Supporting information
Crystallographic Information File (CIF) https://doi.org/10.1107/S0108270102000185/de1177sup1.cif | |
Structure factor file (CIF format) https://doi.org/10.1107/S0108270102000185/de1177Isup2.hkl |
CCDC reference: 183036
Compound (I) was synthesized according to the method of Shah (2000). Full details of the synthesis will be published elsewhere by Dave et al. Thin needle-shaped crystals of (I) were grown from chloroform-ethanol solution by slow evaporation. The crystal density was measured by flotation in an aqueous potassium iodide solution.
H atoms were treated as riding, with C—H = 0.93–0.98 Å, and Uiso(H) = 1.5Ueq(C) for methyl H and 1.2Ueq(C) for all others. Are these the correct constraints?
Data collection: CAD-4 Software (Enraf-Nonius, 1989); cell refinement: CAD-4 Software (Enraf-Nonius, 1989); data reduction: MolEN (Nonius, 1997); program(s) used to solve structure: SHELXS97 (Sheldrick 1990); program(s) used to refine structure: SHELXL97 (Sheldrick 1997); molecular graphics: ORTEPII (Johnson, 1976); software used to prepare material for publication: SHELXL97.
Fig. 1. A view of the molecular structure of (I) showing 50% probability displacement ellipsoids. H atoms have been omitted for clarity. |
C20H18N2O | F(000) = 640 |
Mr = 302.36 | Dx = 1.214 Mg m−3 Dm = 1.204 Mg m−3 Dm measured by flotation |
Monoclinic, P21/c | Cu Kα radiation, λ = 1.54180 Å |
a = 13.738 (3) Å | Cell parameters from 25 reflections |
b = 7.4751 (16) Å | θ = 25–35° |
c = 17.1566 (18) Å | µ = 0.60 mm−1 |
β = 110.117 (13)° | T = 293 K |
V = 1654.4 (5) Å3 | Needle, colourless |
Z = 4 | 0.2 × 0.1 × 0.1 mm |
Enraf-Nonius CAD-4 diffractometer | 2548 reflections with I > 2σ(I) |
Radiation source: fine-focus sealed tube | Rint = 0.029 |
Graphite monochromator | θmax = 68.0°, θmin = 3.4° |
ω/2θ scans | h = 0→16 |
Absorption correction: empirical (using intensity measurements) via ψ scan (North et al., 1968) | k = 0→8 |
Tmin = 0.931, Tmax = 0.942 | l = −20→19 |
3141 measured reflections | 2 standard reflections every 60 min |
3141 independent reflections | intensity decay: 1% |
Refinement on F2 | H-atom parameters constrained |
Least-squares matrix: full | |
R[F2 > 2σ(F2)] = 0.046 | (Δ/σ)max = 0.034 |
wR(F2) = 0.141 | Δρmax = 0.36 or 0.46? e Å−3 |
S = 1.06 | Δρmin = −0.23 or −0.18? e Å−3 |
3010 reflections | Extinction correction: SHELXL97 (Sheldrick, 1997), Fc*=kFc[1+0.001xFc2λ3/sin(2θ)]-1/4 |
220 parameters | Extinction coefficient: 0.0042 (5) |
C20H18N2O | V = 1654.4 (5) Å3 |
Mr = 302.36 | Z = 4 |
Monoclinic, P21/c | Cu Kα radiation |
a = 13.738 (3) Å | µ = 0.60 mm−1 |
b = 7.4751 (16) Å | T = 293 K |
c = 17.1566 (18) Å | 0.2 × 0.1 × 0.1 mm |
β = 110.117 (13)° |
Enraf-Nonius CAD-4 diffractometer | 2548 reflections with I > 2σ(I) |
Absorption correction: empirical (using intensity measurements) via ψ scan (North et al., 1968) | Rint = 0.029 |
Tmin = 0.931, Tmax = 0.942 | 2 standard reflections every 60 min |
3141 measured reflections | intensity decay: 1% |
3141 independent reflections |
R[F2 > 2σ(F2)] = 0.046 | 220 parameters |
wR(F2) = 0.141 | H-atom parameters constrained |
S = 1.06 | Δρmax = 0.36 or 0.46? e Å−3 |
3010 reflections | Δρmin = −0.23 or −0.18? e Å−3 |
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. |
x | y | z | Uiso*/Ueq | Occ. (<1) | |
O1 | 0.77347 (14) | 0.2377 (2) | 0.43189 (12) | 0.0884 (8) | 1.013 (6) |
N1 | 0.82256 (11) | −0.0167 (2) | 0.38443 (9) | 0.0534 (4) | |
N2 | 0.73306 (17) | −0.2059 (3) | 0.56059 (12) | 0.0863 (6) | |
C5 | 0.79970 (13) | −0.1966 (2) | 0.35755 (10) | 0.0479 (4) | |
C4 | 0.70478 (12) | −0.2589 (2) | 0.34080 (10) | 0.0481 (4) | |
H4 | 0.6907 | −0.3777 | 0.3247 | 0.058* | |
C3 | 0.61949 (13) | −0.1424 (2) | 0.34718 (10) | 0.0467 (4) | |
H3 | 0.5991 | −0.0641 | 0.2984 | 0.056* | |
C2 | 0.66548 (14) | −0.0194 (2) | 0.42420 (11) | 0.0512 (4) | |
C1 | 0.75905 (15) | 0.0806 (3) | 0.41405 (12) | 0.0574 (5) | |
C20 | 0.90697 (17) | 0.0788 (3) | 0.36906 (14) | 0.0701 (6) | |
H20A | 0.9462 | −0.0035 | 0.3486 | 0.105* | |
H20B | 0.9514 | 0.1315 | 0.4199 | 0.105* | |
H20C | 0.8788 | 0.1710 | 0.3286 | 0.105* | |
C12 | 0.88503 (13) | −0.3076 (2) | 0.34924 (10) | 0.0485 (4) | |
C13 | 0.97872 (15) | −0.3286 (3) | 0.41369 (12) | 0.0635 (5) | |
H13 | 0.9908 | −0.2663 | 0.4631 | 0.076* | |
C14 | 1.05406 (16) | −0.4411 (3) | 0.40519 (14) | 0.0718 (6) | |
H14 | 1.1167 | −0.4534 | 0.4488 | 0.086* | |
C15 | 1.03754 (16) | −0.5343 (3) | 0.33358 (15) | 0.0681 (6) | |
H15 | 1.0883 | −0.6111 | 0.3285 | 0.082* | |
C16 | 0.94566 (16) | −0.5142 (3) | 0.26894 (13) | 0.0630 (5) | |
H16 | 0.9343 | −0.5772 | 0.2199 | 0.076* | |
C17 | 0.86985 (14) | −0.4009 (2) | 0.27638 (11) | 0.0521 (4) | |
H17 | 0.8081 | −0.3872 | 0.2320 | 0.062* | |
C6 | 0.52275 (12) | −0.2408 (2) | 0.34581 (10) | 0.0453 (4) | |
C7 | 0.52640 (13) | −0.3973 (2) | 0.38960 (11) | 0.0489 (4) | |
H7 | 0.5902 | −0.4470 | 0.4201 | 0.059* | |
C8 | 0.43610 (14) | −0.4808 (3) | 0.38843 (12) | 0.0562 (5) | |
H8 | 0.4398 | −0.5859 | 0.4183 | 0.067* | |
C9 | 0.34157 (14) | −0.4106 (3) | 0.34381 (13) | 0.0640 (5) | |
H9 | 0.2811 | −0.4657 | 0.3443 | 0.077* | |
C10 | 0.33677 (15) | −0.2578 (3) | 0.29831 (17) | 0.0760 (7) | |
H10 | 0.2727 | −0.2106 | 0.2666 | 0.091* | |
C11 | 0.42643 (15) | −0.1743 (3) | 0.29955 (14) | 0.0661 (5) | |
H11 | 0.4221 | −0.0707 | 0.2685 | 0.079* | |
C18 | 0.70442 (15) | −0.1267 (3) | 0.50049 (12) | 0.0578 (5) | |
C19 | 0.58755 (18) | 0.1167 (3) | 0.43317 (15) | 0.0721 (6) | |
H19A | 0.5620 | 0.1874 | 0.3836 | 0.108* | |
H19B | 0.6207 | 0.1932 | 0.4797 | 0.108* | |
H19C | 0.5308 | 0.0552 | 0.4419 | 0.108* |
U11 | U22 | U33 | U12 | U13 | U23 | |
O1 | 0.1115 (14) | 0.0549 (10) | 0.1181 (15) | −0.0284 (8) | 0.0642 (11) | −0.0326 (9) |
N1 | 0.0604 (9) | 0.0502 (9) | 0.0569 (8) | −0.0159 (7) | 0.0296 (7) | −0.0095 (7) |
N2 | 0.0972 (15) | 0.1006 (16) | 0.0549 (10) | −0.0116 (12) | 0.0182 (10) | 0.0084 (10) |
C5 | 0.0538 (9) | 0.0494 (9) | 0.0436 (8) | −0.0082 (8) | 0.0207 (7) | −0.0062 (7) |
C4 | 0.0505 (9) | 0.0482 (9) | 0.0490 (9) | −0.0073 (7) | 0.0213 (7) | −0.0094 (7) |
C3 | 0.0545 (9) | 0.0441 (9) | 0.0439 (8) | −0.0028 (7) | 0.0199 (7) | 0.0018 (7) |
C2 | 0.0629 (10) | 0.0438 (9) | 0.0532 (9) | −0.0020 (8) | 0.0279 (8) | −0.0031 (7) |
C1 | 0.0715 (12) | 0.0470 (10) | 0.0590 (10) | −0.0134 (9) | 0.0292 (9) | −0.0095 (8) |
C20 | 0.0729 (13) | 0.0680 (13) | 0.0803 (14) | −0.0278 (11) | 0.0404 (11) | −0.0133 (11) |
C12 | 0.0491 (9) | 0.0521 (10) | 0.0494 (9) | −0.0088 (7) | 0.0234 (7) | −0.0033 (7) |
C13 | 0.0592 (11) | 0.0770 (14) | 0.0533 (10) | −0.0043 (10) | 0.0181 (9) | −0.0077 (10) |
C14 | 0.0522 (11) | 0.0845 (15) | 0.0765 (13) | 0.0015 (10) | 0.0193 (10) | 0.0058 (12) |
C15 | 0.0623 (12) | 0.0640 (12) | 0.0905 (15) | 0.0002 (10) | 0.0423 (11) | 0.0014 (11) |
C16 | 0.0734 (13) | 0.0620 (12) | 0.0680 (12) | −0.0109 (10) | 0.0427 (10) | −0.0106 (9) |
C17 | 0.0556 (10) | 0.0564 (10) | 0.0497 (9) | −0.0102 (8) | 0.0251 (8) | −0.0047 (8) |
C6 | 0.0461 (9) | 0.0450 (9) | 0.0459 (8) | 0.0028 (7) | 0.0171 (7) | 0.0025 (7) |
C7 | 0.0468 (9) | 0.0471 (9) | 0.0522 (9) | 0.0036 (7) | 0.0165 (7) | 0.0052 (7) |
C8 | 0.0592 (11) | 0.0522 (10) | 0.0627 (11) | −0.0042 (8) | 0.0280 (9) | 0.0031 (8) |
C9 | 0.0482 (10) | 0.0673 (13) | 0.0819 (13) | −0.0059 (9) | 0.0294 (9) | −0.0069 (11) |
C10 | 0.0444 (10) | 0.0729 (14) | 0.1014 (17) | 0.0118 (10) | 0.0134 (10) | 0.0086 (13) |
C11 | 0.0555 (11) | 0.0585 (12) | 0.0763 (13) | 0.0086 (9) | 0.0122 (9) | 0.0191 (10) |
C18 | 0.0653 (11) | 0.0619 (12) | 0.0491 (10) | −0.0101 (9) | 0.0233 (9) | −0.0072 (9) |
C19 | 0.0837 (14) | 0.0550 (12) | 0.0917 (15) | 0.0041 (10) | 0.0481 (13) | −0.0091 (11) |
O1—C1 | 1.212 (2) | C14—C15 | 1.362 (3) |
N1—C1 | 1.361 (2) | C14—H14 | 0.9300 |
N1—C5 | 1.421 (2) | C15—C16 | 1.372 (3) |
N1—C20 | 1.461 (2) | C15—H15 | 0.9300 |
N2—C18 | 1.135 (3) | C16—C17 | 1.382 (3) |
C5—C4 | 1.320 (2) | C16—H16 | 0.9300 |
C5—C12 | 1.482 (2) | C17—H17 | 0.9300 |
C4—C3 | 1.494 (2) | C6—C11 | 1.380 (2) |
C4—H4 | 0.9300 | C6—C7 | 1.382 (2) |
C3—C6 | 1.512 (2) | C7—C8 | 1.383 (2) |
C3—C2 | 1.555 (2) | C7—H7 | 0.9300 |
C3—H3 | 0.9800 | C8—C9 | 1.366 (3) |
C2—C18 | 1.470 (3) | C8—H8 | 0.9300 |
C2—C19 | 1.522 (3) | C9—C10 | 1.372 (3) |
C2—C1 | 1.548 (2) | C9—H9 | 0.9300 |
C20—H20A | 0.9600 | C10—C11 | 1.374 (3) |
C20—H20B | 0.9600 | C10—H10 | 0.9300 |
C20—H20C | 0.9600 | C11—H11 | 0.9300 |
C12—C17 | 1.383 (2) | C19—H19A | 0.9600 |
C12—C13 | 1.387 (3) | C19—H19B | 0.9600 |
C13—C14 | 1.380 (3) | C19—H19C | 0.9600 |
C13—H13 | 0.9300 | ||
C1—N1—C5 | 121.87 (14) | C15—C14—H14 | 119.7 |
C1—N1—C20 | 117.20 (16) | C13—C14—H14 | 119.7 |
C5—N1—C20 | 120.27 (15) | C14—C15—C16 | 119.7 (2) |
C4—C5—N1 | 119.75 (16) | C14—C15—H15 | 120.2 |
C4—C5—C12 | 122.43 (16) | C16—C15—H15 | 120.2 |
N1—C5—C12 | 117.83 (14) | C15—C16—C17 | 120.35 (19) |
C5—C4—C3 | 121.28 (16) | C15—C16—H16 | 119.8 |
C5—C4—H4 | 119.4 | C17—C16—H16 | 119.8 |
C3—C4—H4 | 119.4 | C16—C17—C12 | 120.56 (18) |
C4—C3—C6 | 115.00 (14) | C16—C17—H17 | 119.7 |
C4—C3—C2 | 107.90 (13) | C12—C17—H17 | 119.7 |
C6—C3—C2 | 113.52 (13) | C11—C6—C7 | 117.76 (16) |
C4—C3—H3 | 106.6 | C11—C6—C3 | 119.83 (16) |
C6—C3—H3 | 106.6 | C7—C6—C3 | 122.41 (15) |
C2—C3—H3 | 106.6 | C6—C7—C8 | 120.64 (16) |
C18—C2—C19 | 109.14 (16) | C6—C7—H7 | 119.7 |
C18—C2—C1 | 107.56 (15) | C8—C7—H7 | 119.7 |
C19—C2—C1 | 109.21 (15) | C9—C8—C7 | 120.66 (18) |
C18—C2—C3 | 110.55 (14) | C9—C8—H8 | 119.7 |
C19—C2—C3 | 112.75 (16) | C7—C8—H8 | 119.7 |
C1—C2—C3 | 107.49 (13) | C8—C9—C10 | 119.31 (18) |
O1—C1—N1 | 122.78 (18) | C8—C9—H9 | 120.3 |
O1—C1—C2 | 120.61 (18) | C10—C9—H9 | 120.3 |
N1—C1—C2 | 116.61 (15) | C9—C10—C11 | 120.10 (18) |
N1—C20—H20A | 109.5 | C9—C10—H10 | 119.9 |
N1—C20—H20B | 109.5 | C11—C10—H10 | 119.9 |
H20A—C20—H20B | 109.5 | C10—C11—C6 | 121.49 (19) |
N1—C20—H20C | 109.5 | C10—C11—H11 | 119.3 |
H20A—C20—H20C | 109.5 | C6—C11—H11 | 119.3 |
H20B—C20—H20C | 109.5 | N2—C18—C2 | 177.8 (2) |
C17—C12—C13 | 118.25 (17) | C2—C19—H19A | 109.5 |
C17—C12—C5 | 119.38 (15) | C2—C19—H19B | 109.5 |
C13—C12—C5 | 122.30 (16) | H19A—C19—H19B | 109.5 |
C14—C13—C12 | 120.59 (19) | C2—C19—H19C | 109.5 |
C14—C13—H13 | 119.7 | H19A—C19—H19C | 109.5 |
C12—C13—H13 | 119.7 | H19B—C19—H19C | 109.5 |
C15—C14—C13 | 120.6 (2) | ||
C1—N1—C5—C4 | 17.3 (3) | C4—C5—C12—C13 | −126.4 (2) |
C20—N1—C5—C4 | −153.18 (18) | N1—C5—C12—C13 | 53.8 (2) |
C1—N1—C5—C12 | −162.90 (16) | C17—C12—C13—C14 | −0.6 (3) |
C20—N1—C5—C12 | 26.7 (2) | C5—C12—C13—C14 | 176.27 (19) |
N1—C5—C4—C3 | 1.8 (2) | C12—C13—C14—C15 | −0.4 (3) |
C12—C5—C4—C3 | −178.07 (14) | C13—C14—C15—C16 | 0.9 (3) |
C5—C4—C3—C6 | −166.41 (16) | C14—C15—C16—C17 | −0.3 (3) |
C5—C4—C3—C2 | −38.6 (2) | C15—C16—C17—C12 | −0.7 (3) |
C4—C3—C2—C18 | −62.22 (18) | C13—C12—C17—C16 | 1.1 (3) |
C6—C3—C2—C18 | 66.46 (19) | C5—C12—C17—C16 | −175.85 (16) |
C4—C3—C2—C19 | 175.31 (15) | C4—C3—C6—C11 | −137.37 (18) |
C6—C3—C2—C19 | −56.0 (2) | C2—C3—C6—C11 | 97.7 (2) |
C4—C3—C2—C1 | 54.90 (18) | C4—C3—C6—C7 | 42.6 (2) |
C6—C3—C2—C1 | −176.42 (14) | C2—C3—C6—C7 | −82.3 (2) |
C5—N1—C1—O1 | −174.80 (19) | C11—C6—C7—C8 | −1.7 (3) |
C20—N1—C1—O1 | −4.1 (3) | C3—C6—C7—C8 | 178.33 (16) |
C5—N1—C1—C2 | 5.1 (3) | C6—C7—C8—C9 | 0.2 (3) |
C20—N1—C1—C2 | 175.85 (17) | C7—C8—C9—C10 | 1.5 (3) |
C18—C2—C1—O1 | −102.1 (2) | C8—C9—C10—C11 | −1.7 (4) |
C19—C2—C1—O1 | 16.2 (3) | C9—C10—C11—C6 | 0.1 (4) |
C3—C2—C1—O1 | 138.8 (2) | C7—C6—C11—C10 | 1.5 (3) |
C18—C2—C1—N1 | 77.9 (2) | C3—C6—C11—C10 | −178.5 (2) |
C19—C2—C1—N1 | −163.74 (18) | C19—C2—C18—N2 | −12 (6) |
C3—C2—C1—N1 | −41.1 (2) | C1—C2—C18—N2 | 107 (6) |
C4—C5—C12—C17 | 50.4 (2) | C3—C2—C18—N2 | −136 (6) |
N1—C5—C12—C17 | −129.39 (17) |
Experimental details
Crystal data | |
Chemical formula | C20H18N2O |
Mr | 302.36 |
Crystal system, space group | Monoclinic, P21/c |
Temperature (K) | 293 |
a, b, c (Å) | 13.738 (3), 7.4751 (16), 17.1566 (18) |
β (°) | 110.117 (13) |
V (Å3) | 1654.4 (5) |
Z | 4 |
Radiation type | Cu Kα |
µ (mm−1) | 0.60 |
Crystal size (mm) | 0.2 × 0.1 × 0.1 |
Data collection | |
Diffractometer | Enraf-Nonius CAD-4 diffractometer |
Absorption correction | Empirical (using intensity measurements) via ψ scan (North et al., 1968) |
Tmin, Tmax | 0.931, 0.942 |
No. of measured, independent and observed [I > 2σ(I)] reflections | 3141, 3141, 2548 |
Rint | 0.029 |
(sin θ/λ)max (Å−1) | 0.601 |
Refinement | |
R[F2 > 2σ(F2)], wR(F2), S | 0.046, 0.141, 1.06 |
No. of reflections | 3010 |
No. of parameters | 220 |
No. of restraints | ? |
H-atom treatment | H-atom parameters constrained |
Δρmax, Δρmin (e Å−3) | 0.36 or 0.46?, −0.23 or −0.18? |
Computer programs: CAD-4 Software (Enraf-Nonius, 1989), MolEN (Nonius, 1997), SHELXS97 (Sheldrick 1990), SHELXL97 (Sheldrick 1997), ORTEPII (Johnson, 1976), SHELXL97.
O1—C1 | 1.212 (2) | C5—C12 | 1.482 (2) |
N1—C1 | 1.361 (2) | C3—C6 | 1.512 (2) |
N1—C5 | 1.421 (2) | C2—C18 | 1.470 (3) |
N1—C20 | 1.461 (2) | C2—C19 | 1.522 (3) |
N2—C18 | 1.135 (3) | C2—C1 | 1.548 (2) |
C1—N1—C5 | 121.87 (14) | C18—C2—C19 | 109.14 (16) |
C5—C4—C3 | 121.28 (16) | C1—C2—C3 | 107.49 (13) |
C4—C3—C2 | 107.90 (13) | N2—C18—C2 | 177.8 (2) |
C5—N1—C1—O1 | −174.80 (19) | N1—C5—C12—C13 | 53.8 (2) |
C18—C2—C1—O1 | −102.1 (2) | C2—C3—C6—C7 | −82.3 (2) |
In order to study the influence of substituents of different sizes upon the structural parameters of the molecule, we report here the crystal structure of 1,3-dimethyl-2-oxo-4,6-diphenyl-1,2,3,4-tetrahydropyridine-3-carbonitrile, (I). The crystal structures of the analogous diethyl and dibenzyl derivatives of tetrahydropyridine have previously been reported (Patel et al., 2001b,c). \sch
Fig. 1. depicts the structure of the molecule of (I), and selected bond lengths and angles are given in Table 1. The structure of (I) has the same characteristic molecular dimensions as observed for similar compounds of this class (Patel et al., 2001b,c). The bonds C1—C2, C2—C3 and C3—C4 [1.548 (2), 1.554 (2) and 1.494 (2) Å, respectively] have predominantly single-bond character, whereas the C1—N1 distance of 1.361 (2) Å is a partial double bond. The lengthening of C2—C3 and C3—C4, and the contraction of the C2—C3—C4 angle, [107.9 (1)°] may be attributed to the comparatively heavy substituent at C3 (a phenyl ring). These values are similar to those found in the other derivatives of THP that we have recently reported (Patel et al., 2001b,c).
The triple-bond character of C18—N2 [1.135 (3) Å], and the C2—C18—N2 angle of 177.8 (2)° defining the linearity of the cyano moiety, are typical of this group of 3-cyano-2-pyridine compounds (Black et al., 1992; Hussain et al., 1996; Patel, 2001a).
The methyl group at C2 is out of the plane of the THP ring by 0.627 (3) Å, whereas that at N1 is coplanar with the THP plane. Both the phenyl rings are planar within themselves. Steric interactions force the phenyl rings out of the THP plane by 49.21 (9) and 65.76 (5)°, respectively. However, these dihedral angles are in a much smaller range than those reported for similar compounds (Patel et al., 2001b,c; Hursthouse et al., 1992).
The torsion angles about C2—C18 are as follows, with values for the diethyl and dibenzyl substituents, respectively, given inside the brackets: C1—C2—C18—N2 = 107 (5)° [-126 (8) and -164 (2)°] and C3—C2—C18—N2 = -136 (5)° [113 (8) and 74 (2)°]. These observations reveal that the molecular conformation in all these compounds differs significantly about the C2—C18 bond.
In summary, these results show that, although the molecule of (I) preserves the same characteristic molecular dimensions as analogous compounds, the molecular conformation is influenced by the size of the substituents. As usual in heterocyclic compounds, weak van der Waals forces are responsible for the stability of the structure.
The three Patel et al. (2001) references each need a unique identifier. The a, b and c used above need careful checking to ensure the correct citation is given in each place. Please provide any corrections necessary.