Trimerization of NaC2N3 to Na3C6N9 in the Solid

Sodium dicyanamide NaC2N3 was found to undergo two phase transitions. According to thermal analysis and temperature-dependent X-ray powder diffractometry, the transition of α-NaC2N3 (1a) to β-NaC2N3 (1b) occurs at 33°C and is displacive. 1a crystallizes in the monoclinic system, space group P21/n (no. 14), with a = 647.7(1), b = 1494.8(3), c = 357.25(7) pm, β = 93.496(1)°, and Z = 4. The structure was solved from powder diffraction data (Cu Kα1, T = 22°C) using direct methods and it was refined by the Rietveld method. The final agreement factors were wRp = 0.072, Rp = 0.053, and RF = 0.074. 1b crystallizes in the orthorhombic system, space group Pbnm (no. 62), with a = 650.15(5), b = 1495.1(2), c = 360.50(3) pm, and Z = 4. The structure was refined by the Rietveld method using the atomic coordinates of 1a as starting values (Mo Kα1, T = 150°C). The final agreement factors were wRp = 0.044, Rp = 0.034, RF = 0.140. The crystal structures of both polymorphs contain sheets of Na+ and N(CN)2- ions which are in 1a nearly and in 1b exactly coplanar. Above 340°C, 1b trimerizes in the solid to Na3C6N9 (2). 2 crystallizes in the monoclinic system, space group P21/n (no. 14), with a = 1104.82(1), b = 2338.06(3), c = 351.616(3) pm, β = 97.9132(9)°, and Z = 4. The structure was solved from synchrotron powder diffraction data (λ = 59.733 pm) using direct methods and it was refined by the Rietveld method. The final agreement factors were wRp = 0.080, Rp = 0.059, and RF = 0.080. The compound contains Na+ and the planar tricyanomelaminate C6N93-. The phase transition from 1b to 2 is reconstructive. It occurs in the solid-state without involvement of other phases or intermediates. The crystal structures of 1b and 2 indicate that there is no preorientation of the N(CN)2- in the solid before their trimerization to C6N93-

Transformation of Ammonium Dicyanamide into Dicyandiamide in the Solid

Ammonium dicyanamide NH4[N(CN)2] was synthesized through aqueous ion exchange. The crystal structure was investigated by single-crystal X-ray diffraction (P21/c, a = 378.67(6) pm, b = 1240.9(3) pm, c = 911.84(14) pm, β = 91.488(18)°, Z = 4). It derives from the CsCl structure type. Medium strong hydrogen bonds between NH4+ and [N(CN)2]- ions are indicative of the observed formation of dicyandiamide H4C2N4 during heating. According to DSC and temperature-dependent X-ray powder diffractometry, this isomerization is exothermic and occurs between 102 and 106°C in the solid. The reaction represents the isolobal analogue to the classical synthesis of urea by heating NH4OCN. While other alkali and alkaline earth dicyanamides undergo trimerization or polymerization of their anions during heating, ammonium dicyanamide thus shows a different reactivity

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

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 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

Ergebnisse der wissenschaftlichen Begleitung des Projekts "Klassenassistenz" an der Grundschule Wesendorf (2019-2023)

Die Grundschule am Lerchenberg, Wesendorf, rief in Zusammenarbeit mit den zuständigen sozialen und politischen Institutionen das Modellprojekt Klassenassistenz ins Leben. Unterstützungspersonen sollten nicht mehr in Einzelfallhilfe einzelnen förderbedürftigen Schüler*innen zur Verfügung stehen, sondern der ganzen Klasse. Als „Klassenassistenz“ sollten sie da Unterstützung geben, wo sie benötigt wurde, unabhängig vom Förderstatus des/der betreffenden Schüler*in. Die wissenschaftliche Begleitung des Projekts Wesendorf sollte mit einer Befragung der am Projekt Beteiligten überprüfen, ob sich durch die Einführung der Klassenassistenzen positive Auswirkungen auf die Schule ergeben. (DIPF/Orig.

Von der Schulbegleitung zur Klassenassistenz. Forschungsaspekte

Das Modellprojekt Klassenassistenz wurde ins Leben gerufen, weil die Erfahrungen der Grundschule am Lerchenberg mit dem „Regelmodell“ der Unterstützung einzelner Schüler*innen mit diagnostiziertem Förderbedarf durch Schulbegleiter*innen wenig zufriedenstellend waren (s. Bratz auf S. 29 in diesem Band). Die Forschungsergebnisse zu dieser Form der Unterstützung sind widersprüchlich, geben aber wertvolle Hinweise, wie Förderung im Kontext inklusiver Regelschulen sinnvoll weiterentwickelt werden könnte. Wir geben einen kurzen Überblick über Forschung zur Schulbegleitung, erläutern welche Folgerungen sich daraus für die Konzeption und die Evaluation des Wesendorfer Modells der Klassenassistenz ergaben und berichten über die Ergebnisse. (DIPF/Orig.

The Code of Protest. Images of Peace in the West German Peace Movements, 1945-1990

The article examines posters produced by the peace movements in the Federal Republic of Germany during the ColdWar, with an analytical focus on the transformation of the iconography of peace in modernity. Was it possible to develop an independent, positive depiction of peace in the context of protests for peace and disarmament? Despite its name, the pictorial selfrepresentation of the campaign ‘Fight against Nuclear Death’ in the late 1950s did not draw on the theme of pending nuclear mass death. The large-scale protest movement in the 1980s against NATO’s 1979 ‘double-track’ decision contrasted female peacefulness with masculine aggression in an emotionally charged pictorial symbolism. At the same time this symbolism marked a break with the pacifist iconographic tradition that had focused on the victims of war. Instead, the movement presented itself with images of demonstrating crowds, as an anticipation of its peaceful ends. Drawing on the concept of asymmetrical communicative ‘codes’ that has been developed in sociological systems theory, the article argues that the iconography of peace in peace movement posters could not develop a genuinely positive vision of peace, since the code of protest can articulate the designation value ‘peace’ only in conjunction with the rejection value ‘war’

Streitkultur und Öffentlichkeit im konfessionellen Zeitalter (Volume 95, Edition 1)

Öffentlich ausgetragene Debatten gehören zu den Grundmerkmalen moderner Gesellschaften. Eine entwickelte »demokratische Streitkultur« gilt mithin geradezu als Voraussetzung für das Funktionieren einer Demokratie. Die dieser Wahrnehmung oft implizit zu Grunde liegende These vom »Strukturwandel der Öffentlichkeit« ist in den letzten Jahrzehnten von Seiten der Geschichtswissenschaft verschiedentlich relativiert worden.Was hatten diese unterschiedlichen Formen des öffentlich ausgetragenen Streits gemeinsam? Inwieweit prägten und strukturierten sie die jeweilige(n) historische(n) Öffentlichkeit(en)? Wie wurde das Phänomen des Streits von den Zeitgenossen jeweils wahrgenommen und bewertet? Lassen sich diesbezüglich signifikante Unterschiede zur Zeit der frühen Reformation oder der Frühaufklärung konstatieren? Gab es so etwas wie eine spezifische Streitkultur des konfessionellen Zeitalters