Ab initio structure determination of phase II of racemic ibuprofen by X-ray powder diffraction

Annealing of the quenched ibuprofen at 258 K yielded a new crystalline form, called phase II. Powder X-ray diffraction patterns of this phase II were recorded with a laboratory diffractometer equipped with an INEL G3000 goniometer and a curved position-sensitive detector CPS120. The starting structural model was found by a Monte-Carlo simulated annealing method. The final structure was obtained through Rietveld refinements with rigid-body constraints for the phenyl group and soft restraints on the other interatomic bond lengths and bond angles. The cell volume is 5% larger than that of the conventional phase I at 258 K. It is also shown that the orientation of the propanoic acid group is drastically changed with respect to phase I, leading to strong modifications of the orientation of the O—H...O hydrogen bonds with respect to the chains of dimers. These structural considerations could explain the metastable character of this phase II.

Computing details

Program(s) used to refine structure: FULLPROF.

(ibuprofen)

Crystal data

C13H18O2	V = 1272.03 (18) Å^3
Mr = 206.28	Z = 4
Monoclinic, P21/c	Dx = 1.077 Mg m^−3
Hall symbol: -P 2ybc	X-ray radiation, λ = 1.540560 Å
a = 12.3794 (9) Å	µ = 0.56 mm^−1
b = 5.8723 (5) Å	T = 258 K
c = 17.5615 (15) Å	white
β = 94.873 (4)°

Data collection

Inel CPS120 diffractometer	Data collection mode: transmission
Radiation source: X-ray tube	Scan method: Stationary detector
Specimen mounting: 0.7 mm diameter Lindemann capillary	2θfixed = 0.2-113.8

Refinement

Rp = 0.040	Profile function: pseudo-Voigt
Rwp = 0.056	59 parameters
Rexp = 0.126	35 restraints
RBragg = 0.058	Background function: linear interpolation between selected points
7579 data points

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

	x	y	z	Uiso*/Ueq
C1	0.5572 (6)	0.6766 (19)	0.4158 (4)	0.00000*
C2	0.6059 (6)	0.8680 (12)	0.3731 (4)	0.00000*
C3	0.6963 (8)	0.967 (2)	0.4240 (6)	0.00000*
C4	0.6641 (11)	0.734 (4)	0.3179 (5)	0.00000*
C5	0.7604 (12)	0.631 (4)	0.3441 (7)	0.00000*
C6	0.8130 (12)	0.489 (3)	0.2963 (5)	0.00000*
C7	0.7692 (11)	0.450 (3)	0.2225 (6)	0.00000*
C8	0.6729 (11)	0.554 (3)	0.1962 (6)	0.00000*
C9	0.6204 (11)	0.696 (3)	0.2439 (5)	0.00000*
C10	0.8061 (12)	0.2511 (11)	0.1767 (6)	0.00000*
C11	0.8511 (5)	0.3890 (10)	0.1113 (4)	0.00000*
C12	0.9146 (10)	0.1809 (16)	0.0890 (7)	0.00000*
C13	0.9565 (7)	0.507 (2)	0.1420 (7)	0.00000*
O1	0.6004 (8)	0.6812 (17)	0.4845 (4)	0.00000*
O2	0.4615 (5)	0.6338 (17)	0.4159 (5)	0.00000*
H1	0.54252	0.59460	0.51577	0.00000*
H2	0.54247	0.95211	0.34572	0.00000*
H3	0.69457	0.93109	0.47765	0.00000*
H4	0.76245	0.93109	0.40258	0.00000*
H5	0.68509	1.14351	0.42214	0.00000*
H6	0.79091	0.64412	0.39829	0.00000*
H7	0.88185	0.41905	0.31802	0.00000*
H8	0.64266	0.52523	0.14303	0.00000*
H9	0.55273	0.76436	0.22148	0.00000*
H10	0.75186	0.17634	0.14443	0.00000*
H11	0.85372	0.14644	0.21091	0.00000*
H12	0.79116	0.42036	0.07230	0.00000*
H13	0.97950	0.16131	0.12476	0.00000*
H14	0.94623	0.20722	0.04049	0.00000*
H15	0.86032	0.06455	0.06744	0.00000*
H16	0.99160	0.58868	0.10212	0.00000*
H17	0.98938	0.39791	0.18671	0.00000*
H18	0.95001	0.64061	0.18671	0.00000*