Dependence of Raman Spectral Intensity on Crystal Size in Organic Nano Energetics

Abstract

Raman spectra for various nitramine energetic compounds were investigated as a function of crystal size at the nanoscale regime. In the case of 2,4,6,8,10,12-hexanitro-2,4,6,8,10,12-hexaazaisowurtzitane (CL-20), there was a linear relationship between intensity of Raman spectra and crystal size. Notably, the Raman modes between 120 cm(-1) and 220 cm(-1) were especially affected, and at the smallest crystal size, were completely eliminated. The Raman spectral intensity of octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine (HMX), like that of CL-20's, depended linearly on crystal size. The Raman spectral intensity of 1,3,5-trinitroperhydro-1,3,5-triazine (RDX), however, was not observably changed by crystal size. A non-nitramine explosive compound, 2,4,6-triamino-1,3,5- trinitrobenzene (TATB), was also investigated. Its spectral intensity was also found to correlate linearly with crystal size, although substantially less so than that of HMX and CL-20. To explain the observed trends, it is hypothesized that disordered molecular arrangement, originating from the crystal surface, may be responsible. In particular, it appears that the thickness of the disordered surface layer is dependent on molecular characteristics, including size and conformational flexibility. Furthermore, as the mean crystal size decreases, the volume fraction of disordered molecules within a specimen increases, consequently, weakening the Raman intensity. These results could have practical benefit for allowing the facile monitoring of crystal size during manufacturing. Finally, these findings could lead to deep insights into the general structure of the surface of crystals.

Raman, Infrared, and Laser-Induced Breakdown Spectroscopy Identification of Particles in Raw Materials

“…The physical difference could account for the dark appearance as well as the weaker Raman spectra. 9 A possible source of the darkness might be rust or other elements, so LIBS analysis was done. Laser-induced breakdown spectroscopy analysis of a dark particle found sodium with carbon, as would be expected from sodium citrate, and no other elements, so there was no indication of contamination.…”

Insights into thiram fungicide: A comparative study of solution and solid phases through Raman, DCDR, and SERS measurements and DFT simulations

“…Previous studies have demonstrated that the intensity of low wavenumber vibrational modes in Raman measurements of crystalline powder samples can be affected by the size of crystal grains. 22,23,48,49 Considering that the observed differences between Thir A and Thir B spectra are mostly subtle and confined to the region below 500 cm À1 , we deem the latter explanation to be the most plausible.…”

“…There are significant differences between the solid‐phase Raman spectra and SERS spectra of thiram, which might be attributed to various individual aspects. For instance, the Raman signal from crystalline powder may be influenced by packing or conformational polymorphism or even by different crystal grain sizes 22,23 . In SERS measurements, the analyte is dissolved in a suitable solvent and allowed to adsorb onto the SERS‐active substrate.…”

“…For instance, the Raman signal from crystalline powder may be influenced by packing or conformational polymorphism or even by different crystal grain sizes. 22,23 In SERS measurements, the analyte is dissolved in a suitable solvent and allowed to adsorb onto the SERS-active substrate. Spectra are then measured from a colloidal solution or from dried deposits formed on the solid surface.…”