are close enough to DNA to be considered bound (by the NMR criterion), and n is the number of sodium ions displaced from DNA by the binding of one M2+ ion. For each of the polyamines and methonium ions investigated here, equations derived from this two-state model yield acceptable fittings of the titration curves if roNa, the number of sodium ions bound per DNA phosphate when no competing cations are present, is assigned a value between 0.6 and 1.00. Within this range, changing the value assigned to roNa does change the best-fitted values of DM determined for these polyamines (DH) and for the methonium ions (DMe) but does not alter the following conclusions about the trends in these parameters. (1) For polyamines and methonium ions of the same m, DH exceeds DMe by factors that are significantly larger for m = 3 and 4 than for m = 5 and 6. (2) DH for m = 3 and 4 is larger than DH for m = 5 and 6. (3) DMe for m = 3 and 4 is smaller than DMe for m = 5 and 6.