Methylsilsesquioxane-Based Aerogel Systems-Insights into the Role of the Formation of Molecular Clusters.

Condensed clusters of hydrolyzed methyltrimethoxysilane (MTMS) were studied using two complementary approaches: (i) Fourier transform infrared (FTIR) spectroscopy was applied along with the hydrolysis and condensation stages of a sol-gel process from the condensation of colloidal suspension of nanoparticles to the solid phase of bulk material; (ii) density functional theory calculations of energies, structural and vibrational data of pertinent MTMS hydrolysis products, specifically, methylsilanetriol-based species with different number of silicon atoms (from two to eight atoms) and different structures/conformations (linear, cyclic, and cage, in a total of 13 structures), were performed at B3LYP/6-311+G(d,p) level of theory. The calculated infrared spectra show two distinct Si-O-Si stretch vibration bands for models of caged structures. The higher-frequency IR band at ca. 1120 cm(-1) is derived from the antisymmetric Si-O-Si stretch vibration mode, while the lower-frequency band at 1035 cm(-1) is due to the symmetric Si-O-Si stretch and is characteristic of the cyclic clusters, being absent in highly symmetric cage structures. The calculated versus the experimental FTIR spectra of poly(methylsilsesquioxane) (PMSQ) dried aerogel in KBr pellet show that cage/cyclic-like structures prevail over ladder structures (linear) in actual PMSQ.

Low-temperature FTIR spectroscopic and theoretical study on an energetic nitroimine: dinitroammeline (DNAM).

This paper presents an overview of recent progress in spectroscopic studies of the energetic nitroimine 4,6-bis(nitroimino)-1,3,5-triazinan-2-one (DNAM), based on experimental and theoretical data. The following topics are considered: variable temperature FTIR spectroscopy (4000-400 cm(-1)) applied to the study of natural and isotopically substituted (deuterated) samples aiming to obtain a successful vibrational assignment of the spectra and to investigate H-bonding interactions; extensive theoretical work based on accurate quantum chemical calculations (ab initio MP2 and DFT/B3LYP; harmonic and anharmonic vibrational calculations) to model and help interpreting the experimental findings, as well as to provide fundamental data on this simple prototype nitroimine that can be used as a starting point to the study of more complex related compounds. This work allowed us to reveal detailed features of the IR spectrum of the title compound, presenting, for the first time, plausible assignments.

Crystal and molecular structure of 4,6-bis(nitroimino)-1,3,5-triazinan-2-one: theoretical and X-ray studies.

This paper provides an overview of recent progress on structural data on the title compound. Theoretical work based on quantum mechanical calculations was performed to gain some understanding on the heterocyclic tautomerism potentially exhibited by the compound. The computational studies encompassed a wide range of tautomers/conformers, allowing the determination of the most probable molecular structure. In the gas phase, the nitroimine tautomers are computed to be substantially more stable than the nitramine tautomers. Among three plausible nitroimine forms, special attention was given to 4,6-bis(nitroimino)-1,3,5-triazinan-2-one, whose crystal structure was unequivocally solved by X-ray diffraction. The crystals are orthorhombic, space group Pnma with a = 6.187(2)A, b = 13.252(5)A, c = 8.802(4)A, and Z = 4. The structure was solved by direct methods and refined to a final R = 0.0326. The molecule has an approximate mirror plane relating the two symmetry related halves. The nitroimine groups are positioned in a syn-syn conformation. The least-squares (LS) plane of the heterocyclic ring and the nitroimine ([double bond]N-NO2) substituent LS plane make an angle of 10.05(11) degrees. The crystal structure is held together via hydrogen bonds that assemble the molecules in chains running along the b-axis. Every H-atom is involved in bifurcated hydrogen bonds.