J. Phys. Chem. B 2008, 112, 13465–13466
13465
COMMENTS Comment on “A Theoretical Investigation of the Interactions between Water Molecules and Ionic Liquids” Jesu´s Rodrı´guez-Otero,*,† Enrique M. Cabaleiro-Lago,‡ ´ ngeles Pen˜a-Gallego† and A Departamento de Quı´mica Fı´sica, Facultade de Quı´mica, UniVersidade de Santiago de Compostela, 15782 Santiago de Compostela, Spain, and Departamento de Quı´mica Fı´sica, Facultade de Ciencias, UniVersidade de Santiago de Compostela. Campus de Lugo. AVda. Alfonso X El Sabio s/n 27002 Lugo, Galicia, Spain ReceiVed: June 16, 2008; ReVised Manuscript ReceiVed: August 13, 2008 Introduction Recently, Wang et al. carried out a theoretical investigation of the interactions between water molecules and ionic liquids.1 As a part of this work, these authors studied the interaction between the PF6- anion and one or two water molecules, reaching the following conclusion: “for the PF6- water system, to our surprise, no stationary point was found at the B3LYP/ 6-31G/ level. Therefore, we optimized the complexes at the HF/6-31G/ level and found two stationary points PF6--W and PF6--2W. However, these structures were proved to be unstable at the B3LYP/6-31G/ and MP2/6-31G/ levels”. This conclusion also seemed surprising to us, because at first glance, the interaction between an anion and a water molecule is expected to be favorable in the gas phase. So, it is very strange that only calculations at the HF level lead to stable complexes. First of all, we thought that maybe this result could be due to an unwise selection of the basis set: the inclusion of diffuse functions is practically compulsory to study anions. However, starting from the HF geometries, we did not have any problem finding a stable complex with both one and two water molecules, even without adding diffuse functions. After optimization, a vibrational analysis was performed with both the MP2 and B3LYP methods in order to confirm that the stationary points corresponded to minima. Basically, the spatial disposition of the complexes is the same as Wang. et al. found only at HF level (Figure 1). The inclusion of diffuse functions leads to very symmetric structures: C2V for PF6--W and D2h for PF6--2W (imposing this symmetry, no imaginary frequency was found). Moreover, we also found a stable complex with three water molecules, PF6--3W (with D3 symmetry). However, the use of the 6-31G/ basis set, with both B3LYP and MP2 methods, leads to more distorted structures where water molecules clearly depart from coplanarity with the F-P-F moiety (the F-P-F-O dihedral angle is around 20-30°). In all cases, the length of the O-HsF hydrogen bond is very similar, and only a very slight lengthening takes place (therefore a weakening of the hydrogen bond) as the number of water molecules is increased (see Table 1). † ‡
Facultade de Quı´mica. Facultade de Ciencias.
Figure 1. Structures of minimum energy found for the PF6- anion with one, two, and three water molecules. The appearance of structures obtained with calculations using diffuse functions is similar with all levels (DFT and ab initio). Calculations without diffuse functions lead to less symmetric structures.
TABLE 1: FsH Bond Length in the PF6-sWater Complexes (Å) //
B3LYP/6-31++G MP2/6-31++G// CCSD/aug-cc-pVDZ
PF6-sW
PF6-s2W
PF6-s3W
2.133 2.132 2.138
2.161 2.156
2.191 2.178
TABLE 2: BSSE-Corrected Interaction Energies (kJ/mol)a B3LYP/6-31G/ B3LYP/6-31++G// MP2/6-31G/ MP2/6-31++G// CCSD(T)/aug-ccpVDZb CCSD(T)/aug-ccpVTZb
PF6-sW
PF6-s2W
PF6-s3W
-42.90 (-40.05) -42.73 (-40.54) -41.95 (-38.89) -42.86 (-40.51) -41.87 (-40.07)
-79.30 (-75.79) -79.50 (-76.53) -78.05 (-74.42) -80.17 (-77.01)
-109.53 (-106.12) -112.43 (-109.04) -108.30 (-104.54) -114.07 (-110.48)
-43.98 (-42.18)
a The data in brackets are the complexation energies (that is, interaction energy plus deformation energy)..2,3 b CCSD/aug-ccpVDZ geometry.
Interaction energies of the complexes were calculated by means of the supermolecule method and employing counterpoise method to avoid basis set superposition error.2,3 According to Table 2, the complexes formed by PF6- and water are very favorable (substantial negative interaction energies are found) and the effect of increasing the number of water molecules is practically additive. It is remarkable that B3LYP results are very similar to those obtained with more expensive methods (MP2 and coupled cluster), for both geometries and energies.4 It is well-known that the B3LYP functional usually does not produce good results for noncovalent interactions. However, when the electrostatic component of these interactions is dominant, B3LYP calculations are able to give satisfactory results, as is the case of the present work.
10.1021/jp8052983 CCC: $40.75 2008 American Chemical Society Published on Web 10/01/2008
13466 J. Phys. Chem. B, Vol. 112, No. 42, 2008 In summary, contrary to the results of Wang et al.,1 we have found stationary points for the interaction between the PF6anion and water. These points were undoubtedly characterized as minima by vibrational analysis for both B3LYP and MP2 methods. Moreover, all the complexes showed significant negative interaction energies, indicating a favorable interaction between the PF6- anion and one, two, or three water molecules. All calculations were performed with Gaussian03.5 Supporting Information Available: Geometries and frequencies of all optimized complexes (B3LYP and MP2). This material is available free of charge via the Internet at http:// pubs.acs.org.
Comments References and Notes (1) Wang, Y.; Li, H.; Han, S. J. Phys. Chem. B 2006, 110, 24646. (2) Boys, S. F.; Bernardi, F. Mol. Phys. 1970, 18, 553. (3) Chalasinski, G.; Szczesniak, M. M. Chem. ReV. 2000, 100, 4227. (4) Due to its high computational cost, coupled cluster calculations have been only carried out for the complex with a single water molecule. (5) Frisch, M. J., et al. Gaussian 03; Gaussian, Inc.: Wallingford, CT, 2004.
JP8052983
