摘要:

X-ray photographs have been made of the two rotational forms of solid cyclopentanol and of the rotational form of dl-camphene. At −30°C, cyclopentanol in rotational form I has a hexagonal structure with a = 5.877 Å and c = 1.607 Å and with two molecules per cell. Agreement with the observed intensities can be obtained by assuming that the molecules are located at (1/3, 2/3, 1/4) and (2/3, 1/3, 3/4) and that they are rotating with spherical symmetry about their centers of mass. At −56°C, cyclopentanol in rotational form II also has a hexagonal structure but with a = 10.1 Å and c = 9.49 Å and with six molecules per cell. Intensity ratios are used to find the ratios of molecular scattering factors for camphene at six temperatures between −83.1° and + 24.0°. A model in which the camphene molecules rotate about their centers of mass with spherical symmetry does not produce agreement with the observed results. A model in which the camphene molecules fluctuate randomly among the twelve positions that result from applying the minimum symmetry operations for the point group in cubic lattice (class 23 or T) can be used to obtain agreement with the observed ratios.

ISSN:0369-1152    

DOI:10.1080/15421406908082949

出版商:Taylor & Francis Group    

年代:1969

数据来源: Taylor

摘要:

X-ray powder photographs are used to calculate cell parameters for cyclohexane, cyclohexanol, and dl-camphene for a wide range of temperature. Comparison with the ordianary bulk density of cyclohexane and cyclohexanol leads to upper limits for the vacancy concentrations in these substances of 0.5% and 0.3%, respectively. Above about 10 °C the bulk density of camphene falls off markedly form what would be expected on the basis of the cell parameters. Comparison of the densities at higher temperatures with those to be expected either on the basis of the X-ray data or on the basis of linear extrapolation of the densities at lower temperatures leads to ΔHϵ= 12.5 ± 0.5 kcal/mol and ΔSϵ= 35 ± 2 e.u./mol for the process of vacancy formation. This value for the entropy implies that about 15 molecules gain additional freedom of motion every time one vacancy is formed. These results lead to predictions for the temperature variation in the ratio of molecular scattering factors for the 110 and 200 lines of camphene that are consistent with the observed ratios.

ISSN:0369-1152    

DOI:10.1080/15421406908082950

出版商:Taylor & Francis Group    

年代:1969

数据来源: Taylor

摘要:

Dielectric, visual, and calorimetric observations are brought together to explicate the relations between the four solid forms of cyclohexanol. Rotational form I is stable from the freezing point of 25 °C down to the transition point with non-rotational form III at −8.3 °C; however, form II can be supercooled well below that temperature. Nonr-rotational form II has a transition point with form I at −28.8 °C and is metastable with respect to form III down to about −64° at which point a transition point occurs between forms II and III. Non-rotational form pre-II has a transition point with I at about −44°, below which it is metastable with respect to form II. The approximate temperatures for the maximum rates of nucleation are as follows: for I → pre-II, −87°, for I → III, − 105°, for pre-II → -II, −75°. The approximate temperatures for the maximum growth velocities are as follows: for I → II, − 43°, for I → pre-II, −60°, for I → III, −32°. Analysis of I → pre-II growth rates following the theory of continuous growth leads to a transition temperature of −43.5°, a heat of activation of 9.3 ± 0.5 kcal/mol, and preexponential factor of 5.1 × 109cm/hr.

ISSN:0369-1152    

DOI:10.1080/15421406908082951

出版商:Taylor & Francis Group    

年代:1969

数据来源: Taylor

摘要:

Reflection spectra of a number of polymethine dyes are presented and analysed. Characteristic features of these spectra appear to be outside the framework of the Lorentz-Lorenz theory. An alternate approach based on an appropriate linear response function for surface excitation is developed. The linear response at a particular frequency involves a sum over all k-dependent Green's functions. Damping is brought in through the exciton-phonon interaction. A model Hamiltonian is introduced which can be diagonalized exactly for the single molecule. For the crystal, this permits the treatment of single phonon effects in the strong coupling limit almost as if they were multiple phonon effects. Structure, mainly associated with damping, in the reflection spectrum is expected and found in those regions of the exciton dispersion curve where the density of momentum states is large. Reasonably good agreement is found between the theoretical and experimental reflection curves.

ISSN:0369-1152    

DOI:10.1080/15421406908082952

出版商:Taylor & Francis Group    

年代:1969

数据来源: Taylor

摘要:

The dominant radiationles decay channel in crystalline tetracene at 300 °K is a fission of an excited singlet into two triplet excitons with a rate constantγS= 1.5 × 10−12± 5% cm3-sec−1. The efficiency of this process at room temperature is estimated as 95%and constitutesan efficient intersystem crossing mechanism. At light intensities I ⩾ 1015quanta-cm−2sec−1(366 μm excitation), the triplet densities at 300 °K are sufficiently high to produce radiative triplet-triplet annihilation or fusion. As the light intensity is increased the quantum efficiency of fluorescence increases, and eventually reaches a constant value (about twice its value in the low intensity region, where fusion is not important). It is assumed that triplet-triplet fusion gives rise to either an excited singlet (rate constant γTS = (4.8 ± 1.2) × 10−10cm3-sec−1), or excited triplet (rate constant γTT = (11 ± 5) × 10−10cm3-sec−1). The effect of a magnetic field H = 4000 gauss on the rate constants is determined. The radiative triplet-triplet fusion constant and γS both decrease in approximately the same manner when a magnetic field is applied. γTT is shown to be at most slightly dependent on H.

ISSN:0369-1152    

DOI:10.1080/15421406908082953

出版商:Taylor & Francis Group    

年代:1969

数据来源: Taylor

摘要:

The polarized second singlet-singlet absorption spectrum of phenanthrene at 3000 Å has been observed in mixed crystals of durene and hexamethlbenzene. In durene at 4 °K, bandwidths of 10cm−1are observed and lattice vilbrations identified. Electron spin resonance measurements of the phenanthrene triplet in durene have shown that the orientation of the phenanthrene is changed from that of the durene. Nevertheless the observed polarization ratios are close to those calculated for a long-axis polarized mixed crystal transition. All the results are in agreement with the assignment of1B2for the second singlet state.

ISSN:0369-1152    

DOI:10.1080/15421406908082954

出版商:Taylor & Francis Group    

年代:1969

数据来源: Taylor

摘要:

The description of order in mesophases in terms of angularly dependent radial distribution functions and in terms of correlation functions in density and oreientation is discussed. The determination of these functions from light scattering and diffraction data is described. The concept of non-randomness of orientation correlations is introduced and its description in terms of non-randomness parameters or correlation functions isn proposed. It is pointed out that mesophases of complex structure may be defined in terms of parameters describing the amplitudes and correlation distances for the correlations in density and in random and non-random orientation. The most striking chages at mesophase transitions are in the randomness of the orientation correlations. The relationship between the correlation functions and the morphology for model systems is illustrated.

ISSN:0369-1152    

DOI:10.1080/15421406908082955

出版商:Taylor & Francis Group    

年代:1969

数据来源: Taylor

摘要:

In the couse of our investigations of liquid crystals as solvents, we wish to report some kinetic data for the Claisen rearrangement. The thermal rearrangement of allyl phenyl ethers (1) proceeds by an intramolecular path to give an ortho-dienone (II) which rapidly enolizes to 2-allylphenols (III).

ISSN:0369-1152    

DOI:10.1080/15421406908082956

出版商:Taylor & Francis Group    

年代:1969

数据来源: Taylor