Differences in Charge Density Distribution and Stability of Two

Article pubs.acs.org/crystal

Differences in Charge Density Distribution and Stability of Two Polymorphs of Benzidine Dihydrochloride Anna A. Hoser,† Katarzyna N. Jarzembska,† Łukasz Dobrzycki,† Matthias J. Gutmann,‡ and Krzysztof Woźniak*,† †

Chemistry Department, University of Warsaw, Pasteura 1, 02-093 Warszawa, Poland Rutherford Appleton Laboratory, ISIS Facility, Chilton Didcot, OX11 OQX Oxfordshire, U.K.

‡

S Supporting Information *

ABSTRACT: Charge density distribution analysis supplemented by energy studies of two known (previously reported) polymorphic forms of benzidine dihydrochloride  triclinic P1̅ (TP) and orthorhombic Pbcn (OP)  is presented. High resolution X-ray diffraction measurement results were additionally related to the theoretically evaluated charge density distribution on the basis of periodic ab initio computations. In the case of TP, collected neutron diffraction structural data were included in the high resolution X-ray multipole refinement procedure. Although TP and OP form generally similar layered crystal architectures, the two structures differ significantly in details, due to slightly different conformations of the benzidinium cation, different packing, and thus intermolecular interactions. This is reflected in charge density distribution, crystal morphology, and growth. For TP, the integrated charge for the chloride anions is equal to ca. −0.90e, while for OP it is −1.07e. In consequence, charges for the benzidinium cation are equal to +1.85e for TP and +2.12e for OP. Different charge distributions for both polymorphs cause significant differences in the electrostatic potential. In the case of OP, the electrostatic potential for the −NH3+ group is substantially higher than for TP. Lower values of charge density at aromatic rings of the benzidinium cation for TP can be associated with significant contribution of stacking type of interactions. TP is slightly (ca. 10−15 kJ·mol−1) more thermodynamically advantageous than OP, however, the difference constitutes just a small percentage of the total cohesive energy derived (ca. −1500 kJ·mol−1). Therefore, both polymorphic forms can be obtained simultaneously from one solution under the same conditions. On the other hand, TP is characterized by a more homogeneous distribution of ionic fragments in its crystal lattice than OP. The straightforward comparison of the crucial interlayer interaction energies and the surface free energy values between both polymorphs shows that in the case of TP crystal faces should more preferably grow, being slightly less stable than in OP. Lower surface free energy in OP, and especially low interlayer interaction energy of (100) slabs, presumably hampers further growth of the initially formed crystals, explaining the difficulties in obtaining OP crystals of the size suitable for neutron diffraction experiments. Naturally, kinetic and solvent effects should not be neglected. Nevertheless, the energy results show that despite the resemblance of the cohesive energy values of polymorphs, the character and features of their crystal architectures may cause significant differences in crystallization mechanisms and crystal quality. It appears that there is little correlation between geometrical parameters of hydrogen bonds and their energies.

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INTRODUCTION Polymorphism is the ability of a substance to form more than one crystal structure. Different polymorphs have the same elemental composition but different unit cells or symmetry of crystals. This phenomenon is of particular importance in pharmacy1 and new functional material design. Thus, the study of polymorphism has recently become one of the major activities in the field of crystal engineering. It employs both experimental and theoretical approaches. The most common experimental methods include single crystal and powder X-ray diffraction, differential scanning calorimetry, infrared and Raman spectroscopy, and, rarely, neutron diffraction. Nevertheless, when single crystals which diffract well enough at high diffraction angles are available, then refinement of multipole model of electron density distribution becomes possible. Such studies require high quality of high resolution X© XXXX American Chemical Society

ray data. While in a standard crystallographic refinement atomic scattering factors are spherical and solely atom positions and atomic displacement parameters (ADPs) are refined, in a charge density approach the used atomic scattering factors are aspherical, which significantly increases the number of refineable parameters. The most often applied formalism in multipole refinement is the Hansen−Coppens approach.2 Within this formalism, it is assumed that the total electron density in a crystal consists of atomic contributions centered at atomic positions. Each individual atomic contribution is defined by the following formula: Received: March 11, 2012 Revised: May 14, 2012

A

dx.doi.org/10.1021/cg300337a | Cryst. Growth Des. XXXX, XXX, XXX−XXX

Crystal Growth & Design

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Figure 1. Atomic displacement parameters (ADPs) obtained for both polymorphs of BE2+ × 2Cl− (a) TP and (b) OP at 100 K. For OP hydrogen atom ADPs were generated by the SHADE2 program.6

Table 1. Crystallographic Data and Neutron Measurement Parameters for the TP Polymorph and Details of High Resolution Xray Data Collection and Structure and Charge Density Refinements for Both Polymorphs formula M (g·mol−1) T (K) λ (Å) space group unit cell dimensions (Å, °)

V (Å3) Z, Dx (g·cm−3) μ (mm−1) F(000) crystal size (mm3) limiting indices

sin(θmax)/λ (Å−1) refl. unique refinement method data, restraints, parameters GOF R indices (all data) ρmax, min

ρ(r) = ρc (r ) + Pvκ 3ρv (κr ) +

TP - neutron data

TP - X-ray data

C12N2H14Cl2 257 100(2) 0.69−6.98 P1̅ a = 6.574(1) b = 7.670(1) c = 12.636(2) α = 85.260(1) β = 76.74(1) γ = 73.82(2) 595.5(2) 2, 1.434 0.428 + 0.014 λ 130.6 2×2×5 −14 ≤ h ≤17 −19 ≤ k ≤20 0 ≤ l ≤ 31 1.8950 4115 F2 4109/0/271 1.14 R1 = 0.077 wR2 = 0.168 2.81, −2.84 fm Å−3

C12N2H14Cl2 257 100(2) 0.71073 P1̅ a = 6.5584(3) b = 7.6553(4) c = 12.6078(6) α = 85.228(4) β = 76.738(4) γ = 73.866(4) 591.72(5) 2, 1.443 0.52 268.0 0.15 × 0.21 × 0.32 −15 ≤ h ≤ 15 −17 ≤ k ≤ 17 −29 ≤ l ≤ 29 1.16 15564 F2 13245/0/675 1.16 R1 = 0.01 wR2 = 0.02 0.17, −0.20 eÅ−3

lmax

l

l=0

m=0

OP - X-ray data C12N2H14Cl2 257 100(2) 0.71073 Pbcn a = 27.4396(14) b = 6.0219(3) c = 7.3047(4)

1207.02(11) 4, 1.415 0.51 536.0 0.15 × 0.22 × 0.28 0 ≤ h ≤ 61 0 ≤ k ≤ 13 0 ≤ l ≤ 16 1.11 7002 F2 5517/0/360 2.58 R1 = 0.02 wR2 = 0.03 0.23, −0.37 eÅ−3

Such experimental charge density studies of different polymorphs of the same substance are quite rare.4 This is because the less stable polymorph is most often less likely to form excellent quality crystals. Additionally, experimental charge density studies require suitable equipment. However, when both conditions are fulfilled, the analysis of the derived charge density distributions of the compared polymorphic forms may lead to some valuable insights, regarding bonding, charges, electrostatic potential, weak interactions, and other related features. Polymorphism may also be studied through computational methods. In order to find out which polymorphic form is more thermodynamically stable, cohesive energy can be evaluated. This is the energy that must be put into a crystal to dissociate it into its constituent molecules. Cohesive and/or lattice energy calculations are especially important in structure prediction.5 Additionally, crystal architecture molecular planes, patterns, and synthons may be energetically characterized, thus providing some information about crystal formation and morphology.

∑ κ′3Rl(κ′r) ∑ Plm ±dlm ±(θ , φ)

where ρc(r) and ρv(r) are spherical core and valence densities, respectively. The third term contains the sum of the angular functions dlm±(θ,φ) and thus models the aspherical deformations. The angular functions dlm±(θ,φ) are real spherical harmonic functions normalized to electron density. The coefficients Pv and Plm± stand for multipole populations of the valence and deformation density multipoles, respectively. The κ and κ′ are scaling parameters introduced to expand or contract the valence and deformation densities. In the Hansen− Coppens formalism, the Pv, Plm±, κ and κ′ are refined together with the atomic coordinates and thermal motion parameters. Once charge density distribution is obtained, it can be analyzed by means of the QTAIM3 approach, and consequently all oneelectron properties of the electron density can be derived. B



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computations, neutron and X-ray data collections, and multipolar refinement schemes are available from the Supporting Information.

Our previous studies on two polymorphic forms of the hydrated 1,8-bis(dimethylamino)naphthalene hydrochloride, DMANH+×2Cl−×H5O2+, show that in the case of such similar crystal structures the discrepancies are observed mainly in the intermolecular interaction features.4h Therefore, we decided to check whether for another set of polymorphs, exhibiting larger differences between both forms, it is possible to observe some more pronounced additional effects. As a model compound, we have chosen benzidine dihydrochloride (abbreviated BE2+×2Cl−), which exists as the triclinic P1̅ polymorph (hereafter abbreviated TP, Figure 1a) and as the orthorhombic Pbcn form (hereafter abbreviated OP, Figure 1b). Both polymorphic forms are characterized by slightly different conformations of the benzidine fragment. Such a difference influences the packing arrangement (or vice versa) and, in consequence, is reflected in the unit cell symmetry. BE2+×2Cl− is a model compound to study architecture of the hybrid crystals. In particular, interrelations between charge density, crystal lattice, and morphology (crystal growth) are of special interest. In this work, we investigate differences in charge density distributions of both polymorphs. We intend to verify to what extent experimental and theoretical charge density analyses are helpful in the studies of polymorphism and how fine details of electron densities can be quantified and analyzed. In the case of the triclinic polymorph, crystals appropriate for neutron measurements were obtained, and neutron data were collected. Such a study is important, as it allows using hydrogen atom positions and ADPs derived from neutron measurement in the multipole refinement procedure against the X-ray scattered intensities (see the details of the TP_N refinement). Unfortunately, in the case of the orthorhombic form, it was not possible to grow crystals large enough for neutron measurements. Thus, we decided to use a method which combines both high and low angle refinements supplemented by ADPs for hydrogen atoms obtained from the SHADE2 server6,7 for both polymorphs8 (these are the TP_HI_SH and OP_HI_SH refinements). In consequence, in this work we present the results of three refinements: TP_N, TP_HI_SH, OP_HI_SH. The experimental charge density analysis is enriched with a set of theoretical computations, which gives insights into the relative stability and crystal formation of the studied polymorphs.

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RESULTS AND DISCUSSION Packing. Both polymorphic structures of benzidinium chloride, that is, TP and OP, had already been a subject of some crystallographic studies.9 It is, however, worth recalling the most relevant structural features in the context of the present contribution. Generally, the TP and OP form alike layered crystal architectures (see Figure 2a,b). Each layer consists of organic

METHODS

Figure 2. Unit cell projections of the triclinic (a) and orthorhombic (b) polymorphs of BE2+×2Cl−. Overlay of benzidinium cation molecules from TP (the angle between benzene rings: 21.91(4)°) and OP (45.32(2)°) (c).

Crystallization conditions for TP and OP polymorphs were already described in our previous paper.9 Single crystal neutron measurement for TP was accomplished using time-of-flight (TOF) Laue diffractometer SXD10 at 100 K at ISIS in Chilton. The integration process was conducted with the SXD2001 program.11 High resolution X-ray measurements for TP and OP were carried out at 100 K on a Bruker AXS KAPPA APEX II ULTRA diffractometer12 with TXS rotating molybdenum anode and multilayer optics. All needed corrections were applied. Indexing, integration, and scaling were performed with the original Bruker Apex II software.12,13 Structures were solved and refined in SHELX.14 Multipolar refinements on F2 were performed in XD200615 and analysis of obtained charge density in XDPROP. Theoretical structure factors were calculated in the CRYSTAL0916 package. This program was also applied for the purpose of the cohesive energy evaluation and additional interlayer interaction studies. Calculations were carried out at the DFT(B3LYP)17 level of theory, employing the 6-31G**18 molecular all-electron basis set. Crystallographic data and neutron and X-ray measurement parameters for both polymorphs are listed in Table 1. Further details of

cations held together via hydrogen bonds linking protonated amino groups and isolated chloride anions, and π···π interactions between the aromatic rings. In turn, chloride anions create sheet motifs, being relatively shifted in the (001) plane and in the (100) plane, in TP and OP, respectively. The negatively charged chloride species are neutralized by protonated amino groups penetrating each of the chloride layers. Nevertheless, the benzidinium cation geometries differ significantly between the T and O polymorphic structures. In the case of OP, this moiety consists of two symmetry related halves (x, y, z and −x, y, 0.5 − z), and the angle between the benzene rings amounts to 45.32(2)°. Whereas for the TP structure, the cation does not exhibit any particular higher C



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Table 2. Geometry of Hydrogen Bonds after Multipolar Refinement with Positions and ADPs of Hydrogen Atoms Derived from Neutron Diffraction Experiments - TP_N (First Numerical Value), and with SHADE2- TP_HI_SH (Second Numerical Value), and for OP_HI_SH (the Lower Part of the Table)a TP

OP

a

N−H···Cl−

D−H

H···A

D···A

D−H···A

SYMM

N1−H1···Cl1 N1−H2···Cl2 N1−H3···Cl1 N2−H4···Cl1 N2−H5···Cl2 N2−H6···Cl2 N1−H1A···Cl1 N1−H1B···Cl1 N1−H1C···Cl1 N1−H1A···Cl1 N1−H1B···Cl1 N1−H1C···Cl1

1.048/1.035 1.049/1.035 1.045/1.035 1.044/1.035 1.051/1.035 1.049/1.035 1.035 1.035 1.035 1.035 1.035 1.035

2.175/2.193 2.130/2.135 2.155/2.170 2.181/2.188 2.188/2.207 2.168/2.189 2.202 2.070 2.243 2.847 3.082 2.818

3.212/3.212 3.144/3.144 3.185/3.185 3.196/3.196 3.233/3.233 3.182/3.182 3.137 3.066 3.184 3.159 3.159 3.159

170.17/167.88 161.83/164.28 168.14/166.19 163.48/164.18 172.63/170.63 161.82/160.13 149.29 160.81 150.41 97.83 84.71 99.52

1 + X, Y, Z 2 − X, 1 − Y, 1 − Z 1 − X, 1 − Y, −Z −X, 1 − Y, 1 − Z X, 1 + Y, Z 1 − X, 1 − Y, 2 − Z −X, Y, 1/2 − Z −X, 1 − Y, −Z −X, 1 + Y, 1/2 − Z 1/2 −X, 1/2 + Y, Z 1/2 −X, 1/2 + Y, Z 1/2 − X, 1/2 + Y, Z

For esd values see the cif files and the Supporting Information.

Figure 3. dnorm mapped on Hirshfeld surface: (a) and (b) both sides of the benzidinium cation for TP, (c) the benzidinium cation from OP, (d) percentage contribution to the Hirshfeld surface area different intermolecular interactions for both polymorphs.

symmetry, and the mentioned angle is substantially smaller (21.91(4)°; see Figure 2c). Having carefully looked at the studied benzidinium chloride structures, one can see that also molecular packing, the mutual arrangement of chloride and benzidine ions in particular, and the interlayer distances, are quite different for the two polymorphs.9 In the case of TP, there are three similar hydrogen bonds falling on each N atom, while in OP, one can distinguish up to six short H···acceptor contacts which can be treated as diverse hydrogen bonds per one N atom, that is, three weak, two medium strength, and one strong (see Table 2). As mentioned before, the distance between planes based on nitrogen atoms of the neighboring amino groups is also substantially different, being equal to 2.82 Å in TP and 3.78 Å in OP. Consequently, the Cl− layer penetration by amino groups is more significant in TP (the N atoms of a cation are only 0.36 Å away from the chloride layer in TP and 0.69 Å in the second polymorph). On the other hand, this distinctly bigger gap between the chloride layers, not being efficiently compensated by the decrease of the averaged Cl···Cl distance in each layer, causes some discrepancy between the bulk

parameters characterizing the two polymorphs. Nevertheless, the molar volume and density are comparable for both polymorphic structures (d = 1.443 and 1.415 g·cm−3, VM = 295.86 and 301.76 Å3, for TP and OP, respectively at T = 100 K).9 Neutron Geometry. The structures of the TP polymorph derived from routine X-ray and neutron diffraction experiments are comparable. The obvious difference concerns the hydrogen atom positions and their ADPs. All C−H and N−H distances are significantly longer in the case of neutron data due to the different nature of diffraction of both radiations. Therefore, the geometry of hydrogen bonds obtained purely from X-ray data differs from the one obtained after refinement with H atom positions and ADPs taken from neutron experiments; see Table 2 and Table 1 in the Supporting Information. Table 1 contains structural parameters of H-bonds obtained by applying single crystal neutron diffraction only. Hirshfeld Surfaces Analysis. Hirshfeld surfaces19 calculated for the TP and OP polymorphs of benzidine dihydrochloride differ significantly from each other due to the differences in the symmetry and conformation of the benzidine D



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Figure 4. Deformation density maps in planes containing atoms from benzidine cation: (a) TP form − plane N1C1N2 (b) OP form − plane N1C2C1. Contour level 0.05 e·Å−3, red − positive, blue − negative, zero level − black dashed line.

Figure 5. Deformation density maps in planes containing chloride anions: (a) TP form (b) OP form. Contour level at 0.05 e·Å−3, red − positive, blue − negative, zero level − black dashed line. (c) and (d) isosurfaces of deformation density for chloride anions at 0.6 e·Å−3 (always left) and 0.75 e·Å−3 (always right) levels.

which amounts to ca. 5%, while in OP is lower than 2% (1.6%). This results from stacking interactions in both structures.

cation. This is clearly visible in Figure 3. More thorough analysis of the shape of Hirshfeld surfaces, which employs analysis of curvedness mapped on HS (see Figure 1 in the Supporting Information), shows some discrepancies among the stacking interactions between the two studied polymorphs. Additionally, in Figure 3, which illustrates the dnorm mapped on Hirshfeld surface, one can easily notice that the red part of HS creates a completely different layout for both polymorphs. This means that the closest contacts to HS are different for both polymorphs. A histogram of percentage contributions of various intermolecular interaction types is shown in Figure 3d. Both polymorphs are dominated by the H···H contacts (the highest contribution to the Hirshfeld surface - almost 40% of all interactions). The contribution of H···Cl contacts is larger in the case of OP (>26%) than for TP (ca. 23%). A similar trend is present for the C···H interactions (ca. 34.1% for OP and ca. 28% for TP). However, there is a greater contribution of the C···C contacts observed for the triclinic polymorph structure

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CHARGE DENSITY ANALYSIS Quality of Refinements. For both polymorphs the Hirshfeld rigid body test20 was performed during the final refinement stages, and the highest values of differences of mean square displacements amounted to 3 × 10−4 Å2 along the C(7)−C(8) bond for the T polymorph and 3 × 10−4 Å2 along the C(1)−C(2) bond for the O polymorph. The maximum and minimum values of residual density are equal to 0.17 and −0.20 e·Å−3 for TP, and to 0.23 and −0.37 e·Å−3 for the O polymorph. Residual density maps for the benzidinium cation and chloride anion planes for both polymorphs are available from the Supporting Information. The Hirshfeld rigid body test results mentioned above as well as residual density values, maps, and other statistical parameters (R and wR factors) clearly show that the charge density distribution was described correctly for the triclinic form. The residual density values are though greater for OP. Such a difference may result from the E



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Figure 6. The charge density (a) and Laplacian (b) values at BCPs for covalent bonds in the benzidinium cation. The OP polymorph - blue line, the TP polymorph: red line − one-half of benzidinium cation numbered as 1 (containing atoms N1C1C2C3C4C5C6) green line − second half of benzidinium cation numbered as 2 (containing atoms N2C7C8C9C10C11C12). HI_SH, N − refinements against experimental SF; TH − refinements against theoretical SF. Tables containing numerical values and their esds for charge density and Laplacian at BCPs are available from the Supporting Information.

relative crystal stability  crystals of the orthorhombic form, presumably less stable or rather more susceptible to defects, were also of a lower quality than crystals of the triclinic polymorph. Consequently, during multipole refinement we used estimated hydrogen atom positions and ADPs, which may also reduce the quality of the refinement. Nevertheless, the

results obtained for the OP polymorph seem to be quite satisfactory. When the resolution is considered up to 0.9 Å−1, the high angle noise is reduced, and therefore extreme residual density values are decreased, and amount to −0.27 and 0.18 e·Å−3. F



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Table 3. BCP Properties of N−H···Cl Hydrogen Bondsa Symm OP

TP

N1−H1A···Cl1

X, Y, Z

N1−H1B···Cl1

X, 1 − Y, 1/2 + Z

N1−H1C···Cl1

X, 1 + Y, Z

N1−H1A···Cl1

1/2 − X, 1/2 + Y, Z

N1−H1B···Cl1

1/2 − X, 1/2 + Y, Z

N1−H1C···Cl1

1/2 − X, 1/2 + Y, Z

N1−H1···Cl1

1 + X, Y, Z

N1−H2···Cl2

2 − X, 1 − Y, 1 − Z

N1−H3···Cl1

1 − X, 1 − Y, −Z

N2−H4···Cl1

−X, 1 − Y, 1 −Z

N2−H5···Cl2

X, 1 + Y, Z

N2−H6···Cl2

1 − X, 1 − Y, 2 − Z

ρBCP

LaplacianBCP

GBCP

VBCP

HBCP

|VBCP|/GBCP

HBCP/ρBCP

0.175 0.173 0.247 0.242 0.169 0.156 0.022 0.052 0.016 0.052 0.028 0.052 0.171 0.174 0.196 0.210 0.213 0.215 0.193 0.199 0.210 0.196 0.197 0.192 0.161 0.166 0.191 0.185 0.185 0.204

1.322 1.492 1.548 1.617 1.302 1.376 0.757 0.967 0.523 0.967 0.640 0.967 0.816 0.895 1.143 1.033 1.028 1.262 1.119 1.139 1.139 0.919 0.957 1.173 0.760 0.820 1.101 1.221 1.342 1.168

0.016 0.017 0.022 0.022 0.015 0.015 0.005 0.008 0.004 0.008 0.005 0.008 0.012 0.013 0.016 0.016 0.016 0.018 0.015 0.016 0.017 0.014 0.015 0.016 0.011 0.012 0.015 0.016 0.016 0.017

−0.018 −0.018 −0.029 −0.028 −0.017 −0.016 −0.003 -0.005 −0.002 −0.005 −0.003 −0.005 −0.015 −0.016 −0.020 −0.021 −0.022 −0.023 −0.019 −0.020 −0.022 −0.019 −0.019 −0.019 −0.014 −0.015 −0.019 −0.019 −0.019 −0.021

−0.002 −0.001 −0.006 −0.006 −0.002 −0.001 0.002 0.002 0.002 0.002 0.002 0.002 −0.003 −0.003 −0.004 −0.005 −0.005 −0.005 −0.004 −0.004 −0.005 −0.005 −0.005 −0.004 −0.003 −0.003 −0.004 −0.003 −0.003 −0.004

1.125 1.074 1.280 1.251 1.109 1.042 0.557 0.671 0.549 0.671 0.597 0.671 1.290 1.266 1.249 1.330 1.340 1.270 1.247 1.260 1.293 1.330 1.318 1.226 1.279 1.270 1.247 1.188 1.153 1.266

−0.075 −0.048 −0.170 −0.156 −0.066 −0.027 0.740 0.322 0.712 0.322 0.459 0.322 −0.136 −0.131 −0.135 −0.169 −0.174 −0.152 −0.133 −0.141 −0.158 −0.162 −0.159 −0.125 −0.128 −0.128 −0.132 −0.107 −0.092 −0.145

a For each BCP for triclinic polymorph the first row contains the experimental X-ray results, obtained after TP_N refinement, and the second row contains results obtained after TP_HI_SH refinement. For the orthorhombic polymorph the first row contains the results obtained after the OP_HI_SH refinement. The last row (italic) contains the results of refinement against theoretical structure factors. Values of charge density are given in e·Å−3, while of Laplacian in e·Å−5, and all other values are in a.u.

Deformation Density. Deformation density maps (Fcalc, multipole model − F calc, spherical model; see Figure 4) show the concentration of charge in the bonding area in the benzidine cation. One can see that there appear only minor differences between the deformation densities of the TP and OP polymorphs in the benzidine cation region. The static deformation density maps derived for the theoretical charge density distribution, that is, obtained on the basis of theoretical structure factors (SF), are almost identical to the experimental ones (available from the Supporting Information). The charge concentration on chloride anions is evident and clearly seen in the deformation density maps (see Figure 5) and isosurfaces. The deformation density maps suggest that the density associated with the chloride anion is not ideally spherical. For TP, it is polarized in the direction of another chloride anion, whereas for OP, the charge density of Cl− is elongated in the direction of the furthest benzidine cation. A different nature of the Cl− anions in both polymorphs is well illustrated by different shapes of the deformation isosurfaces drawn at 0.75 e·Å−3 and 0.6 e·Å−3 levels (see Figure 5c). BCP Properties. The values of charge density and its Laplacian at the bond critical points (BCPs) for the covalent bonds are presented in Figure 6 for both polymorphs. For the triclinic polymorph, two refinement types were conducted: TP_N and TP_HI_SH. In the case of the orthorhombic

polymorph, neutron data were not collected, so only the OP_HI_SH refinement was performed. Considering the TP structure and the covalent bonds, the values of charge density at BCPs determined without application of the neutron data are similar to those obtained after TP_N refinement. Therefore, one may conclude that for such compounds as benzidine dihydrochloride the HI_SH refinement scheme gives reliable results and may be applied for the orthorhombic polymorph. Such a method of refinement gives adequate results although (a) the N−H bond lengths differ quite significantly (standardized neutron distance equals 1.035 Å, whereas the corresponding value obtained from neutron experiment amounts to ca. 1.050 Å), (b) the ADPs estimated for hydrogen atoms are different from those derived from the neutron experiment (the mean similarity index21 value calculated for them equals 0.43; for details see Table 2 and figures evaluated with the Peanut22 program in the Supporting Information). This may have a significant influence on the final electronic parameters. Comparing the results obtained for both polymorphs, it turns out that in the case of the covalent bonds not involving H atoms, the values of charge density at BCPs for OP seem to be higher than the corresponding values obtained for TP. On the other hand, there are no noticeable differences between the polymorphs, concerning the electron density at BCPs for the covalent bonds to hydrogen atoms. It G



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Figure 7. Maps of Laplacian in planes of selected N−H···Cl hydrogen bonds obtained after refinement against SF from experiment: (a) TP_N, (b) TP_HI_SH, (c) OP polymorph.

total energy density at BCPs (HBCP) are slightly negative, which suggests that all N−H···Cl− interactions are attractive. The ratio of the potential energy density to the corresponding kinetic energy density for all H···Cl− interactions is slightly higher than 1, which together with the negative total energy density (HBCP) and a positive Laplacian at BCPs, means that these interactions have a transition nature between the closed shell (CS) and shared shell (SS).24 However, they are close to the borderline with pure closed shell region. For the N−H···Cl− hydrogen bonds with the hydrogen bond angle lower than 100°, all electronic parameters (LaplacianBCP, HBCP, |VBCP|/GBCP; see Table 3) suggest that they belong to the closed shell interactions. One can easily observe that for the orthorhombic polymorph the N1−H1B···Cl1 interaction is characterized by significantly more negative values of energy density and bond degree (BD) than the two other hydrogen bonds from this polymorph. This means that this particular interaction is stronger and more covalent than the rest of the N−H···Cl− bonds for this structure. In the case of TP, the discrepancies in values of energy density and bond degrees for all N−H···Cl− interactions are smaller  it seems that for this polymorph all the N− H···Cl interactions have similar strength and covalence degree. The values of charge density and Laplacian obtained for H···Cl− BCPs after the refinement against theoretical structure factors are slightly higher than the corresponding values obtained after the refinement on experimental structure factors. Nevertheless, the analysis of energy density values obtained for all N−H···Cl− interactions lead to the same conclusions about bonding type as experiment. In order to better understand the nature of N−H···Cl− interactions in both polymorphs, maps of Laplacian in N− H···Cl− planes were analyzed. The maps of the Laplacian obtained for the N−H···Cl− plane after the TP_N and TP_HI_SH refinements have similar features  the Laplacian of electron density for the hydrogen atoms involved in the N− H···Cl interactions is usually more (N1−H2···Cl2) or less (N1−H3···Cl1) polarized in the direction of the chloride anion (see Figure 7 a,b and the Supporting Information). Comparing the two refinements performed for TP, one can see that polarization seems to be more pronounced in the case of the TP_HI_SH refinement. In the case of the orthorhombic polymorph, the polarization of Laplacian of electron density appears only for the strongest and most covalent interaction, that is, for N1−H1B···Cl1. The Laplacian of electron density of H1B in plane N1−H1B···Cl1 has an unusual shape. This may result from the relatively short distance between the interacting

occurs that the N−H bond critical points for the O polymorph are characterized by significantly lower values of the Laplacian than for the TP form. The values of charge density and its Laplacian at BCPs obtained after refinements against the structure factors taken from theoretical calculations in CRYSTAL09 differ from those derived from experiment for both polymorphs. In general, the values of charge density at BCPs obtained from theoretical calculations are lower for bonds between non-hydrogen atoms and higher for X−H bonds than the corresponding values of charge density at BCPs obtained from experiment. The magnitude of such differences is significant. In the case of benzidine cation covalent bonds, estimated esds for the amount of electron density at BCPs are usually not higher than 0.05 e·Å−1, whereas discrepancies between theoretical and experimental values do not exceed 0.1 e·Å−1. Similar discrepancies between the theoretical and experimental results were reported earlier.4h,23 A comparison of topological parameters obtained from theory for both polymorphs does not show any significant differences at BCPs for C−C and C−N bonds. In fact, the only differences are manifested at the N−H BCPs  the theoretical values of charge density are greater, whereas values of Laplacian for these BCPs are lower for the OP polymorph than for the triclinic polymorph. It is also worth stressing that when one compares two halves of the benzidine cation from the triclinic polymorph (T1 and T2), it turns out that the differences between values of charge density and Laplacian at BCPs are minor, that is, in the range of their esds. N−H···Cl− Hydrogen bonds. Geometrical parameters of hydrogen bonds are listed in Table 2, whereas the BCP properties for the H···Cl− interactions can be found in Table 3. A rough analysis of the HB geometry leads to a few conclusions: first of all, the distances between the donor and acceptor of hydrogen bonds are longer for the TP polymorph than for OP. The hydrogen bonds are less linear in OP than in TP. Additionally, for three hydrogen bonds for the O polymorph, the D−H···A angle is lower than 100°. The shortest distance between the donor and acceptor is observed for the N1−H1B···Cl1 hydrogen bond from OP. Obviously, it does not mean that this particular bond is the strongest one. These are electronic properties, which are crucial for weak interactions. From Table 2, it can be found that for hydrogen bonds, for which the hydrogen bond angle is higher than 100°, all the N−H···Cl− BCPs have similar properties  they are characterized by small values of charge density and small positive values of the Laplacian. In consequence, the values of H



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Figure 8. Maps of Laplacian in planes of selected N−H···Cl hydrogen bonds obtained after the refinement at SF from theory, (a) and (b) TP polymorph, (c) and (d) OP polymorph, (a) and (c) 6-31G** basis set and for Cl and N charges neutral at starting point, (b) and (d) 86311G_apra_from 199325 for Cl− and charges equal −1 for chlorine and +1 for nitrogen at starting point.

Table 4. Valence Populations (Monopole Atomic Charges) and Integrated Atomic Charges for TP and OPa TP_HI_SH

a

TP_N

TP_TH

OP_HI_SH

OP_TH

atom

Pval

AIM

Pval

AIM

Pval

AIM

Pval

AIM

Pval

AIM

Cl1 N1 C1 C2 C3 C4 C5 C6 H1 H2 H3 H4 H8 H9 H10 Cl2 N2 C7 C8 C9 C10 C11 C12 H5 H6 H7 H11 H12 H13 H14 NH3+ NH3+ BE2+

−0.90(2) −0.23(3) 0.14(2) 0.03(2) 0.00(2) 0.04(2) −0.01(2) −0.05(2) 0.29(2) 0.32(2) 0.27(2) 0.03(2) 0.06(2) 0.02(2) 0.04(2) −0.89(2) −0.19(4) 0.11(2) 0.03(2) 0.05(2) 0.05(2) −0.04(2) −0.07(3) 0.24(2) 0.28(2) 0.29(2) 0.03(2) 0.01(2) 0.02(2) 0.04(2) 0.64 0.62 1.79

−0.90 −1.14 0.29 0.03 −0.01 0.03 −0.01 −0.08 0.55 0.54 0.50 0.05 0.07 0.04 0.07 −0.90 −1.10 0.25 0.01 0.05 0.06 −0.04 −0.10 0.52 0.50 0.53 0.07 0.02 0.03 0.07 0.45 0.44 1.79

−0.92(2) −0.26(3) 0.12(2) 0.01(2) −0.02(2) 0.02(2) −0.04(2) −0.09(2) 0.31(2) 0.32(2) 0.28(2) 0.07(2) 0.09(2) 0.06(2) 0.10(2) −0.92(2) −0.22(3) 0.09(2) 0.00(2) 0.03(2) 0.03(2) −0.06(2) −0.09(2) 0.27(2) 0.28(2) 0.30(2) 0.08(2) 0.04(2) 0.05(2) 0.07(2) 0.65 0.63 1.84

−0.92 −1.18 0.27 −0.01 −0.04 0.02 −0.05 −0.14 0.57 0.56 0.52 0.10 0.10 0.08 0.14 −0.93 −1.15 0.24 −0.03 0.02 0.05 −0.07 −0.12 0.54 0.51 0.54 0.12 0.05 0.07 0.11 0.47 0.45 1.85

−0.88 0.04 0.03 0.05 0.04 0.01 0.05 0.03 0.19 0.20 0.19 0.02 −0.01 0.00 0.04 −0.88 0.04 0.02 0.05 0.05 0.01 0.05 0.04 0.19 0.20 0.19 0.03 −0.02 0.00 0.03 0.62 0.62 1.76

−0.88 −0.95 0.20 0.06 0.04 0.01 0.06 0.02 0.45 0.46 0.45 0.03 −0.02 −0.01 0.05 0.88 −0.95 0.19 0.06 0.05 0.02 0.05 0.04 0.45 0.46 0.46 0.04 −0.02 0.00 0.05 0.42 0.42 1.76

−1.08(5) −0.38(8) 0.06(5) −0.02(4) −0.03(4) 0.04(4) 0.07(4) −0.06(4) 0.41(3) 0.42(3) 0.36(3) 0.09(3) 0.07(2) 0.02(3) 0.04(3)

−1.07 −1.08 0.22 −0.03 −0.02 0.09 0.03 −0.06 0.61 0.56 0.55 0.09 0.05 0.02 0.04

−0.95 0.03 0.03 0.03 0.04 0.03 0.03 0.03 0.20 0.22 0.20 0.04 0.01 0.01 0.03

−0.96 −0.98 0.22 0.05 0.05 0.04 0.04 0.04 0.48 0.48 0.47 0.04 0.00 0.00 0.03

0.81

0.63

0.65

0.45

2.16

2.13

1.90

1.93

For TP, results of TP_HI_SH and TP_N refinements are presented.

polarization of Laplacian at hydrogen atoms in the direction of the nearest chloride anion was observed. Therefore, we decided to check the refinement against SF obtained with different ionic basis sets. After such a procedure one can observe a polarization of hydrogen atom Laplacian distribution in the direction of the nearest chloride anions similar to the one obtained from experiment (see Figure 8b,d). Integrated Charges and Electrostatic Potential. Monopole and integrated atomic charges for both polymorphs

atoms or it may be an artifact (for OP, due to the lack of neutron data, ADPs for hydrogen atoms were estimated from SHADE2). The analogous maps of Laplacian of electron density in the N−H···Cl− planes were analyzed for the theoretically derived charge densities. First, the Laplacian maps for the N−H···Cl hydrogen bonds were genereted after refinement against SFs obtained with simple atomic 6-31G** basis set applied for all atoms (see Figure 8a,c). In these maps only minor (or no) I



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Figure 9. Electrostatic potential mapped on 0.067 e·Å−3 isovalue of charge density: (a) and (b) TP_N refinement, (c) and (d) TP_HI_SH, (e) OP − both sides of the moieties. Figures created in the MolIso program.26

However, standard deviations for monopole charge for OP are quite high and greater than for TP. Nevertheless, the conclusion that the −NH3+ interactions with the chloride anions are slightly stronger for OP than for TP is in good agreement with geometrical parameters and Hirshfeld surface analysis  the N···Cl distances in the chloride anion layer are usually shorter for OP than for TP, and the percentage contribution of the H···Cl interactions is higher for OP than for TP. The integrated charges obtained after multipole refinement against the theoretical structure factors are similar to those obtained after refinement against the experimental structure factors. Different charge distributions cause significant differences in the electrostatic potential for TP and OP (see Figure 9). For OP, electrostatic potential in the −NH3+ area is significantly higher than for TP. This means that the decrease of charge density in the −NH3+ area is more pronounced for OP. Lower values at rings of the benzidinium cation for TP are associated with significant stacking type of interactions. The final distributions of electrostatic potential obtained with TP_N and TP_HI_SH refinements appear very much alike.

from TP_N, TP_HI_SH, and OP_HI_SH are listed in Table 4. Differences between the values of charges (monopole and integrated) from the TP_N and TP_HI_SH refinements are small. In general, charges at hydrogen atoms have slightly higher values after the TP_N refinement, whereas at the carbon atoms are lower than those obtained from the TP_HI_SH refinement. Nevertheless, the overall absolute values of both monopole and integrated charges at the benzidinium cation and chloride anions are almost equal to each other after the TP_N and TP_HI_SH refinements (see Table 4). A chloride anion charge should amount to ca. −1.00e, whereas for benzidinium cation - ca. +2.00e for both polymorphs. Indeed, the monopole and integrated charges obtained after AIM analysis are close to −1 for chloride anions and close to +2 for the benzidine cation. However, they are not exactly equal to the formal values. More detailed analysis shows that for TP, integrated charges for the chloride anions are equal to ca. −0.90e, whereas for OP −1.07e, and, in consequence, charges for the benzidinium cation equal to +1.85e (after TP_N refinement) and +1.79e (after TP_HI_SH refinement) for TP and +2.12e for OP. Additionally, the integrated charges at −NH3+ are equal to 0.45 for TP and 0.63 for OP. Monopole charges at −NH3+ are equal to 0.63 and 0.81 for TP and OP. Therefore, we may conclude that the −NH3+ interactions with the chloride anions are slightly stronger for OP than for TP.

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COMPUTATIONAL RESULTS

The quality of the charge density results, and the difficulties in growing OP crystals of the size suitable for neutron diffraction measurement, J



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implied a question concerning the mutual stability of the polymorphic forms. To approach this problem a set of periodic computations was carried out, while the results were related to crystal morphologies and other observations. As mentioned previously, both polymorphic structures are ionic and exhibit layered architectures. They both form well-defined crystals with clearly seen facets and sharp edges (see Figure 11). TP, however, seems to be more stable and better shaped. Cohesive energy was calculated for both TP geometries (TP_N and TP_HI_SH) and for OP. All values derived from CRYSTAL09 periodic computations at the DFT(B3LYP)/6-31G** level of theory

Table 6. Stabilization Energy Values Evaluated for the Neutral BE2+ × 2Cl− Speciesa

Table 5. Cohesive Energy Values Evaluated for the TP Geometries (after TP_N and TP_HI_SH Refinements), and for OP (after OP_HI_SH Refinement) polymorph

cohesive energy (kJ·mol−1)

TPNEUTRON TPSHADE2 OPSHADE2

−1508 −1503 −1493

are quite alike (see Table 5). As an initial guess the nitrogen atom charge was set to +1, while for the chlorine anion it was set to −1. The overall bulk energy, that is, the total energy of a unit cell, referred to a benzidine hydrochloride fragment energy, provided as a sum of two chloride anions and benzidinium cation energy contributions. The Grimme dispersion, when needed, and BSSE correction were also taken into account. For more details, see section 6.2 of the Supporting Information. It seems that TP is slightly more thermodynamically advantageous than OP. Nevertheless, the difference is almost negligible (10−15 kJ mol−1), constituting just a small percentage of the total cohesive energy derived (see Table 5). This result supports the possibility of obtaining both polymorphic forms from one solution, when the crystallization kinetics is not determinant, rather than the better quality of TP crystals and their preferable formation, or specific stabilization, compared to OP. Consequently, a more complex analysis is needed to find an answer to the question regarding crystal qualities. Thus, some additional BE2+×2Cl− crystal stabilization energies, in contrast to the previously calculated cohesive energies, were estimated. This time, the derived total bulk energy per unit cell referred to the energy of the whole neutral salt species (BE2+ × 2Cl−). We checked, how this energy varies, depending on the choice of chloride ions neutralizing the organic cation. The results are shown in Table 6. The distances indicate the extracted moiety from the crystal lattice. It occurs that the stabilization energies are more consistent and slightly lower in the case of TP. TP is generally characterized by a more homogeneous distribution of ionic fragments in its crystal lattice than in the case of OP, and it seems that the obtained stabilization energy values reflect this crystal architecture feature. This may also suggest that the TP crystal lattice arrangement is more rigid, and thus less likely defected. Interlayer interactions constitute another aspect of the two polymorphs worth exploring. As clearly seen in Figure 3, in the case of TP there are well-defined molecular layers parallel to the (001) and (101) crystal planes (also shown in Figure 10), and the analogous supramolecular motifs of OP, parallel to the (100) and to (001) crystal planes, respectively. In both cases molecular layers interacting via pointing out amine groups and chloride anions (slab1) are more weakly stabilized than the second type of slabs (slab2), interacting both ionically and dispersively, with much more developed intermolecular contacts involved and shorter interatomic distances. What is worth noting here is that these interactions are substantially stronger in the case of TP (Table 7), which goes along with the Hirshfeld surface comparison between polymorphs. OP exhibits extremely weak (100) slab contacts, which is in good agreement with the interlayer distances, probably affecting its crystal formation. Such weak interactions along the a crystallographic direction may cause the observed lower crystal quality (leading to more frequent crystal defects), as the adjacent molecular layers might be quite easily

polymorph

H atoms

N(1)···Cl (Å)

N(2)···Cl (Å)

stabilization energy (kJ·mol−1)

TP

H2/H5 H2/H6 H3/H5 H1/H4 H2/H4 H1/H5 H1B/H1B H1B/H1A H1B/H1C H1A/H1C −/−

3.145 3.145 3.185 3.212 3.145 3.212 3.068 3.068 3.068 3.136 3.159

3.181 3.197 3.181 3.233 3.233 3.181 3.068 3.136 3.186 3.186 3.159

−442 −432 −446 −424 −440 −441 −412 −419 −420 −419 −499

OP

a

Different chloride ion choices are defined by the corresponding N···Cl distances and interacting hydrogen atoms. mutually shifted, or even separated under more demanding conditions (tensions, solvation). In general, the interlayer energy values, together with the surface free energy magnitudes, should explain to some extent the crystal shape and its growth tendencies. In the case of benzidine hydrochloride, the estimated surface free energy value is, though, just a rough estimation, provided on the basis of classical approaches concerning atomic surfaces of metals, metal oxides, and other crystals of the type (see section 6.3 of the Supporting Information). Nevertheless, it still brings some information regarding the free energy per molecular area and then the relative surface stabilities. The greater the surface free energy, the less stable is the molecular surface and, thus, the corresponding crystal facets are usually less likely formed. Of course, crystal surface stability in solution is usually highly affected by the solvation effects. Having carefully looked at crystal morphologies and their face indices, one can notice that in both cases the {001} crystal facets are present (see Figure 11). In the orthorhombic polymorph this is the best developed, while in TP it forms a medium size, crystal facet. Indeed, the estimated surface free energy of {001}, derived on the basis of a molecular slab attributed to the (001) crystal plane (for details, see Table 7), is more advantageous for OP. Also the corresponding interlayer interaction energy is more attractive in the case of OP, suggesting a better stability and preferable growth of {001} crystal face. The other facets, described as {21̅1} and {221̅} (Figure K



Article

Figure 10. Miller indices of crystal planes indicating parallel molecular slabs in the TP structure, for which the interlayer interaction energies were evaluated. Slabs are marked with the green colored boxes and red colored ellipses. higher for TP than for OP. The {11̅2} crystal facet is usually the most developed one, while {111} is the smallest. {101} is not formed during the early stage of crystal growth, probably due to the kinetics of crystallization process and unfavorable consequences for the other crystal facets resulting from its formation. The comparison of the crucial interlayer interaction energies and the surface free energy values between both polymorphs shows that in the case of TP crystal faces should be more eager to grow (if the solvation/kinetics is neglected), being slightly less stable than in OP. Lower surface free energy in OP, and especially low interlayer interaction energy of (100) slabs, may hamper further growth of the initially formed crystals. This would to some extent explain the difficulties in obtaining larger crystals of this polymorph. Nevertheless, the overall problem of crystal morphology, crystallization mechanism, and thermodynamics regarding crystal architecture is very complex. On the other hand, crystal thermodynamic properties, solvation effects, and crystallization kinetics in general are all factors responsible for the final product - crystal and its properties. Therefore, each of the information gained is valuable. Even though water molecules present in the crystallization solution may influence the crystal facet formation significantly, the collected energy information is in accordance with the experimental observations. Additionally, interactions with water molecules are less significant that those between benzidine dihydrochloride species, thus no solvent is incorporated in the studied crystal structures. The energy results show that despite the resemblance of the cohesive energy values of polymorphs, the character and features of their crystal architectures may cause significant differences in crystallization mechanisms and crystal quality. Similar cohesive energy values support the fact of a simultaneous crystallization of both polymorphs under the same conditions (assuming similar solvent influence on the crystallization of both polymorphs). On the other hand, our study demonstrates the importance of balanced and advantageous stabilization energy (in the crucial crystallographic directions) for the overall crystal stability (that is, thermodynamic, mechanical etc.), and crystallization kinetics.

Table 7. Interlayer Intercation Energies Calculated per one molecule for Slabs Extracted from the Crystal Structures of TP and OPa crystal plane

interlayer interaction energy (kJ·mol−1)

surface free energy (J·m−2)

OP

(001) (11̅2) (111) (101) crystal plane

−90 −173 −134 −263 interlayer interaction energy (kJ·mol−1)

0.34 0.21 0.23 0.26 surface free energy (J·m−2)

slab1 slab2

(100) (001)

−38 −153

0.16 0.18

TP slab1 slab2 slab3 slab4

a

Miller indices indicate crystal planes parallel to the selected molecular slabs. Surface free energy stands for a rough estimation of the slab surface stability.

11), are though difficult to distinguish from the OP crystal structure. On the other hand, all the faces defining TP crystals can be quite easily referred to its crystal architecture features. Although molecular layer stabilization energies are significant as denoted by CRYSTAL09 computations, the roughly estimated surface free energy values are

Figure 11. Crystals and their face indices for (a) TP and (b) OP. L



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Article

CONCLUSIONS Two known polymorphic forms of benzidine dihydrochloride  triclinic P1̅, and orthorhombic Pbcn  were studied using experimental and theoretical quantitative electron density distributions obtained from the high resolution X-ray diffraction measurements and from periodic ab initio computations. Additionally, in the case of the triclinic polymorph, it was possible to collect neutron diffraction structural data, which was subsequently applied in the high resolution X-ray multipole refinement . Although structures of both forms are quite similar (layered crystal architecture), they differ due to slightly different conformations of the benzidinium cation, different packing, and intermolecular interactions. For example, the distances between the donor and acceptor atoms are longer for TP, whereas hydrogen bond angles are less linear for OP. Hirshfeld surfaces calculated for both polymorphs of benzidine dihydrochloride have also different shapes and curvedness, resulting mainly from differences in stacking interactions. In general, the values of charge density at BCPs are very consistent, except for those obtained from theoretical calculations, which are lower for bonds between non-hydrogen atoms and higher for X−H bonds. The integrated chloride anion charges are equal to ca. −0.90e and −1.07e, for TP and OP respectively. Naturally, the complementary charges for the benzidinium cation amount to +1.85e and +2.12e. Different charge distributions for both polymorphs cause significant differences in the electrostatic potential. For instance, the electrostatic potential of the −NH3+ group is substantially higher for OP than for TP. Lower values of charge density at phenyl rings of the benzidinium cation in the case of TP can be associated with a significant contribution of stacking type of interactions. TP is slightly (ca. 10−15 kJ·mol−1) more thermodynamically stable than OP. On the other hand, this difference constitutes just a small percentage of the total cohesive energy derived. Estimated surface free energy of {001}, and corresponding interlayer interaction energy, evaluated on the basis of a molecular slab attributed to the (001) crystal plane, are more gainful for OP than for TP, thus leading to a better stability and preferable growth of the {001} crystal face. Generally lower surface free energy in OP, and especially low interlayer interaction energy of (100) slabs, may obstruct further growth of the initial crystals. However, one should be aware of the solvation effects which may significantly influence the crystallization mechanism and preferences in crystal facet formation. The simple comparison of the crucial interlayer interaction energies and the surface free energy values between both polymorphs shows that in the case of TP crystal faces should be more able to grow, being slightly less stable than in OP. Generally, the energy results show that despite the comparable cohesive energy values of polymorphs, the features of their crystal networks may cause significant differences in crystallisation mechanisms and crystal quality. Additionally, it appears that there is little correlation between geometrical parameters of hydrogen bonds and their energies.

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densities, and complete table of BCP properties obtained from experiments and computations for both polymorphs as well as cif data. This information is available free of charge via the Internet at http://pubs.acs.org/. CCDC 881695−881697 entries contain the supplementary crystallographic data, cif files, for all three refinements (TP − neutron data refinement, TP and OP multipole refinements). These data can be obtained free of charge from the Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_ request/cif.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS X-ray single crystal measurements were accomplished at the Structural Research Laboratory of the Chemistry Department, Warsaw University, Poland, established with financial support from the European Regional Development Fund in the Sectoral Operational Programme “Improvement of the Competitiveness of Enterprises, years 2004−2006”, Project No. WKP_ 1/1.4.3./ 1/2004/72/72/165/2005/U. A.A.H. and K.W. thank FNP for the Master (“Mistrz” in Polish) subsidy. K.W. acknowledges financial support from the Ministry of Science and Higher Education (now NCN) Grant N N204 135138. ISIS neutron data collection grant 810331 (Hydrogen Bonds in Dihydrated Dichloride) is kindly acknowledged. Additionally, K.N.J. gratefully acknowledges the Wrocław Centre for Networking and Supercomputing for providing computer facilities.

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REFERENCES

(1) (a) Rolf, H. Polymorphism in the Pharmaceutical Industry; Wiley: Weinheim, Germany, 2006; (b) Bernstein, J. Polymorphism in Molecular Crystals; Oxford University Press: Oxford, 2002. (2) Hansen, N. K.; Coppens, P. Testing aspherical atom refinements on small-molecule data sets. Acta Crystallogr. 1978, A34, 909−921. (3) Bader, R. F. W. Atoms in Molecules - A Quantum Theory; Oxford University Press: Oxford, 1990. (4) (a) Kulkarni, G. U.; Kumaradhas, P.; Rao, C. N. R. Charge density study of the polymorphs of p-nitrophenol. Chem. Mater. 1998, 10, 3498−3505. (b) Whitten, A. E.; Dittrich, B.; Spackman, M. A.; Turner, P.; Brown, T. C. Dalton Trans. 2004, 23−29. (c) Overgaard, J.; Hibbs, D. E. The experimental electron density in polymorphs A and B of the anti-ulcer drug famotidine. Acta Crystallogr. 2004, A60, 480− 487. (d) Gopalan, R. S.; Kulkarni, G. U.; Rao, C. N. R. An experimental charge density study of the effect of the noncentric crystal field on the molecular properties of organic NLO materials. ChemPhysChem 2000, 1, 127−135. (e) Munshi, P.; Guru Row, T. N. Topological analysis of charge density distribution in concomitant polymorphs of 3-acetylcoumarin, a case of packing polymorphism. Cryst. Growth Des. 2006, 6 (3), 708−718. (f) Schmidtmann, M.; Farrugia, L. J.; Middlemiss, D. S.; Gutmann, M. J.; McIntyre, G. J.; Wilson, C. C. Experimental and theoretical charge density study of polymorphic isonicotinamide-oxalic acid molecular complexes with strong O···H···N hydrogen bonds. J. Phys. Chem. 2009, A (113), 13985−13997. (g) Munshi, P.; Jelsch, C.; Hathwar, V. R.; Row, T. N. G. Experimental and theoretical charge density analysis of polymorphic structures: the case of coumarin 314 dye. Cryst. Growth Des. 2010, 10, 1516−1526. (h) Hoser, A. A.; Dobrzycki, Ł.; Gutmann, M. J.; Woźniak, K. Similarities and differences in two different polymorphs of hydrated 1,8-bis(dimethylamino)naphthalene hydrochloride (DMANH+x2Cl-xH5O2+); experimental and theoretical charge

ASSOCIATED CONTENT

S Supporting Information *

Details of computations, neutron and X-ray measurements, and multipolar refinement scheme. Figures illustrating properties of Hirshfeld surfaces, distribution of residual and deformation M



Article

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