Theoretical studies of borazynes and azaborines

Four isomers of didehydroborazine, B3N3H4, borazyne, and three isomers of azaborine, C4H4BN, are studied by DFT, CCSD, and CCSD(T) computational methods. The singlets of 1,2-borazyne (I) and 1,4-borazyne (IV) have angles about the boron of ca. 150 degrees . In 1,2-azaborine (V), the angles are ca. 140 degrees , while the N angles are ca. 112 degrees except in IV (127 degrees ) and 1,4- azaborine (VII, 120 degrees ). These geometries are almost reversed in the triplets. The 1,3-borazyne (III) shows more bicyclic character than the corresponding m-benzyne, with an N-N distance of 1.7 A. In all cases I was found to be lower in energy than the 2,4-borazyne (II), III, and IV. The order of stability of the azaborines is V > VII > 1,3- azaborine (VI). The nucleus-independent chemical shift (NICS) indicates that all isomers of borazyne are less aromatic than benzene and all isomers of azaborine are about as aromatic as benzene. Time-dependent DFT (TD-DFT) is used to study the excited states of the singlets. The significant structural differences between the singlet and triplet states suggest long phosphoresence lifetimes.     

Iminophosphanes as pseudocarbenes: A theoretical study of product formation

P(III) double bond systems possess two closely spaced frontier orbitals, a π-orbital and a σ-orbital. In the parent iminophosphane the orbital sequence is σ (HOMO) and π* (LUMO). Thus, it is isolabal to a singlet carbene and undergoes self-dimerization via a [2 + 1] cycloaddition. The resulting three membered ring system can be considered as a complex of two iminophosphanes. Contrary to the classical cyclopropanation reaction, in the pseudo-carbene iminophosphane the corresponding [2 + 1] reaction is only slightly exothermic, as established by ab initio calculations at the SCF/CEPA-1 and the MCSCF/ACPF level.     

Consequences of Substitution in the Photoelectron Spectra of [2, 2]Paracyclophanes: Separation of ‘through-space’ and ‘through-bond’ interactions as a consequence of fluorosubstitution

The PE. spectra of [2, 2]paracyclophane ( 1 ), 4-amino[2, 2]paracyclophane ( 2 ) and 1, 1, 2, 2, 9, 9, 10, 10-octafluoro[2, 2]paracyclophane ( 3 ) are presented. The bands corresponding to ejection of the photoelectron from the five highest occupied π-orbitals have been assigned. The ‘observed’ orbital energies (i.e. the negative ionization potentials) are discussed in terms of ‘through space’ and ‘through-bond’ interactions between the semi-localized π-orbitals ( e_1g ) of the benzene moieties and the C, C-σ-orbitals of the ethylene bridges. The PE. spectrum of 3 shows that the fluorine-induced lowering of the C, C-σ-orbital energy effectively ‘turns-off’ the ‘through-bond’ interaction. The resulting pattern of the first four bands confirms the assignment given for 1 . Finally the band shifts induced by an amino group in position 4 are again in agreement with this assignment. Attention is drawn to the phenomenon of ‘orbital switching’ as a consequence of substitution in loosely coupled systems such as 1 .     

Correlation of free-electron molecular orbital energies with π ionization energies of aromatic hydrocarbons

With the introduction of an effective mass m^* into the simple free-electron model, the π-orbital energies of aromatic hydrocarbons containing up to 10 condensed benzene rings correlate closely with the π-band positions of the corresponding photoelectron spectra. For a sample of 70 π-orbitals the linear regression yields a standard error SE = 0.129 eV. Compared to HMO results, the number of accidental degeneracies is drastically reduced and the correlation is significantly improved. The free-electron results also compare favorably to those obtained by the PPP, EH and MINDO/2 methods which were scaled by two additional parameters.     

Photoelectron spectra of polyunsaturated [4,4,2]propellanes

The photoelectron spectra of the six propellanes 1 to 6 have been recorded. Those bands which correspond to ejection of an electron from a π-orbital have been assigned, using the concept of through bond and through space interaction. Homoconjugative interaction between the π-systems of the two butadiene moieties of 6 is of the same order as that of the π-orbitals in norbornadiene.     

The molecular structures and the absorption maxima of the H-chromophores of the indigoid dyes

The molecular structures of the H-chromophore of the indigoid dyes and five other isomers are studied by ab initio MP2/6-31 + G*//HF/6-31 + G* method. The bond angles are affected by the π-electron conjugation. The molecular structures of the H-chromophores and indigoid dyes indicate that the benzene rings and the five-membered rings are structurally important. The absorption maxima of the H-chromophores are successfully calculated by CI-singles-MP2/6-31 + G* theory for the first time and correspond to the HOMO, LUMO transition. All these transitions are the π-π* transitions. Like the indigoid dyes, trans isomers have the bathochromic shifts of the absorption maxima, and the bathochromic shifts are found with the best donor group of ? NH. From these calculations, the absorption maxima of some indigoid dyes can be explained by their H-chromophores qualitatively. © 1995 by John Wiley & Sons, Inc.     

The consequences of σ and π conjugative interactions in mono-, di- and triacetylenes. A photoelectron spectroscopic investigation

The HeI photoelectron spectra of mono-, di-, and triacetylenes are presented. In these compounds the two-centre π-orbitals of the ethynyl groups conjugate with the π-orbitals of double bonds or benzene moieties, or with the Walsh orbitals of three-membered ring systems. Assuming the validity of Koopmans' approximation, the observed energies of the radical cation states reached by electron ejection from π-orbitals can be rationalized in terms of a simple LCBO-MO model in those cases, where the molecule is planar. The corresponding numerical results for the ionization energies are in excellent agreement with experiment, if the three parameters of the model are properly calibrated. In contrast, the bands assigned to ejection from in plane π-orbitals are shifted to lower energies by ca. 0.5 eV with respect to the expectation values derived from the above model, due to ‘through-bond’ interaction with lower lying σ-orbitals. Extensive σ/π mixing occurs in the non planar compounds for all orbitals. The assignments of the spectra of diethynylmethane, 1,4-hexadiyne, 1,2-diethynylethane and of cis- and trans-diethynylcyclopropane are backed by semiempirical SCF calculations. The spectra of the cis and trans isomers of diethynylethyleneoxide and diethynylethylenesulfide are discussed by comparison with the corresponding hydrocarbons and with oxirane and thiirane respectively. Finally, the following topics are considered in detail: (a) The effect of spin orbit coupling on the spectrum of 1-iodo-1-butyne-3-ene; (b) the effect of the essentially free internal rotation in divinylacetylene on the band shapes of its photoelectron spectrum and (c) the relationship between the conjugative properties of ethylenic π-orbitals and of the Walsh-orbitals of cyclopropane.     

Die Photoelektronen-Spektren des Tricyclo[4.2.1.02,5]nonadiens und seines 3,4-Diaza-Analogons. Ein Beitrag zur Kenntnis der Wechselwirkung zwischen den einsamen Elektronenpaaren der cis-konfigurierten Azogruppe

A comparative photoelectron spectroscopical investigation of the title compound ( 11 ) and its 3,4- and 7,8-dihydro derivatives ( 9 and 10 ) indicates that a considerable ‘through bond’ interaction exists between the π-orbitals in 11 . The PE. spectra of the 3,4-diaza-analogue of 10 and 11 , which contain a cis azo group in a four-membered ring, yield a splitting Δ_n (4-memb. ring) = 1.55–1.60 eV between the nitrogen lone-pair orbital energies. This value contrasts with those obtained for a three-membered ring analogue (3,3-dimethyldiazirine ( 5 ), Δ_n (3-memb. ring) = 3.55 eV) and for a five-membered ring analogue (2,3-diazanorbornene ( 7 ) Δ_n(5-memb. ring) = 3.10 eV). The sequence Δ_n (3-memb. ring) > Δ_n (4-memb. ring) Δ_n (5-memb. ring) is satisfyingly reproduced by MINDO/2- and EHT-calculations for the model systems with n = 3,4,5. A similar trend can be deduced from MINDO/2-calculations for cis-diimid where Δ_n becomes minimal for a N?N? H angle φ ≈ 100°, whereas Δ_n for the corresponding trans-structure goes through a maximum in this region. The experimental finding as well as the calculated results confirm the predictions made by Gimarc [15] who attributes the behaviour of Δ_n for cis-azo groups to a ‘through-bond’ interaction of the n ₊-orbital with a lowerlying N? N σ-orbital; this interaction becomes maximal for N?N? R angles of the size present in a fourmembered ring, e.g. in 12 or 13 .     

The π-Orbital Sequence in Norbornadiene and Related Hydrocarbons

A method outlined previously [1] is used to show that in norbornadiene ( 3 ) the b ₂(π) orbital lies above a ₁(π), as predicted by theory. This indicates that in 3 through-space interaction between the two basis π-orbitals π_a and π_b is more important than through-bond interaction. Analysis of the PE.-spectra of 8-isopropylidene-tricyclo[3.2.1.0^2,4]-octane ( 13 ) and the corresponding octene ( 15 ) confirms that the π-orbital π_c of the exocyclic double bond conjugates more strongly with the symmetric Walsh-orbital e _s of the cyclopropano moiety than with the π-orbital π_a of a double bond in the same position.     

Photoelectron-spectroscopic Evidence Concerning “Homo-aromaticity”

The first of the two π-bands in the photoelectron spectrum of cis-cis-cis-1, 4, 7-cyclononatriene (I, symmetry C_3v) shows a Jahn-Teller split. This is consistent with the prediction of molecular orbital theory that the top occupied orbitals of I are e (π) and a ₁(π) respectively. From the difference ?( e (π)) - ?( a ₁(π)) = 0.90 to 0.97 eV a value of β_1,3 = ?0.68 eV = 0.27 β (β = -2.5 eV) is obtained for the homoconjugative interaction of two π-orbitals in I. This value represents almost exclusively through-space interaction between the π-orbitals. Through-bond interaction (hyperconjugation) is a minor effect in I. A comparison of the photoelectron data of bicyclo [4.2.1] nonatriene with those of norbornene and cycloheptadiene shows that homoconjugation (homo-aromaticity) can only be detected by photoelectron spectroscopy if the interacting π-bonds (basis orbitals) are symmetry equivalent or have accidentally (almost) degenerate energies.     

The Electronic Spectrum of Tricyclo[3.3.0.0.2,6]octa-3,7-diene

The unusual electronic spectrum of the title compound is shown to be due to the strong interaction between the π-orbitals of the double bonds and the Walsh-orbitals of the fourmembered ring.     

Near Cancellation of Through Space and Through Bond Interaction in bicyclo[3.2.2]nona-6,8-diene

The photoelectron spectra of bicyclo[3.2.2]nonane and its five dehydrogenated analogues are reported. Analysis of the π-bands reveals that the unsymmetrical diene 5 retains the ‘natural order’ of the π-orbitals present in the lower homologues, i. e. the in-phase below the out-of-phase combination of the basis π-orbitals π_a and π_c. The isomeric diene 4 is the first member of the symmetrical homologous series (I), which inverts this order, so that a ₁(π) (in phase!) lies above b ₂(π) (out-of-phase!).     

An ab initio molecular orbital study of structures and energies of spirocompounds: spiro[3.3]heptane and spiro[3.3]hepta-1,5-diene

Ab initio molecular orbital theory with the STO-3G and 4-31G basis sets is used to determine the equilibrium geometries, enthalpies of formation, strain energies and spiro-interactions for spiro[3.3]heptane and spiro[3.3]hepta -1,5 - diene. For spiro[3.3]heptane, molecular mechanics calculations suggest that the component cyclobutane rings are puckered to a greater extent than in cyclobutane itself. For spiro[3.3]hepta - 1,5 - diene, STO-3G calculations predict that the component cyclobutene rings deviate slightly from an orthogonal arrangement. Spiro-interactions in spiro[3.3]hepta - 1,5 - diene are revealed by comparing the calculated structural parameters and strain energies with those of appropriate reference systems. The π-orbitals in spiro[3.3]hepta -1,5 -diene are predicted to be split by about 0.4 eV.     

The effect of the central metal charge on the reduction half-wave potentials of d6 metal complexes

A SCF MO calculation is made to obtain the energies of the lowest vacant π-orbitals of tris-2,2′-bipyridine complexes of d⁶ transition metals in various oxidation states. Any overlap of a metal t_2g-orbital and a ligand π-orbital is neglected and the metal ion is considered as a source of an electrostatic potential field. The π-electron system of the three ligand molecules is treated as a whole by taking account of the overlap of 2pπ-AO's belonging to different ligand molecules. When it is assumed that a ligand π^*-orbital is occupied by the electron added in the course of reduction, the results of the calculation and the Born equation for solvation energy together lead to a linear relation between the reduction half-wave potentials of the complexes and the sum of the charges on the central metal ion and the ligand nitrogen atoms. This linear relation is confirmed experimentally by using the available data on the reduction half-wave potentials of the tris-bipyridine complexes of the following d⁶ metals: Ir(III), Fe(II), Ru(II), Os(II), Cr(0), Mo(0), V(?I) and Ti(?II).     

Theoretical studies on the diazacyclopentadienylidenes: An analysis of aromatic versus nonaromatic systems and singlet versus triplet reactivity

Ab initio molecular orbital calculations have been carried out on the various electronic states of 2,3- ( 6 ), 2,4- ( 7 ), 2,5- ( 8 ), and 3,4-diazacyclopentadienylidene ( 9 ) at the fully geometry optimized 6–31G* level, with single point calculations being carried out at the MP2/6–31G* level. The calculated geometries are interpreted in terms of the degree of occupancy and the nature of the π and σ-nonbonded MO's. At the 6–31G* level the five π-electron, π,σ-triplet states were calculated to be considerably lower in energy. At the MP2/6–31G* level, however, with 7 the six π-electron singlet state is calculated to lie only slightly above the five π-electron triplet (0.4 kcal mole⁻¹), whereas with 8 and 9 the aromatic six π-electron singlet states are calculated to be lower in energy (9.0 and 8.1 kcal mole⁻¹). With 3 and 9 the aromatic six π-electron σ-triplet states lie only 3.6 and 5.5 kcal mole⁻¹ above the lowest energy states. It is concluded that, in general, the energy gained by having an electron in a lower energy σ-type MO instead of a higher energy π MO effectively offsets the energy gained by having an aromatic π system. The results are discussed in terms of the observed chemistry of 6–9 and their substituted systems.     

Die konjugative Wechselwirkung zwischen π- und Walsh-Orbitalen: Das Photoelektron-Spektrum des Homofulvens

Analysis of the photoelectron spectrum of homofulvene ( 1 ) (spiro [2.4] hepta-4,6-diene) confirms the conclusions previously drawn concerning the direct conjugation between π-and Walsh-orbitals. It is shown that the resonance integral (4) appropriate for the semi-quantitative interaction of these orbitals amounts to ?1.9 eV, i.e. nearly the value for conjugating π-orbitals (β = ?2.4 to ?2.5 eV). This explains the close analogy between the photoelectron spectrum of 1 and that of fulvene.     

Evidence for Charge Delocalization in Diazafluorene Ligands Supporting Low-Valent [Cp*Rh] Complexes**

Ligands based upon the 4,5-diazafluorene core are an important class of emerging ligands in organometallic chemistry, but the structure and electronic properties of these ligands have received less attention than they deserve. Here, we show that 9,9′-dimethyl-4,5-diazafluorene (Me₂daf) can stabilize low-valent complexes through charge delocalization into its conjugated π-system. Using a new platform of [Cp*Rh] complexes with three accessible formal oxidation states (+III, +II, and +I), we show that the methylation in Me₂daf is protective, blocking Brønsted acid-base chemistry commonly encountered with other daf-based ligands. Electronic absorption spectroscopy and single-crystal X-ray diffraction analysis of a family of eleven new compounds, including the unusual Cp*Rh(Me₂daf), reveal features consistent with charge delocalization driven by π-backbonding into the LUMO of Me₂daf, reminiscent of behavior displayed by the workhorse 2,2′-bipyridyl ligand. Taken together with spectrochemical data demonstrating clean conversion between oxidation states, our findings show that 9,9′-dialkylated daf-type ligands are promising building blocks for applications in reductive chemistry and catalysis.     

The impact of [1,2,5]chalcogenazolo[3,4-f]-benzo[1,2,3]triazole structure on the optoelectronic properties of conjugated polymers

Two novel conjugated near-infrared (NIR) absorbing donor–acceptor type copolymers comprising benzodithiophene as the donor and [1,2,5]chalcogenazolo[3,4-f]-benzo[1,2,3]triazole derivatives as the acceptors, spaced with thiophene as the π-bridge, were designed and synthesized via Stille polycondensation reaction. The effect of acceptor strength on optoelectronic properties was targeted and investigated. Branched alkyl chains (the extended 2-octyl-1-dodecyl alkyl chain;  C₈C₁₂) were introduced to 5H-[1,2,3]triazolo[4′,5′:4,5]benzo[1,2-c][1,2,5]thiadiazole and 5H-[1,2,3]triazolo[4′,5′:4,5]benzo[1,2-c][1,2,5]selenadiazole for enhanced solubility of polymers which ease the processability hence device constructions. The strongly electron-withdrawing units lead to a substantial change in the absorption properties via promotion of the intramolecular charge transfer band alongside the π–π* transition. The resultant soluble polymers were characterized via cyclic voltammetry to determine highest occupied molecular orbital and lowest unoccupied molecular orbital energy levels as −5.00 and −3.92 eV for PSBT and −4.86 and −4.04 eV for PSeBT, respectively. Electronic band gaps of the copolymers were calculated as 1.08 eV for PSBT and 0.82 eV for PSeBT, respectively. NIR absorbing copolymers were used to construct electrochromic devices.     

Korrelation zwischen photoelektronen- und elektronen-spektren : Die zuordnung des A2u-π-orbitals im pe-spektrum des benzols

It has been possible to correlate, by a modified PPP-SCF-CI-method on benzene, PE- and UV-spectroscopical measurements with a deviation of < ± 10^?3 [eV]. In this manner we obtained sets of parameters, of general application to aromatic compounds and we concluded, that the controversial ionisation potential of 12·30 [eV] should be assigned to the π-a_2u-orbital of benzene.     

How strained is the “flat” benzene ring in superphane ?

The strain energy in the bowl-shaped benzene ring incorporated in superphane has been estimated by the MO theoretical calculations at 20.6 kcal/mole (the 4–31 G calculations). The π-orbitals on the ring are not parallel to each other, but loss in the π-resonance energy does not appear to be serious. The strain in the σ-framework is considered to be responsible for most of the destabilization.