Saburo Nagakura

Vacuum ultra-violet absorption spectra of various mono-substituted benzenes

The vacuum ultra-violet spectra of various mono-substituted benzenes were measured in the wavelength region of 1550 A to 2200 A in the vapour phase by a recording vacuum ultra-violet spectrophotometer. The compounds studied here are phenols (phenol, anisole, phenetole and thiophenol), halogenobenzenes (fluorobenzene, chlorobenzene, bromobenzene and iodobenzene), and toluene. It was found that four π→π* transition bands appear in the wavelength region of 1550 A to 3000 A for phenol, anisole, phenetole, fluorobenzene, chlorobenzene and bromobenzene. On the other hand, six π→π* transition bands were found in the same wavelength region for thiophenol and iodobenzene. The absorption spectra of the former group are similar to that of benzene itself. On the other hand, the absorption spectra of the latter group are very different from that of benzene and rather similar to those of the anilines studied in a previous paper. From this point of view, the former group may be regarded as the molecules with weak substi...

The Relation between Energy Levels of Substituent Groups and Electron Migration Effects in Some Monosubstituted Benzenes

Determinations of the highest occupied (HB, HS) and the lowest vacant levels (VB, VS) were made with benzene and some substituent groups using experimental knowledge of their ionization potentials and their near ultraviolet absorption spectra. On the basis of the relative height of these levels, monosubstituted benzenes can be classified into following three groups: (1) | VS–HB | | VB–HS | for ortho‐, para‐directing molecules: (3) | VS–HB | ≈ | VB–HS | for styrene. By the aid of a simple quantum‐mechanical treatment, it becomes sure that in the case of (1) the interaction between levels VS and HB is predominant and the migrating electron flows out of benzene into the substituent: on the other hand in the case of (2) the interaction between levels VB and HS is the stronger and the migrating electron moves in the reverse direction: and finally, in the case of (3), both interactions referred to in the foregoing should be taken into account with almost...

Temperature dependence of fluorescence lifetimes of trans-stilbene

Abstract The temperature dependence of the fluorescence lifetimes of trans-stilbene was measured with a picosecond laser and a streak camera. A smooth sigmoidal relation was obtained, quite similar to the temperature dependence of fluorescence quantum yields. The results have clearly shown that the radiative rate is essentially constant in the temperature range from 77 K to 295 K.

Phosphorescence of the Charge‐Transfer Triplet States of Some Molecular Complexes

The phosphorescence spectra of some charge‐transfer complexes were studied in glassy solution at 77°K. Long‐lived emission spectra which were different from the phosphorescence spectra of the constituent molecules were observed for the 1,2,4,5‐tetracyanobenzene complexes with mesitylene, durene, and hexamethylbenzene. The maxima of the long‐lived emission spectra were found to shift to lower frequencies as the ionization potential of the donor was decreased; for example, maxima appear at 19.8, 19.5, and 18.3 (× 103 cm−1) in the complexes with mesitylene, durene, and hexamethylbenzene, respectively. This shift is parallel to that observed for charge‐transfer fluorescence spectra which exhibit a mirror‐image relationship with the corresponding absorption spectra. Similar long‐lived emission spectra were also observed for complexes containing pyromellitic dianhydride, tetrachlorophthalic anhydride, and phthalic anhydride as electron acceptors. A combination of theory and experiment indicates that the long‐li...

Intramolecular Charge Transfer Spectra Observed with Some Compounds Containing the Nitro or the Carbonyl Group

A new method, which proved to be useful for the theoretical study of electron migration for the monosubstituted benzene molecule, is employed to interpret strong absorption bands observed with various unsaturated compounds containing the nitro or the carbonyl group. Consequently, it is shown that these absorptions correspond to the transition between two energy levels produced by the interaction of the highest occupied level of the electron donating group with the lowest vacant one of the electron accepting group, and that theoretical values of transition energies and oscillator strengths evaluated by use of the present method are in very good agreement with the observed values. Furthermore, the present interpretation of these absorptions is discussed in comparison with the earlier resonance viewpoint.

The external magnetic field effect on the singlet sensitized photolysis of dibenzoyl peroxide

Abstract The external magnetic field effect upon the singlet sensitized photolysis of dibenzoyl peroxide in toluene was measured at 20°C in the presence of a field of 0–4.3 T. The yield of phenyl benzoata, the geminate product, decreases by 8% and the yields of free radical products increase by 2% in the presence of a magnetic field of 4.3 T. The decrease in the phenyl benzoate yield due to an external magnetic field is proportional to the square root of the applied strength. The observed magnetic field effect is quatitatively interpreted by the radical-pair theory of CIDNP developed by Kaptein.

Electronic spectra and electronic structures of nitrobenzene and nitromesitylene

Abstract The electronic absorption spectrum of nitrobenzene was measured under various conditions. In gaseous state, the absorption measurement was extended to the vacuum ultraviolet region (200-150 mμ) and a new strong band was found at 164 mμ. Furthermore, it was found that the 193-mμ band is composed of two bands, one of which is accompanied by vibrational structure. By combining these experimental results with a theoretical consideration, the most reliable assignments of the absorption bands of nitrobenzene have been accomplished. The results show that the 240-mμ band can be assigned to the intramolecular charge-transfer band. This assignment was also supported by the fact that the band decreases its intensity to a great extent in nitromesitylene. Furthermore, the present study concluded that there is the 6% contribution of the charge-transfer structure in the ground state of nitrobenzene, although some doubt had been thrown about the existence of the resonance interaction in view of the fact that the x-ray crystal analysis data show no shrinkage of the CN bond length.

External magnetic field effect on the decay rate of benzophenone ketyl radical in a micelle

Abstract The magnetic field effect upon the decay rate of the benzophenone ketyl radical in micellar solution was studied via nanosecond laser photolysis. Its decay is represented by a combination of two exponentials. The rate constant of the slow component was found to decrease from 1.6 × 10 6 s −1 at zero field to 9 × 10 5 s −1 at 700 G.

n → σ* Absorption spectra of saturated organic compounds containing bromine and iodine

Ultra-violet absorption spectra of a total of twenty-two saturated organic iodides and bromides were extensively measured in several polar and nonpolar solvents, in order to clarify the nature of long wavelength bands characteristic of these compounds having lone pair electrons. The results show that the absorption bands appear at about 260 and 200 mμ for alkyl iodides and alkyl bromides, respectively. The oscillator strengths were calculated with LCAO MO and AO approximations for the n → σ* transitions as well as for the first Rydberg transitions of the various alkyl bromides and iodides. Comparison of the calculated and experimental results indicated that the longest wavelength absorption bands for the alkyl halides are not due to the Rydberg transition but due to the n → σ* transition, as suggested by Mulliken. The f-values calculated by the aid of AO approximation were in better agreement with the corresponding experimental values than those by the LCAO method. A sharp strong band around 1940 A was assigned to the first Rydberg transition from the comparison of experimental and calculated oscillator strengths for ethyl iodide chosen as an example. Solvent effect, as well as, alkylation effect on the ultra-violet absorption bands was investigated for the compounds. It was found that the n → σ* bands show a blue shift with the increase of solvent polarity and show a red shift in the order of the primary, secondary and tertiary alkylations of the carbon atom attached to iodine. Absorption spectra of methylene iodide and iodoform, which may be resolved to give four individual bands, were successfully interpreted in terms of the appropriate combination of upper and lower orbitals splitted by mutual interactions between the iodine atoms in these compounds.

ESR Spectra of the Charge‐Transfer Triplet States of Some Molecular Complexes

ESR spectra were measured with various solutions containing 1,2,4,5‐tetracyanobenzene (TCNB) and benzene or methyl‐substituted benzenes. We succeeded in finding the ESR absorption lines due to the Δm = ± 2 and Δm = ± 1 transitions of the lowest triplet excited states of TCNB complexes and also in measuring their lifetimes. From the analysis of the ESR spectral data, we determined the fine‐structure constants D and E for the TCNB–toluene complex in ether–toluene mixed solvent, and D* for the TCNB complexes with benzene, toluene, xylenes, mesitylene, durene, pentamethylbenzene, and hexamethylbenzene. From the theoretical consideration on the D* value, an equation representing its dependence upon the charge‐transfer degree (x) in the lowest triplet excited state of a molecular complex was derived. The combination of the equation with the observed D* values gave the x values for the TCNB complexes, for example 95%, 72%, and 42% for TCNB complexes with hexamethylbenzene, durene, and mesitylene, respectively. T...