Ab initio calculations on urea

Multiconfiguration wave functions constructed from contracted Gaussian-lobe functions have been found for the ground and valence-excited states of urea. ICSCF molecular orbitals of the excited states were used as the parent configurations for the CI calculations except for the ¹A₁(π → π*) state. The ¹A₁(π → π*) state used as its parent configuration an orthogonal linear combination of natural orbitals obtained from the second root of a three-configuration SCF calculation. The lowest excited states are predicted to be the n π → π* and π → π* triplet states. The lowest singlet state is predicted to be the n π → π* state with an energy in good agreement with the one known UV band at 7.2 eV. The π → π* singlet state is predicted to be about 1.9 eV higher, contrary to several previous assignments which assumed the lowest band was a π → π* amide resonance band. The predicted ionization energy of 9.0 eV makes this and higher states autoionizing.     

The T3 (π, π) State of acridine

The 00 band maximum of the transition T₃(π, π^*) ← T₁ (π, π^*) of acridine occurs at ≈ 10200 ± 20 cm^?1 in inert (n-hexane, benzene, CCl₄), at 10220 ± 20 cm^?1 in polar (acetonitrile) and at 10170 ± 50 cm^?1 in hydrogen-bonding (methanol, 2-propanol and alkaline water) solvents. Based on the solvent-independent energy of T₁ (π, π^*), the T₃(π, π^*) state of acridine is estimated at 26050 ± 50 cm^?1 in all the solvents.     

The Synthesis,X‐ray Structure Analysis,and Photophysical Characterization of 4‐[(E)‐2‐(4‐Carboxyphenyl)diazenyl]‐morpholine

4‐[(E)‐2‐(4‐Carboxyphenyl)diazenyl]‐morpholine ( 1 ) was prepared in 33% yield from a coupling reaction between morpholine and the diazonium ion formed from 4‐aminobenzoic acid. X‐ray structural analysis of 1 yielded two important insights into its structure: the geometry of the N―N double bond and the partial delocalization across the linear triazene moiety. The absorption spectra of 1 in dilute acetonitrile and 2‐methyltetrahydrofuran solutions both featured an intense (ε ≈ 20,000 M^?1cm^?1) band centered at 320–324 nm that was assigned as a mixture of π → π* and n → π* transitions. Emission was observed at 383 and 379 nm from dilute acetonitrile and 2‐methyltetrahydrofuran solutions of 1 , respectively, with the latter being red‐shifted to 439 nm at 77 K. Emission lifetime data for compound 1 provided evidence that the emission was a mixture of two excited state transitions.     

On the lower excited electronic states of glyoxal

The polarization of both nπ* absorption bands of glyoxal has been measured in a glass matrix of 2-methyltetrahydrofuran by the photoselection method. The second absorption band in the 30 000 cm^?1 region has been assigned to a ¹A_g → ¹B_g nπ* transition. Its intensity is mainly induced by interaction with the solvent. An absorption band at about 43 000 cm^?1 has been ascribed to a charge transfer transition in complexes of glyoxal and 2-MTHF.     

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.     

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.     

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.     

Electronic absorption spectrum and excited state dipole moment of 2,6-difluoropyridine

The u.v. absorption spectrum of 2,6-difluoropyridine in the region 41 000-34 000 cm⁻¹ in the vapour state and in solution has been recorded and a vibronic analysis made. Only one system of bands arising from the π → π* transition has been observed and the 0,0 band is located at 37 840 cm⁻¹ in the vapour-phase spectrum. The oscillator strength of the band system in solution and the dipole moment in the excited state associated with the transition were determined.     

Vibronic spectra of solutions and sols of copper phthalocyanine

Optical spectra of solutions and sols prepared from finely dispersed crystalline copper phthalocyanine (CuPc) were studied. The CuPc preparation was demonstrated to contain an admixture of an amorphous phase. The amorphous phase proved to be soluble in dioxane and heptane with the formation of a true molecular solution of CuPc. It was found that CuPc molecules are absorbed by polyethylene, polypropylene, polycaproamide, and cellulose triacetate films. The optical spectrum of individual CuPc molecules was demonstrated to differ substantially from those of particles of the pigment. It featured intense vibronic bands belonging to three π → π* transitions typical of aromatic structures and a series of bands characteristics of n → π* transitions involving nitrogen atoms (<29000 cm^?1) but showed no absorption bands characteristic of dispersions of the pigment in the visible spectrum (400–800 cm^?1). It was revealed that the Q-band (λ = 670 nm), assigned in the literature to individual CuPc molecule, in reality belongs to CuPc associates.     

Errata

The π → π* absorption spectrum of thioformaldehyde has been recorded at relatively high pressures and path lengths. The system is quite extensive and in H₂CS displays a progression of bands in an interval of 476cm^?1 which can be followed out to ν′ = 12. This is assigned to the ν′₃ CS stretching mode. A second weaker set of bands is assigned to 2ν′₄ the out-of-plane bending mode. The 725 cm^?1 interval observed here compared to the 711 cm^?1 value of the ã state leads us to the conclusion that the barrier to inversion is less than 50 cm^?1.     

Darstellung und spektroskopische Eigenschaften der gemischtvalenten Di(phthalocyaninato)lanthanide(III)

Synthesis and Spectroscopical Properties of the Mixed-Valent Di(phthalocyaninato)lanthanides(III) Green di(phthalocyaninato)lanthanide(III), [M(Pc)₂] (M = rare earth metal ion: La‥(-Ce, Pm)‥Lu) is prepared by anodic oxidation of (ⁿBu₄N)[M(Pc^2?)₂] dissolved in CH₂Cl₂/(ⁿBu₄N)ClO₄. The UV-Vis-NIR spectra show intense π-π* transitions at ? 15000 cm^?1 and 31000 cm^?1, typical for Pc^2? ligands. Bands at ? 11000 cm^?1 and 22000 cm^?1 indicate the equal presence of a Pc^? π-radical. The metal dependent NIR band between 4000 and 9000 cm^?1 is characteristic for these mixed-valent complexes and assigned to an intervalence transition (b₁ → a₂; D_4d symmetry). Most bands are shifted linearly with the M^III radius. In the IR and resonance Raman (r.r.) spectra the typical vibrations of the Pc^? π-radical are dominant. These are essentially metal independent excepting the C? C and C? N vibrations of the inner (CN)₈ ring. The sym. M? N stretching vibration between 141 (La) and 168 cm^?1 (Lu) is selectively r.r.-enhanced when excited with 1064 nm.     

A configuration interaction study of the spin dipole-dipole parameters for formaldehyde and methylene

The electron spin dipole-dipole contribution to the zero field splitting has been evaluated for the ³A₂ (n → π*) and ³A₁ (π → π*) states of formaldehyde using a CI wave function constructed from contracted Gaussian-lobe functions. The values D = 0.539 cm^?1 and E = 0.031 cm^?1 were obtained for the ³A₂(n → π*) state and D = ?0.588 cm^?1 and E = 0.058 cm^?1 were obtained for the ³A₁ (π → π*) state using the CI wave function constructed from SCF orbitals of the respective parent configurations. An analysis of the effect of CI on the parameters is given for the ³A₂ (n n → π*) state of formaldehyde and the ³B₁ ground state of methylene. Numerical results are given which show that internally consistent self-consistent field orbitals (ICSCF ) are superior to canonical SCF orbitals as a starting point for a CI calculation. Our CI wave function for the ¹A₁ ground state gave an energy of ?114.13658 hartrees which is significantly lower than any previously reported energy calculation. This wave function gives a dipole moment of 2.22 Debye (C⁺O^?) in good agreement with the experimental value of 2.33 ± 0.02 Debye.     

Study of the electronic spectra of 2-fluoro-5-chloropyridine and determination of the excited state dipole moment

The ultraviolet absorption spectrum in the region 300-190 nm in the vapour phase and in solution in different solvents, and the luminescence emission spectra in ethanol and cyclohexane at 77 K have been measured for 2-fluoro-5-chloropyridine and analysed. The molecule shows two systems of absorption bands corresponding to the π→π* transition II and π→π* transition III. The oscillator strength of the two systems of absorption bands in solutions and the excited state dipole moment in the ¹π, π* state have been determined. The half-life of phosphorescence in cyclohexane is measured and found to be 3·6 s.     

The reaction of fluorine atoms with formyl fluoride and the CFO self-reaction at 293 K

A fast-flow apparatus with mass spectrometric detection was used to study the system F + CHFO between 2 and 3.5 mbar total pressure. The rate constant of the primary reaction was evaluated directly to yield at 298 K k₍₁₎ = (8.8 ± 1.4) * 10^?13 cm³ * molecule^?1 * s^?1. Numerical modelling was used to determine the rate constant at 298 K of the subsequent reaction CFO + CFO → CF₂O + CO: k₍₂₎ = (4.9 ± 2.0) * 10^?11 cm³ * molecule^?1 * s^?1. The possible occurrences of secondary reactions, CFO + F + M → CF₂O + M, and CFO + F₂ → CF₂O + F, can be excluded under the present conditions. © 1993 John Wiley & Sons, Inc.     

Absorption Spectra and Photoreactivity of Para-aminobenzophenone by Time-dependent Density Functional Theory

The absorption spectral properties of para-aminobenzophenone (p-ABP) were investigated in gas phase and in solution by time-dependent density functional theory. Calculations suggest that the singlet states vary greatly with the solvent polarities. In various polar solvents, including acetonitrile, methanol, ethanol, dimethyl sulfoxide, and dimethyl formamide, the excited S1 states with charge transfer character result from π→π* transitions. However, in nonpolar solvents, cyclohexane, and benzene, the S1 states are the result of n→π* transitions related to local excitation in the carbonyl group. The excited T1 states were calculated to have ππ* character in various solvents. From the variation of the calculated excited states, the band due to π→π* transition undergoes a redshift with an increase in solvent polarity, while the band due to n→π* transition undergoes a blueshift with an increase in solvent polarity. In addition, the triplet yields and the photoreactivities of p-ABP in various solvents are discussed.     

Darstellung,Eigenschaften und elektronische Raman-Spektren von Bis(chloro)phthalocyaninatoferrat(III), -ruthenat(III) und -osmat(III)

Preparation, Properties and Electronic Raman Spectra of Bis(chloro)-phthalocyaninatoferrate(III), -ruthenate(III) and -osmate(III) Bis(chloro)phthalocyaninatometalates of Fe^III, Ru^III and Os^III [MCl₂Pc(2-)]^?, with an electronic low spin ground state are formed by the reaction of [FeClPc(2-)] resp. H[MX₂Pc(2?)] (M = Ru, Os; X = Cl, I) with excess chloride in weakly coordinating solvents (DMF, THF) and are isolated as (n-Bu₄N) salts. The asym. M? Cl stretch (ν_as(MCl)) is observed in the f.i.r. at 288 cm^?1 (Fe), 295 cm^?1 (Ru), 298 cm^?1 (Os), ν_as(MN) at 330 cm^?1 (Fe), 327 cm^?1 (Ru), and 317 cm^?1 (Os); only ν_s(OsCl) at 311 cm^?1 is resonance Raman (r.r.) enhanced with blue excitation. The m.i.r. and FT-Raman spectra are typical for hexacoordinated phthalocyanines of tervalent metal ions. The UV-vis spectra show besides the characteristic π-π* transitions (B, Q, N, L band) of the Pc ligand a number of extra bands at 12–15 kK and 18–24 kK due to trip-doublet and (Pc→M)CT transitions. The effect of metal substitution is discussed. The r.r. spectra obtained by excitation between the B and Q band (λ₀ = 476.5 nm) are dominated by the intraconfigurational transition Γ₇ Γ ₈ arrising from the spin-orbit splitting of the electronic ground state for Fe^III at 536 cm^?1, for Ru^III at 961 cm^?1 and Os^III at 3 028 cm^?1. Thus the spin-orbit coupling constant increases very greatly down the iron group: Fe^III (357 cm^?1)< Ru^III (641 cm^?1)< Os^III (2 019 cm^?1). The Γ₇ Γ ₈-transition is followed by a very pronounced vibrational finestructure being composed in the r.r. spectra by the coupling with ν_s(MCl), δ(MClN) and the most intense fundamental vibrations of the Pc ligand. In absorption only vibronically induced transitions are observed for the Ru and Os complex at 1 700-2800 rsp. 3100-5800 em^?1 instead of the 0-0 phonon transitions. The most intense lines are attributed to combinations of the intense odd vibrational mo-des at ≈ 740 and 1120 cm^?1 with ν₅(MCI), δ(MClN).     

Electronic absorption spectra of M2L6 compounds containing metal-metal triple bonds of σ2π4 configuration

The electronic absorption spectra of compounds containing metal-metal triple bonds of σ²π⁴ valence electronic configuration are presented and discussed. The lowest-energy transition of M₂L₆ compounds (M = Mo or W, L = CH₂Bu^t or OBu^t) is expected to be the dipole-allowed π → π* (e_u → e_g) transition. This appears to be the case for M₂(CH₂Bu^t)₆ and M₂(OBu^t)₆ compounds, in which the lowest energy absorption bands occur between 26,000 and 28,000 cm⁻¹ (ε = 1.1 x 10³-1.8 x 10³ M⁻¹ cm⁻¹). For M₂(NMe₂)₆ compounds, the lowest energy absorption is not the π → π* transition but is assigned instead to a LMCT transition originating from nitrogen lone-pair orbitals, N_1p → π*, observed at 30,800 cm⁻¹ (ε = 1.4 x 10⁴-1.9 x 10⁴ M⁻¹ cm⁻¹). The π → π* transition is not observed in these compounds, but is presumably masked by the more intense LMCT. These assignments are derived from Xα-SW calculations performed and described by other authors (Bursten et al., J. Am. Chem. Soc. 1980, 102, 4579).     

Molecular orbital theory of the hydrogen bond. 25. Water–uracil complexes in excited n → π states

Hydrogen bonding of uracil with water in excited n → π* states has been investigated by means of ab initio SCF -CI calculations on uracil and water–uracil complexes. Two low-energy excited states arise from n → π* transitions in uracil. The first is due to excitation of the C₄? O group, while the second is associated with excitation of the C₂? O group. In the first n → π* state, hydrogen bonds at O₄ are broken, so that the open water–uracil dimer at O₄ dissociates. The “wobble” dimer, in which a water molecule is essentially free to move between its position in an open structure at N₃? H and a cyclic structure at N₃? H and O₄ in the ground state, collapses to a different “wobble” dimer at N₃? H and O₂ in the excited state. The third dimer, a “wobble” dimer at N₁? H and O₂, remains intact, but is destabilized relative to the ground state. Although hydrogen bonds at O₂ are broken in the second n → π* state, the three water–uracil dimers remain bound. The “wobble” dimer at N₁? H and O₂ changes to an excited open dimer at N₁? H. The “wobble” dimer at N₃? H and O₄ remains intact, and the open dimer at O₄ is further stabilized upon excitation. Dimer blue shifts of n → π* bands are nearly additive in 2:1 and 3:1 water:uracil structures. The fates of the three 2:1 water:uracil trimers and the 3:1 water:uracil tetramer in the first and second n → π* states are determined by the fates of the corresponding excited dimers in these states.     

Photochemische Reaktionen. 65.(vorläufige) Mitteilung. Spezifisch π → π*-induzierte Photoisomerisierungen von 10-Dimethoxymethyl-Δ1,9-octal-2-on. Ein photochemischer Zugang zu [4.4.3]-12-Oxapropellan-Derivaten

Selective n → π* excitation of the α,β-unsaturated enone 1 in hydrocarbon solvents resulted in a deconjugation reaction to 3 , reminiscent of results previously reported for similar systems [2], whereas the photoreactivity of 1 in alcohol solvents at wavelengths >3400 Å was so small that only product 4 has been identified as yet. Excitation of the π → π* transition of compound 1 at 2537 Å initiated additional phototransformations which could not be effected by irradiation in the first absorption band. The [4.4.3]-12-oxapropellane derivative 2 was identified as one of the two new major photo-isomers. A 6:8 mixture of products 2 and 3 , plus about 1 part of an isomer of still unknown structure (see however, the Addendum), were readily formed in hydrocarbon solvents, and a 1:10 ratio of 2 and the unknown product was obtained in methanol. Abstraction of a methoxyl hydrogen by the ketone oxygen is proposed to account for the primary photochemical step in the cyclization to 2 . A hydrogen-deuterium isotope effect of 2.7 was observed in a competitive experiment using 1 and 1-d _6. 34% of one deuterium atom were exchanged for hydrogen when 1-d ₆ was photolyzed to 2-d ₆ in t-butyl alcohol, which suggests an intermediate of type a in the pathway 1 → 2 possessing a readily exchangeable proton. Steric considerations would require a strongly distorted, non-planar excited-state geometry of the enone group of 1 for the oxygen to approach a methoxyl hydrogen. The transformation 1 → 2 represents a novel reaction type in photochemical processes of conjugated enones which are specifically induced by π → π* excitation only.     

Fluorescence à 77 K,rotation interne et structure electronique du styréne

A vibronic analysis of the fluorescence spectrum of styrene in crystalline solution at 77 K corroborates the assignment of the O.O band at 34758 cm^?1 for gas-phase spectra. The torsional frequency of the vinyl group with respect to the phenyl ring is used to evaluate the internal rotation barrier of styrene (5.7 kcal mol^?1). Configuration interaction MO calculations including all the mono and di-excilations within the π-electron system of styrene suggest that the first π—π^* transition has a moment tilted by 10° with respect to the long axis of the molecule; this is in agreement with the rotational structure of the gas-phase band at 34758 cm^?1, the analysis of which results in an A-type band.