Diffuse scattering study of aspirin forms (I) and (II)

Full three-dimensional diffuse scattering data have been recorded for both polymorphic forms [(I) and (II)] of aspirin and these data have been analysed using Monte Carlo computer modelling. The observed scattering in form (I) is well reproduced by a simple harmonic model of thermally induced displacements. The data for form (II) show, in addition to thermal diffuse scattering (TDS) similar to that in form (I), diffuse streaks originating from stacking fault-like defects as well as other effects that can be attributed to strain induced by these defects. The present study has provided strong evidence that the aspirin form (II) structure is a true polymorph with a structure quite distinct from that of form (I). The diffuse scattering evidence presented shows that crystals of form (II) are essentially composed of large single domains of the form (II) lattice with a relatively small volume fraction of intrinsic planar defects or faults comprising misoriented bilayers of molecular dimers. There is evidence of some local aggregation of these defect bilayers to form small included regions of the form (I) structure. Evidence is also presented that shows that the strain effects arise from the mismatch of molecular packing between the defect region and the surrounding form (II) lattice. This occurs at the edges of the planar defects in the direction only.

Computing details

Cell refinement: HKL SCALEPACK (Otwinowski & Minor 1997); data reduction: HKL DENZO and SCALEPACK (Otwinowski & Minor 1997); program(s) used to solve structure: known; program(s) used to refine structure: SHELXL97 (Sheldrick, 1997); molecular graphics: Xtal3.7 (Hall et al., 2001); software used to prepare material for publication: WinGX publication routines (Farrugia, 1999).

(aspirin-form2)

Crystal data

C9H8O4	F(000) = 376
Mr = 180.15	Dx = 1.394 Mg m^−3
Monoclinic, P21/c	Mo Kα radiation, λ = 0.71073 Å
Hall symbol: -P 2ybc	Cell parameters from 8462 reflections
a = 12.2696 (5) Å	θ = 27.5–2.6°
b = 6.5575 (3) Å	µ = 0.11 mm^−1
c = 11.4960 (4) Å	T = 300 K
β = 68.163 (2)°	Prism, colourless
V = 858.58 (6) Å^3	0.4 × 0.2 × 0.1 mm
Z = 4

Data collection

KappaCCD diffractometer	1853 independent reflections
Radiation source: fine-focus sealed tube	1425 reflections with I > 2σ(I)
Graphite monochromator	Rint = 0.031
Detector resolution: 9 pixels mm^-1	θmax = 27.2°, θmin = 3.6°
CCD scans	h = −15→15
Absorption correction: integration Gaussian integration (Coppens, 1970)	k = −5→8
Tmin = 0.968, Tmax = 0.986	l = −14→14
6924 measured reflections

Refinement

Refinement on F^2	Primary atom site location: structure-invariant direct methods
Least-squares matrix: full	Secondary atom site location: difference Fourier map
R[F^2 > 2σ(F^2)] = 0.043	Hydrogen site location: inferred from neighbouring sites
wR(F^2) = 0.129	H-atom parameters constrained
S = 1.09	w = 1/[σ^2(Fo^2) + (0.065P)^2 + 0.127P] where P = (Fo^2 + 2Fc^2)/3
1853 reflections	(Δ/σ)max < 0.001
119 parameters	Δρmax = 0.23 e Å^−3
0 restraints	Δρmin = −0.20 e Å^−3

Special details

Geometry. All e.s.d.'s (except the e.s.d. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell e.s.d.'s are taken into account individually in the estimation of e.s.d.'s in distances, angles and torsion angles; correlations between e.s.d.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell e.s.d.'s is used for estimating e.s.d.'s involving l.s. planes.

Refinement. Refinement of F² against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F², conventional R-factors R are based on F, with F set to zero for negative F². The threshold expression of F² > σ(F²) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F² are statistically about twice as large as those based on F, and R- factors based on ALL data will be even larger.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Ų)

	x	y	z	Uiso*/Ueq
C1	0.15361 (12)	0.5634 (2)	−0.00763 (12)	0.0422 (4)
C2	0.24609 (13)	0.4867 (2)	−0.11120 (12)	0.0445 (4)
C3	0.29856 (14)	0.3035 (3)	−0.10485 (15)	0.0541 (4)
H14	0.3591	0.2535	−0.1751	0.065*
C4	0.26148 (15)	0.1939 (3)	0.00552 (16)	0.0584 (4)
H15	0.2976	0.0709	0.0095	0.070*
C5	0.17144 (16)	0.2657 (3)	0.10958 (16)	0.0552 (4)
H16	0.1468	0.1920	0.1840	0.066*
C6	0.11776 (14)	0.4480 (2)	0.10276 (13)	0.0487 (4)
H17	0.0564	0.4953	0.1732	0.058*
C7	0.09012 (13)	0.7565 (2)	−0.00660 (13)	0.0432 (4)
O8	0.00975 (10)	0.81103 (19)	0.09154 (10)	0.0604 (4)
O9	0.12013 (10)	0.85938 (19)	−0.10910 (10)	0.0580 (3)
H18	0.0796	0.9624	−0.0975	0.087*
O10	0.28510 (9)	0.58717 (18)	−0.22724 (8)	0.0494 (3)
C11	0.36529 (13)	0.7386 (3)	−0.24386 (13)	0.0484 (4)
O12	0.40430 (11)	0.7814 (2)	−0.16633 (10)	0.0627 (4)
C13	0.39556 (17)	0.8367 (4)	−0.36860 (16)	0.0710 (6)
H19	0.4527	0.9420	−0.3783	0.107*
H20	0.3262	0.8952	−0.3747	0.107*
H21	0.4274	0.7364	−0.4332	0.107*

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
C1	0.0385 (7)	0.0489 (8)	0.0387 (7)	−0.0031 (6)	−0.0139 (6)	−0.0013 (6)
C2	0.0417 (8)	0.0524 (9)	0.0382 (7)	−0.0042 (6)	−0.0135 (6)	−0.0022 (6)
C3	0.0480 (9)	0.0567 (10)	0.0522 (9)	0.0053 (7)	−0.0125 (7)	−0.0090 (7)
C4	0.0576 (10)	0.0511 (10)	0.0664 (10)	0.0056 (8)	−0.0229 (8)	0.0009 (8)
C5	0.0554 (10)	0.0561 (10)	0.0522 (9)	−0.0040 (8)	−0.0176 (7)	0.0105 (7)
C6	0.0456 (8)	0.0551 (10)	0.0420 (8)	−0.0014 (7)	−0.0124 (6)	0.0026 (7)
C7	0.0400 (8)	0.0524 (9)	0.0354 (7)	−0.0028 (6)	−0.0117 (6)	−0.0005 (6)
O8	0.0624 (7)	0.0663 (8)	0.0415 (6)	0.0164 (6)	−0.0067 (5)	0.0004 (5)
O9	0.0574 (7)	0.0617 (7)	0.0461 (6)	0.0101 (6)	−0.0091 (5)	0.0100 (5)
O10	0.0477 (6)	0.0635 (7)	0.0343 (5)	−0.0028 (5)	−0.0122 (4)	−0.0018 (4)
C11	0.0409 (8)	0.0624 (10)	0.0370 (7)	0.0025 (7)	−0.0088 (6)	0.0003 (7)
O12	0.0619 (8)	0.0819 (9)	0.0463 (6)	−0.0176 (6)	−0.0226 (6)	0.0077 (6)
C13	0.0651 (11)	0.0989 (15)	0.0470 (9)	−0.0108 (11)	−0.0186 (8)	0.0211 (9)

Geometric parameters (Å, º)

C1—C2	1.3974 (19)	C6—H17	0.9300
C1—C6	1.400 (2)	C7—O8	1.2432 (17)
C1—C7	1.485 (2)	C7—O9	1.2869 (17)
C2—C3	1.378 (2)	O9—H18	0.8200
C2—O10	1.4027 (17)	O10—C11	1.361 (2)
C3—C4	1.380 (2)	C11—O12	1.1915 (19)
C3—H14	0.9300	C11—C13	1.487 (2)
C4—C5	1.375 (2)	C13—H19	0.9600
C4—H15	0.9300	C13—H20	0.9600
C5—C6	1.381 (2)	C13—H21	0.9600
C5—H16	0.9300
C2—C1—C6	117.37 (14)	C5—C6—H17	119.2
C2—C1—C7	124.81 (13)	C1—C6—H17	119.2
C6—C1—C7	117.82 (12)	O8—C7—O9	122.72 (14)
C3—C2—C1	121.05 (14)	O8—C7—C1	119.28 (13)
C3—C2—O10	117.33 (13)	O9—C7—C1	117.99 (12)
C1—C2—O10	121.54 (14)	C7—O9—H18	109.5
C2—C3—C4	120.13 (14)	C11—O10—C2	116.72 (11)
C2—C3—H14	119.9	O12—C11—O10	122.52 (14)
C4—C3—H14	119.9	O12—C11—C13	126.41 (16)
C5—C4—C3	120.37 (16)	O10—C11—C13	111.07 (14)
C5—C4—H15	119.8	C11—C13—H19	109.5
C3—C4—H15	119.8	C11—C13—H20	109.5
C4—C5—C6	119.51 (15)	H19—C13—H20	109.5
C4—C5—H16	120.2	C11—C13—H21	109.5
C6—C5—H16	120.2	H19—C13—H21	109.5
C5—C6—C1	121.55 (14)	H20—C13—H21	109.5