摘要:

We have considered the excitation spectrum arising from the coherent electron–exciton pairing in crystals at low temperatures. For the pairing processes, three different types of excitons have been considered: Frenkel excitons (tightly bound), Wannier–Mott excitons (loosely bound), and excitons of the intermediate binding. Expressions for the gap functions and transition temperatures at which metal-to-nonmetal phase transitions occur have been derived and discussed for each type of pairing. In the electron pairing with the intermediate exciton, the dispersion relations which determine the exciton and the electron–exciton modes are solved numerically and the derived results are graphically presented. The physical picture for the electron–exciton coupled modes is analogous to that for polaritons and magnon–phonon modes in crystals.

ISSN:0008-4042    

DOI:10.1139/v85-229

出版商:NRC Research Press    

年代:1985

数据来源: NRC

摘要:

The multiphoton spectra of all-trans1,3,5-hexatriene, in the region of 6.0 eV, is investigated using highly correlated wavefunctions. On the basis of the computed transition energies, intensities, and polarization ratios, an attempt is made to explain the experimentally observed spectra.

ISSN:0008-4042    

DOI:10.1139/v85-230

出版商:NRC Research Press    

年代:1985

数据来源: NRC

摘要:

Raman spectra of solutions of alkali metal perchlorates in dimethyl sulfoxide (DMSO) in the Cl—O, C—S, and S=O stretching regions, as well as of perchlorates in aqueous solutions in the Cl—O stretching region are reported. The results are discussed in terms of half-bandwidths, relative intensities, and depolarization ratios. For H2O the half-bandwidth of the Cl—O stretching band at ~935 cm−1is almost double the value in DMSO solutions. Solutions of perchlorates in DMSO show two symmetric bands in the Cl—O stretching region, whereas in aqueous solutions only one band is observed. The half-bandwidths in perchlorate solutions in DMSO for the C—S stretching band increase with increase in concentration of perchlorate compared to that of liquid DMSO. The band contours in the S=O stretching region in DMSO solutions also show significant changes. These observations are explained on the basis of formation of ion pairs of metal perchlorates in solutions of DMSO and ion hydrates in the case of aqueous solutions.

ISSN:0008-4042    

DOI:10.1139/v85-231

出版商:NRC Research Press    

年代:1985

数据来源: NRC

摘要:

The amino radicals (•AH) formed by the60Co radiolysis of N2O-saturated 0,05 Msolutions of ethylene diamine tetraacetate (EDTA) at pH 7 and 11.2 and of glycine at pH 11.2 brought about an efficient two-electron reduction of lumiflavin (Fl). The spectra of the products were identical to those formed by photolysis of the same solutions and by reduction of the lumiflavin with •CO2−radicals. The products were reoxidised to flavin by oxygen. The quantum yield for flavin disappearance was 0.52 ± 0.07 and 0.17 ± 0.01 in the presence of EDTA at pH 7 and 11.2 and 0.065 ± 0.008 and 0.17 ± 0.01 for glycine at the same pHs, respectively.The overall two-electron reduction can be explained by the mechanism:The rate constants of reaction [4] were found by pulse radiolysis to be 1.8 ± 0.3 × 109and 1.5 ± 0.3 × 109 M−1s−1for the radicals of glycine and EDTA at pH 7 and 3.6 ± 0.3 × 108 Ms−1for glycine radicals at pH 11.2. The spectrum of •FlH formed by glycine radicals at pH 7 is similar to that produced by•CO2−, but there was some perturbation, which is apparently due to interaction with the amine.The radicals formed from the secondary amines piperazine and diethylamine at pH 11.8 also effected reversible two-electron reduction. However, the radicals from glycine anhydride and the primary amine ethylamine yielded significant amounts of non-oxidisable products. The reaction mechanisms are discussed and effects of pH are considered.

ISSN:0008-4042    

DOI:10.1139/v85-232

出版商:NRC Research Press    

年代:1985

数据来源: NRC

摘要:

Absolute differential Raman scattering coefficients of hydrogen bondedN,N-dimethylformamide (DMF) – water complexes and aN,N-dimethylacetamide (DMA) – water complex are calculated with a variational method within the CNDO approximation using an extended basis set. Changes of Raman intensities caused by intermolecular interactions are modeled in our Raman intensity calculations and the calculated intensities of the amide I (C=O stretching), OCN bending, and N—(Me)2stretching vibration of the DMF (and DMA) – water complexes are compared with the experimental absolute Raman intensities of DMF and DMA dissolved in water. The calculated Raman intensities of characteristic normal modes of DMF and DMA and their 1:2 amide – water model are in good agreement with the experimental data. The results show that it is possible to obtain qualitative information on solute – solvent interactions from Raman intensity calculations of complex molecular systems like hydrated DMF and DMA.

ISSN:0008-4042    

DOI:10.1139/v85-233

出版商:NRC Research Press    

年代:1985

数据来源: NRC

摘要:

Computer access to chemical information by a blind chemist is made possible with a VersaBraille system (Braille Information Processor). The CAS database of the DIALOG system is used in this updating assay. Preliminary trials showed the need to establish a rigorous procedure to avoid loss of data. In this study, a functional method is suggested as a substratum to circumvent costly problems.

ISSN:0008-4042    

DOI:10.1139/v85-234

出版商:NRC Research Press    

年代:1985

数据来源: NRC

摘要:

The photocurrent ofN,N,N′,N′-tetramethyl-p-phcnylcnediamine (TMPD) in 2,2,4-trimethylpentane (isooctane) excited at 5.90 eV has been studied as a function of temperature from 195 to 298 K both in the absence and presence of the electron scavengersn-perfluoroheptane and perfluoromethylcyclohexane. Using an exponential radial distribution function for geminate pair separation distances with range parameter corrected to accommodate its change with temperature, it is found that the photoejection quantum yield of TMPD is essentially independent of temperature from298 K toand then increases slightlyon further cooling to 195 K. In the presence of electron scavengers there is a contribution to the magnitude of the range parameter from the action of the scavenger to pre-thermalize the epithermal electron. The temperature dependence of this contribution is analyzed and found to increase with decreasing temperature.

ISSN:0008-4042    

DOI:10.1139/v85-235

出版商:NRC Research Press    

年代:1985

数据来源: NRC

摘要:

The first electronic absorption system of acetaldehyde was recorded in the vapour phase at room temperature. The many-banded spectrum proved to be very complex and it was only at the extreme red edge of the absorption that the pattern became simple enough to analyze. The major experimental requirement was a high pressure × path length (500 Torr × 168 m). Spectra were interpreted in terms of the torsional mode ν′15and ν″15attached to the ν′14out-of-plane bending mode. Both modes were highly Franck–Condon active and theband at 27240.4 cm−1was not directly observed. The barrier to rotation of the methyl group was 618.5 cm−1. More surprising was the observation that the methyl group undergoes a rotation from aeclipsed configuration to a staggered configuration. The intensity of the ν′14quantum addition and its position in the spectrum suggest that the aldehydic hydrogen is nonplanar in the ã state.

ISSN:0008-4042    

DOI:10.1139/v85-236

出版商:NRC Research Press    

年代:1985

数据来源: NRC

摘要:

One possible specific solvent effect, namely, the π-donor ability is considered in the light of simple intermolecular second order perturbation theory. By using the Polányi–Evans–Bell principle this theory predicts that increasing -π-donor ability of solvent should decrease the reaction rate. This prediction is fully borne out by experimental data observed for the solvent effect on 4 + 2 cycloaddition of tétracyanoethylène to anthracene.

ISSN:0008-4042    

DOI:10.1139/v85-237

出版商:NRC Research Press    

年代:1985

数据来源: NRC

摘要:

Photoion – fluorescence photon coincidence experiments were carried out in order to detect the fluorescence of NH3+which is expected to occur from that part of its à electronic state that lies below the lowest dissociation limit. Hel and Nel sources were used to produce the ions. No NH3+fluorescence was detected and upper limits for its quantum yield under our experimental conditions are given. We show that the lifetime of the à state is probably very long, making it difficult to observe fluorescence. Furthermore, we argue that the molecular parameters and potential energy surfaces of the à andstates are such that resonance limit nonradiative coupling to high rovibrational levels of thestate could be an efficient process for apparent intramolecular quenching of NH3+à state fluorescence.

ISSN:0008-4042    

DOI:10.1139/v85-238

出版商:NRC Research Press    

年代:1985

数据来源: NRC