Single-phase melem (2,5,8-triamino-tri-s-triazine) C6N7(NH2)3 was obtained as a crystalline powder by thermal treatment of different less condensed C-N-H compounds (e.g., melamine C3N3(NH2)3, dicyandiamide H4C2N4, ammonium dicyanamide NH4[N(CN)2], or cyanamide H2CN2, respectively) at temperatures up to 450°C in sealed glass ampules. The crystal structure was determined ab initio by X-ray powder diffractometry (Cu KR1: P21/c (No. 14), a ) 739.92(1) pm, b ) 865.28(3) pm, c ) 1338.16(4) pm, ) 99.912(2)°, and Z ) 4). In the solid, melem consists of nearly planar C6N7(NH2)3 molecules which are arranged into parallel layers with an interplanar distance of 327 pm. Detailed 13 C and 15 N MAS NMR investigations were performed. The presence of the triamino form instead of other possible tautomers was confirmed by a CPPI (cross-polarization combined with polarization inversion) experiment. Furthermore, the compound was characterized using mass spectrometry, vibrational (IR, Raman), and photoluminescence spectroscopy. The structural and vibrational properties of molecular melem were theoretically studied on both the B3LYP and the MP2 level. A structural optimization in the extended state was performed employing density functional methods utilizing LDA and GGA. A good agreement was found between the observed and calculated structural parameters and also for the vibrational frequencies of melem. According to temperature-dependent X-ray powder diffractometry investigations above 560°C, melem transforms into a graphite-like C-N material.Melamine (2,4,6-triamino-s-triazine), C 3 N 3 (NH 2 ) 3 1a, represents an important starting material for several industrial applications, for example, the syntheses of melamine-formaldehyde resins or of fireproof materials. 1,2 Furthermore, it is used for the architecture of supramolecular structures, for example, assemblies built up by cyanuric acid C 3 N 3 (OH) 3 1b or melamine derivatives. [3][4][5] In the past few years, another interest arose in melamine as well as in other compounds, C 3 N 3 X 3 1, containing s-triazine (cyanuric) rings C 3 N 3 , for example, cyanuric chloride C 3 N 3 Cl 3 1c [6][7][8][9][10] or cyanuric azide C 3 N 3 (N 3 ) 3 1d. 11 These are considered to be suitable molecular precursor compounds for the synthesis of graphitic forms of carbon nitride (g-C 3 N 4 ). 6,[12][13][14] In most of the postulated structures of g-C 3 N 4 , s-triazine ring systems are linked through trigonal N atoms forming extended 2D sheets 2 (Scheme 1).Recently, another possible building block for g-C 3 N 4 was taken into account (Scheme 2): tri-s-triazine rings C 6 N 7 which are cross-linked by trigonal N atoms 3. [15][16][17] The possible condensation of three s-triazine rings and the existence of a "cyameluric nucleus" C 6 N 7 4 was first postulated by Pauling † Ludwig-Maximilians-Universität München.

The ground states of magnetic Mott insulators GaV 4 S 8 and GeV 4 S 8 were calculated with full-potential LAPW methods using the LDA + U approach. Both compounds undergo structural distortions from cubic to rhombohedral (GaV 4 S 8 ) or orthorhombic (GeV 4 S 8 ) symmetry at low temperatures. GaV 4 S 8 is ferromagnetic below T C ) 10 K, whereas GeV 4 S 8 shows antiferromagnetic order with T N ) 13 K. The spin structure of GeV 4 S 8 was determined by neutron diffraction and described in the magnetic space group P b mn2 1 . The magnetic propagation vector is [ 1 / 2 , 1 / 2 , 0] relating to the cubic paramagnetic unit cell. The LDA + U calculations (U ) 2 eV) confirm the magnetic insulating ground states for the first time with magnetic moments and energy gaps in very good agreement with the experimental data. This method also reproduces the antiferromagnetic spin ordering of GeV 4 S 8 . The Jahn-Teller instability of the degenerated levels in the V 4 cluster MO drives the structural distortions, depending on the cluster electron count. Our results show how the interplay between the electronic, lattice, and spin degrees of freedom of the V 4 cluster units determines the physical and structural properties of these highly correlated transition-metal sulfides.