Molecular Tuning of the Closed Shell CâHÂ·Â·Â·FâC Hydrogen Bond - The

Article pubs.acs.org/JPCA

Molecular Tuning of the Closed Shell C−H···F−C Hydrogen Bond Norman Lu,*,†,‡ Rebecca M. Ley,† Charles E. Cotton,† Wei-Cheng Chung,‡ Joseph S. Francisco,*,†,§ and Ei-ichi Negishi*,† †

Department of Chemistry and §Department of Earth and Atmospheric Science, Purdue University, 1393 Brown Building, West Lafayette, Indiana 47907-1393, United States ‡ Institute of Organic and Polymeric Materials, National Taipei University of Technology, Taipei 106, Taiwan S Supporting Information *

ABSTRACT: The existence of the rare six-membered and intramolecular C−H···F−C hydrogen-bond has been experimentally proven in the gas phase and in the solid state recently. However, the effect of the substituents on this C−H···F−C hydrogen-bond system has never been reported. In view of the importance of this type of C−H···F−C Hbonding whose weak interaction has been found critical in nanotechnology and biological systems, the nine functional groups composed of electron donating and electron withdrawing groups are inserted into this C−H···F−C interaction to study the group effect on the hydrogen bonding. Group effects on this C−H···F−C H-bonding system have been found, and their effects on the H-bonding system have been found to be tunable.

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Dixon and Smart13,14 pioneered the study of C−H···F−C interactions, and then Dixon and co-workers extended the study to the C−H···OC hydrogen bonding interaction. Furthermore, Scheiner15−17 also explored the C−H···F−C interactions,18 but the C−H···F hydrogen bond was still not totally understood. In view of the importance of this weak Hbonding interaction in nanotechnology and biological systems,19−25 the model shown in Scheme 1 is used to study the C−H···F−C intramolecular interactions and examine group effects on the hydrogen bonding in this system. As mentioned in the introduction, the study of the C−H···F−C system has begun to attract considerable attention, but the studies have been mainly focused on the interaction of C−H···F−C system and its red-shifted or blue-shifted effect. In Scheme 1, the C−

INTRODUCTION

During the past decade, scientific interest has been devoted to the weak hydrogen bond, a noncovalent interaction which manifests itself in many ways in structural, energetic, biological, nanoscaled and engineering chemistry.1,2 Several weak proton donors and proton acceptors, whose interaction energies are within a few kcal mol−1, have been studied, among them the N−H---F,3 C−H---O,1,4 C−H---N,1 and C−H---π1,5 interactions appear to be most important in solvation processes, in nanotechnology, and in supramolecular chemistry. However, C−H···F−C hydrogen bonding6 is still rare and has been difficult to observe without the assistance of other weak forces,7 for example, crystal packing or N-base or O-base hydrogenbonding. Caminati et al.8 observed the C−H···F improper hydrogen-bonding between CF2H2 dimers in the gas phase. However, it is noted that the few9 published experimental observations so far are all for intermolecular C−H···F−C hydrogen bonds, for example, between the C−F bond of CHF3 and the hydrogen atom of oxirane.10 To date, only one account of intramolecular H-bonding of C−H···O type has been reported by Matsuura et al.11 The first fully characterized complex with an intramolecular, C−H···F−C hydrogen bond has been experimentally reported recently by us for the solid state.12 This system in the solid state has been observed to have a contracted C−H bond and a short H···F distance of 2.34(7) Å. In addition, the growing interest on this type of C−H···F−C interactions over the past several years prompted us to theoretically study the effects of substituents on the intramolecular hydrogen bonding interaction in the C−H···F−C system. © XXXX American Chemical Society

Scheme 1. Schematic Drawing of Our Study Model of the C−H···F−C System in the Cisoidal Conformation of CH(X)7F−C2F4−C(Y)1F2Fa

a

Note: The transoidal/cisoidal12 conformation of the model molecule, CH(X)7F−C2F4−C(Y)1F2F, discussed in this paper is also called anti/ syn conformation, respectively.

Received: November 5, 2012 Revised: August 1, 2013

A

dx.doi.org/10.1021/jp404791g | J. Phys. Chem. A XXXX, XXX, XXX−XXX

The Journal of Physical Chemistry A

Article

using the B3LYP and MP2 methods were carried out using the Pople 6-31+G(d,p) basis set. These calculations also affirm the results.

H···F−C system has been connected by the CF2CF2 group as a linker. To our knowledge, the group effect of the substituents on the C−H···F−C interaction has never been systematically studied, and that is the reason why in this study the X and Y substituents are introduced to each of two C atoms forming this C−H···F−C system, respectively. The molecular framework unit studied by Scheiner’s group is only confined to the existence of C−H···F−C interaction from molecular dimers. That framework limits exploration of how the C−H···F−C system can be tuned, if at all, by substitutent groups. The reason why we use the C−H···F−C system from Scheme 1 as a study model is because the CH(X)7F−C2F4− C(Y)1F2F system has both a novel structure and the unique linker which allow us to study and tune the group effects of the substituents on the C−H···F−C interaction. Additionally, this system has experimental relevance, as these compounds can be prepared or designed to study these group effects experimentally. Moreover, our choice of X is to choose examples where X incorporated electron-withdrawing, electron-releasing, and halogen effects into the series to extend the scope of studies beyond just one group, that is, X = CN, and explore if any observable trends are generalizable for the CH(X)7F− C2F4−C(Y)1F2F system.

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RESULTS During this study, three different functionalities of X groups (X = CN, CH3, and Cl) are studied. These results are presented below. The results from X = CN are presented first and as main contents. The groups X = CH3 from the electron-donating group and X = Cl from the halogen group are studied to examine the generality of the observed trends. 1. Bond Length and Bond Frequency. From a preliminary screening study, when X = CN and Y is taken from a variety of electron-withdrawing or electron-donating groups, the H-bonding interaction of the C−H···F−C systematically changes in structure and binding energy with the substituent groups. As for Y, nine substituents are investigated in this study; and they are C2H5, OCH3, CH3, CHCH2, C CH, Br, Cl, F, and NO2. These substituents can be divided into two main groups: (1) the electron donating group, EDG, which includes the methoxy and alkyl groups (OCH3, C2H5, CH3) and the unsaturated alkyl groups (CHCH2, CCH) and (2) the electron withdrawing group, EWG, which includes three halogens (F, Cl, Br) and NO2, a very strong electron withdrawing group. From the Natural Bond Orbital, NBO, contour diagrams, for all nine substituents in the series, it can be clearly seen that there is overlap between the C−H antibonding and F−C bonding orbitals. Take Y = NO2 (EWG) for example, Figure 1 shows the NBO diagrams of the C−H···

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COMPUTATIONAL METHODS Calculations are performed using the GAUSSIAN 09 suite of programs.26 Full geometry optimizations were performed using Schlegel’s method with tolerances of better than 0.001 Å for bond lengths and 0.01° for angles, and with a self-consistent field convergence of at least 10−9 of the density matrix. The residual root-mean-square (rms) forces were less than 10−4 a.u. Density Functional Theory calculations are performed using the hybrid functional containing the 3 parameter exchange functional of Becke and the Lee−Yang−Parr (LYP) correlation functional (B3LYP). Ab Initio calculations are performed using second order Møller−Plesset perturbation theory in the frozen core approximation. Dunning’s augmented split valence correlation-consistent polarized valence double-ζ and triple-ζ quality basis sets (aug-cc-pVDZ and aug-cc-pVTZ) are used for calculations. The Dunning, aug-cc-pVDZ, basis set is used with the MP2 optimization calculations. Single point calculations using the MP2/aug-cc-pVTZ method with the MP2/aug-ccpVDZ geometry are also performed. We have incorporated the MP2 results in the text as well as in the Supporting Information. All potential energy surface scans are carried out at the B3LYP/aug-cc-pVDZ level of theory to identify possible minima and to establish substituent effects on the energetic trends of each species. The geometries from the B3LYP/aug-ccpVDZ are used as initial starting geometries for the MP2/augcc-pVDZ geometry optimizations. Full geometry optimizations are carried out on identified minima corresponding to the transoidal (non-hydrogen bonded) conformation. To verify the nature of the minima as stationary points, frequency calculations on the optimized geometries are performed at each level of theory. An analysis of the stabilizing Natural Bond Orbital interactions representative of the hydrogen bond interaction are calculated for the appropriate species with the NBO 5.9 program27 using second order perturbation theory analysis with 0.1 kcal mol−1 thresholds for donor−acceptor interaction energies using the B3LYP/aug-cc-pVDZ level of theory. Calculations performed at the MP2/aug-cc-pVDZ level of theory are found to show similar trends found for the B3LYP levels of theory; and affirm the results. Additional calculations

Figure 1. (left) 3-D NBO diagram of interaction when Y = NO2 (C: black, H: white, O: red, N: blue and F: yellow) and X = CN; (right) The 2-D electron density contour map of interaction.

F−C system in both 3-dimensional (3- D) and 2-dimensional (2-D) diagrams. When Y = C2H5 (EDG), the overlapping of the orbitals of C−H antibonding and F−C bonding is even more obvious. As mentioned in the introduction, the characteristics of the interaction of the C−H···F−C system are observations of the C−H bond contraction, which results in an increase in C−H stretching vibration; and of the elongation in C−F bond and of the short H···F distance. The MP2/aug-cc-pVDZ results for these nine substituents are listed in Table 1. From these data, one can easily observe that these C−H···F−C interactions are present for all nine functional groups because the distances between H and F atoms in the C−H···F−C system are all smaller than 2.54 Å28 which is the sum of van der Waals radius of H and F atoms. From the H···F distance, r(H···F), a general trend is found, that is, the H···F distances almost increase from the top to the bottom in the series. We note that the range for B



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Table 1. Bond Length (Å) and Vibration Frequency (cm−1) Data of the C−H···F−C System with Nine Substituents (X = CN) Calculated at the MP2/aug-cc-pVDZ Level of Theorya frequency

number reflects the extent of interaction within this intramolecular interaction of the C−H···F−C system. When the value becomes negative, it means that the interaction from the C−H···F−C system becomes red-shifted. The size of the reported blue shifts of other molecules in the literature is from few wavenumbers16 to about 40 wavenumbers.16,18 The reported numbers in the literature fall in a similar order of magnitude to the calculated numbers shown in the Table 1. In other words, when the C−H···F−C interactions are occurring, the C−H vibrational frequency can be either blueshifted or red-shifted although the blue-shifting C−H···F−C interactions are more common than the red-shifting ones from the literature data. Thus, for the descriptions of the C−H···F− C interactions the observations of the C−H bond contraction and of the C−H stretch increasing are only suf ficient but not necessary requirements. That is to say that the two necessary requirements are elongation in the C−F bond and the short H··· F distance. Thus, from Table 2 (entries 1−7), the C−H bond lengths mainly show the blue-shifting H-bonding phenomenon which means the C−H bond distance is shortened when the intramolecular C−H···F−C interaction is formed between the C−H and F−C bonds, for Y = all five EDGs. When Y = Br, Cl (Halogen), the Δr(C−H) values are relatively small (close to zero; entries 6−7). However, when Y = NO2 (very strong EWG), the C−H bond distance becomes elongated (entry 9) and there is a red-shifted C−H···F−C interaction. The rationale is that because the resonance effect of Br and Cl can help to stabilize the C−H···F−C interaction, this interaction shows the blue-shifting effect. In contrast, the NO2 substituent is a very strong EWG which destabilizes the C−H···F−C system, and this causes the red-shifted effect. Plots of H···F distance versus C−H distance and C−1F distance are shown in Figure 2. There is interestingly an anticorrelation between the C−1F and C−H results from this plot, which can be easily observed. The red squares are almost linearly situated. The H···F distance increases with the electronwithdrawing abilities of the substituents, so when Y = NO2, the H···F distance is larger than those of the rest of series. The points indicated by blue diamonds moderately show the linear trend. This means that the H···F distance decreases with the electron-donating abilities from the substituents, so when Y = C2H5, the H···F distance is the smallest among the series. These two trends of the r(C−1F) and r(C−H) show an anticorrelated effect with the group substituents. This result is not an artifact of the theory, as several calculations with different methods (both MP2 and B3LYP) and different basis

distance

entry

Y=

νtrans(C−H)

νcis(C−H)

Δν(C−H)

r(H···F)

1 2 3 4 5 6 7 8 9

C2H5 OCH3 CHCH2 CH3 CCH Br Cl F NO2

3150 3150 3150 3150 3150 3149 3148 3148 3149

3162 3153 3155 3154 3152 3149 3148 3145 3137

11 3 6 3 2 0 0 −3 −12

2.2893 2.3689 2.3309 2.3621 2.3671 2.3719 2.3763 2.4054 2.5204

a

Note: the Scheme 1 system is a cisoidal isomer, and another conformational isomer is the transoidal isomer which cannot form the intramolecular C−H···F−C H-bonding; Δν(C−H) = νcis(C−H) − νtrans(C−H).

values from 2.2893 Å to 2.5204 Å overlaps the observed H···F distance of 2.34(7) Å from experimental solid state studies.12 When Y= -C2H5 (EDG), the C−H frequency is the largest (3162 cm−1) among this series for the cisoidal isomer. When Y= -NO2 (very strong EWG), its C−H frequency is 3137 cm−1 which is the smallest C−H vibration among the series. The C− H frequencies from three halogens which are EWGs with both the resonance and inductive effect fall between the five EDGs (entries 1−5) and the very strong EWG, NO2 (entry 9; with the lowest frequency). It is then fair to say that when X = CN group, the group effects on the C−H···F−C system are easily recognizable. The five EDGs which include C2H5, OCH3, CH3, CHCH2, and CCH substituents are able to augment the H and F interactions via donating the electron density to the system, so their C−H frequencies show the blue-shifting Hbonding phenomenon which then exhibits that the C−H vibration from the C−H···F−C system moves to the higher frequency when compared with that from the transoidal conformer, which is without the intramolecular interaction between the C−H and F−C bonds. The C−H frequency difference, Δν(CH), between the study model system (cisoidal) and its conformational isomer (transoidal), shown in Table 1 gives rise to interesting results. As expected, in Table 1 the Δν(CH) frequency values from entries 1−7 are all positive, but those from entries 8 and 9 become negative. The value sign change from the positive number to the negative

Table 2. Bond Length Data (Å) of the C−H···F−C Interaction with Nine Substituents (X = CN) Calculated at the MP2/aug-ccpVDZ Level of Theorya bond length

a

bond length

entry

Y=

rtrans(C−H)

rcis(C−H)

Δr(C−H)

rtrans(C−1F)

rcis(C−1F)

Δr(C−1F)

1 2 3 4 5 6 7 8 9


1.0994 1.0995 1.0995 1.0995 1.0995 1.0997 1.0997 1.0998 1.0996

1.0983 1.0992 1.0989 1.0992 1.0993 1.0996 1.0996 1.1000 1.1006

−0.0011 −0.0003 −0.0006 −0.0003 −0.0003 −0.0001 −0.0001 0.0002 0.0010

1.3758 1.3733 1.3767 1.3751 1.3698 1.3538 1.3529 1.3431 1.3415

1.3884 1.3839 1.3866 1.3833 1.3786 1.3637 1.3627 1.3524 1.3465

0.0126 0.0106 0.0099 0.0082 0.0088 0.0099 0.0098 0.0093 0.0049

Note: rtrans = transoidal and rcis = cisoidal conformations; Δr(C−H) = rcis(C−H) − rtrans(C−H); Δr(C−1F) = rcis(C−1F) − rtrans(C−1F). C



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Figure 2. Anticorrelation of C−F and C−H bond length vs H···F distance. (red square: C−H bond distance; blue diamond: C−1F bond distance; X = CN) These data were calculated at the MP2/aug-cc-pVDZ level of theory.

Figure 3. (a) Plot of C−7F bond distance versus C−H bond distance. (b) Plot of C−1F bond length versus C−2F bond length (X = CN). [These data were calculated at the MP2/aug-cc-pVDZ level of theory].

C−H distance almost linearly increases with increasing H···F distance; shown in Supporting Information, Figures C1 and D1 are plots for X = CH3 and X = Cl, respectively. 2. Relationship of H···F Distance versus C−H Distance and C−1F Distance. As stated before, the one necessary feature of the C−H···F−C interaction is the observation of the elongation of C−F bonds which include the C−1F, C−2F, and C−7F bonds. Their bond length plots are shown in Figures 3a and b. The plot of C−7F bond distance versus C−H length is shown in Figure 3a. As expected, the C−7F bond distance almost linearly increases with the decrease in C−H bond length. The H and 7F are located in the same carbon, so their opposite effects are easily observed. Additionally, besides the C−1F bond elongation, the C−2F bond length also increases with the increasing the C−1F bond length for all the nine substituents shown in Figure 3b. This trend shows that both C−1F and C−2F bond lengths increase with the electrondonating abilities of the nine substituents in the series. Thus, when Y = C2H5, both C−1F and C−2F bond lengths are 1.3884

sets (Dunning and Pople basis sets) show a similar anticorrelation effect with the group substituents (see Supporting Information). These two trends imply that the effect of nine substituents on the H-bonding interaction in the C−H···F−C system is to tune this interaction. This is the first example which illustrates that the C−H···F−C hydrogen bonding system is tunable. To broaden the scope of this study, X = CH3 and Cl groups have also been studied, to examine the generality of these trends observed for X = CN. The X = CH3 is from the electrondonating group, and X = Cl is from the halogen group. Results for the X = CN system show that MP2/aug-cc-pVDZ and B3LYP/aug-cc-pVDZ show similar trends, and thus the calculations for X = CH3 and Cl groups are performed only at the B3LYP/aug-cc-pVDZ level of theory. All of anticorrelation trends observed in Figure 2 were also seen in these two cases. (two corresponding figures are shown in Supporting Information, Figures C1 and D1). Thus, the C−F distance considerably decreases with increasing H···F distance, and the D



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and 1.3765 Å which are the very long C−F bonds, and when Y = NO2, both C−1F and C−2F bond lengths are 1.3465 and 1.3431 Å which are considered as short C−F bonds. One can tune the bond lengths of C−H, C−1F, and C−2F by simply changing the Y substituents. The one necessary feature of the C−H···F−C interaction is the observation of the elongation of C−F bonds which include the increasing distances of C−1F, C−2F, and C−7F bonds during the H-bonding interaction. Thus, for X = CH3 or Cl, the plot of C−7F bond distance versus C−H length and the plot of the elongation of C−F bonds are also similar to Figure 3a and are shown in Supporting Information, Figures C2(a) and D2(a) for X = CH3 and X = Cl, respectively. The relationship between C−1F and C−2F bond length in Figure 3b) for X = CN has also been exactly observed for the case of X = CH3 or Cl. As shown in Supporting Information, the bond lengths of C−1F and C−2F follow the same trend very well, that is, the C−1F bond distance linearly increases with C−2F bond distance for X = CH3 and Cl groups as shown in Supporting Information, Figures C2(b) and D2(b), respectively. 3. Group Effects on the C−H···F−C Interaction. To verify the interaction in terms of the bond formation, a second order perturbation analysis is performed in the Natural Bond Orbital space at the MP2/aug-cc-pVDZ level of theory with E2 interaction energy threshold of 0.1 kcal mol−1. The E2 interaction energy from the NBO analysis allows the C−H··· F−C interaction to be isolated from other molecular structural changes as a result of substituent substitution, and can give more direct insight into the stabilizing interaction from group substitution. The stabilizing interactions for each substitution in the 6 membered rings at the fully optimized geometry are investigated. As shown in Table 3, the E2 interaction energy

Figure 4. Plot of the second order E2 interaction energy versus the r(H···F) distance when X = CN. (calculated at the MP2/aug-cc-pVDZ level of theory).

The stronger the overlap, the shorter this distance, and the weaker the overlap, the longer this distance. In fact, the results clearly show that the substituent groups can tune the hydrogen bonding interaction in the C−H···F−C system. Note that in the solid state,12 the hydrogen bond distance is 2.34(7) Å. The results in Figure 4 show that molecular tuning of the C−H···F− C interaction is rather significant, ranging from 2.29 to 2.52 Å (see Table 1). Additionally, for X = CH3 or Cl group, the plot of the second order E2 interaction energy versus the r(H···F) distance also follows well the tendency displayed in Figure 4. These two plots, Figures C3 and D3, are shown in Supporting Information for X = CH3 and Cl, respectively. For the very strong EWG, NO2, its result (upper left point) is relatively different from those of other halogens (also considered as EWG) possibly because of its very strong electron-withdrawing ability. Just as Figure 4 illustrates the correlation between the r(H··· F) distance and the overlap (as indicated by the second order E2 interaction energy), we have further explored how much the bond angles are changing to get the shorter r(H···F) interaction distances. The two key angles are the (CC1F) and (CCH) angles. The two angles CC1F and CCH involved in the C−H··· F−C interactions all have sp3 hybridized carbon centers. We find that the (CCH) is close to 109.5 degree, and there is very little variation in these angles (±0.1 degree from 109.5 degree). However, the (CC1F) angle which has the carbon center with the Y group does show a variance, and it trends with the overlap as shown in Table 4. Note how this trend, in general, follows the trend observed in Figure 4. Interestingly, this finding suggests that the tuning of the r(H···F) interaction is not a resonance cooperative effect across the six-membered ring of the CH(X)7F−C2F4− C(Y)1F2F system as shown in Scheme 1, as one might expect, would influence both the (CCH) and (CC1F) angles. It is a rather local tuning, that influences the (CC1F) angle for better interaction with the hydrogen. To examine this last assertion, we looked at these interactions when the −CH2CH2− group is used as a spacer group instead of the −CF2CF2- group. Data in Figure 5 show that there is tuning of the r(H···F) interaction with the Y group, and the variation of the r(H···F) interaction is with the (CC1F) angle and there is no variation with the (CCH) angle. The trend follows that observed for the

Table 3. Summary of the Group Effects on C−H···F−C Interaction Energy for All Nine Groups entry

substituent

sum of E2 (kcal·mol−1)

1 2 3 4 5 6 7 8 9


0.65 0.60 0.54 0.34 0.35 0.41 0.28 0.28 0.14

from this system is the largest for the strong EDG (C2H5), and this value mainly decreases with increasing electron-withdrawing abilities in the series. These results indicate that the interaction of the C−H···F−C system becomes stronger when Y is the most electron-releasing substituent. The more the electron-releasing ability Y is, the stronger the hydrogen bonding interaction in the C−H···F−C system is. Figure 4 shows a plot of the second order E2 interaction energy, which gives a measure of the overlap of the orbitals involved in the hydrogen bonding between the C−H and C−F bonds with the distance between the hydrogen and fluorine atoms. It can be seen from Figure 4 that the distance between the hydrogen and fluorine atoms correlates linearly with the degree of overlap. E



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Table 4. Two Bond Angles of (CC1F) and (CCH) from the C−H···F−C System with Nine Substituents (X = CN) Calculated at the MP2/aug-cc-pVDZ Level of Theory entry

substituent (Y=)

(CC1F) (deg)

(CCH) (deg)

1 2 3 4 5 6 7 8 9


106.0 107.2 106.8 106.9 107.5 108.4 108.5 109.9 110.5

109.4 109.4 109.4 109.4 109.4 109.5 109.5 109.4 109.5

C2H5 being the strongest EDG in the series gives rise to the shortest H···F distance and the longest C−1F bond. In contrast, the NO2 being the strong EWG gives rise to the longest H···F distance and the shortest C−1F bond. From the present work, results show, for the first time, that there is a group effect on the C−H···F−C interaction. A novel finding is that the C−H··· F−C hydrogen bonding interaction from the CH(X)7F−C2F4− C(Y)1F2F system can be molecularly tuned. In addition, when X is equal to other functional groups, for example, CH3 and Cl, all the same trends are also observed for the tunable group effect on the C−H···F−C interaction.

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ASSOCIATED CONTENT

S Supporting Information *

Data from both B3LYP/aug-cc-pVDZ and B3LYP/aug-ccpVTZ calculations that affirm the trends. 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] (N.L.), [email protected] (J.S.F.), [email protected] (E.N.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Authors gratefully acknowledge the financial support from Purdue University and the National Science Council of Taiwan.29

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Figure 5. Plot of r(H···F) vs (CC1F) in a system of CH(X)7F−C2H4− C(Y)1F2F with −CH2CH2− as a spacer group (calculated at the MP2/ aug-cc-pVDZ level of theory).

REFERENCES

(1) Desiraju, G. R.; Steiner, T. The Weak Hydrogen Bond in Structural Chemistry and Biology; Oxford University Press: New York, 1999. (2) Cotton, F. A.; Daniels, L. M.; Jordan, G. T., IV; Murillo, C. A. The Crystal Packing of bis(2,2′-dipyridylamido)cobalt(ii),Co(dpa)2, is Stabilized by C−H···N Bonds: Are There any Real Precedents? Chem. Commun. 1997, 17, 1673−1674. (3) Zhao, X.; Wang, X.-Z.; Jiang, X.-K.; Chen, Y.-Q.; Li, Z.-T.; Chen, G.-J. Hydrazide-Based Quadruply Hydrogen-Bonded Heterodimers. Structure, Assembling Selectivity, and Supramolecular Substitution. J. Am. Chem. Soc. 2003, 125, 15128−15139. (4) Tatamitani, Y.; Liu, B.; Shimada, J.; Ogata, T.; Ottaviani, P.; Maris, A.; Caminati, W.; Alonso, J. L. Weak, Improper, C-O···H-C Hydrogen Bonds in the Dimethyl Ether Dimer. J. Am. Chem. Soc. 2002, 124, 2739−2743. (5) López, J. C.; Caminati, W.; Alonso, J. L. The C-H···π Hydrogen Bond in the Benzene-Trifluoromethane Adduct: A Rotational Study. Angew. Chem., Int. Ed. 2006, 45, 290−293. (6) Lee, H.; Knobler, C. B.; Hawthorne, M. F. Supramolecular Selfassembly Directed by Carborane C−H···F Interactions. Chem. Commun. 2000, 24, 2485−2486. (7) Takahashi, S.; Jukurogi, T.; Katagiri, T.; Uneyama, K. Supramolecular Self-assembly Directed by Carborane C−H···F Interactions Isomorphic Supramolecular Structures via One-dimensional Hydrogen Bonding Motifs in Crystals of Chiral Difluorolactates, Trichlorolactates and Trifluorolactates. CrystEngComm 2006, 8, 320− 326. (8) Caminati, W.; Melandri, S.; Moreschini, P.; Favero, P. G. The CF···H-C “Anti-Hydrogen Bond” in the Gas Phase: Microwave Structure of the Difluoromethane Dimer. Angew. Chem., Int. Ed. 1999, 38, 2924−2925. (9) Delanoye, S. N.; Herrebout, W. A.; van der Veken, B. J. Blue Shifting Hydrogen Bonding in the Complexes of Chlorofluoro Haloforms with Acetone-d6 and Oxirane-d4. J. Am. Chem. Soc. 2002, 124, 11854−11855.

−CF2CF2- spacer group, and supports the generality of these trends. As a note, we did investigate the C−H···F−C hydrogen bond energy for the system by calculating a difference in total energy from transoidal and cisoidal conformations with zero-point energy correction. We found that the C−H···F−C bond energy ranges from 2.1 to 0.6 kcal mol−1 using the MP2/aug-ccpVTZ//MP2/aug-cc-pVDZ level of theory. However, we note that the difference in total energy from transoidal and cisoidal conformations may not entirely reflect the C−H···F−C hydrogen bond energy for the system because of additional hydrogen bonded interaction in the transoidal structure not present in the cisoidal conformation. Nevertheless, these results are consistent with the C−H···F−C interaction energy in polyfluoroacetylene of 0.59 kcal mol−1 and 2-fluoroethanol of 2.05 kcal mol−1 at similar levels of theory.13

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CONCLUSION In conclusion, by studying the group effects on the C−H···F−C interaction, it is found that there is a structural effect of substituent substitution on the C−H···F−C interaction. Moreover, there is a general trend for all the nine substituents which are OCH3, C2H5, CH3, CHCH2, CCH, F, Cl, Br, and NO2. Among them, they can be divided into two main groups as the electron donating group, EDG (OCH3, C2H5, CH3, CHCH2 and CCH), and the electron withdrawing group, EWG (F, Cl, Br, NO2). It is found that the extent of electron-donating ability of the Y group can increase the hydrogen bonding interaction in the C−H···F−C system. The F



Article

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(29) Lu, N. who spent his sabbatical leave at Chemistry Department, Purdue University thanks the National Science Council of Taiwan (100-2918-I-027-001) and National Taipei University of Technology for the financial support.

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