Theoretical study on electronic structures and spectroscopic regularities of ethylated single-walled carbon nanotubes

The electronic structures of a series of ethylated single-walled carbon nanotubes (SWCNTs) were studied using density; function theory (DFT) at B3LYP/6-31G(d) level. The bond vertical to the main axis of the SWCNT was predicted to be the most thermodynamically stable additive site by ethylene. The energy gaps of the ethylated SWCNTs decrease with the decrease in the symmetries after the addition. The C-C and C=C stretching vibrations in the IR spectra of the ethylated SWCNTs, compared with those in the IR spectra of the pristine SWCNTs, are red-shifted. The chemical shifts at 172.9 ppm of the bridged carbon atoms in the NMR spectrum of (3,3)-C₂H₄(v) (C₃₆C₂H₄) are shifted downfield in comparison with those at 144.7 ppm of the same carbon atoms in (3,3) (C₃₆). Meanwhile, (3,3)-C₂H₄(v) (C₃₆C₂H₄) shows a weakened anti-aromaticity owing to a nuclear independent chemical shift (NICS) at 3.6 ppm, relative to the NICS value at 6.3 ppm of (3,3) (C₃₆).     

Theoretical studies on the electronic structures and spectra of single silicon-doped SWCNTs

The equilibrium geometries and electronic structures of a series of SWCNTs doped with a silicon atom were studied by using density function theory (DFT). The most stable doping site of silicon predicted at B3LYP/6-31G(d,p) level was located near the boundary of the SWCNTs. The energy gaps of (3,3) C₄₈, (3,3) C₆₀ and (3,3) C₇₂ were respectively decreased by 0.43, 0.25 and 0.14 eV after doping. Based on the B3LYP/6-31G(d) optimized geometries, the electronic spectra of the doped SWCNTs were computed using the INDO/CIS method. The first UV absorption at 973.9 nm of (5,5)-Si(L) (C₅₉Si) compared with that at 937.5 nm of (5,5) (C₆₀) was red-shifted. The ¹³C NMR spectra and nuclear independent chemical shifts (NICS) of the doped SWCNTs were investigated at B3LYP/6-31G(d) level. The chemical shift at 119.4 of the carbon atom bonded with the silicon atom in (3,3)-Si(L) (C₅₉Si) in comparison with that at 144.1 of the same carbon atom in (3,3) (C₆₀) moved upfield. The tendency of the aromaticity (NICS = −0.1) for (3,3)-Si(H) (C₄₇Si) with respect to that of the anti-aromaticity (NICS = 6.0) for (3,3) (C₄₈) was predicted.      

Electronic and vibrational spectroscopic investigation of phenylacetylene-amine complexes. Evidence for the diversity in the intermolecular structures

Shifts in the electronic transitions for the complexes of phenylacetylene with ammonia, methylamine, and triethylamine clearly indicate the variation in the intermolecular structures of the three complexes. The infrared spectrum of phenylacetylene in the acetylenic C-H stretching region shows Fermi resonance bands, which act as a sensitive tool to probe the intermolecular structures. The IR-UV double resonance spectra of the three complexes are disparate and signify the formation of distinct structures. The formation of C-H...N hydrogen-bonded complex with ammonia and two distinct types of pi complexes with methylamine and triethylamine can be inferred from the analysis of electronic and vibrational spectra in combination with ab initio calculations. These complexes clearly point out the fact that marginal changes in the interacting partner can significantly alter the intermolecular structure.     

Electronic structures and excitonic transitions in nanocrystalline iron-doped tin dioxide diluted magnetic semiconductor films: an optical spectroscopic study

Nanocrystalline iron-doped tin dioxide (Sn(1-x)Fe(x)O(2)) films with x from 0 to 0.2 were prepared on c-sapphire substrates by pulsed laser deposition. X-ray diffraction and Raman scattering analysis show that the films are of the rutile structure at low compositions and an impurity phase related to Fe(2)O(3) appears until the x is up to 0.2, suggesting the general change of lattice structure due to the Fe ion substitution. The dielectric functions are successfully determined from 0.0248 to 6.5 eV using the Lorentz multi-oscillator and Tauc-Lorentz dispersion models in the low and high photon energy regions, respectively. With increasing Fe composition, the highest-frequency transverse optical phonons E(u) shifts towards a lower energy side and can be well described by (608 - 178x) cm(-1). From the transmittance spectra, the fundamental absorption edge is found to be decreased with the Fe composition due to the joint contributions from SnO(2) and Fe(2)O(3). It can be observed that the doped films exhibit evident excitonic excitation features, which are strongly related to the Fe doping. Among them, the 6A(1g)→ 4T(2g) transition contributes to the onset of optical absorption. Moreover, the remarkable intensity reduction and a red-shift trend with the doping composition, except for the pure film, can be testified by the photoluminescence spectra. It can be concluded that the replacement of Sn with the Fe ion could induce the 2p-3d hybridization and result in the electronic band structure modification of the Sn(1-x)Fe(x)O(2) films.     

Electronic states and spectroscopic properties of SiTe and SiTe+

Ab initio based configuration interaction calculations have been carried out to study the low-lying electronic states and spectroscopic properties of the heaviest nonradioactive silicon chalcogenide molecule and its monopositive ion. Spectroscopic constants and potential energy curves of states of both SiTe and SiTe+ within 5 eV are reported. The calculated dissociation energies of SiTe and SiTe+ are 4.41 and 3.52 eV, respectively. Effects of the spin-orbit coupling on the electronic spectrum of both the species are studied in detail. The spin-orbit splitting between the two components of the ground state of SiTe+ is estimated to be 1880 cm(-1). Transitions such as 0+ (II)-X1Sigma(+)0+, 0+ (III)-X1Sigma(+)0+, E1Sigma(+)0+ -X1Sigma(+)0+, and A1Pi1-X1Sigma(+)0+ are predicted to be strong in SiTe. The radiative lifetime of the A1Pi state is less than a microsecond. The X(2)2Pi(1/2)-X(1)2Pi(3/2) transition in SiTe+ is allowed due to spin-orbit mixing. However, it is weak in intensity with a partial lifetime for the X2 state of about 108 ms. The electric dipole moments of both SiTe and SiTe+ in their low-lying states are calculated. The vertical ionization energies for the ionization of the ground-state SiTe to different ionic states are also reported.     

Electronic structures of polydiacetylene backbones

Energies per unit cell for various polydiacetylene backbone sequences are determined. The unrestricted Hartree-Fock approach is used to obtain the lowest energy transitions.     

Electronic structures of antiaromatic molecules

The electronic structures of the planar cyclic polymethines with 4n electrons (the so-called antiaromatic molecules) are investigated by the unrestricted Hartree-Fock approximation. The relative stability and spin structures of singlet and triplet states of the above species are discussed.     

Electronic structures of trans-dioxometal complexes

We have employed computational methods based on density functional theory to elucidate the effects of equatorial ligands on the electronic structures of trans-dioxometal complexes. In complexes with amine (sigma-only) equatorial donors, the (1)A(1 g)(b(2 g))(2)-->(1)E(g)(b(2 g))(1)(e(g))(1) excitation energy increases with metal oxidation state: Mo(IV) < Tc(V) < Ru(vi) and W(IV) < Re(V) < Os(VI). Increasing transition energies are attributed to enhanced oxometal pi-donor interactions in the higher valent central metals. But in complexes with cyanide equatorial donors, the (1)A(1 g)(b(2 g))(2)-->(1)E(g)(b(2 g))(1)(e(g))(1) energy remains roughly independent of metal oxidation state, likely owing to the compensating increased pi-donation from the pi(CN) orbitals to the metal d(xy) orbitals as the oxidation state of the metal increases.     

Electronic structures and properties of twisted polyacenes

The effects of twisting on the electronic structures and properties of polyacenes were studied computationally using DFT methods. Singlet-triplet and HOMO-LUMO gaps and vertical S0-S1 transition energies are marginally affected as a function of end-to-end twist angle. The large twist induced by the phenyl substituents in 9,10,11,20,21,22-hexaphenyltetrabenzo[a,c,l,n]pentacene has little influence on its electronic structure.     

Electronic structures and spectra of cyanoethylene derivatives

Results of two different types of molecular orbital calculations of severalπ-orbital systems containing nitrile groups are reported and compared. The bidentate ligandbis-(methylmercapto) maleonitrile and related molecules are included. Electronic spectra of these molecules are reported and assigned in terms of the calculated energies.     

Electronic structures and thermolyses of 2-tetrazenes

The electronic structures and gas phase thermolyses of the cyclic 2-tetrazenes 2 and 3 and of open chain 1,1,4,4-tetramethyl-2-tetrazene (1) have been studied by photoelectron spectroscopy. While the six-membered ring compound 2 yields 1-methylmethylenamine (6) and nitrogen as fragments, the seven-membered ring compound 3 is contracted to 1,2-dimethylpyrazolidine (11). The acyclic 2-tetrazene 1 prefers disproportionation to 6 and dimethylamine (7). Based on MNDO calculations the tonization potentials of 1–3 were assigned to molecular orbitals. Several conformations of 2 and 3 were calculated. Compound 2 shows a rigid boat conformation with equatorial methyl groups, while 3 can occupy several conformations of similiar energies. The different thermal decompositions of 1– 3 are explained.     

Electronic structures of acyl nitrites and nitrates

The gas phase electronic structures of CM(3)C(O)ONO and CM(3)C(O)ONO(2) (M=H, Cl, F) are studied by photoelectron spectroscopy (PES) combined with the outer valence Green's function (OVGF) calculations at 6-311+G(d, p) basis sets. The highest occupied molecular orbital (HOMO) for each compound is the carbonyl oxygen lone pair (n(O)), the ionizations of these orbitals are associated with the vibrational frequency about 1750 and 1820 cm(-1) reflected on the first band, respectively, for acyl nitrites and nitrate. Comparing with the calculated energies, it can be concluded that the syn conformers with Cs overall symmetry, a planar CC(O)ONO skeleton in nitrites, and a planar CC(O)ON skeleton in nitrates, respectively, are the most stable in the gas phase.     

Electronic structures and spectroscopic properties of mono- and binuclear d(8) complexes: a theoretical exploration on promising phosphorescent materials

The structures of trans-[M(2)(CN)(4)(PH(2)CH(2)PH(2))(2)] (M = Pt (1), Pd (2), and Ni (3)), trans-[Pt(2)X(4)(PH(2)CH(2)PH(2))(2)] (X = Cl (4) and Br (5)), and trans-[M(CN)(2)(PH(3))(2)] (M = Pt (6), Pd (7), and Ni (8)) in the ground state were optimized using the MP2 method. Frequency calculations reveal that the weak metal-metal interaction is essentially attractive for 1, 2, 4, and 5 but not for 3. The TD-DFT calculations associated with the polarized continuum model (PCM) were performed to predict absorption spectra in CH(2)Cl(2) solution. Experimental spectra are well reproduced by our results. With respect to analogous mononuclear d(8) complexes (6-8), a large red shift of the absorption wavelength was calculated for the binuclear d(8) complexes (1-3). Relative to 1 with unsaturated CN- donors, introduction of saturated halogen donors into 4 and 5 changes their electronic structures, especially the HOMO and LUMO. The TD-DFT and subsequent unrestricted MP2 calculations predict that 1 produces the lowest-energy d --> p emission while 2-5 favor the d --> d emissions, agreeing with experimental observations.     

A theoretical and spectroscopic study of conformational structures of piroxicam

Piroxicam (PRX) has been widely studied in an attempt to elucidate the causes and mechanisms of its side effects, mainly the photo-toxicity. In this paper fluorescence spectra in non-protic solvents and different polarities were carried out along with theoretical calculations. Preliminary potential surfaces of the keto and enol forms were obtained at AM1 level of theory providing the most stable conformers, which had their structure re-optimized through the B3LYP/CEP-31G(d,p) method. From the optimized structures, the electronic spectra were calculated using the TD-DFT method in vacuum and including the solvent effect through the PCM method and a single water molecule near PRX. A new potential surface was constructed to the enol tautomer at DFT level and the most stable conformers were submitted to the QST2 calculations. The experimental data showed that in apolar media, the solution fluorescence is raised. Based on conformational analysis for the two tautomers, keto and enol, the results indicated that the PRX-enol is the main tautomer related to the drug fluorescence, which is reinforced by the spectra results, as well as the interconvertion barrier obtained from the QST2 calculations. The results suggest that the PRX one of the enol conformers presents great possibility of involvement in the photo-toxicity mechanisms.     

Electronic structures and spectra of adenine and thymine

The near and vacuum ultraviolet spectra of adenine and thymine were measured with the evaporated film method and in solution. The absorption spectrum of the adenine anion in basic solution was also measured. The assignment of the absorption bands was made by comparing the observed transition energies and intensities with the theoretical results obtained by the LCAO-SCF-CI-method. The results indicated that the longest wavelength band of adenine at 269 m is a ^* transition band. This was supported by the fact that a new band due to the n ^* transition was found at 278 m for the adenine anion. Furthermore, it was shown that the thymine molecule may interact strongly with the solvent molecules and that the electronic structure of thymine in solution may be different from that of the free molecule.

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Zusammenfassung Die UV-Spektren von Adenin und Thymin in Lösung bzw. als dünne Filme wurden bis 160 m, herunter gemessen, außerdem das Spektrum vom Adenin-Anion in basischer Lösung. Auf Grund der Ergebnisse einer LCAO-SCF-CI-Rechnung wurde eine Zuordnung der Banden vorgenommen. Danach ist die langwellige Bande bei 269 m ein ^*-Übergang, die neue Bande des Adenin-Anions bei 278 m dagegen ein n ^*-Übergang. Die Elektronenstruktur von Thymin kann durch das Lösungsmittel stark beeinflußt werden.

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Résumé Les spectres dans l'ultraviolet proche de l'adénine et de la thymine ont été mesurés avec la méthode du film évaporé et en solution. Le spectre d'absorption de l'anion adénine en solution basique a été aussi mesuré. L'identification des bandes d'absorption a été faite en comparant les énergies de transition et les intensités observées avec les résultats théoriques obtenus par la méthode LCAO-SCF-CI. Les résultats indiquent que la bande de plus grande longueur d'onde de l'adénine à 269 m, qui a été considérée comme une transition n ^* par certains auteurs, est une bande de transition ^*. Ceci est confirmé par le fait qu'une nouvelle bande, dûe á la transition n ^*, a été observée à 278 m pour l'anion adénine. De plus, on a montré que la molécule de thymine peut interagir fortement avec les molécules de solvant et que la structure électronique de la thymine en solution peut être différente de celle de la molécule libre.

     

Electronic structure and solvatochromism of merocyanines NMR spectroscopic point of view

(1)H and (13)C NMR spectra of two series of malononitrile-based merocyanines, which possess positive and negative solvatochromism have been in detail investigated in low polar chloroform and polar dimethyl sulfoxide (DMSO). Careful attribution of signals in spectra has been made with the help of two-dimensional NMR experiments (COSY, NOESY, HMBC, and HMQC). Hence, the dependence of merocyanines electronic structure on their chemical structure and solvent nature has been studied by this powerful method. It has been shown that there exists a good correlation between the calculated charges on carbon atoms of a polymethine chain and their chemical shifts in (13)C NMR spectra. The influence of solvent polarity on bond orders for dyes with positive and negative solvatochromism is also observed. The comparison of (13)C NMR spectra of merocyanines and corresponding parent ionic dyes allows to determine their sign of solvatochromism irrespectively of electronic spectra, and also to find the key atoms of chromophore whose signals in (13)C NMR spectra are most informative.     

Electronic structures of aromatic amine N-oxides

The electronic structures of heterocyclic amine N-oxides, nitrones, and nitrile N-oxides were systematically studied by the Pariser-Parr-Pople method. In addition, the intensity of n- ^* transition pertinent to conjugated N-oxide systems has been calculated. The results of these calculations were compared with the experimental ones reported hitherto and also carried out in the present work. The electronic structure and spectra of aromatic amine N-oxides were thus discussed in detail.

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Zusammenfassung Die elektronische Struktur heterozyklischer Amin-N-Oxide, Nitrone und Nitril-N-Oxide wird systematisch mit Hilfe der Pariser-Parr-Pople-Methode untersucht. Au\erdem wurde die IntensitÄt von n- ^*-übergÄngen konjugierter N-Oxide berechnet. Die Resultate wurden mit bekannten sowie eigenen experimentellen Ergebnissen verglichen. Die Elektronenstruktur und die Spektren aromatischer Amin-N-Oxide wurden so im Detail diskutiert.

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Résumé La structure électronique d'amine N-oxydes, de nitrones, et de nitrile N-oxydes ont été étudiés systématiquement par la méthode de Pariser-Parr-Pople. De plus, l'intensité de la transition n- ^* des systémes N-oxydes conjugués a été calculée. Les résultats de ces calculs ont été comparés aux résultats expérimentaux rapportés précédemment et également à ceux du présent travail. La structure électronique et le spectre des N-oxydes d'amines aromatiques ont ainsi été discutés en détail.

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Presented at the 21st (Osaka, April 1968) Annual Meeting of the Chemical Society of Japan, and the Symposium of Structure Chemistry and the Electronic State of Molecules (Tokyo, October 1968).     

Electronic structures of main-group carbene analogues

The electronic structures of 15 group 13-16 carbene analogues are analyzed using various quantum chemical methods and compared to the data obtained for the parent N-heterocyclic carbene (NHC), imidazol-2-ylidene. The results of this study present a uniform analysis of the similarities and differences in the electronic structures of p-block main-group carbene analogues. Though all systems are formally isovalent, the theoretical analyses unambiguously indicate that their electronic structures run the gamut from C=C localized (group 13) to C=N localized (group 16) via intermediate, more delocalized, systems. In particular, neither the stibenium ion nor any of the chalcogenium dications is a direct analogue of imidazol-2-ylidene as they all contain two lone pairs of electrons around the divalent main-group center, instead of the expected one. The reason behind the gradual change in the electronic structure of main-group analogues of imidazol-2-ylidene was traced to the total charge of the systems, which changes from anionic to dicationic when moving from left to right in the periodic table. Results from theoretical analyses of aromaticity show that all group 13-16 analogues of imidazol-2-ylidene display some degree of aromatic character. The heavier group 13 anions benefit the least from pi-electron delocalization, whereas the cationic group 15 systems are on par with the parent carbon system and display only slightly less aromatic character than cyclopentadienide, a true 6pi-electron aromatic species. The sigma-donor and pi-acceptor ability of the different main-group carbene analogues are also evaluated.