Calculation of internal rotation spectra of three molecules with increasing asymmetry factors

Internal rotation energy levels for phenol, fluoracetaldehyde, and difluoracetaldehyde are estimated with two methods: first, by means of the reduced inertia moment formula introduced by Pitzer; second, with use of an internal axis system and by taking into account the influence of the overall rotation through the inertia factor. Two methods have been used to estimate potential barriers (CNDO /2 and PCILO ). Results are related to the asymmetry of the tops. Theoretical far IR spectra for the three molecules are calculated. In the case of phenol, good agreement is found with experimental data.     

Simplification of complex EPR spectra by cepstral analysis

As the Fourier transform of time-series data is known as the spectrum, the Fourier transform of the logarithm of the time-series data is called the cepstrum of the data. When cepstral analysis is applied to free induction decay signals of free radicals showing first-order EPR spectra, the identification of nuclear hyperfine coupling constants becomes simple. In a systematic manner, we have examined how the technique of cepstral analysis is affected by the presence of aliasing, noise, uncertainty in the time origin of the free induction decay, the presence of second-order hyperfine couplings, and the applications of various apodization methods. This technique was then applied to analyze the EPR spectrum of anthraquinone anion radical, and anion radicals of porphycene and tetrapropyl-porphycene, and the hyperfine coupling constants thus obtained were compared with published data. A good agreement was always found. We make a case for the usefulness of cepstral analysis in determining the hyperfine coupling constants of complex EPR spectra of organic free radicals.     

A study of the ethane internal rotation barrier

In an effort to deduce the source of the ethane internal rotation barrier, we have investigated the contributions of exchange energy and orthogonality: two effects that are required by the Pauli principle. Fully antisymmetrized, partially antisymmetrized and non-antisymmetrized optimized orbital product wavefunctions were determined. Results show that the exchange energy contribution to the barrier is negligible only when it is evaluated from energy-localized orbitals; even in this case the small total results from cancellation of large contributions. The barrier is apparently caused by the orthogonality that is required between CH orbitals on opposite ends of the molecule. The CC bond has insignificant participation in this effect.     

Activation analysis by internal conversion electron spectra

The applicability of instrumental activation analysis by the conversion radiation of impurities in matrices with high specific gamma-activity has been studied. An iron-free toroidal magnetic β-spectrometer with 2 per cent resolution and 20 per cent transmission for 4 was used for measurement. The spectrometer was made at the Activation Analysis Laboratory of the Academy of Sciences of the Kazakh SSR. Comparative gamma- and beta-spectra of samples of amorphous selenium thin films with palladium, cadmium, indium and mercury impurities were obtained. It was established that the analysis by the spectra of internal conversion electrons enables to increase the detection sensitivity of these elements in selenium as compared to the gamma-spectrometric method. The procedure of mercury determination in amorphous selenium films was developed and by this method the investigation of the dependence of photoconductive properties of the film upon the total mercury content and its distribution on the film surface was carried out.     

Vibrational spectra and electron diffraction study of the structure and internal rotation of N,N-diethylcyanamide

Structural parameters, internal rotation parameters, and frequencies of normal vibrations of N,N-diethyl-cyanamide were obtained. The geometrical structure and internal rotation were studied by gas phase electron diffractometry. The configuration of the bonds of the amine nitrogen atom were found to be close to pyramidal: λ∈CNC 111.8°, λCNC 109.1°; the main bond lengths are (Å): N∈C 1.174; ∈C?N 1.349; C?N 1.479; C?C 1.536. A map of changes in the conformation energies due to rotation of ethyl groups around the N?C bonds is constructed. Experimental IR and Raman spectra of pure liquid and CCl₄ solution were measured. The frequencies and forms of normal vibrations are calculated, and the force constants are determined.     

Vibrational spectra of hexafluorodisiloxane and internal rotation around the SiO bond. An ab initio study

The structure of hexafluorodisiloxane was optimized at different levels of theory up to MP2/6-31G^*, and the vibrational spectrum was calculated and compared with the experimental one. It was shown that at the Hartree-Fock level the simple SCF/3-21 (O^*) basis set gives satisfactory results. The geometry was also optimized at two C_2v structures which appear to be saddle points on the SiF₃ rotor potential curve. The significant change of the SiOSi angle equilibrium value in the course of the SiF₃ group rotation was explained by assuming repulsion between lone pair electrons of fluorines and oxygen. This effect is also revealed in the predicted change in the v_asSiOSi frequency on going from the C₂ to the C_2v structure. This provides the explanation of some unusual features in the vibrational spectra of hexahalodisiloxanes, namely large gas-solid shifts of v_as SiOSi and splitting of this mode in the matrices.     

An analysis of the vibrational structure of high-resolution UV spectra and long-wave IR Fourier transform spectra in studies of internal rotation in molecules

The methods for analyzing the vibrational structure of high-resolution UV spectra and long-wave IR Fourier transform spectra in studies of internal rotation in α,β-unsaturated carbonyl compounds R₄R₃C=CR₂-COR₁ (R₁ = F, Cl; R₂ = R₃ = R₄ = H, CH₃) are compared. These methods were found to give different experimental values for systems of torsional vibration energy levels up to high quantum numbers, torsional frequencies (0–1 transitions), and anharmonicity coefficients x ₁₁ for trans and cis isomers of the same molecules in the ground electronic state (S ₀). It was shown that the experimental technique for analyzing the vibrational structure of UV spectra excludes the hydrolysis of compounds under study. Taking into account Fermi resonance and numerous Deslandres tables constructed for trans and cis isomers provides reliable determination of values necessary for the construction of internal rotation potential functions, because they are multiply repeated in various Deslandres tables. An analysis of the vibrational structure of UV spectra gives more reliable V _n internal rotation potential function parameters. The V _n parameter values were substantiated by quantum-mechanical calculations performed by other authors.     

Far-infrared spectra and barriers to internal rotation of ethyl nitrate

The far-infrared spectra of gaseous and solid ethyl nitrate, CH₃CH₂ONO₂, have been recorded from 500 to 50 cm⁻¹. The fundamental asymmetric torsion of the trans conformer which has a heavy atom plane has been observed at 112.50 cm⁻¹ with two excited states failing to lower frequencies, and the corresponding fundamental torsion of the gauche conformer was observed at 109.62 cm⁻¹ with two excited states also falling to lower frequencies. The results of a variable temperature Raman study indicate that the trans conformer is more stable than the gauche conformer by 328 ± 96 cm⁻¹ (938 ± 275 cal mol⁻¹). An asymmetric potential function governing the internal rotation about the CH₂O bond is reported which gives a trans to gauche barrier of 894 ± 15 cm⁻¹ (2.56 ± 0.04 kcal mol⁻¹) and a gauche to gauche barrier of 3063 ± 68 cm⁻¹ (8.76 ± 0.20 kcal mol⁻¹) with the trans conformer more stable by 220 ± 148 cm⁻¹ (0.63 ± 0.42 kcal mol⁻¹). Transitions arising from the symmetric CH₃ and NO₂ torsions are observed for both conformers, from which the threefold and twofold periodic barriers to internal rotation have been calculated. For the trans conformer the values are 1002 cm⁻¹ (2.87 kcal mol⁻¹) and 2355 ± 145 cm⁻¹ (6.73 ± 0.42 kcal mol⁻¹) and for the gauche conformer they are 981 cm⁻¹ (2.81 kcal mol⁻¹) and 2736 ± 632 cm⁻¹ (7.82 ± 1.81 kcal mol⁻¹) for the CH₃ and NO₂ rotors, respectively. These results are compared to the corresponding quantities for some similar molecules.     

ESR study of the rotation of peroxy radicals in polymers

The nature and the frequencies of rotation of peroxy radicals in polyethylene and polytetrafluorethylene are determined over a wide temperature range theoretically calculated ESR spectra.     

A study of the spectra of diacetylenic phospholipid polymers in solvents and dispersions

Phospholipids, of which phosphoryl cholines are a specific example, are the most common lipid components of cell membranes. Synthetic acetylenic phospholipids form closed model membranes, vesicles, or liposomes when dispersed in water. Below the temperature at which lipid chain crystallization occurs irradiation of vesicles causes polymerization of the diacetylene groups. In chloroform solution the spectrum of the polymer is independent of temperature. However, when the polymer is dissolved in methanol or dispersed in water with another lipid spectral changes occur. As solution or dispersion is cooled a second peak appears on the long wavelength side of the original absorption and gradually increases in intensity. Evidence that suggests that this peak is associated with aggregated polymer chains is presented. It appears that within the aggregates exciton coupling splits the excited state into two new levels and only transitions to the level of lowest energy are allowed.     

A study of the hyperfine structure from the ligands in the EPR spectra of complex compounds

Journal of Structural Chemistry -     

EPR study of spin labeled brush polymers in organic solvents

Spin-labeled polylactide brush polymers were synthesized via ring-opening metathesis polymerization (ROMP), and nitroxide radicals were incorporated at three different locations of brush polymers: the end and the middle of the backbone, and the end of the side chains (periphery). Electron paramagnetic resonance (EPR) was used to quantitatively probe the macromolecular structure of brush polymers in dilute solutions. The peripheral spin-labels showed significantly higher mobility than the backbone labels, and in dimethylsulfoxide (DMSO), the backbone end labels were shown to be more mobile than the middle labels. Reduction of the nitroxide labels by a polymeric reductant revealed location-dependent reactivity of the nitroxide labels: peripheral nitroxides were much more reactive than the backbone nitroxides. In contrast, almost no difference was observed when a small molecule reductant was used. These results reveal that the dense side chains of brush polymers significantly reduce the interaction of the backbone region with external macromolecules, but allow free diffusion of small molecules.     

Theoretical analysis of the internal rotation in aminoborane and borylphosphine

Using a recently proposed orbital deletion procedure and the block-localized wavefunction method, the rotational barriers in H₂BNH₂ and H₂BPH₂ are analyzed in terms of conjugation, hyperconjugation, steric effect and pyramidalization. With the zero-point energy corrections, the π-binding strengths in the planar H₂BNH₂ and H₂BPH₂ are both around 20 kcal/mol at the HF level using the 6-311+G** basis set. With the deactivation of the π atomic orbitals on the boron atom and the evolution from a planar structure to a 90°-twisted structure, the steric repulsion between the B‐H and the N‐H or P‐H is relieved and moreover, the negative hyperconjugation from the lone electron pair or pairs on the nitrogen or phosphorus atoms to the antibonding orbital ^χ* _B H ₂ of the BH₂ group stabilizes the twisted structure by 7.4(8.8) or 4.0(5.0) kcal/mol at the HF/6-31G*(6-311+G**) level. However, the repulsive interaction between the lone pair(s) and the two BH σ bonds is so prominent that the overall steric effect contributes 20.3(22.9) and 19.3(19.8) kcal/mol to the rotational barriers in H₂BNH₂ and H₂BPH₂ with the 6-31G*(6-311+G**) basis set. The present techniques and analyses may also give some clues to justify the parameterization in the empirical molecular mechanics methods. Received: 17 April 1998 / Accepted: 17 September 1998 / Published online: 1 February 1999     

Ab initio study of the internal rotation in peroxyformic acid

The internal rotation in peroxyformic acid was investigated using the ab initio SCF MO LCAO method with the STO-3G and 4-31G basis-sets and with experimental and optimum values of the geometrical parameters. Both basis sets yield a rather flat double-minimum potential for the COOH torsion, the trans planar form being lightly preferred in comparison with the cis one. The effects on the internal rotation of the geometry relaxation, of the intramolecular hydrogen bond and of the methyl group of peroxyacetic acid are also discussed.