Theoretical study of azide anion addition to nonpolar and polar double and triple bonds

Transition structures for the 1,3-polar addition of azide anion to hydrogen cyanide, formaldimine, nitrogen, cis- andtrans -diazene, ethylene and acetylene were obtained at the MP2/6-31+G^* theoretical model. The additions can be divided into two groups: addition to a triple bond, giving rise to an aromatic heterocyclic product, and addition to a double bond, forming a non-aromatic product. All transition structures correspond to a concerted mechanism for the polar cycloaddition. Symmetrical dienophiles, apart from cis-diazene, give rise to synchronous transition structures. The anomaly is explained in terms of strong n-n repulsion of the reactants in the transition structure. The reactivity of the compounds can be rationalized in terms of the bond orders of the newly forming bonds, from the frontier orbital energy differences and from the charge transfer from the azide anion to the dienophile. The quantitative correlation of the reactivity has been judged on the basis of the activation energies of the reactions calculated at MP2/6-31+G^* and MP3/6-31+G^*. It is predicted that the addition of azide to nitrogen is the slowest and that the additions to hydrogen cyanide and acetylene have the lowest barriers, in agreement with literature data.     

CI calculations concerning the styrene cis-trans photoisomerization

Using minimal basis sets, the ionic and diradical excited states of the twisted styrene molecule have been calculated through extended CI, after appropriate SCF procedures. The lowest excited state is a diradical, and the photoisomerization should not involve the ionic state.     

MNDO calculations of systems containing hydrogen bonds

A modified MNDO method, which can be used in the studies on structures with hydrogen bonds X-H-X, X, X = N, O, is described. Results for a wide range of molecular complexes are reported. Energies of hydrogen bonds are reproduced with useful accuracy. The modified MNDO seems to give more reliable values of hydrogen bond energies and barrier heights of proton transfers than 4-31G ab initio model.     

MNDO calculations of systems with hydrogen bonds S-H

The MNDO method has been modified for the calculation of the properties of complexes with hydrogen bonds S-H-X, X = N, O, F, S. The results obtained are in good agreement with the experimental data.     

cis/trans photoisomerization of secondary thiopeptide bonds

The reversible cis/trans photoisomerization of secondary thiopeptide bonds has been systematically studied with UV-visible absorption, capillary electrophoresis, 1H NMR spectroscopy, and circular dichroism methods. It was found that the concentration of the cis conformers could be increased from less than 1 % in the thermal equilibrated solution to up to 20 % in the photostationary state. The rotational barriers of the thiopeptide bond and the pH dependence of the isomerization rates were also studied. The quantum yields of the trans-->cis and the cis-->trans processes were determined from photokinetic analysis.     

Complexes with hydrogen bonds by the MNDO/M method

Conclusions The results obtained show that the modified variant of the MNDO method developed in the present work makes it possible to obtain sufficiently reliable and accurate values of the energies of H bonds in neutral and charged complexes and provides perfectly satisfactory estimates of the geometric parameters of H bonds and the activation energies of proton-transfer reactions. Taking into account that MNDO calculations require relatively small expenditures of computer time (approximately 1000 times less than do nonempirical calculations in the 4-31G basis), the MNDO/M method can be used to study chemical reactions in aqueous solutions in the framework of a supermolecular approach, as well as to simulate fairly complicated biochemical processes, for example, the reactions in the active site of an enzyme.Lenin Young Communist League Novosibirsk State University. Novosibirsk Institute of Bioorganic Chemistry, Academy of Sciences of the USSR, Siberian Brach. Translated from Zhurnal Strukturnoi Khimii, Vol. 29, No. 2, pp. 31–37, March–April, 1988.     

Restricted rotation about partial C,N double bonds

The restricted rotation about the partial C,N double bond in 2-chloro-6-NR₂-pyran-4-ones is discussed in the light of NMR spectroscopic data and theoretical calculations.Ab initio calculations at the HF/6-31G^* level were carried out using a continuum model to take solvent effects into account. The delocalization of-electron density [described by natural bond orbital analysis (NBO)] was applied to determine the degree of conjugation in the ground state (GS) and in the transition state (TS) for the restricted rotation of the compounds studied. The reason for the different barriers to rotation of the NR₂ substituents (pyrrolidino > dimethylamino > morpholidino > piperidino) at the 2-chloro-pyran-4-one ring appears to be the different steric hindrance of the NR₂ substituents in the GS for the restricted rotation.     

Crosslinking study of polyethers containing pendent double bonds

This article studies the crosslinking reactions of some polyethers containing double bonds on the side chains. Thermal crosslinking takes place in all cases, but is overlapped by the degradation process. So, a catalyst has to be used to obtain the cured material and avoid degradation. The best initiator appears to be dicumyl peroxide and the maximum cross-linking degree is achieved only with 5% (w/w). Thermogravimetric analyses are used to test the quality of the cured materials. IR spectroscopy shows that crosslinking actually takes place by polymerizing pendent double bonds. Furthermore, activation energies for thermal and radical initiated crosslinking are determined by means of dynamic DSC studies. © 1995 John Wiley & Sons, Inc.     

The polar properties of valence bonds

Arguments based on bond-stretching frequencies, nuclear quadrupole coupling constants and bond lengths indicate that bonds having the same strength factor can have different electronic structures. It is then argued that a more useful probe for obtaining an insight into bond character is provided by infrared band intensities which can be analyzed to yield values for the constants occurring in bond moment functions. A new bond moment theory, in which the concept of “rehybridization moments” is introduced, is described. Using this theory, self-consistent values are obtained for the bond moment constants governing the band intensities for the molecules HCN, C₆H₆ and C₂H₄.     

A molecular orbital study of trans-polyenes to 18 double bonds

Calculations involving molecular orbitals have appeared in the chemical literature over the past few years, but all have used smalltrans-polyenes. We now report extended Huckel molecular orbital calculations ontrans-polyenes of up to 18 double bonds (36 carbons and 38 hydrogen atoms). Information obtained from these calculations include total energies, bond gaps, and net charges on each atom. Also found is that the band gap approaches 1.41 eV as the number of double bonds approaches infinity. This value is quite close to reported experimental values. Data are also presented for polyenes assuming equal C-C bond lengths.     

Chiral (cyclopentadienone)iron complexes with a stereogenic plane as pre-catalysts for the asymmetric hydrogenation of polar double bonds

In this paper, we describe a small library of easy-to-prepare chiral (cyclopentadienone)iron pre-catalysts for enantioselective CO and CN hydrogenations. Starting from readily accessible achiral materials, six chiral (cyclopentadienone)iron complexes (1a-f) possessing a stereogenic plane were synthesized in racemic form. Based on the screening of pre-catalysts (±)-1a-f in the hydrogenation of ketones and ketimines, we selected two complexes (1a and 1d) for resolution by semipreparative enantioselective HPLC. The absolute configuration of the separated enantiomers of 1a and 1d was assigned by XRD analysis (1a) and by comparison between experimental and DFT-calculated ECD and ORD spectra (1d). The enantiopure pre-catalysts (S)-1a and (R)-1d were tested in the asymmetric hydrogenation of several ketones and ketimines and showed good activity and modest enantioselectivity, the e.e. values ranging from very low to moderate (54%).     

Anionic polymerization of butadiene: A MNDO study of the potential energy surface of the propagation reaction in polar and non-polar media

The anionic polymerization of butadiene, to both the 1,2- and 1,4-addition products, is an important industrial process. It is known that the reaction can be steered to either the 1,2- or 1,4-product by the addition, or absence, of a complexing solvent such as ether. The goals of the current study were to map the MNDO Potential Energy Surfaces (PES) of the propagation reactions both in the presence and absence of dimethyl ether to gain insight into the factors influencing the reaction's selectivity. Single point ab initio calculations at the DZP level were run on all reactants, products and transition states as a cross check on the MNDO results.     

Respective contributions of polar vs enthalpy effects in the addition/fragmentation of mercaptobenzoxazole-derived thiyl radicals and analogues to double bonds

The formation and the reactivity of three selected sulfur-centered radicals formed from mercaptobenzoxazole, mercaptobenzimidazole, and mercaptobenzothiazole toward four double bonds (methyl acrylate, acrylonitrile, vinyl ether, and vinyl acetate) are investigated. The reversibility of the addition/fragmentation reaction in these widely used photoinitiating systems of radical polymerization was studied, for the first time, through the measurement of the corresponding rate constants by time-resolved laser spectroscopy. The combination of these results with quantum mechanical calculations clearly evidences that, contrary to previous studies on other aryl thiyl radicals, the addition rate constants (ka) are governed here by the polar effects associated with the very high electrophilic character of these radicals. However, interestingly, the back-fragmentation reaction (k-a) is mainly influenced by the enthalpy effects as supported by the relationship between the rate constants and the addition reaction enthalpy DeltaHR. The addition and fragmentation rate constants calculated from the transition state theory (TST) are in satisfactory agreement with the experimental ones. Therefore, molecular orbital (MO) calculations offered new opportunities for a better understanding of the sulfur-centered radical reactivity.