A study on the electronic spectra of vinylpyridines and 1,2-(dipyridyl)ethylenes. Molecular orbital calculations

The electronic absorption spectra of 2-, 3-, and 4-vinylpyridines and 1,2-(2,3-dipyridyl), 1,2-(2,4-dipyridyl), 1,2-(3,4-dipyridyl), and 1,2-(4,4-dipyridyl) ethylenes have been investigated in polar and nonpolar solvents. A correlation has been made between the geometry of the molecule and the observed spectrum. Molecular orbital calculations have been carried out using the INDO/S? CI procedure and a limited geometry optimization. The solvent effect at the MO level has been calculated. MO calculations predicted the existence of nπ* transitions that were not observed experimentally. The wave functions of the different CI states were calculated. The experimental transition energy as well as oscillator strength corresponded satisfactorily with the calculated ones. The observed transitions were assigned according to the results of MO calculations.     

Calculated optical spectrum of model oxyheme complex

The optical spectrum of a model oxyheme complex has been calculated using a new intermediate neglect of differential overlap (INDO-SCF-CI ) method that allows for the inclusion of configuration interaction and transition metals. In addition to the porphyrin π→π* transitions common to all heme proteins, four weak x,y polarized transitions observed only in oxyheme complexes have been calculated and assigned to excitations involving the lowest-empty highly delocalized (Oπ, dπ) orbital. Two broad z-polarized bands observed in the single-crystal polarized absorption spectra of oxymyoglobin and hemoglobin have also been calculated. Controversy exists over the assignment of these transitions and, in particular, over the extent of involvement of the oxygen ligand. Our calculations assign the weaker near-IR visible band mainly to the d σ dπ→ dπ* excitations and the more intense UV band mainly to a_2u → dσ* excitations. While significant participation (25%) of the highly delocalized (Oπ, dπ) virtual orbital is also found, these z-polarized transitions need not be totally unique to oxyheme complexes, in keeping with experimental observation.     

Experimental and theoretical (INDO CI) electronic absorption spectroscopy in the examination of tautomeric phenomena in nitracrine and its nitre isomers

Electronic absorption spectroscopy was employed to examine tautomeric phenomena in N,N-dimethyl-N-(1-nitro-9-acridinyl)-1,3-propanediamine (known as nitracrine—WHO, or Ledakrin in Poland) which exhibits antitumour activity, and also to examine its three nitro isomers. The analysis of the experimental absorption spectra reveals that the compounds exist in the liquid-phase in at least two forms (most probably amino and imino tautomeric forms), remaining in an equilibrium which is strongly affected by the features of a solvent. This was qualitatively confirmed by examining frequencies and oscillator strengths of electronic transitions obtained for the lowest energy structures of a given form by two independent methods based on INDO approximation. The latter methods predict that the strongest transitions occurring below 280 nm have a π→π* origin, whereas the longest wavelength transitions may be either π→π* or n→π* type and cause a shift in electron density within the acridine aromatic system and attached nitro group and exocyclic nitrogen atom.     

ortho‐Fluoroazobenzenes: Visible Light Switches with Very Long‐Lived Z Isomers

Improving the photochemical properties of molecular photoswitches is crucial for the development of light‐responsive systems in materials and life sciences. ortho‐Fluoroazobenzenes are a new class of rationally designed photochromic azo compounds with optimized properties, such as the ability to isomerize with visible light only, high photoconversions, and unprecedented robust bistable character. Introducing σ‐electron‐withdrawing F atoms ortho to the N?N unit leads to both an effective separation of the n→π* bands of the E and Z isomers, thus offering the possibility of using these two transitions for selectively inducing E/Z isomerizations, and greatly enhanced thermal stability of the Z isomers. Additional para‐electron‐withdrawing groups (EWGs) work in concert with ortho‐F atoms, giving rise to enhanced separation of the n→π* transitions. A comprehensive study of the effect of substitution on the key photochemical properties of ortho‐fluoroazobenzenes is reported herein. In particular, the position, number, and nature of the EWGs have been varied, and the visible light photoconversions, quantum yields of isomerization, and thermal stabilities have been measured and rationalized by DFT calculations.     

The deoxydative substitution reactions of nicotinamide and nicotinic acid N-oxides by 1-adamantanethiol in acetic anhydride

The reaction of nicotinamide N-oxide with 1-adamantanethiol in acetic anhydride yielded a mixture of 2-and 6-(1-adamantylthio)nicotinamides (49%, in the ratio of 24:1) and 2-, 5-, and 6-(1-adamantylthio)nicotino-nitriles (18%, in the ratio of 79:1:20). From a reaction of nicotinic acid N-oxide with 1-adamantanethiol, there was isolated 2-(1-adamantylthio)nicotinic acid as the only sulfide in 23% yield. Carbon? sulfur bond cleavage took place when 2-(1-adamantylthio)nicotinic acid, or the corresponding amide or nitrile, were boiled with concentrated hydrochloric acid to furnish 2-mercaptonicotinic acid and 1-chloroadamantane, quantitatively. The reaction of nicotinamide N-oxide alone in acetic anhydride at 135° formed N-acetyl-2-hydroxynicotinamide (61%), 2-hydroxynicotinonitrile (0.5%) and N,N-diacetyl-2-acetoxynicotinamide (0.8%).     

Computational Studies on the Structures of Nanographenes with Various Edge Functionalities

Computational studies have often been carried out on hydrogen-terminated nanographenes (NGs). These structures are, however, far from those deduced from experimental observations, which have suggested armchair edges with two carboxy groups on the edges as dominant. We conducted computational studies on NGs consisting of C₄₂, C₆₀, C₇₈, C₉₆, C₁₄₂, and C₁₇₄ carbon atoms with hydrogen, carboxy, and N-methyl imide-terminated armchair edges. DFT calculations inform distorted basal planes and similar HOMO-LUMO gaps, indicating that the edge oxidation and functionalization do not very influence the electronic structure. Comparison of observed UV-vis spectra of carboxy- and N-octadecyl chain terminated NGs with calculated spectra of model NGs informs the contribution of π-π* transitions on the basal plane to the absorptions in the visible region. A dimeric structure of NG and octadecyl-installed NG demonstrate that both the distorted basal planes and the steric contacts among the functional groups widen the surface-to-surface distance thereby allowing the invasion of solvent molecules between the surfaces. This picture is consistent with the improved solubility of edge-modified NGs.     

The molecular structures and the absorption maxima of the H-chromophores of the indigoid dyes

The molecular structures of the H-chromophore of the indigoid dyes and five other isomers are studied by ab initio MP2/6-31 + G*//HF/6-31 + G* method. The bond angles are affected by the π-electron conjugation. The molecular structures of the H-chromophores and indigoid dyes indicate that the benzene rings and the five-membered rings are structurally important. The absorption maxima of the H-chromophores are successfully calculated by CI-singles-MP2/6-31 + G* theory for the first time and correspond to the HOMO, LUMO transition. All these transitions are the π-π* transitions. Like the indigoid dyes, trans isomers have the bathochromic shifts of the absorption maxima, and the bathochromic shifts are found with the best donor group of ? NH. From these calculations, the absorption maxima of some indigoid dyes can be explained by their H-chromophores qualitatively. © 1995 by John Wiley & Sons, Inc.     

Theoretical considerations of the electronic spectra of methyl flavins

Methyl groups in flavins are best treated by the group-orbital approximation. The pseudo-heteroatom approximation overestimates methyl hyperconjugation with the Pariser–Parr–Pople SCF –MO method. Singlet π → π* transition energies are calculated by various MO methods with differing degrees of sophistication, and the results from the P ? P ? P method agree reasonably with the experimental values. 2- and 4-thioflavin analogs are also treated satisfactorily. The effects of position and number of the methyl groups on the spectra of flavins are described in detail. Rough estimates of the n → π* energies of flavins suggest that the lowest singlet excited state is (π, π*), consistent with the fluorescence and phosphorescence polarization data.     

Two electronic transitions in the near-UV region of six isomeric difluorobenzonitriles—I

A new technique—photoacoustic spectroscope (PAS)—is applied to the study of the electronic transitions in the six isomers 3,5-; 2,4-; 2,5-; 3,4-; 2,3-; and 2,6-difluorobenzonitriles. The PAS spectra are compared with solution spectra recorded. The two π-π* transitions analogous to the benzene strong 200 nm, and weak and forbidden 260 nm transitions could be identified in these molecules. An interesting observation is that the origins of the electronic transitions in these molecules are in fairly good agreement with the additive rule which is routed through different starting points and also a prediction of the origin of meta fluorobenzonitrile at 37,536.1 cm⁻¹ from the data of 3,4- and 2,5-difluorobenzonitriles.The fluorescence spectrum for all six isomers is reported here for the first time.     

Tunable Electronic Properties of a Proton‐Responsive N,N‐Dimethylaminoazobenzene Fullerene (C60) Dyad

A basic N,N‐dimethylaminoazobenzene–fullerene (C₆₀) dyad molecular skeleton is modelled and synthesized. In spite of the myriad use of azobenzene as a photo‐ and electrochromic moiety, the idea presented herein is to adopt a conceptually different path by using it as a bridge in a donor–bridge–acceptor single‐molecular skeleton, connecting the electron acceptor N‐methylfulleropyrrolidine with an electron donor N,N‐dimethylaniline. Addition of trifluoroacetic acid (TFA) results in a drastic colour change of the dyad from yellow to pink in dichloromethane (DCM). The structure of the protonated species are established from electronic spectroscopy and time‐dependent density functional theory (TD‐DFT) calculations. UV/Vis spectroscopic investigations reveal the disappearance of the 409 nm ¹(π→π*) transition with appearance of new features at 520 and 540 nm, attributed to protonated β and α nitrogens, respectively, along with a finite weight of the C₆₀ pyrrolidinic nitrogen. Calculations reveal intermixing of n_(N?N)→π*_(N?N) and charge transfer (CT) transitions in the neutral dyad, whereas, the n_(N?N)→π*_(N?N) transition in the protonated dyad is buried under the dominant ¹(π →π*) feature and is red‐shifted upon Gaussian deconvolution. The experimental binding constants involved in the protonation of N,N‐dimethylanilineazobenzene and the dyad imply an almost equal probability of existence of both α‐ and β‐protonated forms. Larger binding constants for the protonated dyads imply more stable dyad complexes than for the donor counterparts. One of the most significant findings upon protonation resulted in frontier molecular orbital (FMO) switching with the dyad LUMO located on the donor part, evidenced from electrochemical investigations. The appearance of a new peak, prior to the first reduction potential of N‐methylfulleropyrrolidine, clearly indicates location of the first incoming electron on the donor‐centred LUMO of the dyad, corroborated by unrestricted DFT calculations performed on the monoanions of the protonated dyad. The protonation of the basic azo nitrogens thus enables a rational control over the energetics and location of the FMOs, indispensable for electron transport across molecular junctions in realizing futuristic current switching devices.     

The electronic structure and optical activity of conjugated dienes: 1,3-Butadiene and α- and β-phellandrene

The optical activity of conjugated dienes is investigated by means of ab initio SCF–CI calculations. The computed electronic spectrum of trans-1,3-butadiene is shown to be in good agreement with the results of more rigorous calculations of the valence transitions and in satisfactory agreement with experiment. The optical rotatory strengths of the lower electronic transitions of twisted 1,3-butadiene as a function of dihedral angle are presented and simulated CD spectra are produced. The N → V₁ (π₂ → π₃*) transition is predicted to have a positive rotational strength for all dihedral angles that correspond to a right-handed twist of the chromophore, in accord with the empirically deduced “diene rule” although for a twist angle of 60°, the rotatory strength is calculated to be almost zero. The role of the orientation of allylic bonds is investigated in the model system 1-butene in which the rotational strength of the π → π* transition as a function of rotation about the 2,3 bond is determined. The effect of allylic bond disposition in dienes on the optical activity of the long-wavelength π₂ → π₃* transition is simulated by use of the exciton coupling model of Harada and Nakanishi in which two 1-butene molecules with suitable geometries are coupled via interactions of the electric dipole transition moments of their π → π* transitions. The model systems 1,3-butadiene and 1-butene are used to rationalize the apparently anomalous optical activity of (?)-α-phellandrene and (?)-β-phellandrene, both of which should have a diene chromophore with a right-handed twist in their most stable conformers and so should be dextrorotatory. The experimental CD spectrum of α-phellandrene is determined at several temperatures down to ?180°C. The observed variation of the apparent rotational strength of the N → V₁ transition is in good agreement with that predicted by use of the exciton coupling model.     

Acylation of a novel quinoxalinyl substituted pyrazole derivative. synthesis,quantum chemistry calculations,and x-ray structure analysis

The preparation of N-acetylpyrazolyl-quinoxalines and results of semiempirical MO calculations are presented. The structure determination of the isomers 3a, b was achieved by X-ray analysis.     

A comparative analysis of the UV/Vis absorption spectra of nitrobenzaldehydes

In a joint experimental and theoretical study, the UV/Vis absorption spectra of the three isomers (ortho, meta, para) of nitrobenzaldehyde (NBA) were analyzed. Absorption spectra are reported for NBA vapors, cyclohexane and acetonitrile solutions. All spectra are poor in vibronic structure and hardly affected in shape by the surroundings (vapor or solution). Moderate solvatochromic shifts of ～ -0.2 eV are measured. For all isomers vertical transition energies, oscillator strengths, and excited state dipole moments were computed using the MS-CASPT2/CASSCF and CC2 methods. Based on these calculations the experimental transitions were assigned. The spectra of all isomers are characterized by weak (ε(max) ≈ 100 M(-1) cm(-1)) transitions around 350 nm (3.6 eV), arising from nπ* absorptions starting from the lone pairs of the nitro and aldehyde moieties. The next band of intermediate intensity peaking around 300 nm (4.2 eV, ε(max) ≈ 1000 M(-1) cm(-1)) is dominated by ππ* excitations within the arene function. Finally, strong absorptions (ε(max) ≈ 10,000 M(-1) cm(-1)) were observed around 250 nm (5.0 eV) which we ascribe to ππ* excitations involving the nitro and benzene groups.     

Electronic transitions and intramolecular hydrogen bonding in anthralin. UV-VIS linear dichroism spectroscopy and quantum chemical calculations

The electronic and molecular structure of the antipsoriatic drug anthralin (1,8-dihydroxy-9(10H)-anthracenone) is investigated by UV-VIS linear dichroism (LD) spectroscopy in stretched polyethylene and by quantum chemical model calculations. Seven individual electronic transitions are resolved below 47?000 cm⁻¹ and assigned to calculated π–π* transitions. The low-energy region is characterized by a relatively broad band around 28?000 cm⁻¹ that can be assigned to two overlapping, differently polarized electronic transitions involving a considerable degree of intramolecular charge transfer from the phenolic moieties to the carbonyl group. Computed wavenumbers for these transitions depend significantly on the assumed geometrical parameters for the intramolecular H-bonds in anthralin; best agreement with observed data is obtained with a geometry corresponding to strong H-bonding. The calculations also indicate that excited state intramolecular proton transfer (ESIPT) is likely to occur, leading to the prediction of a very large Stoke's shift.     

trans–cis Photoisomerization of Azobenzene‐Conjugated Dithiolato‐Bipyridine Platinum(II) Complexes: Extension of Photoresponse to Longer Wavelengths and Photocontrollable Tristability

Azobenzene derivatives modified with dithiolato‐bipyridine platinum(II) complexes were synthesized, revealing their highly extended photoresponses to the long wavelength region as well as unique photocontrollable tristability. The absorptions of trans‐ 1 and trans‐ 2 with one azobenzene group on the dithiolene and bipyridine ligands, respectively, cover the range from 300 to 700 nm. These absorptions are ascribed, by means of time‐dependent (TD)DFT calculations, to transitions from dithiolene(π) to bipyridine(π*), namely, interligand charge transfer (CT), π–π*, and n–π* transitions of the azobenzene unit, and π–π* transitions of the bipyridine ligand. In addition, only trans‐ 1 shows distinctive electronic bands, assignable to transitions from the dithiolene(π) to azobenzene(π*), defined as intraligand CT. Complex 1 shows photoisomerization behavior opposite to that of azobenzene: trans‐to‐cis and cis‐to‐trans conversions proceed with 405 and 312 nm irradiation, which correspond to excitation with the intraligand CT, and π–π* bands of the azobenzene and bipyridine units, respectively. In contrast, complex 2 shows photoisomerization similar to that of azobenzene: trans‐to‐cis and cis‐to‐trans transformations occur with 365 and 405 nm irradiation, respectively. Irradiation at 578 nm, corresponding to excitation of the interligand CT transitions, results in cis‐to‐trans conversion of both 1 and 2 , which is the longest wavelength ever reported to effect the photoisomerization of the azobenzene group. The absorption and photochromism of 4 , which has azobenzene groups on both the dithiolato and bipyridine ligands, have characteristics quite similar to those of 1 and 2 , which furnishes 4 with photocontrollable tristability in a single molecule using light at 365, 405, and 578 nm. We also clarified that 1 and 2 have high photoisomerization efficiencies, and good thermal stability of the cis forms. Complexes 3 and 5 have almost the identical photoresponse to those of their positional isomers, complexes 2 and 4 .     

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:

相似文献   

马来腈二硫纶·邻菲咯啉镍(II)配合物的电子光谱和理论研究

测量了标题配合物Ni(mnt)(phen)在多种介质中的电子吸收光谱和发射光谱, 使用密度泛函理论的B3LYP方法和分子轨道理论的PM3方法研究了其气态分子几何构型、电子结构和成键, 用ZINDO/S方法通过多组态的组态相互作用(CI)计算解释了实验光谱. 结果表明: 该配合物分子为平面结构, 对称性属于点群C_2v, 基态为自旋三重态, 配位键Ni—N和Ni—S为典型的共价结合, Ni的3d电子反馈效应较显著; 可见区的吸收带和发射带(对应于基态电子组态到较低能量激发态组态的跃迁)本质上属于配体phen到mnt^2－的荷移跃迁(LL'CT), 紫外区的吸收带本质上属于配体的π→π*跃迁.     

Electron structure and spectroscopic properties of yttrium compounds with cinnamic and quinaldic acids

The analysis of electron structure of yttrium(III) complexes with cinnamic and quinaldic acids was carried out. Electron transitions were assigned, and influence of the nature of frontier orbitals on the position of absorption bands in the electron spectra was revealed. The TD-DFT calculation has shown that different ratios of intensities of π–π* and n–π* bands in the electron absorption spectra of the compounds are caused by different natures of frontier orbitals of the complexes. It was found that dipole moments of both complexes increase upon photoexcitation, greater changes being observed in Y(III) quinaldate. The reason of the greater Stokes shift of Y(III) quinaldate was established.     

Differentiation of Isomeric Hydroxypyridine <Emphasis Type="Italic">N</Emphasis>-Oxides Using Metal Complexation and Electrospray Ionization Mass Spectrometry

Differentiation between two isomers of hydroxypyridine N-oxide according to the metal cation adducts generated by electrospray ionization (ESI) was investigated for different metal cations, namely Mg (II), Al (III), Ca (II), Sc (III), Fe (III), Co (II), Ni (II), Cu (II), Zn (II), Ga (III), besides the diatomic cation VO(IV). Protonated molecules of the isomeric hydroxypyridine N-oxides as well as the singly/doubly charged adducts formed from neutral or deprotonated ligands and a doubly/triply charged cation were produced in the gas phase using ESI, recording mass spectra with different metal ions for each isomer. While complex formation was successful for 2-hydroxypyridine N-oxide with trivalent ions, in the case of 3-hydroxypyridine N-oxide, only peaks related to the protonated molecule were present. On the other hand, divalent cations formed specific species for each isomer, giving characteristic spectra in every case. Hence, differentiation was possible irrespective of the metal cation utilized. In addition, quantum chemical calculations at the B3LYP/6-31 + G(d,p) level of theory were performed in order to gain insight into the different complexation of calcium(II) with the isomers of hydroxypyridine N-oxide. The relative stability in the gas phase of the neutral complexes of calcium made up of two ligands, as well as the singly charged and doubly charged complexes, was investigated. The results of these calculations improved the understanding of the differences observed in the mass spectra obtained for each isomer.     

A mass spectrometry‐based method for differentiation of positional isomers of monosubstituted pyrazine N‐oxides using metal ion complexes

A series of 11 pairs of substituted pyrazine N‐oxides, differing in the substituent position, were examined using electrospray ionization mass spectrometry (ESI‐MS) in order to use spectra to assess the differentiation of positional isomers. For each compound, mass spectra were recorded with three different metal cations, namely calcium (II), copper (II) and aluminum (III), with characterization of the observed peaks. Differentiation between regioisomeric N‐oxides has been achieved by comparison of the identity and relative intensities of the peaks originating from the adduct ions formed with the metal ions. Principal component analysis (PCA) has been employed to assist in the interpretation of the results obtained with each metal ion, exploring possible trends according to the nature and position of the substituent in the pyrazine N‐oxide. Copyright © 2015 John Wiley & Sons, Ltd.