Nikolai M. Karasev

Flexibility of the Saturated Five-Membered Ring in 2,5-Pyrrolidinedione (Succinimide): Electron Diffraction and Quantum-Chemical Studies with Use of Vibrational Spectroscopy Data

The flexibility of succinimide molecule has been studied for the first time by quantum-chemical (at the MP2 level with up to the 6-311G(3df,2p) basis sets) and gas-phase electron diffraction (GED) methods using vibrational spectroscopy data from literature. The analysis of vibrational spectra, performed for the molecular model of C(2v) symmetry (predicted by high-level ab initio calculations) using the scaling procedure, has shown that the two out-of-plane ring motions, that is, ring-bending and ring-twisting, are practically pure modes of different symmetry types and can be considered separately. The one-dimensional potential curves for the ring-bending and ring-twisting vibrations calculated at the MP2(full)/6-311G(3df,2p) level could be approximated by harmonic and anharmonic functions, respectively. The diverged energy levels for the ring-twisting vibration and the constant transition frequencies for the bending motion, obtained by the solution of the direct one-dimensional problem for the nonrigid model, demonstrate this statement. In the GED analysis, the succinimide molecule with a large-amplitude ring-twisting motion was described by a dynamic model with the distribution of pseudoconformers according to the calculated potential function taking into account structural relaxation effects from the MP2(full)/6-311G(3df,2p) calculations. This model greatly improved the fit of the GED intensities (R factor decreased from 4.6% for static model to 2.8%). The equilibrium molecular parameters r(e) determined in the dynamic approximation are very close to the corresponding values from the ab initio calculations. At the same time, the parameters of the -CH(2)-CH(2)- fragment involved in the ring-twisting motion deviate considerably from those obtained for the static model (C-C bond length by 0.027 A, =C-C-H, C-C-H, and H-C-C-N angles by up to 7 degrees). The flexibility influence on the C-C bond length is several times larger than the calculated vibrational correction (r(e) - r(a)) as well as the experimental uncertainty.

Molecular structure, conformation, and potential to internal rotation of 2,6- and 3,5-difluoronitrobenzene studied by gas-phase electron diffraction and quantum chemical calculations.

3,5-Difluoronitrobenzene (3,5-DFNB) and 2,6-difluoronitrobenzene (2,6-DFNB) have been studied by gas-phase electron diffraction (GED), MP2 ab initio, and by B3LYP density functional calculations. Refinements of r h1 and r e static and r h1 dynamic GED models were carried out for both molecules. Equilibrium r e structures were determined using anharmonic vibrational corrections to the internuclear distances ( r e - r a) calculated from B3LYP/cc-pVTZ cubic force fields. 3,5-DFNB possesses a planar structure of C 2 v symmetry with the following r e values for bond lengths and bond angles: r(C-C) av = 1.378(4) A, r(C-N) = 1.489(6) A, r(N-O) = 1.217(2) A, r(C-F) = 1.347(5) A, angleC6-C1-C2 = 122.6(6) degrees , angleC1-C2-C3 = 117.3(3) degrees , angleC2-C3-C4 = 123.0(3) degrees , angleC3-C4-C5 = 116.9(6) degrees , angleC-C-N = 118.7(3) degrees , angleC-N-O = 117.3(4) degrees , angleO-N-O = 125.5(7) degrees , angleC-C-F = 118.6(7) degrees . The uncertainties in parentheses are three times the standard deviations. As in the case of nitrobenzene, the barrier to internal rotation of the nitro group in 3,5-DFNB, V 90 = 10 +/- 4 kJ/mol, is substantially lower than that predicted by quantum chemical calculations. The presence of substituents in the ortho positions force the nitro group to rotate about the C-N bond, out of the plane of the benzene ring. For 2,6-DFNB, a nonplanar structure of C 2 symmetry with a torsional angle of phi(C-N) = 53.8(14) degrees and the following r e values for structural parameters was determined by the GED analysis: r(C-C) av = 1.383(5) A, r(C-N) = 1.469(7) A, r(N-O) = 1.212(2) A, r(C-F) = 1.344(4) A, angleC6-C1-C2 = 118.7(5) degrees , angleC1-C2-C3 = 121.2(2) degrees , angleC2-C3-C4 = 119.0(2) degrees , angleC3-C4-C5 = 121.1(4) degrees , angleC-C-N = 120.6(2) degrees , angleC-N-O = 115.7(4) degrees , angleO-N-O = 128.6(7) degrees , angleC-C-F = 118.7(5) degrees . The refinement of a dynamic model led to barriers V 0 = 16.5 +/- 1.5 kJ/mol and V 90 = 2.2 +/- 0.5 kJ/mol, which are in good agreement with values predicted by B3LYP/6-311++G(d,p) and MP2/ cc-pVTZ calculations. The values of C-F bond lengths are similar in both molecules. This is in contrast to the drastic shortening of the C-F bond in the ortho position in 2-fluoronitrobenzene compared to the C-F bond length in the meta and para position in 3- and 4-fluoronitrobenzene observed in an earlier GED study.

A Gas Electron Diffraction Study of the Conformational Composition of 1,3,5-Trimethyl-1,3,5-triazacyclohexane

The conformational composition of 1,3,5-trimethyl-1,3,5-triazacyclohexane was studied by gas electron diffraction and quantum-chemical calculations at the density functional theory (B3LYP) and MP2 levels. Conformers with the general chair, boat, and twist ring forms were considered possible. These structures differed in the arrangement of CH3 groups in the axial (a) and equatorial (e) positions. A chair conformer with the axial orientation of one CH3 group was found to satisfy the electron diffraction data. Its main structural parameters (mean values) were rg(C-N) = 1.463(3) Å, rg(C-H) = 1.117(5) Å, ∠(C-N-C) = 110.91(1)°, and ∠(N-C-N) = 111.1(1)°.