Polymorphism of Cinnamic and α-Truxillic Acids - American Chemical

Polymorphism of Cinnamic and r-Truxillic Acids: New Additions to an Old Story† Abdelmoty,#

Buchholz,‡

Di,#

Guo,#

Iman Vera Li Chengyun Kathleen Volker Enkelmann,‡ Gerhard Wegner,‡ and Bruce M. Foxman*,#

Kowitz,‡

CRYSTAL GROWTH & DESIGN 2005 VOL. 5, NO. 6 2210-2217

Department of Chemistry, Brandeis University, P.O. Box 549110, Waltham, Massachusetts 02454-9110, and Max-Planck-Institut fu¨ r Polymerforschung, Ackermannweg 10, D-55120 Mainz, Germany Received April 18, 2005;

Revised Manuscript Received June 23, 2005

ABSTRACT: X-ray structure determinations have been carried out and analyzed for two conformational polymorphs of R-truxillic acid (2). Polymorph I (cell parameters known since 1959) is isolated upon recrystallization of 2 from acetic acid-petroleum ether solution, while polymorph II (fully characterized in 1993) is the product of a one-phase solid-state reaction of tail-irradiated R-cinnamic acid. For polymorph II, two redeterminations of mixed crystals containing solid solutions of R-cinnamic acid and 2 (68.1 and 89.5%, respectively) were performed. Analysis of the structural parameters of the 1993 and present redeterminations of the polymorph II structures, as well as comparison to data from the Cambridge Structural Database, confirm that claims of errors in the 1993 structure determination of polymorph II are unfounded. X-ray quality crystals of the β-polymorph of trans-cinnamic acid (cell parameters known since 1964) have been obtained from Et2O/petroleum ether solutions; the crystal structure is quite similar to that of 4-chlorocinnamic acid. Introduction [2+2] photodimerizations are among the most extensively studied crystalline-state reactions.1,2 The structural principles of solid-state reactivity, e.g., the topochemical postulate and modified approaches,3,4 useful for the basic design, planning, and understanding of solid-state reactions, have arisen directly from the elegant studies of Schmidt and co-workers. When transcinnamic acid is irradiated with UV light, a facile, stereospecific dimerization occurs, depending upon the polymorph of cinnamic acid chosen and the consequent orientation of reactants. For example, irradiation of β-cinnamic acid gives exclusively a head-to-head dimer [β-truxinic acid (1)], while irradiation of R-cinnamic acid yields only a head-to-tail dimer [R-truxillic acid (2)].5

Irradiation of single crystals of R-cinnamic acid with broadband UV radiation causes single crystals of the reactants to fracture and undergo a phase transition to † Dedicated to Professor J. Michael McBride on the occasion of his 65th birthday. * To whom correspondence should be addressed. E-mail: foxman1@ brandeis.edu. # Brandeis University. ‡ Max-Planck-Institut fu ¨ r Polymerforschung.

a polycrystalline mass as the photoreaction proceeds. However, as we have shown,6 if a photoactive crystal of R-cinnamic acid is exposed to light for which it has a low absorption (irradiation in the tail of the absorption band), the light intensity will be relatively uniform throughout the crystal. Instead of exsolution occurring at the surface of the crystal, the product will be homogeneously distributed, and, in optimal cases, a solid solution, hence a one-phase, crystal-to-crystal reaction, will occur.6,7 On the synthetic side, [2+2] photocycloaddition reactions have also been employed (with and without tail-irradiation) in the solid-state polymerization of diolefin crystals.8 Most recently, the design of a stepwise [2+2] cycloaddition system has led to a beautifully efficient, high-yield, solid-state synthesis of rare and elusive 3- and 5-ladderanes.9 The combined synthetic potential of the [2+2] cycloaddition reaction and the tail-irradiation technique will continue to provide new opportunities to study and exploit these interesting solid-state reactions. In this paper, we return to certain “missing links” in the evolving story of crystalline-state [2+2] cycloadditions. First, we note that the structures of two of the prime players have remained unknown. While the space group and unit cell dimensions of R-truxillic acid (the first polymorph of 2 to be discovered, hereafter polymorph I of 2)10 as well as those of β-cinnamic acid (3)11 have been known for over 40 years, the structures have not yet appeared in the literature. Those structures have been successfully determined and are reported here. Second, we note the assertion12 that there are errors in the X-ray structure determination of the second polymorph of R-truxillic acid (hereafter, polymorph II of 2), obtained via tail-irradiation of R-cinnamic acid in a onephase, crystal-to-crystal reaction.6 In this paper, we therefore report a redetermination (using the Brandeis X-ray facility) of the structures of two laser-irradiated crystals of R-cinnamic acid. Finally, we address the issue

10.1021/cg050160s CCC: $30.25 © 2005 American Chemical Society Published on Web 08/17/2005

Polymorphism of Cinnamic and R-Truxillic Acids

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Table 1. Crystallographic Data compound

2a (polymorph II, 68%)

2b (polymorph II, 90%)

chemical formula formula weight crystal system space group T, K a, Å b, Å c, Å β, ° V, Å3 Z Fcalcd, g cm-3 λ, Å; θmax µ, mm-1 trans coeff (empirical corr) unique reflns; obsd reflns; param Rmerge R Rw GOF

0.319 C9H8O2:0.681 C18H16O4 148.16 (monomer) monoclinic P21/n 294 7.6158(15) 18.640(4) 5.6584(14) 104.838(17) 776.5(3) 2 (truxillic acid) 1.267 0.71073; 24.6° 0.089 0.96-0.99

0.105 C9H8O2:0.895 C18H16O4 148.16 (monomer) monoclinic P21/n 294 7.6462(11) 18.2905(14) 5.6092(12) 106.057(16) 753.9(2) 2 (truxillic acid) 1.296 1.54178; 70° 0.754 0.68-0.93

C18H16O4 296.32 monoclinic C2/c 294 15.9958(12) 5.6586(5) 16.3091(12) 99.615(6) 1455.5(2) 4 1.352 1.54178; 78° 0.782 0.81-0.94

C9H8O2 148.16 monoclinic P21/a 294 31.296(4) 4.0152(17) 6.083(2) 90.146(19) 764.4(4) 4 1.287 0.71073; 24.6° 0.091 0.85-0.98

1305; 684; 110

1430; 1230; 110

1541; 1299; 260

1291; 655;104

0.012 0.0441 0.0542 1.12

0.010 0.0621 0.0751 0.97

0.005 0.0349 0.0447 1.04

0.019 0.0364 0.0443 1.13

of possible errors in the original structure determination of 2; the redetermination studies, as well as information from the Cambridge Structural Database, support the earlier findings.6 Experimental Section Redetermination of the Crystal Structures of Irradiated Single Crystals of trans-Cinnamic Acid. R-transCinnamic acid monomer crystals, varying from 0.3 to 0.7 mm along one edge, were obtained by evaporation of an ether solution of the acid. In Mainz, crystals were irradiated in quartz Petri dishes using an argon laser at 351.1 and 363.8 nm for various time intervals between 15 and 2160 min. Flux densities varied from 80 to 140 mW/cm2. The average amount of dimeric product present in a batch was determined by integration of the NMR spectrum of solutions of the irradiated materials in CDCl3/CF3COOH. In Waltham, two crystals (2a and 2b), both exposed to 351.1 nm irradiation at a flux of 90 mW/cm2, were selected for the redetermination study. Crystals 2a and 2b are mixed crystals, each comprising different relative amounts of R-cinnamic and R-truxillic acids. Crystal 2a was obtained from a batch irradiated for 60 min (average dimer content 69.6%), while 2b was obtained from a batch irradiated for 900 min (average dimer content 94.1%). Crystallographic data for compounds 2a [X-ray result: 68.1(7)% dimer] and 2b [X-ray result: 89.5(6)% dimer] are summarized in Table 1. Data were collected on a Nonius CAD-4 diffractometer equipped with Mo KR (2a) or Cu KR (2b) radiation13 and processed using the Nonius MolEN package.14 The structures were solved by direct methods [SIR-92].15 Full-matrix least squares refinement was carried out using the Oxford University CRYSTALS for Windows system.16 For the cinnamic acid-truxillic acid mixture in crystals of both 2a and 2b, only the carbon atoms R and β to the carboxylate group could be reliably located and refined; the occupancies of atoms (C21, C31 [cinnamic]) and (C2, C3 [truxillic]) were constrained to sum to 1.0. All other non-hydrogen atoms were refined using anisotropic displacement parameters. The carboxylic H atom was located and refined in 2a but not in 2b. Other H atoms were fixed at calculated positions, which were updated following each set of least-squares cycles. Drawings were produced using the Oxford University graphics program CAMERON.17 Preparation and Structure Determination of the Solution-Grown (Recrystallized) Phase of R-Truxillic Acid (2c, Polymorph I). Solid R-trans-cinnamic acid (2.5 g, 17 mmol) was placed in an open Pyrex Petri dish and UVirradiated with a PMR-2500 lamp; the solid acid was stirred

2c (polymorph I)

3

daily to expose fresh surfaces. After 5 days of irradiation, the solid was extracted with acetone. The insoluble colorless residue (0.55 g, 22.2%) melted at 282-283 °C. The 1H spectrum of 2, taken in DMSO-d6, showed a singlet at 7.33 ppm (phenyl H atoms) and two multiplets at 3.80 and 4.27 ppm (cyclobutane H atoms); the carboxylate hydrogen atom appears at 12.18 ppm. Single crystals of polymorph I were grown by slow evaporation of either: (i) a 1:1 acetic acid/ethyl acetate solution, or (ii) a 1:1 acetic acid/petroleum ether solution. Crystals formed out of solution (i) were smaller and of poorer quality than those grown from solution (ii), and thus only X-ray diffraction studies carried out on crystals grown by method (ii) will be reported here. X-ray diffraction data were collected as described above, using Cu KR radiation; details are provided in Table 1. Systematic absences of h0l, l ) 2n + 1 indicated that, upon recrystallization, molecules of 2 crystallize in space group Cc or C2/c; Z ) 4. Details of the structure analysis are presented in Table 1. The structure was solved with some difficulty using SHELXS-86.18 The X-ray structure determination of polymorph I was complicated by the location of the molecule near the 2-fold axis in C2/c. The structure was partially solved (central cyclobutane ring, one phenyl group, and one CO2 group) from the first E-map, followed by successive ∆F maps and the addition of recognizable fragments with satisfactory molecular geometries. The occupancies of all non-hydrogen atoms were fixed at 0.5, and bond lengths (phenyl C-C, cyclobutane C-C, and C-O) were restrained to normal values. Hydrogen atoms were added first at calculated positions but were refined in the final analysis. A single carboxylic hydrogen atom was located on a ∆F map; this hydrogen atom was assigned an occupancy of 1.0, since, owing to the disorder about the 2 axis, it represents both carboxylate hydrogen atoms. The final refinement gave R ) 0.035; Rw ) 0.042. Preparation and Structure Determination of β-Cinnamic Acid (3). A solution of cinnamic acid (1.5 g) in Et2O (25 mL) was stirred for 5 min. A 5-mL aliquot of this solution was transferred to a 15 × 150 mm test tube, and 20 mL of either 30-60 °C or 50-110 °C cold petroleum ether (0-5 °C) was added slowly along the wall of the test tube. The tube was covered with Parafilm and cooled to -20 °C in the freezer. Crystalline β-cinnamic acid formed between the Et2O and petroleum ether layers as diffusion proceeded. After 1 day, long, plate-shaped crystals of the β-phase comprised most of the solid (visual inspection); after 2 days, significant quantities of the R-phase were also present. While this is a reproducible procedure for the preparation of single crystals of the β-polymorph, most crystals were of poor diffraction quality. From time to time, a suitable diffraction-quality single crystal could

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Figure 1. Time-conversion study for laser irradiation of R-cinnamic acid crystals. be obtained by this technique or by allowing the mixed solvent to evaporate from an open beaker.

Results and Discussion Crystal-to-Crystal Photodimerization of R-transCinnamic Acid. When topochemical photoreactions are carried out by irradiating the chromophore with light at its λmax, product formation occurs at high concentrations at the incident surface, and falls off rapidly through the inner bulk of the crystal. This results in a heterogeneous spatial distribution of product molecules. The product lattice, concentrated at the surface of the crystal, reaches its limit of solubility in the original monomer owing to a lattice mismatch between reactant and product. Then the reacting phase and product phase separate, leading to disintegration of the crystal.6 However, if a photoactive crystal is exposed to light for which it has a low absorption, the light intensity will be relatively uniform from the surface throughout the crystal. Product will be homogeneously distributed, and, in optimal cases, a solid solution of reactant and product will obtain.6 This phenomenon was first demonstrated for two cases, the classic photodimerization of R-cinnamic acid and a series of styrylpyrilium salts,6,7 and has been successfully applied in numerous instances in recent years.19 In describing the solid-state reactions of cinnamic acid, we use the familiar terminology developed and elucidated by Thomas20 and Wegner.21 Thus, in a one-phase, topotactic solid-state reaction20,21 (as described here for the tail-irradiation experiments), there is no change of space group, and only a single diffraction pattern is visible at any time during the process; a solid solution of reactant and product is present at intermediate stages of conversion. In a twophase, topotactic solid-state reaction,20,21 there is no solid solution, and the diffraction patterns of reactant and product phases are simultaneously visible.22 The structural results of the original tail-irradiation experiments on the R-phase of cinnamic acid were questioned in 1996,12a and the original criticism has been restated and referred to in a number of recent papers.12b-d To address the structural issues, we have reexamined several aspects of the initial crystallo-

graphic results, as well as carrying out new irradiation experiments and, finally, two fresh structure determinations on tail-irradiated crystals. Crystals were irradiated at two wavelengths, 351.1 and 363.8 nm, and at radiant flux densities ranging from 80 to 140 mW/cm2. Irradiation times varied from 15 min to 36 h; degree of conversion was established by dissolution of the sample in CDCl3/CF3COOH and NMR integration of the mixture. The results are displayed in Figure 1. As might be expected, it is evident that conversion is much more rapid at 351.1 nm and that higher applied power at constant wavelength and time gives a significantly higher yield (after an apparent initial induction period of ca. 6 h at 363.8 nm). We also note an apparent “leveling off” of conversion at 363.8 nm and 100 mW/cm2, but this depends on only a single experiment and should be verified in a new series of experiments using several different irradiation times > 20 h. Crystals were selected from batches irradiated at 351.1 nm at a flux of 90 mW/cm2 for 1 h (crystal 2a) and 15 h (crystal 2b), respectively. As observed previously,6 for the solid solution of R-cinnamic and R-truxillic acid in each structure determination, only the positions of the carbon atoms R and β to the carboxyl moiety are well-resolved; occupancies of these atoms were constrained to sum to 1.0. For 2a and 2b, the crystals selected were found to contain 68.1(7)% and 89.5(6)% R-truxillic acid; these amounts are consistent with the average values obtained from NMR integration of an aliquot of the bulk material. Views of the solid solution of R-cinnamic and R-truxillic acid are shown for 2a and 2b in Figure 2, panels a and b, respectively. Within the experimental error of the diffraction measurement, the positions of the phenyl groups are observed to be the same; attempts to resolve independent positions for two components invariably failed. In prior work, the “phenyl ring tilt”, defined as the angle between the plane of the phenyl ring and the axis along the bond linking the phenyl ring to the double bond or cyclobutane ring, was observed to be 25.5° and 13.6° for the monomer and dimer at 67% conversion.6 For 2a at 68.1% conversion, the corresponding values of 26.5° and 13.0° compare well with those observed

Polymorphism of Cinnamic and R-Truxillic Acids

Crystal Growth & Design, Vol. 5, No. 6, 2005 2213

Figure 2. Views of partially reacted crystals (a) crystal 2a (68.1% conversion) and (b) crystal 2b (89.5% conversion). Yellow carbon atoms correspond to the position of the CdC moiety of residual R-cinnamic acid.

previously. At 67 and 100% conversion, the dihedral angles between the phenyl ring and cyclobutane ring were observed to be 96.9° and 100.9°, respectively.6 For 2a and 2b, the corresponding values are 96.8° (68.1%) and 100.6° (89.5%), respectively. For these parameters, there are thus no significant differences between the 1993 study6 and the present work. We now turn our attention to the packing and intra/ intermolecular contacts for 2a, 2b and LAZRAF,6 the structure of polymorph II of the 100% R-truxillic acid, as deposited in the Cambridge Structural Database.23 For the purposes of this section of the paper, the coordinates of LAZRAF (hereafter, polymorph II) were altered to (a) include only the asymmetric unit of the dimer (Z′ ) 0.5 in P21/n), and (b) bring the polymorph II coordinates to the same asymmetric unit we used in the present work [origin change of (0, 1/2, 1)]. This transformed data forms part of the Supporting Information deposited with this paper and is otherwise unchanged with respect to the original deposition. Inspection of the values of the dimer coordinates in each CIF file shows their similarity, and the nearly-identical values obtained from 2b at 89.5% conversion and polymorph II at 100%. A pair of R-truxillic acid molecules and the hydrogen bonds, an R22(8) ring,24 is shown in Figure 3a, while Figure 3b shows the packing of molecules in the unit cell, with infinite sets of R22(8) rings along the [101] unit cell direction. For polymorph II, the O2-H1‚‚‚O1[2 - x, 1 - y, 2 - z] hydrogen bond has an O‚‚‚O distance of 2.64 Å and an O-H‚‚‚O angle of 158°; for 2a, the values are 2.64 Å and 172°. It was not possible to locate the H atom attached to O in 2b, which also has an O2‚‚‚O1 distance of 2.64 Å. The most significant C-H‚‚‚O interactions (not shown in the figure) are an R22(14) ring, C9-H91‚‚‚O2[(1 - x, 1 - y, 2 - z), C‚‚‚O, 3.51 Å, C-H‚‚‚O, 172°] and a C(5) chain C3-H31‚‚‚O2[(x, y, z - 1), C‚‚‚O, 3.46 Å, C-H‚‚‚O, 134°]. While the carboxylate O‚‚‚O R22(8) ring systems are

Figure 3. (a) Two molecules of 2 (polymorph II) showing the noncoplanarity of the two halves of the R22(8) ring. (b) View of the crystal structure of polymorph II of 2 viewed down the c axis.

centrosymmetric, and, by symmetry, the two “CO2H halves” must of course be parallel, there is no requirement that, in this structure or any other, the two halves will be coplanar. In the structures of polymorph II, 2b and 2a, the planes are displaced by 0.66, 0.63, and 0.44 Å, respectively; the values of 0.66 and 0.63 Å for polymorph II and 2b are consistent with similar degrees of conversion. This feature has been cited as one of the reasons that the structure analysis is incorrect.12 To examine the range of values observed for the relative displacements of the two halves of an R22(8) ring, we searched the Cambridge Structural Database for this feature. After excluding entries with disorder and known errors, and with a requirement that Z′ e 1, 1306 entries were extracted, generating 1416 R22(8) carboxylate rings (some entries had more than one carboxylate group). Both centrosymmetric and noncentrosymmetric R22(8) systems were sampled. A summary of the results is presented in Table 2. More than 80 systems (5.8%) had separations of >0.4 Å between the two ring halves, and, for 26% of the observed systems, the separation was greater than 0.2 Å. The most similar deviation found in a truxillic acid was that for 2,2′-diethoxy-R-truxillic acid (XOSKEV), 0.54 Å (Figure 4);25 other notable molecules include 2-(N-

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Table 2. Distribution of Average Interplanar Separations within R22(8) Rings in Carboxylic Acidsa average displacement of R22(8) ring “halves”, Å

number of entries

0.00-0.05 0.05-0.10 0.10-0.15 0.15-0.20 0.20-0.25 0.25-0.30 0.30-0.35 0.35-0.40 0.40-0.45 0.45-0.50 0.50-0.55 0.55-0.65 0.65-0.80 0.80-0.95 1.06, 1.32

307 311 253 171 118 91 53 28 24 14 13 15 9 7 one each

aSampled

from the Cambridge Structural Database.

Figure 5. (a) A single molecule of 2 (polymorph I), showing the labeling scheme. (b) A pair of molecules of 2 (polymorph I), showing the disordered relationship about the 2-fold axis.

Figure 4. Two molecules of 2,2′-diethoxy-R-truxillic acid (XOSKEV), showing the noncoplanarity of the two halves of the R22(8) ring.

phthalimidomethyl)benzoic acid, (CABPIE), 0.66 Å,26 and 2,2,3,3-tetramethylcyclopropanecarboxylic acid (FORGEY), 0.85 Å.27 For XOSKEV and polymorph II, it seems likely that the deviations are a result of steric strain, resulting either from intramolecular interactions (as also in FORGEY) or the crystal packing of a metastable polymorph, respectively. However, other examples such as CABPIE are less obvious, given the apparent lack of internal strain, and suggest that the potential for distortion of the traditional R22(8) ring, e.g., toward a chair form, is rather soft. Finally, it is also clear that deviations are not required to be large for a truxillic acid: for 4,4′-dimethyl-R-truxillic acid (ZZZLDK01),28 the separation is only 0.034 Å. Further evidence cited that the structure of polymorph II was incorrect was the putative existence of “obvious impossibilities in the X-ray data” in the form of short H‚‚‚H contacts, quoted as H2‚‚‚H5 of 2.033 Å.29 For the structure of polymorph II in the Cambridge Structural Database (LAZRAF), the asymmetric unit is comprised of one-half of an R-truxillic acid molecule; the remaining half is generated by the operation (1 - x, -y, -1 - z). Using the asymmetric unit of the original polymorph II data in CSD, we

do find a contact of 2.033 Å, but this is an intramolecular contact, H2‚‚‚H4(1 - x, -y, -1 - z), between the original half-molecule and the symmetry-related other half. There are no intermolecular H‚‚‚H contacts shorter than 2.37 Å: H5‚‚‚H7(1/2 + x, -1/2 - y, z - 1/2). The observed intramolecular contact of 2.033 Å occurs between a hydrogen atom attached to the ortho-position of the phenyl ring and a hydrogen atom attached to the β-carbon atom of the cyclobutane ring (4). A search of the Cambridge Structural Database revealed 28 molecules containing cyclobutane moieties with phenyl rings attached. For these molecules (including polymorph II), the range of analogous H‚‚‚H distances was 1.902-2.093 Å, and 14 had analogous intramolecular H‚‚‚H distances less than or equal to 2.033 Å. In summary, as we examine the structural parameters of polymorph II, 2a and 2b, we find no unusual or unprecedented, “out-of-range” values. We conclude that the original (and subsequent) determinations of the crystal structure of R-truxillic acid (as well as those of mixed crystals of cinnamic and truxillic acids) produced by tail-irradiation of R-trans-cinnamic acid are correct and that claims that these structures contain12c “easily recognizable errors” or that12d the “data do not withstand standard scrutiny” are unwarranted and incorrect. Crystal structure of Recrystallized R-Truxillic Acid (2c, Polymorph I). Eanes and Donnay published the unit cell parameters of recrystallized truxillic acid (polymorph I) in 1959: space group Cc; a ) 16.09, b ) 5.67, c ) 16.35 Å; β ) 99.50°; Z ) 4.10 Since that time, no reports of the detailed crystal structure of polymorph I have appeared. Crude 2 was obtained by broadband irradiation of solid R-trans-cinnamic acid. Crystals of polymorph I were obtained by recrystallization of crude 2 from 1:1 acetic acid/petroleum ether solution. Initial

Polymorphism of Cinnamic and R-Truxillic Acids

Figure 6. View of the crystal structure of polymorph I of 2 viewed down the b axis.

Figure 7. Crystals of the β (left) and R (right) polymorphs of cinnamic acid.

attempts at solving the structure in Cc invariably failed, and a disordered model in space group C2/c proved to be the correct choice. The structure was solved by starting with a small fragment and slowly adding additional atoms at chemically sensible positions, all with occupancies of 0.5. In the final polymorph I model

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(R ) 3.5%), 2 is disordered about the 2-fold axis in C2/ c. A single molecule of 2 and the labeling scheme are shown in Figure 5a. A pair of disordered molecules, showing that the atoms are well-resolved, is shown in Figure 5b. The undisordered molecule occupies a general position, and thus there are no symmetry requirements. The C-C bonds of the cyclobutane moiety in polymorph I [1.535, 1.561, 1.566, 1.569 (3) Å] are similar to those found in 2b [1.545, 1.586(3) Å] and polymorph II (1.542, 1.597 Å). The cyclobutane is slightly puckered, with, for example, atom C(4) 0.38 Å out of the plane defined by the other three atoms of the four-membered ring. This value is consistent with those observed for other substituted cyclobutanes that do not crystallize on a center of symmetry (e.g., β-truxinic acids, 0.34-0.36 Å;30 transnorpinic acid, 0.42 Å31). A view of the packing and hydrogen bonding in polymorph I is shown in Figure 6. Similar to that observed for 2b, chains of molecules linked by two independent R22(8) carboxylate hydrogen bonds (O21-H211‚‚‚O22[-x, -y, -z], 2.651 Å, 176.3°; O41-H211‚‚‚O42[-x, -y, -z], 2.677 Å, 171.2°) extend along the c axis. There is one significant C-H‚‚‚O interaction, a C(7) chain, which occurs either between translation-related molecules, C12-H121‚‚‚O41[(x, 1 + y, z), C‚‚‚O, 3.25 Å, C-H‚‚‚O, 140°]; or, alternatively, between the corresponding disordered site centered at that location, C32-H321‚‚‚O41[(-x, 1+y, 1/2 - z), C‚‚‚ O, 3.38 Å, C-H‚‚‚O, 164°]. The distances between the two “halves” of the R22(8) systems are 0.26 Å for the ring containing O21, and 0.034 Å for the ring containing O41. There is only one intramolecular contact < 2.4 Å, H361-H41 (2.27 Å) between a hydrogen atom attached to the ortho-position of the phenyl ring and a hydrogen atom attached to the β-carbon atom of the cyclobutane ring, suggesting that, in polymorph I, R-truxillic acid is less strained than in the metastable polymorph II.32 Comparison of Figures 3b and 6 thus shows that, while the linear chain of O-H‚‚‚O hydrogen bonding is rather similar ([001]I ) 16.31 Å and 2 × [101]II ) 16.26 Å), the crystal structure of higher-density polymorph I (1.352 compared to ca. 1.30 g cm-3) is characterized by short (3.4-3.5 Å) π interactions between phenyl groups, while that of metastable32 polymorph II has weak C-H‚‚‚π interactions (C‚‚‚centroid, 3.73 Å; H‚‚‚centroid, 2.79 Å; C-H‚‚‚centroid, 143°). Crystal Structure of β-Cinnamic Acid (3). In 1964, Schmidt and Cohen reported the unit cell parameters of β-cinnamic acid 311 (Table 3); the crystal structure of 3 has not been reported. Crystalline 3 is

Figure 8. View of the crystal structure of β-cinnamic acid viewed down the c axis.

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Table 3. Selected Known β-Phases with Structures Related to β-Cinnamic Acid substituents

Refcode

space gp

a, Å

b, Å

c, Å

β, deg

V, Å3

ref

none none 4-formyl 3,4-dichloro 4-chloro 4-iodo 2-nitro 2,4-dichloro 2,4-dichloro 4-nitro 4-bromo

CINMAC02

P21/a P21/a P21/n P21/c P21/a P21/n P21/a P21/a P21/c P21/a P21/a

31.30 31.30 3.89 3.90 32.81 4.12 31.50 36.80 3.88 31.40 33.00

4.04 4.02 6.83 6.56 3.89 6.27 7.22 6.44 6.44 3.90 3.97

6.05 6.08 31.81 36.84 6.54 34.67 3.76 3.88 36.80 7.20 6.66

90.30 90.15 92.12 91.50 95.94 90.32 91.60 90.50 90.50 90.00 94.60

765.0 764.4 844.4 942.2 830.0 895.8 854.8 919.5 919.5 881.7 869.7

11 this work 35 36 33 34 11 11 36 11 11

COWRIP01 DOPFUJ01 PCTCIN WAMZOZ ZZZNQQ ZZZOBE ZZZOBE01 ZZZOIU ZZZOJI

readily obtained from diethyl ether/petroleum ether solutions; however, most crystals are not suitable for X-ray diffraction studies, and reasonable quality single crystals were obtained only after repeated trials. As can be seen from Figure 7, crystals of 3 are easily distinguished from the R-phase but are of poorer quality and are readily deformed. The crystal structure of 3, shown in Figure 8, consists of R22(8) carboxylate dimers, stacked at an angle of 28.5° to the b axis. As suggested long ago,11 the head-to-head packing is consistent with the formation of β-truxinic acid 1 upon UV-irradiation and differs from the head-to-tail arrangement11 found in R-cinnamic acid (Figure 2). The structure and cell constants are closely related to those reported for 4-chlorocinnamic acid,33 4-iodocinnamic acid,34 and 4-formylcinnamic acid35 (Table 3). For the latter three structures, the short (≈4 Å) axis corresponds to b, a, and a, respectively. Despite the different short axis directions in the three monoclinic structures, the angles between the short axis and the molecular plane are very similar: 26.1, 23.1 and 26.8°, respectively. Table 3 includes cell data for nine compounds, including that of 3, which likely have very closely related structures. Conclusions. X-ray structure determinations have been carried out and analyzed for two conformational polymorphs37 of R-truxillic acid (2). Polymorph I (cell parameters known since 1959) is isolated upon recrystallization of 2 from acetic acid-petroleum ether solution, while polymorph II (fully characterized in 1993) is the product of a one-phase solid-state reaction of tailirradiated R-cinnamic acid. For polymorph II, two redeterminations of mixed crystals containing solid solutions of R-cinnamic acid and 2 (68.1 and 89.5%, respectively) were performed. Analysis of the structural parameters of the 1993 and present redeterminations of the polymorph II structures, as well as comparison to data from CSD, confirms that claims of errors12,29 in the 1993 structure determination of polymorph II6 are unfounded. X-ray quality crystals of the β-polymorph of trans-cinnamic acid (cell parameters known since 1964) have been obtained from Et2O/petroleum ether solutions; the crystal structure is quite similar to that of 4-chlorocinnamic acid. Acknowledgment. We thank the National Science Foundation (Grant DMR-0504000) for partial support of this research. Supporting Information Available: CIF files and spreadsheet-format results of CSD searches. This material is available free of charge via the Internet at http://pubs.acs.org.

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