J. Phys. Chem. B 2000, 104, 10191-10195
10191
Modeling Polymorphism-Solvated Supramolecular Clusters Reveal the Solvent Selection of SHG Active and Inactive Dimorphs Sonika Sharma and T. P. Radhakrishnan* School of Chemistry, UniVersity of Hyderabad, Hyderabad 500 046, India ReceiVed: May 22, 2000
Dimorphic forms of 4-(4-hydroxypiperidinyl)nitrobenzene which are second harmonic generation (SHG) active and inactive are investigated. Crystallization from a variety of solvents clearly demonstrate the preferential formation of the centrosymmetric structure in polar solvents and the noncentrosymmetric structure in nonpolar solvents. Semiempirical quantum chemical calculations including solvation effects on supramolecular fragments of the crystals reveal the solvent selection of the crystalline forms. The potential of this approach to address the general problem of polymorphism in molecular materials is indicated.
Introduction Polymorphism has engaged continued attention because of its implications to areas as diverse as the pharmaceutical industry and materials technology besides its critical relevance to the design of crystal architecture.1 Subtle thermodynamic or kinetic factors tilt the choice in favor of one crystalline form or the other with vital consequences for the solid-state properties. A striking example is the case of centrosymmetric and noncentrosymmetric polymorphs which are incapable or capable, respectively, of even order nonlinear optical responses such as second harmonic generation (SHG).2 Though the general problem of polymorphism has been investigated extensively, the specific factors that drive this phenomenon are poorly understood in most cases. Modeling of crystal structures continues to be a challenge since ab initio computations at the crystal level are expensive and methods based on packing potential energy and force fields are empirical.3 In the case of organic crystals, semiempirical computations at the molecular level may be combined with molecular dynamics simulations of crystal structures.4 The pertinent question of structural difference between the gas and crystalline phases invariably arises in all the approaches. We have considered a significant variation on this theme with specific reference to crystal growth from solution, the common technique for the fabrication of organic crystals. Molecular aggregates provide valuable insight into the bulk crystal structure and the problem of polymorphism.5 We start with the premise that the crystal structure that ultimately evolves is determined as a molecular aggregate grows to a critical size in the solVent medium. Such molecular assemblies should be quite amenable to computational modeling, including solvation effects, and should therefore provide useful insight into the bulk structural preferences. A convenient system for testing the approach outlined above has been realized in the solvent-based dimorphism we discovered in 4-(4-hydroxypiperidinyl)nitrobenzene (HPNB). This material shows the interesting coexistence of conformational polymorphism and conformational isomorphism.6 One crystalline form of this push-pull molecule is centrosymmetric, whereas the other is noncentrosymmetric and exhibits SHG. * To whom correspondence should be addressed. Fax: 91-40-3012460. E-mail:
[email protected].
Most significantly, a systematic and detailed investigation of the crystallization of HPNB in a variety of solvents revealed a clear preference in terms of solvent polaritysthe centrosymmetric and noncentrosymmetric structures are obtained from polar and nonpolar solvents, respectively. We show that the energetics of supramolecular fragments of the crystals modeled using the semiempirical AM1 method,7 including solvation corrections,8 reflect accurately the solvent selection of the polymorphic structures. Experimental and Theoretical Methods HPNB was synthesized following standard procedure9 by the direct addition of 4-hydroxypiperidine to 4-fluoronitrobenzene in potassium carbonate-dimethyl sulfoxide (DMSO). The details of characterization are provided in the Supporting Information. This compound shows good solubility in a wide variety of solvents and was recrystallized from chloroformtoluene, chloroform, toluene, benzene, carbon tetrachloride, water, methanol, ethanol, and acetonitrile to examine the solvent selection of the polymorphic structures. X-ray diffraction data were collected on an Enraf-Nonius MACH3 diffractometer. Data were reduced using Xtal3.4;10 Lorentz and polarization corrections were included. Nonhydrogen atoms were found using the direct method analysis in SHELX-97,11 and after several cycles of refinement, the positions of the hydrogen atoms were calculated and added to the refinement process. The hydroxy H atom in the centrosymmetric crystal was clearly seen in the Fourier map: hence, it was picked and its position refined; the resulting structure was nearly the same as the one where the H atom position was calculated and added to the refinement. Details of the data collection, solution and refinement, anisotropic thermal parameters, and full lists of bond lengths and angles are deposited as Supporting Information. Second harmonic generation from microcrystalline powders was examined using the Kurtz-Perry12 method. Particle sizes were graded using standard sieves. The fundamental beam (1064 nm) of a Q-switched nanosecond-pulsed Nd:YAG laser was used, and the second harmonic signal was detected using a photomultiplier tube (Hamamatsu), monochromator (Jobin-Yvon Model HRS-2), and oscilloscope (Tektronix, Model TDS 210,
10.1021/jp001864+ CCC: $19.00 © 2000 American Chemical Society Published on Web 10/19/2000
10192 J. Phys. Chem. B, Vol. 104, No. 44, 2000
Sharma and Radhakrishnan
60 MHz). SHG was calibrated against signals from microcrystalline urea having a particle size > 150 µm. Computations were carried out using the AM1 method implemented in the MOPAC9313 program package. Solvation effect was included using the conductor-like screening model (COSMO).8 The number of geometric segments per molecule (NSPA) was kept at 60 for all of the calculations. The value of the dielectric constant (EPS) was set equal to 78.39 for water and 2.379 for toluene. Single point calculations at the molecular geometry from the crystal structure as well as partial optimization studies were carried out. The PRECISE option was employed in all the optimizations. Since the geometries in these calculations are not stationary points, thermodynamic calculations are not reliable; hence, enthalpies instead of free energies are used to discuss relative stabilities. Single point ab initio calculations were carried out at the HF/6-31G* and B3LYP/631G* levels using the Gaussian94 program package.14 Solvation calculations used the Onsager as well as the polarized continuum model (PCM). Complete computational data and the partially optimized geometries are provided in the Supporting Information. Results and Discussion HPNB crystals grown in 2-propanol have been reported15 to be centrosymmetric. We observed weak SHG in HPNB crystallized from chloroform-toluene16 suggesting the presence of a noncentrosymmetric polymorph. Careful examination of these crystals revealed two morphologies, needles incapable of SHG (HPNB-C) and plates showing SHG (HPNB-N). Crystallization from polar solventsswater, acetonitrile, methanol, and ethanols gave SHG inactive material, whereas nonpolar solventss toluene, benzene, and carbon tetrachloridesyielded exclusively SHG active material. Chloroform is found to be a borderline solvent, providing a mixture of the two. These experiments clearly showed that the solvent polarity is the driving force behind the selection of the polymorphic structure. The crystallization from acetonitrile helped to rule out the possibility that the H-bonding nature of the solvent is responsible for the formation of the SHG inactive structure. We have carried out a detailed Kurtz-Perry experiment on microcrystalline powder obtained from toluene; it showed a phase-matchable SHG of 3 U (1 U ) SHG of urea). HPNB crystallized from benzene and carbon tetrachloride showed similar SHG. We have determined the crystal structures of the two polymorphs. HPNB-C belongs to the P21/c space group17 and has the reported structure.15 HPNB-N belongs to the noncentrosymmetric P21 space group with two molecules in the asymmetric unit.17 Neither structure includes solvent molecules. Several crystals grown from water and toluene were examined, and they indexed to the unit cells of HPNB-C and HPNB-N, respectively. In all the molecular structures (Figure 1), piperidinyl N shows a small pyramidalization based on which nitrophenyl group is designated as axial in HPNB-C and as axial and equatorial in the two molecules in HPNB-N. Including the hydroxy group orientation, the conformer in HPNB-C can be labeled (a,e) and those in HPNB-N as (a,a) and (e,e). HPNB-C shows H-bonded chains along the [101] direction (O8- - -H16′ ) 2.020 Å, O8- - -H16′-O16′ ) 169.1°). A Coulombic interaction between the nitro group N atom and the hydroxyl group O atom (N7‚‚‚O16′′ ) 3.254 Å) may be visualized as linking adjacent H-bonded chains leading to a sheet formation in the ac plane (Figure 2). The adjacent layers are related by the center of inversion. In HPNB-N H-bonded chains of alternating (a,a) and (e,e) conformers run in the [101h] direction
Figure 1. Molecular geometries of HPNB from single-crystal X-ray study. (a, top) The (a,e) conformer in HPNB-C (dihedral angle C15N10-C1-C11 ) +171.2°), (b, middle) the (a,a) conformer in HPNB-N (dihedral angle C15-N10-C1-C11 ) +151.2°), and (c, bottom) the (e,e) conformer in HPNB-N (dihedral angle C31-N26-C17-C27 ) -155.1°).
(Figure 3). The H bonds are alternately formed by the axial hydroxy group (O25- - -H16′ ) 2.107 Å, O25- - -H16′-O16′ ) 162.6°) and the equatorial hydroxy group (O8′′- - -H32 ) 2.154 Å, O8′′- - -H32-O32 ) 156.8°) with the nitro groups of the near neighbor molecules. These chains in the ac plane are related by the screw rotation to chains running in the opposite direction in a parallel plane. The nearly antiparallel orientation of the major hyperpolarizability components of molecules in adjacent planes leads to the weak SHG in HPNB-N. We have carried out semiempirical AM1 as well as ab initio calculations on the molecular geometries extracted from the crystal structures. The enthalpy or energy are found to increase as, (a,e) < (a,a) < (e,e) (Table 1). Correction for solvation (water
Solvent Selection of SHG Dimorphs
J. Phys. Chem. B, Vol. 104, No. 44, 2000 10193
TABLE 1: AM1 and ab Initio Calculated Dipole Moments, µ, of Different Conformations of HPNB in the Gas Phase and Enthalpy of Formation, ∆Hf (or Energies, E), in the Gas Phase and with Water and Toluene Solvationa method (solvation model) AM1 (COSMO) HF/6-31G* (PCM) [Onsager]b
B3LYP/6-31G* (PCM)
∆Hf (kcal/mol) [E (Hartrees)]
conformer
µ (D) (gas phase)
gas phase
water
toluene
(a,e) (e,e) (a,a) (a,e)
9.019 8.074 7.614 9.817
158.8 167.8 167.0 -757.860 155
(e,e)
8.591
-757.847 179
(a,a)
8.290
-757.848 356
(a,e) (e,e) (a,a)
9.747 8.600 8.430
-762.454 097 -762.437 616 -762.440 062
137.7 146.7 147.1 -757.884 908 [-757.872 993] -757.871 071 [-757.857 981] -757.870 203 [-757.856 897] -762.476 627 -762.459 246 -762.459 876
149.5 158.5 158.2 -757.867 147 [-757.865 503] -757.853 826 [-757.851 614] -757.854 437 [-757.851 964] -762.460 371 -762.443 570 -762.445 566
a
The single point calculations used geometries of the (a,a) and (e,e) conformers in HPNB-C crystal and that of the (a,e) conformer in HPNB-N crystal. b Values in square brackets are the energies with the Onsager model for solvation correction. The radii used for the (a,a), (e,e), and (a,e) conformers are respectively 4.91, 4.72, and 4.83 Å.
Figure 2. Crystal packing in HPNB-C showing the H bonds (- - -) and the electrostatic interactions (‚‚‚). N atoms are shown as filled circles.
and toluene) in both the semiempirical and ab initio calculations using different solvation models retains (a,e) as the stablest. Full geometry optimization of these and the (e,a) conformer were also carried out at the AM1 level. It showed the trend, (a,e) < (a,a) < (e,e) < (e,a), with the ∆Hf, being -25.2, -24.7, -24.0, and -23.6 kcal/mol, respectively. This is consistent with the absence of the (e,a) conformer in the polymorphs. The calculated (AM1 and ab initio) dipole moments of the three conformers increase as (a,a) < (e,e) < (a,e), as expected from the relative extents of dipole component cancellation. Similar trends were also found for the dipole moments calculated for the solvated systems. The dipole moment trends are consistent with the stabilization of the (a,e) conformer in polar solvents and the (a,a) and (e,e) conformers in the nonpolar solvents. However, the calculated enthalpies of the conformers do not reflect the solvent selection of the structures. These calculations suggested the need to investigate molecular clusters to gain insight into the bulk structural preferences. The general agreement between the AM1 and ab initio results above
Figure 3. Crystal packing in HPNB-N showing the H bonds (- - -). N atoms are shown as filled circles.
provided confidence in the semiempirical method. We have investigated the clusters using the AM1 procedure since ab initio calculations would be computationally expensive. The H atom positions alone were optimized in the following AM1 calculations, fixing the non-H atom framework as in the crystals. The solvation calculations were carried out with water and toluene representing the two sets of solvents in which HPNB crystallizations were investigated. The three monomer conformations and several clusters extracted from HPNB-C and HPNB-N were considered. For the clusters, we have started with dimers and successively doubled them to tetramers and octamers. This allowed a systematic comparison of the supramolecular fragments of increasing size in the two polymorphic structures. Since numerous choices exist for the clusters, we have restricted the investigation to selected sets of supramolecular entities having similar dipole moments. This may be justified by the fact that the comparison of the energetics (including solvation correction) of finite size clusters possessing widely different polarity is inappropriate, especially since HPNB-C has zero net polarity and HPNB-N would show only weak polarity due to the near
10194 J. Phys. Chem. B, Vol. 104, No. 44, 2000
Sharma and Radhakrishnan Between the high dipole moment tetramers (Figure 4), in the gas phase and in toluene T2 is stabler, but in water T1 becomes stabler! In the low dipole moment ones, T3 is consistently stabler. Thus the high dipole moment pair reflects the solvent stabilization in agreement with the polymorph formation with the clusters from the centric and noncentric crystals preferred in water and in toluene, respectively. Considering the octamers with high dipole moment, in the gas phase and in toluene O2 is stabler, whereas in water O1 is stabilized. The low dipole moment pair, O3 and O4 shows the same trends. Thus, independent of the dipolar nature, the stabilizations of the octamers exactly parallel the solvent preferences of the polymorphs. Interestingly, the stablest octameric clusters in water and toluene are O3 from HPNB-C and O4 from HPNB-N, respectively. These results clearly indicate that as we proceed from monomers, to dimers, to tetramers, and finally to octamers, the relative stabilizations within pairs of clusters extracted from HPNB-C and HPNB-N smoothly approach the solvent preferences displayed by the corresponding bulk crystal lattices. Conclusion
Figure 4. Schematic diagram of the tetramers studied.
TABLE 2: AM1 Dipole Moment, µ, in the Gas Phase and Enthalpy of Formation, ∆Hf, in the Gas Phase and with Water and Toluene Solvation, of HPNB Conformers and Molecular Clusters Extracted from HPNB-C and HPNB-N Crystals ∆Hf (kcal/mol)
designation (source)
µ (D) (gas phase)
gas phase
water
toluene
(a,e) (HPNB-C) (e,e) (HPNB-N) (a,a) (HPNB-N) D1 (HPNB-C) D2 (HPNB-C) D3 (HPNB-N) T1 (HPNB-C) T2 (HPNB-N) T3 (HPNB-C) T4 (HPNB-N) O1 (HPNB-C) O2 (HPNB-N) O3 (HPNB-C) O4 (HPNB-N)
8.468 8.017 7.963 18.851 17.176 17.032 33.743 33.632 0.270 1.547 68.094 67.081 2.087 3.044
-15.5 -13.7 -13.1 -34.8 -30.5 -30.8 -53.9 -59.3 -77.0 -69.3 -101.2 -116.8 -114.4 -134.1
-38.3 -36.4 -34.5 -73.7 -73.0 -67.9 -131.4 -126.0 -143.9 -128.7 -247.5 -242.2 -259.8 -235.8
-25.4 -23.6 -22.5 -51.9 -48.9 -47.3 -87.2 -88.9 -106.8 -95.9 -164.1 -172.3 -177.1 -179.6
cancellation of the molecular dipoles in the lattice. As an example of the clusters studied, we provide in Figure 4 a schematic diagram of the tetramers. The calculated enthalpies of formation and dipole moments are collected in Table 2. The enthalpies of formation of the monomers reduce considerably with respect to the values in Table 1 because of the optimization of the C-H and O-H bonds; however, these values are higher than the enthaplies of formation of the fully optimized structures. Among the monomers (a,e) is again the stablest in the gas phase as well as with either solvent. Among the dimers, D1 (the H-bonded pair) and D2 (the pair with the Coulombic interaction) from HPNB-C are stabilized over D3 (the H-bonded pair) from HPNB-N in water as well as in toluene. However, it should be noted that the relative stabilizations in toluene are less than that in water.
We have systematically investigated the solvent selection of SHG active and inactive crystal structures of HPNB. The welldefined HPNB dimorph growth conditions in terms of solvent polarity has enabled convenient modeling. The following observations about HPNB are notable: (i) Even at the stage of clusters as small as octamers, solvent selection of the bulk structure is clearly revealed. (ii) Computationally inexpensive methods such as the AM1 and simple solvation models using the dielectric continuum approach are successful in demonstrating the solvent stabilizations. (iii) The energy differences between polymorphs are usually marginal;3b however, at the supramolecular fragment level they are amplified, allowing definitive comparisons. It is also clear that thermodynamic factors control the dimorph selection in HPNB. The intuitive idea that the directions for solid-state assembly are determined at an early stage of molecular aggregation in the solvent medium is demonstrated. The present study that rationalizes the dimorphism in HPNB shows that appropriate modeling of solvated molecular clusters provides insight into the crystalline architecture that finally evolves. The level of computation and the solvation model that should be employed and the cluster sizes that must be considered for specific cases of polymorphism need to be extensively explored. The approach developed in this study provides directions for novel strategies to address the larger problem of crystal structure prediction. Acknowledgment. Financial support from the Department of Science and Technology (Swarnajayanti Fellowship) and the use of the National Single Crystal Diffractometer Facility funded by the DST at the School of Chemistry, University of Hyderabad, are gratefully acknowledged. S.S. thanks the Council of Scientific and Industrial Research for a Senior Research Fellowship. We thank Mr. P. Sharma for help with some of the computations. Supporting Information Available: Details of synthesis and characterization, powder SHG studies, X-ray structure determination, computational details, and results of semiempirical and ab initio studies. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) (a) Threlfall, T. L. Analyst 1995, 120, 2435. (b) Bernstein, J.; Davey, R. J.; Henck, J. O. Angew. Chem., Int. Ed. Engl. 1999, 38, 3440.
Solvent Selection of SHG Dimorphs (2) (a) Tabei, H.; Kurihara, T.; Kaino, T. Appl. Phys. Lett. 1987, 50, 1855. (b) Wang, Y.; Tam, W.; Stevenson, S. H.; Clement, R. A.; Calabrese, J. Chem. Phys. Lett. 1988, 148, 136. (c) Shimizu, Y.; Uemiya, T.; Yoshie, N.; Nakatsu, K.; Nogami, T.; Shirota, Y. In Nonlinear Optics of Organics and Semiconductors; Kobayashi, T., Eds.; Springer Proceedings in Physics,. Vol. 36; Springer-Verlag: Berlin, 1989; p 210. (d) Huang, K.; Britton, D.; Etter, M. C.; Byrn, S. R. J. Mater. Chem. 1995, 5, 379. (e) Voigt-Martin, I. G.; Li, G.; Yakimanski, A.; Schulz, G.; Wolff, J. J. J. Am. Chem. Soc. 1996, 118, 12830. (f) Ravi, M.; Rao, D. N.; Cohen, S.; Agranat, I.; Radhakrishnan, T. P. J. Mater. Chem. 1996, 6, 1853. (g) Pan, F.; Bosshard, C.; Wong, M. S.; Serbutoviez, C.; Schenk, K.; Gramlich, V.; Gu¨nter, P. Chem. Mater. 1997, 9, 1328. (3) (a) Pertsin, A. J.; Kitaigorodsky, A. I. The Atom-Atom Potential Method; Springer-Verlag: Berlin, 1987. (b) Gavezzotti, A. J. Am. Chem. Soc. 1991, 113, 4622. (4) (a) Stockton, G. W.; Godfrey, R.; Hitchcock, P.; Mendelsohn, R.; Mowery, P. C.; Rajan, S.; Walker, A. F. J. Chem. Soc., Perkin Trans. 2 1998, 2061. (b) Starbuck, J.; Docherty, R.; Charlton, M. H.; Buttar, D. J. Chem. Soc., Perkin Trans. 2 1999, 677. (5) (a) Etter, M. C. Acc. Chem. Res. 1990, 23, 120. (b) Braga, D.; Grepioni, F. Acc. Chem. Res. 1994, 27, 51. (6) Bilton, C.; Howard, J. A. K.; Madhavi, N. N. L.; Nangia, A.; Desiraju, G. R.; Allen, F. H.; Wilson, C. C. Chem. Commun. 1999, 1675. (7) Dewar, M. J. S.; Zoebisch, E. G.; Healy, E. F.; Stewart, J. J. P. J. Am. Chem. Soc. 1985, 107, 3902. (8) Klamt, A.; Schu¨u¨rmann, G. J. Chem. Soc., Perkin Trans. 2 1993, 799.
J. Phys. Chem. B, Vol. 104, No. 44, 2000 10195 (9) Taylor, E. C.; Skotnicki, J. S. Synthesis 1981, 606. (10) Hall, S. R., King, G. S. D., Stewart J. M., Eds. Xtal3.4; University of Western Australia: 1995. (11) Sheldrick, G. M. SHELX-97; University of Go¨ttingen: Go¨ttingen, Germany, 1997. (12) Kurtz, S. K.; Perry, T. T. J. Appl. Phys. 1968, 39, 3798. (13) MOPAC93; Fujitsu Inc., 1993. (14) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Gill, P. M. W.; Johnson, B. G.; Robb, M. A.; Cheeseman, J. R.; Keith, T.; Petersson, G. A.; Montgomery, J. A.; Raghavachari, K.; Al-Laham, M. A.; Zakrzewski, V. G.; Ortiz, J. V.; Foresman, J. B.; Cioslowski, J.; Stefanov, B. B.; Nanayakkara, A.; Challacombe, M.; Peng, C. Y.; Ayala, P. Y.; Chen, W.; Wong, M. W.; Andres, J. L.; Replogle, E. S.; Gomperts, R.; Martin, R. L.; Fox, D. J.; Binkley, J. S.; Defrees, D. J.; Baker, J.; Stewart, J. P.; HeadGordon, M.; Gonzalez, C; Pople, J. A. Gaussian94, Revision D.2; Gaussian, Inc.: Pittsburgh, PA, 1995. (15) Tomlin, D. W.; Bunning, T. J.; Price, G. E.; Fratini, A. V.; Adams, W. W. Acta Crystallogr. C 1996, 52, 1000. (16) A convenient solvent for crystallization of p-nitroanilines. See: Gangopadhyay, P.; Rao, S. V.; Rao, D. N.; Radhakrishnan, T. P. J. Mater. Chem. 1999, 9, 1699. (17) See Supporting Information for our crystal data on HPNB-C. Crystal data for HPNB-N: C22H28N4O6, M ) 444.48, monoclinic, a ) 9.5423(9), b ) 11.674(3), c ) 9.7854(11) Å, β ) 98.285(8)o, V ) 1078.7(3) Å3, T ) 298 K, space group P21 (No. 4), Z ) 2, µ(Mo KR) ) 0.101 mm-1, no. of unique reflections ) 2595, no. of reflections with I > σI ) 1616, R ) 0.0448 (I > 2σI).
