Article pubs.acs.org/JPCA
Moving Atoms and Small Molecules out of Open Containers Michael L. McKee* Department of Chemistry and Biochemistry, Auburn University, 179 Chemistry Building, Alabama 36849, United States S Supporting Information *
ABSTRACT: Density functional theory with the M05-2X exchange/correlation functional is used to study the barriers for expulsion of atoms and small molecules (N2, CO, H2, Ar, Kr, Xe, H2O) out of open fullerenes (I20) and related molecular containers (C40H20, [5]beltene, cucurbit[5]uril). The reactions are examples where dispersion plays a critical role in determining the barrier heights. Calculations are compared with experimental kinetic data for N2@I20, CO@I20, and Xe@cucurbit[5]uril (Xe@CB[5]). Comparing the four molecular containers, the activation barriers for escape of an atom or small molecule correlate with the binding energies. A new open-fullerene model container C40H20 (C40) was constructed from C60 with a constriction at both ends formed by five methylene groups around the rim. The activation barriers for escape of N2 and CO from the model container are similar to those from the I20 open-cage fullerene. In the case of H2O@C40, charge analysis reveals an interesting charge transfer at the transition state as the escaping guest is “squeezed” out of the host container.
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For “loose-fitting” substrates within the endohedral complexes, computed frequencies could be CO ∼ N2 > Kr > Xe.
H 2O@CB[5](aq) + Xe(g) → Xe@CB[5](aq) + H 2O(l) (1)
Thus, whereas the calculations do not provide quantitative agreement with experiment in water, they do predict that Xe will bind significantly with CB[5] in water at 298 K (eq 1, ΔG(aq) = 0.1 kcal/mol) and that exchange in water (Xe@[CB5](aq) → CB[5](aq) + Xe(g)), should be slow ΔG‡(aq) = 27.8 kcal/mol).
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CONCLUSIONS Escape of atoms and molecules from OCFs are examples of dispersion-controlled reactions. Density functional theory with dispersion-aware exchange/correlation functions such as M052X is necessary to compute realistic activation barriers. From the transition structures and thermodynamic parameters, equilibrium constants can be computed for moving atoms and small molecules into and out of open cages. Four OCF have been considered, I20, C40, [5]beltene, and CB[5], where substrates include N2, CO, Ar, Kr, Xe, H2, and H2O. Calculations of N2@I20, CO@I20, and Xe@CB[5] (in water) have been compared with experimental results.
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ASSOCIATED CONTENT
S Supporting Information *
Table S1 contains total energies, zero-point energies, heat capacity corrections, and BSSE for the OCF species. Table S2 contains the Cartesian coordinates optimized at the M05-2X/631G(d) level. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We acknowledge the Alabama Supercomputer Center for providing time. This article is dedicated to Professor Peter Politzer, who had made numerous contributions to the analysis and interpretation of wave functions.
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REFERENCES
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Figure 6. Curbit[5]uril (CB[5]) with N2, CO, H2, and Kr inside. The plot of Xe@CB[5] (not shown) looks very similar to Kr@CB[5]. 2370
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