Hiroaki Baba

Configuration Analysis in the LCAO Molecular Orbital Theory

A procedure is described which makes it possible to analyze and interpret the results of calculations performed by means of the LCAO–SCF molecular orbital method including configuration interaction (the Pariser–Parr–Pople method) in the language of the isoconjugate‐hydrocarbon model, localized‐orbital model, or other appropriate models. A general process is shown for expressing the wavefunctions of any electronic configurations or states based on the SCF orbitals by the corresponding wavefunctions based on “reference” orbitals. Specific formulas are given for expanding the wavefunctions of lower singlet and triplet configurations of the former type in terms of the configuration wavefunctions of the latter type. The π‐electronic states of the phenol molecule are dealt with to illustrate the present analysis.

Electronic Spectra and Hydrogen Bonding. I. Phenol and Naphthols

The effect of hydrogen bonding on the electronic absorption spectra of phenol, α naphthol, and β naphthol has been investigated with particular attention to the relation between the nature of electronic transitions and their behavior in hydrogen bond formation. The spectra were obtained down to 2000 A in isooctane solution in the presence of varying concentrations of dioxane. From the analysis of the observed spectra, the equilibrium constants for the hydrogen bonds and the spectra of the hydrogen‐bonded species were determined. The hydrogen bond energies are given for the ground and excited states of the solute molecules. The experimental results clearly indicate that effects of hydrogen bonding on electronic spectra differ markedly with transitions. Both the frequency shifts and the intensity changes differ in magnitude and even in sign according to the properties of the transitions concerned. It is shown that the transition at 47 000 cm−1 of α naphthol is displaced to higher frequencies upon formation ...

Proton transfer in p-nitrophenol-triethylamine system in aprotic solvents

Abstract The interaction of p-nitrophenol with triethylamine in various aprotic solvents was studied quantitatively by means of electronic absorption spectroscopy, with special regard to proton transfer. The acid-base interaction leads to different types of complexes, depending upon the dielectric constant of the solvents. In 1,2-dichloroethane as the solvent, proton transfer takes place from the nitrophenol to the amine to yield a hydrogen-bonded ion pair, which is in equilibrium with a simple hydrogen-bonded complex. The enthalpy change determined from the spectral data indicates that the ion pair is more stable by 3.1 kcal mole in the ground state. Schematic double-minimum potential curves are given for the proton in the excited state as well as in the ground state of the acid-base system. A calculation was made to estimate the solvation energy of the complexes on the basis of the reaction field theory. The result suggests that in a medium of dielectric constant unity the ion pair should be less stable by 5.0 kcal/mole in the ground state, but more stable by 7.8 kcal mole in the excited state. In acetonitrile, only a proton-transferred species was observed which was assigned to a solvent-separated ion pair.

Vapor‐phase fluorescence emissions of pyridine and its methyl derivatives: Excitation‐energy dependence of nonradiative electronic relaxation

Vapor‐phase fluorescence from the S1 (n, π*) states of pyridine‐h5 and ‐d5, α‐ and β‐picolines, and 2,6‐lutidine has been studied. The fluorescence quantum yields of these compounds are, respectively, 5.9×10−5, 6.0×10−5, 3.5×10−5, 5.4×10−5, and 2.5×10−5 for excitation to the zero‐point level of S1. The quantum yields of the first four compounds decrease sharply as the excitation energy is raised to the value corresponding to the S2 (π, π*) state, whereas the yield of 2,6‐lutidine is fairly constant throughout the S1←S0 and S2←S0 absorption regions. The fluorescence quantum yields and their excitation‐energy dependence undergo no significant change upon pressure increase from 0.5 to 20 Torr or addition of a foreign gas up to 760 Torr. The rate constants for the intersystem crossing (kISC) and the other nonradiative processes (kFQ) are evaluated as functions of the excitation energy for pyridine‐h5 and α‐picoline with the aid of the intersystem‐crossing yield values which are available from the literature. ...

Fluorescence from the Second Excited Singlet State and Radiationless Processes in Pyrene Vapor

The fluorescence spectrum originating from the second excited singlet state S2 of pyrene in the vapor phase at 170°C and 0.21 mm Hg has been studied with particular attention to changes of emission characteristics with the excitation energy. The fluorescence from the first excited singlet state S1 has a comparatively constant quantum yield Φ1F (between 0.14 and 0.08), regardless of the excitation energy. In contrast to this, the quantum yield Φ2F of the fluorescence from S2 increases rapidly with excitation into successively higher singlet states. The ratio Φ2F/Φ1F is of the order of 0.001 for excitation into S2, but it is larger than 0.1 for excitation into the fourth excited singlet state S4. Addition of cyclohexane as an inert foreign gas to the pyrene vapor results in a marked decrease in Φ2F/Φ1F, which seems to reach a constant value at high pressures of cyclohexane (〉60 mm Hg). From an analysis of the experimental data and a consideration of the kinetics of radiative and radiationless processes in a...

Interpretation of electronic spectra by configuration analysis: Absorption spectra of monosubstituted naphthalenes

Abstract The electronic absorption spectra of four monosubstituted naphthalenes, α-, β-naphthols, and α-, β-naphthylamines have been investigated by means of configuration analysis with particular attention to the dependence of spectra on the position of substitution and on the electron-donating power of the substituent. The results of molecular orbital calculations based on the Pariser-Parr-Pople method are analyzed in terms of locally excited states and intramolecular charge-transfer configurations. The characteristic changes in location and polarization of the Lb, La, and Bb bands caused by substitution at the α- or β-position are adequately explained by the analysis. Two strong absorption bands of α-substituted naphthalenes, which appear in place of the Bb band of naphthalene, are shown to result from a mixing of the B 3u + (B b ) and A g − states. The amino group exerts a great influence on the electronic structure of the parent molecule, so that the Bb band cannot be identified in the spectrum of β-naphthylamine.

Vibronic coupling and intramolecular dynamics of pyrene as revealed by the S0→S2 excitation spectrum in a supersonic jet

Abstract The fluorescence excitation spectrum of pyrene obtained in a supersonic jet for the S 0 →S 2 transition shows a complicated structure due to the interaction of discrete levels of S 2 with the quasi-continuous levels belonging to S 1 . The intensity distribution pattern in this region has been evaluated from quantum-mechanically calculated quantities, such as the vibronic coupling integrals, and Franck-Condon factors deduced from independent experiments. The dynamics in pyrene following excitation into S 2 are discussed.

Rotational effects on fluorescence quantum yields of pyrazine and pyrimidine in the vapor phase

Abstract Fluorescence-excitation and quantum-yield spectra were measured along the rotational contours of the 0-0 and 10a o 1 bands of pyrazine and the 0-0 band of pyrimidine in the vapor phase at low pressure. A marked variation in quantum yield was observed. The yield spectra for the 0-0 bands exhibit a very sharp peak near the band origin. It is suggested that the S–T coupling is enhanced with increasing rotation.

Electronic spectra and intramolecular dynamics of phenanthrene in a supersonic free jet

The fluorescence excitation spectra and the fluorescence decays of phenanthrene-h10 and phenanthrene-d10 in a supersonic free jet have been measured with excitation across the S 0 → S 1 and S 0 → S 2 absorption regions. The fluorescence excitation spectra of the isolated phenanthrene molecule are compared with the absorption spectra in low temperature matrices. The presence of a 2-type out-of-plane vibrational bands in the S 0 → S 2 absorption spectra indicates that a 1 B 1(πσ*) state lies near the S 2 state. The S 2 → S 1 internal conversion rates at various vibronic levels of S 2 for both compounds have been evaluated from the analysis of profile and width of the individual vibronic bands in the excitation spectra. The excess vibrational energy dependence of the S 2 → S 1 internal conversion rate is the opposite of that shown by the total decay rate in S 1. The excess vibrational energy dependence of the electronic decay rate and the profile of the vibronic absorption bands belonging to the S 0 → S 2 tr...

Molecular association and emission spectra of benzoic acid

Abstract The emission spectra of benzoic acid in isopentane-methylcyclohexane rigid glass solution at 77°K have been studied with particular regard to effects of dimer formation on the spectra. The monomer of benzoic acid shows phosphorescence alone with a high quantum yield ΦPM of 0.70, whereas the dimer emits not only phosphorescence (ΦPD = 0.46) but also fluorescence of fairly high intensity (ΦFD = 0.25). It is inferred that the benzoic acid monomer has an ( n, π ∗ ) triplet state between its lowest excited singlet and lowest triplet states, and this results in efficient intersystem crossing and hence in the nonfluorescent nature of the monomer. The appearance of fluorescence in the dimer is explained on the assumption that the ( n, π ∗ ) triplet state rises above the level of the lowest excited singlet owing to the hydrogen bonds involved in the dimerization.