Acridine form IX

We report a new polymorph of acridine, C₁₃H₉N, denoted form IX, obtained as thin needles by slow evaporation of a toluene solution. The structure was solved and refined from powder X-ray data. The structures of five unsolvated forms were previously known, but this is only the second with one mol­ecule in the asymmetric unit. The melting point [differential scanning calorimetry (DSC) onset] and heat of fusion are 108.8 (3) °C and 19.2 (4) kJ mol⁻¹, respectively.

1. Chemical context

With the crystal structures of five forms already reported, acridine is already one of the more prolifically polymorphic mol­ecules known [see Phillips (1956), Phillips et al. (1960), Mei & Wolf (2004), Braga et al. (2010), Kupka et al. (2012), and Lusi et al. (2015)]; two additional forms have been described, but structures were not reported, by Herbstein & Schmidt (1955) and Braga et al. (2010). This large number of observed forms seems particularly noteworthy in view of the fact that the mol­ecule has zero degrees of flexibility, although perhaps counterintuitively, some 40 rigid mol­ecules are observed to be polymorphic (Cruz-Cabeza & Bernstein, 2013).

2. Structural commentary

The form described here was previously predicted by Price & Price (unpublished) using CrystalPredictor (Karamertzanis & Panti­lides, 2005) to generate a crystal energy landscape, limited to one independent mol­ecule in the asymmetric unit cell in the most common space groups. These were relaxed to mechanically stable structures with DMACRYS (Price et al., 2010). This new form corresponded to one of two structures with the lowest computed lattice energy. Further details are available in Schur et al. (2019). Geometry details for form IX are given in Table 1.

Table 1
Selected geometric parameters (Å, °)

N1—C10	1.315 (18)	C5—C12	1.388 (6)
N1—C13	1.317 (18)	C6—C7	1.366 (17)
C1—C2	1.37 (3)	C6—C12	1.436 (12)
C1—C10	1.44 (3)	C7—C8	1.41 (3)
C2—C3	1.41 (2)	C8—C9	1.36 (4)
C3—C4	1.367 (16)	C9—C13	1.44 (4)
C4—C11	1.435 (9)	C10—C11	1.444 (7)
C5—C11	1.389 (5)	C12—C13	1.443 (11)
C10—N1—C13	116.9 (7)	N1—C10—C11	124.5 (8)
C2—C1—C10	121.3 (12)	C1—C10—C11	116.6 (11)
C1—C2—C3	121.7 (17)	C4—C11—C5	122.5 (5)
C2—C3—C4	119.7 (13)	C4—C11—C10	120.0 (7)
C3—C4—C11	120.8 (10)	C5—C11—C10	117.6 (5)
C11—C5—C12	118.9 (4)	C5—C12—C6	122.4 (8)
C7—C6—C12	120.9 (14)	C5—C12—C13	117.7 (6)
C6—C7—C8	119.4 (15)	C6—C12—C13	119.8 (8)
C7—C8—C9	122 (2)	N1—C13—C9	119.0 (13)
C8—C9—C13	121.3 (18)	N1—C13—C12	124.5 (9)
N1—C10—C1	118.9 (10)	C9—C13—C12	116.5 (13)

3. Supra­molecular features

The four mol­ecules in the unit cell are connected by a cycle of C⋯H (2.81 Å) and N⋯H (2.73 Å) contacts that are shorter than the sum of the van der Waals radii. There is also an H⋯H inter­action of 2.29 Å.

4. Synthesis and crystallization

Crystals were grown by slow evaporation from a toluene solution. Thin needles of form IX samples were taken from the walls of crystallization vials. The material was gently crushed and loaded into a glass capillary for powder diffraction measurements. Further details are available in Schur (2013).

5. Refinement details

Crystal data, data collection and structure refinement details are summarized in Table 2. Data were collected at the high resolution powder diffractometer at the National Synchrotron Light Source beamline X16C, operated in step scanning mode. X-rays of wavelength 0.69979 Å were selected by a Si(111) channel cut monochromator. Diffracted X-rays were selected by a Ge(111) analyzer before an NaI(Tl) scintillation detector. The sample of form IX was obtained concomitantly with forms III (1.4%) and VII (1.1%), which were included in the Rietveld fit, with atomic positions fixed at literature values.

Table 2
Experimental details

Crystal data
Chemical formula	C13H9N
Mr	179.21
Crystal system, space group	Monoclinic, P21/n
Temperature (K)	295
a, b, c (Å)	11.28453 (11), 12.38182 (12), 6.67905 (9)
β (°)	92.0618 (6)
V (Å^3)	932.61 (2)
Z	4
Radiation type	Synchrotron, λ = 0.699789 Å
μ (mm^−1)	0.08
Specimen shape, size (mm)	Cylinder, 8 × 1
Data collection
Diffractometer	Huber 401 diffractometer, Ge(111) analyzer crystal
Specimen mounting	1 mm glass capillary, spun during data collection
Data collection mode	Transmission
Scan method	Step
2θ values (°)	2θmin = 2, 2θmax = 35, 2θstep = 0.005
Refinement
R factors and goodness of fit	Rp = 0.041, Rwp = 0.050, Rexp = 0.028, RBragg = 0.011, χ^2 = 3.183
No. of parameters	81
No. of restraints	12
H-atom treatment	H-atom parameters not refined
Computer programs: TOPAS-Academic (Coelho, 2016) and Mercury (Macrae et al., 2008).

The mol­ecule was defined by a z-matrix for refinement. Mirror symmetry was imposed on bond distances and angles; 7 distances, 6 angles, and 11 torsions were refined. There is a single isotropic displacement parameter for all C and N atoms; that of H atoms is 1.5 times greater. All H atoms are tethered.

Standard uncertainties were calculated by a bootstrap method, described in Coelho (2016). As such, they reflect the propagation of statistical errors from the raw data and do not take account of systematic errors. Realistic estimates of the precision of measurements are somewhat larger.

The Rietveld refinement plot is shown in Fig. 1. Fig. 2 illustrates the atom-labeling scheme, and Fig. 3 shows the three-dimensional structure, with short inter­molecular inter­actions shown as broken lines.

Figure 1 The acridine mol­ecule in form IX, with atom labels and 50% probability displacement spheres.

Figure 2 Rietveld plot of acridine form IX. Red dots are measured intensities, black line is the fit, and the blue trace at the bottom is the difference plot, measured minus fit. Note the two vertical scale changes. Vertical tick lines show allowed peak positions of form IX peaks. Fit includes two impurity phases: 1.4% form III and 1.1% form VII. Tick marks were omitted for clarity.

Figure 3 Packing diagram of acridine form IX. Close inter­molecular inter­actions (less than the sum of van der Waals radii) are marked in turquoise dashed lines.

The refinement model included preferred orientation parameter 1.08 in the (100) direction (March, 1932; Dollase, 1986), and anisotropic microstrain broadening (Stephens, 1999).

Supporting information

Computing details

Data collection: spec; cell refinement: TOPAS-Academic (Coelho, 2016); data reduction: TOPAS-Academic (Coelho, 2016); program(s) used to solve structure: TOPAS-Academic (Coelho, 2016); program(s) used to refine structure: TOPAS-Academic (Coelho, 2016); molecular graphics: Mercury (Macrae et al., 2008).

Acridine

Crystal data

C13H9N	Z = 4
Mr = 179.21	Dx = 1.276 Mg m^−3
Monoclinic, P21/n	Synchrotron radiation, λ = 0.699789 Å
a = 11.28453 (11) Å	µ = 0.08 mm^−1
b = 12.38182 (12) Å	T = 295 K
c = 6.67905 (9) Å	Particle morphology: thin needles
β = 92.0618 (6)°	yellow-white
V = 932.61 (2) Å^3	cylinder, 8 × 1 mm

Data collection

Huber 401 diffractometer, Ge(111) analyzer crystal	Data collection mode: transmission
Radiation source: National Synchrotron Light Source	Scan method: step
Channel cut Si(111) monochromator	2θmin = 2°, 2θmax = 35°, 2θstep = 0.005°
Specimen mounting: 1 mm glass capillary, spun during data collection

Refinement

Least-squares matrix: full	12 restraints
Rp = 0.041	22 constraints
Rwp = 0.050	H-atom parameters not refined
Rexp = 0.028	Weighting scheme based on measured s.u.'s
RBragg = 0.011	(Δ/σ)max = 0.02
6601 data points	Background function: 9th order Chebyshev plus broad pseudo-Voigt
Profile function: Convolution of Gaussian and Lorentzian, with anisotropic strain broadening per Stephens (1999).	Preferred orientation correction: March parameter 1.084 in (1 0 0) direction
81 parameters

Special details

Refinement. Mirror symmetry imposed on bond distances and angles.

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

	x	y	z	Uiso*/Ueq
N1	0.1540 (5)	0.1053 (19)	0.045 (2)	0.0474 (5)*
C1	0.1213 (11)	0.003 (3)	−0.252 (4)	0.0474 (5)*
C2	0.1640 (14)	−0.048 (2)	−0.417 (3)	0.0474 (5)*
C3	0.2853 (15)	−0.0476 (11)	−0.4573 (15)	0.0474 (5)*
C4	0.3633 (9)	0.0068 (7)	−0.3327 (10)	0.0474 (5)*
C5	0.4001 (2)	0.1154 (2)	−0.0256 (3)	0.0474 (5)*
C6	0.4263 (13)	0.2290 (7)	0.2776 (12)	0.0474 (5)*
C7	0.379 (2)	0.2748 (11)	0.4427 (15)	0.0474 (5)*
C8	0.256 (2)	0.266 (2)	0.471 (3)	0.0474 (5)*
C9	0.1830 (18)	0.210 (4)	0.341 (6)	0.0474 (5)*
C10	0.1992 (5)	0.0588 (10)	−0.1120 (15)	0.0474 (5)*
C11	0.3234 (4)	0.0603 (4)	−0.1568 (6)	0.0474 (5)*
C12	0.3543 (7)	0.1655 (3)	0.1406 (7)	0.0474 (5)*
C13	0.2282 (7)	0.1578 (9)	0.1666 (17)	0.0474 (5)*
H1	0.0388 (12)	0.002 (4)	−0.229 (6)	0.0711 (8)*
H2	0.1103 (18)	−0.086 (2)	−0.504 (4)	0.0711 (8)*
H3	0.313 (2)	−0.0838 (17)	−0.572 (2)	0.0711 (8)*
H4	0.4453 (10)	0.0071 (13)	−0.3603 (18)	0.0711 (8)*
H5	0.4825 (3)	0.1185 (6)	−0.0491 (8)	0.0711 (8)*
H6	0.5086 (12)	0.2370 (14)	0.256 (2)	0.0711 (8)*
H7	0.428 (3)	0.315 (2)	0.535 (3)	0.0711 (8)*
H8	0.224 (3)	0.299 (3)	0.586 (4)	0.0711 (8)*
H9	0.1006 (19)	0.205 (6)	0.364 (7)	0.0711 (8)*

Geometric parameters (Å, º)

N1—C10	1.315 (18)	C9—C13	1.44 (4)
N1—C13	1.317 (18)	C10—C11	1.444 (7)
C1—C2	1.37 (3)	C12—C13	1.443 (11)
C1—C10	1.44 (3)	C1—H1	0.95
C2—C3	1.41 (2)	C2—H2	0.95
C3—C4	1.367 (16)	C3—H3	0.95
C4—C11	1.435 (9)	C4—H4	0.95
C5—C11	1.389 (5)	C5—H5	0.95
C5—C12	1.388 (6)	C6—H6	0.95
C6—C7	1.366 (17)	C7—H7	0.95
C6—C12	1.436 (12)	C8—H8	0.95
C7—C8	1.41 (3)	C9—H9	0.95
C8—C9	1.36 (4)
C10—N1—C13	116.9 (7)	N1—C13—C12	124.5 (9)
C2—C1—C10	121.3 (12)	C9—C13—C12	116.5 (13)
C1—C2—C3	121.7 (17)	C2—C1—H1	120 (4)
C2—C3—C4	119.7 (13)	C10—C1—H1	119 (4)
C3—C4—C11	120.8 (10)	C1—C2—H2	119 (2)
C11—C5—C12	118.9 (4)	C3—C2—H2	119 (2)
C7—C6—C12	120.9 (14)	C2—C3—H3	120 (2)
C6—C7—C8	119.4 (15)	C4—C3—H3	120 (2)
C7—C8—C9	122 (2)	C3—C4—H4	119.6 (12)
C8—C9—C13	121.3 (18)	C11—C4—H4	119.6 (11)
N1—C10—C1	118.9 (10)	C11—C5—H5	120.5 (4)
N1—C10—C11	124.5 (8)	C12—C5—H5	120.6 (5)
C1—C10—C11	116.6 (11)	C7—C6—H6	119.5 (15)
C4—C11—C5	122.5 (5)	C12—C6—H6	119.6 (12)
C4—C11—C10	120.0 (7)	C6—C7—H7	120 (3)
C5—C11—C10	117.6 (5)	C8—C7—H7	120 (2)
C5—C12—C6	122.4 (8)	C7—C8—H8	119 (3)
C5—C12—C13	117.7 (6)	C9—C8—H8	120 (3)
C6—C12—C13	119.8 (8)	C8—C9—H9	120 (5)
N1—C13—C9	119.0 (13)	C13—C9—H9	119 (5)

 

Footnotes

‡Deceased.