Molecular orbital calculations on the ethylene episulfide molecule and its isomers

Systematic non-empirical and semi-empirical molecular orbital studies have been performed on the sulfur atom + ethylene system and its reaction products thiirane and vinylmercaptan. The results are in general agreement with earlier qualitative and semiquantitative rationalizations of the reaction but also bring to light a number of novel features.     

An ab initio study of halogen—olefin molecular complexes

A systematic computational study, using the 3–21G basis set, of the F₂/CH₂H₄ system shows that several molecular complex configurations can exist and that the axial-perpendicular type is not the most stable. Full geometry optimization results for F₂/C₂H₄ and Cl₂/C₂H₄ in axial-perpendicular configurations are reported and shown to be improvements over previous calculations.     

The π-donating ability of heteroatoms in α-substituted methyl cations

An ab initio SCF geometry optimization on the simple cations H₂X, with X = F, Cl, NH₂ and PH₂ has been performed at the split valence shell (4-31G) level. The computed optimum conformations correspond in each case to a structure in which all atoms lie in the same plane. Comparison of the computed charge distributions reveals that the third period heteroatom (Cl and P) is a better π electron donor than the corresponding second period analogue (F and N). This result parallels that obtained recently [Can. J. Chem.53, 1144 (1975)] for S and O in H₂OH and H₂SH, but contradicts current notions based on assumed values of the C_π-X_π overlap integrals. It is shown here by explicit calculations of overlap integrals that these assumptions are not always correct. Furthermore, it is shown that arguments based only on overlap are necessarily incomplete since they neglect terms like the energy difference between the interacting orbitals which can play a dominant role. The relative importance of such terms is discussed for these species.     

Study of the quality of Gaussian basis sets for carbon and silicon: Calculations on methane and silane

A number of Gaussian basis sets for carbon and silicon have been examined in terms of the one-electron properties of methane and silane. The convergence of the properties to their limiting values is not monotonic but, in general, a representation that involves five Gaussian functions per occupied atomic orbital on the heavy atom is sufficient to closely approach the limits. A relationship between the sizes and partitioned electronic energies is shown to hold to a good approximation for the Boys spatially localized molecular orbitals employed in this study.     

Gaussian wave functions for CH3 and NH

Ground state single determinant LCAO-MO-SCF wave functions, using a large contracted Gaussian basis set (6^s, 2^p, 1^d/3^s, 1^p), have been computed for the 9 electron molecular systems of CH₃ and NH. The minimum energies obtained using Roothaan's open shell SCF procedure for the planar equilibrium geometries were ?39.5703 Hartree for CH₃ and ?55.8945 Hartree for NH. Additional properties such as electron populations and multiple moments were calculated from the planar wave functions.     

Conformational energy surfaces of triplet-state isomeric methyloxiranes

A conformational study was carried out on the three ring-opened structures of triplet methyloxirane with a minimal Gaussian basis set, within the unrestricted Hartree–Fock framework. For the two structures energy surfaces E(θ₁, θ₂) were generated, where θ₁ measures the methyl rotation and θ₂ is associated with the torsion about the other C? C bond. For the third structure an energy hypersurface E(θ₁, θ₂, θ₃) was generated, where energy was a function of methyl rotation θ₁ and two nonequivalent C? O rotations θ₂ and θ₃. Analysis of the surfaces revealed the locations and relative energies of the critical points (minima, saddle points, and maxima). The overall stereochemical finding was that these ring-opened triplet C₃H₆O species possessed rather flexible structures.     

Amidicity change as a significant driving force and thermodynamic selection rule of transamidation reactions. A synergy between experiment and theory

Although essential in medicinal and industrial chemistry, transamidation reactions are still poorly understood mechanistically and in particular in terms of the extreme nature for their proceeding either very smoothly or not occurring at all. As yet, there exists no qualitative rule to predict the outcome of an amide interacting with an amine, with quantitative evaluations far from being established. In this paper we aim to clarify the thermodynamic selection rule and driving force of transamidation reactions based on amidicity value, measuring numerically the amide bond strength, toward providing a relatively simple protocol for practicing organic chemists to predict the outcome of an experiment. The change of amidicity over the course of a reaction made it possible to see that the process is favorable or unfavorable. This recently evaluated driving force of amidicity behaves analogously to the driving force of aromaticity in other organic reactions. This paper presents a successful comparison between empirical synthetic results and relevant computational characterizations, for a variety of transamidation reactions, all toward a synergy between experiments and theory. In this paper, we are re-examining experimentally and theoretically earlier experimental findings in relation to transamidation reactions and interpreting them from the aspect of amidicity change and stabilization enthalpies.     

The influence of exocyclic phosphorous substituents on the intrinsic stability of four-membered heterophosphetes: a theoretical study

Four-membered heterophosphetes such as oxaphosphetes, thiaphosphetes, and azaphosphetes have long been considered desirable target molecules for organic chemists because of their interesting structural features. In spite of extensive investigation, only one azaphosphete and one thiaphosphete have been synthesized to date. In this paper, two possible conformers of these four-membered rings, as well as their open-ring phosphorane forms with a set of exocyclic substituents and a few ring heteroatoms were studied by molecular computations. The results suggested that the relative stability of these compounds is strongly dependent on the electronic effect of the exocyclic P-substituents. Three different types of exocyclic substituents X were recognized. However, only the strong electron-withdrawing substituents (X=F, CN, OCN, SCN) were able to stabilize the ring forms, providing the possibility to design stable heterophosphetes on the basis of the present computational results.     

Hydrogen bonds: relation between lengths and electron densities at bond critical points

The electron densities for a number of molecules with either inter- or intra-molecular hydrogen bonds are analyzed using the theory of atoms in molecules. The levels of theory used include second order M?ller Plesset and density functional methods. The molecules investigated ranges from small molecules/ions to an alanine octa-peptide. The hydrogen bond length, BL, varies from 1.15 to 3.01 ? and ρ_b, the electron density at the bond critical point, spans the interval 0.0033 to 0.168 (au). We find that the data may be represented by the relation ), where A and B are empirical constants. The relation is compared to a similar relation derived from solid state experiments. Since the ρ_b values are related to the bond strengths, this general relationship may useful for hydrogen bond studies.     

para‐Nitroblebbistatin,the Non‐Cytotoxic and Photostable Myosin II Inhibitor

Blebbistatin, the best characterized myosin II‐inhibitor, is commonly used to study the biological roles of various myosin II isoforms. Despite its popularity, the use of blebbistatin is greatly hindered by its blue‐light sensitivity, resulting in phototoxicity and photoconversion of the molecule. Additionally, blebbistatin has serious cytotoxic side effects even in the absence of irradiation, which may easily lead to the misinterpretation of experimental results since the cytotoxicity‐derived phenotype could be attributed to the inhibition of the myosin II function. Here we report the synthesis as well as the in vitro and in vivo characterization of a photostable, C15 nitro derivative of blebbistatin with unaffected myosin II inhibitory properties. Importantly, para‐nitroblebbistatin is neither phototoxic nor cytotoxic, as shown by cellular and animal tests; therefore it can serve as an unrestricted and complete replacement of blebbistatin both in vitro and in vivo.