Ab initio SCF CL study of the electronic spectrum of nitroso-methane

Configuration interaction studies of ground, n_ → π^*, n₊ → π^*, and π → π^* electronically excited states are reported for nitroso-methane in its eclipsed equilibrium geometry. The first (n_ → π^*) and the second (n⁺ → π^*) singlet states are calculated at 2.17 and 7.14 eV. it is shown that a significant delocalization of the nonbonding orbitals on the nitrogen and oxygen is responsible for the large energy gap between these two states. The two lowest triplet states occur at 1.29 and 5.39 eV and are of n_ → π^* and π → π^* origin.     

Electronic absorption of carboxylic acids and their anions

Electronic spectra of formic, acetic, mono-, di-, trichloro- and trifluoroacetic, glycolic, cyanoacetic, pivalic, α-methoxyacetic, lactic, oxalic, tartaric and citric acids and betaine and of corresponding anions were recorded. The acid forms of all the carboxylic acids studied show a medium-strong π → π^* and a weak n → π^* absorption band, the latter in the 220–250-nm region. The corresponding anions (or the completely dissociated forms of polybasic acids) show the π → π^* absorption bands, but no indication of a shoulder corresponding to a n → π^* transition. Changes in the absorbance in the wavelength region corresponding to the n → π^* transition with addition of alkali metal hydroxides can be used for end-point detection in titrations. Changes of these absorbances in solutions of buffers or strong acids can be used for pK determinations. A pK value of 0.89 (at μ = 0.5) was found for dichloroacetic acid; approximate pK values were established by means of the H_o acidity scale for trichloroacetic acid (—0.80) and trifluoroacetic acid (—0.92). Finally, absorbances in the 220–250-nm region can be used for determination of carboxylic acids in solutions of strong acids, and some buffers, like phosphate or borate.     

Induced circular dichroism of azulene

The π→π^* absorption bands of azulene show circular dichroism in the presence of (+)-nopinone.     

The electronic absorption spectrum and excited state dipole moment of 3-fluoropyridine

The electronic absorption spectrum of 3-fluoropyridine in the vapour state and in solutions in different solvents in the region 3000-1900 Å has been measured and analysed. Three systems of absorption bands; n→π* transition I, π→π* transition II and π→π* transition III are identified. The oscillator strength of the absorption band systems due to the π→π* transition II and π→π* transition III and the excited state dipole moments associated with these transitions have been determined by the solvent-shift method.     

Photochemische Reaktionen. 62. Mitteilung [1]. Die Photofragmentierung von O-acetyljervin

On ultraviolet irradiation O-acetyljervine ( 1 ) is subjected to several parallel fragmentations. From the complex reaction mixtures obtained in a variety of solvents (dioxan, tetrahydrofuran, acetonitrile, iso-octane, benzene) the major alicyclic products 6 – 8 and the heterocyclic compounds 12 – 16 have been isolated. Products 6 – 8 undergo further photochemical changes, e.g., decarbonylation of 7 to 9 and hydrolytic cleavage of 8 to 10 . These photofragmentations are initiated almost specifically upon selective π → π* excitation at 2537 Å with a quantum yield of Φ²⁵³⁷ = 0.145 for conversion of starting material. Reaction upon irradiation in the long-wavelength n → π* absorption band is very much less efficient (Φ³⁶⁶⁰ = 0.611 · 10^?3, both determinations for O-trimethylsilyl-jervine ( 2 ) in tetrahydrofuran). A high degree of photostability is observed also at 2537 Å on N-protonation of O-acetyljervine ( 1 ) in acetic acid. Furthermore, reactivity is greatly reduced for the N-methyl ( 3 ) and N-acetyl ( 4 ) derivatives in neutral solvents at 2537 Å. N-Chloro-O-acetyljervine ( 5 ) in dioxan at 2537 Å gave preferentially O-acetyljervine hydrochloride.     

Circular dichroism spectra of straight chain mono-olefins. Assignment of ethylene transitions

The circular dichroism spectra of three optically active alkenes, S, 3-methyl-1-pentene, S,4-methyl-1-hexene and S,5-methyl-1-heptane have been measured in the spectral region 200-140 nm in the gas phase. An intense CD band which does not have a well defined analogue in the absorption was observed in all three compounds and is assigned as π → σ_(CH)^*. A second prominent CD band at higher energy is assigned as σ_(CH) → π^*. The assignments of low frequency CD bands are discussed.     

Application of the MC SCF method to the π → π* excitation energies of ethylene

The MC SCF method is employed to calculate the N → T and N → V π → π^* vertical excitation energies of ethylene. To obtain accurate excitation energies it is found to be necessary to utilize an expanded valence space containing two π and two π^* orbitals. Relatively small MC SCF calculations, allowing at most one-electron excitations from the sigma space, are found to yield excitation energies and spatial extents of the excited states in excellent agreement with the predictions of large multi-reference or iterative-natural-orbital CI calculations. These results show that within an MC SCF framework σ-σ correlation is unimportant for describing the π → π^* processes. We also conclude that the neglect of the effects of unlinked cluster terms in some of the CI calculations may have introduced small, but important, errors in the excitation energies and predictions of the spatial extent of the V state.     

The far ultraviolet spectrum of thioformaldehyde

The electronic absorption spectrum of thioformaldehyde has been recorded from 2200 to 1800 Å. Four electronic transitions have been identified in the spectrum and have been assigned to the π → π*, n → 4s, n → 4p_y and n → 4p_z electron promotions.     

Theoretical solvent effects on molecular structure and spectroscopic properties through the virtual-charge model. Application to the inverted solvatochromy of benzophenone n → π* and π → π* absorption bands

Non-specific solvent effects have been introduced within the virtual-charge model of Constanciel and Tapia: the definition of solvatons for a reference state allows us to apply this method to the study of all states (ground and excited) of the molecule. This model, which has been developed here within the CNDO/S scheme, allows a quantitative evaluation of solvatochromy of electronic transitions. Moreover, relationships between bond lengths and bond population indexes are used to describe geometrical variations upon solvation. The calculations are applied to the study of solvent effects on the molecular structure and the n → π^* and π → π^* absorption bands of the benzophenone molecule. The results of the model are in good agreement with solvent-dependent experimental data and it can possibly be used when the experimental approach is difficult.     

Electronic spectra of 2-methoxypyridine and 2-methoxy-d3-pyridine

The electronic absorption spectrum of 2-methoxypyridine in the vapour and solution phases and 2-methoxy-d₃-pyridine in the vapour state in the region 3000–2450 Å and the luminescence spectra of 2-methoxypyridine in ethanol at 77 K have been measured and analysed. The oscillator strength of the absorption band system due to the π → π* transition and the excited state dipole moment in the ¹π,π* state have been estimated for 2-methoxypyridine.     

Excited electronic states of the fluorobenzenes by variable angle electron impact spectroscopy

Electron-impact excitation spectra of benzene, fluorobenzene, o-difluorobenzene, 1,3,5-trifluorobenzene, 1,2,3,4-tetrafluorobenzene, pentafluorobenzene, and hexafluorobenzene have been measured at impact energies of 50 eV and either 25 eV or 30 eV, and scattering angles from 5° to 80°. Each molecule shows an absorption maximum at about 3.9 eV corresponding to a singlet → triplet, π → π^*, transition. In benzene, fluorobenzene, o-difluorobenzene, and 1,3,5-trifluorobenzene, an additional singlet → triplet transition was detected at about 5.6 eV. Three singlet → singlet transitions analogous to the 4.90, 6.20, and 6.95 eV transitions in benzene are seen in each of the fluorine-substituted molecules. The more highly substituted compounds exhibit an additional singlet → singlet transition that is most clearly observed in the hexafluorobenzene spectrum with a peak at 5.32 eV.     

Absorption and magnetic circular dichroism studies on thiepins

2,7-di-tert-butyl and 2,7-di-tert-butyl-4-ethoxy-carbonylthiepins exhibit a similar spectroscopic profile in their absorption and the magnetic circular dichroism spectra. A quantum-mechanical calculation based on the INDO method gave a theoretical interpretation of the spectra, which were all ascribed to the π→π^* electronic transitions characteristic of the thiepin skeleton.     

Electronic structure of the radicals NF and NCI. II. Potential energy curves for NCI

Large-scale MRD CI calculations are employed to determine the potential energy curves for the three lowest X³Σ⁻, a¹Δ and b¹Σ⁺ states of NCl with π^*2 configuration, for the higher states arising from σ → π^*, π → σ^* and π → π* excitation and for the ²Π ion. Numerous avoided crossings are found to occur between curves in the region of internuclear separations treated. A comparison with the corresponding curves for the radicals NF, O₂ and SO is undertaken and the basic similarities and distinctions are discussed. The curves in NF and NCl are quite similar, with generally lower energy gaps in the second-row radical; they can to a large extent be constructed from the homonuclear counterpart by mixing of g and u symmetry and by taking into account the different multiplicities in the dissociation products N + Cl or F and O + O or S.     

Quenching of triplet states of diazines by H-atom donors. Formation of azyl radicals

The laser flash photolysis of pyrazine in water and in organic solvents has been examined. The ³(n, π^*) state in water has absorption bands at 230, ≈ 260, ≈ 295, ≈ 640, 700 and 810 nm, and decays with k = 2.2 × 10⁵ sec^?1. It is quenched by oxygen with k_q = 3.2 × 10⁹ M^?1 sec^?1 and by various H-atom donors, e.g., k_q = 1.3 × 10⁸ M^?1 sec^?1 for isopropyl alcohol. On reaction with H-atom donors, the chemistry of ³(n, π^*) pyrazine produces the neutral pyrazyl-radical and the dihydro radical cation, whose characteristic absorption spectra have been identified. These results are discussed by comparison with ³(π, π^*) diazines and with ³(n, π^*) aromatic carbonyl compounds.     

Anomalous solvent shift effect and observation of phosphorescence in bilirubin

Bilirubin IX-α has a large extinction coefficient but shows a weak blue-shift in solvents of increasing dielectric constant. A blue-shift is typical of an n → π^* transition, and is here interpreted in terms of the amide group present in the terminal pyrrole rings. Compounds undergoing n → π^* transitions usually form triplet states. With bilirubin, an emission is observed at 77 K; evidence is presented that this may be phosphorescence from an excited triplet state. The energy of this triplet level is 230 kJ mole^?1, thus bilirubin should be capable of sensitizing the formation of ¹Δ_g O₂.     

A molecular orbital study of lithium ion association with bases. The excited carbonyl bases R2CO

SCF CI calculations have been performed to investigate Li^XXX association with excited bases R₂CO. Although association leads to large increases in n → π¹ transition energies, the complexes R₂COLi^XXX remain bound in the n → π¹ state, but are destabilized relative to the ground state. In the Li^XXX-urea complex, the n → π¹ A₂, state lies slightly above a charge-transfer π → σ^* A₂ state.     

Direct quantum chemical calculation of rotatory strengths of methylpyrrolidones : Model compounds containing a peptide group

CNDO/S method is used to compute the rotatory strengths of L-5-methylpyrrolid-2-one and S-3-methylpyrrolid-2-one, cyclic compounds containing a peptide group. The observed circular dichroism (CD) bands in the regions of 185 and 220 nm corresponded to π → π^* and n → π^* transitions, respectively. The computed rotatory strengths indicate that nonplanarity of pyrrolidone ring makes a substantial contribution to the CD of compounds under consideration. The comparison of computed and experimental rotatory strengths suggests that both compounds have a nonplanar ring conformation.     

Triplet states of fluorocarbonyl compounds determined by trapped electron spectroscopy

Ion cyclotron resonance trapped electron spectra for carbonylfluoride, trifluoroacetyl fluoride, hexafluoroacetone, acetyl fluoride and I,I.I-trifluoroacetone are reported. Comparisons of data obtained for ³(π → π^*) vertical excitation energies are used in conjunction with photoelectron spectroscopic data to deduce the effects of fluorine and trifluoromethyl groups on the energies of σ, π and π^* orbitals associated with the carbonyl group.     

The 1A1 π→π* state of formaldehyde

The ¹A₁ π→π^* state of formaldehyde is predicted to be 11.2 eV above the ground state and not diffuse.     

Triplet excited states of nitronaphthalenes. III. 1,4-dinitronaphthalene

Nanosecond flash photolysis of 1,4-dinitronaphthalene (1,4-DNO₂N) in aerated and deaerated solvents shows a transient species with absorption maximum at 545 nm. The maximum of the transient absorption is independent of solvent polarity and its lifetime seems to be a function of the hydrogen donor efficiency of the solvent. The transient absorption is attributed to the lowest excited triplet state of 1,4-DNO₂N. The reactivity of this state for hydrogen abstraction from tributyl tin hydride (Bu₃SnH), K_q = 3.8 × 10⁸M^?1 sec, is almost equal to that of nitrobezene triplet state which has been characterized as an n → π* state. Based on spectroscopic and kinetic evidence obtained in the present work, the triplet state of 1,4-DNO₂N behaves as an n → π* state in nonpolar solvents, while in polar solvents the state is predominantly n → π* with a small amount of intramolecular charge transfer character.