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.     

Electronic structure of benzaldehyde: A comparative study of the lowest excited singlet π* ← π and π* ← n states

VE-PPP, CNDO/2, and CNDO/s-CI methods have been used to investigate the electronic spectrum and structure of benzaldehyde. Electronic charge distributions and bond orders in the ground and lowest excited singlet π* ← π and π* ← n states of the molecule have been studied. The molecule has been shown to be nonplanar in the lowest π* ← n excited singlet state, in agreement with the conclusions drawn from the study of vibrational spectra. Dipole moments in both excited states have been shown to be larger than the ground-state value. Thus, the ambiguity in the experimental result for the π* ← π n excited singlet state dipole moment has been resolved. It has been shown that the n orbital is mainly localized on the CHO group. Furthermore, charge distributions, dipole moments, and molecular geometries are shown to be very different in the excited singlet π* ← π and π* ← n states.     

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.     

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.     

n→π*-Übergänge in Dicarbonylverbindungen Der Einfluss der n,n- und π*, π*-Wechselwirkung

By CNDO-CI calculations we have found that dicarbonyl compounds exhibit only two n → π* transitions in the visible or near UV. region, instead of four as expected from simpler MO-models. The dominant features of the long-wavelength electronic spectra may be characterized by the relative energy of the two n and the two lowest π* orbitals. In general we distinguish between three cases:

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Intramolecular electronic communication in a dimethylaminoazobenzene–fullerene C60 dyad: An experimental and TD‐DFT study

An electronically push–pull type dimethylaminoazobenzene–fullerene C₆₀ hybrid was designed and synthesized by tailoring N,N‐dimethylaniline as an electron donating auxochrome that intensified charge density on the β‐azonitrogen, and on N‐methylfulleropyrrolidine (NMFP) as an electron acceptor at the 4 and 4′ positions of the azobenzene moiety, respectively. The absorption and charge transfer behavior of the hybrid donor‐bridge‐acceptor dyad were studied experimentally and by performing TD‐DFT calculations. The TD‐DFT predicted charge transfer interactions of the dyad ranging from 747 to 601 nm were experimentally observed in the UV‐vis spectra at 721 nm in toluene and dichloromethane. A 149 mV anodic shift in the first reduction potential of the N?N group of the dyad in comparison with the model aminoazobenzene derivative further supported the phenomenon. Analysis of the charge transfer band through the orbital picture revealed charge displacement from the n_(N?N) (nonbonding) and π _(N?N) type orbitals centered on the donor part to the purely fullerene centered LUMOs and LUMO+n orbitals, delocalized over the entire molecule. The imposed electronic perturbations on the aminoazobenzene moiety upon coupling it with C₆₀ were analyzed by comparing the TD‐DFT predicted and experimentally observed electronic transition energies of the dyad with the model compounds, NMFP and (E)‐N,N‐dimethyl‐4‐(p‐tolyldiazenyl)aniline (AZNME). The n_(N?N) → π*_(N?N) and π_(N?N) → π*_(N?N) transitions of the dyad were bathochromically shifted with a significant charge transfer character. The shifting of π_(N?N) → π*_(N?N) excitation energy closer to the n → π*_(N?N) in comparison with the model aminoazobenzene emphasized the predominant existence of charge separated quinonoid‐like ground state electronic structure. Increasing solvent polarity introduced hyperchromic effect in the π_(N?N) → π*_(N?N) electronic transition at the expense of transitions involved with benzenic states, and the extent of intensity borrowing was quantified adopting the Gaussian deconvolution method. On a comparative scale, the predicted excitation energies were in reasonable agreement with the observed values, demonstrating the efficiency of TD‐DFT in predicting the localized and the charge transfer nature of transitions involved with large electronically asymmetric molecules with HOMO and LUMO centered on different parts of the molecular framework. © 2009 Wiley Periodicals, Inc. J Comput Chem, 2010     

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.     

Tayloring standard TDDFT approaches for computing UV/Vis transitions in thiocarbonyl chromophores

We report the development of an accurate computational procedure for the calculation of the n → π* (λ_max?1) and π → π* (λ_max?2) transitions of a set of thiocarbonyl derivatives. To ensure converged results, all calculations are carried out using the 6‐311+G(2df,p) basis set for time‐dependent calculations, and the 6‐311G(2df,p) for the ground‐state geometrical optimization. Starting with two hybrids, PBE0 and B3LYP, the Hartree–Fock exchange percentage (α) used is optimized in order to reach excitation energies that fit experimental data. It turns out that BLYP(α) is the more adequate functional for calibration. For the n → π* excitation, the optimal α value lies in the 0.10–0.20 interval, whereas for the π → π* process setting α equal to 0.10 provides the most accurate results. The corresponding mean absolute errors (MAE) are limited to 17 nm for λ_max?1, and to 10 nm for λ_max?2, allowing a consistent and accurate prediction of both transitions. We also assess the merits of the ZINDO//AM1 scheme and it turns out that the semi‐empirical method only provides a poor prediction of the λ_max of thiocarbonyl derivatives, especially for the n → π* transition. © 2007 Wiley Periodicals, Inc. Int J Quantum Chem, 2008     

Computational Photochemistry of the Azobenzene Scaffold of Sudan I and Orange II Dyes: Excited‐State Proton Transfer and Deactivation via Conical Intersections

We employed the complete active space self‐consistent field (CASSCF) and its multistate second‐order perturbation (MS‐CASPT2) methods to explore the photochemical mechanism of 2‐hydroxyazobenzene, the molecular scaffold of Sudan I and Orange II dyes. It was found that the excited‐state intramolecular proton transfer (ESIPT) along the bright diabatic ¹ππ* state is barrierless and ultrafast. Along this diabatic ¹ππ* relaxation path, the system can jump to the dark ¹nπ* state via the ¹ππ*/¹nπ* crossing point. However, ESIPT in this dark state is largely inhibited owing to a sizeable barrier. We also found two deactivation channels that decay ¹ππ* keto and ¹nπ* enol species to the ground state via two energetically accessible S₁/S₀ conical intersections. Finally, we encountered an interesting phenomenon in the excited‐state hydrogen‐bonding strength: it is reinforced in the ¹ππ* state, whereas it is reduced in the ¹nπ* state. The present work sets the stage for understanding the photophysics and photochemistry of Sudan I–IV, Orange II, Ponceau 2R, Ponceau 4R, and azo violet.     

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.     

Rydberg or Valence? The Long‐Standing Question in the UV Absorption Spectrum of 1,1′‐Bicyclohexylidene

The electronic excited states of the olefin 1,1′‐bicylohexylidene (BCH) are investigated using multiconfigurational complete active space self‐consistent‐field second order perturbation theory in its multi‐state version (MS‐CASPT2). Our calculations undoubtedly show that the bulk of the intensity of the two unusually intense bands of the UV absorption of BCH measured with maxima at 5.95 eV and 6.82 eV in the vapor phase are due to a single ππ* valence excitation. Sharp peaks reported in the vicinity of the low‐energy feature in the gas phase correspond to the beginning of the π3s_R Rydberg series. By locating the origin of the ππ* band at 5.63 eV, the intensity and broadening of the observed bands and their presence in solid phase is explained as the vibrational structure of the valence ππ* transition, which underlies the Rydberg manifold as a quasi‐continuum.     

π-Bond Configurations,Conformations, and Dynamic Behaviour of Benzo[c]octalene and Dibenzo[c,j]octalene

The π-bond configurations, the conformations, and the dynamic behaviour of dibenzo [c,j]octalene (2) and of benzo [c]octalene (3) have been investigated by ¹³C-NMR. spectroscopy at different temperatures. Dibenzooctalene was found to present π-bond fixation in the octalene unit as in 2b ; with this π-bond fixation the molecule is not planar and takes two different conformations which are rapidly interconverted by inversion of one cyclooctatetraene ring. Monobenzooctalene (3) also presents π-bond fixation in the octalene unit but exists as two valence isomers, 3b and 3c. Isomer 3c dominates the dynamic equilibrium. With this π-bond configuration, the molecule is chiral but undergoes several isodynamic processes, namely inversion of the cyclooctatriene and/or of the cyclooctatetraene ring. The valence isomer 3b can have two different conformations which are rapidly interconverted by inversion of one cyclooctatetraene ring. The interconversion 3c ? 3b implies the occurrence of a π-bond shift process; this process affects the ¹³C-NMR. lineshape above 50°.     

Effect of Solvents on the Spectra of Some β-Diketones

Acetoacetanilide, benzoylacetanilide and their derivatives have been examined in ultraviolet region in a series of solvents covering a broad polarity range e. e. from chloroform (Z, 63.2) to methanol (Z .83.6). Transition energies and oscillator strengths have been calculated and transition energy (E_T) has been plotted against Z-values, a new empirical measurement of solvent polarity. A linear relationship was observed between the transition energy and Z-values for π → π* and n → π* transitions. These transitions are identified as charge transfer (c-t) transitions and with the solvents having carbonyl oxygen and sulphur atom a c-t complex formation has been suggested. Strong electron-donating substituents on phenyl group of the nitrogen atom also showed a weak to moderate n → π* transitions. These substituents have no influence on the position of the λ_max in the same solvent. Stabilization energy of the excited state of these ligands and hence the dipole moments of the excited states have been calculated in comparison with pyridinium iodide. Solvent sensitivities of these ligands have also been calculated.     

π* ← n electronic absorption spectrum of 2,6-dichloropyrazine

The π* ← n electronic absorption system of 2,6-dichloropyrazine, corresponding to the ¹B_3u ← ¹A_1g transition of pyrazine, has been recorded in the vapour phase and in solution in cyclohexane. A vibrational analysis of this system has been proposed and it is shown that vibronic interaction between two excited states of 2,6-dichloropyrazine exists. Another system is observed in the solution spectrum of this molecule in cyclohexane and it is shown to be a π* ← π transition analogous to the ¹B_2u ← ¹A_1g transition in pyrazine.     

S1←S0 vibronic spectrum and structure of fluoral in the S1 state

The vibronic absorption spectrum of fluoral vapor was studied in the region of the S₁←S₀ electronic transition (313–360 nm). The origin O₀ ⁰⁺) of the transition (29419 cm⁻¹) and a number of fundamental frequencies in the S₀ and S₁ states were determined. The character of intensity distribution in the spectral bands indicates that the electronic excitation leads to significant change of the CF₃ group orientation relative to the molecular frame. Moreover, it was found that the carbonyl fragment of the molecule in the S₁ state has pyramidal structure (in contrast, the carbonyl fragment of the fluoral molecule in the S₀ state is planar). The experimental torsion and inversion energy levels were used for the calculation of internal rotation and inversion potential functions of fluoral molecule in the S₁ state. The potential barriers to internal rotation and inversion were found to be 1270 cm⁻¹ (15.2 kJ mol⁻¹) and 550 cm⁻¹ (6.6 kJ mol⁻¹), respectively. The conformational changes caused by S₁←S₀ electronic excitation in the fluoral molecule are similar to those observed in acetaldehyde and biacetyl molecules and differ from the conformational behavior of hexafluorobiacetyl molecule. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 2, pp. 294–299, February, 1998.     

Zur Deutung des UV.-Spektrums von Pyridin-N-oxid

From measurements of the influence of an electric field on the absorption spectrum of pyridine-N-oxide it is concluded that the 330 nm band is polarized perpendicular to the dipole moment, while the 280 nm transition moment lies parallel. Furthermore from these experiments the dipole moments in both excited states have been determined (Table 1). PARISER -PARR -POPLE -calculations as well as CNDO-calculations admit an assignment of the 330 nm band to an A₁ → B₁, π → π* transition and of the 280 nm band to an A₁ → A₁, π → π* transition. Thereby energy, polarization, intensity of the transition, and the dipole moments of the excited states have been taken into consideration. This assignment does not exclude the possibility of a weak n-π* transition at approximately the same wavelength as the A₁ → B₁ transition.     

The Importance of Ligand-Induced Backdonation in the Stabilization of Square Planar d10 Nickel π-Complexes

The electronic nature of Ni π-complexes is underexplored even though these complexes have been widely postulated as intermediates in organometallic chemistry. Herein, the geometric and electronic structure of a series of nickel π-complexes, Ni(dtbpe)(X) (dtbpe=1,2-bis(di-tert-butyl)phosphinoethane; X=alkene or carbonyl containing π-ligands), is probed using a combination of ³¹P NMR, Ni K-edge XAS, Ni K_β XES, and DFT calculations. These complexes are best described as square planar d¹⁰ complexes with π-backbonding acting as the dominant contributor to M−L bonding to the π-ligand. The degree of backbonding correlates with ²J_PP from NMR and the energy of the Ni 1s→4p_z pre-edge in the Ni K-edge XAS data, and is determined by the energy of the π*_ip ligand acceptor orbital. Thus, unactivated olefinic ligands tend to be poor π-acids whereas ketones, aldehydes, and esters allow for greater backbonding. However, backbonding is still significant even in cases in which metal contributions are minor. In such cases, backbonding is dominated by charge donation from the diphosphine, which allows for strong backdonation, although the metal centre retains a formal d¹⁰ electronic configuration. This ligand-induced backbonding can be formally described as a 3-centre-4-electron (3c-4e) interaction, in which the nickel centre mediates charge transfer from the phosphine σ-donors to the π*_ip ligand acceptor orbital. The implications of this bonding motif are described with respect to both structure and reactivity.     

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.     

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.     

Spezifisch π→π*-induzierte Reaktionen von γ-Dimethoxymethylcyclohexen-2-onen: 1,3-Umlagerung und Wasserstoffabstraktion durch das α-Kohlenstoffatom

When α,β-unsaturated γ-dimethoxymethyl cyclohexenones are excited to the S₂(π,π*) state, certain unimolecular reactions can be observed to compete with S₂ → S₁ internal conversion. These reactions do not occur from the S₁(n,π*) or the lowest T(π,π* and n,π*) states. They comprise the radical elimination of the formylacetal substituent (cf. 8 , 9 → 32 + 33 ), γ → α formylacetal migration (cf. 6 → 27 , 8 → 30 , 9 → 34 , 12 → 37 ), and a cyclization process involving the transfer of a methoxyl hydrogen to the α carbon and ring closure at the β position (cf. 6 → 28 , 8 → 31 , 12 → 38 , 20 → 40 + 41 ). The quantum yield of the ring closure 20a → 40a + 41a is 0.016 at ≤ 0.05M concentration. It is independent of the excitation wavelength within the π→π* absorption band (238–254 nm), but Φ ( 40a + 41a ) decreases at higher concentrations. According to the experimental data the reactive species of these specifically π→π*-induced transformations is placed energetically higher than the S₁(n,π*) state, and it is either identical with the thermally equilibrated S₂(n,π*) state, or reached via this latter state. The linear dienone 14 undergoes a similar π→π*-induced cyclization (→ 42 ) whereas the benzohomologue 26 proved unreactive, and the dienone 22 at both n → π and π→π* excitation only gives rise to rearrangements generally characteristic of cross-conjugated cyclohexadienones.