Cooperative and Diminutive Unusual Weak Bonding In F3CXÂ·Â·Â·HMgH

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Cooperative and Diminutive Unusual Weak Bonding In F3CX · · · HMgH · · · Y and F3CX · · · Y · · · HMgH Trimers (X ) Cl, Br; Y ) HCN, and HNC) Mohammad Solimannejad* and Masumeh Malekani Quantum Chemistry Group, Department of Chemistry, Faculty of Sciences, Arak UniVersity, Arak 38156-8-8349, Iran

Ibon Alkorta Instituto de Quı´mica Me´dica (CSIC), Juan de la CierVa, 3; 28006-Madrid, Spain ReceiVed: August 11, 2010; ReVised Manuscript ReceiVed: October 1, 2010

MP2 calculations with cc-pVTZ basis set were used to analyze intermolecular interactions in F3CX · · · HMgH · · · Y and F3CX · · · Y · · · HMgH triads (X ) Cl, Br; Y ) HCN, and HNC) which are connecting with three kinds of unusual weak interactions, namely halogen-hydride, dihydrogen, and σ-hole. To understand the properties of the systems better, the corresponding dyads are also studied. Molecular geometries, binding energies, and infrared spectra of monomers, dyads, and triads were investigated at the MP2/cc-pVTZ computational level. Particular attention is given to parameters such as cooperative energies, cooperative dipole moments, and many-body interaction energies. Those complexes with simultaneous presence of a σ-hole bond and a dihydrogen bond show cooperativity energy ranging between -1.02 and -2.31 kJ mol-1, whereas those with a halogen-hydride bond and a dihydrogen bond are diminutive, with this energetic effect between 0.1 and 0.63 kJ mol-1. The electronic properties of the complexes have been analyzed using the molecular electrostatic potential (MEP), the electron density shift maps, and the parameters derived from the atoms in molecules (AIM) methodology. 1. Introduction Noncovalent interactions between molecules play a very important role in supramolecular chemistry, molecular biology, and materials science.1 Although research has traditionally focused on the more common hydrogen-bonded (HB) interactions, more recently, interest has grown in other types of intermolecular interactions, namely, halogen bonding, σ-hole bonding, dihydrogen bonding, and hydride bonding, among others. Halogen bonding describes a directional interaction between covalently bound halogen atoms (X) and Lewis bases (A). Several excellent reviews on halogen bonding are now available2,3 together with a recent book on halogen bonding.4 Recently, metal hydride-halogen interactions have been reported in literature.5-8 When a half-filled p orbital participates in forming a covalent bond, its electron normally tends to be somewhat localized in the internuclear region, thereby diminishing the electronic density in the outer (noninvolved) lobe of that orbital. This electron-deficient outer lobe of a half-filled p orbital involved in a covalent bond is called a “σ-hole”.9 Positive σ-holes have now been found computationally on the outer surfaces of Group V, VI, and VII atoms in numerous molecules.10-13 Halogen bonding is a subset of σ-hole bonding. It is increasingly recognized that σ-hole bonding, especially involving Group VII, occurs widely in biological systems.10,11 There is also considerable interest and activity in applying it in crystal engineering.14,15 In the last years, an important number of new chemical groups have been added to those suitable to form hydrogen bonding.16,17 * To whom correspondence [email protected].

should

be

addressed.

E-mail:

One of the most interesting cases corresponds to those HB where the interaction is between two hydrogens, one hydric with partial negative charge and another protic with positive charge.18-22 This special case of HB has been named a dihydrogen bond (DHB).23 As an extension of the DHB, the possibility that hydrogen atoms with electron excess can interact with electron deficient atoms has been proposed theoretically. This interaction was originally named an inverse hydrogen bond24 and more recently as a hydride bond.25 Recently, an article concerning cooperativity in multiple unusual weak bonds with hydrogen bonds, hydric bonds, DHBs, halogen bonds, and ion-π interactions has been published.26 The importance of cooperativity in the structure of aggregates including those found biological systems is still not well understood. In addition, it has been shown that the cooperative and anticooperative, or diminutive, effects are highly dependent on the geometrical disposition of the interacting molecules.27,28 Thus, careful studies in simple models are of interested in order to extend their conclusion to larger ones. Herein, we designed some simple structures including halogen-hydride, dihydrogen, and σ-hole bonding. Thus, we have selected two trifluoromethylhaloderivatives (F3CX, X ) Cl and Br) due to their implication in atmosphere chemistry and green house effect:29 MgH2, since it is a suitable hydride for experimental studies and has been proposed as a potential hydrogen storage material,30 and the HCN/HNC isomers that are prototypes of linear hydrogen bond donor/acceptors. We performed a theoretical study on the eight F3CX · · · HMgH · · · Y and F3CX · · · Y · · · HMgH (X ) Cl, Br; Y ) HCN, and HNC) triads with the aim of investigating the effect of halogen-hydride and σ-hole bonding on a DHB and the cooperativity between them. To our

10.1021/jp1075687  2010 American Chemical Society Published on Web 10/26/2010

Bonding In F3CX · · · HMgH · · · Y and F3CX · · · Y · · · HMgH Trimers SCHEME 1: Disposition of the Monomers within the Complexes

best knowledge, study of cooperativity in triads with halogenhydride, σ-hole, and dihydrogen bonding is reported here for the first time. 2. Computational Details Structures of the monomers and the complexes were optimized and characterized by frequency computations at the MP2/ cc-pVTZ computational level. In a very recent paper, Riley et al. pointed out that this method provides very good estimates of geometries and energies for noncovalent complexes.31 Calculations were performed using the Gaussian 03 system of codes.32 The interaction energy has been calculated as the difference of the total energy of the complexes and the sum of the isolated monomers in their minima configuration. The full counterpoise (CP) method33 was used to correct the interaction energy from the inherent basis set superposition error (BSSE). The atoms in molecules (AIM) methodology34 has been used to analyze the electron density of the systems considered at the MP2/cc-pVTZ computational level. The topological analysis has been carried out with the AIMAll program.35 The atomic charges have been obtained by integration of the electron density in the atomic basins. As a measure of the quality of the integration, the value of the integrated Laplacian has been used. Values of this parameter smaller than 1 × 10-3 for all the atoms of a system have shown to provide small average error in the total charge of the system.36

J. Phys. Chem. A, Vol. 114, No. 45, 2010 12107 The molecular electrostatic potential (MEP) and total electron densities have been evaluated with the Gaussian 03 facilities. Electron density difference maps are obtained as the difference of the electron density of the complex and the electron density of the monomers with their geometry within the complex. 3. Results and Discussion 3.1. Geometries. The systems studied form stable triads with C3V symmetry (Scheme 1). The bond angle between halogen, hydrogen, and nitrogen (carbon) atoms that involved in interactions are 180°. The intermolecular distances are 3.08-3.24 Å for X · · · C(N) σ-hole bonds and 2.80-2.94 Å for halogenhydride bonds. The intermolecular distances for dihydrogen contacts range between 1.66-1.94 Å (Table 1). For the systems connecting with σ-hole and DHBs, the X · · · C(N) and H · · · H distances in triads are smaller than the corresponding values in dyads. In the triads that form with halogen-hydride and DHBs, X · · · H and H · · · H distances are larger with respect to values in dyads. This trend can be interpreted as a cooperative effect of DHB with σ-hole bonds and diminutive of DHB with halogen-hydride bonds. In the studied triads, the decrease in X · · · C(N) and H · · · H distances is larger than the increases in X · · · H and H · · · H distances upon triad formations. This shows that the effect of cooperativity of σ-hole bonds on DHBs is more pronounced than that of diminutive of halogen-hydride bonds on DHBs. 3.2. Energies. The interaction energy in the dyads can be regarded as the energy difference between the complex and the monomers: Ei(A-B) ) EAB - (EA + EB) and the corresponding value in the triads (Ei(A-B-C)) is calculated in the similar way. Ei(AB, T) and Ei(BC, T) are the interaction energies of AB and BC dyads while they are in the geometry of triads. Table 2 gives the interaction of eight studied triads and respective dyads. All results were corrected for BSSE by using the full counterpoise method. As shown in Table 2, the binding energy of halogen-hydride and σ-hole bonds ranges from 4.65 to 7.38 kJ mol-1, and 6.46 to 9.40 kJ mol-1, respectively, and that of DHBs is in the range of 15.06-25.50 kJ mol-1, that is, the interaction energies of DHBs are generally much larger than those of σ-hole and halogen-hydride bonds.

TABLE 1: Intermolecular Distances (R) in the Investigated Triads (T) and Dyads (D)a

a

triads (A · · · B · · · C)

R(AB, T)

R(AB)

∆RAB

R(BC, T)

R(BC)

∆RBC

F3CBr · · · CNH · · · HMgH F3CBr · · · NCH · · · HMgH F3CBr · · · HMgH · · · HNC F3CBr · · · HMgH · · · HCN F3CCl · · · CNH · · · HMgH F3CCl · · · NCH · · · HMgH F3CCl · · · HMgH · · · HNC F3CCl · · · HMgH · · · HCN

3.166 3.083 2.816 2.805 3.241 3.110 2.941 2.935

3.220 3.119 2.771 2.771 3.286 3.140 2.893 2.893

-0.054 -0.036 0.045 0.034 -0.045 -0.030 0.048 0.042

1.632 1.912 1.671 1.952 1.643 1.922 1.666 1.948

1.661 1.947 1.661 1.947 1.661 1.947 1.661 1.947

-0.029 -0.034 0.010 0.006 -0.018 -0.025 0.005 0.001

∆R are changes relative to the respective dyads.

TABLE 2: Interaction Energies (Ei, kJ mol-1) of Halogen-Hydride, Dihydrogen, and σ-Hole Bonding in the Investigated Dyads and Triads (T) at the MP2/cc-pVTZ Level triads (A · · · B · · · C)

Ei(A-B-C)

Ei(A-B)

Ei(B-C)

Ei(AB, T)

Ei(BC, T)

ECOOP


-37.19 -25.62 -31.34 -21.36 -33.47 -22.48 -29.21 -19.06

-9.40 -9.08 -7.38 -7.38 -6.46 -6.57 -4.65 -4.65

-25.50 -15.06 -25.50 -15.06 -25.50 -15.06 -25.50 -15.06

-8.44 -8.81 -7.38 -7.41 -5.69 -6.38 -4.62 -4.65

-25.39 -14.98 -25.51 -15.05 -25.45 -15.01 -25.51 -15.05

-2.28 -1.48 1.54 1.08 -1.50 -1.01 0.93 0.64

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TABLE 3: Decomposition of Interaction Energy (kJ mol-1) of the Investigated Triads triads (A · · · B · · · C) F3CBr · · · CNH · · · HMgH F3CBr · · · NCH · · · HMgH F3CBr · · · HMgH · · · HNC F3CBr · · · HMgH · · · HCN F3CCl · · · CNH · · · HMgH F3CCl · · · NCH · · · HMgH F3CCl · · · HMgH · · · HNC F3CCl · · · HMgH · · · HCN

ER

∆EA-B ∆EB-C ∆EA-C ∆EA-B-C -9.61 -9.22 -7.49 -7.51 -6.67 -6.73 -4.71 -4.72

-26.26 -15.16 -26.07 -15.16 -26.22 -15.17 -26.11 -15.17

-0.41 -0.28 0.37 0.27 -0.28 -0.25 0.25 0.18

-2.14 -1.40 1.23 0.85 -1.34 -0.71 0.71 0.49

1.23 0.44 0.63 0.19 1.05 0.38 0.64 0.17

The calculated interaction energies of the trimers and the three constituent dimers are gathered in Table 2. An energetic cooperativity parameter has been calculated using eq 137

Ecoop ) Ei(A-B-C) - Ei(A-B) - Ei(B-C)

(1)

where Ei(A-B-C) is the interaction energy of the trimer, and Ei(A-B) and Ei(B-C) are the interaction energy of the isolated dimers within their corresponding minima configuration. In all cases studied, a favorable cooperativity is observed for triads that show simultaneously a σ-hole bond and a DHB. In the same way, diminutive is observed for those cases where a halogen-hydride and DHB is present. This cooperative energy is much smaller that in complexes with hydrogen bonding or simultaneous hydrogen bond and a halogen bond.28 3.3. Many-Body Interaction Analysis. The two- and threebody contributions to total binding energy are obtained by manybody analysis.38,39 The two-body terms (∆EA-B, ∆EA-C, and ∆EB-C) can be calculated as the binding energy of each molecular pair in the geometry of triad minus the energy sum of the monomers, all of which are frozen in the geometry of the triad. The three-body term ∆EA-B-C is calculated as the total binding energy of the triad minus the interaction energy of each pair of monomers with all of them frozen in the geometry of the triad using eq 2.40

∆EA-B-C ) Ei(ABC) - ∆EA-B - ∆EA-C - ∆EB-C

(2) The total relaxation energy (ER) is defined as the energy sum of the monomers with all of then frozen in the geometry of the triads minus the energy sum of the optimized monomers. Thus, the total binding energy of the triad is obtained using eq 3.40

Ei(A-B-C) ) ∆EA-B + ∆EA-C + ∆EB-C + ∆EA-B-C + ER

(3)

The results are presented in Table 3, in which all energies are corrected for BSSE. F3CBr · · · CNH · · · HMgH (ECOOP ) -2.28) and F3CCl · · · HMgH · · · HCN (ECOOP ) 0.64) are the most and the least stable triads among studied complexes, respectively. As seen in Table 3, two-body interaction energy in some cases provides the largest contribution of the total interaction energy, up to 80%. For all triads, the two-body interaction energies ∆EA-B and ∆EB-C are attractive, that is, they make a positive contribution to the total interaction energy. ∆EA-C is attractive for triads with cooperativity and repulsive for triads that are diminutive. For all triads, ∆EA-C is the smallest two-body-interaction term, which is consistent with the largest distance between them.

Figure 1. ER (kJ/mol) vs Ei(A-B-C).

TABLE 4: Frequency Shifts ∆ν˜ (cm-1) and Intensity Ratios γ of H-C (H-N) Stretching Vibration in the Studied Triads and the Corresponding Dyads Relative to Those in the Isolated HCN and HNC Moleculesa,b triads

∆ν˜ triad ∆ν˜ dyad ∆∆ν˜ Ab γtriad γdyad ∆γ


-350 -134 -289 -106 -333 -127 -296 -108

-304 -111 -304 -111 -304 -111 -304 -111

-46 -23 16 5 -29 -16 8 3

1.15 1.20 0.95 0.95 1.09 1.14 0.97 0.97

7.02 6.82 5.67 5.40 6.47 6.34 5.62 5.36

5.44 5.19 5.44 5.19 5.44 5.19 5.44 5.19

1.58 1.64 0.23 0.21 1.03 1.15 0.18 0.17

∆∆ν˜ ) ∆ν˜ triad - ∆ν˜ dyad, Ab )∆ν˜ triad/ ∆ν˜ dyad, and ∆γ ) γtriad γdyad. b ν˜ of H-C and N-H in isolated HCN and HNC are 3476 and 3837 cm-1, respectively. a

The three-body interaction energy ∆EA-B-C is attractive in triads with cooperativity and repulsive for triads that are diminutive. This situation is similar to two-body interaction energies ∆EA-C. The contribution of the three-body interaction energy is much smaller than that of two-body interaction energy. The relaxation energy can be taken as a measure of the degree of strain that drives the distortion of the ternary system. As seen in Table 3, the relaxation energy is positive; that is, it makes destabilization contribution to the total interaction energy of the triads. The relaxation energy is largest for F3CBr · · · CNH · · · HMgH and is smallest in F3CCl · · · HMgH · · · HCN, which is in line with order of stabilities in these triads. Thus, an acceptable linear correlation can be found between the total interaction energy and the relaxation energy (Figure 1). 3.4. Vibrational Analysis. Table 4 lists the frequency shift and intensity ratio of the HC(HN) stretching vibration in the triads and dyads relative to those in the isolated HCN(HNC) molecules. As expected, the formation of complexes results in a red-shift and intensity enhancement of the HC(HN) stretching vibration in the infrared spectra. Whether in the triads or in the dyads, the changing sequence of the HCN(HNC) stretching frequency is consistent with that of its intensity enhancement. We adopt a cooperativity factor41 to evaluate the cooperativity between unusual weak interactions in the present study. The cooperativity factor Ab is calculated42 using eq 4.

Ab ) ∆ν˜ triad /∆ν˜ dyad

(4)

The results for the HC(HN) stretching are listed in Table 4. The cooperativity factor ranges from 0.95 to 1.20. Those complexes with Ab values larger than 1 are the ones with positive

Bonding In F3CX · · · HMgH · · · Y and F3CX · · · Y · · · HMgH Trimers

J. Phys. Chem. A, Vol. 114, No. 45, 2010 12109 TABLE 6: Changes in AIM Parameters of Triads Relative to Respective Dyads

Figure 2. ECOOP (kJ/mol) vs Ab parameter. NCH are CNH clusters are represented with black and white square, respectively.

TABLE 5: Coopertivity of Dipole Moments (Debye) in the Investigated Triads triads (A · · · B · · · C)

∆(ABC)

∆(AB)

∆(BC)

coop-dipole


2.27 1.89 0.52 0.16 1.91 1.59 0.78 0.43

0.81 0.85 0.78 0.78 0.56 0.61 0.49 0.49

1.17 0.83 1.17 0.83 1.17 0.83 1.17 0.83

0.30 0.22 0.14 0.12 0.19 0.15 0.11 0.09

energy cooperativity, whereas the ones with values smaller than 1 are the complexes with diminutive energies. In fact, linear correlations between the ECOOP and the Ab parameters can be obtained for each of the NCH and CNH families (Figure 2). The variation of the intensity of the H-X in the HB donor, has been shown to be related to the interaction energy of the complex.43 In the cases studied, the intensity ratio with respect to the isolated monomers of the H-C (H-N) stretching vibration, γtriad and γdyad, show large values for all the triads and dyads considered. The difference between the γtriad and γdyad, ∆γ, is greater than 1 for the complexes with cooperativity and is less that 1 for those with diminutive effect. 3.5. Dipole Moments. Among the electronic properties considered, we have study the effect of the complex formation on the dipole moment value of title triads (Table 5). A cooperativity parameter has been defined for the dipole moment enhancement due to the complex formation, eq 5:

triads (A · · · B · · · C)

∆FAB

∆∇2AB

∆FBC

∆∇2BC


0.0013 0.0008 -0.0010 -0.0008 0.0008 0.0005 -0.0007 -0.0006

0.0039 0.0032 -0.0024 -0.0018 0.0030 0.0026 -0.0021 -0.0019

0.0015 0.0010 -0.0006 -0.0002 0.0009 0.0007 -0.0003 -0.0001

0.0009 0.0017 -0.0002 -0.0003 0.0006 0.0012 -0.0001 -0.0001

energetic cooperativity than for the ones than present negative energetic cooperativity. 3.5. Electron Density Analysis. The values of the electron density in the intermolecular bcp’s show a clear dependency with the interatomic distance. Thus, exponential relationships can be obtained for the DHB interactions in the dimer and triads obtained here, in agreement with previous reports on HB complexes.44,45 Table 6 lists the variation in electron density and Laplacian of electron density at two bond critical points located between molecules A, B, and C. The enhancement in electron density and Laplacian of electron density is observed for triads with cooperativity, but in the triads with diminutive bonding, a reduction of electron density and Laplacian of electron density upon triads formation is observed. Good linear correlations are obtained between the variation of the electron density in the intermolecular bcp versus the energetic cooperativity (Figure 3). The calculated charges of the molecules within the triads are gathered in Table 7. In all the cases, the F3CX, NCH, and CNH molecules gain electron density while the HMgH one lost it.

coop-dipole ) ∆dipole(ABC) - ∆dipole(AB) ∆dipole(BC) (5) where the corresponding ∆dipole is calculated as the difference between the dipole moment in the cluster and the vectorial sum of the isolated monomers in their geometry of minimum energy configuration. The module of the variations of the dipole moments is gathered in Table 5. The triads with large values of ∆dipole(ABC) are those with energetic cooperativity, whereas those with values smaller than 1 D are the ones that present diminutive energetics. It should be noted than the large values of the ∆dipole(ABC) are associated with the dipole moment of the F3CX and HCN/HNC molecules pointing in the same direction within the triad, whereas in the small values of ∆dipole(ABC) the direction of dipole moment of one molecule is the opposite to the other one. The dipole cooperativity is in general small, between 0.30 and 0.09 D, being slightly larger for those system that show

Figure 3. ∆F (au) vs ECOOP (kJ/mol). Black and white triangles represent the AB and BC intermolecular bcp’s, respectively.

TABLE 7: Total Charge of the Molecules (e) within the Triads Calculated with the AIM Methodology triads (A · · · B · · · C)

Q(A)

Q(B)

Q(C)


-0.014 -0.005 -0.017 -0.017 -0.010 -0.004 -0.011 -0.011

-0.027 -0.019 0.053 0.039 -0.030 -0.020 0.048 0.033

0.041 0.024 -0.036 -0.022 0.040 0.024 -0.037 -0.022

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Figure 4. Electron difference maps of two of the triads considered in the present article. Light blue and green regions represent loss and gain of electron density ((0.0004 e/bohr3), respectively.

TABLE 8: Molecular Electrostatic Minima (au) Associated to the Extreme Moieties of the MgH2, HCN, and HNC Molecules and the Corresponding Dimers system

H extreme

HMgH HNC HCN HMgH · · · HNC HMgH · · · HCN

-0.044 -0.028 -0.033

Figure 5. Molecular electrostatic isosurfaces ((0.025 au) of the isolated CNH and MgH2 molecules and its complex. Negative and positive regions are shown in red and yellow colors, respectively. The values of the minima are indicated.

C/N extreme -0.075 -0.068 -0.091 -0.079

The HMgH molecule became more positively charge when it is surrounded by two molecules than when it is at the end of the chain with the same two molecules. The electron density difference maps (Figure 4) of the triads show that while in those cases where the MgH2 is in one extreme, the lost/gain regions are alternated, with a region of electron density lost in the side of the MgH2 molecule and a region of gain around the fluorine atoms of the CF3X ones. Important electron displacements are observed from the MgH2 toward the region where the DHB interaction takes place and in the CNH/HCN molecule toward the σ-hole one. In the case of the triads with MgH2 in the middle, the variations in the CF3X and HCN/HNC molecules is smaller than in the previous case discussed, whereas in MgH2 a significant transfer of electron toward the outer regions of this molecule is observed. 3.6. Molecular Electrostatic Potentials. The a priori analysis of the molecular electrostatic potential (MEP) of isolated molecules has been recognize for a long time as a useful tool for proposing the formation of weak complexes based on its sign and magnitude.46 In fact, linear correlations have been between the MEP and binding energy in hydrogen bonded complexes.47,48 More recently, it has been used to explain the cooperativity effect in hydrogen bonded clusters.49 The triads studied here can be considered as the interaction of F3CX with electron donors. The value of the interacting negative MEP region of electron donor can provide some clues of the strength of the interaction. The values of the minima region of the interacting CNH(NCH) · · · HMgH dimers and the corresponding isolated monomers are gathered in Table 8 and Figure 5. The MEP minima associated to the hydrogen atoms of the MgH2 molecule are smaller than those of the CNH and NCH molecules within the dimers. Thus, it is expected that the interaction of the CNH/NCH · · · HMgH with F3CX will be more favorable when the X atom interacts with the C/N extreme than when the interaction will be the X · · · H one. In addition, if the minima values of the MEP in the isolated molecules are compared to those in the CNH/NCH · · · HMgH dimers, it can be observed that the absolute values increase for the C/N extreme and decrease for the H one. Thus, the

interaction of the isolated HMgH molecule with the F3CX ones will be stronger than when the HMgH molecule is involved in the mentioned dimers. The opposite should happened with the interaction of the CNH/NCH molecules with F3CX, where the larger binding energy should be observed in the trimers. All of these results are in agreement with the diminutive/cooperativity observed in the trimers. 4. Conclusions Ab intio calculations at MP2/cc-pVTZ level were used to explore the cooperativity in F3CX · · · HMgH · · · HCN(HNC) and F3CX · · · NCH(CNH) · · · HMgH (X ) Cl and Br) triads. The equilibrium structures, vibrational spectra, energetics, and cooperative effect on the properties of the complexes were analyzed. Those triads where the MgH2 molecule is located at the end of the chain show energetic cooperativity, whereas when this molecule is located in the middle, the obtained cluster is diminutive. Similar conclusions are obtained from the analysis of the intermolecular distances, dipole moments, and frequency shift. These results can be explained based on the analysis a priori of the MEP of the interacting subsystems. The findings are helpful for understanding the cooperative and competitive role of halogen-hydride, dihydrogen, and σ-hole bonding in molecular recognition, crystal engineering, and biological systems. Supporting Information Available: Cartesian coordinated of optimized structures at MP2/cc-pVTZ computational level. This material is available free of charge via the Internet at http:// pubs.acs.org. References and Notes (1) Muller-Dethlefs, K.; Hobza, P. Chem. ReV. 2000, 100, 143. (2) Metrangolo, P.; Neukirch, H.; Pilati, T.; Resnati, G. Acc. Chem. Res. 2005, 38, 386. (3) Fourmigue´, M. Curr. Opin. Solid State Mater. Sci. 2009, 13, 36. (4) Metrangolo, P.; Resnati, G., Eds. Halogen Bonding: Fundamentals and Applications of Structural Bonds; Berlin: Springer; 2007; Vol. 126. (5) Lipkowski, P.; Grabowski, S. J.; Leszczynski, J. J. Phys. Chem. A 2006, 110, 10296. (6) Li, Q.; Yuan, H.; Jing, B.; Liu, Z.; Li, W.; Cheng, J.; Gong, B.; Sun, J. J. Mol. Struct (THEOCHEM) 2010, 942, 145. (7) Li, Q.-Z.; Dong, X.; Jing, B.; Li, W.-Z.; Cheng, J.-B.; Gong, B.A.; Yu, Z.-W. J. Comput. Chem. 2010, 31, 1662. (8) Solimannejad, M.; Malekani, M.; Alkorta, I. J. Mol. Struct (THEOCHEM) 2010, 955, 140.

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Cooperative and Diminutive Unusual Weak Bonding In F3CXÂ·Â·Â·HMgH

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