On the correlation between photoelectron and electronic spectra in trans- azomethane

A possibility of correlating electronic and photoelectron spectra is discussed, using trans-azomethane as an example. The Coulomb and exchange integrals required were obtained by three semi-empirical SCF-methods: MINDO/2, CNDO/2, and a modified CNDO method. The orbital energies were taken as minus the corresponding experimental ionization potentials. The sequence of the transition energies ΔE (n_s → π*) Δ E (n_a → π*) < ΔE (π → π*) is found to be different from the ionization potential sequence IP (n_s) < IP (π) < IP (n_a), in agreement with previous spectroscopic studies; the results support the latest view that the π → π* transition of the azo group occurs at around 12 eV.     

Calculations on the electronic spectra of anilino,phenoxyl and benzyl radicals

The electronic spectra of benzyl, anilino and phenoxyl have been calculated, using two well known SCF-MO methods. Good agreement is found with experiment in all instances. However, the calculations still cannot explain the red shift produced by the addition of an extra electron to benzyl.

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Zesummenfassung Die Elektronenspektren des Benzyl-, Anilino- und Phenoxylradikals sind mit Hilfe zweier wohlbekannter SCF-MO-Methoden berechnet worden. Die Übereinstimmung mit den Experimenten ist überall gut. Die Rechnungen können jedoch nicht die Rotverschiebung erklären, die beim Hinzufügen eines Elektrons zum Benzylradikal erzeugt wird.

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Resume On calcule les spectres électroniques des radicaux benzyle, anilino et phénoxyle par deux méthodes SCF-MO bien connues. L'accord avec l'expérience est bon partout. Cependant, ces calculs n'expliquent pas l'effet bathocrome de l'addition d'un électron au benzyle.

     

Theoretical study on the structures and electronic spectra of TCNE2-.

Investigations into the charge-separated states and electron-transfer transitions in tetracyanoethylene (TCNE) complexes have recently generated much interest. In this work we present theoretical calculations showing that the most stable structure of the dianion TCNE2- has D2d symmetry in vacuum as well as in the solvents dichloromethane and acetonitrile. By means of the coupled cluster linear response, we compute the vertical electronic spectrum in both the gas phase and solution. The theoretical results are compared to the experimental data and good agreement is achieved.     

CNDO,INDO and RCNDO-CI calculations on the electronic spectra of saturated hydrocarbons

CNDO, INDO and RCNDO (CNDO including higher (Rydberg) atomic orbitals in the basis) calculations completed by first order configuration interaction were performed on straight chain and branched chain paraffins.The results interpret reasonably the main characteristics of the observed electronic spectra. The importance of outer atomic orbitals is stressed and it is found that the first singlet-singlet transition of highly branched paraffins leads to an excited state with considerable Rydberg character.

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Zusammenfassung Rechnungen vom Typ CNDO, INDO und RCNDO (CNDO mit höheren (Rydberg) Atomorbitalen in der Basis) unter Einschluß von Konfigurationswechselwirkung 1. Ordnung wurden für unverzweigte und verzweigte gesättigte Kohlenwasserstoffe durchgeführt.Die Resultate lassen eine Deutung der Hauptcharakteristika der beobachteten Elektronen-spektren zu. Die Wichtigkeit der äußeren Atomorbitale ist zu betonen. Die ersten Singulett-Singulett-Übergänge der stark verzweigten Paraffine führen zu angeregten Zuständen mit beträchtlichem Rydbergcharakter.

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Résumé Des calculs CNDO, INDO et RCNDO (ayant des orbitales atomiques supérieures dans la base) ont été effectués sur les paraffines normales et ramifiées tenant compte de l'intéraction de configuration de premier ordre jusqu'à 30 configurations.Les résultats interprêtent d'une manière raisonnable les spectres électroniques mesurés. L'importance des orbitales atomiques supérieures apparaît clairement. La première bande observée des hydrocarbures hautement ramifiés comme le néopentane ou l'isobutane est attribuée à une transition dont l'état excité possède un fort caractère Rydberg.

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The research for this paper was supported in part by the Defense Research Board of Canada; Grant Number 9530-63.     

The electronic structure,spectra, and reactivity of nitrophenols

The all-valence-electron CNDO /2 calculations were performed for the three isomeric nitrophenols. Using the newly derived σ-core charges and subsequently revising the valence-state ionization potentials and one-center two-electron repulsion integrals, Pariser–Parr–Pople (PPP ) CI calculations were performed on the title compounds following the Nishimoto–Forster scheme. A better agreement between theory and experiment has been observed in spectral assignments compared to the conventional PPP approach. Information from the CNDO /2 calculations was used to obtain useful electronic structural parameters and to get a quantitative insight into the chemical reactivity of these molecules. All the results were compared with the basic compounds, phenol and nitrobenzene. The electronic spectra of these isomers were recorded in both polar and nonpolar solvents.     

The electronic absorption spectra and the electronic structures of cytosine,isocytosine, and their anions and cations

The electronic absorption spectra of cytosine and isocytosine have been measured in several solvents at room temperature, and the temperature and pH dependencies of the electronic absorption spectra in aqueous solution have also been examined, special attention being paid to the existence of tautomeric forms. Consequently, it is concluded that cytosine in aqueous solution takes 2-keto-6-amino (3H) form (I) at room temperature, but with rising temperature 2-keto-6-imino form (III) coexists with form I, the latter being determined to be more stable by 5.5 kcal/mole than the former, and that cytosine takes form I and form III in trimethyl phosphate and in acetonitrile, respectively. The change in stability of the tautomeric forms by different solvents has been discussed with regard to their calculated dipole moments due to-electrons and the polarity of the solvents.The-electron structures of several tautomeric forms of cytosine (5 tautomeric forms) and isocytosine (3 tautomeric forms) and of their ions have been calculated by combining the configuration interaction with the Pariser-Parr-Pople SCF MO method. The transition energies, oscillator strengths,-electron densities,-bond orders and dipole moments have been evaluated. Concerning the transition energy and oscillator strength, the comparison has been made between the theoretical and observed values, showing that the agreement between them is improved in the present calculation compared with previous calculations.

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Zusammenfassung Die Elektronen-Absorptionsspektren von Cytosin und Isocytosin wurden in verschiedenen Lösungsmitteln bei Raumtemperatur gemessen; in wäßriger Lösung wurden auch Temperatur und pH-Abhängigkeiten untersucht, wobei besonders auf die Existenz von Tautomeren geachtet wurde. Es ergab sich, daß Cytosin in wäßriger Lösung bei gewöhnlicher Temperatur in der 2-Keto-6-amino-Form vorliegt, aber bei steigender Temperatur daneben auch die 2-Keto-6-imino auftritt. Letztere dürfte etwa 5,5 Kcal/Mol stabiler sein. Cytosin in Trimethylphosphat und Acetonitril liegt in Form I bzw. Form II vor. Die unterschiedlichen Stabilitäten in verschiedenen Lösungsmitteln wurden in Hinblick auf die berechneten Dipolmomente, die vom-Elektronensystem und der Polarität des Lösungsmittels herrühren, diskutiert.Die-Elektronenstrukturen von fünf tautomeren Formen von Cytosin und von Isocytosin (drei tautomere Formen) sowie von deren Ionen wurden mittels einer Kombination von Konfigurationswechselwirkung und PPP-SCF-MO-Verfahren berechnet. Übergangsenergien, Oszillatorenstärken,-Elektronendichten und Bindungsordnungen sowie Dipolmomente wurden berechnet, wobei sich bei den beiden ersten Größen Verbesserungen gegenüber früheren Rechnungen ergaben.

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Résumé Les spectres d'absorption électronique de la cytosine et de l'isocytosine ont été obtenus à température ambiante dans différents solvants; en milieu aqueux la dépendance de la température et du pH a été étudiée, permettant des conclusions sur les problèmes de tautomérie. Il apparaît qu'en milieu aqueux, à température ordinaire, la cytosine se trouve sous la forme 2-céto, 6-amino, mais qu'à température plus élevée elle se trouve aussi sous la forme 2-céto,6-imino. Cette dernière est plus stable d'environ 5,5 Kcal/Mol. Un équilibre analogue se produit dans le triméthylphosphate et l'acétonitrile. Les différentes stabilités dans les différents solvants sont discutées en fonction des moments dipolaires calculés, qui proviennent du système d'électrons et de la polarité du milieu.La structure électronique de cinq formes tautomères de la cytosine et de trois formes tautomères de l'isocytosine ainsi que des ions correspondants a été calculée par une méthode S.C.F. M.O. P.P.P. avec I.C. Les énergies de transition, les forces d'oscillateur, les charges et les indices de liaison des électrons sont calculés, avec de meilleurs résultats que ceux obtenus précédemment pour les deux premières grandeurs.

     

The polarized electronic spectra and electric field spectra of benzo-diazoles. II. 2,1,3-benzothiadiazole

The S₁ ← S₀ absorption spectra of 2,1,3-benzothiadiazole (BTD) have been measured at 4.2 K in four different host crystals: naphthalene, durene, p-dichlorobenzene (DCB) and p-dibromobenzene. Detailed vibrational analyses are given for BTD imbedded in napthalene and DCB. The polarization measurements show that the S₁ state has B₂ symmetry, like its selenium analogue (BSD). The transition is dominated by a single totally symmetric mode - 484 cm^?1. The Herzberg-Teller coupling contributes only a very small fraction of the total intensity. The Stark measurements of a DCB sample containing both BTD and BSD enabled us to compare the charge distribution of BTD and BSD in the state S₁. The Stark splittings of BTD are 17% greater than the splittings of BSD. Reorganization of the σ-core during the excitation is used to explain the difference. The drastic change in dipole moment upon excitation implies that the S₁←S₀ transitions of BTD and BSD are not localized in the six-membered ring as suggested by previous workers. Weak phosphorescence of BTD in napthalene and DCB and singlet-triplet absorption spectrum of neat BTD have been observed. The heavy atom effect of spin-orbit coupling is to explain the ST absorption intensity of BTD and BSD.     

On the additivity of bond dipole moments. Stark effect studies of the rotationally resolved electronic spectra of aniline, benzonitrile, and aminobenzonitrile

We study the influence of an applied electric field on the fully resolved electronic spectra of aniline (AN), benzonitrile (BN), and 4-aminobenzonitrile (ABN) in the gas phase. Using these data, we test the commonly held but rarely proven assumption that the total dipole moment of a polyatomic molecule is the vector sum of bond dipole moments, localized in different parts of the molecule. We find that μ_a(ABN)≈μ_a(AN)+μ_a(BN) in the excited S₁ state, but not in the ground S₀ state. Possible reasons for this non-additivity are discussed.     

The polarized electronic spectra and electric field spectra of benzo-diazoles. I. 2,1,3-benzoselenadiazole

The analyses of the polarized spectra of the S₁ - S_o transition of 2,1,3-benzoselenadiazole (BSD) dispersed in two different host crystals: naphthalene and p-dichlorobenzene have yielded (1) the assignment of the S₁ state as B₂, and (2) the vibronic coupling activity by b₂ fundamentals. VibronicaUy induced intensity by b₂ fundamentals accounts for only a small fraction of the overall intensity, but the intensity of phonon sidebands amounts to about 80% of the total intensity. The phosphorescence spectrum and the singlet-triplet absorption have been observed. Electric field spectra have been used to (1) measure the change of dipole moments upon excitation: 1Δμ(S₁ - S_o)I = 3.35 D and IΔμ(T₁- S_o) I = 2.26 D, (2) establish the identity of low frequency bands (19 - 111 cm^?1) as phonon sidebands, and (3) probe the nature of vibronic activity.     

On an IC electronic balance control system and its impact on the calibration

The contribution deals with an “1974 integrated circuit” version of an electronic damping and nulling system, adapted to a torsion balance which now is more than twenty years old. The discussion of calibration results reveal that masses of about 0.5 mg, in certain cases, can be intercompared with a relative precision of 3 × 10^?5, indicating that the balance sensibility is better than 2 × 10^?8 g. As the deflection sensitivity of the balance is 1.3 × 10^?4 deg/μg the position sensor on the balance can detect, over a few minutes, average displacements of about 4 × 10^?7 cm (i.e. ～ 40 Å).     

Photoelectron spectra,electronic and spatial structure of phenylhydrazones

Theoretical and Experimental Chemistry -     

Marked difference in the electronic structure of cyanide-ligated ferric protoglobins and myoglobin due to heme ruffling

Electron paramagnetic resonance experiments reveal a significant difference between the principal g values (and hence ligand-field parameters) of the ferric cyanide-ligated form of different variants of the protoglobin of Methanosarcina acetivorans (MaPgb) and of horse heart myoglobin (hhMb). The largest principal g value of the ferric cyanide-ligated MaPgb variants is found to be significantly lower than for any of the other globins reported so far. This is at least partially caused by the strong heme distortions as proven by the determination of the hyperfine interaction of the heme nitrogens and mesoprotons. Furthermore, the experiments confirm recent theoretical predictions [Forti, F.; Boechi, L., Bikiel, D., Martí, M.A.; Nardini, M.; Bolognesi, M.; Viappiani, C.; Estrin, D.; Luque, F. J. J. Phys. Chem. B2011, 115, 13771-13780] that Phe(G8)145 plays a crucial role in the ligand modulation in MaPgb. Finally, the influence of the N-terminal 20 amino-acid chain on the heme pocket in these protoglobins is also proven.     

On the ab initio evaluation of the solvent shift of electronic absorption spectra

Solvent shift in water of π → π^* transitions of the carbonyl group is evaluated making use of a recently proposed computdtional procedure for the continuum solvent model. Comparisons with the discrete model and with composite discrete-continuum procedures are performed.     

DFT study on the geometric, electronic structure and Raman spectra of 5,15-diphenylporphine

The ground state geometric, electronic structure and Raman spectra of 5,15-diphenylporphine (H(2)DPP) have been studied using B3LYP/6-31G(d) method and compared with that of well-studied free base porphine (H(2)P) and meso-tetraphenylporphine (H(2)TPP). Calculation shows that 5,15-substitution causes remarkable in-plane distortion, whereas the resulting out-of-plane distortion is negligible. The calculated electronic structure of H(2)DPP is consistent with the absorption spectra compared with H(2)P and H(2)TPP. The calculated vibrational frequencies of H(2)DPP scaled with a single factor of 0.971 agree well with experimental data (the rms error is 8.0 cm(-1)). The assignment of experimental Raman bands of H(2)DPP was discussed on the basis of theoretical calculation and the comparison with that of H(2)P and H(2)TPP. The splitting of some vibrational modes involving the motion of C(m) atom, such as nu(1), nu(8), and nu(10), was observed and was attributed to the diversification of the environment around C(m) atoms. As the shift of absorption peaks, the shift of some structure-sensitive Raman bands of H(2)DPP form that of H(2)TPP and H(2)P was attributed to the in-plane nuclear reorganization (IPNR) induced by phenyl-substitution, though the contribution of nonplanarity mechanism could not be excluded completely.     

Vibrational spectra,force fields and electronic structures of sydnones

Normal vibrational analyses of 3-methylsydnone and 3-methylsydnone-d₄ have been performed. On the basis of the set of force constants obtained, the electronic distribution within the mesoionic ring is evaluated and compared to the results of MO calculations. The π-bond order of the sydnone carbonyl group is shown to be lower than that in alicyclic esters and the unusually high “carbonyl stretching” frequency is due to the contributions from other coordinates to this mode. The splitting of the “carbonyl stretching” band observed in the spectra of 3-methylsydnone and of related derivatives is explained by the strong kinematic coupling between mesoionic bonds.     

NMR spectra of free-base porphine, porphyrazine, phthalocyanine and naphthalocyanine as well as their metal complexes: density functional calculations

Nuclear magnetic shielding tensors of porphine have been calculated at density functional B3LYP and PBE level using the gauge independent atomic orbital (GIAO) method. The geometries used were optimized using the 6-31G(d) basis set and the NMR calculations were performed using 6-31G(d) and 6-311G(d,p) basis sets, respectively. The calculated NMR shielding tensors and chemical shifts of porphine are compared with previous calculations as well as experimental data and satisfying results are obtained. Further NMR calculations are extended to metal-free and metallo-porphyrazine, -phthalocyanine, and -naphthalocyanine for the first time and the results are compared with experimental data available. The chemical shifts of the atoms in these compounds are assigned according to the experimental data available.     

Theoretical study on the structures and electronic spectra of C120On (n=1,2)

ZINDO series calculations have been carried out to study the double‐cage oxides C₁₂₀O_n (n=1,2). The results show that the formation of a furan ring by the bridge‐bond between the two cages connected the two C₆₀ fullerene units and formed the C₁₂₀O with C_2v symmetry. C₁₂₀O₂ has two isomers with C_2v symmetry depending on either 6–6 or 6–5 connection between the two cages. Two furan rings and a pure four‐member ring form in this molecule. The formation of C₁₂₀O assuages the constraint of epoxide structure in C₆₀O, shortens the distance of the monomers, and produces some finite interaction between the two balls. More bonding in C₁₂₀O₂ shortens the distance of the two cages further and brings about stronger interaction. However, the two cages in C₁₂₀O_n (n=1,2) behave somehow independently that the electronic spectra of C₁₂₀O_n (n=1,2) are similar to those of C₆₀. The 6–6 connection isomer of C₁₂₀O₂ is more stable; its spectra are in good agreement with those of the experiment. The calculated electronic spectra of C₁₂₀O not only are in good agreement with the experiment in the ultraviolet region but also get some weak peaks in the visible region (>400 nm) not observed in experiment. © 2000 John Wiley & Sons, Inc. Int J Quant Chem 79: 291–307, 2000