3-Amino-1,2,4-triazine

In the crystal structure of 3-amino-1,2,4-triazine, C₃H₄N₄, the mol­ecules form hydrogen-bonded chains that are almost parallel to the b axis (3.2°), and which are inclined to the a and c axes by ∼21 and ∼69°, respectively. The distortion of the 1,2,4-triazine ring in the crystal is compared with gas-phase ab initio molecular-orbital calculations.

S1. Comment

The present study is a continuation of our investigation of the characterization of the hydrogen bonds formed by triazine derivatives in the solid state (Janczak & Kubiak, 1999; Janczak & Perpétuo, 2001a,b,c,d, 2002; Perpétuo & Janczak, 2002). Triazine and its derivatives, as well as its organic and inorganic complexes or salts, can, via multiple hydrogen bonds, develop supramolecular structures by self-assembly of components which contain complementary arrays of hydrogen-bonding sites (Mathias et al., 1994; Zerkowski & Whitesides, 1994; MacDonald & Whitesides, 1994; Row, 1999; Krische & Lehn, 2000; Sherrington & Taskinen, 2001). In order to expand the understanding of the solid-state physical-organic chemistry of compounds containing multiple and different hydrogen-bonding systems, we have studied the solid-state structure of 3-amino-1,2,4-triazine, (I). Additionally, the geometry of the molecule has been compared with the ab initio fully optimized geometry calculated at the HF/6–31 G(d,p) level (Frisch et al., 1995), and these results are presented here. The ab initio molecular-orbital calculation was carried out on the isolated and non-interacting molecule. \sch

The planar six-membered aromatic ring of (I) (Fig. 1) is significantly distorted from the ideal hexagonal form, with the internal C3—N4—C5 angle significantly smaller than 120°. This is a result of the steric effect of a lone-pair electron, predicted by the valence-shell electron-pair repulsion theory (VSEPR; Gillespie, 1963, 1992). Although the other two ring N atoms also have a lone-pair electron, the C6—N1—N2 and C3—N2—N1 angles are less distorted from 120° than the angle containing only one N atom (C3—N4—C5). This is undoubtedly due to the direct bond between the two N atoms (N1—N2), which partially reduces the steric effect of the lone-pair electron. Additionally, the steric effect of the lone-pair electron at the N1 and N2 ring atoms is reduced due to hydrogen bonds, in which both ring N atoms are involved as acceptor (Fig. 2).

The ab initio optimized geometry calculated for (I) (Fig. 3) shows a similar correlation between the C—N—C, N—C—N, N—N—C and N—C—C angles within the ring to that found in the crystal. Thus, the ring distortion results mainly from the steric effect of the lone-pair electrons on the ring N atoms and, to a lesser degree, from the hydrogen-bonding system and crystal packing. The values of the N—N, N—C and C—C bond lengths within the 1,2,4-triazine ring in the crystal are comparable with those found in the crystals of other N-heteroaromatic derivatives (Allen et al., 1987), but are slightly longer than those in the optimized gas-phase molecule, while the C—NH₂ bond is longer in the optimized molecule than in the crystal.

In the crystal of (I) (Fig. 2), the molecules are parallel to each other. Each 3-amino-1,2,4-triazine molecule is involved in four hydrogen bonds, in two as a donor and in the other two as an acceptor. The hydrogen-bonded molecules form chains that are almost parallel to the b axis (3.2°), and which are inclined by ~21 and ~69° to the a and c axes, respectively. The chains of hydrogen-bonded molecules are parallel to each other, forming a stacking structure. Within one stack, the molecules are separated by a distance of ~2.88 Å. This distance is significantly shorter than the distance between the π-aromatic ring systems (Pauling, 1960) and indicates a strong π–π interaction between the 1,2,4-triazine rings within the stacks (Fig. 2).

S2. Experimental

Crystals of (I) were obtained by sublimation under vacuum and at the temperature gradient. Please clarify.

Computing details

Data collection: XSCANS (Siemens, 1991); cell refinement: XSCANS; data reduction: XSCANS; program(s) used to solve structure: SHELXS97 (Sheldrick, 1990a); program(s) used to refine structure: SHELXL97 (Sheldrick, 1997); molecular graphics: SHELXTL (Sheldrick, 1990b); software used to prepare material for publication: SHELXL97.

Figures

	Fig. 1. A view of the molecule of (I), with 50% probability displacement ellipsoids. H atoms are shown as small spheres of arbitrary radii.
	Fig. 2. A projection of the structure of (I) along [100], showing molecules connected into chains by N—H···N interactions.
	Fig. 3. The results of the optimized gas-phase calculation on 3-amino-1,2,4-triazine. Bond lengths are given in Å and angles in °.

3-amino-1,2,4-triazine

Crystal data

C3H4N4	Z = 2
Mr = 96.10	F(000) = 100
Triclinic, P1	Dx = 1.473 Mg m^−3 Dm = 1.47 Mg m^−3 Dm measured by flotation
Hall symbol: -P 1	Melting point = 448–449 K
a = 5.225 (1) Å	Mo Kα radiation, λ = 0.71073 Å
b = 6.166 (1) Å	Cell parameters from 28 reflections
c = 7.079 (1) Å	θ = 8–14°
α = 84.03 (3)°	µ = 0.11 mm^−1
β = 76.88 (3)°	T = 295 K
γ = 77.81 (3)°	Parallelepiped, light yellow
V = 216.73 (7) Å^3	0.34 × 0.22 × 0.18 mm

Data collection

Siemens P4 diffractometer	668 reflections with I > 2σ(I)
Radiation source: fine-focus sealed tube	Rint = 0.013
Graphite monochromator	θmax = 28.2°, θmin = 3.0°
ω/2θ scans	h = −6→6
Absorption correction: analytical face-indexed (SHELXTL; Sheldrick, 1990)	k = −6→8
Tmin = 0.972, Tmax = 0.981	l = −9→9
2005 measured reflections	2 standard reflections every 50 reflections
1021 independent reflections	intensity decay: 0.7%

Refinement

Refinement on F^2	Secondary atom site location: difference Fourier map
Least-squares matrix: full	Hydrogen site location: refall
R[F^2 > 2σ(F^2)] = 0.034	All H-atom parameters refined
wR(F^2) = 0.083	w = 1/[σ^2(Fo^2) + (0.0238P)^2 + 0.0537P] where P = (Fo^2 + 2Fc^2)/3
S = 1.01	(Δ/σ)max = 0.002
1021 reflections	Δρmax = 0.18 e Å^−3
81 parameters	Δρmin = −0.16 e Å^−3
0 restraints	Extinction correction: SHELXL97 (Sheldrick, 1997), Fc^*=kFc[1+0.001xFc^2λ^3/sin(2θ)]^-1/4
Primary atom site location: structure-invariant direct methods	Extinction coefficient: 0.061 (12)

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	0.0622 (2)	0.86837 (18)	0.21037 (17)	0.0517 (4)
N2	0.2138 (2)	0.67434 (17)	0.15160 (16)	0.0443 (3)
C3	0.1087 (2)	0.4881 (2)	0.20347 (18)	0.0388 (3)
N4	−0.1390 (2)	0.48287 (18)	0.31155 (16)	0.0431 (3)
C5	−0.2827 (3)	0.6771 (2)	0.3636 (2)	0.0478 (4)
C6	−0.1826 (3)	0.8721 (3)	0.3126 (2)	0.0561 (4)
N7	0.2662 (3)	0.29792 (19)	0.14369 (19)	0.0500 (3)
H5	−0.464 (3)	0.682 (2)	0.443 (2)	0.050 (4)*
H1	0.204 (3)	0.173 (3)	0.161 (2)	0.049 (4)*
H2	0.411 (3)	0.306 (2)	0.064 (2)	0.051 (4)*
H6	−0.296 (3)	1.026 (3)	0.358 (2)	0.062 (4)*

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
N1	0.0538 (7)	0.0364 (6)	0.0614 (8)	−0.0142 (5)	0.0021 (6)	−0.0076 (5)
N2	0.0423 (6)	0.0340 (6)	0.0539 (7)	−0.0122 (5)	0.0011 (5)	−0.0049 (5)
C3	0.0401 (7)	0.0347 (7)	0.0410 (7)	−0.0115 (5)	−0.0035 (5)	−0.0020 (5)
N4	0.0399 (6)	0.0406 (6)	0.0475 (6)	−0.0142 (5)	−0.0005 (5)	−0.0025 (5)
C5	0.0386 (7)	0.0477 (8)	0.0532 (8)	−0.0104 (6)	0.0021 (6)	−0.0072 (6)
C6	0.0528 (9)	0.0425 (8)	0.0649 (10)	−0.0065 (7)	0.0051 (7)	−0.0109 (7)
N7	0.0463 (7)	0.0332 (6)	0.0642 (8)	−0.0138 (5)	0.0075 (6)	−0.0046 (5)

Geometric parameters (Å, º)

N1—C6	1.316 (2)	C5—C6	1.393 (2)
N1—N2	1.336 (2)	C5—H5	0.981 (15)
N2—C3	1.357 (2)	C6—H6	1.044 (15)
C3—N7	1.330 (2)	N7—H1	0.883 (16)
C3—N4	1.351 (2)	N7—H2	0.842 (16)
N4—C5	1.311 (2)
C6—N1—N2	119.1 (1)	C6—C5—H5	120.1 (8)
N1—N2—C3	118.1 (1)	N1—C6—C5	121.0 (1)
N7—C3—N4	118.6 (1)	N1—C6—H6	117.5 (9)
N7—C3—N2	116.4 (1)	C5—C6—H6	121.5 (9)
N4—C3—N2	124.9 (1)	C3—N7—H1	120.8 (9)
C5—N4—C3	115.0 (1)	C3—N7—H2	117.2 (10)
N4—C5—C6	121.8 (1)	H1—N7—H2	120.1 (14)
N4—C5—H5	118.1 (8)

Hydrogen-bond geometry (Å, º)

D—H···A	D—H	H···A	D···A	D—H···A
N7—H1···N1^i	0.88 (2)	2.13 (2)	3.013 (2)	178 (2)
N7—H2···N2^ii	0.84 (2)	2.21 (2)	3.054 (2)	178 (2)

Symmetry codes: (i) x, y−1, z; (ii) −x+1, −y+1, −z.