Internal rotation and equilibrium structure of the 2-methyl-2-nitropropane molecule from joint processing of gas phase electron diffraction data,vibrational and microwave spectroscopy data,and quantum chemical calculation results

The structure and internal rotation of the 2-methyl-2-nitropropane molecule is studied by electron diffraction and quantum chemical calculations with the use of microwave and vibrational spectroscopy data. The electron diffraction data are analyzed within the general intramolecular anharmonic force field model and the quantum chemical pseudoconformer model, considering the adiabatic separation of the degree of freedom of large amplitude motion, i.e., the internal rotation of the NO₂ group. The equilibrium eclipsed configuration of the C _s symmetry molecule has the following experimental bond lengths and valence angles: r _e(N=O) = 1.226//1.226(8) Å, r _e(C–N)//r _e(C–C) = 1.520//1.515/1,521(4) Å, ∠_еC–C–N = = 109.1/106,1(8)°, ∠_еO=N=O = 124.2(6)°, ∠_eC–C–H_avg = 110(3)°. The equilibrium geometry parameters are well consistent with MP2/cc-pVTZ quantum chemical calculations and microwave spectroscopy data. The thermally average parameters previously obtained within the small vibration model show a satisfactory agreement with the new results. The electron diffraction data used in this work do not allow a reliable determination of the barrier to internal rotation. However, at a barrier of 203(2) cal/mol, which is derived from the microwave study, it follows from the electron diffraction data that the equilibrium configuration must correspond to an eclipsed arrangement of C–C and N=O bonds, which is also consistent with the results of quantum chemical calculations of various levels.     

Hexamethylenetetramine (urotropine) C6H12N2: Interpreting the vibrational spectra of -d0 and -d12 isotopomers by scaling the quantum-chemical force field

The equilibrium structure and quadratic and cubic force fields of the urotropine molecule are calculated at the MP2 (full)/cc-pVTZ level. Pulay scaling of the quadratic force field allows unambiguous interpretation of the vibrational spectra of -d₀ and -d₁₂ urotropines. A reliable matrix for the quadratic force constants of urotropine is obtained which may be used to determine the parameters of the equilibrium structure of the urotropine molecule by means of gas-phase electron diffraction.     

Design of the optimal sector device and determination of its parameters in gas electron diffraction

This work presents a way to select the optimal form of the sector lobe and practical algorithms to determine the sector function of the device in use. Knowledge of the sector function enhances the quality of drawing a background line and ensures the division of multiplicative and additive noise components, thus increasing the reliability of structural parameters determined by gas electron diffraction.     

Quantum-chemical study of the silver trifluoroacetate dimer

The results of quantum-chemical calculations of the formation energy, equilibrium structure, and potential surface sections along the nonrigid degrees of freedom of the silver trifluoroacetate dimer are presented. Calculations were performed by the B3LYP method with the cc-pVTZ correlation-coherent basis for C, O, and F atoms using the basis and relativistic effective core potentials Stuttgart 1997 RSC for Ag atoms, and, for comparison, by the HF method in the 6-31G(d) basis and MP2 method in the 6-311G(df) basis for C, O, and F atoms using the basis and relativistic effective core potentials SBKJC for Ag atoms. The eight-membered ring is a rigid planar fragment with a bond order of 0.2 between the silver nuclei. The nearly free internal rotation of the CF₃ group affects the geometrical parameters of the ring. It was substantiated that in electron diffraction experiments, the difficulties of interpretation could be explained not only by the presence of decomposition products in the sample, but also by possible oligomerization of silver trifluoroacetate.     

New method for calculating the moments of thermally averaged densities of distribution of internuclear distances in polyatomic molecules using the Bloch integral equation

A convenient method is suggested for calculating thermally averaged powers of the normal vibrational coordinates Q _i by iteratively solving the Bloch integral equation with an anharmonic function of potential energy using multidimensional Hermite polynomials. Analytical formulas of the first approximation regarding anharmonicity constant have been obtained for the following moments of thermally averaged density: 〈Q ₁〉, 〈 Q ₁ ² 〉, 〈Q ₁ Q ₂〉, 〈Q ₁ ³ 〉 〈Q ₁ ³ 〉, 〈Q ₁ Q ₂ Q ₃〉, 〈Q ₁ ⁴ 〉, 〈Q ₁ ² Q ₂ ² 〉, 〈Q ₁ Q _2/3〉, 〈Q ₁ Q ₂ Q ₃ ² 〉, 〈 Q 1 Q ₂ Q ₃ Q ₄〉.