H. Sponer

Analysis of the Near Ultraviolet Electronic Transition of Benzene

The absorption spectrum of benzene at 2800–2200A has been analyzed. The vibrational structure of the system has been found to be in agreement with the selection rules for a forbidden transition (1A1g→1B2u). The transition becomes possible when vibrations of type Eg+ distort the molecule. Only carbon frequencies have been found to be effective. The interpreted data are collected in series on page 211. The analysis is corroborated by comparison with the absorption of solid benzene at —259°C, with the absorption spectrum of heavy benzene and with the fluorescence spectra of both benzenes.

Near Ultraviolet Absorption of Pyridine Vapor

The absorption spectrum of pyridine at 3000–2500A has been studied in the first order of a 3‐m grating spectrograph. The band system represents an electronic transition A1→B1 (using symmetry C2v for pyridine). Several progressions of totally symmetric vibrations are observed. The band 34769 cm−1 is taken as 0,0 band. Besides carbon ring vibrations the occurrence of hydrogen vibrations is established and discussed.

Absorption Studies of the Vapors of the Three Isomeric Picolines in the Near Ultraviolet

Studies of the near ultraviolet absorption spectra of α‐, β‐, and γ‐picoline vapor have been made in low and medium dispersion. Conclusive evidence is found for two electronic transitions in the vicinities of 35500 and 38500 cm−1. The former transition is interpreted as an allowed A1—B2 transition resulting from the excitation of an sp2 nonbonding electron of the nitrogen atom into the first unfilled π‐orbital of the ring. The latter transition is considered an allowed A1—B1 transition resulting from the excitation of a π ring electron. The A1—B2 system exhibits narrow headless bands similar to the first absorption bands in pyridine. It is of weak intensity comparable to that of the forbidden A1g—B2u transition in benzene. The A1—B1 system, which is much stronger, consists of broad diffuse bands. There is evidence that this system may be overlapped by another transition in β‐picoline.

Wavelength Shifts in the near Ultraviolet Spectra of Fluorinated Benzenes

Results on wavelength shifts of the 0,0 bands in the near ultraviolet absorption spectra of a number of fluorinated benzenes are presented, and a brief discussion in terms of the inductive effect and the migration effect is added.

Triplet‐Singlet Emission Spectra of Solid Toluene at 4°K and 77°K and in EPA Solution at 77°K

Phosphorescence of toluene has been studied at 4°K and 77°K in its solid phases (crystalline and amorphous) and in solid solution (EPA) at 77°K. The spectrum of the crystal at 4°K (3460‐4530 A) consists of sharp bands, and weaker diffuse bands which resemble in arrangement the rigid glass spectrum at 77°K. This emission has a lifetime of about 8 sec. The spectrum of solid toluene at 77°K (3950‐4900 A) has broad diffuse bands, and an estimated lifetime of ∼10‐2‐10‐3 sec. Its vibrational structure differs from that of the spectra at 4°K and in EPA, but resembles much the structure of the phosphorescence spectrum of benzaldehyde. The origin of the various spectra and the differences between them are discussed. The suggestion is made that benzaldehyde is produced photochemically and that its spectrum appears by a process of sensitization.

Triplet—Singlet Luminescence from Methylated Benzenes in the Crystalline State and in Rigid Glass Solutions

In continuation of earlier studies, triplet—singlet emission of the following methylated benzenes is reported in their solid phases: mesitylene, durene, hexamethylbenzene. The spectra were obtained in rigid EPA solutions at 77°K, and in the crystalline state at 77° and at 4°K. Two different spectra were found for mesitylene and durene crystals, one at shorter wavelengths representing the genuine triplet—singlet emission of the substance, and the other at longer wavelengths belonging to the lowest phosphorescence of an oxidation product. Hexamethylbenzene crystal apparently exhibits only one spectrum at any temperature. No solution spectrum was obtained for it because of its poor solubility. For an interpretation of the observations, various processes which take place in these crystals upon light illumination are discussed.

Delayed Fluorescence in Naphthalene Crystals at 4°K

The fluorescence of naphthalene crystals has been observed through a phosphoroscope indicating that a part of this emission has a lifetime greater than 10—3 sec at 4.2°K. It is suggested that this phenomenon is connected with the photoconductivity observed in crystals of aromatic compounds and has its origin in the storage of energy by an electron trapping mechanism.

Triplet-singlet emission spectra of benzene in a crystalline matrix of cyclohexane at 4·2°K and 77°K

Abstract The triplet-singlet emission of benzene in a crystalline matrix of cyclohexane was found to consist of a great number of well-defined bands between 3388 and 4712 A which are particularly sharp at 4°K. The pattern is that of a band system built upon three “zero” levels the middle of which is the strongest and hence acts as origin for the main bands of the spectrum. The vibrational analysis is fairly straightforward and is in agreement with a 3 B 1 u → 1 A 1 g or 3 B 2 u → 1 A 1 g transition. It is suggested that the two main “zero” levels represent positions of benzenes of slightly different energies in the cyclohexane lattice. Occurrence of ordered groups of pure and mixed composition in the liquid mixture benzene-cyclohexane are considered important for the crystallization process of the mixture.

Note on the Ultraviolet Absorption Systems of Benzene Vapor

An attempt has been made to give a coherent interpretation of the absorption systems of benzene vapor in the far ultraviolet including the Rydberg series. The following assignments are proposed: A forbidden transition of symmetry 1A1g→1B1u is suggested for the bands at 2050 to 1850A and an allowed 1A1g→1E—u transition for the much more intense bands at 1850 to 1650A. The continuous background in the latter region may possibly be due to a transition involving a C–H dissociation. The observed two Rydberg series can both be assigned to allowed transitions of symmetry 1A1g→1E—u.

Spectroscopic Studies in the Near Ultraviolet of the Three Isomeric Dimethylbenzene Vapors. II. Absorption Spectra of Meta and Ortho Dimethylbenzene

The vapor absorption spectra of meta‐ and ortho‐xylene were obtained with a 3‐meter grating spectrograph. The m‐xylene spectrum has the least number of bands and the most diffuse appearance. Its 0, 0 band is located at 36955 cm−1. The o‐xylene spectrum consists of many very sharp bands, several of which must be assigned to v—v transitions. Its 0, 0 band is found at 37308 cm−1. Vibrational frequencies of 965, 675 and 470 cm−1 are found in the upper level of the meta‐spectrum and are correlated with the ground‐state frequencies of 995, 725, and 510 cm−1. In the ortho spectrum ground‐state vibrational frequencies of 1233, 1054, and 736 cm−1 are correlated with the excited state values 1195, 939, and 692 cm−1, respectively. Assignments of the different frequencies are discussed. The Tesla luminescence spectrum of meta‐xylene is interpreted along with the absorption spectrum.