The electronic spectra and excited state dipole moment of 2-fluoropyridine

The electronic absorption spectrum in the vapour state and in solution in different solvents in the region 3000–1900 Å and the fluorescence and phosphorescence emission spectra in ethanol or cyclohexane at 77 K have been studied for 2-fluoropyridine and analysed. Two systems of absorption band corresponding to the π→π* transition II and π→π* transition III have been observed and the excited state dipole moments have been determined from the solvent-induced shifts of the electronic absorption bands. The half-life of phosphorescence in cyclohexane at 77 K is found to be 3.5 s.     

(d-p)π共轭作用对簇合物Mo_2S_4(dtp)_2和Mo_3S_4(dtp)_4·Py电子光谱的影响

比较了含有配位不饱和过渡金属原子簇合物Mo_2S_4(dtp)_2,(Ⅰ)和Mo_3S_4(dtp)_4·Py,(Ⅱ)的紫外—可见电子吸收光谱实验数据和半经验的量子化学方法INDO计算得到的电子跃迁能,讨论了(d-p)π型共轭作用对其电子吸收光谱位移的影响。结果表明,过渡金属簇合物中电子吸收光谱位移,与有机共轭体系有着类似的变化规律,即随着共轭π键数目增加,π-π电子跃迁的吸收光谱红移。     

g-CN体系的准粒子能带结构和光学特性

利用多体格林函数理论,本文研究了二维CN体系(包括triazine和tri-s-triazine)的激发态特性。通过GW方法,我们计算了准粒子的能量。考虑电子-空穴相互作用,通过求解Bethe-Salpeter方程,我们获得了激发态能量和光谱。我们发现,在这两种CN体系的价带中,σ轨道和π轨道之间的交换作用非常强烈。由于占据的σ轨道和π轨道之间的准粒子修正量非常不同,因此,为了得到准确的带隙值和光谱,我们需要对这两种轨道开展精确的GW计算。与单层的CN体系相比,双层结构中层与层之间的范德华相互作用使带隙值降低了0.6 eV,而光吸收谱红移了0.2 eV,这是由于双层结构具有更小的激子束缚能。我们计算的吸收峰的位置与实验结果符合很好。实验中的吸收峰主要是由深能级的π轨道到π^*轨道的跃迁形成的。π→π^*跃迁和σ→π^*跃迁之间的耦合能够在长波长范围产生弱的吸收尾巴,如果调整入射光的极化方向,由σ→π^*跃迁产生的高强度的吸收峰将会在更低能量处出现。     

Calculation of the nitrogen screening constants of some N-heterocycles

Absolute nitrogen screening constants and chemical shifts of some N-heterocycles are calculated within the CNDO/S level of approximation to Pople's model. Satisfactory agreement between calculated and observed nitrogen chemical shifts is obtained in most cases. The application of the results to the study of tautomeric equilibria is discussed. ‘Pyridine-type’ nitrogen chemical shifts are dominated by n → π* transitions together with significant contributions from π → σ* and σ → π* transitions whereas the latter two are the dominant ones for ‘pyrrole-type’ nitrogen nuclei.     

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.     

A theoretical interpretation of 13C screening data in unsaturated molecules

Carbon-13 screening constants are calculated within the INDO/S level of approximation to Pople's model. Satisfactory agreement is obtained in most cases between the calculated and observed screening results. An analysis of the contributions of the π → σ*, σ → π* and σ → σ* transitions to the paramagnetic term shows that a linear relationship between ¹³C chemical shifts and the lowest energy transition is not present. The average excitation energies are found to vary appreciably among the molecules studied.     

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.     

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.     

Electronic absorption spectra of benzoquinone and its hydroxy substituents and effect of solvents on their spectra

The electronic absorption spectra of 1,4-benzoquinone (BQ) and its 2,5-dihydroxy and tetrahydroxy derivatives have been studied in detail. The interpretation of the electronic bands is made on the basis of PPP and CNDO calculations. It is found that the pi --> pi* transitions are well predicted by the PPP method. The predictions of the CNDO method are however superior both in their accuracy as well as ability to predict the n --> pi* transitions. The effect of solvents on the electronic absorption bands have also been investigated in detail. Linear correlations are found between the solvent's dielectric constant and wavelength of the absorption bands. The solvent shifts are explained on the basis of the polarities of the solute and solvent molecules as well as due to hydrogen bonding.     

Electronic structure of the pyrimidine and purine components of nucleic acids in their ground and lower excited singlet and triplet states

Quantum-chemical calculation of the energies of the electronic transitions and the electronic structures of the neutral and ionic species of the nucleic acids components in their ground and lower excited singlet and triplet ππ* and nπ* states has been carried out in the all-valence-electron approximation CNDO /S . The results of the calculation allow one to identify the most photoreactive sites of the molecules and to consider the dependence of the location of these sites on the ionic state of the molecules. The calculated data are compared with our previous results obtained in a π-electron approximation. The individual absorption spectra of various ionic and tautomeric species of the nucleic acids components obtained by us earlier have been decomposed into bands corresponding to separate electronic transitions. As a rule, there is a good agreement between the calculated data in the two approximations and the experimental results.     

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.     

EFFECT OF DIELECTRIC CONSTANTS OF SOLVENTS ON THE BATHOCHROMIC SHIFTS IN RETINAL ISOMERS STUDIED THROUGH CARBON-13 NMR AND CNDO/2 METHODS

Abstract— The carbon-13 NMR and UV absorption spectra of all- trans -retinal have been obtained as a function of dielectric constant of solvent. The chemical shift data have been analyzed by the CNDO/2 MO calculation with the 'solvaton' model and Pople's theory for carbon-13 shielding constant. It has been shown that the changes in chemical shifts are correlated with the electron densities on the corresponding carbon atoms. Considering this result, the ground state properties of all- trans -retinal have been discussed. An increase in the electronic charge alternation and a decrease in the bond alternation have been found along the polyene-chain with an increase of dielectric constant. The electronic structure of 11- cis -retinal has been compared with that of all- trans -retinal in solution. It is suggested that the solvent-induced red-shift of absorption maxima is caused by the π-electron derealization in the vicinity of the terminal oxygen atom. Finally, on the basis of these results, the environmental effect on the chromophore in rhodopsin has been discussed.     

The intensities of low-energy electronic absorption transitions of some carbonyls and thiocarbonyls

The intensities of low-energy electronic transitions for some carbonyls and thiocarbonyls have been calculated from CNDO wavefunctions.Quite good agreement with experimental results has been obtained, where the latter are available. A satisfactory approximation for calculating intensities employs only one-center integrals. From the calculated trends in oscillator strengths, the absorption of thiophosgene at 4.46 eV can be identified as the π→π* ¹A₁←X?¹A₁ system. Another, very weak, system of thiophosgene at ≈ 3.9 eV is tentatively assigned to an n→π* ¹A₁ ← X?¹ A₁ transition, with the n orbital localized on the chlorine atoms.     

X-Ray determination of the effective charges on sulfur and phosphorus atoms in chemical compounds

The dependencies of Kα line shifts of the P and S atoms on effective charges of the atoms, calculated by CNDO/S, CNDO/2, INDO, and MNDO methods in the minimal and extended basis sets, have been investigated for model compounds. Good linear relationships between Kα shifts and atomic charges have been obtained for all cases investigated except for CNDO/2 and INDO calculations in the extended basis set. It is found that the PS bond ionicity is a nearly constant value, whereas the P S bond ionicity increases linearly with an increase in the positive charge of the central P atom. It is shown that the P and S atom charge in sulfides and phosphines containing no substituents with a strong M-effect at the central atom depend only on inductive substituent effects. Also, the positive charge on the central atom increases with the growth of the π-donor properties of substituents in sulfides and phosphines containing substituents exerting a strong M-effect, as well as in phosphoryl and thiophosphoryl compounds and sulfones.     

Electronic properties of N(5)-ethyl flavinium ion

We investigated the electronic properties of N(5)-ethyl flavinium perchlorate (Et-Fl(+)) and compared them to those of its parent compound, 3-methyllumiflavin (Fl). Absorption and fluorescence spectra of Fl and Et-Fl(+) exhibit similar spectral features, but the absorption energy of Et-Fl(+) is substantially lower than that of Fl. We calculated the absorption signatures of Fl and Et-Fl(+) using time-dependent density functional theory (TD-DFT) methods and found that the main absorption bands of Fl and Et-Fl(+) are (π,π*) transitions for the S(1) and S(3) excited states. Furthermore, calculations predict that the S(2) state has (n,π*) character. Using cyclic voltammetry and a simplistic consideration of the orbital energies, we compared the HOMO/LUMO energies of Fl and Et-Fl(+). We found that both HOMO and LUMO orbitals of Et-Fl(+) are stabilized relative to those in Fl, although the stabilization of the LUMO level was more pronounced. Visible and mid-IR pump-probe experiments demonstrate that Et-Fl(+) exhibits a shorter excited-state lifetime (590 ps) relative to that of Fl (several nanoseconds), possibly due to faster thermal deactivation in Et-Fl(+), as dictated by the energy gap law. Furthermore, we observed a fast (23-30 ps) S(2) → S(0) internal conversion in transient absorption spectra of both Fl and Et-Fl(+) in experiments that utilized pump excitations with higher energy.     

吡啶酮系偶氮类化合物可见吸收光谱的预测

利用密度泛函的B3LYP方法, 在6-311G*基组水平对吡啶酮系偶氮类化合物进行构型优化, 并进行了自然键轨道(NBO)分析, 然后用TDDFT方法和ZINDO/S方法分别计算了它们的可见吸收光谱, 结果均与实验值十分吻合. 通过对比发现, 对于最高吸收波长的计算, ZINDO/S能以较快的速度得到较好的结果. 在用ZINDO/S计算的过程中, 回归分析发现π-π重叠加权因子(OWF_π-π)与染料分子吡啶环上两个羰基氧原子平均电荷Z_O有较好的线性关系: OWF_π-π＝0.11425－1.04178ZO, 这一关系不仅可从量子化学的角度进行解释, 而且可用于同类染料可见吸收光谱的预测. 分子轨道的研究表明, 这些化合物的最高可见吸收波长主要对应着共轭体系中给电子体到受电子体的电子跃迁.     

Investigation of the charge state of heteroatoms in organic compounds of third-row elements by x-ray fluorescence spectroscopy 10. Correlations between values of the shifts of the kα lines of sulfur,phosphorus, and chlorine and the values of the effective charges of these atoms determined from semiempirical quantum-chemical calculations

A set of correlation equations which relate the experimental values of the shifts of the E K lines (E=S, P, Cl) for various classes of E-containing compounds to the values of the effective charges of the atoms of E determined from quantum-chemical calculations by the CNDO/S, CNDO/2, INDO, MINDO/3, and MNDO methods in the minimal basis and the CNDO/S method in an expanded basis has been obtained. It has been established that the INDO method in the minimal basis is best for calibrating the values of (E K) on the basis of the values of the effective charges of the atoms, while the CNDO/2 method and the INDO in the expanded basis are practically unsuitable for determining the values of the effective charges of S and P atoms.For previous communication, see [1].Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 2, pp. 403–410, February, 1991.     

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.     

Nature of the intense near-IR absorption and unusual broad UV-visible-NIR spectra of azulenocyanines: density functional theory studies

The nature of the broad and intense near-IR (NIR) absorptions of azulenocyanine (H(2)Azc) in the range of 720-1300 nm due to the π→π* transitions is clearly revealed on the basis of density functional theory (DFT) and time-dependent DFT (TD-DFT) calculations. The electron moving between peripheral conjugated systems and the eighteen electron-π-conjugated core due to different types of electron transitions has also been clarified. Because of the small gap between the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) of all four H(2)Azc isomers, their Q absorption bands in the region of 850-1300 nm are significantly red-shifted relative to its isoelectronic analogue naphthalocyanine H(2)Nc. Observation of the new absorption bands between Q and Soret bands from 720 to 1050 nm for H(2)Azc is considered to be the result of electron transitions from the fused azulene rings to the 18 electron-π-conjugated core. In addition, the introduction of electron-withdrawing or electron-donating substituents onto the periphery of the azulenocyanine ring is revealed to further reduce the LUMO-HOMO gap, leading to a further red-shifted broad and intense NIR absorption band from 900 to 1700 nm for H(2)(F(8)Azc) and H(2)[(NH(2))(8)Azc].