ISSN:0026-8976    

DOI:10.1080/00268977400100281

出版商:Taylor & Francis Group    

年代:1974

数据来源: Taylor

摘要:

The results of the phosphorescence microwave double resonance studies of the lowest excited triplet (T1) states of acetophenone, benzaldehyde and their derivatives in various mixed crystals are presented and analysed. The zero field splittings (ZFS), the total decay rates and the relative radiative decay rates and populating rates were determined for a series of compounds in different host crystals. The correlations between ZFS, the decay rates and the energy separation between the3nπ* and3ππ* states were carefully examined and the results were explained in terms of the mixing of the3nπ* and3ππ* states. The mixing coefficients of the3nπ* states in theT1states were estimated from the decay rates. Changes of the ZFS, decay rates, vibrational structures of the phosphorescence spectra of the3ππ* aromatic carbonyl molecules were also analysed. The mechanisms responsible for theS1→T1andT1→S0processes of the3ππ* states of these aromatic carbonyl molecules are discussed in view of the obtained results.

ISSN:0026-8976    

DOI:10.1080/00268977400100291

出版商:Taylor & Francis Group    

年代:1974

数据来源: Taylor

摘要:

Detailed single-crystal E.P.R. studies of the lowest excited triplet states (T1) of a series of aromatic carbonyl molecules, such as acetophenone, substituted acetophenones and substituted benzaldehydes, were made using 1,4-dibromobenzene and durene as hosts. Signs and magnitudes of the zero field splittings (ZFS), directions of the principal axes of the ZF tensor, and the orientations of the guest molecules with respect to the host molecules were determined for a series of aromatic carbonyl molecules. The results obtained are consistent with the3ππ* assignment of theT1states of the systems studied here. Comparisons of the observed ZFS and those estimated from the Hayashi-Nagakura analysis were made for several systems. Hyperfine interactions and spin distributions were investigated in the case of 4-fluoroacetophenone. The large spin density at the 4 position (ρ4= 0·28) was discussed in terms of the3ππ* assignment and compared with those of the related triplet state molecules and radical anions.

ISSN:0026-8976    

DOI:10.1080/00268977400100301

出版商:Taylor & Francis Group    

年代:1974

数据来源: Taylor

摘要:

The temperature dependence of the phosphorescence spectra, decay rates and theS0→T2(3nπ*) absorption spectra were studied for aromatic carbonyl molecules in mixed organic crystals. The energy separation, ΔET, between the3ππ* lowest excited triplet (T1) states and the higher3π*states (T2) was estimated for several systems from the temperature dependence of the phosphorescence spectra and decay rates. It was found that the decay rates of the aromatic carbonyl molecules in the mixed crystal systems studied are determined by (1) thermal population to the3nπ* states and (2) increased radiationless transition rates at higher temperatures.

ISSN:0026-8976    

DOI:10.1080/00268977400100311

出版商:Taylor & Francis Group    

年代:1974

数据来源: Taylor

摘要:

Theoretical values of the bond-length dependence of the nuclear spin-spin couplingJ(HD) in the HD molecule have been obtained by Schulman and Kaufman. Although scope exists for their improvement they are the only results available and are sufficiently close to the true values to justify the making of predictions of the coupling in the isotopomers of HD and the temperature dependences of the coupling in these compounds. HT appears to be the most promising compound to study experimentally, and the predicted spin-spin coupling increase of about 1·5 Hz on increasing the temperature from 100 K to 450 K should be measurable. The negative isotope effect on the reduced spin-spin coupling of the hydrogen molecule predicted theoretically is of the same sign as that suggested for primary isotope effects from experimental investigations.

ISSN:0026-8976    

DOI:10.1080/00268977400100321

出版商:Taylor & Francis Group    

年代:1974

数据来源: Taylor

摘要:

This paper reports the study of the Raman autocorrelation functions and molecular motion of HCl in the liquid and the paraelectric solid phases. It is shown that both the molecular reorientation and other types of interaction such as the hydrogen bonding have very important effects on influencing the time evolution of the Raman correlation functions in HCl. In contrast to the theory for weakly interacting diatomic molecules, it is pointed out that in strongly interacting molecular systems the N.M.R.T1process due to the intramolecular magnetic dipole-dipole interaction will not yield the same correlation time as that deduced directly from the depolarized Raman correlation function. Despite the fact that hydrogen bonding occurs in the liquid and solid HCl, from the available Raman and N.M.R.T1data it was shown that large step rotational diffusion is present; however, the diffusional process is modulated by the hydrogen bond rupture-reformation process.

ISSN:0026-8976    

DOI:10.1080/00268977400100331

出版商:Taylor & Francis Group    

年代:1974

数据来源: Taylor

摘要:

A theory for the rate of decay of triplet-state excitons by their interaction with acoustic phonons is developed under the assumption that only the equilibrium internuclear distance between molecules is changed when one of them is excited. The decay rate is found to be proportional to the square of the gradient of the exchange energy |·J/·r|2. A procedure for obtaining·J/·ris outlined, and numerical results are presented for two benzene molecules in a geometry similar to that of the phenyl rings of adjacent molecules in crystalline 1,2,4,5-tetrachlorobenzene. The calculated exchange interaction energy |J|=16·7 cm-1is an order of magnitude larger than the exciton frequency bandwidth Δv=1·3 cm-1recently reported by Francis and Harris, while the calculated exciton decay rate ∼1011s-1, being comparable to Δv, is much too large to be compatible with the observations. A postulated Franck-Condon reduction of |J| arising from an intramolecular lattice distortion bound to the exciton is necessary to reconcile these theoretical results with the experiment.

ISSN:0026-8976    

DOI:10.1080/00268977400100341

出版商:Taylor & Francis Group    

年代:1974

数据来源: Taylor

摘要:

The E.S.R. spectrum shape of a dynamic biradical system is calculated by means of the density spin-matrix formalism. The biradical is treated as a system which has two conformations characterized by the different values of the exchange integralsJ=J1andJ=J2and the lifetimes τ1and τ2respectively. The influence of both the rate and the strength of exchange interactions on the shape of the spectrum is discussed. Criteria to distinguish the spectra of the cases of both slow and fast exchange are discussed.

ISSN:0026-8976    

DOI:10.1080/00268977400100351

出版商:Taylor & Francis Group    

年代:1974

数据来源: Taylor

摘要:

The photodimerization of anthracene sandwich pairs embedded in dianthracene single crystals has been studied by absorption and fluorescence spectroscopy over a wide range of temperature (6–295 K). An excimer state has been found to be the intermediate state in the formation of the dianthracene. The fluorescence efficiency of the sandwich pair is unity at very low temperature while the quantum yield for the dimerization process is unity at high temperatures. The thermal activation energy for the photodimerization process has been determined directly as well as from temperature dependence measurements of the fluorescence intensity. The two methods yield 590 ± 25 cm-1and 611 ± 20 cm-1respectively for this energy. Corresponding measurements were also carried out with the perdeuterated anthracene sandwich dimer. These show a marked deuterium effect.

ISSN:0026-8976    

DOI:10.1080/00268977400100361

出版商:Taylor & Francis Group    

年代:1974

数据来源: Taylor

摘要:

E.S.R. Spectra of the radicals produced from the γ-irradiated diborane (6) in polycrystal or single crystal have been investigated. From the analysis of the results of the single crystal, the radical was identified as neutral diboranyl, ·B2H5, produced by extracting one of the two bridging hydrogen atoms of diborane. The major portion of the spin density on the radical is equally shared by the two boron atoms, a considerable portion exists on the bridging hydrogen atom, and a small portion with negative sign on the four terminal hydrogens. The orbitals of the unpaired electron on the two borons are in the same plane at an angle of 73°. These results are in good agreement with those from INDO calculation for the radicals. The structure of the radical is discussed in comparison with some other radicals having a similar structure.

ISSN:0026-8976    

DOI:10.1080/00268977400100371

出版商:Taylor & Francis Group    

年代:1974

数据来源: Taylor