Melem (2,5,8-triamino-tri-s-triazine), an important intermediate during condensation of melamine rings to graphitic carbon nitride: Synthesis, structure determination by x-ray powder diffractometry, solid-state NMR, and theoretical studies

Article abstract of DOI:10.1021/ja0357689

Single-phase melem (2,5,8-triamino-tri-s-triazine) C₆N ₇(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 NH ₄[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 Kα₁: P2₁/c (No. 14), a = 739.92(1) pm, b = 865.28(3) pm, c = 1338.16(4) pm, β = 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 ¹³C and ¹⁵N 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.

Full text of DOI:10.1021/ja0357689

Published on Web 08/02/2003
Melem (2,5,8-Triamino-tri-s-triazine), an Important Intermediate
during Condensation of Melamine Rings to Graphitic Carbon
Nitride: Synthesis, Structure Determination by X-ray Powder
Diffractometry, Solid-State NMR, and Theoretical Studies
Barbara Ju¨rgens,† Elisabeth Irran,‡ Ju¨rgen Senker,† Peter Kroll,§ Helen Mu¨ller,† and
Wolfgang Schnick*,†
Contribution from the Ludwig-Maximilians-UniVersita¨t Mu¨nchen, Department Chemie,
Butenandtstrasse 5-13 (D), D-81377 Mu¨nchen, Germany, Institut fu¨r Chemie - Anorganische
Festko¨rperchemie, Sekr. C2, TU Berlin, Strasse des 17. Juni 135, D-10623 Berlin, Germany, and
Institut fu¨r Anorganische Chemie, RWTH Aachen, Professor-Pirlet-Strasse 1, D-52056 Aachen,
Germany
Received April 24, 2003; E-mail: wolfgang.schnick@uni-muenchen.de
Abstract: 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, â ) 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 13C and
15N 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), C3N3(NH2)3 1a, repre-
sents an important starting material for several industrial
applications, for example, the syntheses of melamine-formal-
dehyde 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 C3N3(OH)3 1b or melamine
derivatives.3-5 In the past few years, another interest arose in
melamine as well as in other compounds, C3N3X3 1, containing
s-triazine (cyanuric) rings C3N3, for example, cyanuric chloride
C3N3Cl3 1c6-10 or cyanuric azide C3N3(N3)3 1d.11 These are
considered to be suitable molecular precursor compounds for
the synthesis of graphitic forms of carbon nitride (g-C3N4).6,12-14
In most of the postulated structures of g-C3N4, s-triazine ring
systems are linked through trigonal N atoms forming extended
2D sheets 2 (Scheme 1).
Recently, another possible building block for g-C3N4 was
taken into account (Scheme 2): tri-s-triazine rings C6N7 which
are cross-linked by trigonal N atoms 3.15-17 The possible
condensation of three s-triazine rings and the existence of a
“cyameluric nucleus” C6N7 4 was first postulated by Pauling
(6) Alves, I.; Demazeau, G.; Tanguy, B.; Weill, F. Solid State Commun. 1999,
109, 697.
(7) Zhang, Z.; Leinenweber, K.; Bauer, M.; Garvie, L. A. J., McMillan, P. F.;
Wolf, G. H. J. Am. Chem. Soc. 2001, 123, 7788.
† Ludwig-Maximilians-Universita¨t Mu¨nchen.
‡ TU Berlin.
(8) Wolf, G. H.; Bauer, M.; Leinenweber, K.; Garvie, L. A. J.; Zhang, Z. NATO
Sci. Ser., II: Mathematics, Physics and Chemistry 2001, 48 (Frontiers of
High-Pressure Research II: Application of High Pressure to Low-
Dimensional NoVel Electronic Materials), 29.
§ RWTH Aachen.
(1) Allcock, H. R.; Lampe, F. W. Contemporary Polymer Chemistry; Prentice
Hall: New Jersey, 1990.
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Dekker: New York, 1973-1979; Vols. 1-5.
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Monographs in Supramolecular Chemistry, issue 7; Cambridge: New York,
2000.
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A. D. New J. Chem. 2002, 26, 508.
9
10288
J. AM. CHEM. SOC. 2003, 125, 10288-10300
10.1021/ja0357689 CCC: $25.00 © 2003 American Chemical Society

Study of Melem (2,5,8-Triamino-tri-s-triazine)
A R T I C L E S
Scheme 1. s-Triazines
Scheme 2. Tri-s-triazines
and Sturdivant.18 On the basis of this complex ring system,
further substitutional derivatives were predicted in analogy to
compounds containing the cyanuric ring, for example, cyame-
luric acid C6N7(OH)3 4a which is related to cyanuric acid C3N3-
(OH)3 or hydromelonic acid C6N7(NCNH)3 4b which represents
the analogue to postulated tricyanomelamine C3N3(NCNH)3
1e.19 Many attempts were performed to synthesize and char-
acterize cyameluric derivatives; however, the existence of the
C6N7 ring system has been proven lately with the crystal
structure of tri-s-triazine C6N7H3 4c.20-21 Very recently, Kroke
et al. described the first functionalized derivative, trichloro-tri-
s-triazine C6N7Cl3 4d.17
In analogy to 2 where 2,4,6-triamino-s-triazine represents the
repeating unit, 2,5,8-triamino-tri-s-triazine 4e can be considered
as the repeating unit of 3. Synonyms for 2,5,8-triamino-tri-s-
triazine are 2,5,8-triamino-1,3,4,6,7,9,9b-heptaazaphenalene or
2,5,8-triamino-1,3,4,6,7,9-hexaazacycl[3.3.3]azine and 2,5,8-
triamino-s-heptazine. The trivial name melem for compound 4e
dates back to Liebig who performed different experiments with
carbon nitrides:22 While heating potassium thiocyanate with
ammonium chloride, he obtained 1a which he named melamine.
Later, the formation of melamine by different other experiments
was observed, for example, by heating cyanamide H2CN2 5,
ammonium dicyanamide NH4[N(CN)2] 6, or dicyandiamide
H4C2N4 7 (Scheme 3).23-25
The pyrolysis of melamine as well as its behavior under
pressure were investigated, and condensation processes (Scheme
4) leading to so-called melam 8, melem 4e, and melon 9 were
postulated.26-30 For melamine, structural data have been re-
ported. Neither the existence of melam nor the existence of
melem and melon could be confirmed by crystal structure
determinations as yet.31,32
(22) Liebig, J. Ann. Chem. 1834, 10, 1.
(23) Bieling, H.; Radu¨chel, M.; Wenzel, G.; Beyer, H. J. Prakt. Chem. 1965,
28, 325.
(24) Ju¨rgens, B.; Ho¨ppe, H. A.; Irran, E.; Schnick, W. Inorg. Chem. 2002, 41,
4849.
(25) Franklin, E. C. J. Am. Chem. Soc. 1922, 44, 486.
(26) May, H. J. Appl. Chem. 1959, 340.
(27) van der Plaats, G.; Soons, H.; Snellings, R. Proc. Eur. Symp. Therm. Anal.
1981, 2, 215.
(18) Pauling, L.; Sturdivant, J. H. Proc. Natl. Acad. Sci. U.S.A. 1937, 23, 615.
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(20) Hosmane, R. S.; Rossman, M. A.; Leonard, N. J. J. Am. Chem. Soc. 1982,
104, 5497.
(21) Shahbaz, M.; Urano, S.; LeBreton, P. R.; Rossman, M. A.; Hosmane, R.
S.; Leonard, N. J. J. Am. Chem. Soc. 1984, 106, 2805.
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J.; Wang, X. C.; Liu, J.; Zou, G. T. Chem. Phys. Lett. 2003, 368, 668.
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9
J. AM. CHEM. SOC. VOL. 125, NO. 34, 2003 10289

A R T I C L E S
Ju¨rgens et al.
Scheme 3. Formation of Melamine 1a
Scheme 4. Postulated Condensation of Melamine 1a
to 470 °C (heating rate: 1 °C min-1) in a sealed glass ampule (outer
diameter, 16 mm; inner diameter, 12 mm; length, 120 mm). After the
mixture was cooled, NaCl as well as a yellow insoluble polymer
remained at the bottom of the ampule. At the top, a beige reaction
product was found containing melamine. Subsequent sublimation (220
°C, 1 Pa) of this product leads to nearly pure melamine with a
significantly higher content of 15N within both the amino groups and
the s-triazine ring.
Herein, we report on the synthesis of melem 4e obtained in
preparative amounts, its crystal structure determination, and
spectroscopic investigation. Thus, we confirm the existence of
a fundamental C-N-H molecule, that has been postulated a
long time ago.
Experimental Section
Preparation of Melem C6N7(NH2)3. Melem 4e was synthesized by
heating cyanamide 5 (Fluka, g98%) or ammonium dicyanamide 6 (for
preparation, see ref 24) or dicyandiamide 7 (Avocado, 99%) or
melamine 1a (Fluka, purum, g99% (NT)). The commercial products
were used as purchased: 80 mg of starting material (1.90 mmol of 5,
0.95 mmol of 6, 0.95 mmol of 7, or 0.63 mmol of 1a, respectively)
was filled into a glass ampule (outer diameter, 16 mm; inner diameter,
12 mm). The ampule was sealed at a length of 120 mm and heated to
450 °C (heating rate: 1 °C min-1). After about 5 h at this temperature,
the ampule was slowly (2 °C min-1) cooled to room temperature.
After the ampule was opened, the typical smell of ammonia was
detected. At the top of the ampule, colorless crystals were found which
were identified by X-ray powder diffractometry as sublimated melamine.
At the bottom, a white-beige powder containing melem was isolated.
The product has formed with a yield of ca. 60%. It is not moisture
sensitive. Anal. calcd for melem: H, 2.75; C, 33.03; N, 64.22. Found:
H, 2.98; C, 32.62; N, 62.04.
Samples of C315N3(15NH2)3 were used to prepare 15N-enriched melem
C615N7(15NH2)3, which contains 15N atoms within the tri-s-triazine ring
as well as within the amino groups.
Physical Measurements. X-ray Diffraction. X-ray powder diffrac-
tion data were used for the crystal structure determination because no
single crystals of melem were available. The microcrystalline samples
were enclosed in glass capillaries of diameter 0.3 mm. The diffraction
investigations were carried out in Debye-Scherrer geometry. The
diffraction data were collected on a conventional powder diffractometer
(STOE Stadi P, Cu KR1 radiation).
Temperature-dependent X-ray powder diffraction experiments were
performed on a STOE Stadi P powder diffractometer (Mo KR1) with
a computer controlled STOE furnace: Samples of melamine and of
melem, respectively, were enclosed in silica capillaries and heated from
room temperature to 700 °C in the angular range 3° e 2θ e 30°.
Powder diffraction patterns were recorded in steps of 20 °C. Addition-
ally, a sample of melem was cooled with a 600 Series Cryostream
Cooler (Oxford Cryosystems) from room temperature to -140 °C. The
cooling procedure was interrupted in steps of 20 °C to record diffraction
patterns at constant temperatures.
Preparation of 15N-Enriched Samples of Melamine and Melem.
For 15N MAS NMR investigations, a special synthesis was developed
to prepare 15N-enriched samples: 54.4 mg (0.20 mmol) of Na3[C6N9]
(for preparation, see ref 33) was heated with 15.1 mg (0.28 mmol) of
sublimated 15NH4Cl (Promochem, 98 at. % enriched) under argon up
Thermoanalytical Investigations. A DSC curve of melamine was
recorded with a DSC 141 (Setaram): 17.906 mg (0.142 mmol) of
melamine 1a was filled under argon into a steel pressure crucible and
heated to 500 °C (heating rate: 5 °C min-1). Additionally, the thermal
effects during cooling were recorded. The use of special pressure
(32) Varghese, J. N.; O’Connell, A. M.; Maslen, E. N. Acta Crystallogr. 1977,
B33, 2102.
(33) Ju¨rgens, B.; Irran, E.; Schneider, J.; Schnick, W. Inorg. Chem. 2000, 39,
665.
9
10290 J. AM. CHEM. SOC. VOL. 125, NO. 34, 2003

Study of Melem (2,5,8-Triamino-tri-s-triazine)
A R T I C L E S
Figure 1. Observed (crosses) and calculated (line) X-ray powder diffraction patterns as well as difference profiles of the Rietveld refinement of melem 4e
(STOE Stadi P, λ ) 154.06 pm).
crucibles was necessary because conventional alumina crucibles burst
due to evolution of ammonia.
Calculations. Methods. The theoretical assessment of the structural
properties of C6N7(NH2)3 is based on a detailed comparison of
computational results for both the molecular and the extended state.
The molecular-orbital calculations were performed using the Gaussian
quantum chemistry software package.35 Beckes three-parameter hybrid
functional (B3LYP)36,37 was employed as well as the second-order
Møller-Plesset perturbation theory (MP2).38 Dunning’s correlation-
consistent basis set (cc-pVDZ) was used.39 A Pople-type basis set (6-
311++G**) yielded very similar results.
Solid-State MAS NMR Spectroscopy. 13C and 15N MAS NMR
spectra of melamine and melem were recorded at room temperature
with a conventional impulse spectrometer DSX 500 Avance (Bruker)
operating at 500 MHz. For recording the 15N MAS NMR spectra, 15N-
enriched samples of both compounds were used.
The samples were filled into zirconia rotors with a diameter of 4
mm and mounted in a standard double-resonance MAS probe (Bruker).
The signals were referenced to trimethylsilane (TMS) (13C) and
nitromethane (15N), respectively. Rotation frequencies between 3 and
7 kHz were chosen.
The structural optimizations in the extended state were done using
the Vienna ab initio simulation package (VASP).40-43 Ultrasoft pseudo-
potentials were employed for the atoms, and the exchange-correlation
energy of the valence electrons was treated at the DFT level using both
the local density approximation (LDA)44 and the generalized-gradient
approximation (GGA).45 The wave function was expanded into a plane
wave basis set using a cutoff energy of 400 eV; for the integration
over the Brillouin Zone, a Monkhorst-Pack 2 × 2 × 2 k-point mesh
was used.46 Initial positions of atoms were the crystal coordinates
supplied by the refinement of the X-ray diffraction data. During
relaxation, all crystal coordinates were optimized while keeping space
group symmetry and experimental lattice constants fixed. Releasing
the latter constraint showed the usual trends of LDA and GGA tending
to smaller and larger volumes, each by 4%, respectively.
A ramped cross-polarization sequence was employed to excite both
13C and 15N nuclei via the proton bath where the power of the 1H
radiation was linearly varied about 50%. A CPPI (cross-polarization
combined with polarization inversion) experiment was performed to
investigate the bonding and the position of the hydrogen atoms of
melamine and melem, respectively. For these experiments, rotation
frequencies of 3.7 kHz (melamine) and 5 kHz (melem) and an initial
1
contact time of 30 ms before inverting the sign of the H radiation
were used. The data collection of all experiments was performed
applying broadband proton decoupling via a TPPM sequence.34
Vibrational Spectroscopy. FTIR spectra of melamine and melem
were obtained at room temperature by using a Bruker IFS 66v/S
spectrometer with DTGS detector. The samples were thoroughly mixed
with dried KBr (5 mg of sample, 500 mg of KBr). The preparation
procedures were performed in a glovebox under dried argon atmosphere.
The spectra were collected in a range from 400 to 4000 cm-1 with a
resolution of 2 cm-1. During the measurement, the sample chamber
was evacuated.
(34) Bennett, A. E.; Rienstra, C. M.; Auger, M.; Lakshimi, K. V.; Griffin, R.
G. J. Chem. Phys. 1995, 103, 6951.
(35) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M.
A.; Cheeseman, J. R.; Zakrzewski, V. G.; Montgomery, J. A., Jr.; Stratmann,
R. E.; Burant, J. C.; Dapprich, S.; Millam, J. M.; Daniels, A. D.; Kudin,
K. N.; Strain, M. C.; Farkas, O.; Tomasi, J.; Barone, V.; Cossi, M.; Cammi,
R.; Mennucci, B.; Pomelli, C.; Adamo, C.; Clifford, S.; Ochterski, J.;
Petersson, G. A.; Ayala, P. Y.; Cui, Q.; Morokuma, K.; Malick, D. K.;
Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.; Cioslowski, J.; Ortiz,
J. V.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, I.;
Gomperts, R.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng,
C. Y.; Nanayakkara, A.; Gonzalez, C.; Challacombe, M.; Gill, P. M. W.;
Johnson, B. G.; Chen, W.; Wong, M. W.; Andres, J. L.; Head-Gordon,
M.; Replogle, E. S.; Pople, J. A. Gaussian 98, revision A.11; Gaussian,
Inc.: Pittsburgh, PA, 1998.
For FT-Raman measurements, samples of melamine and melem were
filled into glass capillaries of 0.5 mm diameter. The spectra were excited
by a Bruker FRA 106/S module with a Nd:YAG laser (λ ) 1064 nm)
scanning a range from 0 to 3500 cm-1
.
Photoluminescence Spectroscopy. Photoluminescence spectra were
recorded with a spectrofluorimeter FL900 (Edinburgh Instruments) with
a Xe lamp as the light source and a Hamamatsu photomultiplier.
BaMgAl10O17:Eu which has a quantum efficiency of 90% at 254 nm
was used as reference. The transfer function of the spectrometer has
been calibrated over the entire frequency range of the measurements
utilizing reference phosphors.
Mass Spectrometry. Mass spectra were obtained with a JEOL
MStation JMS 700. The source was operated at a temperature of
200 °C.
(36) Becke, A. D. Phys. ReV. A 1988, 38, 3098.
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(38) Møller, C.; Plesset, M. S. Phys. ReV. 1934, 46, 618.
(39) Dunning, T. H. J. Chem. Phys. 1989, 90, 1007.
(40) Kresse, G.; Hafner, J. Phys. ReV. B 1993, 47, 558.
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9
J. AM. CHEM. SOC. VOL. 125, NO. 34, 2003 10291

A R T I C L E S
Ju¨rgens et al.
Table 1. Crystallographic Data for Melem C6N7(NH2)3
Table 2. Atomic Coordinates and Displacement Factors (in pm2)
for Melem C6N7(NH2)3; All Atoms in Wyckoff Position 4e
formula
H6C6N10
a
Mr/g mol-1
crystal system
space group
powder diffractometer
radiation; λ/pm
temperature/°C
lattice parameters
a/pm
218.18
atom
x
y
z
U
iso
monoclinic
P21/c (no. 14)
Stoe STADI P
Cu KR1; 154.06
22
C(i)1
C(i)2
C(i)3
C(e)4
C(e)5
C(e)6
N(i)1
N(c)2
N(c)3
N(c)4
N(c)5
N(c)6
N(c)7
N(t)8
N(t)9
N(t)10
H1
0.765(2)
0.675(2)
0.867(2)
0.602(2)
0.773(2)
0.959(2)
0.771(1)
0.695(1)
0.581(1)
0.659(1)
0.870(1)
0.956(1)
0.883(1)
0.511(1)
0.766(1)
1.056(1)
0.542(5)
0.427(3)
0.679(4)
0.846(3)
1.081(5)
1.091(4)
0.203(2)
-0.069(2)
0.046(2)
0.093(1)
-0.195(1)
0.294(1)
0.065(2)
0.221(1)
-0.044(1)
-0.196(1)
-0.083(1)
0.169(1)
0.447(1)
0.453(1)
0.593(1)
0.3119(9)
0.5924(9)
0.5903(8)
0.5019(9)
0.3509(7)
0.3644(8)
0.5017(8)
0.6515(9)
0.6443(8)
0.4956(7)
0.2226(6)
0.6460(7)
0.6356(7)
0.183(1)
0.204(1)
0.633(2)
0.692(2)
0.696(7)
0.602(1)
440(20)
440(20)
440(20)
440(20)
440(20)
440(20)
320(10)
320(10)
320(10)
320(10)
320(10)
320(10)
320(10)
320(10)
320(10)
320(10)
250
739.92(1)
b/pm
865.28(3)
c/pm
1338.16(4)
â/deg
99.912(2)
843.95(4)
4
1.717
5° e 2θ e 80°
7500
V/106 pm3
Z
F
calc/g cm-3
0.315(1)
profile range
no. of data points
observed reflections
structural parameters
profile parameters
ø2
0.1046(8)
-0.3282(9)
0.413(1)
0.164(4)
0.047(3)
-0.384(4)
-0.349(2)
0.416(2)
515
68
18
0.883
H2
H3
H4
H5
250
250
250
250
structure refinement
R-values
Rietveld refinement (GSAS)47
wRp
Rp
RF
0.056
0.042
0.079
H6
0.479(2)
250
a Uiso is defined as exp(-8π2Uiso sin2 θ/λ2); Uiso values of H atoms are
not refined.
Scheme 5. One Melem Molecule in the Crystal Structure of
C6N7(NH2)3
Results and Discussion
a
Powder Diffractometry and Structure Refinement. The
diffraction pattern of a room-temperature measurement of melem
was indexed with a monoclinic unit cell (Figure 1, Table 1).
The extinction rules unequivocally indicated the space group
P21/c. A structure solution with direct methods was not possible.
To solve the structure, the profile and lattice constants were
refined by the LeBail method. For these calculations and also
for the Rietveld refinement, the program GSAS was employed.47
The crystal structure was solved by a combination of trial-and-
error and rigid-body methods. The atomic positions of the
molecule neglecting the outermost NH2 groups were calculated
in a random position in the unit cell. The distances between
the atoms were restrained with a very high weight, so that nearly
a rigid body was formed. As a reference for the atomic distances,
the data of C6N7Cl3 4d were taken from Kroke et al.17 The
positions of all atoms were refined simultaneously. When the
a The labeling of the atoms is indicated. The molecule adopts ap-
proximately symmetry D3h.
the labeling of the different C and N positions of the melem
molecule is illustrated.
Crystal Structure. Solid melem 4e consists of C6N7(NH2)3
molecules interconnected by extended hydrogen bridges. Com-
parable to the situation found in tri-s-triazine and trichloro-tri-
s-triazine, each molecule contains a cyameluric nucleus C6N7
of three anellated s-triazine rings. In the case of melem, the
structure is completed by three terminal N atoms bound to
positions 2, 5, and 8 of the tri-s-triazine C6N7 nucleus.
In the asymmetric unit, there is one complete C6N7(NH2)3
molecule (Figure 2). Two differently orientated layers of parallel
molecules alternate along the direction of the c-axis. The planes
of the planar molecules are slightly tilted to the (10-1) plane.
Layers of C6N7(NH2)3 molecules are formed which are parallel
to this plane. Neighboring molecules form an angle of about
40°. The interlayer distance is 327 pm. This value is comparable
to the respective distances in melamine (320 pm - 340 pm), in
the C-N polymer [C6N9H4]Cl (322 pm), as well as in pure
graphite (334 pm). The relatively short interlayer distance
observed in melem may be explained by intermolecular π‚‚‚π
interactions between the aromatic C6N7 nuclei.
2
refinement reached a local minimum (RF ≈ 60%), the weight
of the restraints was reduced to zero, and therefore the atoms
randomized and were released to positions of high electron
density. The weight was then increased again so the molecule
was formed again. This procedure was repeated twice, and then
2
the molecule reached a position where it gave a low RF value
(∼20%). Refinement with a low weight gave an R-value of
about 15%. A subsequent Fourier map clearly revealed the
positions of the N atoms of the outermost NH2 groups. This
result nicely illustrates the reliability of the position and
orientation of the melem molecule in the cell. The H atoms of
the NH2 groups could not be found by difference Fourier
synthesis. Their positions were calculated by the program
SHELX48 and refined with restraints. Details of the structure
determination and refinement are listed in Table 1, and the
refined atomic coordinates are given in Table 2. In Scheme 5,
(47) Larson, A. C.; von Dreele, R. B. General Structure Analysis System, Los
Alamos National Laboratory Report LAUR 86-748, 1990.
(48) Sheldrick, G. M. SHELXTL, V 5.10 Crystallographic System; Bruker AXS
Analytical X-ray Instruments Inc.: Madison, 1997.
The ring system of C6N7(NH2)3 is quite planar; the sums of
the bond angles around each atom are 360°. The rings are
9
10292 J. AM. CHEM. SOC. VOL. 125, NO. 34, 2003

Study of Melem (2,5,8-Triamino-tri-s-triazine)
A R T I C L E S
Figure 2. Crystal structure of melem 4e.
Table 3. Atomic Distances (in pm) and Angles (in deg) in
C6N7(NH2)3 (Distances N(t)-H and Angles C(e)-N(t)-H and
H-N(t)-H Are Constrained)
C(i)1-N(i)1 139(2) N(i)1-C(i)1-N(c)2 126(2) C(i)1-N(i)1-C(i)2 118(2)
C(i)1-N(c)2 131(1) N(i)1-C(i)1-N(c)7 113(2) C(i)1-N(i)1-C(i)3 124(2)
C(i)1-N(c)7 139(1) N(c)2-C(i)1-N(c)7 119(2) C(i)2-N(i)1-C(i)3 117(1)
C(i)2-N(i)1 146(2) N(i)1-C(i)2-N(c)3 115(1) C(i)1-N(c)2-C(e)4 111(1)
C(i)2-N(c)3 128(2) N(i)1-C(i)2-N(c)4 122(1) C(i)2-N(c)3-C(e)4 121(1)
C(i)2-N(c)4 130(2) N(c)3-C(i)2-N(c)4 122(2) C(i)2-N(c)4-C(e)5 110(1)
C(i)3-N(i)1 132(2) N(i)1-C(i)3-N(c)5 127(2) C(i)3-N(c)5-C(e)5 107(1)
C(i)3-N(c)5 135(1) N(i)1-C(i)3-N(c)6 120(1) C(i)3-N(c)6-C(e)6 115(1)
C(i)3-N(c)6 137(2) N(c)5-C(i)3-N(c)6 113(1) C(i)1-N(c)7-C(e)6 119(1)
C(e)4-N(c)2 136(1) N(c)2-C(e)4-N(c)3 126(1)
C(e)4-N(c)3 140(1) N(c)2-C(e)4-N(t)8 117(2)
C(e)4-N(t)8 127(1) N(c)3-C(e)4-N(t)8 117(2)
C(e)5-N(c)4 135(2) N(c)4-C(e)5-N(c)5 135(1)
C(e)5-N(c)5 138(1) N(c)4-C(e)5-N(t)9 113(2)
C(e)5-N(t)9 136(1) N(c)5-C(e)5-N(t)9 111(2)
C(e)6-N(c)6 130(1) N(c)6-C(e)6-N(c)7 128(1)
C(e)6-N(c)7 131(2) N(c)6-C(e)6-N(t)10 116(2)
C(e)6-N(t)10 134(1) N(c)7-C(e)6-N(t)10 116(2)
distorted in a way similar to that in tri-s-triazine 4c20 and
trichloro-tri-s-triazine 4d.17 The average distance between N(i)1
and the neighboring C(i) atoms (139 pm) is slightly longer than
the average distances of C(i)-N(c) (133 pm) and C(e)-N(c)
(135 pm) within the cyameluric ring. As is also found in tri-
s-triazine and trichloro-tri-s-triazine, the average angle N(c)-
C(e)-N(c) of the ring system is larger (130°), and the average
C(i)-N(c)-C(e) angle is smaller than 120° (114°), while all
other average angles are 120°. The deviations of the individual
distances and angles (Table 3) from the average values can be
explained by the lower accuracy of the X-ray powder diffrac-
tometry as compared to single-crystal structure determinations.
Nevertheless, it is quite remarkable that, on the basis of
conventional powder diffraction data, a relatively complex
structure could be solved and refined unambiguously (large cell
volume of 844 × 106 pm3, 16 atoms in the asymmetric unit, 68
structural and 18 profile parameters) without the use of a high-
resolution diffraction experiment.
Figure 3. Hydrogen bonds between melem molecules. The molecules in
the middle and top right belong to the same layer.
ously. However, there are three intermolecular distances
N‚‚‚N which are in the range of the hydrogen bonds found in
melamine. The first two intermolecular distances N(c)5-
N(t)10 and N(c)6-N(t)9 amount to 285(2) and 317(1) pm,
respectively. Thus, they are predestined for hydrogen bonding.
The two hydrogen bonds mentioned above connect neighboring
molecules within one layer parallel (10-1). Furthermore, a third
hydrogen bond is formed from N(t)10 with a short intermo-
lecular distance of 300(2) pm to a N(c)7 atom of a melem
molecule within a different layer. For N(t)8, a hydrogen bond
to N(c)3 can be assumed because of the distance N(t)8-
H‚‚‚N(c)3 of 328 pm. Altogether, there are eight hydrogen bonds
per C6N7(NH2)3 molecule (Figure 3). Thus, there are as many
hydrogen bonds as were found in melamine. In the latter, four
pairs of parallel hydrogen bonds between the molecules occur.
According to temperature-dependent X-ray diffractometry
within the temperature range from -140 to 520 °C, the lattice
parameters steadily increase (a 5.7%, c 3.9%, â 2.4%). In
contrast, parameter b is nearly constant with only a minor
increase of 1.4%.
The average distance between the C(e) atoms and the N(t)
atoms of the NH2 groups is 132 pm, which is in the range of
the corresponding distances found in melamine.31,32 The crystal-
lographic point symmetry of the molecule is C1, although it
comes close to D3h.
As the position of the H atoms could not be obtained
experimentally, hydrogen bonds cannot be identified unambigu-
9
J. AM. CHEM. SOC. VOL. 125, NO. 34, 2003 10293

A R T I C L E S
Ju¨rgens et al.
Scheme 6. 13C and 15N Chemical Shifts Observed for Melamine
1a and Melem 4e
15N MAS NMR Spectroscopic Characterization. A 15N
MAS NMR spectrum of melamine in natural abundance was
performed by Damodaran et al. The authors observed resonances
at 83.9, 87.4, 90.9, 171.6, and 173.7 ppm (referenced to liquid
ammonia) according to values at -296.3, -292.8, -289.3,
-208.6, and -206.5 ppm (referenced to nitromethane).49
In this work, a 15N MAS NMR spectrum of 15N (>90%)-
enriched melamine was recorded (Figure 5, Scheme 6). In the
spectrum, resonances of the three amino groups are observed
at -298.7, -295.1, and -291.4 ppm. The signals at -211.0
and -208.8 ppm with a 2.0(2):1 intensity ratio are assigned to
the ring N atoms of the s-triazine ring. These results are in
agreement with those found by Damodaran et al. for melamine
as well as for those of substituted triamino-s-triazines.51
The signals of the ring N atoms of nonsubstituted s-triazine
C3N3H3 dissolved in DMSO are observed at -98.5 ppm.52 The
significant difference of about 110 ppm between the shifts found
for the N ring atoms of s-triazine and melamine is caused
presumably by the influence of the three amino groups in
melamine. The same influence of amino groups is observed for
tri-s-triazine and the respective triamino-substituted derivative
melem: In the 15N MAS NMR spectrum of tri-s-triazine
dissolved in DMSO, the ring N atoms are observed at 133.8
ppm (referenced to DMF) which corresponds to -141.4 ppm
(referenced to nitromethane).21
Figure 4. 13C MAS NMR spectra of melamine 1a (top) and melem 4e
(bottom). ×’s indicate spinning sidebands.
Solid-State MAS NMR Characterization. 13C MAS NMR
Spectroscopic Characterization. The 13C MAS NMR spectrum
of melamine (Figure 4) reveals two signals in a 1.9(1):1 intensity
ratio at 167.5 and 169.2 ppm. Normally, cross-polarization
experiments are not reliable for quantitative intensity analysis,
but they give a rough clue for the number of atoms which cause
a signal. The observed shifts are slightly different from those
reported by Damodaran et al. who found a similar intensity ratio
for signals at 166.2 and 167.8 ppm.49 In melamine dissolved in
DMSO, the carbon atoms become magnetically equivalent, and
only one shift is observed at 167.1 ppm.50 The observed values
are in good agreement with those found for sp2-hybridized
carbon atoms in the s-triazine ring systems of other carbon
nitride materials.7,10 An overview of the observed chemical shifts
for melamine and melem is given in Scheme 6.
The 13C MAS NMR spectrum of melem (Figure 4) exhibits
two signal groups. The resonances for the C(e)N2(NH2) groups
observed at 164.3 and 166.4 ppm with a 1.9(1):1 intensity ratio
are comparable to those found for melamine. In contrast, the
carbon atoms CN2X (X ) H, Cl) in tri-s-triazine and 2,5,8-
trichloro-tri-s-triazine are shifted to higher values (171.6 ppm
The amino groups of melem provide a significant shift to
lower values: In the 15N MAS NMR spectrum of melem,
resonances of the N(c) atoms within the cyameluric ring are
observed at -197.1, -199.6, -202.0, -203.4, and -205.3 ppm
with intensity ratios of 1:1.0(1):1.1(1):1.2(1):2.3(1). The intensi-
ties indicate the presence of six N(c) atoms within the cyame-
luric nucleus. Even for the central atom N(i), the influence of
the amino groups is observed: In tri-s-triazine, the shift of the
central 15N atom is found at 81.8 ppm (referenced to DMF) or
-193.4 ppm (referenced to nitromethane). In the triamino
21
in C6N7H3 and 175.0 ppm in C6N7Cl317). The second signal
group in the spectrum of melem belongs to the C(i)N3 groups
of the cyameluric nucleus: These are observed at 155.1 and
156.0 ppm, which are comparable to the signals found for
C6N7H3 (159.7 ppm)21 and C6N7Cl3 (158.2 ppm).17
(49) Damodaran, K.; Sanjayan, G. J.; Rajamohanan, P. R.; Ganapathy, S.;
Ganesh, K. N. Org. Lett. 2001, 3, 1921.
(50) Pouchert, C. J. The Aldrich Library of NMR Spectra, 2nd ed.; Aldrich
Chemical Co.: Milwaukee, WI, 1983.
(51) Amm, M.; Platzer, N.; Guilhem, J.; Bouchet, J. P.; Volland, J. P. Magn.
Reson. Chem. 1998, 36, 587.
(52) Stefaniak, L.; Roberts, J. D.; Witanowski, M.; Webb, G. A. Org. Magn.
Reson. 1984, 22, 201.
9
10294 J. AM. CHEM. SOC. VOL. 125, NO. 34, 2003

Study of Melem (2,5,8-Triamino-tri-s-triazine)
A R T I C L E S
Scheme 7. Possible Tautomers of Melamine 1a and Melem 4e
derivative melem, the resonance is observed at -234.4 ppm.
The lowest shifts in the 15N MAS NMR spectrum of melem
are caused by the amino groups at -267.1, -281.0, and -281.6
ppm.
CPPI (Cross-Polarization Combined with Polarization
Inversion) Experiment. For melamine as well as for melem,
the occurrence of different tautomeric forms seems to be possible
(Scheme 7). In the case of melamine, the presence of
tautomer 1a(I) was confirmed by X-ray and neutron diffrac-
tion.31,32
For melem 4e, a localization of the H atoms by X-ray powder
diffractometry was not possible. To determine which tautomer
4e(I)-(IV) occurs in the solid, a CPPI experiment was
recorded.53,54 This modified CP sequence allows one to dif-
ferentiate various N sites due to the existence of NH2 and NH
groups as well as of tertiary N atoms carrying no proton. During
the sequence, the magnetization starts from an optimum value.
It decreases and becomes negative with increasing inversion
time.53,54 The descent of the normalized intensities by increasing
inversion times can be described by eq 1.
ti
3ti
2
2n
n + 1
Ms(ti) ) Mo(tC)
exp -
+
exp -
+
(
)
(
)
[
n + 1
TD
2TD
ti2
-
- 1 (1)
2
(
)
]
TC
The parameter n characterizes the number of protons which are
directly bound to the N atoms. TC is related to dipolar coupling
to nearby protons leading to a coherent transfer of polarization,
while TD describes the decay caused by isotropic spin diffusion
(TC , TD).
Figure 5. 15N MAS NMR spectra of melamine 1a (top) and melem 4e
(bottom). ×’s indicate spinning sidebands; an impurity is marked with O.
n (n ) 0, 1, 2) given in eq 1. In the first hundred micro-
seconds, the intensity decreases rapidly because of the transfer
of magnetization to nearby protons. In a second time-regime,
the curve decreases with a much slower rate due to spin
diffusion. The crossover of the normalized intensity is observed
The experimental polarization inversion curves for melamine
and melem are given in Figure 6. The intensity of all resonances
is normalized with respect to their intensities for ti ) 0. Thus,
the starting point of the resulting hyperbola is 1 in all cases of
2
at [2/(n + 1)] - 1 corresponding to exp(-3ti2/TC ) (ti . TC)
and exp(-ti/TD) ≈ 1 (ti , TD). In the case of n ) 1, the
crossover region is expected at 0; for n ) 2, it should occur
at -1/3.
(53) Gervais, C.; Maquet, J.; Babonneau, F.; Duriez, C.; Framery, E.; Vaultier,
M.; Florian, P.; Massiot, D. Chem. Mater. 2001, 13, 1700.
(54) Gervais, C.; Babonneau, F.; Maquet, J.; Bonhomme, C.; Massiot, D.;
Framery, E.; Vaultier, M. Magn. Reson. Chem. 1998, 36, 407.
9
J. AM. CHEM. SOC. VOL. 125, NO. 34, 2003 10295

A R T I C L E S
Ju¨rgens et al.
to those of the NH2 groups of melamine, as they have a
crossover nearby -0.3. No curve is found with a crossover
region at about 0, thus indicating that no imido N atoms occur
in the molecule. Therefore, the tautomer 4e(I) describes the
correct bonding situation as well as the existence of three amino
groups in melem.
For the resonances which represent the ring N(c) atoms,
different parameters TD are observed which can be divided into
three groups. For the resonances at -197.1 and -202.0 ppm,
the longest TD values (∼6.0 ms) are observed. For the signals
at -199.6 and -203.4 ppm, TD values were determined to be
4.4 and 4.9 ms. The shortest time constants (TD ≈ 3.1 ms) are
found for the two N(c) atoms represented by the resonance at
-205.3 ppm. According to the theory outlined above, the
magnitude of TD is influenced by the strength of proton nitrogen
1
contacts. The shorter the closest H15N contact is, the smaller
the time constant TD becomes. Thus, the variations of TD for
the resonances mentioned above may now permit an assignment.
According to the crystal structure determination, the atoms N(c)5
and N(c)7 are involved in the strongest (shortest) hydrogen
bonds. Therefore, the resonance at -205.3 ppm can be assigned
to N(c)5 and N(c)7. The observed TD values are in a range
comparable to that of those found in polyborazilene and borane-
ammonia complexes by the same CPPI experiment.53,54
The resonance at -234.2 ppm has a TD value of 15 ms; this
slow decay indicates the extremely long distance between the
N atom and the nearest protons. Therefore, this signal is
caused by the N(i)1 atom.
Apart from the results concerning tautomers of melem, the
CPPI experiment unequivocally confirms the above-mentioned
assignment of the different resonances to the different kinds of
N atoms, N(c), N(i), and N(t).
Figure 6. Normalized signal intensities versus inversion time ti of the 15
CPPI MAS NMR experiments for melamine 1a (upper graph) and melem
4e (lower graph).
N
Vibrational Spectroscopic Characterization. According to
the general formula 3N - 6 (N: number of atoms), melem C6N7-
(NH2)3 has 60 fundamental modes of vibration. These can be
divided into two parts: 21 vibrations are associated with the
three NH2 groups of melem, which is comparable to the situation
found for melamine with its three amino groups.55,56 Each NH2
group has six vibrational modes: the stretching vibrations νas-
NH2 and νsNH2, the bending and rocking vibrations δNH2 and
FNH2 which are in-plane, as well as the out-of-plane twisting
and wagging vibrations τNH2 and ωNH2. The remaining 39
modes are skeletal vibrations belonging to the C6N7 nucleus
which are found in trichloro-tri-s-triazine, too.17 Apart from the
vibrations of the C6N7 nucleus and the amino groups in the
spectra of melem, vibrations belonging to the three C(e)-N(t)-
H2 groups should appear. In Figure 7, the FTIR and FT-Raman
spectra of melem are compared to those of melamine; the
observed frequencies and an assignment are given Table 6. In
the region above 2800 cm-1, the spectra of both compounds
are very similar due to the three amino groups and their
respective νN-H stretching vibrations. For the cyameluric
nucleus, similar vibrations are expected as found in the IR
spectrum of the chlorinated derivative 4d; for example, the
vibrations at 1610 (vs), 1505 (vs), 1310 (vs), and 825 (m) cm-1
are observed for 4d, and signals at 1606 (vs), 1469 (vs), 1304
(m), and 802 (s) are found in the FTIR spectrum of melem 4e.
To test the significance of a CPPI experiment for the structure
determination of melem, this experiment was first applied to
melamine which unequivocally crystallizes in the amino form31,32
(Scheme 7). Thus, in the case of melamine, two different cases
of polarization decays are to be expected. Because no hydrogen
atom is covalently bound to the ring atoms, a slow polarization
decay should occur for these resonances at both short and long
ti. For the nitrogen atoms with two hydrogen atoms covalently
bound (n ) 2), the polarization is rapidly transferred back to
the protons for small ti. For larger ti, the polarization transfer
mechanism changes to spin diffusion, and, therefore, a crossover
to a slow decay will be observed at a polarization of -1/3. The
experimental CPPI curves for melamine are in excellent
agreement with these conclusions. The polarization of the nuclei
with σiso ) -208.8 and -211.0 ppm decreases slowly without
a crossover section, whereas for the polarization of the nuclei
with σiso ) -291.4, -295.1, -298.7 ppm, a crossover is
observed at -0.3. Consequently, the former and the latter
resonances unambiguously can be assigned to the nitrogen atoms
in the ring and the amino groups, respectively.
The curves for melem are comparable: The respective curves
of the six resonances at -234.2, -205.3, -203.4, -202.0,
-199.6, and -197.1 ppm decrease quite slowly. These cor-
respond to the seven nonprotonated N(c) atoms within the
cyameluric nucleus. Furthermore, the curves of the resonances
at -267.1, -281.0, and -281.6 ppm exhibit behavior similar
(55) Jeremy Jones, W.; Orville-Thomas, W. J. Trans. Faraday Soc. 1959, 55,
203.
(56) Wang, Y.-L.; Mebel, A. M.; Wu, C.-J.; Chen, Y.-T.; Lin, C.-E.; Jiang,
J.-C. J. Chem. Soc., Faraday Trans. 1997, 93, 3445.
9
10296 J. AM. CHEM. SOC. VOL. 125, NO. 34, 2003

Study of Melem (2,5,8-Triamino-tri-s-triazine)
A R T I C L E S
Table 5. Bond Lengths (in pm) and Angles (in deg) for the
a
Calculated Crystal Structure of C6N7(NH2)3
bond
GGA
LDA
C(i)-N(i)
C(i)-N(c)
C(e)-N(c)
C(e)-N(t)
N-H
140.6(1)
132.7(3)
135.6(4)
133.3(8)
102.7(10)
139.6(1)
131.8(3)
134.7(5)
132.2(9)
103.8(15)
N(c)-C(i)-N(i)
N(t)-C(e)-N(c)
C(e)-N(c)-C(i)
C(i)-N(i)-C(i)
N(c)-C(i)-N(c)
N(c)-C(e)-N(c)
119.5(6)
117.1(10)
117.6(7)
120.0 (1)
121.0 (2)
125.7 (6)
119.3(4)
117.6(11)
118.2(5)
120.0 (3)
121.3 (3)
124.8 (8)
a The bond lengths and angles are averaged over all chemically identical
bonds, with the standared deviation given in parantheses.
Table 6. Calculated Wavenumbers (in cm-1, at the B3LYP/
cc-pVDZ Level of Theory) of the C6N7(NH2)3 Molecule in the Gas
Phase (Calculated Relative Intensities Truncated to Full Integer
Values) in Comparison to Those Found in the Vibrational Spectra
of Melem 4ea
calc. rel intensity
ν˜ 4e (observed,
ν˜ 4e (calculated,
this work)
this work)
IR
Ra
assignment
3488 s (IR), 3498 w (Ra)
3746
3746
3596
3595
99
0
0
131
0
591
68
E′
A2′
A1′
E′
3428 s (IR), 3430 w (Ra)
3330-3090 vbr (IR, Ra)
3070-3020 br (IR, Ra)
1677 m (IR), 1677 w (Ra)
230
1700
1671
1624
1578
1547
1531
1483
1396
1319
1289
1179
1173
1068
1058
1003
826
862
0
1529
676
0
0
366
0
7
0
1
0
21
0
0
58
0
4
10
4
16
3
0
0
8
0
0
1
28
0
0
2
0
0
E′
A1′
E′
1606 vs, br (IR)
1581 m (Ra)
E′
A1′
A2′
E′
A1′
E′
A2′
E′
A1′
E′
A2′
E′
A2′′
E′
Figure 7. FTIR (upper graph) and FT-Raman (lower graph) spectra of
melamine 1a and melem 4e.
1469 s (IR)
1304 m (IR)
1259 w (IR)
1196 w (IR)
1154 w (IR), 1155 s (Ra)
938 w, br (IR)
Table 4. Bond Lengths (in pm) and Angles (in deg) for the Free
C6N7(NH2)3 Molecule (Values Are Calculated Using Both DFT
(B3LYP Hybrid Functional) and Post-Hartree-Fock Methods
(MP2))
bond
B3LYP
MP2
C(i)-N(i)
C(i)-N(c)
C(e)-N(c)
C(e)-N(t)
N-H
141.2
133.0
134.8
135.0
101.0
141.1
133.5
135.0
135.0
101.1
802 s (IR)
786
776
749
742
728
654
601
600
582
0
2
0
0
0
0
0
0
0
0
9
0
0
4
0
0
E′′
A2′′
A1′
E′′
A2′
E′′
A1′′
E′
N(c)-C(i)-N(i)
N(t)-C(e)-N(c)
C(e)-N(c)-C(i)
C(i)-N(i)-C(i)
N(c)-C(i)-N(c)
N(c)-C(e)-N(c)
119.6
115.8
116.3
120
120.9
128.3
119.9
115.5
115.6
120
120.2
129.0
545 s (Ra)
531
449
0
0
49
7
A1′
E′
391
289
255
249
223
128
121
111
0
3
80
0
0
491
0
7
0
3
0
0
0
0
1
0
A2′
E′
Photoluminescence Spectroscopy. In the excitation spectrum
of melem, a maximum at λmax, exc ) 288 nm (T ) 20 °C) occurs.
A2′′
E′′
A1′′
A2′′
E′′
A2′′
E′′
The emission spectrum exhibits a maximum at λmax, emission
)
366 nm with a relatively high quantum efficiency of 40%
(Figure 8). A significant shift of the emission maximum is
observed as compared to the chlorinated derivative 4d (λmax, abs
) 310 nm, λmax, emission ) 466 nm).17
93
0
0
Mass Spectrometric Characterization. The formula C6N10H6
of melem was confirmed by mass spectrometry. Apart from the
basis peak (m/z 218) which represents the melem molecule,
further fragments are observed. MS (DEI+, 70 eV) m/z: 219
(MH+, 10%), 218 (M+, 100%), 178 (M+ - NCN, 3%), 177
(M+ - NCNH, 33%), 172 (M+ - CN2H6, 8%), 135 ([C3N7H]+,
a vbr, very broad; br, broad; s, strong; m, middle; w, weak.
4%), 127 ([C3N6H7]+, 5%), 109 ([C3N5H3]+, 5%), 108
([C3N5H2]+, 7%), 68 ([C2N3H2]+, 21%), 43 ([CN2H3]+, 5%),
42 ([CN2H2]+, 2%). MS, HR: C6N10H6: m/z (calc.) ) 218.0777,
m/z (obs.) ) 218.0756.
9
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A R T I C L E S
Ju¨rgens et al.
lengths for C(i)-N(c) of 129.3 and 135.4 pm and for C(e)-
N(c) of 139.0 and 141.6 pm, the longer distances always to the
protonated N(c) atoms. The terminal imido groups become much
closer bonded; C(e)-N(t) is 127.5 pm. The data clearly indicate
a strong localization of the previously delocalized bonding
within the molecule. The total energy of this tautomeric form
comes out 133 kJ mol-1 less favorable than that for D3h
symmetry 4e(I). Further on, for tautomer 4e(II) which exhibits
Cs symmetry, a total energy of about 54 kJ mol-1 less favorable
than that for D3h was calculated. Hence, the calculations support
the amino tautomer 4e(I), as was independently proven by the
CPPI NMR experiment discussed above.
Within the crystal structure of melem C6N7(NH2)3, the
optimized molecular geometry came out quite similar in
comparison with the free molecule in the gas phase, its local
symmetry being close to D3h with only minor distortions. Bond
lengths and angles are given in Table 5. There is a remarkable
agreement between the results for the crystal structure and those
obtained for the free molecule. Noteworthy, the terminal amino
groups all became almost coplanar with the C-N cyameluric
nucleus of the molecule (Figure 2). Some particular effects are
present, most of which are attributed to the hydrogen bonding
between different molecules. The N-H bond distances are
somewhat elongated within one particular amino group. The
atom N(t)10 of the amino group also participates to the largest
extent in the intermolecular hydrogen bonding. Moreover, while
dihedral angles calculated within the C6N7 cyameluric nucleus
do not deviate more than 2° from an ideal planar conformation,
dihedrals involving hydrogen atoms indicate rotation of the
almost planar terminal amino groups of up to 13°. However,
the hydrogen-bond acceptors of the molecule, the N(c) atoms,
appear unaffected by this kind of interaction.
Figure 8. Photoluminescence (emission) spectrum of melem 4e.
Thermal Behavior of Melamine. Different thermoanalytical
investigations were performed to study the formation of melem
during heating melamine but also to investigate the temperature
stability of melem at higher temperatures.
The DSC curve of melamine shows a sharp endothermic
signal at 363 °C typical for a melting process. During the cooling
of the sample from 500 °C to room temperature, a sharp
exothermic signal is observed at 317 °C. Therefore, the signal
detected during heating is assigned to a reversible melting
process which exhibits a significant hysteresis.
Temperature-dependent X-ray powder diffraction of melamine
indicates several phase transitions. Between 320 and 340 °C,
melamine transforms into an unknown phase. Between 360 and
380 °C, a further transition occurs which is accompanied by
the formation of melem. By further heating above 520 °C, the
reflections of melem disappear, and only a broad reflection at
d ) 337 pm is observed.
The vibrational frequencies of the free C6N7(NH2)3 molecule
were calculated using the B3LYP hybrid functional. Out of the
60 eigenmodes of the D3h molecule, 7 transform as A1′, 2 as
A1′′, 5 as A2′′, 6 as A2′, 14 as E′, and 6 as E′′. A2′′ and E′
modes are IR-active, A1′, E′, and E′′ are Raman-active, and A1′′
and A2′ are silent (inactive) modes. The eigenvalues are given
in Table 6 together with the calculated relative intensities. The
primed modes retain the planarity of the molecule, while double-
primed modes are out-of-plane vibrations and predominantly
located at lower wavenumbers. Basically, the calculated results
differ by not more than 2-4% from the experimental values
given in Figure 7, an agreement which is rather good and quite
typical for calculations of such systems. Apparently, the six
modes with the highest wavenumbers, N-H stretching vibra-
tions above 3595 cm-1, come out too high from the calculation
and do not match their experimental counterparts, which are
between 3000 and 3500 cm-1. The reason is found in the
secondary interaction of the extended hydrogen-bonding net-
work, which causes both an elongation of N-H bonds and a
slight “out-of-plane” rotation of the amino groups, leading to a
decrease of the N-H stretching force constant and a significant
broadening of the signals.
In further experiments, melamine was heated in sealed glass
ampules to different temperatures between 380 and 500 °C.
After being cooled, the reaction products remaining at the
bottom of the ampules were analyzed by X-ray powder
diffractometry. Apart from the reflections of the starting material
melamine and the herein described melem, the reflections of at
least two further intermediates were observed in the powder
patterns which could not be indexed so far.
Thermal Behavior of Melem. According to temperature-
dependent X-ray powder diffraction experiments between 560
and 580 °C, the reflections of melem become blurred, and above
ca. 580 °C only a broad reflection at about d ) 340 pm occurs,
a value that is typical for graphite-like C-N materials.6
Accordingly, melem is stable up to 560 °C. Above that
temperature, it transforms into a C-N-H polymer without
melting.
Results of the Calculations. The single free C6N7(NH2)3
molecule adopts D3h symmetry. Optimizations with distorted
initial geometry such as pyramidalized terminal amino groups
or localized double bonds within the ring structure readily
converged toward the high symmetrical conformation. The
calculated bonds lengths and angles are given in Table 4.
Apparently, the applied DFT and post-Hartree-Fock methods
yield almost similar results.
Summary
To account for the possibility of different tautomeric forms
of melem (as discussed in more detail within the NMR section),
one proton from each amino group was shifted to a N atom of
the C6N7 nucleus. Assuming C3h symmetry, we found bond
The occurrence of different intermediates during the con-
densation process of melamine according to Scheme 4 is
conceivable: A condensation of two molecules of melamine
under formation of the C6N7 ring system might be necessary.
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10298 J. AM. CHEM. SOC. VOL. 125, NO. 34, 2003

Study of Melem (2,5,8-Triamino-tri-s-triazine)
A R T I C L E S
Scheme 8. Possible Formation of Melem As Suggested by May26
Scheme 9. Possible Formation of Melem by Reaction of Melamine 1a with Dicyandiamide 7
Another possible formation of melem was proposed by May:
During the sublimation of melamine, which starts at about
350 °C, decomposition of some melamine molecules into the
monomer cyanamide H2NCN 5 could take place. These
molecules may react with nonsublimated melamine, leading to
melem (Scheme 8).
Furthermore, another reaction mechanism comparable to the
formation of tri-s-triazine described by Hosmane et al. may be
possible.20 Accordingly, the formation of melem starts from
dicyandiamide: Some dicyandiamide molecules may transform
into melamine which could react with nonreacted dicyandiamide
molecules (Scheme 9).
Additionally, the existence of further intermediates during
the condensation process is probable. For control of the
condensation reaction, the specific temperature and pressure
conditions during the reaction have to be adjusted carefully.
A possible way to clarify the reaction mechanism is the partial
doping of the 15N positions within the starting material
melamine. By 15N MAS NMR investigations, it might be
possible to determine which types of N atoms form the ring
atoms N(c) and N(i) or the amino N(t) atoms, respectively.
Additionally, 15N doping of the different N positions within
cyanamide 5, ammonium dicyanamide 6, and dicyandiamide 7
should be helpful to understand the formation of melem 4e.
In this work, we could prove the existence of melem as an
important intermediate during the thermal condensation of
melamine as well as of other simple C-N-H precursor
compounds. The fact that melem (triamino-tri-s-triazine) readily
forms during heating of melamine (triamino-s-triazine) might
be indicative of a generally higher thermodynamic stability of
tri-s-triazine derivatives and maybe even their oligomers and
polymers as compared to those deriving from s-triazine.
Therefore, it seems to be rather unlikely that within the further
condensation of melem on the way to g-C3N4, sheets made up
of s-triazine rings (connected by N atoms, structure 2, Scheme
1) are formed. Therefore, we believe that graphite-like C-N
materials as well as g-C3N4 itself formed by condensation of
melamine presumably contain tri-s-triazine (connected by N
atoms, structure 3, Scheme 2) instead of s-triazine rings
(structure 2, Scheme 1). This assumption is also supported by
26
9
J. AM. CHEM. SOC. VOL. 125, NO. 34, 2003 10299

A R T I C L E S
Ju¨rgens et al.
DFT calculations, which indicate a significantly higher stability
LMU Mu¨nchen), and especially with Prof. Edwin Kroke
(Universita¨t Konstanz), are gratefully acknowledged. Financial
support by the Fonds der Chemischen Industrie, the Bundesmi-
nisterium fu¨r Bildung und Forschung, and the Deutsche For-
schungsgemeinschaft is also gratefully acknowledged.
of g-C3N4 based on tri-s-triazine building units.17
Acknowledgment. The authors are indebted to the following
people for conducting the physical measurements: Dagmar
Ewald and Dr. Gerd Fischer (mass spectrometry), Sascha Correll
(temperature-dependent X-ray powder diffractometry), and
Stefan Rannabauer (differential scanning calorimetry) (all
Department Chemie, LMU Mu¨nchen), as well as Dr. Thomas
Ju¨stel and Dieter Wa¨dow (Philips Research Laboratories
Aachen) for the photoluminescence spectroscopy. Fruitful
discussions with Prof. Hendrik Zipse (Department Chemie,
Supporting Information Available: Crystallographic infor-
mation (CIF). This material is available free of charge via the
Internet at http://pubs.acs.org.
JA0357689
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10300 J. AM. CHEM. SOC. VOL. 125, NO. 34, 2003