Nature of Cl···Cl Intermolecular Interactions via Experimental and

Dec 8, 2010 - Monalisa Bai , Sajesh P. Thomas , Ranjith Kottokkaran , Susanta K. Nayak , Praveen C. Ramamurthy , and T. N. Guru Row. Crystal Growth & ...
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13434

J. Phys. Chem. A 2010, 114, 13434–13441

Nature of Cl · · · Cl Intermolecular Interactions via Experimental and Theoretical Charge Density Analysis: Correlation of Polar Flattening Effects with Geometry Venkatesha R. Hathwar and Tayur N. Guru Row* Solid State and Structural Chemistry Unit, Indian Institute of Science, Bangalore-560012, India ReceiVed: October 20, 2010; ReVised Manuscript ReceiVed: NoVember 17, 2010

The experimental charge density distribution in three compounds, 2-chloro-3-quinolinyl methanol, 2-chloro3-hydroxypyridine, and 2-chloro-3-chloromethyl-8-methylquinoline, has been obtained using high-resolution X-ray diffraction data collected at 100 K based on the aspherical multipole modeling of electron density. These compounds represent type I (cis), type I (trans), and type II geometries, respectively, as defined for short Cl · · · Cl interactions. The experimental results are compared with the theoretical charge densities using theoretical structure factors obtained from a periodic quantum calculation at the B3LYP/6-31G** level. The topological features derived from the Bader’s theory of atoms in molecules (AIM) approach unequivocally suggest that both cis and trans type I geometries show decreased repulsion, whereas type II geometry is attractive based on the nature of polar flattening of the electron density around the Cl atom. Introduction Halogen · · · halogen interactions provide weak but highly directional packing motifs, which aid in the evaluation of supramolecular assemblies in solid state.1 These interactions are ubiquitously found in the packing of organic molecules2 as well as in the construction of layered structures which exhibit solidstate reactivity.3 Halogen · · · halogen (C-X1 · · · X2-C) short contacts are characterized in terms of three parameters, Rij ) X1 · · · X2 and two angles θ1 ) C-X1 · · · X2 and θ2 ) X1 · · · X2C.2b,e,4 Contacts with θ1 = θ2 are referred to as type I, where as contacts with θ1 = 180° and θ2 = 90° are referred to type II interactions (Scheme 1). In an effort to evaluate the preference to noncentrosymmetric crystallization of dipolar molecules, two types of geometries were invoked, L geometry, which occurs across a 21 screw axis or a glide plane, and V geometry across a two-fold rotation axis.2e A short halogen · · · halogen contact is regarded as (i) a donor-acceptor interaction, (ii) a secondary interaction, (iii) due to electron-transfer interaction, (iv) an interaction between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO), and (v) a combination of any of these.5 A statistical analysis2f of the crystal structures of halogenated hydrocarbons using the Cambridge structural database (CSD) shows that the number of contacts between the halogen atoms is greater than the number of contacts expected from the exposed area of the halogen atom alone. Further crystallographic and theoretical studies prove that the strength of the halogen · · · halogen contacts decreases as I · · · I > Br · · · Br > Cl · · · Cl; and their relative strengths decrease as a function of the hybridization of the ipso carbon atom in the following order sp2 > sp > sp3, and the strength is dependent on the extent of electronegativity associated with the substituent atom.2f Also, the likelihood of generation of type II contact increases over that of the type I contact upon proceeding from Cl to I as the polarization effects increase.2c In a recent report,6 the structure-directing ability of halogen · · · halogen interactions in assembling β sheets into 2D layers in a series of bis-amides and their cocrystals has been

explored. In situ cryo crystallization studies of 2-chloroaniline7 generate Cl · · · Cl contacts along the 31 screw axis, resulting in a helical supramolecular motif (generally referred to as an X3 synthon). Because X · · · X contacts are of several types, it is sometimes not possible to characterize them based on geometrical considerations only, and the use of an independent technique like the mechanical behavior offers clearer insight into the nature of such interactions.8 Ever since the structures of halogen molecules, in particular, Cl2, Br2, and I2 were determined,9 the nature of packing of molecules in the crystal structure suggested deviations from normal van der Waals type structures. These diatomic molecules crystallize in the orthorhombic space group Cmca and are layered with all atoms in planes parallel to (100). This feature ensures the formation of intermolecular contacts less than the sum of van der Waals radii. It is of interest to note that these intermolecular interactions between the halogen atoms have been discussed and debated for several decades;2 however, no systematic analysis has been carried out so far to assess unambiguously whether such interactions are attractive or repulsive. The layered orthorhombic crystal structure of the halogens cannot be anticipated to possess isotropic potentials with only quadrapole-quadrapole interactions, like in the structures of N2, NO, and CO, which crystallize in cubic space SCHEME 1: Schematic Representation of Halogen · · · Halogen Type I (trans and cis Geometry) Contacts and Type II (L Geometry) Contacts

* To whom correspondence should be addressed. E-mail: ssctng@ sscu.iisc.ernet.in. Telephone: +91-80-22962796. Fax: +91-80-23601310.

10.1021/jp1100413  2010 American Chemical Society Published on Web 12/08/2010

Nature of Cl · · · Cl Intermolecular Interactions SCHEME 2: Schematic Diagram of (a) 2-Chloro-3-quinolinyl Methanol (VCL1), (b) 2-Chloro-3-hydroxypyridine (VCL2), and (c) 2-Chloro-3-chloromethyl-8-methylquinoline (VCL3)

group Pa3j.9a Potential energy models preferentially needed anisotropic terms to be added, and in this context, two possible models were generated. The first model is based on a postulate that anisotropic nonbonded radii are to be associated with the Cl atom in the solid chlorine molecule.10 The other model proposes that halogen atoms are weakly bonded in molecular crystals with an estimated energy for Cl · · · Cl interactions about 3% of the energy of the Cl-Cl covalent bond.11 The two models proposed look at the anisotropic features differently; the Williams model11 considers the extent of polarization among the participating halogen atoms, whereas the Nyburg model10 treats this effect as due to anisotropy in the van der Waals radius of each halogen atom. This allows for the description of the interpretation of the effect either as attractive based on the Williams model or as decreased repulsion based on the Nyburg model.2b The interpretation of Cl · · · Cl interactions both from experimental and theoretical consideration have supported either of the two models2b,f over the years, but a clear picture has not been arrived at either in terms of geometrical considerations2b or in terms of lattice energies.2d A recent charge-density-based analysis of type II contacts12 demonstrates that these interactions are clearly attractive. Charge density analysis of crystalline materials by using highresolution X-ray diffraction data has become routine and enables derivation of reliable one-electron properties associated with the electron density.13 The Hansen and Coppens multipole14 formalism is the most popular methodology used to model charge density features and the atoms in molecules (AIM) approach15 provides the tools for the evaluation of properties based on the derived model density distributions. This article describes the experimental charge density distribution based on the aspherical multipole modeling of electron density in three compounds, 2-chloro-3-quinolinyl methanol (VCL1), 2-chloro-3-hydroxypyridine (VCL2), and 2-chloro-3chloromethyl-8-methylquinoline (VCL3) (Scheme 2). The experimental results are compared with the theoretical charge densities using theoretical structure factors obtained from periodic quantum calculation at the B3LYP/6-31G** level. The three compounds are chosen to represent all possible orientations of the Cl · · · Cl contacts (Scheme 1). Of these, VCL1 and VCL2 represent two possible orientations for type I contacts referred to as trans and cis, respectively, and the third compound VCL3 represents the type II contact.

J. Phys. Chem. A, Vol. 114, No. 51, 2010 13435 Experimental Section The compounds were synthesized based on known procedures,16 and good-quality single crystals were grown by slow evaporation from methanol at room temperature. All crystals were colorless. Single crystals of size ∼0.3 mm were selected under a polarizing microscope and affixed to Hampton Research cryoloops using paratone-N oil for data collection. The crystals were cooled to 100(2) K with a liquid nitrogen stream using an Oxford cryosystem. The high-resolution X-ray diffraction data sets were collected on a Bruker AXS Kappa Apex CCD diffractometer using Mo KR radiation. The crystal-to-detector distance was fixed at 40 mm for all of the crystals. Complete redundant data were collected in all of the compounds by strategies generated by program COSMO in the Bruker software.17 The scan width per frame was ∆ω ) 0.5°. The cell refinement and data reduction were carried out using the SAINTPLUS,17 and numerical absorption correction was performed by face indexing. The sorting, scaling, and merging were carried out by using SORTAV.18 The crystal structures were solved by direct methods using SHELXS9719 and refined in the spherical atom approximation (based on F2) by using SHELXL9719 included in the complete WinGX package suite.20 The ORTEP21 diagrams (Figure 1) were generated using POVRay.22 The packing diagrams (Figure 1) depicting the Cl · · · Cl intermolecular interactions were generated using the package Mercury.23 The experimental and all crystallographic details are summarized in Table 1. (a) Multipole Refinement. The charge density modeling and refinement was performed with XD200624 using the Hansen and Coppens multipole formalism.14 It allows describing of the atomic electron density as a superposition of pseudoatoms as follows

Fatom(r) ) Fcore(r) + Pvalκ3Fval(κr) + lmax

∑ l)0

l

κ′3Rl(κ′r)

∑ Plm(ylm((θ, φ)

m)0

where Fcore and Fval represent the spherical core and valence unitary electron density, respectively. Pval is the valence population parameter and gives an estimation of the net atomic charge q ) Nval - Pval, where Nval is the number of valence electrons in a free neural atom, ylm represents multipolar spherical harmonic functions of order l in real form, Rnl are Slater type radial functions, and Plm are the multipolar populations. The coefficients κ and κ′ describe the contraction-expansion for the spherical and multipolar valence densities, respectively. The structure factors were derived from the Su, Coppens, and Macchi wave functions.25 Atomic displacement parameters of hydrogen atoms were obtained using the SHADE2 approach.26 The multipolar nonspherical atom refinements were carried out with the full-matrix least-squares program with XD2006.24 The function minimized was ∑ w(|F0| - K|Fc|)2 for all reflections with I/σ(I) > 3. Initially, the scale factor was refined against the whole resolution range of diffraction data. The positional and anisotropic thermal displacement parameters of the nonhydrogen atoms were refined against the reflections with sin θ/λ > 0.7 Å-1. The C-H bond lengths were constrained to the values determined by neutron diffraction experiments27 (Car-H ) 1.077 Å, Cmethylene-H ) 1.092 Å, and Cmethyl-H ) 1.059 Å). For nonhydrogen atoms, the scale, positional and thermal displacement parameters, Pval, Plm,, κ, and κ′, were allowed to refine in a stepwise manner, until the convergence was reached. The

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Figure 1. ORTEP and packing diagrams depicitng Cl · · · Cl intermolecular contacts in the compounds (a) VCL1, (b) VCL2, and (c) VCL3. ORTEP diagrams are drawn with 50% ellipsoidal probability, and H atoms are shown with anisotropic displacement parameters obtained from SHADE226 analysis.

multipole expansion was truncated at the octupole level (l ) 3) for non-hydrogen atoms. The most significant multipoles correspond to ylm terms with lm( ) 10, 20, 21+, 22+, 22-, 30 and were allowed to refine for the Cl atom for all three structures. Appropriate local site symmetry constraints and chemical constraints were imposed on the multipole populations of all of the non-hydrogen atoms during the multipolar refinement. Anisotropic thermal parameters for H atoms were fixed to the values obtained from SHADE2,26 analysis and only monopole and bond directed dipole (dz) and quadrupole (q3z2-1) components were allowed to refine. Because the space group of compound VCL2 is noncentrosymmetric (Fdd2), the origin was fixed at the chlorine atom Cl1 during the multipolar refinements. Further, the absolute configuration for this crystal structure was confirmed based on the value of the Flack parameter (Table 1). It is to be pointed out that Gram-Charlier expansion refinement up to third order was carried out for chlorine atoms to account for anharmonicity effects for compounds VCL1 and VCL3, resulting in better residual characteristics (Supporting Information Figures S4 and S12). However, VCL2 did not show any significant improvement, and hence, the results on VCL2

do not include third-order Gram-Charlier refinement on thermal parameters associated with the chlorine atom. (b) Theoretical Calculations. Single-point periodic quantum calculations were carried out using CRYSTAL0628 with the geometry obtained from the experimental charge density refinement as input. The SCF calculations were performed using the B3LYP/6-31G** level of theory. This basis set provides reliable and consistent results in studying the intermolecular interactions.29 The shrinking factors (IS1, IS2, and IS3) along with the reciprocal lattice vectors were set to 4 (30 k-points in the irreducible Brillouin zone). The bielectronic Coulomb and exchange series values for the truncation parameter were set as ITOL1 ) ITOL2 ) ITOL3 ) ITOL4 ) 8 and ITOL5 ) 17, respectively. The level shifter was set to 0.7 hartree for better convergence. Upon convergence on energy (∼10-6 Hartree), the periodic wave functions were obtained, and the XFAC module was used to generate the theoretical structure factors at the same resolutions as that observed from the experiments. The atomic positions were held fixed to the values obtained from the experimental charge density values during the multipolar refinement with theoretical structure factors. All theoretical structure factors were assigned unit weights during the refine-

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TABLE 1: Crystallographic Data and Experimental Details compound VCL1

VCL2

formula formula weight crystal system space group a (Å) b (Å) c (Å) R (°) β (°) γ (°) volume (Å3), Z calculated density F(000) absorption coefficient (mm-1) T (K) λ (Å) (sin θ/λ)max (Å-1) Rint measured reflections unique reflections overall completeness

C10H8ClNO 193.62 triclinic P-1 7.088(2) 7.220(2) 9.140(2) 92.55(2) 106.95(1) 110.32(2) 414.0(2), 2 1.554 200 0.411 100(2) 0.71073 1.08 0.0229 76156 8707 99.6%

R(F2), wR(F2) (all data) R(F2), wR(F2) (I > 2σ(I)) GoF Flack parameter

0.0241, 0.0685 0.0207, 0.0659 1.090 -

refinement based on observed reflections [I > 3σ(I)] total number of parameters R(F), R(wF) R(F2), wR(F2) GoF maximum DMSA values (Å2) ∆Fmin, ∆Fmax (e Å-3) CCDC number

Multipole F2 7701 365 0.0109, 0.0151 0.0168, 0.0299 1.010 5(2) × 10-4 at C(2)-C(10) bond -0.118, 0.160 768017

C5H4ClNO 129.54 orthorhombic Fdd2 23.1021(8) 25.1758(9) 3.7363(1) 90 90 90 2173.1(1), 16 1.584 1056 0.582 100(2) 0.71073 1.08 0.0244 36006 5626 99.9% Spherical Atom Refinement 0.0277, 0.0621 0.0235, 0.0609 1.075 0.02(2)

ments based on the methodology followed in the literature.30 The thermal displacement parameters were set to zero to consider a static model, and multipolar refinements of the theoretical data were carried out up to the same levels as that used for the experimental charge density modeling to compare the obtained results with experimental structure factors. Results and Discussion (a) Crystal Structures. The details of crystal structure determination along with parameters of multipolar refinement for all three compounds are listed in Table 1. The ORTEP diagrams showing thermal ellipsoids at the 50% probability level along with the atom labeling of the molecules and corresponding packing diagrams specifically indicating Cl · · · Cl intermolecular contacts are displayed in Figure 1. The detailed discussions on geometrical analysis of VCL1,16 VCL3,16 and VCL22e based on X-ray diffraction are already reported in the literature. The compounds represent the three types of geometries with VCL1 in a type I trans Cl · · · Cl geometry (3.5747(2)Å and θ1 ) θ2 ) 150.6°), VCL2 showing a type I cis geometry (3.3172(1)Å and θ1 ) θ2 ) 158.7°), and VCL3 with a type II geometry (3.4668(2)Å with θ1 ) 168.3° and θ2 ) 103.6°). In addition, VCL1 is stabilized by a strong O-H · · · N and weak C-H · · · O and C-H · · · π intermolecular interactions (Supporting Information Figure S1). Strong O-H · · · N and weak C-H · · · O intermolecular hydrogen bonds form a tetrameric building unit in VCL2 (Supporting Information Figure S2). It is of interest to

Refinement F2 4889 245 0.0152, 0.0189 0.0186, 0.0365 1.017 6(2) × 10-4 at N(1)-C(5) bond -0.112, 0.129 768018

VCL3 C11H9Cl2N 226.09 monoclinic P21/c 14.980(2) 4.5662(7) 14.858(2) 90 94.87(1) 90 1012.6(3), 4 1.483 464 0.596 100(2) 0.71073 1.08 0.0261 76000 10681 100% 0.0530, 0.1036 0.0321, 0.0925 1.018 F2 7393 367 0.0202, 0.0185 0.0249, 0.0370 1.165 14(2) × 10-4 at C(2)-C(10) bond -0.133, 0.187 768019

note that in the crystal structure of VCL3, an additional secondary C-H · · · Cl intermolecular contact develops perpendicular to the direction of Cl · · · Cl contacts (Supporting Information Figure S3). (b) Multipole Modeling, Deformation Densities, and Topological Properties. The Hirshfeld rigid bond test31 was applied to all covalent bonds involving non-hydrogen atoms during the multipole refinements in all three compounds. The values of maximum differences of mean-square displacement amplitudes (DMSDA) are found to be 5(2) × 10-4 Å2 at C(2)-C(10) for VCL1, 6(2) × 10-4 Å2 at N(1)-C(5) for VCL2, and 14(2) × 10-4 Å2 at C(2)-C(10) for VCL3. The residual electron densities calculated (with I > 3σ(I)) over the asymmetric unit are almost featureless (minimum and maximum densities of -0.118-0.160 e Å-3 for VCL1, -0.112-0.129 e Å-3 for VCL2, and -0.133-0.187 e Å-3 for VCL3). These residual densities are comparable to the values reported earlier for hexachlorobenzene (C6Cl6),12 supporting the correctness of the model. The residual electron density maps, static deformation density maps, Laplacian maps for all three molecules, and also the Laplacian maps for the Cl · · · Cl interaction region are given in the Supporting Information (Figures S4-S15). The static deformation density maps obtained from experimental and theoretical multipolar refinements show good agreement with each other and display all of the expected subtle bonding features, except for Cl atoms (Supporting Information Figures S5, S9, and S13). The aspherical nature of the charge density

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Figure 2. 3D static deformation density maps from theoretical charge density calculations for Cl · · · Cl intermolecular interactions of the compounds (a) VCL1, (b) VCL2, and (c) VCL3. Blue and red colors represent positive and negative values, respectively. The ∆F(r) isosurfaces are drawn at (0.01 e Å-3.

Figure 3. The Laplacian maps with (3,-1) critical points (CP) are shown for compounds (a) VCL1, (b) VCL2, and (c) VCL3. The contours are drawn on the logarithmic scale.

distribution at the Cl atom resulting in the polar flattening effect is better observed in the 3D static deformation density maps for Cl · · · Cl intermolecular interaction regions for all three compounds (Figure 2). The corresponding Laplacian maps show (3,-1) bond critical points (Figure 3) and topological values (Table 2) indicating the closed-shell nature of interactions. The topological values of all covalent bonds (interaction lengths (Rij), electron densities (Fb), and Laplacians (32Fb)) obtained for both experiment and theoretical calculations are listed (Supporting Information Table S1), and the values show good agreement, demonstrating that both methodologies provide

a consistent measure of the topological properties of the charge densities for all three compounds. The topological analysis of the total electron density F(r) and other BCP properties were evaluated for Cl · · · Cl intermolecular interactions using the XDPROP module.24 Further, a careful analysis of the topological features of Cl · · · Cl intermolecular regions brings out the following observations. The experimental Cl · · · Cl interaction lengths for all three compounds (Rij) are shorter than the sum of the van der Waals radii with the distances of 3.5747(2) Å for VCL1, 3.3172(1) Å for VCL2, and 3.4668(2) Å for VCL3. The Fcp and 32Fcp are in the range of 0.03-0.06 e Å-3 and

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TABLE 2: Topological Values of the Cl · · · Cl Interaction at the BCPa

compound VCL1 VCL2 VCL3 a

C-X1 · · · X2-C C1-Cl1 · · · Cl1-C1 (-x,-y+2,-z) (trans geometry) C1-Cl1 · · · Cl1-C1 (-x+2,-y,z) (cis geometry) C1-Cl1 · · · Cl2-C10 (-x,-y-1,-z+1) (L geometry)

Rij (Å)b

θ1/θ2 (°)b

3.5747(2) 3.5746 150.6/150.6 (type I) 3.3172(1) 3.3172 158.7/158.7 (type I) 3.4668(2) 3.4667 168.3/103.6 (type II)

Fcp 32Fcp (e Å-3) (e Å-5)

ε

G(rcp) V(rcp) E(rcp) (kJ mol-1 (kJ mol-1 (kJ mol-1 |V(rcp)|/ Eint G(rcp) (kJ mol-1) bohr-3) bohr-3) bohr-3)

0.03 0.04

0.41 0.44

0.11 0.11

7.8 9.1

-4.9 -6.1

2.9 3.0

0.62 0.67

1.8 2.9

0.05 0.06

0.66 0.72

0.02 0.03

13.5 15.4

-9.2 -11.2

4.3 4.2

0.68 0.73

10.4 15.8

0.03 0.05

0.47 0.57

0.03 0.07

9.0 11.8

-5.6 -7.9

3.4 3.9

0.63 0.68

4.8 7.9

The values from CRYSTAL06 using B3LYP/6-31G** are given in italics. b Rij ) X1 · · · X2, θ1 ) C-X1 · · · X2, and θ2 ) X1 · · · X2-C.

0.41-0.72 e Å-5, respectively, (Table 2) and correspond to closed-shell interactions. Further, these values are in good agreement with the reported values in the literature12 for weak intermolecular interactions. The corresponding values of Fcp and 32Fcp obtained from theoretical calculations using CRYSTAL0628 are in excellent agreement (Table 2; shown in italics) with the experimental values. The values of kinetic energy density, G(rcp), and the potential energy density, V(rcp), calculated based on the work of Abramov and Espinosa,32 for type I (trans) are 7.8(9.1) and -4.9(-6.1) kJ mol-1 bohr-3 (Table 2), while they are 13.5(15.4) and -9.2(-11.2) kJ mol-1 bohr-3 (Table 2), respectively, for the type I (cis) geometry. The total energy density, E(rcp), the sum of kinetic and potential energy densities at BCP, is 2.9(3.0) kJ mol-1 bohr-3 for VCL1 (which shows type I trans geometry) and 4.3(4.2) kJ mol-1 bohr-3 for VCL2 (which shows type I cis geometry). The ratio of |V(rcp)|/G(rcp) in both cases is less than 1, representing closed-shell interactions.32 The static 3D deformation density maps for type I (both trans and cis) unambiguously support the features required for the Nyburg model10 by indicating decreased repulsion due to anisotropy of the electron density around the Cl atom (Figure 2a and b). The depletion of the spherical distribution at the Cl atom is clearly seen in the Laplacian maps (Figure 3a and b). Recently, halogen bonding has been described as an interaction between a region of charge depletion (CD), referred to as a hole on the halogen atom, and a region of charge concentration (CC), referred to as a lump on another molecule.33 A similar description emerges upon closely examining the 3D deformation density maps in the Cl · · · Cl interaction topology. In type I (both cis and trans), the hole corresponding to the CD at one of the Cl atoms (shown in red) faces the hole corresponding to the Cl atom participating in the Cl · · · Cl interaction, while the lumps (CC region) on each Cl atom (shown in blue) face away from the interaction region (Figure 2a and b). Thus, polar flattening emerges as the main reason for the Cl · · · Cl interaction, supporting the Nyborg model. The values of G(rcp) and V(rcp) are 9.0(11.8) and -5.6(-7.9) kJ mol-1 bohr-3 for the compound VCL3 (Table 2). The total energy density, E(rcp), the sum of kinetic and potential energy densities at BCP, is 3.4(3.9) kJ mol-1 bohr-3 for VCL3 (which shows a type II geometry). The ratio of |V(rcp)|/G(rcp) is less than 1, representing closed-shell interactions.32 The static 3D deformation density map unambiguously supports the features required for the Willams model11 with the CD region hole (shown in red) facing the CC region lump (shown in blue), directly resulting in a δ+ · · · δ- type of interaction (Figure 2c). Also, the Laplacian map (Figure 3c) supports this observation by indicating that polar flattening induces a δ+ · · · δ- type of interaction. Thus, electrostatic interaction holds the two chlorine

atoms together in this type II interaction geometry, supporting the earlier observation in the literature.12b The total interaction energy (Eint) was calculated using XD200624 for all Cl · · · Cl interactions as a pairwise interaction energy between neighboring molecules involving these two chlorine atoms, as discussed in the literature.34 These calculations include dispersion, exchange-repulsion, and electrostatic terms. Williams and Cox’s potential35 is used for calculation of dispersion and exchange-repulsion terms. The total interaction energies (Eint) are 1.8(2.9) kJ mol-1 for type I (trans), 10.4(15.8) kJ mol-1 for type I (cis), and 4.8(7.9) kJ mol-1 for type II geometries. It is of interest to note that the values of the energy densities and the total interaction energies (Eint) are comparable with recently reported I · · · I intermolecular contacts.36 It is to be noted that there are other intermolecular interactions, in particular, the strong O-H · · · N, which will have a significant contribution to the energies of packing of the molecules. The topological values of these additional interactions in all compounds are listed in Table 3. It is to be expected that this would have an influence on the numerical values obtained for the energies and also the topological features in the Cl · · · Cl interaction region. However, the trends are clearly indicative of a conclusive decision on the nature of Cl · · · Cl interactions. In addition, the somewhat significant differences observed in the features of the deformation density maps in both experimental and theoretical studies need to be addressed (Supporting Information Figures S5, S9, and S13). It was pointed out earlier that the third harmonic expansion on the chlorine atom in the multipole refinement strategy resulted in better residual density maps for VCL1 and VCL3 (Supporting Information Figures S4 and S12). However, these corrections do not show significant improvement in the static deformation density features of experimental multipolar modeling. Even though both maps from experiment and theory support the observation of polar flattening, the quantitative comparison in terms of the contour strengths is still elusive. Such differences are seen in many earlier studies,30 and there is no plausible explanation given. These differences could arise due to many factors like (1) inadequacy of the theoretical basis set and (2) limitation on the flexibility of the basis functions in the multipole refinement strategy.30d The importance of the bias of radial fit functions has also been discussed in detail.30e However, it remains clear that both experimental and theoretical maps support the observations made to differentiate between type I (cis and trans) and type II Cl · · · Cl interactions. (c) Electrostatic Potential Isosurfaces. The electrostatic potential is generated using the XD200624 on an isolated molecule extracted from the crystal lattice. The analysis of the electrostatic potential (ESP) on the molecular surfaces was

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TABLE 3: Topological Values of Hydrogen Bonds Present in All Three Compoundsa compound

bond

Rij(Å)

Fcp (e Å-3)

32Fcp (e Å-5)

λ1

λ2

λ3

ε

G(rcp) au

VCL1

N1 · · · X1-H1O (x+1,y,z) O1 · · · X2-H3 (-x-1,-y+1,-z+1) O1 · · · X2-H4 (-x-1,-y+1,-z+1) N1 · · · X2-H7 (-x+1,-y+2,-z+1) N1 · · · X11-H1O (x-1/4,-y+1/4,z-1/4) O1 · · · X16-H4 (-x+3/4,-y-1/4,z+1/4) Cl1 · · · X11-H3 (x-1/4,-y+1/4,z-1/4) Cl1 · · · X16-H4 (-x+3/4,-y-1/4,z+1/4) Cl1 · · · X4-H4 (x,-y-3/2,z-1/2) Cl1 · · · X4-H3 (x,-y-3/2,z-1/2) Cl2 · · · X2-H10B (-x,y+1/2,-z+3/2)

2.0642(1) 2.0719 2.6223(2) 2.6315 2.4762(2) 2.4846 2.5763(1) 2.5809 1.7901(3) 1.8012 2.5161(4) 2.5142 2.9320(5) 2.9370 2.8152(3) 2.8152 2.8496(1) 2.8550 2.7590(2) 2.7610 2.7801(1) 2.7800

0.11 0.09 0.03 0.03 0.04 0.03 0.04 0.04 0.24 0.19 0.02 0.02 0.02 0.02 0.02 0.03 0.02 0.02 0.03 0.03 0.03 0.02

1.84 1.97 0.52 0.49 0.74 0.65 0.67 0.68 2.29 4.00 0.65 0.61 0.34 0.38 0.43 0.55 0.37 0.47 0.48 0.52 0.44 0.72

-0.58 -0.53 -0.11 -0.10 -0.16 -0.14 -0.14 -0.16 -1.64 -1.21 -0.11 -0.10 -0.06 -0.08 -0.08 -0.11 -0.10 -0.10 -0.12 -0.11 -0.14 -0.06

-0.51 -0.42 -0.10 -0.09 -0.15 -0.13 -0.13 -0.14 -1.50 -1.08 -0.08 -0.08 -0.05 -0.07 -0.06 -0.09 -0.08 -0.07 -0.11 -0.09 -0.13 -0.05

2.94 2.93 0.74 0.68 1.05 0.91 0.94 0.96 5.44 1.70 0.84 0.80 0.46 0.51 0.57 0.75 0.55 0.64 0.72 0.71 0.72 0.83

0.14 0.25 0.12 0.19 0.05 0.07 0.06 0.14 0.10 0.12 0.37 0.24 0.09 0.14 0.40 0.30 0.25 0.30 0.08 0.22 0.07 0.11

38.9 40.1 10.2 9.3 14.5 12.6 13.1 13.3 63.5 87.9 12.1 11.5 6.5 7.2 8.2 10.6 7.3 9.0 9.6 10.0 8.9 13.5

VCL2

VCL3

a

V(rcp) au -27.7 -26.6 -6.0 -5.5 -8.8 -7.4 -8.1 -8.0 -61.6 -66.9 -6.6 -6.5 -3.7 -4.1 -4.5 -6.2 -4.5 -5.2 -6.3 -5.9 -5.8 -7.3

The values from CRYSTAL06 using B3LYP/6-31G** method are given in italics.

Figure 4. Electrostatic potential isosurface maps from experimental charge density calculations of the compounds (a) VCL1, (b) VCL2, and (c) VCL3 drawn at an isosurface value of (0.5 e Å-3. Blue and red colors represent electropositive and electronegative regions, respectively.

performed to highlight the effect of crystalline environment and also to point out the distribution of electron density in the molecule using the program MOLISO.37 Electropositive isosurface regions (Figure 4) are indicated by blue surfaces, whereas electronegative regions are displayed as red surfaces.

The green surfaces indicate neutral potential surfaces. The electrostatic potential around the Cl atom is depicting the polar flattening effects as the blue surface is pointing along the bond axis, whereas the yellowish surface (less negative than the red surfaces) is perpendicular to the bond axis. The

Nature of Cl · · · Cl Intermolecular Interactions hydrogen atoms are positively polarized, and oxygen atoms are negatively polarized, as indicated by blue and red surfaces, respectively. These features also support the conclusions on the nature of Cl · · · Cl intermolecular interactions. It may be pointed out that the ESP isosurfaces from theoretical charge density calculations are also in one-toone correspondence with experimental values (Supporting Information Figure S17). Conclusion The analysis of all three possible geometries of Cl · · · Cl intermolecular contacts in terms of both experimental and theoretical charge density measurements followed by careful topological evaluation of properties leads to a clear understanding of the nature of such interactions, in particular, the features associated with the geometrical requirements of type I and type II contacts. The results correlate the factors associated with decreased repulsion and the attractive nature of Cl · · · Cl intermolecular contacts in terms of derived one-electron properties, electrostatic potentials, and 3D static deformation densities with these geometries. We believe the work described in this article has given a solid platform for a complete understanding of halogen interactions resulting in energetically favorable arrangements in the crystal lattice. Acknowledgment. We thank Dr. Nawaz Khan and Dr. Binoy Saha for providing the samples and Dr. P. Munshi for useful discussions and help in the multipole refinements. The authors also thank the Indian Institute of Science for the X-ray diffraction facility. V.R.H. thanks IISc for research fellowships and CSIR, India, for a travel grant to attend the MSSC2010 workshop at Imperial College, London. T.N.G.R. thanks the Department of Science and Technology, New Delhi, for financial support. Supporting Information Available: Crystallographic information files (CIF) for all studied compounds, packing diagrams of all compounds depicting intermolecular interactions, residual maps, static deformation density maps, Laplacian maps, 3D Static deformation density maps from experimental charge density analysis, electrostatic potential isosurface maps from theoretical charge density calculations, and tables of topological features of all covalent bonds for all studied compounds. This material is available free of charge via the Internet at http:// pubs.acs.org. References and Notes (1) Desiraju, G. R. Crystal Engineering: The Design of Organic Solids; Elsevier: Amsterdam, The Netherlands, 1989. (2) (a) Sarma, J. A. R. P.; Desiraju, G. R. Acc. Chem. Res. 1986, 19, 222–228. (b) Desiraju, G. R.; Parthasarathy, R. J. Am. Chem. Soc. 1989, 111, 8725–8726. (c) Pedireddi, V. R.; Reddy, D. S.; Goud, B. S.; Craig, D. C.; Rae, A. D.; Desiraju, G. R. J. Chem. Soc., Perkin Trans. 2 1993, 2353–2360. (d) Price, S. L.; Stone, A. J.; Lucas, J.; Rowland, R. S.; Thornley, A. E. J. Am. Chem. Soc. 1994, 116, 4910–4918. (e) Saha, B. K.; Nangia, A.; Nicoud, J. F. Cryst. Growth Des. 2006, 6, 1278–1281. (f) Awwadi, F. F.; Willett, R. D.; Peterson, K. A.; Twamley, B. Chem.sEur. J. 2006, 12, 8952–8960. (3) (a) Desiraju, G. R. In Organic Solid State Chemistry; Desiraju, G. R., Ed.; Elsevier: Amsterdam, The Netherlands, 1987; pp 519-546. (b) Matsumoto, A.; Tanaka, T.; Tsubouchi, T.; Tashiro, K.; Saragai, S.; Nakamoto, S. J. Am. Chem. Soc. 2002, 124, 8891–8902. (4) (a) Sakurai, T.; Sundaralingam, M.; Jeffrey, G. A. Acta Crystallogr. 1963, 16, 354–363. (b) Ramasubbu, N.; Parthasarathy, R.; Murray-Rust, P. J. Am. Chem. Soc. 1986, 108, 4308–4314. (5) (a) Rosenfield, R. E., Jr.; Parthasarathy, R.; Dunitz, J. D. J. Am. Chem. Soc. 1977, 99, 4860–4862. (b) Guru Row, T. N.; Parthasarathy, R.

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