Degenerate thermal ignition of gaseous suspensions

Study has been made of the effect of a reduction of the heat of reaction per unit mass on the critical conditions for ignition of a gaseous suspension. Calculations are presented to show the relation between critical ignition temperature and the particle diameter and concentration.     

Molecular structures of acetylene derivatives of tin: Part III. Gas phase electron diffraction study of tetrakis(trifluoropropynyl)-tin,Sn(CC-CF3)4

A gas phase electron diffraction study of tetrakis(trifluoropropynyl)tin is reported. The model, based on T_d symmetry for the carbon—tin skeleton and C_3v symmetry for the CF₃ groups, refines to the following parameters (bond lengths, r_a, in nm; valence angles in degrees): Sn—C0.2070(7), CC 0.1215(6), C—C 0.1460(7), C—F 0.1343(2), CCF 111.3(0.2). The uncertainties (given in parentheses) represent three times the standard deviation values. The results obtained point to practically free rotation of the CF₃ groups. The presence of electronegative CF₃ causes shortening of the Sn-C bonds in Sn(CC—CF₃)₄ from Me₃SnCCH and Me₃ SnCCSnMe₃. The triple CC bond length is larger than in hexafluoro-2-butyne and nearly the same as in dimethylacetylene.     

Gas-phase electron diffraction study of tris(trimethylstannyl)amine,N(SnMe3)3

The geometrical parameters of tris(trimethylstannyl)amine have been determined by gas-phase electron diffraction. The _a structure has been refined using mean amplitude values calculated from the force fields of a number of tin derivatives.The experimental data are consistent with a planar bond configuration at the nitrogen in N(SnMe₃)₃. The final set of geometrical parameters is as follows (average bond distances, r_g , in Å, angles in degrees): SnC 2.166(5), SnN 2.038(3), CH 1.117(17). NSnC 108.5(1.5), SnCH 112.1(1.6). Mean amplitude values have been varied for those distances which give considerable contributions to scattering.The results obtained fill a gap in the knowledge of structures of Group IV element μ-nitrido derivatives. They confirm the conclusion that lowering of ligand MR_n electro-negativity weakens the tendency to deviation from planarity in the central fragment NM₃. This tendency may be considered as a manifestation of the second-order Jahn-Teller effect.     

The Mills—Nixon and anomeric effects in 2,3-dihydro-1H-1,3,2-benzodiazaphospholes: a gas-phase electron diffraction study of C6H4(NMe)2PCl using quantum-chemical data and “a priori” force field

Geometric parameters and the force fields of the molecule of 2-chloro-1,3-dimethyl-2,3-dihydro-1H-1,3,2-benzodiazaphosphole, C₆H₄(NMe)₂PCl (1), and the 1,3-dimethyl-2,3-dihydro-1H-1,3,2-benzodiazaphospholium cation, C₆H₄(NMe)₂P⁺ (2), containing a dicoordinated P atom were calculated by the restricted Hartree—Fock (RHF) method using the 6-31G* and 6-311G** basis sets. Vibrational characteristics of the internuclear distances in molecule 1 were calculated using the a priori scaled force field. Structural analysis of the gas-phase electron diffraction data for this molecule was performed with inclusion of nonlinear kinematic effects at the first-order level of perturbation theory. Substantial lengthening of the P—Cl bond and conformational peculiarities of the phosphole fragment of molecule 1 are consistent with the expected manifestations of the anomeric effect. The type of partial localization of the -bonds in the benzene fragment (relative weights of two resonance Kekule structures, or the Mills—Nixon effect) changes on going from covalent to ionized state of the P atom in the annelated five-membered ring.     

Ab initio calculations of the structure and harmonic force fields for the amine forms of dinitramine and methyldinitramine. Vibrational spectra and their interpretation using a scaling procedure

Ab initio geometries and vibrational spectra have been calculated for the amine structures of dinitramine and methyldinitramine, HN(NO₂)₂ and CH₃N(NO₂)₂. It is shown at the RHF and MP2 levels with the use of the 6-31G^* and 6-31G^** basis sets that these molecules have different symmetries in their equilibrium states,C _sandC ₁ respectively. The quantum chemical RHF/6-31G^* force fields were scaled with the set of transferable factors previously obtained by the authors to assign the available experimental vibrational bands and predict the positions of bands for the unmeasured spectral regions. Some common patterns of the geometrical parameters, vibrational spectra, and force fields of the simplest nitramines are discussed.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 11, pp. 2135–2147, November, 1995.The authors are grateful to the Russian Foundation for Basic Research (Project No. 93-03-4410) and to the International Science Foundation (Grant No. MQXOOO) for financial support of works fulfilled at the Department of Chemistry, M. V. Lomonosov Moscow State University. The authors also acknowledge the support of the Scientific Technical Program Universities of Russia.     

Quantum-chemical calculations for silyl- and alkyl-pseudohalides X3YNCZ (X = H or CH3; Y = C or Si; Z = O,S, or Se) and electron diffraction study of the equilibrium molecular structure of (CH3)3SiNCSe

The equilibrium structures and force fields of the twelve simplest silyl- and alkyl-pseudo halides are calculated by means of B3LYP and MP2(full) quantum-chemical methods with the use of the aug-cc-pVTZ basis. Some regularities in their structure are established. Using these data, the equilibrium structure of the (CH₃)₃SiNCSe molecule with symmetry C _3v is described experimentally for the first time via gas electron diffraction. The following values of the main r _e parameters are determined (uncertainty 3σ is in parentheses): C=Se, 1.709(14) Å; N=C, 1.190(10) Å; N-Si, 1.767(15) Å; Si-C, 1.847(13) Å; N-Si-C, 106.4°; C-Si-C, 112.4°.     

Interpreting the vibrational spectra of uracil molecules and their deuterated isotopomers using a scaled quantum-chemical quadratic force field

The quadratic force field of the uracil molecule is obtained by MP2(full) calculations using the cc-pVTZ and cc-pVQZ basis sets. Under the assumption that the most stable diketone form of the uracil molecule has a flat configuration with C _s symmetry, the available vibrational gas-phase spectra of uracil and the matrix isolation spectra of its seven N-, C-, and mixed N,C-deuterated derivatives are analyzed jointly for the first time by using Pulay??s force field scaling. Band assignments suggested earlier are corrected. It is shown that sets of 14 scaling factors allow us to reproduce the adjusted interpretation of the spectra and to obtain the most reliable quadratic force constant matrix for uracil among those available, based on joint consideration of the experimental and quantum-chemical calculation results.     

Quantum-chemical calculations and electron diffraction study of the equilibrium molecular structure of vitamin K3

The equilibrium molecular structure of 2-methyl-1,4-naphthoquinone (vitamin K₃) having C _s symmetry is experimentally characterized for the first time by means of gas-phase electron diffraction using quantum-chemical calculations and data on the vibrational spectra of related compounds.