View Article Online / Journal Homepage / Table of Contents for this issue COMMUNICATION www.rsc.org/crystengcomm | CrystEngComm NO32…NO32 and NO32…p interactions in the crystal of urea nitrate{{ Yulia V. Nelyubina, Konstantin A. Lyssenko,* Denis G. Golovanov and Mikhail Yu. Antipin Downloaded by University of Tennessee at Knoxville on 21 March 2013 Published on 30 August 2007 on http://pubs.rsc.org | doi:10.1039/B709180J Received 18th June 2007, Accepted 23rd August 2007 First published as an Advance Article on the web 30th August 2007 DOI: 10.1039/b709180j The geometrical and energy aspects of the NO32…NO32 and NO32…p-system interactions were analyzed in the framework of the charge density investigation in crystalline urea nitrate. The results obtained have revealed that relatively weak anion– anion and anion–p (cation) contacts contribute greatly to the stabilization of the layered structure for this particular ionic material. It’s commonly assumed that in ionic materials the predominant contacts are cation–anion ones due to the attractive electrostatic component of these interactions. At the same time, there is a number of both experimental and theoretical reports indicating the occurrence in solids of short-range anion–anion contacts that can occasionally be identified on the basis of some physico-chemical data (see, e.g., ref. 1 and references therein). This bond formation is not only a consequence of the geometry of neighboring ions, namely, due to the forced shortened distance, but resulted from the competition among them to attract electrons. The detailed analysis of charge density in ionic crystals has revealed the presence of anion–anion interactions for point-charge ions (such as halides in rock-salts of NaCl type) as well as for rather bulky ones.2 Considering the formally repulsive character of the contacts between two anions one can expect their energy to be extremely small as compared to the cation–anion ones. However, in some cases the crystal packing of the particular ionic material is partly affected by such weak interactions. Thus, the importance of anion–anion contacts in crystalline alkali halides has already been discussed.3 Furthermore, as was recently reported, similar Cl2…Cl2 interactions were found in the crystal of hydroxylammonium chloride.4 The anion–anion contacts playing a significant role in the crystal packing, were also observed for the series of 1,3-dialkyl-4,5-bis(3amino-guanidino)imidazolidine-2-one salts where the formation of centrosymmetric nitrate–nitrate dimers leads to a racemic structure.5 It should be noted that according to the Cambridge Structural Database (CSD) the occurrence of Cl2…Cl2 interactions on the basis of the geometrical criteria can be expected only for 76 compounds, which is not comparable with total number of chlorine salts.4 At the same time, nitrate…nitrate contacts (with a O…O separation within the range of 2.7–3.3 Å) are observed for A. N. Nesmeyanov Institute of Organoelemental Compounds of the Russian Academy of Sciences, 119991, Vavilov Str., 28, Moscow, Russia.. E-mail: kostya@xrlab.ineos.ac.ru; Fax: 495 135 5085; Tel: 495 135 9214 { CCDC reference number 651076. For crystallographic data in CIF or other electronic format see DOI: 10.1039/b709180j { Electronic supplementary information (ESI) available: Additional experimental details. See DOI: 10.1039/b709180j This journal is ß The Royal Society of Chemistry 2007 23% (539 of 2324 ordered structures with the R-factor ,0.075) of nitrate-containing crystals. As a result, based on rough analysis of CSD we can suggest that consideration of NO32…NO32 interactions on both qualitative and quantitative levels is of great importance for the description of the corresponding salts. Apparently, the geometrical data alone cannot serve as an unambigious indicator of chemical bonding. It is important to indicate that earlier the interionic O…O bonds were reported and analyzed in the crystal of danburite (CaB2Si2O8),6 where 11 bonding interactions for the O…O distance in the range of 2.32–3.36 Å were found. Moreover, the trinitromethanide C(NO2)32 and dinitramide N(NO2)22 ions are known to exhibit the intraionic O…O bonds between NO2-groups.7 In order to analyze whether the above NO32…NO32 contacts are bonding or are repulsive in nature, in the current communication we performed a detailed charge density r(r) investigation in the crystal of urea nitrate (1). The choice of 1 was due to the presence in the solid state of NO32…NO32 interactions (the O…O separation is close to the average value for the above distance range) as well as the availability of the crystalline material.8 The additional interest to 1 was caused by the close resemblance of the cation under consideration and the guanidinium one in corresponding salts, which was proven to be a very effective receptor for anions.9 Another important point is the occurrence of a nitrate…p-system of urea contacts in the crystalline 1. Moreover, the latter is geometrically similar to an analogous nitrate…guanidinium interaction found in the crystal of tetrakis(guanidinium) tris(3-sulfonatophenyl)phosphine nitrate.10 The former is of interest as a model of an anion…p system (see ref. 11). The fact that the nitrate anion in 1 simultaneously participates in the formation of two different types of interactions gives us the possibility to evaluate their energetics and analyze their role in the crystal structure formation. To accomplish this we performed the topological analysis of the r(r) function in 1 by means of Bader’s Atoms in Molecule (AIM)12 theory on the basis of the high-resolution X-ray diffraction (XRD) data.13 In accordance with previous investigations8 the crystal of 1 is ‘‘built’’ by NO32 anions and urea cations protonated via O(1) atoms. The latter results in the elongation of the CLO bond up to 1.3077(3) Å as compared with crystalline urea (1.2565(5) Å).14 The difference in bonding graphs of cation 1 and urea molecule also affects the C–N separation, which is equal to 1.3173(4) and 1.3211(4) Å in the case of the former and 1.3384(4) Å for the latter. The ions in the crystal of 1 are held together by the O–H/N– H…O hydrogen bonds of intermediate strength (the O/N…O distances are 2.6037(4)–2.9688(4) Å with the smallest value in the case of the O(1)–H(1O)…O(4) one) thus forming the H-bonded sheets in such a way that any ion is surrounded by 4 counter-ions CrystEngComm, 2007, 9, 991–996 | 991 Downloaded by University of Tennessee at Knoxville on 21 March 2013 Published on 30 August 2007 on http://pubs.rsc.org | doi:10.1039/B709180J View Article Online Fig. 1 The fragment of the H-bonded sheet of 1 in the atom representation by thermal ellipsoids at p = 80%. The additional O…N contacts are drawn by green dash lines. and 2 ones with the same charge (Fig. 1). The realization of the former also leads to the occurrence of additional O(1)…N(1) contacts (the N…O separation is 3.0312(4) Å). The above supramolecular associates are assembled into a 3-D framework through the nitrate…nitrate and nitrate…p-system interactions. Analyzing the bonding pattern in 1 more closely one can bring up the urea+…(NO32)2…urea+ ‘tetramers’ (Fig. 2A) each interlinking four H-bonded sheets that leads to the layered structure where the sheets are shifted relative to the neighboring ones (Fig. 2B). It is important to indicate that the corresponding interatomic distances O(3)…O(39) and O(3)…C(10) are very close to each other and equal to 3.0273(4) and 3.0201(4) Å, respectively. For comparison, the O…C separation in the crystal of tetrakis(guanidinium) tris(3-sulfonatophenyl)phosphine nitrate10 is 3.087 Å. Therefore, based on the geometrical parameters of these contacts and the fact that they play the same role in the formation of crystal Fig. 2 The tetramer formed through the NO3…NO3 and NO3…p-system contacts (A) and the fragment of the crystal packing for 1 representing the formation of layered structure (B). 992 | CrystEngComm, 2007, 9, 991–996 packing of 1, one can expect their strength to be very similar. As a consequence, it was the point of interest to investigate whether the NO32…NO32 and NO32…p-system contacts observed in the solid correspond to attractive interactions or, in contrast, are forced in nature. To accomplish this, we have performed the search for bond critical points CPs (3, 21) (hereinafter BCPs) in the interatomic area. Thus, the values of the r(r) function in the BCPs for C–N/C– O bonds (2.503–2.536/2.402 e Å23) in 1 as well as the ellipticity (e), which serves as a measure of the p component, indicate the partial double-bond character of the C–N ones (0.29–0.30) while the C(1)–O(1) bond (0.14) is almost a single one. For comparison, the C–N/C–O bonds in the urea molecule16 are characterized by nearly equal values of both r(r) and e being 2.538/2.536 e Å23 and 0.19/0.15, respectively. As was expected, the BCPs were localized for all of the above H-bonds as well as for shortened O(3)…O(39) and O(3)…C(1) contacts in the crystal of 1. The latter can be considered as an O…p interaction because the CLN bonds in urea cation exhibit a pronounced degree of double bond character and the lone pair of oxygen is directed to the p-orbital of the carbon atom (see ref. 11). In addition, a small number of BCPs attributed to the considerably long cation–cation and anion–anion interactions was also found (Table 1). In line with the topological parameters in the corresponding BCPs the interionic interactions are all of a closed-shell type (the positive +2r(r) values vary in the broad range of 0.23–2.73 e Å25, and electron energy densities he(r) are 0.00069– 0.00567 a.u.) with the exception of the O(1)–H(1O)…O(4) H–bond (+2r(r) = 5.75 e Å25, he(r) = 20.00227 a.u.) corresponding to the intermediate type of interatomic interactions (Table 1). It is to be mentioned that in the case of crystalline urea14,15 where the molecules are assembled by N–H…O and weak N…N contacts the absence of an acidic proton at the carbonyl group results in predominantly electrostatic nature of the hydrogen bonds. Along with the BCPs we have also located a number of ring CPs (3, +1) in the centers of H-bonded cycles, such as those observed for the cation–anion pair (Fig. 3), and cage CPs (3, +3) caused by the three-dimensional framework in the solid of 1. The characteristic set of critical points satisfies the Morse equation.12 The chemical bonding pattern for 1, in particular, the formation of the above ‘tetramer’ and occurrence of unusual nitrate…nitrate and urea+…urea+ contacts, is also sustained by the ‘‘easy-to-see’’ description of the r(r) distribution provided by the qualitative analysis of the deformation electron density (DED). Thus, the DED distribution in the plane of the ionic H-bonded pair was found to be characterized by the expected features (Fig. 3). The DED peaks attributed to electron lone pairs (LPs) are located in the vicinity of each oxygen atom and directed toward the hydrogens of NH2-fragments that is common for the H-bonds.16 The depletion of the density between the interacting atoms is indicative of the substantial contribution of the electrostatic component. The analogous trend is observed for the ‘tetramer’ in the plane formed by N(3)O(3) groups of both nitrates. In this case the corresponding contacts can be described in terms of the ‘‘peak-tohole’’ formalism, i.e. the DED peaks localized along the line of the N–O bond (Fig. 4A) and LP of the O(3) atom (Fig. 4B) are directed toward the areas of the electron density depletion near the oxygen and the carbon atoms, respectively. As a result of such This journal is ß The Royal Society of Chemistry 2007 View Article Online Downloaded by University of Tennessee at Knoxville on 21 March 2013 Published on 30 August 2007 on http://pubs.rsc.org | doi:10.1039/B709180J Table 1 Topological parameters of the r(r) in the CPs (3, 21) for the intermolecular contacts in the crystal of 1 Interaction R/Å r(r)/e Å23 +2r(r)/e Å25 2v(r)/a.u. he (r)/a.u. Econt/kcal mol21 O(2)…H(1NB) O(3)…H(2NB) O(4)…H(1OA)a O(2)…H(1NAB) O(3)…H(2NAC) O(1)…N(1D) O(3)…O(39) O(3)…C(10) O(4)…O(3B) O(4)…O(2-) O(1)…N(1E) O(4)…H(2NBC) O(4)…H(2NB9) O(2)…H(2NAF) N(1)…H(2NBF) 1.957(5) 1.905(4) 1.597(4) 1.959(5) 1.967(5) 3.0312(4) 3.0273(4) 3.0201(4) 3.0814(4) 3.4776(4) 3.2602(4) 2.677(5) 3.006(5) 3.095(5) 3.100(5) 0.126(1) 0.183(1) 0.364(1) 0.114(1) 0.140(1) 0.047(1) 0.048(1) 0.051(1) 0.027(2) 0.014(2) 0.037(2) 0.047(2) 0.028(2) 0.020(2) 0.035(2) 2.73(1) 2.53(1) 5.75(1) 2.69(1) 2.56(1) 0.89(1) 0.81(1) 0.73(1) 0.55(2) 0.23(2) 0.53(2) 0.68(1) 0.40(2) 0.35(2) 0.46(2) 0.01703 0.02278 0.06419 0.01570 0.01787 0.00450 0.00429 0.00418 0.00247 0.00098 0.00281 0.00385 0.00200 0.00155 0.00249 0.00567 0.00174 20.00227 0.00608 0.00433 0.00234 0.00203 0.00168 0.00160 0.00069 0.00136 0.00162 0.00105 0.00102 0.00112 5.3 7.2 20.1 4.9 5.6 1.4 1.4 1.3 0.8 0.3 0.9 1.2 0.6 0.5 0.8 a The H(1AO) atom is obtained from the basic one by the symmetry operation x, y 2 2, z; H(1NAB) and O(3B) by 2x + 2, y 2 0.5, 2z + 1.5; H(2NAC) and H(2NBC) by 2x + 1, y 2 0.5, 2z + 0.5; N(1D) by 2x + 2, y + 0.5, 2z + 1.5; O(39) and H(2NB9) by 2x + 1, 2y, 2z + 1; C(10) by x, 2y + 0.5, z 2 0.5; O(2-) by x, 2y 2 0.5, z 2 0.5; N(1E) by 2x + 2, 2y + 1, 2z + 1; H(2NAF) and H(2NBF) by x, 2y + 0.5, z + 0.5. Along with the anion–anion contact, nitrate participates in the formation of the bifurcated interaction with the next-nearest NO32 neighbors (Fig. 5A). The O…O separations for O(4)…O(3B) and O(4)…O(2-) contacts are equal to 3.0814(4) and 3.4776(4) Å. The examination of the DED distribution in the region of the latter (Fig. 5B) has revealed that both O…O contacts are, to some Fig. 3 The static deformation electron density in the plane of the cation– anion pair in the crystal of 1. Contours are drawn with a 0.1 e Å23 interval, the negative contours are dashed. a similarity, the described supramolecular associate is an ideal system for the comparative analysis of the analogous by nature but different by charges nitrate…nitrate and nitrate…p-system types of interactions at the quantitative level. Fig. 4 The DED in the area of the O(3)…O(39) (A) and O(3)…p-system (B) interactions. Contours are drawn with 0.1 e Å23 interval, the negative contours are dashed. This journal is ß The Royal Society of Chemistry 2007 Fig. 5 The fragment of the crystal packing in 1 representing the formation of additional nitrate…nitrate contacts (A) and the DED in the plane formed by O(4), O(2-) and O(3B) atoms of NO2-groups (B) (contours are drawn with 0.1 e Å23 interval, the negative contours are dashed). CrystEngComm, 2007, 9, 991–996 | 993 Downloaded by University of Tennessee at Knoxville on 21 March 2013 Published on 30 August 2007 on http://pubs.rsc.org | doi:10.1039/B709180J View Article Online extent, of a ‘‘peak-to-hole’’ type. However, since the degree of their directionality is open to questions, one can assume that the O(4)…O(2-) interaction is much weaker than the second one which, in turn, should be extremely weak as compared to the H-bonds in the crystal of 1. It is to be noted that the former interlinks the H-bonded sheets and can thus be the consequence of the layer structure of 1. Furthermore, it seems that even the O(4)…O(3B) contact is resulted from the ‘‘constricting effect’’ of the relatively strong H-bonds. In addition to these interlayer contacts the BCP and its associated bond path were located for the cation–cation O(1)…N(1E) interaction with the O…N distance equal to 3.2602(4) Å. For comparison, see the above value in the case of the similar O…N interaction between the urea fragments lying in the plane of the sheet. Accordingly, the interatomic separation for the O(4)…O(2-) contact is considerably smaller than for the O(1)…N(1E) one. Moreover, in contrast to the described O…O interaction, the DED distribution in the area of the latter (Fig. 6) is attributed to the ‘‘peak-to-peak’’ type, i.e. overlap of two electron lone pairs. The possibility to estimate the integrated strength of the interionic contacts can be provided by the evaluation of charge transfer accompanying their formation. To accomplish this we determined the atomic basins (V) surrounded by a zero-flux surface and integrated r(r) over V to obtain atomic charges. Although the integrated Langrangian (L(r) = 21/4+2r(r)) for every V has to be zero,12 reasonably small numbers with an average value of 0.8 6 1024 a.u. and maximum L(r) observed for the C(1) atom (2.8 6 1024 a.u.) were obtained. The atomic charges estimated according to this procedure lead to the value of charge transfer 0.53 e with the charge leakage equal to 0.001 e. Such redistribution of charge density indicates that the cation– anion interactions are characterized by the relatively high energies. It should be noted that the charge transfer in a recently analyzed hydroxylammonium chloride4 was 0.72 e. The absolute values of atomic charges for 1 in comparison with those for the crystalline urea can give us more specific information about the bonded pattern in 1. Thus, the obtained values in the case of the cation 1/urea molecule14 were found to be 20.97/21.18 e for oxygen; 1.38/1.67 e for carbon; 21.03 and 20.95/21.21 e for nitrogen and 0.36 4 0.43/0.4840.49 e for hydrogen atoms, respectively. The hydrogen atom of the OH group is characterized by the maximum positive value equal to 0.45 e. Fig. 6 The DED in the area of the O(1)…N(1E) interaction. Contours are drawn with 0.1 e Å23 interval, the negative contours are dashed. 994 | CrystEngComm, 2007, 9, 991–996 The difference for the O and N charges agrees well with the occurrence of electrostatic O…N contacts, which can be described as a transfer of the oxygen’s LP to the nitrogen, e.g. for the O(1)…N(1E) contact the ONC angle is 78.1(2)u. A similar tendency is observed for the C(1) atom due to the transfer of electron density from the anion. The variation of charges in the case of the N and H atoms apparently results from the formation of N–H…O bonds surpassing those in an urea crystal by both number and strength.14,15 For information, the values of the atomic charges in the nitrate anion are +0.58 e in the case of N(3) and 20.33, 20.35 and 20.37 e for O(4), O(2) and O(3) atoms. It should be noted that the latter correlates with the difference in the atomic energy values (Eat) obtained by integration of he(r)12 over V for the corresponding oxygens (DEat between the O(2)/O(3) and O(4) atoms is 20.096/20.183 a.u.). In addition to the atomic charges in the crystal of 1, we also estimated the atomic volumes according to an analogous procedure. Since the integration was carried out numerically, it is important to indicate that their sum in the crystal (117.02 Å3) reproduces the unit cell volume per molecule (117.17 Å3) with the relative error 0.1%. It is to be mentioned that the total volumes of NH2 groups (22.13 and 21.96 Å3 for N(1) and N(2)) are smaller than those of the urea molecule (25.4 Å3).14 This also proves the multiplicity of contacts formed by the latter. However, drawing any conclusion based on the comparison of the same values for the CO fragment (20.40 Å3 in 1 and 21.93 Å3 in urea crystal) is complicated by the protonated character of urea in the solid 1. The energy of the above interionic contacts (Econt), in particular, the nitrate…nitrate and urea+…urea+ ones, in the crystal of 1 was estimated by means of Espinosa’s correlation scheme17—the semiquantitive relationship between Econt and the value of the v(r) function in the BCP. Thus, the Econt for N–H…O hydrogen bonds varies in the range of 0.5–7.2 kcal mol21 (Table 1) with the greatest one corresponding to the shortest contact. The maximal Econt (20.1 kcal mol21) is observed in the case of the O(1)– H(1)…O(4) interaction. It is to be mentioned that the energy of intermolecular H-bonds in the crystalline urea amounts to 4.6 kcal mol21.14 The values of Econt for the additional anion– anion (Fig. 5) and cation–cation (Fig. 6) interactions in 1 were found to be 0.3–0.9 kcal mol21. As expected, the energies of the O(3)…O(3) and O(3)…p-system contacts are very similar and equal to 1.4 and 1.3 kcal mol21. For comparison, the Econt of the O…O interactions in danburite6 is in the range of 1.1– 16.3 kcal mol21 with the maximum one for the shortest (2.32 Å) distance. In the case of the crystalline MgO the corresponding value (O…O separation is 2.98 Å)2 amounts to 3.2 kcal mol21. Such a decrease of the O…O energy in 1 as compared to MgO and danburite is clearly a consequence of the lower charge density on oxygens in the NO32 moiety, i.e. a less effective overlap of valence shells between the O atoms. The obtained Econt values lead to the value of the total energy for nitrate…nitrate, urea+…urea+ and nitrate…urea+ interactions equal to 2.4, 3.1 and 46.8 kcal mol21 which is in good agreement with the charge transfer accompanying the formation of interionic H-bonds. An analogous tendency is observed for the strength of the contacts which participate in the formation of the H-bonded sheets (47.3 kcal mol21), and those interlinking the above associates (5.0 kcal mol21). However, the latter amounts to 11% of the overall Econt for the sheets and is thus independent of the This journal is ß The Royal Society of Chemistry 2007 Downloaded by University of Tennessee at Knoxville on 21 March 2013 Published on 30 August 2007 on http://pubs.rsc.org | doi:10.1039/B709180J View Article Online charge of interacting species. It is to be mentioned that, due to the ionic character of 1, the sum of estimated energies, in contrast to the molecular crystals (see ref. 18), cannot be directly compared with the lattice energy. The latter by definition includes the large contribution from the charge redistribution upon the dissociation of crystalline solids.19 Moreover, the other energy parameters— those due to dispersion and repulsion—must also be taken into consideration. Thus, according to both the qualitative analysis of the charge density distribution and the energetics of the chemical bonding in 1 the studied structure can be described as a layered one, with the energy of the contacts in the sheets being 10 times greater than those of the interlayer. Furthermore, the bonding between layers in 1 on the quantitative level is comparable with the graphite one. On the other hand the ideal complementarity of urea+ and nitrate ions in the layer allows us to expect the presence of such associates in the corresponding solutions. As a result, one can assume that the urea nitrate serves as a suitable model for investigation of the graphite intercalation and/or forms cocrystals of graphite intercalate type. Topological analysis of the electron density distribution in the crystal of 1 indicates that the occurrence of NO32…NO32 bonds is not a manifestation of the ‘‘constricting effect’’ of the strong cation…anion contacts but resulted from not fully electrostatic nature of interaction between like-charged moieties. At the same time, the statistical analysis of CSD performed has revealed a more pronounced tendency of the nitrate salts to display such type of contacts in comparison with chloride-containing compounds. Accordingly, the nitrate…nitrate and nitrate…p-system interactions affect the crystal packing of the particular ionic material and even contribute greatly to the stabilization of its layered structure. Therefore, in spite of the considerable weakness of anion–anion interactions they may and do play an important role in the formation of a particular crystal pattern and thus determine the mutual disposition of strongly bounded supramolecular entities. Experimental Crystals of 1 (CH5N3O4, M = 123.08) are monoclinic, space group P21/c, at 100 K: a = 7.9761(1), b = 8.2004(1), c = 7.3668(1) Å, b = 103.414(1)u, V = 468.697(10) Å3, Z = 4 (Z9 = 1), dcalc = 1.744 g cm23, m(Mo Ka) = 1.74 cm21, F(000) = 256. The intensities of 33142 reflections were measured with a Bruker SMART APEX2 CCD diffractometer [l(Mo Ka) = 0.71072 Å, v-scans, 2h , 110u] and 5960 independent reflections [Rint = 0.0320] were used in further refinement. The structure was solved by a direct method and refined by the full-matrix least-squares technique against F2 in the anisotropic–isotropic approximation. The hydrogen atoms were located using the Fourier synthesis of electron density and refined in the isotropic approximation. For 1 the refinement converged to wR2 = 0.0985 and GOF = 1.000 for all independent reflections (R1 = 0.0330 was calculated against F for 4570 observed reflections with I . 2s(I)). All calculations were performed using SHELXTL PLUS 5.0. The multipole refinement was carried out within the Hansen– Coppens formalism20 using the XD program package21 with the core and valence electron density derived from wave functions This journal is ß The Royal Society of Chemistry 2007 fitted to a relativistic Dirac–Fock solution.22 Before the refinement, the O–H and N–H bond distances were normalised to the values obtained from the neutron data.8 The refinement was carried out against F and converged to R = 0.0229, Rw = 0.0221 and GOF = 0.68 for 4526 merged reflections with I . 3s(I). The total electron density function was positive everywhere and the maxima of residual electron density located in the vicinity of nuclei were not more than 0.09 e Å23. Topological analysis of the experimental r(r) function was carried out using the WINXPRO program package.23 Acknowledgements This study was financially supported by the Russian Foundation for Basic Research (Project 06-03-32557), the Foundation of the President of the Russian Federation (Federal Program for the Support of Leading Scientific Schools, Grant NSh 1060.2003.30, and Young Doctors, Grant MK-1054.2005.3) and the Russian Science Support Foundation. Notes and references 1 M. A. Carvajal, I. Garcia-Yoldi and J. J. Novoa, J. Mol. Struct. (THEOCHEM), 2005, 727, 181. 2 S. Gottlicher and A. Vegas, Acta Crystallogr., Sect. B, 1988, 44, 362; V. Tsirelson, Yu. Abramov, V. Zavodnik, A. Stash, E. Belokoneva, J. Stahn, U. Pietsch and D. Feil, Struct. Chem., 1998, 9, 249. 3 L. Pauling, The nature of the chemical bond, and the structure of molecules and crystals, 3rd edn, Cornell University Press, Ithaca, NY, 1960; A. M. Pendás, A. Costales and V. Luaña, J. Phys. 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Hansen, XD – A Computer Program Package for Multipole Refinement 996 | CrystEngComm, 2007, 9, 991–996 This journal is ß The Royal Society of Chemistry 2007