Different packing in three polymorphs of 2,4,6-trimethoxy-1,3,5-triazine

2,4,6-Trimethoxy-1,3,5-triazine was found to exhibit three different polymorphs. The α-polymorph undergoes reversible phase transformation to the β-polymorph at 340 K with an enthalpy of 3.9 kJ mol⁻¹. The heat of fusion of the β-polymorph at 404 K is 18.1 kJ mol⁻¹. The low-tempreature phase (α-polymorph) crystallizes in the orthorhombic space group Pnma. The high-tempreature phase (β-polymorph) can be obtained from the melt and cystallizes in the monoclinic space group P2₁. The γ-polymorph is obtained by crystallization from a 1:1 mixture of methanol and methylene­chloride from hydrolyzed 2,4-dimethoxy-6-oxybenzophenone-1,3,5-triazine. The γ-polymorph melts at 409 K with an enthalpy of 11.4 kJ mol⁻¹. The γ-polymorph crystallizes in the trigonal space group R3c. The molecules occupy a crystallographic threefold axis. Molecules of 2,4,6-tri­methoxy-1,3,5-triazine are planar in all three polymorphs. The major difference between the three polymorphs is the mode of packing of the molecules in the crystal.

Computing details

Data collection: Collect (Nonius, 2001); cell refinement: DENZO SMN (Otwinowski & Minor 1997); data reduction: DENZO SMN (Otwinowski & Minor 1997); program(s) used to solve structure: SHELXS97 (Sheldrick, 1990); program(s) used to refine structure: SHELXL97 (Sheldrick, 1997); molecular graphics: ORTEP-3 for Windows (Farrugia, 1997).

Figures

ooo(gama)

Crystal data

C6H9N3O3	Dx = 1.412 Mg m^−3
Mr = 171.16	Mo Kα radiation, λ = 0.71073 Å
Trigonal, R3c	Cell parameters from 488 reflections
Hall symbol: R 3 -2"c	θ = 1.8–25.1°
a = 14.027 (1) Å	µ = 0.12 mm^−1
c = 7.088 (1) Å	T = 293 K
V = 1207.8 (2) Å^3	Hexagonal rods, colorless
Z = 6	0.31 × 0.24 × 0.15 mm
F(000) = 540

Data collection

Nonius KappaCCD diffractometer	204 reflections with I > 2σ(I)
Radiation source: fine-focus sealed tube	Rint = 0.012
Graphite monochromator	θmax = 25.1°, θmin = 5.3°
Detector resolution: 95 pixels mm^-1	h = 0→16
ω–scans	k = −13→0
439 measured reflections	l = −8→8
238 independent reflections

Refinement

Refinement on F^2	Secondary atom site location: difference Fourier map
Least-squares matrix: full	Hydrogen site location: inferred from neighbouring sites
R[F^2 > 2σ(F^2)] = 0.032	H atoms treated by a mixture of independent and constrained refinement
wR(F^2) = 0.082	w = 1/[σ^2(Fo^2) + (0.0441P)^2 + 0.2582P] where P = (Fo^2 + 2Fc^2)/3
S = 1.12	(Δ/σ)max < 0.001
238 reflections	Δρmax = 0.09 e Å^−3
40 parameters	Δρmin = −0.09 e Å^−3
1 restraint	Absolute structure: Flack H D (1983), Acta Cryst. A39, 876-881
Primary atom site location: structure-invariant direct methods	Absolute structure parameter: 0 (10)

Special details

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.

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

	x	y	z	Uiso*/Ueq
N1	0.0372 (2)	−0.07375 (19)	0.4141 (3)	0.0596 (8)
C2	0.1031 (2)	0.0353 (2)	0.4148 (3)	0.0574 (10)
O3	0.20930 (18)	0.06550 (15)	0.4181 (5)	0.0833 (9)
C4	0.2881 (3)	0.1831 (3)	0.4224 (7)	0.0990 (16)
H4A	0.3616	0.1946	0.4245	0.114 (16)*
H4B	0.2761	0.2150	0.5333	0.118 (16)*
H4C	0.2788	0.2174	0.3121	0.125 (17)*

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
N1	0.043 (2)	0.0442 (14)	0.0917 (19)	0.0222 (16)	0.0030 (11)	0.0013 (11)
C2	0.0416 (16)	0.050 (2)	0.080 (2)	0.0225 (14)	0.0004 (13)	0.0034 (13)
O3	0.0398 (12)	0.0504 (13)	0.156 (2)	0.0197 (10)	0.0047 (12)	0.0067 (13)
C4	0.045 (2)	0.056 (2)	0.182 (5)	0.0152 (16)	0.005 (2)	0.007 (3)

Geometric parameters (Å, º)

N1—C2^i	1.315 (4)	O3—C4	1.457 (4)
N1—C2	1.335 (4)	C4—H4A	0.9600
C2—N1^ii	1.315 (4)	C4—H4B	0.9600
C2—O3	1.330 (3)	C4—H4C	0.9600
C2^i—N1—C2	112.6 (3)	O3—C4—H4B	109.5
N1^ii—C2—O3	119.7 (2)	H4A—C4—H4B	109.5
N1^ii—C2—N1	127.4 (3)	O3—C4—H4C	109.5
O3—C2—N1	112.9 (2)	H4A—C4—H4C	109.5
C2—O3—C4	117.1 (3)	H4B—C4—H4C	109.5
O3—C4—H4A	109.5
C2^i—N1—C2—N1^ii	−0.8 (5)	N1^ii—C2—O3—C4	0.5 (4)
C2^i—N1—C2—O3	178.7 (2)	N1—C2—O3—C4	−179.0 (3)

Symmetry codes: (i) −x+y, −x, z; (ii) −y, x−y, z.