On the Polarity of Buckminsterfullerene with a Water Molecule Inside Bernd Ensing,* ,† Francesca Costanzo, ‡ and Pier Luigi Silvestrelli ‡,§ † Van’t Hoff Institute for Molecular Sciences, Universiteit van Amsterdam, Science Park 904, 1098 XH, Amsterdam, The Netherlands ‡ Dipartimento di Fisica e Astronomia “G. Galilei”, Universita’ di Padova, via Marzolo 8, I-35131 Padova, Italy § DEMOCRITOS National Simulation Center, Trieste, Italy ABSTRACT: Since the recent achievement of Kurotobi and Murata to capture a water molecule in a C 60 fullerene (Science 2011, 333, 613), there has been a debate about the properties of this H 2 O@C 60 complex. In particular, the polarity of the complex, which is thought to be underlying the easy separation of H 2 O@C 60 from the empty fullerene by HPLC, was calculated and found to be almost equal to that of an isolated water molecule. Here we present our detailed analysis of the charge distribution of the water-encapsulated C 60 complex, which shows that the polarity of the complex is, with 0.5 ± 0.1 D, indeed substantial, but significantly smaller than that of H 2 O. This may have important implications for the aim to design water-soluble and biocompatible fullerenes. ■ INTRODUCTION Endohedral fullerenes are fascinating molecular complexes that consist of a nanoscale carbon cage and an encapsulated guest atom, ion, or molecule. They have opened a new way to study small molecular species in confined isolation but also provide an interesting means to alter the properties of the otherwise rather inert fullerenes. Recently, Kurotobi and Murata found a synthetic route to surgically insert a single water molecule into the most common fullerene, the C 60 “buckyball”. 1 This is a remarkable feat considering that water under normal conditions prefers to exist in a hydrogen-bond-forming hydrophilic environment. As the strong dipole moment of the water molecule is expected to polarize the symmetric nonpolar C 60 cage, the H 2 O@C 60 complex may be an important step forward toward the aim to make fullerenes that are water-soluble and biocompatible. 2,3 Among the extensive analysis that Kurotobi and Murata performed to characterize the H 2 O@C 60 moiety, they showed that the complex can be relatively easily separated from the empty fullerene by liquid chromatography, which is a first indication that indeed the outside properties of the carbon cage have changed after encapsulating a water molecule. They also performed density functional theory (DFT) calculations and computed that the dipole moment of the H 2 O@C 60 complex is 2.03 Da surprisingly high value considering that it is equal to that of an isolated water molecule (2.02 D) at the same M06- 2X/6-311G(2d,p) level of theory. 1 While this result was very recently confirmed by Bucher 4 using the DFT-BLYP-D functional (2.0 D), it is in sharp contrast to the result of earlier work by Ramachandran and Sathyamurthy, who found 0.54 D using Hartree-Fock and MP2, 5 and by Yagi and Watanabe, who found 0.61 D using DFT-B3LYP. 6 Balch attributes this discrepancy to the lower level of sophistication of the earlier calculations. 2 See Table 1 for a compilation of the published values for the dipole moment at the different levels of theory. Here we present our careful analysis of the dipole moment of the H 2 O@C 60 complex in order to resolve the inconsistency among the different literature results of the dipole moment. For this aim, we reproduced the previous ab initio estimates and performed additional benchmark DFT calculations. We also analyze the effect of the water encapsulation on the electronic density distribution of the C 60 cage. Using ab initio molecular dynamics simulations, we measure the dipole distribution and the relaxation of the orientational time correlation at finite temperatures. ■ METHOD We performed all-electron Hartree-Fock (HF) and second- order Møller-Plesset perturbation theory (MP2) calculations as well as density functional theory (DFT) calculations with, among others, the B3LYP 7 and M06-2X 8 hybrid-functionals using the Gaussian program. 9 The CPMD program 10 was used to perform DFT calculations and DFT molecular dynamics (MD) simulations using pure GGA functionals augmented with Grimme’s 13 dispersion interactions. We used norm-conserving Troullier-Martins pseudopotentials with a plane waves cutoff Received: November 11, 2012 Published: November 17, 2012 Table 1. Literature Values for the Dipole Moment (D) method basis set H 2 O@C 60 H 2 O ref M06-2X 6-311G(2d,p) 2.03 2.02 1 BLYP-D 80 Ry 2.0 1.86 4 HF 6-31G 0.54 2.5 5 B3LYP cc-pVDZ/STO-3G 0.61 1.94 6 Article pubs.acs.org/JPCA © 2012 American Chemical Society 12184 dx.doi.org/10.1021/jp311161q | J. Phys. Chem. A 2012, 116, 12184-12188