The X-ray and neutron crystal structure of 2,4,6-triamino-1,3,5-triazine (melamine)

Abstract

The crystal structure of 2,4,6-triamino-1,3,5-triazine has been investigated with extensive three-dimensional X-ray (sin 0/,t < 0.91) and neutron (sin 0/h < 0.84 A -I) diffraction data. The structure is significantly nonplanar with the amine groups deviating by up to 0-10 A from the mean plane through the ring. The molecule does not behave like a rigid body, the out-of-plane mean-square amplitudes of the ring nitrogens being about 12% greater than those of the C atoms. The positional parameters for the C and N atoms from the X-ray refinement are generally similar to the neutron values, but the thermal parameters for these atoms show a systematic bias as a result of the neglect of the non-spherical symmetry of the valence density. The X-ray H positions are displaced along the N-H bonds by amounts which, while generally similar to those reported for similar systems, are also related to the internal modes of vibration in the structure. The internal motions of the H atoms, as indicated by the differences between thermal ellipsoids for the H and amine N atoms, are related to the hydrogen bonding. The amplitudes for the hydrogen-bonded atoms are relatively lower normal to the N--H bonds, and larger in the directions corresponding to the N-H stretching mode. This exaggerates the foreshortening of the N-H bonds, as has been predicted by Coulson

Structure of Melaminium Dihydrogenpyrophosphate and Its Formation from Melaminium Dihydrogenphosphate Studied with Powder Diffraction Data, Solid-State NMR, and Theoretical Calculations

“…Nevertheless, significant distortions of the melamines suggest that both rings are protonated. In contrast to, for example, crystalline melamine 31 but similar as in MP, in MPy the aromatic π-system is distorted because of proton transfer from the (pyro)phosphates to one of the endocyclic melamine nitrogens. In particular, this leads to a bond length alteration with C-N distances varying in the range 1.28-1.47 Å.…”

“…The melamine packing in MPy is quite common for melamine complexes and salts. While in pure melamine 31 each melamine molecule binds through two hydrogen bonds to three neighbors, leading to a complex threedimensional melamine network, in the presence of other moieties melamine shows a typical tendency to self-assemble via double hydrogen bonds (N‚‚‚N ≈ 3 Å) into infinite chains or "ribbons", stacked parallel to each other because of π-π stacking effects. 7,14,[41][42][43][44][45][46][47][48][49][50] In these ribbons each melamine molecule is bonded to two melamine neighbors only because one of melamine's endocyclic nitrogens is hydrogen bonded to one of the other structural moieties, and therefore not available for hydrogen bonding to another (third) melamine.…”

Transformation of Ammonium Dicyanamide into Dicyandiamide in the Solid

“…24 The powder diffraction patterns of the heated samples are in good agreement (Figure 4) with those simulated from structural data of dicyandiamide and melamine, respectively. 25,26 The Further investigations are in progress to get a detailed picture of the mechanism of these solid-state transformations. 33 …”

Electron-density studies. V. The electron density in melamine (2,4,6-triamino-s-triazine) with and without exponent refinement

“…1. Diagram of the melamine structure based on the neutron parameters of Varghese, O'Connell & Maslen (1977). …”

“…INDO -an INDO calculation by Varghese (1974). STO-3G (SM)-an STO-3G calculation with standard molecular exponents by Chandler & Spackman (1975) A semi-empirical INDO calculation was carried out, Atom with the geometry determined by the neutron diffraction experiment (Varghese, O'Connell & Maslen, 1977). H(1) The resulting 'valence populations' (gross Mulliken H(2) atomic populations less two for C and N atoms) are H(3) H(4) shown in Table 1 together with those of two other H(5) semi-empirical (Santry, 1974) and one ab initio calcula-H(6) tion (Chandler & Spackman, 1975).…”