Photochemistry of β,γ-enones-VIII: On the remarkable photostability of some β,γ.β',γ'-dienones and the 1,3-acyl shift photoreactivity of two β,γ.γ',δ'-dienones

The direct irradiation of the β,γ.β',γ'-dienones 1–5 and the β,γ.γ',δ'-dienones (E)-6a, (E)-7a and 8a at λ 300 nm has been studied. The β,γ.β,γ'-dienones 1–5 are remarkable photostable for λ ? 300 nm, even upon prolonged irradiation, in contrast to simple β,γ-enones which upon irradiation exhibit α-cleavage, γ-hydrogen abstraction, (E)-(Z) isomerization and oxetane formation. The observed photostability of the β,γ.β',γ'-dienones is rationalized in terms of a rapid radiationless decay of the excited singlet state, enhanced by CT-interaction between the carbonyl ¹(n-π*) state and the homoconjugated 1,4-diene moiety, which precludes fluorescence, photochemical reactions and intersystem crossing (ISC).The β,γ.γ',δ'-dienones (E)-(6a), (E)-7a and 8a exhibit only a 1,3-acyl shift (1,3-AS) without (E)-(Z) isomerization of the alkenyl moiety, to yield (E)-6b, (E)-7b and 8b. It is concluded that the 1,3-AS proceeds from the ¹(n-π*) state with a rate which is very large relative to the rate of ISC to the ³(n-π*) state, thus precluding any internal triplet energy transfer (1TET) from the ³(n-π*) to the ³(π-π*) state which would manifest itself by (E)-(Z) isomerization.     

Theoretical study of the vertical electron affinity and ionization potentials of C3

The electron affinity and first three ionization potentials of C₃ are calculated using the multiconfigurational SCF and configuration interaction methods and by Möller-Plesset perturbation theory. Whereas Koopmans' theorem and SCF calculations indicate that the first cation state is ²Π_u, upon inclusion of correlation effects both the ²Σ_u and ²Σ_g cation states are found to lie lower in energy. CI calculations indicate that the ground state (²Π_g) anion is stable by 1.74 eV. Allowing for the error in the calculated electron affinity of the carbon atom, C₃^? is estimated to be stable by 2.0 eV, in excellent agreement with the 2.05 eV value determined from recent photodetachment measurements. No excited anion states are found to be bound at the equilibrium geometry of the neutral molecule.     

Theoretical Foundations Of Purely Semi-Empirical Quantum Chemistry. III. Repulsion Integrals

Results of ab initio calculations of correlation energies are used to estimate the correlation contribution to true repulsion integrals γpq. The one-center true γpp and the distance dependence of the true γpq are shown to be in semi-quantitative agreement with the empirical resonance integrals used in pi-electron theories. The relationship between the true γpp and the difference between true molecular ionization potentials and electron affinities is also discussed.     

Charge-transfer absorption spectra between tetracyclo[3.2.0.02,7.04,6]heptane and π-type electron acceptors

Charge-transfer transition energies, association constants, and molar extinction coefficients of complexes between tetracyclo[3.2.0.0^2,7.0^4,6]heptane(quadricyclane) and olefine type or quinone type acceptors were measured in methylene chloride at 20°. An excellent linear relationship (r = 0·9938) with a slope of 0·96 was observed in plots of ν_max(CT) for a series of complexes of quadricyclane against ν_max(CT) for the corresponding complexes of N,N-dimethyl-aniline, indicating that quadricyclane forms electron donor-acceptor complexes of weak interactions. The first ionization potential, estimated from the hyperbolic relationship between the charge-transfer transition energy (E_CT) and the ionization potential, was an exceptionally low value (8·28–8·32 eV) for a saturated hydrocarbon and was indeed in the same order of magnitude with that of norbornadiene, which was well reproduced by EHM and MINDO/1 calculations applying Koopmans' theorem to the calculated HOMO energy.     

Simple derivation of the potential energy gradient for an arbitrary electronic wave function

The magnitude of reorganization energies in the photoelectron (PE ) spectra of various transition metal compounds with Mn, Fe, and Ni as 3d center is studied by means of a variable INDO Hamiltonian. The Koopmans defects are analyzed as a function of the one-electron resonance integral β and as function of the one- and two-center electron–electron interaction integrals. β has the property of an inverse coupling constant; reorganization effects are enlarged with reduced β values. In the limit of very small resonance integrals a reduction of the calculated Koopmans defects due to modified localization properties of the orbital wave function is encountered. The two-center electron-electron interaction integrals γ have been calculated via an exponential formula with a variable range parameter. In the limit of long-range potentials with flattened γ; gradients a significant reduction of relaxation and correlation is diagnozed; large defects are predicted in the short-range limit with steep gradients in the repulsion potential. The one-center Coulomb and exchange integrals (γ, K) have been modified by a multiplicative factor. With enlarged one-center integrals enhanced Koopmans defects are encountered. The reorganization energies are determined by means of a Green's function approach with a renormalized approximation for the self-energy part.     

Assignment of the four lowest ionized states of p-benzoquinone and the question of “lone pair splitting” in this system

The lowest ionized states of p-benzoquinone are assigned to be n_/²B_3g, n₄/²B_2u, π₁/²B_3u, and π₂/²B_1g in order of increasing energy. This sequence of states confirms Turner's original assignment and invalidates subsequent proposals. The origin of the difficulties with these assignments are traced back to the non-validity of Koopmans' theorem for the “oxygen lone pair” ionizations (i.e., ionization to the n_/²B_3g and n₊/²B_2u states).     

Overlap population and the homopolar dipole

The expression {[^S_AB/(1 + S_AB)]R/2} 2e is shown to be an upper limit approximation to the homopolar dipole. It results from adding the overlap population 4C₁C₂S_AB to the atom with the smaller orbital radius. Dipole moments calculated from this partitioning are closer to the experimental values than those computed with equally sophisticated techniques from the standard gross atomic populations. Atomic charges are identical to Löwdin's in the limiting case when S_ABR/2 = $\text{x}$_AB, but the calculations are over non-orthogonalized AO's.     

Conformation of some ribofuranosyl imidazole and uracil nucleosides: a 1H and 13C nmr study

100 and 220 MHz ¹H NMR spectra, recorded in DMSO-d₆ solution and including some measurements at 343 K, of three anomeric pairs of novel 2', 3'-O-isopropylidene nucleosides (two ribofuranosyl imidazoles, 1 and 2, and one uridine 3, all related to intermediates in denovo nucleotide biosynthesis) have been analysed, mostly as overlapping ABX spin systems. Chemical shifts and couplings have enabled the α- and β-anomers to be identified; the sugar-ring vicinal H-H coupling constants indicate that the 2',3'-ketal blocking group locks the α and β anomers in predominantly S and approximately 1:1 N/S ribofuranoside conformations, respectively. Among the ¹³C shifts reported for 3 in DMSO-d₆, those of the 2', 3'-O-isopropylidene CH₃ in the α anomer (shift difference 1.4ppm) are around 1.6 ppm upfield of those for the β anomer (which have a shift difference of 1.8 ppm).     

Photoelectron spectroscopic study of simple hydrogen-bonded dimers. II. The methanol dimer

HeI photoelectron spectra of a supersonic jet of methanol vapor have been obtained by using the temperature-controlled supersonic nozzle beam photoelectron spectrometer recently constructed in our laboratory. A HeI spectrum attributable to the methanol dimer (CH₃OH)₂ has been deduced by spectrum stripping. The first ten vertical ionization energies and the first adiabatic ionization energy of (CH₃OH)₂ have been determined from the stripped spectrum. Ab initio SCF MO calculations of ionization energies have also been carried out for (CH₃OH)₂ on the basis of Koopmans' theorem. The lower bound of the dissociation energy of (CH₃OH)₂⁺ has been estimated to be 1.2 ± 0.2 eV from the adiabatic ionization energies of the monomer and dimer. The equilibrium structure of (CH₃OH)₂ is also discussed.     

Ab Initio calculation on BF3 and BCl3

Ab initio calculation are performed on the BF₃ and BCl₃ molecules, and the Koopmans' theorem ionization potentials are compared with the experimental ones obtained from photoelectron spectra.     

Photoelectron spectra of hydrogen peroxide and hydrogen disulphide: ab initio calculations

Koopmans' theorem ionization potentials from ab initio gaussian basis set calculations on cis, trans and skew forms of HOOH and HSSH are compared with experimental values from photoelectron spectroscopy.     

Fluorine core esca linewidths in CH3F and CF4

Ab initio calculations within the Hartree-Fock formation have been carried out on potential energy surfaces of the ground and the F_1s hole states of CH₃F and CF₄ in order to investigate linewidths of their ESCA spectra. The calculations show that potential energy surfaces of both hole states have dissociative character and can be approximated by straight lines in the region of interest. A simple formula for the ESCA fwhm linewidth is derived which yields results in good agreement with experiment. Theoretically derived relaxation and Koopmans' energies have been investigated as a function of geometry.     

Double-ionization energies to ground and excited states of the tungsten hexacarbonyl dication

Tungsten hexacarbonyl was investigated by double-charge-transfer (DCT) spectroscoPy, and the double-ionization energies to ground and electronically excited states of W(CO) ₆ ²⁺ determined. The double-ionization energies corresponding to the first two distinct peaks in the spectra are 22.8 ± 0.3 eV and 28.5 ± 0.3 eV, but numerous overlapping peaks at higher energies are evident. It is shown that the DeI spectra can explain the main features of a previously determined (J. Am. Soc. Mass Spectrom. 1990, 1, 16–27) internal energy distribution curve for W(CO) ₆ ²⁺ ions formed by 70-eV electron ionization of W(CO)₆ molecules.     

Tuning the reactivity of mononuclear nonheme manganese(iv)-oxo complexes by triflic acid

Triflic acid (HOTf)-bound nonheme Mn(iv)-oxo complexes, [(L)Mn^IV(O)]²⁺–(HOTf)₂ (L = N4Py and Bn-TPEN; N4Py = N,N-bis(2-pyridylmethyl)-N-bis(2-pyridyl)methylamine and Bn-TPEN = N-benzyl-N,N′,N′-tris(2-pyridylmethyl)ethane-1,2-diamine), were synthesized by adding HOTf to the solutions of the [(L)Mn^IV(O)]²⁺ complexes and were characterized by various spectroscopies. The one-electron reduction potentials of the Mn^IV(O) complexes exhibited a significant positive shift upon binding of HOTf. The driving force dependences of electron transfer (ET) from electron donors to the Mn^IV(O) and Mn^IV(O)–(HOTf)₂ complexes were examined and evaluated in light of the Marcus theory of ET to determine the reorganization energies of ET. The smaller reorganization energies and much more positive reduction potentials of the [(L)Mn^IV(O)]²⁺–(HOTf)₂ complexes resulted in greatly enhanced oxidation capacity towards one-electron reductants and para-X-substituted-thioanisoles. The reactivities of the Mn(iv)-oxo complexes were markedly enhanced by binding of HOTf, such as a 6.4 × 10⁵-fold increase in the oxygen atom transfer (OAT) reaction (i.e., sulfoxidation). Such a remarkable acceleration in the OAT reaction results from the enhancement of ET from para-X-substituted-thioanisoles to the Mn^IV(O) complexes as revealed by the unified ET driving force dependence of the rate constants of OAT and ET reactions of [(L)Mn^IV(O)]²⁺–(HOTf)₂. In contrast, deceleration was observed in the rate of H-atom transfer (HAT) reaction of [(L)Mn^IV(O)]²⁺–(HOTf)₂ complexes with 1,4-cyclohexadiene as compared with those of the [(L)Mn^IV(O)]²⁺ complexes. Thus, the binding of two HOTf molecules to the Mn^IV(O) moiety resulted in remarkable acceleration of the ET rate when the ET is thermodynamically feasible. When the ET reaction is highly endergonic, the rate of the HAT reaction is decelerated due to the steric effect of the counter anion of HOTf.     

13C-NMR-Spektren von γ-substituierten pentamethincyanin-farbstoffen mit indolenin-endgruppen

The ¹³C-resonances of fourteen γ-substituted pentamethincyanine dyes 1 with indolenine nuclei have been measured and assigned. The effect of the γ-substituents R on the ¹³C chemical shifts is compared with the corresponding effect of substituents R of monosubstituted benzenes and 1-substituted trans-butadienes. The chemical shifts of the C²-methincarbons, which are separated by four bonds from R, correlate with Hammett's substituent parameters σ_para. A significant substituent effect through nine covalent bonds has been observed.     

Correlation between thermal electron transfer in solution and photoelectron emission

The activation free energy for electron transfer in solution or at electrodes is correlated to the corresponding Franck—Condon determined reorganization free energy R_m for photoelectron emission. Excellent to fair agreement is obtained between the activation free energies predicted from R_m and experimental values. Data are given for V²⁺, Cr²⁺, Mn²⁺, Fe²⁺, Co²⁺, and Fe(CN)^4?₆ in aqueous solution.     

Contributions to cytochrome c inner- and outer-sphere reorganization energy

Cytochromes c are small water-soluble proteins that catalyze electron transfer in metabolism and energy conversion processes. Hydrogenobacter thermophilus cytochrome c₅₅₂ presents a curious case in displaying fluxionality of its heme axial methionine ligand; this behavior is altered by single point mutation of the Q64 residue to N64 or V64, which fixes the ligand in a single configuration. The reorganization energy (λ) of these cytochrome c₅₅₂ variants is experimentally determined using a combination of rotating disc electrochemistry, chronoamperometry and cyclic voltammetry. The differences between the λ determined from these complementary techniques helps to deconvolute the contribution of the active site and its immediate environment to the overall λ (λ_Total). The experimentally determined λ values in conjunction with DFT calculations indicate that the differences in λ among the protein variants are mainly due to the differences in contributions from the protein environment and not just inner-sphere λ. DFT calculations indicate that the position of residue 64, responsible for the orientation of the axial methionine, determines the geometric relaxation of the redox active molecular orbital (RAMO). The orientation of the RAMO with respect to the heme is key to determining electron transfer coupling (H_AB) which results in higher ET rates in the wild-type protein relative to the Q64V mutant despite a 150 mV higher λ_Total in the former.

Efficient delocalization of the redox-active molecular orbital (RAMO) in HtWT results in an increase in H_AB value which in turn accelerates the electron transfer (ET) rate in spite of the higher reorganization energy (λ) than the HtQ64V mutant.     

Indice de liaison en méthode CNDO. Relation entre indice de liaison et distance de liaison

The use of p_AB = Σ_μ^A Σ_ν^B _μν S_μν as bond index in the CNDO approximation is theoretically justified as it is an elaborated form of Coulson's bond order and an approximation of Roby's shared electron density. This bond index correlates with diatomic energy E_AB, a measure of binding energy, and with interatomic distance R_AB, for several bonds involving hydrogen and atoms of the n = 2 period. As an example of utilization of p_AB bond lengths in butadiene and a series of ethylenic compounds are determined.     

Calculation of vertical ionization potentials of difluoramine by perturbation corrections to koopmans' theorem

The vertical ionization potentials of difluoramine are calculated by perturbation corrections to Koopmans' theorem. The calculation shows that difluoraimine has three overlapping bands between 15 and 16 eV. The calculated results compare well with the experimental values. The photoelectron spectrum of difluoramine is compared with that of OF₂ and CH₂F₂.