Konstantin B. Borisenko

Barrier to internal rotation of the nitro group in ortho-nitrophenols from gas-phase electron diffraction

Abstract The barrier heights to the internal rotation of the nitro group in 2-nitroresorcinol, 2-nitrophenol, 4,6-dinitroresorcinol, and nitrobenzene have been determined by the application of a dynamic model to the electron diffraction data. With Monte Carlo optimization, the results are invariant to the choice of a particular initial parameter set. The internal motion of the nitro group in 2-nitroresorcinol, 2-nitrophenol, and 4,6-dinitroresorcinol is hindered by higher barriers than in nitrobenzene, as a consequence of intramolecular hydrogen bond formation.

Geometrical consequences of intermolecular hydrogen bond formation in the formic acid and acetic acid dimers from ab initio MO calculations

Abstract Ab initio molecular orbital calculations on monomer/dimer formic and acetic acids have been performed with the 6–31G∗ and 6–31G∗∗ basis sets, including the electron correlation using second-order Moller-Plesset perturbation theory. Vibrational frequency analyses confirmed the stability of all the computed structures. The computed geometrical changes in dimeric formic and dimeric acetic acids, as compared to their monomers, are in good agreement with the notion of resonance-assisted intermolecular hydrogen bond formation. Trends in the structural changes obtained from electron diffraction were in general confirmed by the calculations, although they failed to predict the difference in the CC bond length observed in the monomeric and dimeric forms of acetic acid. However, the experimental change in the OH bond length upon hydrogen bond formation seems definitely exaggerated. Taking into account our previous results on 2-nitroresorcinol and 2-nitrophenol, this change is now expected to be not larger than 0.02 A.

Geometrical consequences of fluorination and intermolecular hydrogen bond formation in fluoroformic acid and trifluoroacetic acid dimers from ab initio MO calculations

Abstract The molecular structures of fluoroformic and trifluoroacetic acids and their dimers have been determined by ab initio molecular orbital calculations including electron correlation at the MP2 level with standard 6-31G∗ and 6-31 + G∗ basis sets. Fluorination introduces geometrical changes as expected for a strong electron withdrawing substituent. Considerable inter-molecular interactions were detected in the dimers accompanied by pronounced structural changes compared with the monomers, similarly to those observed for formic acid and acetic acid monomers/dimers, and described as consequences of resonance-assisted hydrogen bonding. The hydrogen bonding in the fluoroformic acid dimer appears to be somewhat stronger than in the trifluoroacetic acid dimer.

Intramolecular hydrogen bonding and molecular structure of 2-iminomethyl-phenol and molecular structure of iminomethyl-benzene from ab initio MO calculations

Abstract Ab initio calculations, including electron correlation (MP2), have been performed for iminomethyl-benzene and 2-iminomethyl-phenol. They indicate the existence of two (syn and anti) stable conformers of 2-iminomethyl-phenol. The principal syn conformer is characterized by considerable hydrogen bonding between the iminomethyl and hydroxy groups, accompanied by pronounced structural changes in the rest of the molecule, as compared with phenol and iminomethyl-benzene calculated at the same theoretical level. These structural changes are consistent with the notion of resonance-assisted hydrogen bonding, also observed in a series of nitrophenols and similar molecules. The anti conformer of 2-iminomethyl-phenol has 41.8 kJ mol−1 higher energy than the syn conformer. The internal rotation of iminomethyl group in 2-iminomethyl-phenol is hindered by a considerably higher barrier, 55.9 kJ mol−1, due to intramolecular hydrogen bond formation, than that in iminomethyl-benzene, 27.6 kJ mol−1, according to the calculations at the HF/6-31G∗ level. The amino tautomer form of 2-iminomethyl-phenol has 25.0 kJ mol−1 (HF/6-31G∗) or 30.6 kJ mol−1 (MP2(FC)/6-31G∗) higher energy than the imino form. The energy of the structural changes as compared to iminomethyl-benzene and phenol and the energy of the intramolecular hydrogen bond formation in 2-iminomethyl-phenol was estimated to be −28.8 kJ mol−1 at the HF/6-31G∗ level and −36.8 kJ mol−1 at the MP2(FC)/6-31G∗ level of theory.

Molecular structure and intramolecular motion of hexamethyldisiloxane from gas-phase electron diffraction

Abstract The molecular structure of hexamethyldisiloxane has been reinvestigated by electron diffraction. The two kinds of large amplitude motion in the molecule, the Si–O–Si bending and the torsion about the Si–O bonds, were treated independently in terms of their potential functions. In the equilibrium structure the molecule has probably C 2v symmetry with staggered C 3 Si–O–Si fragments and a nonlinear Si–O–Si linkage with an angle of 155°. The barrier to linearization was obtained to be at least 10 kJ mol −1 . In addition, there is free or nearly free internal rotation of the trimethylsilyl groups in hexamethyldisiloxane. In terms of r g bond lengths the electron diffraction analysis yielded Si–O 1.639±0.003 A, Si–C 1.871±0.004 A, and the r a angle Si–O–Si 152±2°.

Preparation of N(SeCl)2+X–(X = SbCl6or FeCl4), F3CCSeNSeCCF3+SbCl6–, F3CCSeNSeCCF3, F3CCSeNSeCCF3and F3CCSeSeC(CF3)C(CF3)SeSeCCF3. Electron diffraction study of F3CCSeSeCCF3and crystal structure of the eight-membered heterocycle F3CCSeSeC(CF3)C(CF3)SeSeCCF3

The salts N(SeCl)2+SbCl6–1 and N(SeCl)2+FeCl4–2 were synthesized by reaction of SeCl3+ X–(X = SbCl6 or FeCl) with N(SiMe3)3; 1 was also formed by reaction of Se2NCl3 with SbCl5. Reaction of 1 with SnCl2 and F3CCCCF3 led to the formation of F3C[graphic omitted]CF3+ SbCl6–3. In this reaction the Se2N + cation is a likely intermediate because SnCl2 seems to be essential for chloride abstraction in the first reaction step to generate Se2N+in situ which then adds F3CCCCF3 to yield 3. Compound 3 is a useful building block to generate selenium compounds such as F3C[graphic omitted]CF34, F3C[graphic omitted]CF35 and F3C[graphic omitted]CF36. The heterocycle 5 was shown by electron diffraction to have an approximately planar four-membered ring structure. The structure of compound 6 was determined by X-ray crystallography: orthorhombic, space group Pbca, a= 10.1920(21), b= 13.0615(20) and c= 22.050(5)A. In order to rationalize the structures of 5 and the cation F3C[graphic omitted]CF3+, ab initio calculations were made on model compounds in which the CF3 groups were replaced by a fluorine atom (i.e.F[graphic omitted]F for 5and F[graphic omitted]F+ for the cation in 3). In addition, mass spectrometric experiments were performed in order to examine the structures and stabilities of the unligated cation F3C[graphic omitted]CF3+ as well as its neutral counterpart. The existence of the neutral radical 4 was established by means of neutralization–reionization mass spectrometry.

Electron diffraction study of the nitrogen dioxide molecular structure at 294, 480 and 691 K

Abstract Gas-phase electron diffraction experiments have been carried out on NO 2 at three temperatures, viz., 294, 480, and 691 K. Applying the necessary vibrational corrections, the equilibrium structure ( r e ) of NO 2 was determined from the 480 K data where it was the sole component of the vapor, 1.195(3) A and 133.7(2)∘. This information was then utilized in the analysis of the mixture of NO 2 (82%) and N 2 O 4 (18%) at 294 K. The NO bond of NO 2 is shorter than that of N 2 O 4 by 0.008 A, and the ONO angle opens slightly in the dimer as compared with NO 2 . The barrier height to internal rotation in N 2 O 4 is estimated to be about 40 kJ mol −1 from the electron diffraction data. There is a slight increase of the average NO bond length ( r g ) at higher temperatures.

Molecular structure and intramolecular hydrogen bonding in 4,6-dinitroresorcinol and 2,5-dinitrohydroquinone from ab initio molecular orbital calculations

Abstract Ab initio molecular orbital calculations using second-order Moller-Plesset (MP2) perturbation theory with the 6-31G ∗ basis set have been performed for 4,6-dinitroresorcinol and 2,5-dinitrohydroquinone. Both molecules are characterized by considerable hydrogen bonding between the nitro and hydroxy groups; pronounced structural changes in other parts of the molecules are observed when compared with the parent phenol and nitrobenzene molecules calculated at the same computational level. These structural changes are consistent with the notion of resonance-assisted hydrogen bonding, also observed in a series of other o -nitrophenols and similar molecules. The geometrical and energetic characteristics of the structures indicate somewhat weaker hydrogen bonding in 2,5-dinitrohydroquinone than in 4,6-dinitroresorcinol, which may be interpreted by the difference in the mutual orientation of the substituent pairs in the two compounds.

MONTE CARLO GLOBAL OPTIMIZATION IN THE REFINEMENT OF MOLECULAR STRUCTURE PARAMETERS FROM GAS-PHASE ELECTRON DIFFRACTION DATA

Abstract An automated approach to the least-squares optimization, a so-called multi-start Monte Carlo global optimization, of the molecular parameters from gas-phase electron diffraction data has been proposed and tested on some previously studied molecules. The results are compared with those obtained with the conventional single-start optimization method.