The application of strictly localized geminals to the description of chemical bonds

A theory is developed in which closed-shell molecules are viewed as systems of weakly interacting chemical bonds. Composite-particle creation operators obtained by an appropriate quasiparticle transformation are used to create the wave function of two-electron bonds. These quasiparticles are bosons, since they are composed of two electrons, but the total many-electron wave function is properly antisymmetric. The internal structure of the quasi-Bose-particles is affected by inductive interbond interactions. Delocalization and dispersion interactions between different bonds are neglected, thus the approach corresponds to a first-order many-body PT (Perturbation Theory) with a correlated, but fully localized, reference state. The whole formalism is developed ab initio. The nonorthogonality problem is handled by a biorthogonal formulation. To illustrate the effectiveness of the model, numerical calculations are reported.     

Vibronic coupling in cyclopentadienyl radical: a method for calculation of vibronic coupling constant and vibronic coupling density analysis

A method of calculation of vibronic or electron-phonon coupling constant is presented for a Jahn-Teller molecule, cyclopentadienyl radical. It is pointed out that symmetry breaking at degenerate point and violation of Hellmann-Feynman theorem occur in the calculations based on a single Slater determinant. In order to overcome these difficulties, the electronic wave functions are calculated using generalized restricted Hartree-Fock and complete active space self-consistent-field method and the couplings are computed as matrix elements of the electronic operator of the vibronic coupling. Our result agrees well with the experimental and theoretical values. A concept of vibronic coupling density is proposed in order to explain the order of magnitude of the coupling constant from view of the electronic and vibrational structures. It illustrates the local properties of the coupling and enables us to control the interaction. It could open a way to the engineering of vibronic interactions.     

Spin-orbit vibronic coupling in 3Pi states of linear triatomic molecules

The Renner-Teller vibronic-coupling problem of a 3Pi electronic state of a linear molecule is analyzed with the inclusion of the spin-orbit coupling of the 3Pi electronic state, employing the microscopic (Breit-Pauli) spin-orbit coupling operator for the two unpaired electrons. The 6x6 Hamiltonian matrix in a diabatic spin-electronic basis is obtained by an expansion of the molecular Hamiltonian in powers of the bending amplitude. The symmetry properties of the Hamiltonian with respect to the time-reversal operator and the relativistic vibronic angular momentum operator are analyzed. It is shown that there exists a linear vibronic-coupling term of spin-orbit origin, which has not been considered so far in the Renner-Teller theory of 3Pi electronic states. While two of the six adiabatic electronic wave functions do not exhibit a geometric phase, the other four carry nontrivial topological phases which depend on the radius of the integration contour. The spectroscopic effects of the linear spin-orbit vibronic-coupling mechanism have been analyzed by numerical calculations of the vibronic spectrum for selected model examples.     

Fermi level alignment in self-assembled molecular layers: the effect of coupling chemistry

Photoelectron spectroscopy was used to explore changes in Fermi level alignment, within the pi-pi* gap, arising from modifications to the coupling chemistry of conjugated phenylene ethynylene oligomers to the Au surface. Self-assembled monolayers were formed employing either thiol (4,4'-ethynylphenyl-1-benzenethiol or OPE-T) or isocyanide (4,4'-ethynylphenyl-1-benzeneisocyanide or OPE-NC) coupling. The electronic density of states in the valence region of the two systems are nearly identical with the exception of a shift to higher binding energy by about 0.5 eV for OPE-NC. Corresponding shifts appear in C(1s) spectra and in the threshold near E(F). The lack of change in the optical absorption suggests that a rigid shift of the Fermi level within the pi-pi* gap is the major effect of modifying the coupling chemistry. Qualitative consideration of bonding in each case is used to suggest the influence of chemisorption-induced charge transfer as a potential explanation. Connections to other theoretical and experimental work on the effects of varying coupling chemistries are also discussed.     

En route to a molecular sheaf: active metal template synthesis of a [3]rotaxane with two axles threaded through one ring

We report that a 2,2':6',2″-terpyridylmacrocycle-Ni complex can efficiently mediate the threading of two alkyl chains with bulky end groups in an active metal template sp(3)-carbon-to-sp(3)-carbon homocoupling reaction, resulting in a rare example of a doubly threaded [3]rotaxane in up to 51% yield. The unusual architecture is confirmed by X-ray crystallography (the first time that a one-ring-two-thread [3]rotaxane has been characterized in the solid state) and is found to be stable with respect to dethreading despite the large ring size of the macrocycle. Through such active template reactions, in principle, a macrocycle should be able to assemble as many axles in its cavity as the size of the ring and the stoppers will allow. A general method for threading multiple axles through a macrocycle adds significantly to the tools available for the synthesis of different types of rotaxane architectures.     

Regioselectivity in multiple cycloadditions to fullerene C60: vibronic coupling density analysis

We theoretically investigate multiple cycloadditions of butadienes to fullerene C₆₀ with the concept of vibronic coupling density (VCD) as a reactivity index. Recently, we have clarified that C₆₀ bears ethylene moieties at its reactive sites as functional groups, on the basis of VCD. In this study, we calculate the VCDs of C₆₀ adducts with n butadienes (n=1, …, 5) and show that the ethylene moieties are conserved with almost the same distributions as those in C₆₀. The VCD analysis predicts that the hexakisadduct with a six-fold axis is finally obtained, which is consistent with experimental observations.     

The interaction of chemical bonds. III. Perturbed strictly localized geminals in LMO basis

The formalism of strictly localized geminals (SLGs ) is summarized. It is shown that the SLG wave function serves as an appropriate multiconfigurational reference state that can easily be improved by perturbational, CI - or coupled cluster-type procedures. The possibility of expanding the geminals in the basis set of localized Hartree-Fock molecular orbitals (LMOs ) is discussed. Sample calculations on H₄, CH₄, H₂O, and He…?He systems are reported. © 1994 John Wiley & Sons, Inc.     

Triplet excitation transfer through the walls of hemicarcerands: dependence of the electronic coupling on the size of the molecular cage.

Triplet excitation transfer from biacetyl trapped inside three hemicarcerands of different size (1, 2, and 3) to acceptors in the surrounding medium was investigated. The largest hemiracerand 1 employs four butyl linkers and the intermediate hemicarcerand 2 four o-xylyl linkers. The smallest hemicarcerand 3 contains only three methylene linkers. Both neat liquid triplet acceptors and acceptors dispersed in solvents were used. The primary objective of this work was to determine the dependence of the energy transfer rate on the size and the electronic structure of the molecular cages. There is a pronounced, more than 10-fold, increase of triplet energy transfer rates with decreasing size of the cage. The corresponding electronic coupling, /V/, increases approximately by a factor of approximately 3.5 from the largest hemicarceplex 1 to the smallest hemicarceplex 3. This increase of the electronic interaction is similar to that observed in covalently bound systems when the distance between the triplet donor and the acceptor is reduced by one carbon-carbon sigma-bond. The electronic structure of the hemicarcerand appears to be of secondary importance, at least when T(1) states of the donor and the acceptor are far from a resonance with the T(1) state of the cage. A very good agreement between the results obtained in neat acceptors and in solution was found, indicating that the association between the acceptors and the molecular cages is negligible, if at all present. An unexpectedly large interaction between the guest and the polarizable walls of the hemicarcerands manifested by emission red-shifts was observed in all cases. This suggests that the entrapment within the molecular cage gives rise to an environment considerably different from that of a single molecule in the gas phase. An interesting correlation between the magnitude of the phosphorescence spectral shift, Deltanu(0-0), and the guest-to-external acceptor electronic coupling, /V/, was found.     

Intramolecular aryl-aryl coupling of fluoroarenes through Al2O3-mediated HF elimination

A strategy for effective intramolecular aryl-aryl coupling of fluoroarenes through Al(2)O(3)-mediated HF elimination is reported. It is demonstrated that the C-F bond, which is widely believed to be the most passive functionality in organic chemistry, can be reconsidered as a useful functional group allowing very effective C-C bond formation. The solid-state strategy presented in this study opens the possibility for facile syntheses of insoluble extended polycyclic aromatic hydrocarbons.     

Native chemical ligation through in situ O to S acyl shift

[reaction: see text] A novel strategy to generate thioester peptides compatible with Fmoc chemistry is presented. Peptide-C(alpha)oxy-(2-mercapto-1-carboxyamide)ethyl ester undergoes an O to S acyl shift during ligation and the newly formed thioester intermediate reacts with an N-terminal cysteine fragment generating a product with native amide bond at the ligation site.     

The effect of spin-orbit coupling on fast neutral chemical reaction O(3P)+CH3-->CH3O

The effect of nonadiabatic transitions through the spin-orbit couplings has been investigated on the fast neutral reaction, O((3)P)+CH(3)-->CH(3)O. Adiabatic potential energies and the spin-orbit coupling terms have been evaluated for the four electronic states of CH(3)O ((2)E, (2)A(2), (4)E, and (4)A(2)) that correlate with the O((3)P)+CH(3) asymptote, as a function of CO distance and OCH angle under the C(3v) symmetry, by ab initio electronic structure calculations using multireference internally contracted single and double excitation configuration interaction method with the 6-311G(2df,2pd) basis sets. Multistate quantum reactive scattering calculations have been carried out with the use of thus obtained potential energies and spin-orbit coupling matrices, based on the generalized R-matrix propagation method. The calculated thermal rate constants show a slight positive dependence on temperature in a range between 50 and 2000 K, supporting the previous experimental results. It is shown that the spin-orbit coupled excited states give rise to reflections over the centrifugal barrier due to the quantum interference. Classical capture calculations yield larger rate constants due to the neglect of quantum reflections. It is concluded that the effect of nonadiabatic transitions is of minor importance on the overall reactivity in this reaction.     

A method to reduce the size of the vibronic basis employed in the simulation of spectra using the multimode vibronic coupling approximation

In the time-independent multimode approach for the determination of vibronic spectra involving strongly coupled electronic states, the equilibrium geometry and normal modes of the reference or precursor state are usually employed as the basis for the multimode expansion. This basis, while easily constructed, is generally ill-suited for determining the eigenstates of the observed species. Employing a more computationally effective basis requires the evaluation of Franck-Condon overlap integrals. Using established generalized Hermite polynomial generating function formalisms, an algorithm is developed that can efficiently determine the enormous requisite number of these overlap integrals. It is found that this flexibility in the choice of multimode basis can significantly reduce the size of the basis needed to obtain converged spectral simulations. The previously reported spectrum of the ethoxy (C(2)H(5)O) radical serves as an example of the efficacy of the new technique.     

Comparative study of weak interactions in molecular crystals: H-H bonds vs hydrogen bonds

The crystal structures of tetraphenylphosphonium squarate, bianthrone, and bis(benzophenone)azine are shown to contain a variety of C-H(delta+)...(delta+)H-C interactions, as well as a variety of C-H...O and C-H...C(pi) interactions. Each of these molecules possesses interactions that can possibly be characterized as either H-H bonds or weak hydrogen bonds based on the first four criteria proposed by Koch and Popelier. These interactions have been completely characterized topologically after the multipole refinement of the structures. It appears that weak interactions of the form C-H(delta+)...(delta+)H-C possess certain correlations between the various properties of the electron density at the bond critical points. The coexistence of the three types of interactions makes it possible to establish similarities and differences in the correlations of these weak interactions. This all leads to a better understanding of H-H interactions and how they fit into the hierarchy of weak interactions.     

Intermediate vibronic coupling in charge transfer states: comprehensive calculation of electronic excitations in sexithiophene crystal

A comprehensive theory of linear vibronic coupling in a coupled manifold of Frenkel and charge-transfer states in an infinite molecular crystal is presented and applied for sexithiophene. The approach, valid in the intermediate-coupling regime, includes up to three-particle terms of the Philpott expansion, with the vibronic wavefunctions represented in the Lang-Firsov basis. As a stringent test, the scheme is used to reproduce the complete set of available sexithiophene absorption and electroabsorption spectra within a unified theoretical framework. The input is based primarily on independent calculations and to some extent on independent experiments, with explicit fitting contained within the limits set by the estimated inherent errors of a priori parameter estimates. Reasonably good quantitative agreement with experimental spectra is achieved. The results resolve some existing interpretational ambiguities and expose some peculiarities of electric field effect on vibronic eigenstates of Frenkel parentage, highlighting the role of charge-transfer interactions.     

Reaction mechanism in the mechanochemical synthesis of dibenzophenazine: application of vibronic coupling density analysis

The reaction mechanism for mechanochemical synthesis of dibenzophenazine was theoretically investigated in terms of the vibronic coupling density, which describes the interactions between electrons and nuclear motions. The concept theoretically indicates experimentally observed reactive sites that cannot be explained by the conventional frontier orbital theory. The results of vibronic coupling density analysis suggested the difference between reaction mechanisms under thermal and mechanochemical conditions.