Supporting information
Crystallographic Information File (CIF) https://doi.org/10.1107/S0108270102019996/sk1598sup1.cif | |
Structure factor file (CIF format) https://doi.org/10.1107/S0108270102019996/sk1598Isup2.hkl |
CCDC reference: 204034
A commercial sample of benzanilide (Source?) was crystallized from an ethanol solution at ambient temperature.
The space group P1 was selected, and confirmed by the subsequent structure analysis. The ADDSYM option in PLATON (Spek, 2002) revealed no additional symmetry. For the minor orientational component, the two rings were constrained to be planar regular hexagons, with C—C distances of 1.390 Å, and the remaining distances involving C, N and O atoms were tied to the corresponding distances in the major component. The non-H atoms in the minor component were all refined isotropically. A common isotropic displacement parameter was applied to atoms C31—C36, a second common isotropic displacement parameter to atoms C41—C46, and individual isotropic parameters to atoms N31, C37 and O31. The site occupancy factors for the two orientations then refined to 0.839 (5) and 0.161 (5), respectively. The H atoms were treated as riding, with C—H distances of 0.95 Å and N—H distances of 0.88 Å.
Data collection: KappaCCD Server Software (Nonius, 1997); cell refinement: DENZO-SMN (Otwinowski & Minor, 1997); data reduction: DENZO-SMN; program(s) used to solve structure: SHELXS97 (Sheldrick, 1997); program(s) used to refine structure: SHELXL97 (Sheldrick, 1997); molecular graphics: PLATON (Spek, 2002); software used to prepare material for publication: SHELXL97 and PRPKAPPA (Ferguson, 1999).
C13H11NO | Z = 2 |
Mr = 197.23 | F(000) = 208 |
Triclinic, P1 | Dx = 1.359 Mg m−3 |
Hall symbol: -P 1 | Mo Kα radiation, λ = 0.71073 Å |
a = 5.3406 (5) Å | Cell parameters from 2140 reflections |
b = 7.7727 (7) Å | θ = 3.5–27.5° |
c = 12.3901 (15) Å | µ = 0.09 mm−1 |
α = 72.702 (3)° | T = 120 K |
β = 79.389 (3)° | Block, colourless |
γ = 89.914 (5)° | 0.08 × 0.06 × 0.04 mm |
V = 481.89 (9) Å3 |
Nonius KappaCCD area-detector diffractometer | 2140 independent reflections |
Radiation source: fine-focus sealed X-ray tube | 1092 reflections with I > 2σ(I) |
Graphite monochromator | Rint = 0.079 |
ϕ scans, and ω scans with κ offsets | θmax = 27.5°, θmin = 3.5° |
Absorption correction: multi-scan (DENZO-SMN; Otwinowski & Minor, 1997) | h = −6→6 |
Tmin = 0.989, Tmax = 0.997 | k = −10→10 |
7033 measured reflections | l = −15→15 |
Refinement on F2 | Primary atom site location: structure-invariant direct methods |
Least-squares matrix: full | Secondary atom site location: difference Fourier map |
R[F2 > 2σ(F2)] = 0.061 | Hydrogen site location: inferred from neighbouring sites |
wR(F2) = 0.160 | H-atom parameters constrained |
S = 0.96 | w = 1/[σ2(Fo2) + (0.0761P)2] where P = (Fo2 + 2Fc2)/3 |
2140 reflections | (Δ/σ)max < 0.001 |
159 parameters | Δρmax = 0.27 e Å−3 |
4 restraints | Δρmin = −0.25 e Å−3 |
C13H11NO | γ = 89.914 (5)° |
Mr = 197.23 | V = 481.89 (9) Å3 |
Triclinic, P1 | Z = 2 |
a = 5.3406 (5) Å | Mo Kα radiation |
b = 7.7727 (7) Å | µ = 0.09 mm−1 |
c = 12.3901 (15) Å | T = 120 K |
α = 72.702 (3)° | 0.08 × 0.06 × 0.04 mm |
β = 79.389 (3)° |
Nonius KappaCCD area-detector diffractometer | 2140 independent reflections |
Absorption correction: multi-scan (DENZO-SMN; Otwinowski & Minor, 1997) | 1092 reflections with I > 2σ(I) |
Tmin = 0.989, Tmax = 0.997 | Rint = 0.079 |
7033 measured reflections |
R[F2 > 2σ(F2)] = 0.061 | 4 restraints |
wR(F2) = 0.160 | H-atom parameters constrained |
S = 0.96 | Δρmax = 0.27 e Å−3 |
2140 reflections | Δρmin = −0.25 e Å−3 |
159 parameters |
x | y | z | Uiso*/Ueq | Occ. (<1) | |
O11 | −0.1228 (5) | 0.7445 (8) | 0.0032 (3) | 0.0310 (9) | 0.839 (5) |
C11 | 0.3558 (7) | 0.7580 (4) | −0.1536 (2) | 0.0205 (7) | 0.839 (5) |
C12 | 0.1972 (7) | 0.8436 (5) | −0.2274 (3) | 0.0218 (8) | 0.839 (5) |
C13 | 0.2568 (10) | 0.8509 (8) | −0.3418 (3) | 0.0265 (9) | 0.839 (5) |
C14 | 0.4726 (13) | 0.7742 (11) | −0.3849 (4) | 0.0287 (10) | 0.839 (5) |
C15 | 0.6307 (9) | 0.6894 (8) | −0.3111 (4) | 0.0274 (9) | 0.839 (5) |
C16 | 0.5744 (6) | 0.6806 (4) | −0.1961 (3) | 0.0240 (8) | 0.839 (5) |
C17 | 0.0825 (5) | 0.7454 (4) | 0.0341 (3) | 0.0233 (7) | 0.839 (5) |
N11 | 0.3126 (4) | 0.7517 (3) | −0.0365 (2) | 0.0236 (6) | 0.839 (5) |
C21 | 0.0876 (7) | 0.7398 (4) | 0.1551 (2) | 0.0237 (8) | 0.839 (5) |
C22 | −0.1161 (6) | 0.6534 (5) | 0.2412 (3) | 0.0257 (8) | 0.839 (5) |
C23 | −0.1225 (10) | 0.6464 (10) | 0.3541 (4) | 0.0277 (9) | 0.839 (5) |
C24 | 0.0742 (16) | 0.7284 (14) | 0.3831 (4) | 0.0305 (10) | 0.839 (5) |
C25 | 0.2777 (11) | 0.8169 (9) | 0.2974 (4) | 0.0318 (11) | 0.839 (5) |
C26 | 0.2871 (7) | 0.8208 (5) | 0.1846 (3) | 0.0261 (9) | 0.839 (5) |
N31 | 0.2751 (7) | 0.7473 (5) | 0.0386 (3) | 0.049 (4)* | 0.161 (5) |
C37 | 0.129 (3) | 0.753 (3) | −0.0384 (11) | 0.041 (4)* | 0.161 (5) |
O31 | −0.108 (3) | 0.744 (5) | −0.022 (3) | 0.041 (4)* | 0.161 (5) |
C31 | 0.187 (3) | 0.737 (3) | 0.1560 (8) | 0.026 (4)* | 0.161 (5) |
C32 | −0.038 (3) | 0.649 (3) | 0.2266 (17) | 0.026 (4)* | 0.161 (5) |
C33 | −0.094 (5) | 0.645 (5) | 0.3413 (17) | 0.026 (4)* | 0.161 (5) |
C34 | 0.074 (7) | 0.730 (6) | 0.3855 (14) | 0.026 (4)* | 0.161 (5) |
C35 | 0.299 (6) | 0.817 (5) | 0.3149 (19) | 0.026 (4)* | 0.161 (5) |
C36 | 0.355 (3) | 0.821 (3) | 0.2001 (16) | 0.026 (4)* | 0.161 (5) |
C41 | 0.254 (3) | 0.761 (3) | −0.1574 (13) | 0.036 (4)* | 0.161 (5) |
C42 | 0.135 (4) | 0.852 (3) | −0.247 (2) | 0.036 (4)* | 0.161 (5) |
C43 | 0.246 (6) | 0.864 (5) | −0.3602 (17) | 0.036 (4)* | 0.161 (5) |
C44 | 0.475 (7) | 0.784 (6) | −0.3831 (15) | 0.036 (4)* | 0.161 (5) |
C45 | 0.594 (5) | 0.693 (5) | −0.293 (2) | 0.036 (4)* | 0.161 (5) |
C46 | 0.483 (3) | 0.681 (3) | −0.1803 (18) | 0.036 (4)* | 0.161 (5) |
H12 | 0.0488 | 0.8969 | −0.1994 | 0.026* | 0.839 (5) |
H13 | 0.1478 | 0.9096 | −0.3923 | 0.032* | 0.839 (5) |
H14 | 0.5109 | 0.7800 | −0.4639 | 0.034* | 0.839 (5) |
H15 | 0.7791 | 0.6366 | −0.3396 | 0.033* | 0.839 (5) |
H16 | 0.6840 | 0.6221 | −0.1458 | 0.029* | 0.839 (5) |
H11 | 0.4488 | 0.7518 | −0.0059 | 0.028* | 0.839 (5) |
H22 | −0.2533 | 0.5982 | 0.2221 | 0.031* | 0.839 (5) |
H23 | −0.2623 | 0.5850 | 0.4123 | 0.033* | 0.839 (5) |
H24 | 0.0699 | 0.7241 | 0.4608 | 0.037* | 0.839 (5) |
H25 | 0.4118 | 0.8753 | 0.3165 | 0.038* | 0.839 (5) |
H26 | 0.4297 | 0.8788 | 0.1269 | 0.031* | 0.839 (5) |
H31 | 0.4412 | 0.7501 | 0.0146 | 0.059* | 0.161 (5) |
H32 | −0.1528 | 0.5913 | 0.1964 | 0.031* | 0.161 (5) |
H33 | −0.2479 | 0.5853 | 0.3896 | 0.031* | 0.161 (5) |
H34 | 0.0354 | 0.7272 | 0.4639 | 0.031* | 0.161 (5) |
H35 | 0.4138 | 0.8750 | 0.3451 | 0.031* | 0.161 (5) |
H36 | 0.5088 | 0.8809 | 0.1519 | 0.031* | 0.161 (5) |
H42 | −0.0215 | 0.9059 | −0.2317 | 0.044* | 0.161 (5) |
H43 | 0.1652 | 0.9259 | −0.4217 | 0.044* | 0.161 (5) |
H44 | 0.5511 | 0.7926 | −0.4602 | 0.044* | 0.161 (5) |
H45 | 0.7504 | 0.6392 | −0.3088 | 0.044* | 0.161 (5) |
H46 | 0.5637 | 0.6191 | −0.1188 | 0.044* | 0.161 (5) |
U11 | U22 | U33 | U12 | U13 | U23 | |
O11 | 0.0221 (13) | 0.0532 (15) | 0.021 (2) | 0.0018 (10) | −0.0043 (10) | −0.0152 (18) |
C11 | 0.0197 (16) | 0.0194 (15) | 0.0212 (17) | −0.0035 (14) | −0.0025 (12) | −0.0054 (12) |
C12 | 0.0187 (16) | 0.0250 (16) | 0.025 (2) | 0.0027 (13) | −0.0078 (14) | −0.0097 (15) |
C13 | 0.0266 (17) | 0.0258 (19) | 0.026 (2) | −0.0021 (13) | −0.0098 (17) | −0.0039 (17) |
C14 | 0.0342 (18) | 0.027 (2) | 0.0248 (18) | −0.0052 (14) | −0.0004 (13) | −0.0114 (13) |
C15 | 0.0263 (19) | 0.0255 (17) | 0.031 (2) | 0.0004 (15) | −0.0001 (15) | −0.0127 (17) |
C16 | 0.0217 (17) | 0.0210 (15) | 0.028 (2) | 0.0014 (14) | −0.0065 (14) | −0.0046 (13) |
C17 | 0.0206 (15) | 0.0218 (14) | 0.0262 (18) | 0.0005 (11) | −0.0018 (12) | −0.0069 (13) |
N11 | 0.0166 (12) | 0.0306 (14) | 0.0234 (15) | 0.0005 (10) | −0.0053 (10) | −0.0070 (11) |
C21 | 0.0216 (16) | 0.0278 (17) | 0.0229 (19) | 0.0056 (16) | −0.0051 (14) | −0.0091 (13) |
C22 | 0.0181 (17) | 0.0275 (17) | 0.033 (2) | 0.0048 (16) | −0.0058 (15) | −0.0108 (14) |
C23 | 0.0251 (19) | 0.0312 (18) | 0.025 (2) | 0.0043 (15) | −0.0003 (13) | −0.0080 (16) |
C24 | 0.0377 (19) | 0.0330 (19) | 0.0221 (18) | 0.0098 (14) | −0.0079 (13) | −0.0089 (13) |
C25 | 0.034 (2) | 0.0312 (18) | 0.036 (3) | 0.0015 (15) | −0.0141 (19) | −0.014 (2) |
C26 | 0.0212 (17) | 0.0257 (17) | 0.0298 (19) | 0.0006 (16) | −0.0050 (14) | −0.0059 (14) |
O11—C17 | 1.227 (4) | N31—C37 | 1.331 (9) |
C11—C12 | 1.385 (4) | N31—C31 | 1.422 (9) |
C11—C16 | 1.400 (4) | N31—H31 | 0.88 |
C11—N11 | 1.412 (4) | C37—O31 | 1.241 (10) |
C12—C13 | 1.379 (4) | C37—C41 | 1.487 (10) |
C12—H12 | 0.95 | C31—C32 | 1.39 |
C13—C14 | 1.389 (4) | C31—C36 | 1.39 |
C13—H13 | 0.95 | C32—C33 | 1.39 |
C14—C15 | 1.381 (5) | C32—H32 | 0.95 |
C14—H14 | 0.95 | C33—C34 | 1.39 |
C15—C16 | 1.382 (5) | C33—H33 | 0.95 |
C15—H15 | 0.95 | C34—C35 | 1.39 |
C16—H16 | 0.95 | C34—H34 | 0.95 |
C17—N11 | 1.362 (3) | C35—C36 | 1.39 |
C17—C21 | 1.491 (4) | C35—H35 | 0.95 |
N11—H11 | 0.88 | C36—H36 | 0.95 |
C21—C22 | 1.386 (4) | C41—C42 | 1.39 |
C21—C26 | 1.396 (4) | C41—C46 | 1.39 |
C22—C23 | 1.378 (5) | C42—C43 | 1.39 |
C22—H22 | 0.95 | C42—H42 | 0.95 |
C23—C24 | 1.384 (5) | C43—C44 | 1.39 |
C23—H23 | 0.95 | C43—H43 | 0.95 |
C24—C25 | 1.388 (4) | C44—C45 | 1.39 |
C24—H24 | 0.95 | C44—H44 | 0.95 |
C25—C26 | 1.380 (4) | C45—C46 | 1.39 |
C25—H25 | 0.95 | C45—H45 | 0.95 |
C26—H26 | 0.95 | C46—H46 | 0.95 |
C12—C11—C16 | 119.7 (3) | C37—N31—C31 | 125.7 (9) |
C12—C11—N11 | 122.5 (3) | C37—N31—H31 | 117.1 |
C16—C11—N11 | 117.8 (3) | C31—N31—H31 | 117.1 |
C13—C12—C11 | 119.3 (3) | O31—C37—N31 | 127.7 (18) |
C13—C12—H12 | 120.3 | O31—C37—C41 | 113.7 (17) |
C11—C12—H12 | 120.3 | N31—C37—C41 | 118.5 (14) |
C12—C13—C14 | 121.5 (3) | C32—C31—C36 | 120.0 |
C12—C13—H13 | 119.3 | C32—C31—N31 | 126.7 (13) |
C14—C13—H13 | 119.3 | C36—C31—N31 | 113.3 (13) |
C15—C14—C13 | 119.0 (3) | C31—C32—C33 | 120.0 |
C15—C14—H14 | 120.5 | C31—C32—H32 | 120.0 |
C13—C14—H14 | 120.5 | C33—C32—H32 | 120.0 |
C14—C15—C16 | 120.4 (3) | C32—C33—C34 | 120.0 |
C14—C15—H15 | 119.8 | C32—C33—H33 | 120.0 |
C16—C15—H15 | 119.8 | C34—C33—H33 | 120.0 |
C15—C16—C11 | 120.1 (3) | C33—C34—C35 | 120.0 |
C15—C16—H16 | 120.0 | C33—C34—H34 | 120.0 |
C11—C16—H16 | 120.0 | C35—C34—H34 | 120.0 |
O11—C17—N11 | 123.6 (3) | C36—C35—C34 | 120.0 |
O11—C17—C21 | 119.7 (3) | C36—C35—H35 | 120.0 |
N11—C17—C21 | 116.7 (3) | C34—C35—H35 | 120.0 |
C17—N11—C11 | 126.9 (2) | C35—C36—C31 | 120.0 |
C17—N11—H11 | 116.5 | C35—C36—H36 | 120.0 |
C11—N11—H11 | 116.5 | C31—C36—H36 | 120.0 |
C22—C21—C26 | 118.8 (3) | C42—C41—C46 | 120.0 |
C22—C21—C17 | 118.6 (3) | C42—C41—C37 | 117.9 (17) |
C26—C21—C17 | 122.6 (3) | C46—C41—C37 | 122.1 (17) |
C23—C22—C21 | 121.0 (3) | C41—C42—C43 | 120.0 |
C23—C22—H22 | 119.5 | C41—C42—H42 | 120.0 |
C21—C22—H22 | 119.5 | C43—C42—H42 | 120.0 |
C22—C23—C24 | 120.2 (3) | C42—C43—C44 | 120.0 |
C22—C23—H23 | 119.9 | C42—C43—H43 | 120.0 |
C24—C23—H23 | 119.9 | C44—C43—H43 | 120.0 |
C23—C24—C25 | 119.3 (3) | C43—C44—C45 | 120.0 |
C23—C24—H24 | 120.3 | C43—C44—H44 | 120.0 |
C25—C24—H24 | 120.3 | C45—C44—H44 | 120.0 |
C26—C25—C24 | 120.6 (3) | C46—C45—C44 | 120.0 |
C26—C25—H25 | 119.7 | C46—C45—H45 | 120.0 |
C24—C25—H25 | 119.7 | C44—C45—H45 | 120.0 |
C25—C26—C21 | 120.1 (3) | C45—C46—C41 | 120.0 |
C25—C26—H26 | 119.9 | C45—C46—H46 | 120.0 |
C21—C26—H26 | 119.9 | C41—C46—H46 | 120.0 |
C11—N11—C17—C21 | −179.8 (3) | N11—C17—C21—C22 | −150.7 (3) |
C12—C11—N11—C17 | 32.5 (5) |
D—H···A | D—H | H···A | D···A | D—H···A |
N11—H11···O11i | 0.88 | 2.31 | 3.141 (4) | 157 |
N11—H11···O31i | 0.88 | 2.34 | 3.13 (2) | 150 |
N31—H31···O11i | 0.88 | 2.31 | 3.17 (3) | 165 |
N31—H31···O31i | 0.88 | 2.37 | 3.24 (3) | 172 |
Symmetry code: (i) x+1, y, z. |
Experimental details
Crystal data | |
Chemical formula | C13H11NO |
Mr | 197.23 |
Crystal system, space group | Triclinic, P1 |
Temperature (K) | 120 |
a, b, c (Å) | 5.3406 (5), 7.7727 (7), 12.3901 (15) |
α, β, γ (°) | 72.702 (3), 79.389 (3), 89.914 (5) |
V (Å3) | 481.89 (9) |
Z | 2 |
Radiation type | Mo Kα |
µ (mm−1) | 0.09 |
Crystal size (mm) | 0.08 × 0.06 × 0.04 |
Data collection | |
Diffractometer | Nonius KappaCCD area-detector diffractometer |
Absorption correction | Multi-scan (DENZO-SMN; Otwinowski & Minor, 1997) |
Tmin, Tmax | 0.989, 0.997 |
No. of measured, independent and observed [I > 2σ(I)] reflections | 7033, 2140, 1092 |
Rint | 0.079 |
(sin θ/λ)max (Å−1) | 0.650 |
Refinement | |
R[F2 > 2σ(F2)], wR(F2), S | 0.061, 0.160, 0.96 |
No. of reflections | 2140 |
No. of parameters | 159 |
No. of restraints | 4 |
H-atom treatment | H-atom parameters constrained |
Δρmax, Δρmin (e Å−3) | 0.27, −0.25 |
Computer programs: KappaCCD Server Software (Nonius, 1997), DENZO-SMN (Otwinowski & Minor, 1997), DENZO-SMN, SHELXS97 (Sheldrick, 1997), SHELXL97 (Sheldrick, 1997), PLATON (Spek, 2002), SHELXL97 and PRPKAPPA (Ferguson, 1999).
C11—N11—C17—C21 | −179.8 (3) | N11—C17—C21—C22 | −150.7 (3) |
C12—C11—N11—C17 | 32.5 (5) |
D—H···A | D—H | H···A | D···A | D—H···A |
N11—H11···O11i | 0.88 | 2.31 | 3.141 (4) | 157 |
N11—H11···O31i | 0.88 | 2.34 | 3.13 (2) | 150 |
N31—H31···O11i | 0.88 | 2.31 | 3.17 (3) | 165 |
N31—H31···O31i | 0.88 | 2.37 | 3.24 (3) | 172 |
Symmetry code: (i) x+1, y, z. |
The crystal structure of benzanilide, PhNHCOPh, (I), was first reported many years ago (Kashino et al., 1979). These authors reported an ambient temperature study of a C2/c phase crystallized from ethanol, which has Z' = 1/2 with the molecules disordered across twofold rotation axes, such that the O atom and the H atom bonded to N both lie on the twofold axes. A unique H···O contact thus characterizes the N—H···O hydrogen bonds, which link the molecules into a C(4) chain generated by translation along the shortest axis of the cell, b = 5.323 (3) Å. \sch
In this paper, we report the structure of a triclinic (P1, Z' = 1) polymorph of (I), also crystallized from ethanol. In this polymorph, the molecules (Fig. 1) lie in general positions, but it was clear early in the refinement that there was some orientational disorder in the structure. Some 16% of the molecular sites are occupied by a second orientation, which is approximately related to the major orientation by a 180° rotation of the molecule about a line in the plane of the central HNCO unit lying normal to the C14···C24 line. Hence atoms C14 and C44 have almost coincident sites, as do atoms C24 and C34. Similarly, atom pairs O11 and O21, and H11 and H21, occupy sites which are almost coincident (Fig. 2).
The molecules of (I) are linked into C(4) chains generated by translation along the [100] direction (Fig. 3), and because of the close proximity of the alternative donor and acceptor sites, hydrogen bonds can be formed between any two adjacent molecules within the chain, regardless of whether they adopt the major or minor orientation (Table 2). Hence the hydrogen bonding imposes no necessary correlation between the molecular orientations at adjacent sites within the chain, although this may be imposed by non-bonded C···C contacts.
Within the molecule of (I), the bond lengths and angles present no unusual features. The central C—N(H)—C(O)—C fragment is essentially planar and the phenyl rings are each twisted by ca 30° away from this plane (Table 1). The conformation in the P1 polymorph thus resembles that in the C2/c polymorph, where the dihedral angles between the rings and the central unit are both ca 31°.
The density reported for the C2/c polymorph (1.321 Mg m−3) is somewhat lower than that found here for the P1 form. This suggests that the P1 form may be thermodynamically more stable (Burger & Ramberger, 1979). The ambient temperature unit cell reported for the C2/c polymorph [a = 24.34 (4), b = 5.325 (3) and c = 8.012 (8) Å, and β = 107.2 (3)°; Kashino et al., 1979] can be readily related to the P1 cell by the transformation (0, −1, 0/ 0, 0, −1/ 1/2, 0, 0). In the C2/c polymorph, the molecules lie across twofold rotation axes, with the centroid of the reference molecule at (0, 0.205, 1/4), while in the P1 polymorph, the centroid of the reference molecule is at approximately (0.198, 0.749, −0.002), so that the projections of the two structures down their short axes are very similar (Fig. 4). However, alternate chains in the [100] direction in the C2/c structure are related by the C-centring operation and hence suffer a shift of b/2 between adjacent chains in this direction, whereas no such shifts occur in the P1 polymorph.
In addition to these translations, the very different disorder ratios (50:50 in C2/c and 84:16 in P1) effectively preclude the possibility of a simple displacive phase transformation between the two polymorphs, as this would require at least 34% of the molecules to change orientation on conversion of either phase to the other. Since the molecules are ca 11.2 Å in length, and longer if the van der Waals surfaces are included, the end-over-end rotation involved in this change of orientation would require each such molecule to sweep out, unhindered, a disc of area nearly 100 Å2.
As in the P1 polymorph, there is no necessary correlation between the orientations of adjacent molecules in the C2/c polymorph imposed by the hydrogen bonding. However, it was found (Kashino et al., 1979) that non-bonded C···C contacts imposed correlation within, but not between, the [010] chains. Kashino et al. (1979) discussed the C2/c structure in terms of the fully ordered structures which could arise in each of the sub-groups of C2/c (Cc, C1, P21/c and P21/n); the representation in Fig. 4 b here is in fact the C1 sub-structure.
In our recent structural studies, we have encountered several examples of concomitant polymorphism. Thus, when crystallized from ethanol, 2-iodo-4-nitro aniline yields a mixture of triclinic (P1, Z' = 1) and orthorhombic (Pbca, Z' = 1) crystals which have slightly different colours (McWilliam et al., 2001). Crystallization of ethyl N-(2-amino-6-benzyloxy-5-nitrosopyrimidin-2-yl)-3-aminopropanoate from acetonitrile-ethanol-water (1/1/1 by volume) yields a mixture of two monoclinic polymorphs, one blue (P21/c, Z' = 1) and the other pink (P21, Z' = 2) (Quesada et al., 2002), where the conformations of the three independent molecules of the nitrosopyrimidine are all different, so that these concomitant polymorphs are also conformational polymorphs. The 1:1 adduct formed between (S)-malic acid and 4,4'-bipyridyl crystallizes from methanol as a mixture of triclinic (P1, Z' = 1) and monoclinic (C2, Z' = 1) polymorphs (Farrell et al., 2002). Finally, benzanilide crystallizes from ethanol in both a monoclinic form (C2/c, Z' = 0.5; Kashino et al., 1979) and the triclinic form (P1, Z' = 1) reported here.
We emphasize that in none of these systems had there been any attempt to engineer such polymorphic behaviour, nor was this behaviour being specifically sought after. Instead, each pair of polymorphs was identified serendipitously. In the cases of 2-iodo-4-nitroaniline and of the nitrosopyrimidine, the identification of the two forms was facilitated by their differences in colour, but in the other examples, identification depended solely upon careful scrutiny of the crystalline samples and observation of more than one crystal habit, followed in every case by careful manual separation.
Our identification, essentially by chance, of four such examples within a rather short space of time suggests to us that the phenomenon of concomitant polymorphism may, in fact, be a rather common one, certainly far more common than the current literature (for a review, see Bernstein et al., 1999) tends to suggest, but one which goes largely unnoticed. On the other hand, we note the recent report on 3,6,13,16-tetrabromo-2,7,12,17-tetrapropylporphycene, where monoclinic prisms and triclinic plates crystallize concurrently from dichloromethane-hexane (Aritome et al., 2002).