摘要:

The crystal and molecular structures of 2-methyl-2-phenylindolin-3-one-1-oxyl and 2-methyl-2-phenyl-1,3-indandione are reported and the geometry of the molecules is discussed in relation to that reported in the literature for analogous systems. The two compounds are isomorphs and consequently it was possible to obtain crystals mixed in any ratio. Crystals of the ketone with 1% of the nitroxide radical have been studied by EPR and ENDOR spectroscopy. The hyperfine tensors of the protons and of the nitrogen nucleus, and thegtensor have been determined. The principal directions of the magnetic interactions have been compared with those obtained from the crystal structure. The presence of two diastereometric conformations of the radical in the mixed crystals is discussed.

ISSN:0026-8976    

DOI:10.1080/00268979100101011

出版商:Taylor & Francis Group    

年代:1991

数据来源: Taylor

摘要:

The EPR spectrum due to radical pairs was studied in mixed crystals containing 5% of a nitroxide indolinone radical and 95% of the isoelectronic ketone. Each radical pair gives rise to a pair of lines in the wings of the main spectrum. The first order analysis of the angular dependence of the fine splittings gave the electron-electron dipolar tensors due to three different pairs. The comparison of the experimental tensors with the calculated ones allowed us to identify the radical pairs in the crystal structure. The temperature dependencies of the EPR intensities of two types of radical pairs were analysed and the singlet-triplet energy separations were obtained. The magnetic susceptibility of the pure radical crystal has been determined and compared with the results of the EPR of the mixed crystals. An ENDOR investigation was performed on the radical pairs atT= 5K. The spectra can be explained by taking into account the inter radical electron-nuclear dipolar interaction.

ISSN:0026-8976    

DOI:10.1080/00268979100101021

出版商:Taylor & Francis Group    

年代:1991

数据来源: Taylor

摘要:

Mandelstam-Brillouin scattering data in mixed isomeric alcohols n-pentanol (nPe-OH) and 2-methyl-2-butanol (2Me-2BuOH) are presented. The hypersonic velocity and normalized absorption are measured as a function of the scattering angle, in the temperature range from -15°C to +45°C, and as a function of n-PeOH molar fraction going from the pure n-PeOH to the pure 2Me-2BuOH. The experimental results confirm the existence of a shear relaxation phenomenon in the GHz region, that has been previously detected in pure liquids. The temperature dependence of the relaxation time τsand of the shear modulusG∞evaluated within viscoelastic liquid models, support the existence of open heterostructures in the 2-methyl-2-butanol rich phase. A detailed analysis is also carried out for the measured shear viscosity ηs, whose temperature dependence becomes more and more non-Arrhenian by adding normal pentanol (n-PeOH) to its stereoisomer (2Me-2BuOH). A comparison between the shear flow mean activation enthalpy and the high frequency shear relaxation enthalpy allows us to demonstrate the more ‘fragile’ behaviour of 2Me-2BuOH and of its more concentrated mixtures with respect to that of n-PeOH, confirming that the observed non-ideal behaviour in the mixing process can be associated with the existence of H-bond imposed structural aggregates that are in dynamic equilibrium.

ISSN:0026-8976    

DOI:10.1080/00268979100101031

出版商:Taylor & Francis Group    

年代:1991

数据来源: Taylor

摘要:

Electric polarizabilities, dipole and quadrupole moments of the molecules LiH, LiF, LiCl, NaH, NaF, NaCl, BeO, BeS, MgO, MgS, BeH, BeF, BeCl, MgH, MgF, MgCl, LiO, LiS, NaO and NaS are computed in a uniform basis at the correlated level of theory. The results are rationalized in terms of the properties of ion pairs, and the stability of the isolated anions: the hydrides, oxides and sulphides show large and positive correlation corrections to the polarizability in CASSCF, and erratic convergence of the MBPT series. The open-shell alkaline-earth hydrides and halides in the series all exhibit negative polarizability anisotropy.

ISSN:0026-8976    

DOI:10.1080/00268979100101041

出版商:Taylor & Francis Group    

年代:1991

数据来源: Taylor

摘要:

When account is taken of the vibrational dependence of the potential function, the usual condition of detailed balance taken with respect to the vibrational-rotational frequencies does not apply, and model spectral functions satisfying this condition are inaccurate. A model correlation function is derived that deals with the case of vibration-dependent potentials. The parameters of the model are computed from relations involving the spectral moments. This model is shown to give spectral shapes, for a number of examples involving the fundamental and first overtone bands of H2-He and H2-H2at room temperature and 1000 K, in excellent agreement with spectra calculated quantum mechanically from first principles. The model is useful for predicting the collision-induced vibrational spectra for these and other systems for which the vibration-dependent induced dipole and potential are known.

ISSN:0026-8976    

DOI:10.1080/00268979100101051

出版商:Taylor & Francis Group    

年代:1991

数据来源: Taylor

摘要:

The intramolecular and intermolecular contributions to the longitudinal current fluctuations of the hydrogens of liquid water have been computed by molecular dynamics simulation on the TIP4P model at 245 K. This indicates that the recently discovered high frequency (∼ 165 THz) ‘optical-like’ collective mode of the hydrogens is due to the intermolecular contributions and that these and the intramolecular part cancel each other in the frequency range 20–120 THz. Conversely, they mutually reinforce outside this region to produce the acoustical (∼8 THz) and ‘optical like’ collective bands, in the low-kregion. The high frequency mode is rooted in the librational motion of the molecules. This is shown by a comparison of the intermolecular part spectrum with that of the interparticle correlation function of the projection of the angular velocity over the principal axes of inertia of the molecule. In particular, we are able to show that the high frequency band is almost entirely due to correlations of the rotation of the tagged molecule and that of the cluster of neighbours around the axis normal to the dipole, in the molecular plane. Large cage effects are also observed in the translational dynamics, and the interparticle centre of mass velocity correlation function is found to be almost equal to the time propagated auto-correlation function.

ISSN:0026-8976    

DOI:10.1080/00268979100101061

出版商:Taylor & Francis Group    

年代:1991

数据来源: Taylor

摘要:

Time-of-flight neutron diffraction measurements were carried out on pure liquid water in the high-Qregion and at high temperatures between 25 and 200°C. The observed data at high scattering angle (ca.90°) have been analyzed to determine the intramolecular distancesrODandrDDby solving the problems of the inelasticity corrections. They show no temperature dependence, within experimental error, between 25 and 200°C:rOD= 0·970 ± 0·005 Å andrDD= 1·55 ± 0·01 Å. The conclusion reached in this work is consistent with the results of high frequency Raman scattering. The intramolecular structure of water molecule is unchanged by temperature, but its structure can be changed by ion-water interaction as shown in previous work.

ISSN:0026-8976    

DOI:10.1080/00268979100101071

出版商:Taylor & Francis Group    

年代:1991

数据来源: Taylor

摘要:

Tests of several closures for the Ornstein-Zernike equation in the case of hard spheres are considered. The best of these is extended to the case of molecular fluids and solved for a hard homonuclear diatomic fluid. The theory used is a modification of an earlier theory originally proposed by Verlet for the hard sphere system, which we denote as the VM theory. The results are compared with computer simulation data and with the results of the Percus-Yevick (PY) and Nonspherical Bridge Function (NSB) theories. The VM theory produces excellent results for the compressibility factor and the spherical harmonic coefficients of the pair distribution function up to a packing fraction of 0·47 and a reduced elongation of 1·0. The NSB results are not as accurate, and the PY results are less accurate still. The bridge function given by the VM, NSB, and PY closures are compared with that extracted from computer simulation pair distribution function data. The VM theory gives good agreement and is similarly superior to the NSB and PY theories.

ISSN:0026-8976    

DOI:10.1080/00268979100101081

出版商:Taylor & Francis Group    

年代:1991

数据来源: Taylor

摘要:

The temperature dependence of the dynamics of the van der Waals complex formed between benzene and hexafluorobenzene has been investigated in its solid states. The measurements fall into two regimes. At low temperatures,T⩽ 1>00 K, using inelastic neutron scattering discrete librational transitions between pairs of molecules have been observed. At higher temperatures,T⩾ 205 K, we lose this discrete structure and obtain the phonon spectrum. Both sets of spectra are rationalized using relatively simple molecular models.

ISSN:0026-8976    

DOI:10.1080/00268979100101091

出版商:Taylor & Francis Group    

年代:1991

数据来源: Taylor

摘要:

Three phase transitions have been identified in the system C6H6+ C6F6, at 205, 251 and 274 K. We believe these phase transitions are the result of increasing amounts of orientational disorder in this material. The molecular dynamics of the lowest temperature phase have been studied in some detail and are shown to consist of sixfold reorientations of the molecule about its C6axis. From the temperature dependence of this thermally activated reorientation we obtainEa= -9·4 ± 0·5 kJ mol-1andv0= 3·9 ± 0·6 meV (9·4 × 1011Hz).

ISSN:0026-8976    

DOI:10.1080/00268979100101101

出版商:Taylor & Francis Group    

年代:1991

数据来源: Taylor