An STO-3G molecular orbital investigation of planar m- and p-dihydroxybenzene: Fully optimized molecular structures

STO-3G fully optimized molecular structures have been calculated for various planar conformers of m- and p-dihydroxybenzene (resorcinol and hydroquinone, respectively). Superposition of fully optimized phenol structures has been used to facilitate rapid gradient optimization. The theoretical structures are discussed with reference to available experimental data. It has been found that the STO-3G model also yields useful and reasonably accurate information with respect to crystal structure determinations.     

An STO-3G molecular orbital investigation of planar m- and p-methoxyphenol: Fully optimized molecular structures

STO-3G fully optimized molecular structures of planar m- and p-methoxyphenol have been determined. It has been found that superposition of fully optimized parent structures facilitates a rapid gradient optimization. In addition, it is shown that the superposition structures already represent an energetically almost fully relaxed situation. Structural variations have been observed which parallel previous observations regarding the performance of the STO-3G level of accuracy. Accordingly, STO-3G computations provide a reasonably inexpensive means of obtaining fairly accurate and consistent theoretical information about the equilibrium structures of substituted benzenes.     

An STO-3G molecular orbital investigation of planar m- and p-dimethoxybenzene: Fully optimized molecular structures

The STO-3G fully optimized molecular structures of planar m- and p-dimethoxybenzene have been determined using gradient optimization with initialization from superposition structures. It has been found that, in accordance with previous investigations of similar molecular systems, the superposition reference structures are already almost fully relaxed energetically. Structural parameter variations have been found, which parallel the results obtained from previous studies on other hydroxy- and methoxysubstituted benzenes. Comparisons with experimental data indicate that theoretically computed structures may also be of use for crystal structure investigations.     

The STO-3G transition structure of the diels-alder reaction

A transition structure with C_S symmetry for the Diels-Alder reaction of butadiene with ethylene has been obtained with the STO-3G basis set. Stereochemical consequences are discussed.     

N2O4 complexes with electron donor molecules. An STO-3G study

The quantum chemical calculations for N₂O₄ complexes with some electron-donor molecules: CH₃NO₂, CH₃COOH, CH₃CN, C₆H₆ and with another molecule of N₂O₄ show that the donor molecule lies under the N₂O₄ plane and its electron-donor atom points to the center of N-N bond of N₂O₄ (or near this center). The electron charge is transferred from the donor to N₂O₄ molecule, mostly to the N-N bond of N₂O₄.     

Anab initio investigation of rotation and inversion barriers in formylphosphine

Rotation about N–CO bonds in amides has been extensively investigated, but a corresponding barrier to rotation about the P–CO bond in an acylphosphine has yet to be observed. In the present 4-31Gab initio study of formylphosphine, rotation barriers of 9.6 and 13.5 kJ mol^–1 and a phosphorus pyramidal inversion barrier of 108.0 kJ mol^–1 are predicted. A comparison of STO-3G and STO-3G^* barriers suggests that polarization functions are not needed to describe rotation in this system.

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Eine ab initio Untersuchung von Rotations- und Inversions-Barrieren in Formylphosphin

Zusammenfassung Die Rotation um N–CO-Bindungen in Amiden wurde bisher intensiv untersucht, eine entsprechende Rotationsbarriere für Drehungen um die P–CO-Bindung in Acylphosphinen wurde jedoch nicht beobachtet. Eine 4-31Gab initio-Untersuchung an Formylphosphin ergibt Rotationsbarrieren von 9,6 und 13,5 kJ mol^–1 und eine pyramidale Inversionsbarriere von 108,0 kJ mol^–1 als Voraussage. Ein Vergleich der STO-3G und STO-3G^* Barrieren legt nahe, daß Polarisationsfunktionen für die Beschreibung der Rotation in diesen Systemen nicht nötig sind.

     

NMR studies of barriers to rotation in N-nitrosamines

The potential energy barrier to rotation about the NN bond in dimethylnitrosamine (I), N,N′ dinitroso hexahydro pyrimidine (III), and N,N′N″ trinitroso hexahydro 1,3,5 triazine (IV) were determined by calculation of the lineshapes at different temperatures between 345 K and 515 K. An estimate of ΔG3 was made at coalescence for N-nitroso piperidine (II). The potential barriers increase in the order IV < III < II ∼ I which is interpreted in terms of the electron withdrawing effect of the nitrosamine group.     

Conformational origin of glassy-state relaxation and ductility in aromatic polycarbonates

A new two-state conformational transition is proposed to explain the large, low-temperature mechanical loss peak seen in glassy polycarbonates. Restricted Hartree Fock ab initio calculations at the 6–31G7 level for diphenyl carbonate (DPC), a key model compound of bisphenol-A polycarbonate, reveal two inequivalent trans-trans carbonate-ring conformations both of which will exist in solution, melt or glassy states. These calculations appear to be the first high level ones (with full geometry optimization) reported for DPC, and the findings are consistent with earlier ab initio results for phenyl formate and other smaller model compounds and also with single-crystal X-ray data for DPC and oligomers. In addition to a trans-trans conformer of DPC with both phenyl rings on the same side of the carbonate unit (called the ‘syn’ conformer) which is seen in the crystalline state of DPC, an ‘anti’ conformer of lower energy is found, which has its two phenyl rings located on opposite sides of the plane of the carbonate unit. Analysis of these calculated ground state geometries and energies as well as experimental single crystal X-ray results indicates that the ‘anti’ conformer has the lowest energy in the gas phase and solution, while the ‘syn’ conformation is stabilized relative to the ‘anti’ in the bulk, probably because of aromatic ring interactions between neighbour chain segments. In the glassy state of either DPC or polycarbonate, one expects a nearly random mixture of ‘syn/anti’ conformers, and the prominent low-temperature mechanical loss peak observed in many polycarbonates is consistent with a molecular level two-state process consisting of ‘syn/ anti’ carbonate conformer conversions. These conformational transitions must involve rotation and translation of both the carbonate units and, most importantly, the neighbouring phenyl groups. The possible influence of these conformational changes and the accompanying correlated molecular motions on polymer ductility and ageing is briefly discussed.     

An investigation in the field of aromatic heterocycles

The electronic structures of benzo[c][1,2,5]oxadiazole, benzo[c][1,2,5]thiadiazole, benzo[c][1,2,5]selenadiazole, naphtho[1,2-c][1,2,5]oxadiazole, naphtho[1,2-c][1,2,5]thiadizole, naphtho[1,2-c][1,2,5]selenadiazole, naphtho[2,3-c][1,2,5]oxadiazole, naphtho-[2,3-c][1,2,5]thiadiazole, and naphtho[2,3-c][1,2,5]selenadiazole have been investigated in the -electronic approximation by the Pariser-Parr-Pople method. Molecular diagrams have been calculated and the energies of the first singlet transitions have been calculated. A correspondence has been found between the calculated results and those obtained by experiment (UV spectra, reactivities, etc.).For Communication XVIII, see [1].Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 11, pp. 1473–1478, November, 1973.In conclusion, we express our gratitude to D. A. Bochvar and A. A. Bagatur'yants for a discussion of the results obtained.     

Electronic states and barriers to internal rotation in silaallenes

The electronic states and barriers to internal rotation in allene ( 1a ), 1-silaallene ( 2a ), and 2-silaallene ( 3a ) are investigated computationally using ab-initio molecular orbital methods. Planar geometries with two-, three-, and four-π-electron configurations have been considered as possible transition states ( 1b–3d ). Structures have been optimized at the Hartree–Fock level with a small split valence basis set (3-21G) and higher level calculations with basis sets of split valence (6-31G ) and split valence plus polarization function (6-31G *) quality include correlation energy estimates from Møller–Plesset second-and third-order perturbation theory. The electronic barrier to internal rotation in allene is estimated near 53 kcal/mol whereas the corresponding barriers in 1-silaallene and 2-silaallene are considerably smaller, ca. 35 and 20 kcal/mol, respectively. The transition states are predicted to possess bent geometries in all three molecules with open-shell singlet, three-π-electron configurations in 1 and 2 ( 1c, 2c ) but a closed-shell singlet, two-π-electron configuration in 3 (3d) .     

Vibrational analyses and barriers to internal rotation of CH3CCl2F and CH3CHClF

The i.r. spectra of gaseous and solid CH₃CCl₂F and CH₃CHClF have been recorded from 140 to 4000 cm⁻¹. The corresponding Raman spectra of the liquids have also been recorded and depolarization values have been measured for the CH₃CCl₂F molecule. Also the Raman spectrum of gaseous CH₃CHClF has been recorded. All spectra have been interpreted in detail and the 18 normal vibrations of both molecules have been characterized. Hot bands of the internal torsional mode for CH₃CCl₂F were observed at 248(2 ← 1) and 232 cm⁻¹ (3 ← 2) in the far-i.r. spectrum of the gas. These frequencies are in good agreement with those calculated using the 4·42 kcal/mole barrier obtained in an earlier Raman study. The internal torsional mode for CH₃CHClF was observed at 261 cm⁻¹ in the spectra of the solid and the three-fold periodic barrier was calculated to be 4·38 kcal/mole. The barriers are compared in a series of chloro, fluoro and bromo substituted ethane derivatives.     

Theoretical study of the barriers to internal rotation in nitrous acid

Nitrous acid, HONO, has been studied for three geometries by the ab initio LCAO SCF MO method with a basis of accurate gaussian atomic orbitals. The trans geometry is correctly predicted to be most stable, lying about 2 kcal/mole lower than the cis form, and 9 kcal/mole lower than the 90° form (experimental estimates being 0.4 and 11.6 kcal/mole, respectively). Population analysis, dipole moment components, and properties related to nuclear-nuclear and nuclear-electron potentials all show a partial breaking of the hydroxyl oxygen-nitrogen bond at 90° compared to cis and trans, as well as the effects of electronic rearrangement for nuclear screening in the high nuclear repulsion cis form. The cis to 90° barrier is dominated by the attractive components of the total energy, while the trans to 90° one is dominated by repulsive components, in agreement with our analysis and an earlier prediction by Allen.     

An investigation of energy migration in luminescent diluted Gd3+ systems

The luminescence of yttrium-diluted Gd³⁺ compounds containing efficient trap centers is reported and discussed. By using a rough approach it is possible to derive the critical distance for energy transfer between Gd³⁺ ions. This approach implies a critical Gd³⁺ concentration above which the Gd³⁺ emission is quenched. This concentration can be related to the number of Gd³⁺ neighbors involved in the energy migration process among the Gd³⁺ ions. It is shown that the critical distance for energy transfer between Gd³⁺ depends on the ionic character of the compound. In fluorides this distance is approximately 5 Å, in oxides approximately 6.5 Å. For Gd³⁺ ions in oxides on a (nearly) centrosymmetric site, this distance is approximately 5 Å, as in the fluorides. These latter phenomena can be explained concerning the type of interaction between the Gd³⁺ ions.     

Conformational analysis,barriers to internal rotation and vibrational assignment for dimethylethylamine

The IR (50–3500 cm^?1) and Raman (20–3500 cm^?1) spectra have been recorded for gaseous and solid dimethylethylamine. Additionally, the Raman spectrum of the liquid has been recorded and qualitative depolarization values have been obtained. Due to the fact that three distinct Raman lines disappear on going from the fluid phases to the solid state, it is concluded that the molecule exists as a mixture of the gauche and trans conformers in the fluid phases with the gauche conformer being more stable and the only one present in the spectra of the unannealed solid. From the temperature study of the Raman spectrum of the liquid a rough estimate of 3.9 kcal mol^?1 has been obtained for ΔH. Relying mainly on group frequencies and relative intensities of the IR and Raman lines, a complete vibrational assignment is proposed for the gauche conformer. The potential functions for the three methyl rotors have been obtained, and the barriers to internal rotation for the two CH₃ rotors attached to the nitrogen atom have been calculated to be 3.51 and 3.43 kcal mol^?1, whereas the barrier for the CH₃ rotor of the ethyl group has been calculated to be 3.71 kcal mol^?1. The asymmetric torsional mode for the gauche conformer has been observed in both the IR and Raman spectra of the gas at 105 cm^?1 with at least one hot band at a lower frequency. Since the corresponding mode has not been observed for the trans conformer, it is not possible to obtain the potential function for the asymmetric rotation although estimates on the magnitudes of some of the terms have been made. Significant changes occur in the low-frequency IR and Raman spectra of the solid with repeated annealing; several possible reasons for these changes are discussed and one possible explanation is that a conformational change is taking place in the solid where the trans form is stabilized by crystal packing forces. These results are compared to the corresponding quantities for some similar amines.     

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.