Some calculations on single-wall carbon nanotubes

Francisco Torrens

Institut Universitari de Ciència Molecular, Universitat de València

Dr. Moliner 50, E‑46100 Burjassot (València), Spain

Francisco.Torrens@uv.es

 

The aqueous solubility S_w of C_60-70-82, van der Waals dimerљ (C₆₀)₂ and chemically reduced C₆₀H₆₀ is calculated with our program based on AQUAFAC model. C₆₀-I_h has a especially symmetric truncated-icosahedral structure. C₇₀-D_5h is similar to C₆₀, with the 10 extraљ atoms inserted in a band of hexagons around the middle of the truncated icosahedron, producing a prolate, ellipsoidal structure. A substructure of C₇₀, where the five non‑equivalent atoms are labelled a-e, shows that atoms a-dљ join one pentagon with two hexagons, while atom eљ joins three hexagons. Some endohedralљ C₆₀H₆₀ isomers with one or more C-H bonds pointing inside the cavity result more stable than their all-outљ counterparts. The calculations refer to AM1 optimum number of inside H atoms and geometry, which has 10 endohedral H atoms. The negative Gibbs free energy of solvation in water -DG_solv,water slightly increases from C_60-82. However, -DG_{solv,1‑octanol} increases from C_60-82. 1‑Octanol-, cyclohexane- and chloroform-water partition coefficients (P_o, P_ch and P_cf) increase by seven orders of magnitude with the number of atoms. For (C₆₀)₂-C_5h, it is considered the possibility that the aqueous phase be entirely assigned to the monomer. C₆₀H₆₀ results show that no important effect on logPљ is expected related to the all-exoљ (I_h) or partially endoљ (C₁) position of H atoms. A comparative study is carried out with programs SCAP and CDHI (logP_o), and SCAP and a method by Leo and Hansch (logP_ch-cf). LogP_o-ch-cf allow calculating molecular lipophilicity patterns, which show that, for a given atom, normalized logP_o-ch-cf are sensitive to the presence in the molecule of other atoms-groups, e.g., C₇₀, where logP_a-c atomic contributions are greater than logP_d-e are, what can be explained because the distances from the nearest pentagon vary gradually from aљ to e. By contrast, CDHI does not differentiate atoms a-e. The molecular lipophilicity pattern (MLP) is the 10´normalized logP_o map of C₇₀. Lipophilicity monotonically decreases as the distance from the nearest pentagon increases. LogP_o correlates quadratically with the distance from the nearest pentagon. For single-wall carbon nanotubes (SWNT) of chiral (lattice) indices (n,m), logS_w and logP_o-ch-cf correlate with (n² + nm + m²)^1/2, which is proportional to SWNT diameter. (10,10), the most favourite SWNT, presents consistency between a relatively small aqueous solubility and great P_o-ch-cf. The partition of SCAP logPљ for (17,0)-(10,10) shows that the contribution of the trivalent atoms aљ is smaller that that of the divalent atoms bљ is. However, CDHI does not differentiate atoms a-b. (17,0)-(10,10) MLPs show that divalent atoms bљ present the greatest lipophilicity. Although the solubility in organic solvents is predicted greater than in water, absolute solubility in organic solvents is estimated extremely small, which is supported by the fact that there are few solvents for SWNTs. SWNT solubility is studied in various solvents, finding a class of non‑H‑bonding Lewis bases with good solubility. Solvents group in three classes. Although a Lewis basicity without H‑donors was suggested experimentally to be an important condition for a good solvent, this may be a necessary but not sufficient condition since DMSO, a bad solvent, is an exception. Poly(a,4‑benzyl), 1,2,3,4‑tetramethylbenzene (TMB), 1,2,3,5‑TMB, 1,2,4,5‑TMB and poly(1,2,4,5‑TMB) (PTMB) behave as class 2. PTMB is an ultrastrong solvent of C₆₀ (at least 10 times stronger than the best known solvents). SWNTs in some organic solvents are positively charged, while in water-Triton X are negative. CycloD‑Glcp_n are less hydrophilic than D‑Glcp_n are. CycloD‑Glcp_n lipophilicity shows that hydrophobic effect is important in D‑Glcp_n-inclusion complexes with hydrophobic guests.