Vibrational spectra and structure of RDX and its 13C- and 15N-labeled derivatives: A theoretical and experimental study (original) (raw)
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Theoretical and Experimental Vibrational and NMR Studies of Α and Β-RDX
2008
Cyclic nitramine hexahydro-1,3,5-trinitro-s-triazine (RDX), is an important energetic ingredient for propellants and explosives. To understand RDX explosive nature, the molecular structure needs to be examined in detail. Solid state RDX exist as three polymorphs: α, β and γ. The α-form is the stable polymorph at room temperature with Cs symmetry, β-polymorph has a molecular symmetry of C3v and γpolymorph may assume one of three orthorhombic structures: D2h, C2v or D2. Solid state Infrared and Raman spectra of α-RDX and C and N (ring) enriched isotopomers were recorded and fundamental frequencies were assigned using isotopic frequency shifts. Changes in vibrational signals associated with isotopic substitutions provide nearly unambiguous assignments of vibrational spectra of α-RDX, particularly where exact nature of vibrational modes had been either vague or contradictory. Assignments of the vibrational modes for α and β-RDX were made through comparisons between experimental and dens...
Journal of Molecular Structure: …, 2007
Optimized geometrical parameters are obtained for triamantane, iso-tetramantane and cyclohexamantane (X = C, Si, Ge, Sn) using B3LYP method with 6-311G** and/or LANL2DZ basis set. For carbon compounds Hartree-Fock (HF) method is also used for the study. The harmonic vibrational frequencies are obtained for all the molecules at their respective optimized geometries. The calculated adamantane frequencies are fitted to the experimental ones of adamantane-d 0 molecule to get the scale factors for C-H stretching frequencies. For all the other modes the scale factors obtained earlier [G. Ramachandran, S. Manogaran, J. Mol. Struct. THEOCHEM, 766 (2006) 125] by fitting all the isotopomers are used. The scale factors thus obtained are used to predict the fundamental frequencies of all carbon compounds and compared to the available experimental Raman frequencies. The assignments are proposed for all fundamentals of the title compounds using normal coordinate analysis.
Journal of Molecular Structure, 2010
The diperoxide 3,6-diphenyl-1,2,4,5-tetroxane was synthesized and its infrared and Raman spectra were measured. The structural stability and the vibrational spectra of 1,2,4,5-tetroxane and 3,6-diphenyl-1,2,4,5-tetroxane were investigated by the DFT-B3LYP and the ab initio MP2 calculations with the 6-311G ** basis set. From the calculations, the chair form of 1,2,4,5-tetroxane was predicted to be about 9, 20, and 50 kcal/mol lower in energy than the twist, boat, and planar structures, respectively. The ring inversion in 1,2,4,5-tetroxane is concluded to take place most probably through the twist structure of the ring. In the case of 3,6-diphenyl-1,2,4,5-tetroxane, the planar-equatorial (P eq) conformation was predicted by both levels of theory to be the lowest energy and predominant form of the molecule. The planar perpendicular and axial equatorial interconversion barriers were calculated to be about 2 and 12 kcal/mol, respectively. Reliable vibrational assignments were provided for the spectra of 1,2,4,5-tetroxane and 3,6-diphenyl-1,2,4,5-tetroxane by combining experimental and theoretical data of the two molecules.
Vibrational spectroscopic and quantum chemical studies of the trioxatriangulenium carbocation
Journal of Molecular Structure, 2000
Raman and infrared (IR) spectra of the stable carbocation trioxatriangulenium (TOTA) are reported. In a theoretical effort, ab initio and density function theory (DFT) methods are used to determine the vibrational spectra and the optimized geometry of TOTA. The theoretical results are compared with the experimentally obtained spectra and with the previously reported X-ray crystal structure. It is concluded that DFT calculations based on the non-local exchange-correlation functionals (UB-LYP) yields the most accurate description of the vibrational spectra and the geometry of TOTA. As a result of the calculations, it appears that the positive charge in the TOTA carbocation is substantially delocalized. ᭧
Journal of Molecular Structure, 2011
The molecular geometry, vibrational frequencies, gauge including atomic orbital (GIAO) 1H and 13C chemical shift values and several thermodynamic parameters of 6-(2-hydroxyethyl)-2,3,4-triphenyl-2,6-dihydro-7H-pyrazolo[3,4-d]pyridazin-7-one in the ground state have been calculated by using the Hartree–Fock (HF) and density functional methods (B3LYP) with 6–31G(d) basis set. The results of the optimised molecular structure are presented and compared with the experimental X-ray diffraction. The calculated results show that the optimised geometries can well reproduce the crystal structural parameters and the theoretical vibrational frequencies, and 1H and 13C NMR chemical shift values show good agreement with experimental data. The computed vibrational frequencies are used to determine the types of molecular motions associated with each of the experimental bands observed. To determine conformational flexibility, molecular energy profile of the title compound was obtained by semi-empirical (AM1) with respect to selected degree of torsional freedom, which were varied from −180° to +180° in steps of 10°. Besides, molecular electrostatic potential (MEP), frontier molecular orbitals (FMO), and thermodynamic properties were performed at HF and DFT levels of theory.
Triatom: programs for the calculation of ro-vibrational spectra of triatomic molecules
Computer Physics Communications, 1993
The TRIATOM program suite calculates energy levels, wavefunctions, and where appropriate, dipole transition moments and spectra, for rotating and vibrating triatomic molecules. Potential energy, and where necessary, dipole surfaces must be provided. The programs use an exact (within the Born-Oppenheimer approximation) Hamiltonian, offer a choice of several body-fixed, internal coordinate systems based on two distances and an included angle and employ basis function expansions of orthogonal polynomials. The calculations are variational, and rotational excitation is treated using an efficient two-step algorithm. Constituent programs are TRIATOM which solves the vibrational problem and also performs the first step for rotationally excited systems. SELECT which optionally preselects basis functions for TRIATOM. ROTLEVD which performs the second step for rotationally excited states. DIPOLE computes either line or band transition intensities. SPECTRA uses the data generated by the other programs to give simulated absorption or emission spectra.
Journal of Chemical Sciences, 2008
The FTIR and FT Raman spectra of dacarbazine were recorded in the regions 4000–400 and 3500–100 cm−1, respectively. The optimized geometry, wavenumber, polarizability and several thermodynamic properties of dacarbazine were studied using ab initio Hartree-Fock, MP2 and DFT methods. A complete vibrational assignment aided by the theoretical harmonic wavenumber analysis was proposed. The calculated harmonic vibrational frequencies were compared with experimental FTIR and FT Raman spectra. Based on the comparison between calculated and experimental results and the comparison with related molecules, assignments of fundamental vibrational modes were made. The X-ray geometry and experimental frequencies were compared with the results of theoretical calculations.