V.A. Sipachev

Local centrifugal distortions caused by internal motions of molecules

Abstract The problem of centrifugal distortions caused by intramolecular vibrational motions is considered at the level of classical perturbation theory. A scheme of the computational algorithm is described and the results obtained for several typical objects of structural studies are given.

Gas-phase electron diffraction study of μ4-oxohexa-μ-nitratotetraberyllium

Abstract Gaseous μ 4 -oxohexa-μ-nitratotetraberyllium, Be 4 O(NO 3 ) 6 , has been studied by electron diffraction at a temperature of ≈160°C. The intensity data were analysed in terms of a model having T symmetry. The molecular force field was estimated with the aid of a recently developed procedure to provide a satisfactory fit to 29 experimental frequencies and 39 amplitudes within the framework of a 24-parameter force field model. In the final stage of the structural analysis the amplitudes were varied in groups formed on the basis of spectroscopic calculations. The principal geometrical parameters refined to the following values ( r g structure, bond lengths in A, angles in degrees): BeO 1 (O 1 is the central μ 4 -oxygen atom), 1.665(21); BeO, 1.620(8); NO, 1.185(5); NO, 1.298(4); η (the angle of rotation of the NO 3 group about the twofold axis), 25.2(1.8); ∠ONO, 117.0(0.9) (the uncertainties are presented by 2.5 times the standard deviation values, plus a scale error).

A comparison of amplitudes and shrinkage corrections for C6Cl3(NO2)3 calculated using conventional and new procedures

Abstract A harmonic force field for C6Cl3(NO2)3 has been estimated, and the calculated and experimental vibrational frequencies are compared. The vibrational amplitudes and shrinkage corrections have been calculated using the conventional technique based on normal coordinate analysis (1) and the new technique of taking into consideration non-linear terms in the transformation between the Cartesian and internal coordinates (2). The shrinkage corrections for bonded distances calculated using technique (2) are substantially (an order of magnitude) smaller than those calculated with the conventional procedure (1). The opposite effect is observed for non-bonded distances depending on internal rotations, which is more consistent with the physical sense of the influence of vibrational motions on the molecular structure.

Fragmentation of μ4-oxohexa-μ-nitratotetraberyllium, Be4O(NO)3)6, under electron impact

Abstract The behaviour of Be4O(NO3)6 under electron impact is similar to that of its carboxylato analogues, Be4O(RCO2)6 where R is H, alkyl or halogenated alkyl. In all these systems, the dissociation of the molecular ions is dominated by steric interactions. The major fragmentations involve the elimination of N2O5 or (RCO)2O and Be(NO3)2 or Be(RCO2)2 from the ions [M-NO3]+ or [M-RCO2]+. The results obtained confirm the structural similarity of the nitrato complex to tetra-nuclear beryllium oxocarboxylates.

Fragmentation of group IV metal tetranitrates and oxovanadium nitrate under electron impact

Abstract The mass spectra of TI(NO3)4, Zr(NO3)4, Hf(NO3)4, Sn(NO3)4 and VO(NO3)3 are reported. The heaviest ions in all the spectra are those formed by the loss of one NO3 group from the molecular ions. Further fragmentation involves change of metal or nitrogen valences. Splitting off of nitrogen reduction products is accompanied by the formation of fragment ions with peroxo structures.

A calculation procedure for corefinement of spectroscopic and electron diffraction experiments

Abstract Perturbation theory equations are applied in the refinement of molecular force fields on the basis of vibrational spectroscopic and electron diffraction data. Computer realization of this approach is far less costly than numerical evaluation of Jacobi matrix elements, which makes the procedure suitable for handling large molecules. Apart from refined force field parameters, correlations between spectroscopic amplitudes are determined for further treatment of s M( s ) curves. The combined inverse problem is solved by sequential optimizations of the molecular geometry and vibrational Hamiltonian.

Ab initio calculations of the structure, vibrational spectra, and thermodynamic properties of silver trifluoroacetates

Abstract The structure and properties of monomeric and dimeric silver trifluoroacetates were calculated at different levels of approximation, from RHF with the 3-21G basis set for all atoms to MP2 with the SBKJC and Stutrgart RSC relativistic effective core potentials for silver and the 6-311G (df) basis set for the other atoms. The monomer was found to exist as two conformers in the gas phase, in a more stable conformer, the Ag atom formed coordination bonds with two carboxyl oxygens, and a less stable conformer contained O–Ag–F bridges and the CO double bond. The CF3 group freely rotated in the monomer without Ag–F bonds and experienced hindered rotations in the dimer. The theoretically predicted vibrational spectrum of the dimer and the thermodynamic functions of the dimerization reaction were consistent with the available experimental data.

Mean vibrational amplitudes of 1,3-butadiene, acrolein and glyoxal

Abstract The mean vibrational amplitudes of 1,3-butadiene, acrolein, and glyoxal have been calculated by explicitly including the effects of kinematic anharmonicity, and are compared with the corresponding amplitudes with this effect neglected. The scaled and unscaled quantum-mechanical force fields calculated at the correlated MP2/631G ∗ //MP2/6-31G ∗ computational level were used. The calculated results are also compared with the available experimental data.

Fragmentation of zirconium and hafnium tetraperchlorates and copper perchlorate under electron impact

Abstract The mass spectra of Zr(ClO4)4, Hf(ClO4)4 and CU(ClO4)2 are reported. The major fragmentation path for Zr and Hf compounds involves the loss of ClO4· followed by the elimination of Cl2O7 (Cl2O6) and, eventually ClO3·(ClO2·). The copper perchlorate molecular ion dissociates to Cu+ by sequentially splitting off ClO4· radicals. Transformations of coordination polyhedra during fragmentation are predetermined by metal redox properties. The mass spectra were employed to roughly estimate the extent of thermal dissociation of initial molecules under mass-spectrometric experiment conditions.