On the Difference of the Properties between the Blue-Shifting Halogen Bond and theBlue-Shifting Hydrogen Bond

Weizhou Wang, Yu Zhang, and Baoming Ji*College of Chemistry and Chemical Engineering, Luoyang Normal UniVersity, Luoyang 471022, China

ReceiVed: April 17, 2010; ReVised Manuscript ReceiVed: June 06, 2010

The difference of the properties between the blue-shifting halogen bond and the blue-shifitng hydrogen bondhas been investigated at the M06/6-311++G(d,p) theory level. It was found that the three lone electron pairson the halogen atom play an important role for the difference of the properties between the blue-shiftinghalogen bond and the blue-shifitng hydrogen bond. The additional electron-density transfer from the threelone electron pairs on the halogen atom to the remote part of the halogen atom donor (i.e., the antibondingorbitals of the bonds other than X-Hal) makes the blue-shifting halogen bond much more ubiquitous in thehalogen-bonded complexes than the blue-shifitng hydrogen bond in the hydrogen-bonded complexes, and italso makes the values of the X-Hal bond contraction generally much larger than the values of the X-Hbond contraction. The difference of the properties between the X-Cl · · ·Y blue-shifting halogen bond andthe X-Br · · ·Y blue-shifting halogen bond was also discussed.

Introduction

Noncovalent bond, a type of chemical bond that does notinvolve the sharing of pairs of electrons, plays a critical role innature, which has made it the focus of attention for a long time.Among all kinds of noncovalent interactions, the hydrogen bond(X-H · · ·Y; X and Y are electronegative atoms or groups) isundoubtedly the most important one. In recent years, the halogenbond (X-Hal · · ·Y; Hal ) Cl, Br, or I) has also been found toplay a dominant role in many fields, although not as importantas the hydrogen bond.1-20 Many studies have revealed that theproperties of the halogen bond are pretty similar to the propertiesof the hydrogen bond. For instance, in addition to the conven-tional red-shifting hydrogen bond, there are many unconven-tional blue-shifting hydrogen bonds;21 similarly, the unconven-tional blue-shifting halogen bonds have also been found in manycomplexes.6 After all, the electronic structure of the halogenatom is quite different from that of the hydrogen atom. Theremust be some difference in the properties between the hydrogenand halogen bonds.

Comparing the computational and experimental results of thehydrogen bond with those of halogen bond,6,11,20-22 one canclearly find that the blue-shifting halogen bond occurs morefrequently in the halogen-bonded complexes than the blue-shifting hydrogen bond in the hydrogen-bonded complexes.Furthermore, in the halogen-bonded complexes, the X-Cl · · ·Yblue-shifting halogen bond occurs more frequently than theX-Br · · ·Y blue-shifting halogen bond. The other finding is thatthe magnitude of the contraction of the X-Hal bond in the blue-shifting halogen bond is generally considerable. For example,a large contraction (0.0347 Å) of the C-Cl bond was observedin the halogen-bonded complex F3C-Cl · · ·OH-.20 However,the magnitude of the contraction of the X-H bond in the blue-shifting hydrogen bond is seldom over 0.0100 Å.21 It is alsoobserved that the values of the C-Cl bond contraction arealways larger than the corresponding ones of the C-Br bondcontraction upon the blue-shifting halogen bond formation.

Table 1 lists a series of values of the bond-length change uponthe halogen bond or the hydrogen bond formation calculated atthe M06/6-311++G(d,p) theory level. It can be seen from Table1 that the C-Cl bond contraction is quite outstanding, especiallyin the negatively charged complexes. Evidently, the values inTable 1 confirm our above-mentioned findings once again. Mostof the halogen-bonded complexes in Table 1 have also beeninvestigated theoretically at the MP2/aug-cc-pVTZ level oftheory.20 The values of the bond-length change at the MP2/aug-cc-pVTZ theory level are very similar to those reported here,which means that the above-mentioned findings are independentof the computational methods.

Then, an important question is: what is the origin of thedifference of the properties between the blue-shifting hydrogenbond and the blue-shifting halogen bond? In recent years, thereare many different explanations for the origin of the blue-shiftinghydrogen bond and the blue-shifting halogen bond.11,21 Ourpurpose in the present study is not to give a new explanationfor the origin of the blue-shifting hydrogen bond or the blue-shifting halogen bond but to try to find the answer to the problemmentioned above. Certainly, the study on the origin of thedifference of the properties between the blue-shifting hydrogenbond and the blue-shifting halogen bond will help us furtherunderstand the nature of the bond-length change upon complexformation.

Computational Details

Structures of the monomers and the complexes were opti-mized and characterized by frequency computations at the M06/6-311++G(d,p) theory level. M06 is a newly developed hybridfunctional of Truhlar and Zhao, which gives a good performancefor the noncovalent interactions.23 Natural bond orbital (NBO)analyses are done for the systems studied in this article also atthe M06/6-311++G(d,p) theory level, so that the role of theelectron-density transfer involving the formation of the hydrogenbond or the halogen bond can be quantitatively evaluated.24

Unless otherwise stated, all values given in this article wereobtained from the M06/6-311++G(d,p) calculations. To avoid

* To whom correspondence should be addressed. Tel: +86-379-65523821. Fax: +86-379-65523821.

J. Phys. Chem. A 2010, 114, 7257–7260 7257

10.1021/jp103457u 2010 American Chemical SocietyPublished on Web 06/15/2010

possible computational artifact, the MP2/cc-pVTZ calculationswere also performed for some systems. In a very recent paper,Riley et al. pointed out that this method provides very goodestimates of geometries and energies for the noncovalentcomplexes.25

All of the calculations were performed using the Gaussian09 suite of programs.26 The NBO analyses were carried out withthe built-in subroutines of the Gaussian 09 program. Here wenote that the value of the bond-length change is given as thedifference of the bond length between the complex and themonomer, so that a negative value of the bond-length changerefers to a bond contraction upon complex formation and apositive value of the bond-length change refers to a bondelongation upon complex formation.

Results and Discussion

In the halogen atom donor of the halogen-bonded complex,the halogen atom has three nonbonding pairs of electrons inthe outer level, whereas the hydrogen atom in the hydrogendonor of the hydrogen-bonded complex has no lone electronpair. Therefore, it is reasonable to assume that the three loneelectron pairs of the halogen atom may play a dominant rolefor the difference of the properties between the blue-shiftinghalogen bond and the blue-shifting hydrogen bond. In thenegative electric field region of the electron donor Y, the electrondensity of the three lone pairs on the halogen atom is transferredto the remote part (i.e., the antibonding orbitals of the bondsother than X-Hal) of the halogen atom donor. According toBerlin’s theorem,22,27 the electron density decrease in the lone-pair region of the halogen atom will lead to the decrease in theelectrostatic Hellmann-Feynman force acting on the halogennucleus and, consequently, the contraction of the X-Hal bond.As a comparison, there is no such electron-density transfer inthe hydrogen-bonded complex. Therefore, it can be concludedhere that the additional electron-density transfer from the lone-pair region of the halogen atom to the remote part of the halogenatom donor makes the blue-shifting halogen bond much moreubiquitous in the halogen-bonded complexes than the blue-shifitng hydrogen bond in the hydrogen-bonded complexes andalso makes the values of the X-Hal bond contraction generallymuch larger than the values of the X-H bond contraction.

To check the validity of the explanation for the difference ofthe properties between the blue-shifting halogen bond and theblue-shifting hydrogen bond, we designed a series of halogen-bonded complexes formed by CCl4 with one, two, three, and

four NH3, respectively. The geometries of the designed com-plexes are shown in Figure 1. The idea is very simple: by placingmore and more NH3 molecules around the CCl4 molecule, wecan gradually prevent the electron density in the lone-pair regionof the halogen atom Cl1 from being transferred to the σ*antibonding orbitals of the C-Cl2, C-Cl3, and C-Cl4 bonds;then, the C-Cl1 bond will become more and more long,according to the above explanation. The calculated results aregiven in Table 2. As expected, the C-Cl1 bond becomes moreand more long from CCl4 · · ·NH3 to CCl4 · · · (NH3)4. Upon theformation of complexes CCl4 · · ·NH3 and CCl4 · · · (NH3)2, theC-Cl1 bond is contracted, and it is elongated upon the formationof complexes CCl4 · · · (NH3)3 and CCl4 · · · (NH3)4. It also can beseen from Table 2 that the results calculated at the M06/6-311++G(d,p) theory level are very similar to those calculatedat the MP2/cc-pVTZ theory level, which again demonstratesthe reliability of the M06/6-311++G(d,p) method for the presentstudy. Table 2 also lists the values of the second-orderperturbation stabilization energies (∆E2) of some significantdonor-acceptor orbital interactions for the four halogen-bondedcomplexes. According to the NBO theory, the larger values of∆E2 mean more electron-density transfer between the corre-sponding donor and acceptor orbitals. First, it is noticed in Table2 that the values of ∆E2 for the complex CCl4 · · ·NH3 are all

TABLE 1: X-Cl(Br) Bond-Length Changes (∆rX-Cl(Br), angstroms) upon Halogen Bond Formation versus X-H Bond-LengthChanges (∆rX-H, angstroms) upon Hydrogen Bond Formation at the M06/6-311++G(d,p) Theory Level

complex ∆rX-Cl complex ∆rX-Br complex ∆rX-H

F2BCl · · ·NH3 -0.0044 F2BBr · · ·NH3 -0.0019 F2BH · · ·NH3 -0.0005F3CCl · · ·NH3 -0.0057 F3CBr · · ·NH3 -0.0001 F3CH · · ·NH3 +0.0039F3SiCl · · ·NH3 -0.0048 F3SiBr · · ·NH3 +0.0002 F3SiH · · ·NH3 +0.0015F2BCl · · ·F- -0.0270 F2BBr · · ·F- -0.0074 F2BH · · ·F- +0.0281F2BCl · · ·Cl- -0.0232 F2BBr · · ·Cl- -0.0147 F2BH · · ·Cl- +0.0011F2BCl · · ·Br- -0.0215 F2BBr · · ·Br- -0.0144 F2BH · · ·Br- +0.0000F3CCl · · ·F- -0.0090 F3CBr · · ·F- +0.0358 F3CH · · ·F- +0.0627F3CCl · · ·Cl- -0.0157 F3CBr · · ·Cl- +0.0157 F3CH · · ·Cl- +0.0174F3CCl · · ·Br- -0.0145 F3CBr · · ·Br- +0.0181 F3CH · · ·Br- +0.0142F3SiCl · · ·F- -0.0153 F3SiBr · · ·F- +0.0185 F3SiH · · ·F- +0.7561F3SiCl · · ·Cl- -0.0208 F3SiBr · · ·Cl- -0.0012 F3SiH · · ·Cl- +0.0186F3SiCl · · ·Br- -0.0184 F3SiBr · · ·Br- -0.0012 F3SiH · · ·Br- +0.0137F3CCl · · ·NC- -0.0223 F3CBr · · ·NC- +0.0050 F3CH · · ·NC- +0.0181F3SiCl · · ·NC- -0.0226 F3SiBr · · ·NC- -0.0071 F3SiH · · ·NC- +0.0160F3CCl · · ·CN- -0.0092 F3CBr · · ·CN- +0.0439 F3CH · · ·CN- +0.0220F3SiCl · · ·CN- -0.0178 F3SiBr · · ·CN- +0.0069 F3SiH · · ·CN- +0.0211

Figure 1. Geometries of the complexes formed by CCl4 with one,two, three, and four NH3, respectively.

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larger than the corresponding ones for the monomer CCl4. Thisis understandable because in the negative electric field regionof the electron donor NH3, more electron density in the lone-pair region of the Cl1 will be transferred to the σ* antibondingorbitals of the C-Cl2, C-Cl3, and C-Cl4 bonds. By addingmore and more NH3 molecules around the CCl4 molecule, wefound that the values of ∆E2 become more and more small,which indicates that the electron-density transfer of Cl1 is indeedprevented. The decrease in the NBO charge on Cl1 fromCCl4 · · ·NH3 to CCl4 · · · (NH3)4 further confirms the increase inthe electron density in the lone-pair region of Cl1. Here theresults of the C-Cl1 bond length change, and the correspondingelectron-density transfer of Cl1 clearly shows the validity of theabove explanation for the origin of the difference of theproperties between the blue-shifting halogen bond and the blue-shifting hydrogen bond.

The change of electrostatic interaction, intermolecular charge-transfer interaction (LP(N1)f σ*(C-Cl1)), or electron-densitytransfer from the lone-pair orbitals of Cl2, Cl3 and Cl4 to theC-Cl1 σ* antibonding orbitals will also affect the C-Cl1 bond-length change upon complex formation. To exclude these effects,we studied the following three complexes: Cl- · · ·CH4,Cl- · · ·ClCH3, and Cl- · · ·BrCH3 (Figure 2). The distancebetween Cl- and H (Cl, Br) was kept constant at 7.0 Å toeliminate the effects of electrostatic interaction and intermo-lecular charge-transfer interaction. Note that at such longdistance, the intermolecular charge-transfer interaction can beneglected. The atoms Cl2, Cl3, and Cl4 of the complexes inFigure 1 were changed to H here to get rid of the effect of theelectron-density transfer from the lone-pair orbitals of Cl2, Cl3,and Cl4 to the C-Cl1 σ* antibonding orbitals. As shown inFigure 2, the C-H, C-Cl, and C-Br bonds are all contractedupon complex formation. The absolute value of C-Cl bondcontraction is larger than that of the C-Br bond contraction,

and the absolute value of C-H bond contraction is the smallestone, which is in agreement with our findings mentioned above.Also, the electron-density transfer from the lone-pair orbitalsof Cl (Br) to the C-H σ* antibonding orbitals can be evaluatedby the second-order perturbation stabilization energies (Table3). Upon the complex Cl- · · ·ClCH3 or Cl- · · ·BrCH3 formation,the values of ∆E2[LPCl1(Br1) f σ*(C-H)] all increase, andthe increasing magnitude of ∆E2 in Cl- · · ·ClCH3 is larger thanthe corresponding value in Cl- · · ·BrCH3. The correlationbetween the electron-density transfer and the bond-length changeis very obvious. It is the electron-density transfer from the lone-pair orbitals of Cl (Br) to the C-H σ* antibonding orbitals thatdetermines the extent of the C-H, C-Cl, or C-Br bondcontraction upon complex formation. Again, the important roleof the three lone electron pairs on the halogen atom for thedifference of the properties between the blue-shifting halogenbond and the blue-shifting hydrogen bond is confirmed.

For the diatomic halogen molecules such as Cl2 and Br2, theyare always involved in the formation of the red-shifting halogenbond but not the blue-shifting halogen bond. For example, whenCl2 interacts with C6H6, C2H4, H2O, NH3, and Cl-, respectively,to form the halogen-bonded complexes, the Cl-Cl bond iselongated by 0.0198, 0.0335, 0.0170, 0.0578, and 0.3459 Å,respectively. Upon the formation of theses halogen-bondedcomplexes, the lone electron pairs on Cl find nowhere to go, sothe intermolecular charge-transfer interaction (LP(Y) fσ*(Cl-Cl)) plays a dominant role and leads to the elongationof the Cl-Cl bond. Only in this case are the properties of thehalogen bond similar to those of the hydrogen bond.

Conclusions

In this work, we employed the recently developed densityfunctional M06 and NBO theories to study the origin of thedifference of the properties between the blue-shifting halogenbond and the blue-shifitng hydrogen bond. In the negativeelectric field region of the electron donor Y, the electron density

TABLE 2: C-Cl1 Bond-Length Changes (∆r, angstroms) upon Complex Formation, Distances between N1 and Cl1 (angstroms),Number of Imaginary Frequencies (Nimg), NBO Charges on Cl1, and the Second-Order Perturbation Stabilization Energies(∆E2, kilocalories per mole) for Some Significant Donor-Acceptor Orbital Interactions of the Four Halogen-Bonded Complexes

property CCl4 CCl4 · · ·NH3 CCl4 · · · (NH3)2 CCl4 · · · (NH3)3 CCl4 · · · (NH3)4

M06/6-311++G(d,p)symmetry Td C3V C2V C3V Td

Nimg 0 0 2 3 0∆r(C-Cl1) -0.0068 -0.0027 0.0028 0.0071d(N1 · · ·Cl1) 2.8959 2.9371 2.9614 2.9868∆E2[LP(Cl1) f σ*(C-Cl2)] 6.71 7.47 7.29 6.68 6.51∆E2[LP(Cl1) f σ*(C-Cl3)] 7.18 7.47 7.29 6.68 6.57∆E2[LP(Cl1) f σ*(C-Cl4)] 9.26 9.96 8.84 9.80 8.72q(Cl1) 0.0750 0.1081 0.0916 0.0757 0.0608∆E2[LP(N1) f σ*(C-Cl1)] 3.89 3.38 3.11 2.85MP2/cc-pVTZ∆r(C-Cl1) -0.0071 -0.0032 0.0007 0.0047d(N1 · · ·Cl1) 2.9761 3.0149 3.0525 3.0916

Figure 2. Geometries of the complexes Cl- · · ·CH4, Cl- · · ·ClCH3, andCl- · · ·BrCH3. ∆r (angstroms) is the bond-length change upon complexformation. d (angstroms) is the distance between two atoms.

TABLE 3: Second-Order Perturbation StabilizationEnergies (∆E2, kilocalories per mole) for Some SignificantDonor-Acceptor Orbital Interactions at the M06/6-311++G(d,p) Theory Level

property ClCH3

Cl- · · ·ClCH3 BrCH3

Cl- · · ·BrCH3

∆E2[LP(2)Cl1(Br1) f σ*(C-H2)] 3.69 3.98 2.71 2.89∆E2[LP(2)Cl1(Br1) f σ*(C-H3)] 3.69 3.94 2.71 2.88∆E2[LP(3)Cl1(Br1) f σ*(C-H2)] 4.92 5.29 3.61 3.85∆E2[LP(3)Cl1(Br1) f σ*(C-H3)] 1.23 1.30 0.90 0.96∆E2[LP(3)Cl1(Br1) f σ*(C-H4)] 1.23 1.34 0.90 0.96

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in the lone-pair region of the halogen atom is transferred to theremote part of the halogen atom donor. The decrease in theelectron density in the lone-pair region of the halogen atom leadsto Hal moving to X and to the contraction of X-Hal bond.The H atom does not have lone electron pairs; therefore, thereis no such electron-density transfer upon the blue-shiftinghydrogen bond formation. This explains why the blue-shiftinghalogen bond is much more ubiquitous in the halogen-bondedcomplexes than the blue-shifting hydrogen bond in the hydrogen-boned complexes and why the values of the X-Hal bondcontraction are generally much more larger than the values ofthe X-H bond contraction. The difference of the propertiesbetween the X-Cl · · ·Y blue-shifting halogen bond and theX-Br · · ·Y blue-shifting halogen bond can also be rationalizedby the magnitude of the electron-density transfer in the lone-pair regions of Cl and Br. We hope that these findings will shedlight on the origin of the blue-shifting halogen bond and theblue-shifting hydrogen bond.

Acknowledgment. This project was supported by the NaturalScience Foundation of Luoyang Normal University. It was alsosupported by the Scientific Research Foundation for theReturned Overseas Chinese Scholars, Ministry of Education ofChina, and the Natural Science Foundation of Henan Educa-tional Committee.

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