Helena Dodziuk

Molecular modeling study of hydrogen storage in carbon nanotubes

Abstract Molecular mechanics calculations and molecular dynamics simulations were carried out for the systems consisting of (5,5)armchair, (9,0)zigzag, (7,3)chiral nanotubes and for the nanotube bundle built of seven (5,5)armchair nanotubes and hydrogen molecules with the aim to analyze the possibility of the use of nanotubes as hydrogen containers. Contrary to earlier calculations in which the condition of the nanotubes rigidity was imposed and hydrogen molecules were treated as spheres, the CVFF and ESFF force fields used do not imply such limitations. The results of the calculations seem to indicate that high hydrogen content in the nanotubes cannot be achieved through physisorption.

Symmetry-Adapted Perturbation Theory Applied to Endohedral Fullerene Complexes: A Stability Study of H2@C60 and 2H2@C60.

Because of difficulties in a description of host-guest interactions, various theoretical methods predict different numbers of hydrogen molecules which can be inserted into the C60 cavity, ranging from one to more than 20. On the other hand, only one H2 molecule inside the C60 fullerene has been detected experimentally. Moreover, a recently synthesized H2@C70 complex prevails in the mixture formed with 2H2@C70. To get a deeper insight into the stability of the complexes created from C60 and hydrogen molecules, we carried out highly accurate calculations for complexes of one or two hydrogen molecules with fullerene applying symmetry-adapted perturbation theory (SAPT) and a large TZVPP basis set for selected points on the potential energy surfaces of H2@C60 and 2H2@C60. The electron correlation in the host and guests has been treated by density functional theory. Our calculations yield the stability of the recently synthesized H2@C60 complex. In addition, for all tried positions of the H2 dimer inside the C60 cage, the 2H2@C60 complex has been characterized by a positive interaction energy corresponding to the instability of this species. Contrary to the conclusions of several theoretical studies, this finding, as well as model considerations and literature experimental data, indicates that only one hydrogen molecule can reside inside the C60 cage. The calculated energy components have been analyzed to identify the most important contributions to the interaction energy. Supermolecular interaction energies obtained with MP2, SCS-MP2, and DFT+Disp methods are also reported and compared to those of DFT-SAPT. The DFT-SAPT interaction energy has also been calculated for several points on the potential energy surface for a larger 2H2@C70 complex, confirming, in agreement with recent experimental findings, that this species is stable. The DFT-SAPT approach has been used for the first time to obtain interaction energies for van der Waals endohedral complexes, demonstrating that the method is capable of handling such difficult cases.

Steric effects in conjugated systems: I. Monosubstituted methylbutadienes

Abstract The optimum geometry of, and strain energy in, butadiene, 2-methylbutadiene, and cis - and trans -butadiene were calculated using the Wiberg scheme. A semiquantitative pattern was obtained of deformations induced in the butadiene molecule by a methyl group substituted in various positions. The barriers to methyl group rotation calculated with two different sets of non-bonded potential functions are much less in cis - than in trans -pentadiene. Similarly in 2-methylbutadiene the barrier is much higher than that in trans -pentadiene.

Water solubilization, determination of the number of different types of single-wall carbon nanotubes and their partial separation with respect to diameters by complexation with η-cyclodextrin

Complexation of single-wall carbon nanotubes with 12-membered cyclodextrins enables not only their solubilization in water but also their partial separation with respect to diameters and determination of the number of nanotube types on the basis of NMR spectra.

Small Molecules in C60 and C70: Which Complexes Could Be Stabilized?

The recent syntheses of complexes involving some small molecules in opened fullerenes and those of hydrogen molecule(s) in C60 and C70 are accompanied in the literature by numerous computations for endohedral fullerene complexes which cope with the problem of the stability of these complexes. In this contribution, stabilization energies of endohedral complexes of C60 and C70 with H2, N2, CO, HCN, H2O, H2S, NH3, CH4, CO2, C2H2, H2CO, and CH3OH guests have been estimated using symmetry-adapted perturbation theory, which, contrary to the standard DFT and some other approaches, correctly describes the dispersion contribution of the host-guest interactions. On the basis of these calculations, the endohedral complexes with all these guests were found stable in the larger fullerene, while the C60 cage was found too small to host the latter four molecules. Except for H2 and H2CO, a stabilization effect for most guests in the C60 cage is about 30 kJ/mol. For H2 and H2O guests, a typical supramolecular effect is observed; namely, the stabilization in the smaller cage is equal to or larger than that in the larger C70 host. Except for the water molecule where the induction interaction plays a non-negligible role, in all complexes the main stabilization effect comes from the dispersion interaction. The information on the stability of hypothetical endohedral fullerene complexes and physical factors contributing to it can be of importance in designing future experiments contributing to their applications.

13 C NMR differentiation of diastereoisomeric complexes of cis-decalin with β-cyclodextrin

13 C NMR spectra show chiral discrimination between two invertomers of cis-decalin complexed with β-cyclodextrin.

A proposal for a modification of the Cahn, Ingold and Prelog classification of chirality

Abstract In spite of its wide usage the Cahn, Ingold and Prelog (CIP) classification of chirality has some deficiencies. In particular, there is no uniform generally accepted system of designation of chirality sense and some descriptors used can be interpreted in contradictory ways, as it happens in the cyclophanes case. Therefore, a modification of the classification is proposed here with the purpose to remove the deficiencies of the CIP classification changing as little as possible in existing practice. The importance of so called conformational chirality underestimated both in the classification and in chemical practice is stressed.

Molecular mechanics calculations of molecular and chiral recognition by cyclodextrins. Is it reliable? The selective complexation of decalins by β-cyclodextrin

Abstract Molecular mechanics calculations are frequently used to rationalize experimental findings concerning molecular and chiral recognition by cyclodextrins although the reliability of the method in general and the influence of choice of the parameters in particular has not almost been analyzed. In this work molecular and chiral recognition of decalin isomers by β-cyclodextrin was studied using amber , cvff , cff91 , and mmx force fields and five values of dielectric constant ϵ =1, 2, 4, 10, 20. Most calculations yielded consistently lower stability of the complex with trans -decalin. However, in most cases the absolute value of the energy difference between the stabilization energies of the complexes with the latter isomer and the one with cis -decalin closer to it, |ΔΔ E molec |, characterizing molecular recognition was found smaller than the corresponding value involving cis -decalin enantiomers, |ΔΔ E chir |=|Δ E M - cis −Δ E P - cis |, characterizing chiral recognition. The calculations indicate no preference for complexation with either M - cis or P - cis decalin enantiomer. Therefore, we believe that MM is not suitable for reliable analysis of chiral recognition by cyclodextrins. Although the ϵ value of 1 was found unreliable for molecular mechanics calculations using all FF under study, our preliminary results of molecular dynamics calculations in water for the complexes of camphor and pinene enantiomers with α-cyclodextrin using the latter value yielded qualitatively correct results.

Structure of cyclodextrins and their complexes: Part 2. Do cyclodextrins have a rigid truncated-cone structure?2

Abstract Model molecular mechanics calculations using the MM2 force field reveal high complexity of the energy hypersurface of α-cyclodextrin. In spite of limited accuracy of the calculations carried out under many simplifying assumptions, the results obtained seem to indicate that the isolated molecule is highly flexible. This conclusion seems to be valid for cyclodextrin solutions but the flexibility of the host is expected to be restricted in complexes. Such a qualitative conclusion is in agreement with NMR studies of cyclodextrins and their complexes with aromatic guests on the one hand and with their ability to form complexes with guest molecules of various shapes on the other.

Ab initio calculations of the NMR spectra of [1.1.1] propellane and bicyclo [1.1.1] pentane

New high-level ab initio values of all the NMR shielding and spin–spin coupling constants in [1.1.1]propellane and bicyclo[1.1.1]pentane are obtained and compared with previous theoretical results and available experimental data. Electron correlation effects are taken into account and large basis sets suitable for NMR studies are applied. The results for the shielding constants depend primarily on the basis set, while the spin–spin coupling constants, in particular for the carbon–carbon coupling, are very sensitive to electron correlation effects. The computed NMR parameters agree well with the known experimental values, with the exception of the bridgehead–bridgehead carbon coupling constant in [1.1.1]propellane. Our best estimates of this constant differ from the experimental value for a substituted [1.1.1]propellane, and possible explanations of this discrepancy are discussed.