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CONTRIBUTED ARTICLE 11.
The Raman Spectrum of Anton Hammerl,a
Gerhard Holl,b a Institute of
Inorganic Chemistry, b
Wehrwissenschaftliches Institut für Werk-,
Introduction During the last 8 years, significant advances have been made in the area of inorganic azides, as indicated by the number of recent reviews covering various aspects of the subject.[1-3] In contrast to the chemistry of many hydrazinium salts, which has been extensively explored in the last 30 years,[4,5] detailed studies into the chemistry of ionic hydrazinium azides have only recently come to fruition.[6,7] We have recently become interested in the preparation of hydrazinium azides and their possible application as high-energy-density materials (HEDM) for military use.[8,9] Here we report on the variable temperature Raman spectrum of hydrazinium azide, [N2H5]+[N3]-, which was prepared in high yield from anhydrous hydrazine and HN3 in diethylether solution at 20°C. Experimental Anhydrous hydrazine was prepared from hydrazinium hydrate (Merck) by distillation over NaOH and BaO and dried over Na.[6,10] Hydrazoic acid, HN3, was prepared in a neat reaction from stearic acid (Aldrich) and sodium azide (Aldrich).[11] A solution (25 mL, c = 2 mol L-1) of HN3 in ether was dropped into the hydrazine solution (1.02 g, 31.9 mmol) within 30 min at 0°C. The precipitated hydrazinium azide, [N2H5]+[N3], was filtered off and sublimed in vacuum. Yield: 1.91 g (80%).[6,10,11] mp: 75 °C Raman (200 mW, RT): microanalysis: calcd.: 6.71% H, 93.29% N. The Raman spectra were recorded with a Perkin Elmer 2000 NIR FT Raman spectrometer fitted with a Nd-YAG laser (1064 nm) using the 90° geometry. The low temperature spectra were recorded using a Ventacon low temperature cell and the high temperature spectra were recorded using a Ventacon high temperature cell. All samples were flame-sealed under N2 in 5 mm Raman capillaries. Discussion of the Raman Spectra To get an overview over the temperature dependency of the Raman spectra of hydrazinium
azide, [N2H5]+[N3]-, we first
recorded Raman spectra in the temperature region between -100°C and +80°C in intervals
of 20°C at a resolution of 4 cm-1 (Figure 1). There is no significant change
in the spectra between -100°C and 0°C. Between 0°C and 20°C there is a slight shift of
the symmetric azide peak at about 1350 cm-1 (see Experimental). Above 50°C the
spectra get very noisy which may partly be attributed to the high volatility of the
compound at this temperature which caused some sublimation in the sealed sample tube.
To study this phenomenon in more detail we again recorded Raman spectra
in the temperature region between -30°C and +10°C with a resolution of 2 cm-1
(Figure 3). At -30°C the peak at 1346 cm-1 has only marginal intensity whereas
the peak at 1338 cm-1 predominates the region of the symmetric NNN-
stretch. Heating up the sample gradually to +5°C does not cause any significant change in
the spectra. However, at +10°C the higher-frequency peak at 1346 cm-1 begins
to increase and at +15°C the low-frequency peak at 1338 cm-1 has completely
disappeared. The 148 cm-1 shifts with increasing temperature to 134 cm-1
at +15°C. This is not surprising since one would expect the hydrogen bridges to get
longer with increasing temperature and therefore a low-frequency shift of this peak is in
agreement with this assumption. The change of the symmetric azide peak at about 1350 cm-1
indicates a change in the coordination of the azide anion. Presumably at higher
temperature there are more long hydrogen bridges whereas at low temperature there is only
one (or fewer) shorter and stronger hydrogen bridges.
The observed phase transition in the Raman spectra was also detected in
the DSC spectrum at +4.7°C. The DSC spectrum (Figure 4) shows that the peak is rather
broad which indicated a slow phase transition over a certain temperature range which again
is in good accord with the observed Raman spectra.
Acknowledgment We are indebted to and thank Professor Patrick Hendra for many valuable discussions and the Fonds der Chemischen Industrie for financial support of this work. References
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(½) = 183 Hz) ppm.
(cm–1)
= 3358 (s), 3284 (s, n(NH2)),
3153 (br, m) , 3054 (s, br, 
)), 2033 (s, n(
)), 1593 (m, br), 1504 (m, br, d(
)), 
(cm–1) = 3259 (vw, n(NH2)), 1347 (s, 
