Supporting information
Crystallographic Information File (CIF) https://doi.org/10.1107/S1600536802015763/bt6174sup1.cif | |
Structure factor file (CIF format) https://doi.org/10.1107/S1600536802015763/bt6174Isup2.hkl |
CCDC reference: 139913
The title compound was synthesized according to the method of Mrozek, Karolak-Wojciechowska, Bsiri & Barbe (2000).
As the collected data were relatively weak, there was a large proportion of reflections with low intensities, and thus some of the reflections were marked as unobserved. This affects the fraction of unique reflections observed (out to θ=80.91°), which is equal to 62.6%. All hydrogen atoms were placed in calculated positions and treated as riding on the adjacent carbon atom. The methyl group was allowed to rotate about its local threefold axis.
Data collection: KM4 Software (Kuma 1993); cell refinement: KM4 Software; data reduction: DATAPROC (Gałdecki et al., 1998); program(s) used to solve structure: SHELXS97 (Sheldrick, 1990a); program(s) used to refine structure: SHELXL97 (Sheldrick, 1997); molecular graphics: XP in SHELXTL/PC (Sheldrick, 1990b) ORTEP-3 (Farrugia 1997); software used to prepare material for publication: SHELXL97.
Fig. 1. Molecular structure of (I). Displacement ellipsoids are drawn at the 50% probability level. |
C20H14N2O2S | F(000) = 720 |
Mr = 346.39 | Dx = 1.392 Mg m−3 |
Monoclinic, P21/n | Cu Kα radiation, λ = 1.54178 Å |
a = 9.826 (2) Å | Cell parameters from 25 reflections |
b = 7.171 (1) Å | θ = 10–35° |
c = 23.941 (5) Å | µ = 1.87 mm−1 |
β = 101.57 (3)° | T = 293 K |
V = 1652.7 (5) Å3 | Needle, colourless |
Z = 4 | 0.5 × 0.2 × 0.1 mm |
Kuma KM4 diffractometer | 1802 reflections with I > 2σ(I) |
Radiation source: fine-focus sealed tube | Rint = 0.063 |
Graphite monochromator | θmax = 80.9°, θmin = 3.8° |
ω–2θ scans | h = −8→0 |
Absorption correction: numerical X-RED (Stoe & Cie, 1999) | k = −9→0 |
Tmin = 0.421, Tmax = 0.872 | l = −27→30 |
2670 measured reflections | 2 standard reflections every 100 reflections |
2469 independent reflections | intensity decay: 2% |
Refinement on F2 | Secondary atom site location: structure-invariant direct methods |
Least-squares matrix: full | Hydrogen site location: inferred from neighbouring sites |
R[F2 > 2σ(F2)] = 0.058 | H atoms treated by a mixture of independent and constrained refinement |
wR(F2) = 0.166 | w = 1/[σ2(Fo2) + (0.0874P)2] where P = (Fo2 + 2Fc2)/3 |
S = 1.11 | (Δ/σ)max < 0.001 |
2469 reflections | Δρmax = 0.38 e Å−3 |
228 parameters | Δρmin = −0.40 e Å−3 |
0 restraints | Extinction correction: SHELXL, Fc*=kFc[1+0.001xFc2λ3/sin(2θ)]-1/4 |
Primary atom site location: structure-invariant direct methods | Extinction coefficient: 0.0118 (10) |
C20H14N2O2S | V = 1652.7 (5) Å3 |
Mr = 346.39 | Z = 4 |
Monoclinic, P21/n | Cu Kα radiation |
a = 9.826 (2) Å | µ = 1.87 mm−1 |
b = 7.171 (1) Å | T = 293 K |
c = 23.941 (5) Å | 0.5 × 0.2 × 0.1 mm |
β = 101.57 (3)° |
Kuma KM4 diffractometer | 1802 reflections with I > 2σ(I) |
Absorption correction: numerical X-RED (Stoe & Cie, 1999) | Rint = 0.063 |
Tmin = 0.421, Tmax = 0.872 | 2 standard reflections every 100 reflections |
2670 measured reflections | intensity decay: 2% |
2469 independent reflections |
R[F2 > 2σ(F2)] = 0.058 | 0 restraints |
wR(F2) = 0.166 | H atoms treated by a mixture of independent and constrained refinement |
S = 1.11 | Δρmax = 0.38 e Å−3 |
2469 reflections | Δρmin = −0.40 e Å−3 |
228 parameters |
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. |
x | y | z | Uiso*/Ueq | ||
S | 0.19697 (11) | 0.72766 (14) | 0.16491 (4) | 0.0714 (4) | |
C9 | 0.0842 (4) | 0.7569 (4) | 0.09724 (14) | 0.0526 (9) | |
C11 | −0.0543 (4) | 0.8047 (4) | 0.09442 (14) | 0.0518 (9) | |
C8 | −0.1123 (5) | 0.8445 (5) | 0.14292 (17) | 0.0698 (11) | |
H8 | −0.0556 | 0.8436 | 0.1790 | 0.084* | |
C7 | −0.2480 (6) | 0.8833 (6) | 0.1371 (2) | 0.0825 (13) | |
H7 | −0.2839 | 0.9081 | 0.1695 | 0.099* | |
C6 | −0.3380 (5) | 0.8874 (6) | 0.0828 (2) | 0.0801 (13) | |
H6 | −0.4318 | 0.9142 | 0.0798 | 0.096* | |
C5 | −0.2872 (4) | 0.8522 (5) | 0.03560 (18) | 0.0655 (10) | |
H5 | −0.3464 | 0.8557 | 0.0001 | 0.079* | |
C12 | −0.1440 (4) | 0.8098 (4) | 0.03940 (15) | 0.0517 (9) | |
N10 | −0.1001 (3) | 0.7748 (3) | −0.00914 (12) | 0.0535 (7) | |
C13 | 0.0342 (4) | 0.7332 (4) | −0.00580 (14) | 0.0525 (9) | |
C4 | 0.0800 (5) | 0.6944 (5) | −0.05782 (16) | 0.0670 (11) | |
H4 | 0.0165 | 0.6997 | −0.0923 | 0.080* | |
C3 | 0.2129 (5) | 0.6505 (5) | −0.0577 (2) | 0.0756 (12) | |
H3 | 0.2402 | 0.6245 | −0.0919 | 0.091* | |
C2 | 0.3111 (5) | 0.6435 (5) | −0.0061 (2) | 0.0784 (12) | |
H2 | 0.4035 | 0.6174 | −0.0068 | 0.094* | |
C1 | 0.2728 (4) | 0.6745 (5) | 0.04453 (19) | 0.0658 (11) | |
H1 | 0.3388 | 0.6654 | 0.0782 | 0.079* | |
C14 | 0.1326 (4) | 0.7209 (4) | 0.04678 (15) | 0.0525 (9) | |
C1' | 0.3072 (4) | 0.9238 (4) | 0.17037 (13) | 0.0516 (8) | |
C6' | 0.2824 (4) | 1.0684 (4) | 0.13049 (14) | 0.0554 (9) | |
H6' | 0.2051 | 1.0636 | 0.1009 | 0.067* | |
C5' | 0.3716 (4) | 1.2177 (4) | 0.13475 (14) | 0.0540 (9) | |
H5' | 0.3558 | 1.3141 | 0.1082 | 0.065* | |
C4' | 0.4844 (4) | 1.2213 (5) | 0.17896 (14) | 0.0517 (8) | |
N8' | 0.5826 (4) | 1.3758 (5) | 0.18233 (15) | 0.0695 (9) | |
O1 | 0.5573 (3) | 1.5005 (4) | 0.14802 (14) | 0.0989 (11) | |
O2 | 0.6870 (4) | 1.3714 (5) | 0.21948 (15) | 0.1162 (13) | |
C3' | 0.5085 (3) | 1.0822 (5) | 0.21942 (13) | 0.0513 (9) | |
H3' | 0.5852 | 1.0903 | 0.2492 | 0.062* | |
C2' | 0.4203 (4) | 0.9308 (5) | 0.21634 (13) | 0.0514 (8) | |
C7' | 0.4442 (4) | 0.7806 (5) | 0.26112 (15) | 0.0713 (11) | |
H71' | 0.5313 | 0.8019 | 0.2867 | 0.107* | |
H72' | 0.4460 | 0.6612 | 0.2431 | 0.107* | |
H73' | 0.3705 | 0.7830 | 0.2821 | 0.107* |
U11 | U22 | U33 | U12 | U13 | U23 | |
S | 0.0763 (8) | 0.0656 (5) | 0.0621 (6) | −0.0179 (5) | −0.0104 (5) | 0.0228 (4) |
C9 | 0.055 (3) | 0.0424 (15) | 0.056 (2) | −0.0151 (15) | 0.0011 (17) | 0.0066 (13) |
C11 | 0.052 (3) | 0.0443 (16) | 0.057 (2) | −0.0093 (15) | 0.0073 (17) | 0.0043 (13) |
C8 | 0.075 (3) | 0.072 (2) | 0.065 (2) | 0.004 (2) | 0.021 (2) | 0.0040 (18) |
C7 | 0.092 (4) | 0.070 (2) | 0.094 (3) | −0.011 (2) | 0.041 (3) | −0.001 (2) |
C6 | 0.062 (3) | 0.073 (3) | 0.111 (4) | 0.004 (2) | 0.030 (3) | 0.002 (2) |
C5 | 0.052 (3) | 0.0516 (18) | 0.089 (3) | −0.0009 (17) | 0.006 (2) | 0.0072 (18) |
C12 | 0.051 (3) | 0.0386 (15) | 0.062 (2) | −0.0002 (14) | 0.0035 (17) | 0.0037 (13) |
N10 | 0.054 (2) | 0.0443 (13) | 0.0581 (17) | −0.0095 (13) | 0.0028 (14) | 0.0032 (11) |
C13 | 0.062 (3) | 0.0405 (15) | 0.0542 (19) | −0.0080 (15) | 0.0086 (17) | 0.0014 (13) |
C4 | 0.086 (3) | 0.054 (2) | 0.061 (2) | 0.004 (2) | 0.016 (2) | 0.0016 (16) |
C3 | 0.085 (4) | 0.054 (2) | 0.096 (3) | −0.003 (2) | 0.037 (3) | −0.004 (2) |
C2 | 0.063 (3) | 0.059 (2) | 0.118 (4) | 0.000 (2) | 0.032 (3) | 0.001 (2) |
C1 | 0.057 (3) | 0.0474 (18) | 0.091 (3) | −0.0066 (17) | 0.009 (2) | 0.0094 (17) |
C14 | 0.049 (3) | 0.0412 (14) | 0.065 (2) | −0.0047 (14) | 0.0052 (17) | 0.0069 (14) |
C1' | 0.051 (2) | 0.0544 (17) | 0.0473 (18) | 0.0044 (15) | 0.0045 (16) | 0.0064 (13) |
C6' | 0.053 (2) | 0.0532 (17) | 0.0532 (19) | −0.0020 (16) | −0.0063 (17) | 0.0042 (14) |
C5' | 0.059 (2) | 0.0443 (15) | 0.0547 (19) | −0.0054 (15) | 0.0015 (17) | 0.0016 (13) |
C4' | 0.048 (2) | 0.0537 (17) | 0.0536 (19) | −0.0051 (15) | 0.0101 (17) | −0.0062 (14) |
N8' | 0.063 (2) | 0.070 (2) | 0.072 (2) | −0.0147 (17) | 0.0056 (19) | −0.0083 (16) |
O1 | 0.092 (2) | 0.078 (2) | 0.117 (3) | −0.0264 (18) | 0.000 (2) | 0.0229 (18) |
O2 | 0.085 (3) | 0.140 (3) | 0.105 (3) | −0.046 (2) | −0.027 (2) | 0.012 (2) |
C3' | 0.041 (2) | 0.0633 (19) | 0.0454 (18) | 0.0024 (16) | −0.0010 (15) | −0.0095 (14) |
C2' | 0.052 (2) | 0.0571 (18) | 0.0420 (17) | 0.0103 (16) | 0.0031 (16) | 0.0019 (13) |
C7' | 0.079 (3) | 0.073 (2) | 0.054 (2) | 0.004 (2) | −0.006 (2) | 0.0160 (17) |
S—C1' | 1.764 (4) | C3—H3 | 0.9300 |
S—C9 | 1.782 (3) | C2—C1 | 1.359 (5) |
C9—C11 | 1.392 (5) | C2—H2 | 0.9300 |
C9—C14 | 1.408 (5) | C1—C14 | 1.429 (5) |
C11—C8 | 1.421 (5) | C1—H1 | 0.9300 |
C11—C12 | 1.431 (5) | C1'—C6' | 1.397 (4) |
C8—C7 | 1.342 (6) | C1'—C2' | 1.399 (4) |
C8—H8 | 0.9300 | C6'—C5' | 1.374 (4) |
C7—C6 | 1.418 (6) | C6'—H6' | 0.9300 |
C7—H7 | 0.9300 | C5'—C4' | 1.370 (5) |
C6—C5 | 1.349 (5) | C5'—H5' | 0.9300 |
C6—H6 | 0.9300 | C4'—C3' | 1.377 (4) |
C5—C12 | 1.424 (5) | C4'—N8' | 1.461 (4) |
C5—H5 | 0.9300 | N8'—O1 | 1.206 (4) |
C12—N10 | 1.342 (4) | N8'—O2 | 1.216 (4) |
N10—C13 | 1.339 (5) | C3'—C2' | 1.382 (5) |
C13—C14 | 1.428 (5) | C3'—H3' | 0.9300 |
C13—C4 | 1.434 (5) | C2'—C7' | 1.504 (4) |
C4—C3 | 1.342 (5) | C7'—H71' | 0.9600 |
C4—H4 | 0.9300 | C7'—H72' | 0.9600 |
C3—C2 | 1.408 (6) | C7'—H73' | 0.9600 |
C1'—S—C9 | 103.54 (14) | C3—C2—H2 | 119.6 |
C11—C9—C14 | 119.9 (3) | C2—C1—C14 | 120.9 (4) |
C11—C9—S | 119.8 (3) | C2—C1—H1 | 119.5 |
C14—C9—S | 120.2 (3) | C14—C1—H1 | 119.5 |
C9—C11—C8 | 123.9 (3) | C9—C14—C13 | 117.4 (3) |
C9—C11—C12 | 117.8 (3) | C9—C14—C1 | 124.7 (3) |
C8—C11—C12 | 118.2 (4) | C13—C14—C1 | 117.9 (3) |
C7—C8—C11 | 120.7 (4) | C6'—C1'—C2' | 120.9 (3) |
C7—C8—H8 | 119.7 | C6'—C1'—S | 121.4 (3) |
C11—C8—H8 | 119.7 | C2'—C1'—S | 117.8 (2) |
C8—C7—C6 | 121.5 (4) | C5'—C6'—C1' | 120.4 (3) |
C8—C7—H7 | 119.2 | C5'—C6'—H6' | 119.8 |
C6—C7—H7 | 119.2 | C1'—C6'—H6' | 119.8 |
C5—C6—C7 | 119.8 (4) | C4'—C5'—C6' | 118.5 (3) |
C5—C6—H6 | 120.1 | C4'—C5'—H5' | 120.8 |
C7—C6—H6 | 120.1 | C6'—C5'—H5' | 120.8 |
C6—C5—C12 | 120.9 (4) | C5'—C4'—C3' | 121.9 (3) |
C6—C5—H5 | 119.5 | C5'—C4'—N8' | 118.7 (3) |
C12—C5—H5 | 119.5 | C3'—C4'—N8' | 119.4 (3) |
N10—C12—C5 | 118.0 (3) | O1—N8'—O2 | 123.0 (4) |
N10—C12—C11 | 123.1 (3) | O1—N8'—C4' | 118.8 (3) |
C5—C12—C11 | 118.9 (4) | O2—N8'—C4' | 118.2 (3) |
C13—N10—C12 | 118.3 (3) | C4'—C3'—C2' | 120.9 (3) |
N10—C13—C14 | 123.4 (3) | C4'—C3'—H3' | 119.5 |
N10—C13—C4 | 117.9 (3) | C2'—C3'—H3' | 119.5 |
C14—C13—C4 | 118.6 (4) | C3'—C2'—C1' | 117.4 (3) |
C3—C4—C13 | 121.2 (4) | C3'—C2'—C7' | 121.2 (3) |
C3—C4—H4 | 119.4 | C1'—C2'—C7' | 121.4 (3) |
C13—C4—H4 | 119.4 | C2'—C7'—H71' | 109.5 |
C4—C3—C2 | 120.4 (4) | C2'—C7'—H72' | 109.5 |
C4—C3—H3 | 119.8 | H71'—C7'—H72' | 109.5 |
C2—C3—H3 | 119.8 | C2'—C7'—H73' | 109.5 |
C1—C2—C3 | 120.9 (4) | H71'—C7'—H73' | 109.5 |
C1—C2—H2 | 119.6 | H72'—C7'—H73' | 109.5 |
C1'—S—C9—C11 | −108.7 (2) | C11—C9—C14—C1 | 178.6 (3) |
C1'—S—C9—C14 | 75.1 (3) | S—C9—C14—C1 | −5.2 (4) |
C14—C9—C11—C8 | −179.6 (3) | N10—C13—C14—C9 | −0.7 (4) |
S—C9—C11—C8 | 4.2 (4) | C4—C13—C14—C9 | −179.0 (3) |
C14—C9—C11—C12 | 1.9 (4) | N10—C13—C14—C1 | 179.7 (3) |
S—C9—C11—C12 | −174.3 (2) | C4—C13—C14—C1 | 1.4 (4) |
C9—C11—C8—C7 | −177.6 (3) | C2—C1—C14—C9 | −179.4 (3) |
C12—C11—C8—C7 | 0.8 (5) | C2—C1—C14—C13 | 0.1 (5) |
C11—C8—C7—C6 | −0.4 (6) | C9—S—C1'—C6' | 8.1 (3) |
C8—C7—C6—C5 | −0.2 (7) | C9—S—C1'—C2' | −172.3 (3) |
C7—C6—C5—C12 | 0.4 (6) | C2'—C1'—C6'—C5' | 2.1 (5) |
C6—C5—C12—N10 | 179.5 (3) | S—C1'—C6'—C5' | −178.4 (3) |
C6—C5—C12—C11 | 0.1 (5) | C1'—C6'—C5'—C4' | −0.2 (5) |
C9—C11—C12—N10 | −1.5 (4) | C6'—C5'—C4'—C3' | −1.4 (5) |
C8—C11—C12—N10 | 179.9 (3) | C6'—C5'—C4'—N8' | 177.5 (3) |
C9—C11—C12—C5 | 177.9 (3) | C5'—C4'—N8'—O1 | 4.1 (5) |
C8—C11—C12—C5 | −0.7 (4) | C3'—C4'—N8'—O1 | −177.0 (3) |
C5—C12—N10—C13 | −179.4 (3) | C5'—C4'—N8'—O2 | −175.2 (4) |
C11—C12—N10—C13 | 0.0 (4) | C3'—C4'—N8'—O2 | 3.7 (5) |
C12—N10—C13—C14 | 1.2 (4) | C5'—C4'—C3'—C2' | 1.1 (5) |
C12—N10—C13—C4 | 179.5 (3) | N8'—C4'—C3'—C2' | −177.7 (3) |
N10—C13—C4—C3 | −179.6 (3) | C4'—C3'—C2'—C1' | 0.7 (5) |
C14—C13—C4—C3 | −1.2 (5) | C4'—C3'—C2'—C7' | −178.6 (3) |
C13—C4—C3—C2 | −0.7 (6) | C6'—C1'—C2'—C3' | −2.3 (5) |
C4—C3—C2—C1 | 2.3 (6) | S—C1'—C2'—C3' | 178.1 (2) |
C3—C2—C1—C14 | −2.0 (6) | C6'—C1'—C2'—C7' | 177.0 (3) |
C11—C9—C14—C13 | −0.9 (4) | S—C1'—C2'—C7' | −2.6 (4) |
S—C9—C14—C13 | 175.3 (2) |
Experimental details
Crystal data | |
Chemical formula | C20H14N2O2S |
Mr | 346.39 |
Crystal system, space group | Monoclinic, P21/n |
Temperature (K) | 293 |
a, b, c (Å) | 9.826 (2), 7.171 (1), 23.941 (5) |
β (°) | 101.57 (3) |
V (Å3) | 1652.7 (5) |
Z | 4 |
Radiation type | Cu Kα |
µ (mm−1) | 1.87 |
Crystal size (mm) | 0.5 × 0.2 × 0.1 |
Data collection | |
Diffractometer | Kuma KM4 diffractometer |
Absorption correction | Numerical X-RED (Stoe & Cie, 1999) |
Tmin, Tmax | 0.421, 0.872 |
No. of measured, independent and observed [I > 2σ(I)] reflections | 2670, 2469, 1802 |
Rint | 0.063 |
(sin θ/λ)max (Å−1) | 0.640 |
Refinement | |
R[F2 > 2σ(F2)], wR(F2), S | 0.058, 0.166, 1.11 |
No. of reflections | 2469 |
No. of parameters | 228 |
H-atom treatment | H atoms treated by a mixture of independent and constrained refinement |
Δρmax, Δρmin (e Å−3) | 0.38, −0.40 |
Computer programs: KM4 Software (Kuma 1993), KM4 Software, DATAPROC (Gałdecki et al., 1998), SHELXS97 (Sheldrick, 1990a), SHELXL97 (Sheldrick, 1997), XP in SHELXTL/PC (Sheldrick, 1990b) ORTEP-3 (Farrugia 1997), SHELXL97.
C9—C11 | 1.392 (5) | C12—N10 | 1.342 (4) |
C9—C14 | 1.408 (5) | N10—C13 | 1.339 (5) |
C11—C8 | 1.421 (5) | C13—C14 | 1.428 (5) |
C11—C12 | 1.431 (5) | C13—C4 | 1.434 (5) |
C8—C7 | 1.342 (6) | C4—C3 | 1.342 (5) |
C7—C6 | 1.418 (6) | C3—C2 | 1.408 (6) |
C6—C5 | 1.349 (5) | C2—C1 | 1.359 (5) |
C5—C12 | 1.424 (5) | C1—C14 | 1.429 (5) |
Acridine derivatives are currently delivered to animals as antihelminthics agents (Durchheimer et al., 1980). In addition, acridine derivatives have shown in vitro activity against protozoan-like Trypanosoma cruzi (Ngadi et al., 1993) and Leishmania donovani (Messa-Valle et al., 1996). This work has been undertaken in the context of our studies on acridine derivatives with potential pharmacological properties (Karolak-Wojciechowska et al., 1996; Karolak-Wojciechowska et al., 1998). It was especially important to perform an X-ray analysis on single crystals of 9-(thio-2-methyl-4-nitrophenyl) acridine, (I), to obtain atomic coordinates which could be used as a starting point for further molecular modelling. The molecular geometry of(I) is presented in Fig. 1. The molecule consists of two cyclic moieties, an acridine and a phenyl ring. The rings are joined via a sulfur bridge at C9 (S1—C9 = 1.782 (3) and S1—C1' = 1.764 (4) Å). The phenyl ring in (I) is nearly perpendicular to the acridine moiety, with a dihedral of 75.01 (7)°. The molecules of (I) stack head-to-tail along the y-axis, with a distance between molecules of about 4.381 Å. All bond distances and angles have typical values. The acridine skeleton is slightly bent, with dihedral angles between the two terminal carborings A1 and A3 and the heterocycle A2 1.2 (1) and 0.5 (1)°, respectively. As has been shown for various hydrocarbon systems, such small deviations from planarity of an aromatic species does not affect the aromaticity significantly (Krygowski et al., 2000; Krygowski & Cyranski, 2001). It should also be noted that such a slight deformation of the acridine skeleton is in accordance with data for 34 C9-substituted acridines retrieved from the Cambridge Structural Database (Allen & Kennard, 1993). Most of the molecules found include an acridine moiety with a very slight boat conformation. Only two structures found in the CSD contain the acridine in a chair conformation. The average value of the dihedral angles between the external and internal rings for the 34 molecules is 1.5°. Taking into consideration the interest in hydrocarbons and the aromaticity of their aza analogues (Krygowski et al., 2000; Krygowski & Cyranski, 2001) we decided to investigate quantitatively the aromaticity in acridines (Mrozek,Karolak-Wojciechowska, Amiel & Barbe, 2000a,b) by calculating the HOMA index (Krygowski, 1993) for each acridine ring in (I); its values are 0.568 and 0.524 for the carborings, and 0.816 for the heterocycle. To confirm these results, calculations of the HOMA index were performed for all 34 C9-substituted acridines found in the CSD (Set 1; HOMAAV = 0.744 and HOMAA2 = 0.510) and as an extension of our statistical research for 18 unsubstituted acridines (Set 2; HOMAAV = 0.810 and HOMAA2 = 0.601) retrieved additionally. For the terminal rings A1 and A3, the results of the HOMA calculations are given as an averaged HOMA index (HOMAAV) whereas HOMAA2 is the value of HOMA index for internal heterocyclic ring A2. The aromaticity of the internal heterocycle is notably higher than for the external homorings. A similar effect is observed not only for the acridine parent molecule anthracene, where the HOMA index values are 0.638 for the external rings and 0.763 for the internal one, but also for different aza-substitued benzoides (Krygowski et al., 2000; Krygowski & Cyranski, 2001; Cyranski & Krygowski, 1996) and may be attributed to topological conditions. Moreover, the replacement of CH by N usually leads to an increase in the aromaticity of the system. This conclusion is confirmed by the fact that the HOMA indices for the central acridine ring are higher or, at least, comparable to those for anthracene.