Showing papers on "HOMO/LUMO published in 1989"

Theory of photoinduced intramolecular electron transfer in condensed media

Abstract: In this paper, we are concerned with the theoretical treatment of intramolecular electron transfer (ET). In particular we are interested in the effect of the chemical groups connecting between donor and acceptor groups on electron transfer. We treat two cases. Type 1 refers to the situation where the energy difference between the LUMO of the connecting groups and the LUMO of the donor group is much larger than vibrational energies. Type 2 refers to the situation where the energy difference between the LUMO of the connecting groups and the LUMO of the donor group is comparable to vibrational energies. We show that Type 1 ET is similar to the Raman scattering while Type 2 ET is similar to the resonance Raman scattering.

Alkoxide Clusters of Molybdenum and Tungsten as Templates for Organometallic Chemistry: Comparison with Carbonyl Clusters of the Later Transition Elements

Abstract: Alkoxide and carbonyl ligands complement each other because they both behave as “π buffers” to transition metals. Alkoxides, which are π donors, stabilize early transition metals in high oxidation states by donating electrons into vacant dπ orbitals, whereas carbonyls, which are π acceptors, stabilize later transition elements in their lower oxidation states by accepting electrons from filled dπ orbitals. Both ligands readily form bridges that span MM bonds. In solution fluxional processes that involve bridge–terminal ligand exchange are common to both alkoxide and carbonyl ligands. The fragments [W(OR)3], [CpW(CO)2], [Co(CO)3], and CH are related by the isolobal analogy. Thus the compounds [(RO)3W W(OR)3], [Cp(CO)2WW(CO)2Cp], hypothetical [(CO)3CoCo(CO)3], and HCCH are isolobal. Alkoxide and carbonyl cluster compounds often exhibit striking similarities with respect to substrate binding—e.g., [W3(μ3-CR)(OR′)9] versus [Co3(μ3-CR)(CO)9] and [W4(C)(NMe)(OiPr)12] versus [Fe4(C)(CO)13]—but differ with respect to MM bonding. The carbonyl clusters use eg-type orbitals for MM bonding whereas the alkoxide clusters employ t2g-type orbitals. Another point of difference involves electronic saturation. In general, each metal atom in a metal carbonyl cluster has an 18-electron count; thus, activation of the cluster often requires thermal or photochemical CO expulsion or MM bond homolysis. Alkoxide clusters, on the other hand, behave as electronically unsaturated species because the π electrons are ligand-centered and the LUMO metal-centered. Also, access to the metal centers may be sterically controlled in metal alkoxide clusters by choice of alkoxide groups whereas ancillary ligands such as tertiary phosphanes or cyclopentadienes must be introduced if steric factors are to be modified in carbonyl clusters. A comparison of the reactivity of alkynes and ethylene with dinuclear alkoxide and carbonyl compounds is presented. For the carbonyl compounds CO ligand loss is a prerequisite for substrate uptake and subsequent activation. For [M2(OR)6] compounds (M = Mo and W) the nature of substrate uptake and activation is dependent upon the choice of M and R, leading to a more diverse chemistry.

Solvent effects resulting from frontier molecular-orbital interactions. Part 2. The Diels–Alder reaction between 5-substituted 1,4-naphthoquinones and 2,3-dimethylbutadiene

Abstract: The rates of the Diels–Alder reactions between 2,3-dimethylbutadiene and 5-substituted 1,4-naphthoquinones in different solvents have been measured and found to give hyperbolic correlations with the acceptor numbers of the solvents. This can be interpreted in terms of FMO interactions between the solvent, acting as an electrophile, and naphthoquinones, which influence the LUMO energy of the solvated quinones. Linearization of the hyperbolic functions and comparison of their slopes suggest that the substituent has little influence on the solvent effect.

Interchromophoric homoconjugation effect and intramolecular charge-transfer transition of the triptycene system containing a tetracyanoquinodimethane chromophore

Abstract: The mechanism of the intramolecular charge-transfer (CT) transition of 2,2′-(9,10-dihydro-9,10-o-benzenoanthracene-1,4-diyliidene)bispropanedinitrile (1), a triptycene system containing a tetracyanoquinodimethane (TCNQ) chromophore, has been clarified by the π-electron SCF-Cl-dipole velocity (DV) MO calculation. The intramolecular CT transition of (1)(λmax 535 nm, log Iµ 3.40) accepts absorption intensity from the intense π→π* transition of the TCNQ chromophore (λmax 409 nm, log Iµ 4.66), as follows. The HOMO of the TCNQ group mixes with the HOMO of two benzene groups, by the interchromophoric homoconjugation of a through-space interaction, to build up the new HOMO of the combined system. The CT transition from the new HOMO to the LUMO of TCNQ moiety is partially allowed because the transition includes part of the allowed TCNQ transition. The intensity of the CT transition therefore directly reflects the degree of mixing between the HOMOs of TCNQ and two benzene moieties. The SCF-Cl-DV MO calculation corroborates the present mechanism. When the ratio of β(homoconjugation)/β(aromatic) is 35%, the calculated curve is in good agreement with the observed electronic spectrum: Calc., λmax 537 nm (log Iµ 3.50), λmax 406 nm (log Iµ 4.59); Found, λmax 535 nm (log Iµ 3.40), λmax 409 nm (log Iµ 4.66). As illustrated in Figure 5, the HOMO of the total system obviously contains the HOMO of the TCNQ group. From the results of the calculation the interchromophoric homoconjugation effect β(homo) of the triptycene system was estimated to be ca. 30% of the regular aromatic conjugation β(arom).

Spectral Sensitization of Silver Halides: The Last Twenty Years

Abstract: The progress in spectral sensitization of silver halides in the last twenty years is altogether reflected ill the work published by Paul Gilmal/. Photo-induced electron and IlDle injection betweetJ the silver halide and the spectral sensitizing dye, as well as betweel/ sensitizer and supersensitizer, are the essential events ill spectral sellsitization. These phenomena are dependent on the relatiollships between the valence and conduction band ellergy of the silver halide all the one Iland and between the HOMO and LUMO energies of the spectral sensitizer on the oilIer hand, where the relative position of the dye’s energy levels can be evalrlated from its electrochemical reduction and oxidation half-wave potential. In the future, expanding activities are to be expected in the investigation of the kinetics of the competing primary processes whicll ultimately determine spectral sellsitization.

The Ambident Nucleophilic Center. Stereochemical Consequences of HOMO‐LUMO and HOMO‐HOMO Dominant Processes

Abstract: The nitrogen centers of pyridine, quinoline, pyrrole anion, carbazole anion, and acetonitrile possess, in each case, two energetically non-equivalent electron pairs: one, a non-bonding orbital, lies in the molecular plane or on the bond axis; the other is part of a π-type orbital, and is perpendicular to the molecular plane or the bond axis. Simple molecular orbital considerations predict that the π-type orbital will always be the higher lying and, therefore, the preferred donor orbital in a HOMO-LUMO controlled frontier molecular orbital interaction with the HOMO and LUMO of an electrophilic reagent. This stereoelectronic effect seems to have been observed experimentally in the case of carbazole anion, but not in the cases of quinoline or acetonitrile (present work). The reasons for the different behavior of these nucleophiles have been analyzed using the semi-empirical procedure AMI. It is pointed out that the lower lying non-bonding orbital can control the stereochemical course of the reaction when HOMO-HOMO interactions dominate. This will occur when the non-equivalent electron pairs of the nucleophile lie close to the HOMO of the electrophilic reagent, as is the case in the reactions of pyridine, quinoline, and acetonitrile with methyl choride or trimethyloxonium cation. Transition structures for N-methylation of acetonitrile and pyrrole anion have been located and are, respectively, linear and planar. However, a 45° bend of the pyrrole transition structure leads to only a 29-fold rate retardation at 25°C, compared to the 300-fold retardation calculated for acetonitrile.

Mndo molecular orbital analyses of models for proanthocyanidin-methylolphenol reactions

Abstract: The MNDO molecular orbital method has been applied to molecular structures that model or react with proanthocyanidins. Atom net charge distribution, structural parameters, heats of formation, and ionization potentials are evaluated for: (+)-catechin, (−)-epicatechin, catechol, resorcinol, phloroglucinol, o- and p-methylolphenol, and o-and p-benzoquinone methide in an investigation of chemical reactivity under different pH conditions. Other MO calculations are reviewed, and the optimized MNDO MO parameters are compared with existing literature data. The total net and HOMO electronic charge distribution is evaluated in terms of nucleophilicity and chemical reactivity. The HOMO frontier orbitals in polyhydroxybenzene and the LUMO orbitals in the benzoquinone methides play an important role in chemical reactivity.

Electronic structure of Friedel-Crafts catalysts, an ab initio study of the RF-BF3 adducts

Abstract: By means of ab initio calculations using several basis sets (STO-3G, 3-21G, and 6-31 +G), the minimum-energy structures for the RF–BF3 intermediates (R = H, Me, and Et) of the Friedel–Crafts alkylation reaction have been obtained. The role of the catalyst in the process through an analysis of dissociation energies, atomic charges, and LUMO energies of reaction intermediates, and the same quantities for the RF species, has been studied.

Studies on the relationship between the MO indices and anticholinesterase activity of suberogorgin

Abstract: The molecular orbital (MO) reactivity indices of a new potent reversible anticholinesterase (anti-ChE) agent, Suberogorgin (S), and its derivatives (S-E; S-OH, α and β forms; S-H, α and β forms also) were calculated using the MNDO method; and their anti-ChE activities against Torpediniforms Narcline timilei (Torpedo) acetylcholinesterase (AChE) were studied. It was found that the energy of lowest unoccupied MO (E lumo ), nucleophilic frontier orbital charges (f n r) of C4 and C5 (double bond) and the net charges of C9 (carbonyl carbon) andC13 (carboxyl carbon) were related to the anti-ChE activity of S. By esterification of the carboxyl group, hydrogenation of the double bond or reduction of the carbonyl group, the MO reactivity indices will be changed and, then, the anti-ChE activities will be weakened. On the basis of the structure-activity relationship (SAR) of S and its derivatives, the active groups of S were sought and a possible schematic diagram of the interaction of S with the active sites of AChE was suggested.

Selectivities of Aromatic Methyl Metalation. Unexpectedly Strong Interactions of Carbanions with Aromatic Rings

Abstract: The site selectivities of the metalation reactions of dimethylnaphthalenes and dimethylanthracenes followed by MeOD quenching were investigated experimentally and theoretically. The 2(3)-methyl groups were found to be more effectively metalated than expected from the LUMO amplitudes of the aromatic rings. The MNDO calculations suggested that the naphthylmethyl and anthrylmethyl anions are not the aromatic rings perturbed by the carbanions, but the significantly different conjugated rings doubly bonded to the exocyclic carbon atoms. The unexpectedly strong interactions of the carbanion centers with the aromatic rings can account for the discrepancies between the selectivities and the LUMO amplitudes.

NH-tautomerism effects and isocyclic substitution in alkylporphyrins and their chemical dimers

Abstract: A study has been made of the absorption spectra, circular dichroism in a magnetic field (MCD), and polarized fluorescence of individual NH-tautomers of free-base porphyrins with asymmetric substitution. The four-orbital model of Gouterman and the perimeter model of Michl have been used in demonstrating that, in spite of the basic differences in type of electronic spectra of the two NH-tautomers of each compound, their absorption spectra in the visible region are described by a system of two linear oscillators X and Y. The fluorescence polarization spectra of the tautomers in the region of band III are extremely sensitive to changes in the side substituents in the isocycle. From an analysis of the sign sequence of electronic bands in the MCD spectra, it has been established that these compounds may be classed as “hard” chromophores with identical type of orbital splitting for the HOMO (b1 and b2) and LUMO (c2 and c2) in the two tautomers of one and the same compound. A specific role has been found for the keto group in the isocycle, as manifested in inversion of the sign sequence of electronic bands in the MCD spectra of NH-tautomers with a cyclopentanone ring.

Compared electronic structure of negative ions M p C{ /n - }: I. Normal elements in Hückel theory

Abstract: Negative cluster ions M p C− n (M normal element, n < 10, p = 1–4) produced by various experimental techniques from carbides show in their emission intensities a very strong evenodd effect according to the parity of the carbon atom number n. This is in particular the case when M = N, F, Cl (p = 1), M = H, Al, Si, S (p = 1, 2) or M = B (p = 1–4). The largest intensities of M P C− p ions always take place for even n except in the cases of NC− n , B2C− n and Al2C− n , for which the maxima of emission occur for odd n. This oscillating behaviour corresponds to alternations in the stability of the clusters which are mainly due to the fact that, in Pitzer and Clementi model (linear chains in the sp hybridization within the framework of Huckel theory), the HOMO (highest occupied molecular orbital) of the clusters lies in a double degenerate π level band: a cluster with a complete HOMO is always more stable than a cluster with a nearly empty HOMO. This result involves that the total number of π electrons is the main factor governing the parity of the stability alternations. Accordingly, since the knowledge of the π electron number requires the determination of the σ electron number too, these alternations enable us to infer a very likely electronic structure of the ions.

Photophysical, Photochemical, and Electrochemical Properties of Mononuclear and Dinuclear Ruthenium( 11) Complexes Containing 2,2'-Bipyridine and the

Abstract: compared with those of Ru(bpy):+. For both 1 and 2, the lowest energy absorption band and the luminescence band are attributed to Ru - bpy metal-to-ligand charge-transfer (MLCT) singlet and triplet excited states, respectively. Electrochemical oxidation is centered on the metal(s) and electrochemical reduction is centered on the ligands, with bpy being reduced at potentials less negative than those of bpt-. Because of the stronger o-donor ability of bpt- compared with bpy, the Ru - bpy CT absorption and emission bands of 1 (AmPb = 480 nm and A,,," = 678 nm in CH$N at room temperature) are red-shifted compared with those of the parent Ru(bpy):+ complex. Both absorption and emission move to higher energies in going from 1 to 2 (for 2: Ub = 453 nm and Amm = 648 nm). In nitrile rigid matrix at 77 K the emission lifetimes are 2.8 and 3.6 ps for 1 and 2, respectively. For both complexes increasing temperature causes a strong decrease of the emission lifetime (0.16 ps for 1 and 0.10 ps for 2 at room temperature), but the In (1/~) vs 1/T plots between 77 and 298 K are quite different in the two cases, indicating that (i) the luminescent excited state of 1 is much more sensitive than that of 2 to the melting of the solvent matrix and (ii) the luminescent excited state of 2, but not that of 1, undergoes a thermally activated (AE = 3200 cm-I) radiationless transition to an upper, short-lived excited state (presumably, a triplet metal-centered level 3MC). In agreement with the expectation based on the temperature dependence of the luminescence lifetime, 2, but not 1, undergoes a photodissociation reaction in CHzCl2 solutions containing CI- ions. Such a photoreaction leads to 1 and Ru(bpy)zClz. All the observed properties show that in 2 the lowest unoccupied molecular orbital does not belong to the bpt- bridging ligand but to the terminal bpy ligands. Because of the nonequivalence of the coordinating positions of the bridging ligand, in 2 the MLCT and MC levels related to the two Ru(bpy)z2+ units are not isoenergetic. It is found that the lowest excited state of 2 (responsible for the luminescence) is a 'MLCT level centered on one R~(bpy)~~+ unit, whereas photochemistry takes place from a 3MC level centered on the other Ru(bpy)p unit.

Alkoxide Clusters of Molybdenum and Tungsten as Templates for Organometallic Chemistry: Comparison with Carbonyl Clusters of the Later Transition Elements

Abstract: Alkoxide and carbonyl ligands complement each other because they both behave as “π buffers” to transition metals. Alkoxides, which are π donors, stabilize early transition metals in high oxidation states by donating electrons into vacant dπ orbitals, whereas carbonyls, which are π acceptors, stabilize later transition elements in their lower oxidation states by accepting electrons from filled dπ orbitals. Both ligands readily form bridges that span MM bonds. In solution fluxional processes that involve bridge–terminal ligand exchange are common to both alkoxide and carbonyl ligands. The fragments [W(OR)3], [CpW(CO)2], [Co(CO)3], and CH are related by the isolobal analogy. Thus the compounds [(RO)3W W(OR)3], [Cp(CO)2WW(CO)2Cp], hypothetical [(CO)3CoCo(CO)3], and HCCH are isolobal. Alkoxide and carbonyl cluster compounds often exhibit striking similarities with respect to substrate binding—e.g., [W3(μ3-CR)(OR′)9] versus [Co3(μ3-CR)(CO)9] and [W4(C)(NMe)(OiPr)12] versus [Fe4(C)(CO)13]—but differ with respect to MM bonding. The carbonyl clusters use eg-type orbitals for MM bonding whereas the alkoxide clusters employ t2g-type orbitals. Another point of difference involves electronic saturation. In general, each metal atom in a metal carbonyl cluster has an 18-electron count; thus, activation of the cluster often requires thermal or photochemical CO expulsion or MM bond homolysis. Alkoxide clusters, on the other hand, behave as electronically unsaturated species because the π electrons are ligand-centered and the LUMO metal-centered. Also, access to the metal centers may be sterically controlled in metal alkoxide clusters by choice of alkoxide groups whereas ancillary ligands such as tertiary phosphanes or cyclopentadienes must be introduced if steric factors are to be modified in carbonyl clusters. A comparison of the reactivity of alkynes and ethylene with dinuclear alkoxide and carbonyl compounds is presented. For the carbonyl compounds CO ligand loss is a prerequisite for substrate uptake and subsequent activation. For [M2(OR)6] compounds (M = Mo and W) the nature of substrate uptake and activation is dependent upon the choice of M and R, leading to a more diverse chemistry.