π* ← n and π* ← π absorption spectra of aminopyrazine

π* ← n and π* ← π absorption spectra of aminopyrazine have been recorded and analysed assuming C_s symmetry for the molecule.     

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.     

Sn ← S1 and Tn ← T1 absorption spectra of highly purified chrysene in solution

Time-resolved S_n ← S₁ and T_n ← T₁ absorption spectra were observed for highly purified chrysene in THF solution. Formerly assigned to the S₄ ← S₁ band located in the 17200–17600 cm⁻¹ (581-568 nm) region. S_n ← S₁ was reassigned to S₆ ← S₁. The S₄ ← S¹, S₅ ← S₁, S₇ ← S¹ and S₈ ← S₁ bands were also observed at 13500 cm⁻¹ (740 nm). 15700 cm⁻¹ (635 nm). 19000 cm⁻¹ (525 nm), and 20400 cm⁻¹ (490 nm), respectively. The relevant molar extinction coefficients were 7100 (S₄ ← S₁), 15000 (S₅ ← S¹), 14000 (S₆ ← S₁), 19000 (S₇ ← S₁), and 14000 M⁻¹ cm⁻¹ (S₈← S₁).     

Davydov splitting in the polarized absorption spectra of single crystals of some copper complexes of α-amino acids

This paper presents the absorption spectra observed with polarized light of the single crystals of the copper complexes of L-alanine and DL--aminobutyric acid.The spectra are interpreted by the crystal-field theory.

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Zusammenfassung Es werden die Absorptionsspektren angegeben, welche mit Hilfe polarisierten Lichts an Einkristallen der Kupferkomplexe von L-Alanin und DL--AminobuttersÄure beobachtet wurden.Die Spektren wurden an Hand der Kristallfeldtheorie gedeutet.

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Résumé On donne les spectres d'absorption observés, à l'aide de lumière polarisée, aux cristaux uniques des complexes de cuivre du L-alanine et de l'acide DL--aminobutyrique.Les spectres sont interprétés suivant la théorie du champ cristallin.

     

A simple algorithm for the calculation of the π ionization energies of substituted benzenes

A simple algorithm, based on HMO approach is used for calculating Π-ionization potentials (Π-IP's) of polysubstituted benzenes. The parameters required for the method are determined from Π-IP's of monosubstituted benzenes. For 50 disubstituted benzenes the first three values of the Π-IPś are obtained with root mean square (rms) error of 0.19 eV. Some examples of application are presented, illustrating the possibility of fast and quite accurate estimation of Π-IP's for polysubstituted benzenes.     

The photoacoustic spectra of substituted benzenes in the near infrared region—I

The photoacoustic spectra of three benzonitriles, five acetophenones and three benylbromides were recorded using the commercial EDT OAS 400 photoacoustic spectrometer in the near i.r. region, i.e. 3850–10000 cm⁻¹ (2.6−1.0 μm). A combined study of the mid and far i.r. absorption spectra with the near i.r. PAS spectra led to a provisional assignment of the observed PAS peaks as combinations of fundamentals and overtone bands.     

New description of the substituent effect on electronic spectra by means of substituent constants—IV. Charge transfer spectra of EDA complexes of tetracyanoethylene with meta-disubstituted benzenes

Charge transfer spectra of EDA complexes composed of TCNE and various kinds of m-disubstituted benzenes are discussed on the basis of a general equation, theoretically derived to express the substituent effect on electronic spectra. Molecular orbital calculation shows that the HOMO of the substituted benzenes is divided into two groups. One has a₂-like character and the other b₂-like, so that the substituent effect on the CT spectra has been also classified into two groups, since the CT spectral character is different in the two. This viewpoint is supported by the application of the general equation to the CT spectra. Also, we have applied the equation successfully to the other typical π-π or n-σ type EDA complexes.     

Evolution of thermal disorder in the absorption and Raman spectra of all-trans-β-carotene

The temperature-dependent visible absorption and resonant Raman spectra of all-trans-β-carotene extremely diluted in dimethyl sulfoxide are investigated to clarify the effects of temperature on conjugated polyenes. The visible absorption and Raman spectra are found to blueshift with increasing temperature. The blueshift in polyenes is attributed to the decrease in the liquid density and the concomitant decrease in the refractive index by the Lorentz–Lorenz relation. We demonstrate that visible absorption is reproduced well by simple Franck–Condon analysis using a single Huang–Rhys factor over a temperature range. The analysis reveals features of temperature dependence in terms of important spectral parameters, such as line width, Raman scattering cross section and Huang–Rhys factor.     

Comparison of the electronic absorption spectra of the isomeric 1,1′- and 2,2′-binaphthyl spiro [cyclophosphazene] derivatives

It has been found that the u.v.-spectrum of the monospiro [(2,2′-dioxybinaphthyl-1,1′) cyclophosphazene] (3) closely resembles those of the other previously described 7-membered 2,2′-bridged 1,1′-binaphthyl derivatives. An hitherto unreported effect of the associated influence of α-substitution, β-coupling and the next cyclization on the u.v. absorption of naphthalene chromophore has been shown by the comparison of the electronic spectra of 1-naphthol (5), 1,1′-dihydroxybinaphthyl-2,2′ (6) and monospiro [(1,1′-dioxybinaphthyl-2,2′) cyclophosphazene] (7). Exclusively strong resonance interaction between the conjugated naphthalene rings was evidenced for 7.     

On the solvatochromism of the n ↔ π* electronic transitions in ketones

The solvatochromism of the n ? π* electronic transitions of acetone, which is determined in this work by means of absorption and emission spectroscopy, has been studied using the pure solvent scales for polarizability, dipolarity and acidity of the medium. From these analyses, the necessary reduction of the dipole moment and the increase of the polarizability for acetone on electronic excitation are evaluated using Abe's approach (Bull. Chem. Soc. Jpn. 1966, 39, 936). The influence that the increase of the aromatic structure (cf. acetophenone) and the lengthening of the size of the alkyl substituents R- (cf. R-CO-R) cause on the solvatochromism of acetone is discussed. Also, we have shown that the photophysical model proposed by Pimentel (J. Am. Chem. Soc. 1957, 79, 3323), which is widely accepted for explaining the n → π* blue shift phenomenon for hydrogen-bonded complexes, is mistaken due to ignoring the acid-base changes undergone on electronic excitation; accordingly, a new photophysical model has been proposed.     

Bond Fukui functions as descriptor of the electron density reorganization in π conjugated systems

The bond Fukui function is introduced and tested as a new reactivity index capable of predicting the evolution of bond breaking and formation processes during an organic reaction involving π conjugated systems. As an illustration, we examine many cases where substituted ethylenes and dienes may respond to different reagents to yield cycloaddition, Michael addition, and other reactions at double bonds.     

Solvent effects on electronic absorption spectra of nitrochlorobenzenes,nitrophenols and nitroanilines—I. Studies in nonpolar solvents

Estimation of dipole moments and polarizabilities in various excited states of o-, m- and p-nitrochlorobenzenes, nitrophenols, nitroanilines and the corresponding monosubstituted benzenes was carried out using solvatochromic shifts in electronic absorption spectra making use of equations based on McRae's and Abe's approaches. Comparison of the observed μ_es with those obtained from addition of group moments indicates that, in o- and m-nitrophenols and o- and m-nitroanilines, the electronic transitions have their origin in the donor groups. The excited states of p-nitrophenol and p-nitroaniline appear to have comparable contributions from the excited states of both the donor as well as the acceptor group due to the large extent of resonance interaction.     

Solvent effects on electronic absorption spectra of nitrochlorobenzenes,nitrophenols and nitroanilines—II. Studies in polar solvents

A general method for estimating the specific solute—solvent interaction energies (E_s) from analysis of solvent effects on electronic absorption spectra has been outlined. The E_s values for a number of mono- and disubstituted benzenes in a variety of solvents such as water, alcohols, chloroalkanes, ether and acetonitrile have been estimated and the results are discussed in relation to the interacting groups in the solute and solvent molecules. These interactions, which can be classified as H bonding or EDA type, are mainly electrostatic in nature. These studies indicate that, in the case of disubstituted benzenes, the intramolecular electronic interactions are stronger than the solute—solvent interactions.     

Reproduction and interpretation of the UV–vis spectra of some flavonoids

The UV–vis spectra of 15 naturally occurring flavonoids, including flavones, flavonols, and isoflavones, were simulated and compared to literature experimental results. For this purpose, the TDDFT method in conjunction with the B3LYP-D3 and B3LYP functionals was applied. In spite of some discrepancy between the structural predictions, the differences between the TDDFT/B3LYP and TDDFT/B3LYP-D3 results are negligible. The performance of both functionals in reproducing the UV–vis spectra of the selected flavonoids is satisfactory. The NLMO clusters, i.e. molecular moieties characterised with distinguished electron density, were created. The combined results from the TDDFT and NLMO approaches were employed to perform deep analysis of the spectra. It was found that the transitions from the bonding NLMO clusters to the LUMO cluster are favoured at longer wavelengths, whereas the transitions from the HOMO and HOMO?1 clusters to the antibonding NLMO clusters are favoured at shorter wavelengths. All flavonoids show the HOMO → LUMO and HOMO?1 → LUMO transitions at long wavelengths, and majority of them show the HOMO → LUMO+1 transition at moderate wavelengths. This investigation confirms our earlier finding that the TDDFT and NBO approaches are complementary, implying that the results from the two theories can be combined to better understand the redistribution of electron density upon excitation.     

On the Distribution of π Bonds in Cyclofusene

We present a type of coronafusene termed cyclofusene, in which each hexacycle shares exactly two nonadjacent edges with other hexacycles. Cyclofusene has exactly four configurations of bonds such that each bond belongs to the inner or outer boundary. In each of these configurations, the outer boundary has six more bonds than the inner boundary. The number of shared bonds in any mixed configuration is even. Let m be the number of shared bonds in a mixed configuration for a cyclofusene with exactly k linear chains. Then m k. Furthermore, there exists a mixed configuration with exactly k shared bonds.