Symmetry-breaking phenomena in metalloporphyrin pi-cation radicals

Density functional theory (DFT) calculations of the energetics, molecular structures, and spin density profiles of metalloporphyrin pi-cation radicals suggest that the common practice of describing these radicals in terms of a universal A(1u)/A(2u) dichotomy is often not justified, confirming a possibility first foreseen by Prendergast and Spiro (ref 15) over a decade ago on the basis of vibrational spectroscopy and semiempirical calculations. Because of near-degeneracy of the a(1u) and a(2u) HOMOs of many metalloporphyrins, the cation radicals derived from these compounds undergo a pseudo-Jahn-Teller (pJT) distortion and are, therefore, best described as (2)A(u) with reference to the C(4h) point group, rather than as (2)A(1u) (D(4h) or (2)A(2u) (D(4h)). We find that the porphyrin cation radicals undergo a pJT distortion if the energy difference between the (2)A(1u) and (2)A(2u) pi-cation radicals, optimized under D(4h) symmetry constraints, is less than 0.15 eV. According to this criterion, metallo-porphine and metallo-OEP pi-cation radicals should always be pJT-distorted and metallo-meso-tetrahalogenoporphyrin radicals should not. For [Zn(TPP(*))](+), the (2)A(1u)/(2)A(2u) energy difference is almost exactly at the threshold of 0.15 eV, consistent with the experimental observation of both symmetry-broken and undistorted structures for this species. The (2)A(1u)/(2)A(2u) energy difference (when the molecular geometries are optimized under a D(4h) symmetry constraint) also appears to govern whether the real pJT-distorted cation radical is more A(1u)- or A(2u)-like in terms of its spin density profile. Because many metalloporphyrin pi-cation radicals exist as cofacial dimers in the crystalline phase, we examined the symmetries and structures of the model compounds [[Zn(P)](2)](+,2+) by means of DFT geometry optimizations. The results showed that dimerization has relatively little impact on the bond length alternation in the individual rings. A final interesting result, consistent with experiment, is that the bond length alternation in the delocalized mixed-valence dimer [[Zn(P)](2)](+) is about half that found for [[Zn(P)](2)](2+).     

Theoretical analysis of core size effect in metalloporphyrins

Density functional theory has been applied to a series of unsubstituted planar metalloporphyrins (MPs) to elucidate how geometry and frequencies correlate with the metal-nitrogen distance, referred to as the core size. Different transition metals can invoke expansion or contraction of the porphyrin core due to electronic effects resulting from the amount of d-electron pairing as well as occupancy of the d(x(2)(-y(2))) orbital. A full vibrational analysis consisting of all in-plane and out-of-plane frequencies was carried out, and the resulting modes were plotted against core size for a linear analysis and grouped within symmetry blocks. The modes were separated according to planarity, and all modes with a large slope and best fit greater than 0.8 were considered sensitive to metal-nitrogen distances. All planar skeletal modes above 1450 cm(-1), including the pyrolle ring deformations, are found to be core-size sensitive. The most significant out-of-plane modes sensitive to core size are gamma(8) and gamma(9), which are infrared active and grouped within the A(2u) symmetry block. The present work also opens possible quantitative applications for the correlation of spectroscopic properties of MPs and heme proteins with actual structural parameters.     

The effect of axial Mg ligation on the geometry and spin density distribution of chlorophyll and bacteriochlorophyll cation free radical models: a density functional study.

Density functional calculations are performed on models of chlorophyll and bacteriochlorophyll to examine the effect of Mg ligation on the geometry and spin density distribution of the cation free radicals formed. It is shown that, whereas the properties of the bacteriochlorophyll model can be explained on the basis of the electron density distribution of the highest occupied molecular orbital (HOMO), for the chlorophyll model the geometry and spin density properties of the ligated species do not follow this trend. For the ligated chlorophyll models it is shown that, due to the closeness in energy of the HOMO and HOMO-1 orbitals, a Jahn-Teller distortion occurs on one-electron oxidation, leading to an admixed hybrid orbital for the cation radical form. Orbital mixing is shown to lead to significant changes in the geometry and spin density distribution of the cation free radical formed. It is also shown that orbital mixing does not lead to an increase in the magnitude of the (14)N hyperfine couplings thereby invalidating reports in the literature which have dismissed mixed orbital states for the primary donor cation radicals of photosynthetic reaction centers based on this criterion.     

A Generalized Unpaired Electron Spin Density Equation and Organic Hyperconjugation Mechanism

A large class of stereochemcial and related interactions in organic chemistry are repulsive and others are attractive, but the relative orientation of two methyl groups and the amount of energy required to twist one relative to the other (the hindered rotation energy barriers), or the alignment of such a group with respect to a conjugated ring to which it is attached (widely attributed to a mechanism called “hyperconjugation”) are estimated to be small in compared with the total energy of the molecule. We used theories of both isotropic and anisotropic proton hyperfine interactions in the π‐electron systems developed in the early sixties. They are approximated by the magnetic dipole nteractions between each proton and an electron spin magnetization that is distributed in 2s and 2p Slater atomic orbitals center on carbon atoms. We have extended these theories to the non‐planar olefinic cation radicals, which are very important in biochemistry as well as in petroleum catalysis. A three dimensional electron spin density equation has been developed in this paper to handle some Jahn‐Teller vibronic molecules. The new electron spin density equation related the observed proton hyperfine splittings to the non‐planar structures of the open‐chain alkene cation radicals generated by radiolysis and various chemical oxidation methods. The spin densities and the conformational calculations based on valence bond theory and symmetry principles are compared with some more elaborated molecular orbital calculations in the literature. The localized valence bond approaches are better in accord with our experimental results. The anomalous line‐width effect of the four methyl groups observed in the 2,3‐dimethyl‐2‐butene cation radicals also confirmed the positive sign of the electron‐proton hyperfine constant of hyper‐conjugation mechanism. A methyl substituent attached to a conjugated molecule often behaves as if it formed part of the region of conjugation; the charge appears to flow from the methyl group into the π electron system and it may also give rise to an appreciable dipole moment. Methylation also gives rise to an appreciable dipole moment, and the resultant red shift of electronic absorption bands is of some importance in the design of dye molecules.     

Exploring the Jahn-Teller and pseudo-Jahn-Teller conical intersections in the ethane radical cation

We report a theoretical account on the static and dynamic aspects of the Jahn-Teller (JT) and pseudo-Jahn-Teller (PJT) interactions in the ground and first excited electronic states of the ethane radical cation. The findings are compared with the experimental photoionization spectrum of ethane. The present theoretical approach is based on a model diabatic Hamiltonian and with the parameters derived from ab initio calculations. The optimized geometry of ethane in its electronic ground state (1A1g) revealed an equilibrium staggered conformation belonging to the D3d symmetry point group. At the vertical configuration, the ethane radical cation belongs to this symmetry point group. The ground and low-lying electronic states of this radical cation are of 2Eg, 2A1g, 2Eu, and 2A2u symmetries. Elementary symmetry selection rule suggests that the degenerate electronic states of the radical cation are prone to the JT distortion when perturbed along the degenerate vibrational modes of eg symmetry. The 2A1g state is estimated to be approximately 0.345 eV above the 2Eg state and approximately 2.405 eV below the 2Eu state at the vertical configuration. The symmetry selection rule also suggests PJT crossings of the 2A1g and the 2Eg electronic states of the radical cation along the vibrational modes of eg symmetry and such crossings appear to be energetically favorable also. The irregular vibrational progressions, with numerous shoulders and small peaks, observed below 12.55 eV in the experimental recording are manifestations of the dynamic (E x e)-JT effect. Our findings revealed that the PJT activity of the degenerate vibrational modes is particularly strong in the 2Eg-2A1g electronic manifold which leads to a broad and diffuse structure of the observed photoelectron band.     

Structure,epr parameters,and reactivity of organic free radicals from a density functional approach

Summary A recently introduced density functional incorporating gradient corrections and some Hartree-Fock exchange has been used to study the structures, properties, and reactivity of representative organic free radicals. A general theoretical model has been introduced, in which standardized grid, functional, and orbital basis set are used to compute geometrical parameters, vibrational frequencies, and one-electron properties. The results are compared with available experimental data from diatomic to polyatomic radicals. All the geometric and electronic parameters compare favourably with available experimental data and with the results of refined post Hartree-Fock computations. Also the thermodynamics and kinetics of a representative unimolecular reaction (isomerization of formaldehyde radical cation) are well reproduced. These findings together with the very favourable scaling of the computations with the number of electrons suggest that the density functional approach is a promising theoretical tool for the study of relationships between structure and properties of large free radicals.     

Propagator quantum chemical study of <Emphasis Type="Italic">S</Emphasis>-<Emphasis Type="Italic">cis</Emphasis>-(<Emphasis Type="Italic">Z</Emphasis>)-2-(2-formylethenyl)pyrrole: electronic structure and aspects of intramolecular hydrogen bond manifestation in ionization spectra

For the purpose of investigation of the electronic structures of functionalized pyrroles with potential biological activity the electronic structures and ionization spectra of S-cis-(Z)-2-(2-formylethenyl)pyrrole (FP) were calculated by the propagator quantum chemical method. The calculations were performed using the third-order algebraic diagrammatic construction method (ADC(3)) for one-particle Green´s function (electronic propagator) and the 6–31G** basis set. Going from FP (possessing the intramolecular hydrogen bond H?O) to its conformation FPR (without H?O bonding), the O1s-ionization energy and the ionization energy of the σ-type lone electron pair orbital of the O atom decrease by ~0.2 eV, which is a consequence of stopping the electron density transfer from the O atom. A strong electron density transfer through the hydrogen bond from the O atom to the NH group occurs in the nitrogen core level ionization spectrum as evidenced by a lower N1s-ionization energy of FP (by ~0.7 eV ) compared to that of FPR. The valence shell ionization spectra of FP and FPR calculated using the ADC(3) method are characterized by a high density of the satellite states. The results obtained indicate that the electronic structures of the compounds of the considered class are characterized by pronounced effects of electron correlation.     

Radical cation generation from singlet and triplet excited states of all-trans-lycopene in chloroform

On direct photoexcitation, subpicosecond time-resolved absorption spectroscopy revealed that the 1B(u)-type singlet excited state of all-trans-lycopene in chloroform was about seven times more efficient than all-trans-beta-carotene in generating the radical cation. The time constant of radical cation generation from the 1B(u)-type state was found to be approximately 0.14 ps, a value that was comparable for the two carotenoids. On anthracene-sensitized triplet excitation, radical cation generation was found to be much less efficient for lycopene than for beta-carotene. A slow rising phase (20-30 micros) in the bleaching of ground-state absorption was common for both lycopene and beta-carotene in chloroform and was ascribed to an efficient secondary reaction with a solvent radical leading to the formation of carotenoid radical cations. The reverse ordering in the tendency of the excited states of different multiplicities for the two carotenoids to generate radical cations is discussed in relation to the two carotenoids as scavengers of free radicals.     

Phase separation in manganites induced by orbital-ordering strains

Doped manganite perovskites AMnO(3) exhibit a rich variety of electronic properties, resulting from the interplay of charge (Mn(3+)/Mn(4+)), spin (Mn magnetic moment) and orbital (Mn(3+) Jahn-Teller distortion) degrees of freedom. Magnetisation measurements and ESR spectra have been used to study a series of eight AMnO(3) perovskites, in which the A cation sites are occupied by a distribution of 70% trivalent lanthanide and 30% divalent Ca, Sr or Ba ions. These all have a mean A cation radius of 1.20 Angstrom but different values of the cation size variance sigma(2). A change from orbital disorder to order (cooperative Jahn-Teller distortions) was previously found in the insulating regime at sigma(2) = approximately 0.005 Angstrom(2). This work has shown that co-existence of the orbitally ordered and disordered phases is found in sigma(2)= 0.0016-0.0040 Angstrom(2) samples, with a difference of 40 K between their Curie temperatures. This is ascribed to competition between orbital ordering and microstructural lattice strains. At larger sigma(2) > 0.005 Angstrom(2), the orbital ordering strains are dominant and only this phase is observed. This intermediate temperature phase segregation is one of many strain-driven separation phenomena in manganites.     

Structure and bonding in SnWO4, PbWO4, and BiVO4: lone pairs vs inert pairs

Three ternary oxides, SnWO4, PbWO4, and BiVO4, containing p-block cations with ns2np0 electron configurations, so-called lone pair cations, have been studied theoretically using density functional theory and UV-visible diffuse reflectance spectroscopy. The computations reveal significant differences in the underlying electronic structures that are responsible for the varied crystal chemistry of the lone pair cations. The filled 5s orbitals of the Sn2+ ion interact strongly with the 2p orbitals of oxygen, which leads to a significant destabilization of symmetric structures (scheelite and zircon) favored by electrostatic forces. The destabilizing effect of this interaction can be significantly reduced by lowering the symmetry of the Sn2+ site to enable the antibonding Sn 5s-O 2p states to mix with the unfilled Sn 5p orbitals. This interaction produces a localized, nonbonding state at the top of the valence band that corresponds closely with the classical notion of a stereoactive electron lone pair. In compounds containing Pb2+ and Bi3+ the relativistic contraction of the 6s orbital reduces its interaction with oxygen, effectively diminishing its role in shaping the crystal chemical preferences of these ions. In PbWO4 this leads to a stabilization of the symmetric scheelite structure. In the case of BiVO4 the energy of the Bi 6s orbital is further stabilized. Despite this stabilization, the driving force for a stereoactive lone pair distortion appears to be enhanced. The energies of structures exhibiting distorted Bi3+ environments are competitive with structures that possess symmetric Bi3+ environments. Nevertheless, the "lone pair" that results associated with a distorted Bi3+ environment in BiVO4 is more diffuse than the Sn2+ lone pair in beta-SnWO4. Furthermore, the distortion has a much smaller impact on the electronic structure near the Fermi level.     

Influence of Meso Substituents on Electronic States of (Oxoferryl)porphyrin pi-Cation Radicals

A series of (oxoferryl)porphyrin pi-cation radicals generated from porphyrins substituted at the meso positions with highly electron-withdrawing aryl groups has been characterized: tetrakis-5,10,15,20-(2,6-dichlorophenyl)-, 5-(2-chloro-6-nitrophenyl)-10,15,20-tris(2,6-dichlorophenyl)-, and 5-(2,6-dinitrophenyl)-10,15,20-tris(2,6-dichlorophenyl)porphyrins (porphyrins 1-3, respectively). The physical-chemical properties of the oxidized complexes of 1-3 are compared to those of two (oxoferryl)porphyrin pi-cation radical complexes substituted with electron-releasing aryl groups: tetramesitylporphyrin (TMP) and 2-iodotetramesitylporphyrin (2-iodoTMP). While all of the complexes examined show close correspondance in a number of spectroscopic parameters, some significant differences were observed. In contrast to observations for the oxidized complexes of TMP and 2-iodoTMP, the resonance Raman marker bands nu(2) and nu(11), which are indicators of symmetry state of porphyrin pi-cation radicals of 1-3, do not show the expected downfrequency shifts for oxidation to compound I analogs in a(2u) symmetry states. The upfield hyperfine NMR shifts of the pyrrole beta-proton signals of the compound I analogs of 1-3 are much larger than those for TMP and 2-iodoTMP. These data may be explained by admixture of some a(1u) character into the ground state of radical cations of 1-3, consistent with the hypothesis that electron-withdrawing meso substituents lower the energy of the a(2u) molecular orbital, favoring an a(1u) admixture.     

Theoretical analysis of the Jahn-Teller distortions in tetrathiolato iron(II) complexes

Crystallographic studies of [Fe(SR)(4)](2-) (R is an alkyl or aryl residue) have shown that the Fe(II)S(4) cores of these complexes have (pseudo) D2d symmetry. Here we analyze the possibility that these structures result from a Jahn-Teller (JT) distortion that arises from the e(3z(2) - r(2), x(2) - y(2)) orbital ground state of Fe(II) in T(d)symmetry. Special attention is paid to the influence of the second-nearest neighbors of Fe, which lowers the symmetry and reduces the full JT effect to a smaller, pseudo JT effect (PJT). To estimate the size of the PJT distortion, we have determined the vibronic parameters and orbital state energies for a number of [Fe(SR)(4)](2-) models using density functional theory (DFT). Subsequently, this information is used for evaluating the adiabatic potential surfaces in the space of the JT-active coordinates of the FeS(4) moiety. The surfaces reveal that the JT effect of Fe(II) is completely quenched by the tetrathiolate coordination.     

Electron correlation in the GK state of the hydrogen molecule

The second excited (1)Sigma(g)(+) state of the hydrogen molecule, the so-called GK state, has a potential energy curve with double minima. At the united atom limit it converges to the 1s3d configuration of He. At large internuclear distances R, it dissociates to two separated atoms, one in the ground state and another in the 2p excited state. Radial pair density calculations and natural orbital analyses reveal unusual effect of electron correlation around the K minimum of the potential energy curve. As R>2.0 a.u., a natural orbital of sigma(u) symmetry joins the two natural orbitals of sigma(g) symmetry at smaller R. The average interelectronic distance decreases as the internuclear distance increases from R=2.0 to 3.0 a.u. Around R=3.0 a.u. the singly peaked pair density curve splits into two peaks. The inner peak can be attributed to the formation of the ionic electron configuration (1s)(2), where both 1s electrons are on the same nucleus. As the two 1s electrons run into different nuclei, one of the two 1s electrons is promoted to the 2p state, which results in the outer peak in the pair density curve. The Rydberg 1s2p configuration persists as the nuclei stretch, and becomes dominant at large R where four natural orbitals, two of sigma(g) and two of sigma(u) symmetry, become responsible.     

Theoretical study of the electronic structure of HXY/XYH radicals (X[Double Bond]C,Si;Y[Double Bond]O,S)

The electronic structures of the HXY/XYH compounds (X[Double Bond]C,Si;Y[Double Bond]O,S) on the (2)A(') electronic ground state were investigated by applying the natural bond orbital (NBO) method to the computed B3LYP6-311G(**) wave functions. Different localized structures are proposed for the HXY and XYH isomers and the central XY unit is described as intermediate between a double and a triple bond in HCO, HCS, HSiO, and HSiS, similar to a double bond in COH, CSH, and SiSH, and clearly a single bond in SiOH. Through the comparison between the NBO results for the diatomic and hydrogenated compounds, the energy preferences on each pair of isomers and the computed geometrical parameters are explained. According to the structures proposed, the HXY compounds are sigma radicals with the spin density distributed along the molecular framework, while the XYH compounds are pi radicals with most of the unpaired spin located on an almost pure p orbital of the X atom. Finally, the amounts of spin density on natural atomic orbitals provided by the NBO method are used to explain the computed values of the isotropic and anisotropic hyperfine coupling constants.     

Excited state relaxation dynamics of the zinc(II) tetraphenylporphine cation radical

A new technique employed to study the photophysical properties of the zinc(II) tetraphenylporphine cation radical is reported. It employs a combination of controlled potential coulometry and femtosecond absorption spectrometry. The fast transient lifetime of 17 ps of the pi-cation species originates in very efficient mixing of the a(2u) HOMO cation orbital that places electronic density mainly on pyrrolic nitrogens and metal d-orbitals. That explains the lack of any emission of the cationic species. This nonradiative decay process might elucidate the processes taking place in photosynthetic systems when an electron is removed from the porphyrinic moiety and the hole is produced.     

Theoretical investigation on the electronic and geometric structure of GaN2+ and GaN4+

The electronic and geometric structures of gallium dinitride cation, GaN2+ and gallium tetranitride cation, GaN4+ were systematically studied by employing density functional theory (DFT-B3LYP) and perturbation theory (MP2, MP4) in conjunction with large basis sets, (aug-)cc-pVxZ, x = T, Q. A total of 7 structures for GaN2+ and 24 for GaN4+ were identified, corresponding to minima, transition states, and saddle points. We report geometries and dissociation energies for all the above structures as well as potential energy profiles, potential energy surfaces, and bonding mechanisms for some low-lying electronic states. The calculated dissociation energy (De) of the ground state of GaN2+, X1Sigma+, is 5.6 kcal/mol with respect to Ga+(1S) + N2(X1Sigmag+) and that of the excited state, ?3Pi, is 24.8 kcal/mol with respect to Ga+(3P) + N2(X1Sigmag+). The ground state and the first excited minimum of GaN4+ are of 1A1(C2v) and 3B1(C2v) symmetry with corresponding De of 11.0 and 43.7 kcal/mol with respect to Ga+(1S) + 2N2(X1Sigmag+) for X1A1 and Ga+(3P) + 2N2(X1Sigmag+) for 3B1.     

Orbital symmetry as a tool for understanding the bonding in Krossing's cation

The geometric and electronic structure of Krossing's cation, Ag(eta(2)-P(4))(2)(+), which shows an unexpected planar coordination environment at the metal center and D(2)(h) symmetry both in solution and in the solid state, have been investigated using density functional theory and orbital-symmetry-based energy decomposition. The analysis reveals that the contribution from electrostatic interactions to the bond energy is greater than that of orbital interactions. Partitioning of the latter term into the irreducible representations shows that, in addition to the 5s orbital, 5p orbitals of silver act as acceptor orbitals for electron donation from sigma(P-P) orbitals (a(1)(g), b(1)(u)) and n(P) orbitals (b(3)(u)). Back-donation from the 4d(10) closed shell of Ag into sigma orbitals of the pnictogen cages (b(2)(g)) is also important. However, this contribution is shown not to determine the D(2)(h) structure, contradicting conclusions from the pioneering study of the title cation (J. Am. Chem.Soc. 2001, 123, 4603). The contributions from the irreducible representations to the stabilizing orbital interactions in the D(2)(h) structure and in its D(2)(d)-symmetric conformer are analogous, indicating that the planar coordination environment at the metal center in Ag(eta(2)-P(4))(2)(+) is induced by intermolecular rather than by intramolecular interactions. Because ethylene coordination to a metal ion is an elementary reaction step in industrial processes, the bonding in Ag(C(2)H(4))(2)(+) has been analyzed as well and compared to that in Krossing's cation. Surprisingly, similar contributions to the bond energies and an involvement of metal 4d and 5p orbitals have been found, whereas a recent atoms in molecules analysis suggested that the metal-ligand interactions in silver(I) olefin complexes fundamentally differ from those in tetrahedro P(4) complexes. The only qualitative difference between the bonding patterns in Ag(eta(2)-P(4))(2)(+) and Ag(C(2)H(4))(2)(+) is the negligible energy contribution from the b(3)(u) irreducible representation in the ethylene complex because a respective symmetry-adapted linear combination of ligand orbitals is not available.     

Special stability of cationic MPb12+ clusters and superalkali character of neutral MPb12 clusters (M = B, Al, Ga, In, and Tl)

The electronic structures and stabilities of cationic MPb12+ clusters (M = B, Al, Ga, In, and Tl) with 50 valence electrons are investigated within density functional theory. It is shown that, at the B3LYP/cc-pVDZ(-PP) and BPW91/cc-pVDZ(-PP) levels of theory, the structures of MPb12+ with icosahedra (I(h)) symmetry are energetically favorable, and their high stabilities may arise from the closed-shell nature of the pi subsystems which are subject to the 2(N(pi + 1)2 rule with N(pi = 1). In addition, the possessing of large nucleus-independent chemical shifts of the five kinds of clusters reflects the common aromatic character of these clusters. From the comparison of our studies on the binding energies and the highest occupied molecular orbital and the lowest unoccupied molecular orbital energy gaps, the cluster AlPb12+ has higher stability than the others and this is consistent with the recent mass-spectrometric discovery of Al-doped Pb(n)+ clusters, in which AlPb12+ is highly abundant. The same methods are used to search for the structures of the neutral MPb12 clusters. The calculations reveal that the most stable geometries of the BPb12 and GaPb12 clusters have I(h) symmetry, the AlPb12 and InPb12 clusters have T(h) symmetry, and the TlPb12 cluster has C5v symmetry. Furthermore, the vertical ionization potentials of the neutral MPb12 clusters are smaller than that of some alkali atoms, indicating that the neutral MPb12 clusters possess superalkali character.     

Computational investigation of the vibrational and electronic states of S2N2

The structures and vibrational frequencies of the ground and excited states of S(2)N(2) have been calculated using density functional (DF) methods. Time-dependent DF theory (TDDFT) has been used to calculate the excitation energies of the lowest 20 singlet-singlet transitions using a variety of methods. All computational methods predict a small highest occupied molecular orbital (HOMO)-lowest unoccupied molecular orbital (LUMO) gap. There is some disagreement in the ordering of the b(2g) and b(3g) pi orbitals. This is reflected in the ordering of the B(2u) and B(3u) states from the TDDFT calculations. The excitation energies and oscillator strengths strongly suggest it is the transitions to these states that are responsible for the experimental electronic spectrum. The calculated geometries and vibrational frequencies for these two states show that both have C(2v) equilibrium structures. Modelling of the vibrational progressions and band shapes suggest that the ordering of the states is B(2u)相似文献   

C40H2各种可能异构体稳定性的理论研究

采用AM1和PM3两种半经验方法,对D_5d对称性的C₄₀及C₄₀H₂所有可能异构体的几何构型进行了非限制对称性全优化,得到51种稳定异构体,在此基础上研究了氢的加成反应规律及本体C₄₀和最稳定及最不稳定C₄₀H₂异构体的红外光谱,讨论了影响C₄₀(D_5d)氢加成异构体稳定性及加成位置选择性的三种主要因素:(1)C₄₀本体几何结构;(2)共轭效应;(3)电荷分布影响.