Localized Molecular Orbital Analysis Research Articles

An ab Initio MO Study on the Transformation of Acetylene to Vinylidene in the Coordination Sphere of Rhodium(I). The Intra- and Intermolecular Proton Transfer Mechanism

The transformation of a rhodium(I) η2-alkyne model complex RhCl(PH3)2(HC⋮CH) (A) into the vinylidene form RhCl(PH3)2(CCH2) (E) has been examined by ab initio theoretical calculations using MP2 level geometry optimizations and localized molecular orbital (LMO) analysis. The vinylidene form E has been found to be 7.8 kcal/mol more stable than A. The previously found intraligand 1,2-hydrogen shift mechanism in the Ru(II)-coordinated alkyne−vinylidene isomerization is not relevant for the present Rh system. The reaction proceeds via the oxidative addition product RhCl(PH3)2(H)(C⋮CH) (C), followed by a bimolecular hydrogen shift from the metal to the terminal carbon of a second molecule rather than by intramolecular 1,3-hydrogen transfer. The LMO analysis of the transition state of the unimolecular 1,3-hydrogen shift indicates that the hydrogen moves as a proton while it interacts with the three centers simultaneously, i.e., Rh, Cα, and Cβ in the transition state. The hydrogen was analyzed to migrate also as a proton in the bimolecular mechanism. The barrier of the bimolecular pathway has been further calculated for a more realistic system with substituted phosphines, RhCl(PiPr3)2(H)(C⋮CH), using the integrated MO + MM (MP2:MM3) method. It was concluded that in the real system with substituents on both the phosphines and the alkyne, RhCl(PiPr3)2(HC⋮CR), the bimolecular hydrogen shift is still favored by ca. 15 kcal/mol in free energy of activation; unimolecular 1,3-H migration should become important in special cases like solid state isomerizations.

Localized molecular orbital studies on B4Cl4, 1,5‐C2B3H5 and BH2‐(n = 6–10, 12)

AbstractThe wave functions, level energies and Mülliken population analysis of localized molecular orbitals (LMO's) for B4Cl4, 1,5‐C2B3H5 and the closo‐BnH2‐n (n = 6–10, 12) are calculated by using the Edmiston‐Reudenberg energy localisation scheme under the CNDO/2 approximation in order to reveal the nature of quasi‐aromaticity of the closo‐BnH2‐n (n > 5). It has been found that all the B‐H or B‐Cl LMO's are highly localised between the B and H (or Cl) atoms, corresponding to B—H or B—Cl σ‐bond, while the Bn framework bonding is formed mainly by the three‐centered two‐electron B‐B‐B bonds on the polyhedral faces. In the cases of B4Cl4 and 1,5‐C2B3H5, these three‐centered B‐B‐B bonds just fill their polyhedral faces; however, for the framework bonding of the closo‐BnKn2‐ (n > 5), the valence electron deficiency leads to the delocalisation of their three‐centered B‐B‐B bonds, and as delocalisability of this three‐centered B‐B‐B bond increases, some three‐centered B‐B‐B bonds are farther delocalized to become a four‐centered B‐B‐B‐B bond. It is important that for the closo‐BnHn2‐ (n > 5), the sequence of this delocalisability of the three‐centered B—B—B bond is consistent with the degree of quasiaromaticity of the closo‐boranes.

Theoretical Studies of Mg(1S, 3P) Atom Reaction Mechanisms with HF, H2O, NH3, HCl, H2S, and PH3 Molecules

Abstract The reaction mechanisms of Mg (1S and 3P) atom and XH molecules (X = F, OH, NH2, Cl, SH, and PH2) were studied by ab initio molecular orbital methods. A localized molecular orbital centroid analysis along the reaction coordinate shows that the Mg(1S) atom insertion into the X–H bonds occurs through the pull–push mechanism. The activation barriers for the Mg(1S) atom insertion are approximately the same values for each groups (X = F and Cl, OH and SH, and NH2 and PH2). For the Mg(3P) atom reactions with above six compounds, the hydrogen abstraction (produces MgH and H) and/or the exchange reaction (produces MgX and H) were studied. Both reactions of Mg(3P) atom are the same one with two-step mechanism. The first step is an electron transfer from a Mg atom into the X–H bonds, which corresponds the transition state. The activation barrier height of the reaction is explained by adiabatic electron affinity of the X–H bonds. The second step is the formation of the products (Mg–X+H or Mg–H+X), which is clarified by the difference of Mg–X and Mg–H bond energies.

Ab initio calculation of 14N and 35Cl nuclear quadrupole coupling constants in aziridine and Cl-aziridine

Ab initio calculations of electric field gradients (EFG) at 14N and 35Cl nuclei and conversion to the corresponding nuclear quadrupole coupling constants are reported for aziridine and Cl-aziridine molecules. The structural parameters for Cl-aziridine used for these calculations are obtained as a result of gradient optimization at the HF/4–31G ∗ level. The mutual orientation of the molecular axes and principal axes of the EFG tensor is presented. To evaluate the main contribution from various bonds and lone pairs to the EFG tensor components, a localized molecular orbital (LMO) analysis is made. The comparison of the results of the calculation with experiment is discussed.

Characterization of aluminum atom insertion mechanisms into fluorine-hydrogen, oxygen-hydrogen, nitrogen-hydrogen, chlorine-hydrogen, sulfur-hydrogen and phosphorus-hydrogen bonds by ab initio MO methods

The insertion mechanisms of an aluminum atom into the X-H bond (X=F, O, N, Cl, S, and P) of six compounds, HF, H 2 O, NH 3 , HCl, H 2 S, and PH 3 , were investigated with ab initio molecular orbital methods. Each compound forms an addition complex with an Al atom. A localized molecular orbital centroid analysis along the reaction coordinate shows that the insertion is a two-step reaction.

The structure and bonding in group IV [1.1.1]Propellanes

The structures of Group IV [1.1.1]propellanes are determined using ab initio all-electron and effective core potential methods. The bonding in these systems is examined using the theory of atoms in molecules and localized molecular orbital analysis. Singlet-triplet splittings are also calculated. Comparisons with the corresponding bicyclopentanes are made. The bonding interaction between the two bridgehead atoms decreases upon descending the group. Further, the similarities in the internuclear bridgehead region between the propellanes and bicyclopentanes increase upon descending the group.

ChemInform Abstract: COMPARATIVE ANALYSIS OF THE ELECTRONIC STRUCTURE OF POSITIONAL ISOMERS: INDOLE-ISOINDOLE

On the basis of an analysis of the canonical and localized molecular orbitals of indole and isoindole, calculated in the SCF and CNDO/2 approximations, as well as an x-ray crystallographic investigation of 2-methyl-isoindole, a comparison of the electronic structure of the positional isomers was made. The 10π-electronic system of isoindole is more integral than for indole: isoindole is a single 10π-electronic system with an appreciable localization of the bonds in the carbocyclic portion of the bicycle; the electronic structure of indole can be represented in a first approximation as the aggregate of three weakly interacting π-subsystems: the benzene ring, the double bond between the α- and β-carbon atoms, and the free electron pair of the nitrogen atom.

Strictly localized molecular orbitals

Some possibilities and limitations of the strictly localized model of molecular wave functions are discussed in the light of CNDO/2 calculations. The analysis of non-orthogonal localized molecular orbitals obtained from usual SCF wavefunctions gives some insight into the effects omitted from the strictly localized model.

Localized atomic orbitals for atoms in molecules. I. Methodology

A method has been devised for the construction of localized atomic orbitals (LAOs) for atoms in molecules. The LAOs reflect the atomic hybridizations suggested by the localized molecular orbital (LMO) analyses of Edmiston and Ruedenberg. The procedure involves definition of a new function, L(Aa‖A?), a measure of a particular type of exchange energy associated with the distribution between AOs a and ? on atom A. The terms L(Aa)=L(Aa‖Aa) constitute an atomic localization sum, L(A), which is required to be a maximum in the LAO basis on A. The properties of the components of L(A) are explored so as to achieve maximal computational efficiency. The maximization process is contingent upon an iterative sequence of 2×2 rotations. In this paper it is shown that the method will yield satisfactory results for all closed shell free atoms plus for the F atom in the HF molecule. The minimal basis of LAOs on F in HF are partitioned as (1) a core orbital with a population of about two electrons, (2) three trigonally equivalent lone pair orbitals each with a population of about two electrons, and (3) a bonding orbital pointing toward H with a population of somewhat less than one electron. The results match both classical and LMO predictions. In general, the LAO and LMO methods, although independent, yield complementary pictures of localized valence and bonding, as is shown further in the second publication of this series. There are, however, several features of the LAO method for molecules which may make it an attractive alternative to the LMO process, including computational time requirements for larger systems and a much preferable description of the valence in C2.

Localized atomic orbitals for atoms in molecules. II. Diatomic molecules

Using a previously described method, localized atomic orbitals (LAOs) for atoms in molecules are found for the atoms Li, B, C, N, O, and F in the diatomic molecules LiH, Li2, LiF, BH, B2, BF, C2, CO, NH, N2, and F2. In all instances LAOs partition into sets of core, lone pair, and bonding orbitals which, except in C2, are arranged according to the hybridization schemes suggested by the localized molecular orbital (LMO) analyses of Edmiston and Ruedenberg. C2 is of special importance as the LMO result for this molecule is unorthodox and difficult to interpret whereas the LAO description is more lucid and enlightening. Generally, core LAOs are doubly occupied and lone pair LAOs are either unoccupied (Li) or doubly occupied. Bonding is usually described principally as the interaction of bonding LAOs on the adjacent, bonded atoms. Exceptions to these generalities are found for C2 (not a Lewis compound) and NH (considered in an excited state). The composition of core LAOs for atoms in molecules is found to be a function primarily of the atom itself and is rather insensitive to chemical environment of the atom. On the other hand, composition of the valence LAOs is found to vary in a meaningful manner reflecting changes in both the number and orbital arrangement of valence electrons.

Theory of lone pairs. II. A moment analysis of localized molecular orbitals in ten‐electron hydrides

AbstractAb initio double‐zeta quality molecular orbital calculations have been carried out on an extensive series of ten‐electron hydrides. The Edmiston–Ruedenberg energy‐localized molecular orbitals were calculated and the total molecular and localized orbital densities analyzed in terms of dipole moments, contour plots, and a simplified model for the orbital density involving analysis of the first and second moments. The simplified moment analysis model may be easily visualized in terms of threedimensional geometric objects, spheres, and ellipsoids. The model summarizes the information on the effective functional distribution inherent in the more detailed orbital contour plots in a clear and concise manner.