Motohiko Koyanagi

Effect of dispersion forces of solventsII. On the O-O band of the near ultraviolet absorption spectrum of benzene in fluid solutions

Abstract London dispersion forces due to surrounding solvent molecules often affectremarkably an electronically forbidden transition of a solute molecule. The 2600 A absorption system of benzene has been studied in some detail. The second order perturbation theory has been applied to explain the molecular interactions between solute and solvent molecules. Transition polarizabilities and oscillator strengths induced by dispersion effects of surrounding solvent molecules have been calculated. Applications have also been made to the 2000 A absorption system of benzene, the 3200 A system of naphthalene, and the 4000–5000 A band system of p -benzoquinone.

3nπ* spectra of benzaldehyde. I. Vapor phosphorescence☆

Abstract Vibrational analysis of the vapor 3 nπ* phosphorescence for three isotopic benzaldehydes (B- h 6 , B-1 d 1 , B-R d 5 ) shows that the out-of-plane aldehyde hydrogen wagging vibration (ν 26 ) is the most active non-totally symmetric mode in the spectrum. Since the intensity of 26 2 0 ⪢ 26 1 0 the mechanism of ν 26 activity is primarily as a Franck—Condon mode. The only other out-of-plane mode definitely attributed to the vapor phosphorescence is the weakly active CHO torsional vibration (ν 36 ) with I (36 1 0 ) > I (26 1 0 ). Other Franck—Condon modes are ν 7 , ν 25 , ν 20 , ν 17 and ν 8 .

n, π∗ electronic states of p-benzoquinone; The T-S emission spectrum in mixed crystals

Abstract In order to scrutinize the lowest triplet n, π ∗ state of p-benzoquinone, we have observed its T-S emission and absorption spectra in a crystal, in an isotopically mixed crystal, and in p-dihalobenzene crystals at various temperatures between 1.45 and 77°K. Important evidence was found that leads to a conclusion that the phosphorescence process may have been an orbitally and spin forbidden 3B1g-1Ag transition. Although it is still multiplicity forbidden, the phosphorescence has been allowed vibronically through perturbation of four b1u fundamentals (except a CH stretching mode) and two b3u fundamentals of the out-of-plane CH and ring distortion modes. The contribution of the b1u CO stretching mode to the vibronic perturbation was greatest and that of the b1u ring bending mode was second. A detailed analysis of the T-S absorption of the p-benzoquinone crystal and the T-S emission of p-benzoquinone-h4 in an isotopically mixed crystal predicted that the 3B1g level is the lowest triplet, being located at 18 624 cm−1, which is lower about 320 cm−1 than the 3Au level. Comparison of the f-value and the observed lifetime (6.4 × 10−4 sec in an isotopically mixed crystal) of the T-S transition indicated there should be anomalous nonradiative processes in the triplet state of p-benzoquinone. For comparison, the phosphorescence spectrum of toluquinone has also been observed in p-dihalobenzenes.

On the origin of the Ham bands in the spectrum of benzene in fluid solution

The spectrum of benzene has been studied in various fluid solutions at various temperatures in the ultra-violet region and at room temperature in the infra-red, and the relations between the spectral behaviour and the intensity of the Ham bands have been examined. No positive evidence for assigning the bands to the 3E1u−1A1g transition has been found, and it is believed that the bands belong to the lowest 1B2u−1A1g transition. No evidence has been found that the appearance of the bands is due to chemical bond forces or charge-transfer bond forces between benzene and solvent molecules. It has been concluded that the intensity of the Ham bands has a close relation to the intensity of the anomalous infra-red bands of benzene 1177(e2g) and 850(e1g) cm−1 vibrations, and that the origin of the Ham bands is due to dispersion forces between benzene and solvent molecules. This conclusion is valid only if the solution is fluid.

Interstate interactions between T1 and T2 of benzaldehydes in crystalline matrices

Abstract A modulation polarization technique has been applied to phosphorescence studies of several benzaldehydes in mixed crystals at 4.2 K. The result shows that two types of interstate couplings are mainly operative for 3ππ* → S0 phosphorescence processes: They are (1) second order vibronic-spin orbit mixings through out-of-plane vibrations and (2) direct environmental mixings between 3ππ* and 3ππ* states.

Stark effects on the phosphorescence spectra of p-benzoquinone

Stark effects on the phosphorescence spectra of p-benzoquinone have been studied in naphthalene crystals at temperatures between 1.7 and 30 K. The experimental data establish that the band splitting of 24 cm−1 arises from a double minimum potential in the T1 state.

Direct evidence of photochemical α-cleavage of benzoin in fluid solutions

Abstract By means of optical absorption, 1 NMR, and transient EPR techniques, the fate of diluted benzoin upon light irradiation to its S 1 (nπ*) state has been investigated in methylcyclohexane and benzene solutions at room temperature. The CIDEP spectrum of benzoin is observed for the first time, and the intermediate radicals involved are assigned. The overall results show that the main scheme of the photochemical reactions is the α-cleavage occurring in the excited triplet state of benzoin, as proved in the almost net emission pattern of the CIDEP spectra. A stoichiometric reaction leading to effective benzaldehyde formation is established for the benzoin solutions.

Interstate couplings in the triplet manifold of 9‐xanthone

Using a heat‐pulse modulation technique, phosphorescence spectra of 9‐xanthone have been studied in n‐pentane at 77 and 2.1 K. The three legitimate phosphorescent levels with different lifetimes are detected at 25 751, 25 766, and 25 774 cm−1 above the zero‐vibrational level of the ground state. The origin of these multiple levels is explained in terms of spin–orbit interaction superimposed on environmental electrostatic coupling between T1(ππ*) and T2(nπ*). The two coupling constants obtained are: (for the spin–orbit coupling) ‖〈1ξ‖Hso‖2η〉‖ =15.70 cm−1 (ξ=x, η=y or vice versa); (for environmental direct coupling) 〈1ζ‖Henv‖2ζ〉 =3.14 cm−1 (ζ=x, y, or z). For comparison, the T←S phosphorescence excitation spectrum, measured in the same matrix, are in good agreement with the calculated ones.

Tangled ← S spectra of isotopic benzaldehydes☆

Abstract The T 1.2 ← S 0 phosphorescence excitation spectra of benzaldehyde and its deuterated derivatives have been obtained in methyl benzoate and acetophenone hosts at 4.2 K. The observed tangled vibronic bands have been analyzed in terms of Born—Oppenheimer vibronic wavefunctions of both the excited ππ* and nπ* triplet states. Two perturbation terms are employed in this calculations: one accounts for the vibronic interactions, the other for direct environmental coupling interactions between T 1 (ππ*) and T 2 (nπ*) vibronic levels. In these hosts the spectra show sparse tangledness indicating that the most important resonances are between the zero-order vibrational level of T 2 (nπ*) and zero-order low-frequency out-of-plane vibronic levels of T 1 (ππ*). Substitution of the aldehyde hydrogen by deuterium produces a dramatic effect on the spectral intensity distribution, but little effect on the band positions. We conclude that the activity of the T 1 (ππ*) origin band is mainly controlled by direct environmental perturbations between T 1 (ππ*) and T 2 (nπ*) with a minor contribution from the first-order spin—orbit coupling interaction between T 1 (ππ*) and S 1 (ππ*). The lowest excited state energy level consists of an almost pure vibrationless level of T 1 (ππ*). The vibrationless origin level of T 2 (nπ*), however, is difficult to identify in the T ← S spectrum because this level interacts strongly with several out-of-plane vibronic levels of T 1 (ππ*) in resonance with T 2 0 (nπ a ; 0-0). Diagonalization of the secular determinant in terms of a basis set made up of many 3 ππ* and 3 nπ* Born-Oppenheimer vibronic levels shows that the Condon and Born-Oppenheimer approximations are inappropriate for discussing the intensities and designating the assignments.

Simulated T ← S spectrum of 2,4,5-trimethylbenzaldehyde in durene☆

Abstract The T 1,2 ← S 0 phosphorescence excitation spectrum of 2,4,5-trimethylbenzaldehyde in durene has been simulated using forty-five zero-order Born-Oppenheimer product states of which thirty-two belong to T 1 (ππ*), the others to T 2 (nπ*). The spectrum is very complicated in the region 400–600 cm −1 above the T 1 (ππ*) ←3 S 0 origin band at 24150 cm −1 . In this tangled region conventional vibrational analysis is not useful. Several comments on the physical properties of the excited triplet states of 2,4,5-trimethylbenzaldehyde are given.