Index of content:
Volume 46, Issue 7, 01 April 1967
46(1967); http://dx.doi.org/10.1063/1.1841068View Description Hide Description
The glass‐transition temperatures of 34 liquids have been measured by differential thermal analysis, and the results compared with the values predicted from viscosity measurements by the equation lnη = A″+B″/(T—T 0). In those liquids which have two non‐Arrhenius viscosity regions, there is good agreement between observed Tg values and those predicted from the lower‐temperature form of this equation. In those liquids which have only one observable non‐Arrhenius region, measuredTg values are usually lower than those predicted from viscosity measurements. It is suggested that this is caused by a change in non‐Arrhenius viscosity behavior at higher viscosities which are unattainable experimentally.
46(1967); http://dx.doi.org/10.1063/1.1841069View Description Hide Description
The Anderson—Tsao and Curnutte impact theory of pressure broadening is applied to microwave rotational lines of symmetric‐top molecules. The effect of elastic and inelastic collisions on the optical cross section is examined. Simple but accurate expressions are obtained for microwavelinewidths due to a variety of intermolecular interactions. The theory is applied to existing microwavelinewidths, which happen to involve interactions between polar molecules. The theoretical width of the J = 1→2 line of PF3 is in good agreement with experiment, although the agreement is less satisfactory for the J = 0→1 line of CH3F and CHF3, and poor for the J = 1→2 line of CHF3. However, the theory satisfactorily accounts for widths of the NH3 3–3 inversion line broadened by CHF3 as well as other symmetric‐top molecules.
High‐Temperature Vaporization and Thermodynamics of the Titanium Oxides. I. Vaporization Characteristics of the Crystalline Phases46(1967); http://dx.doi.org/10.1063/1.1841070View Description Hide Description
The vaporization behavior of the titanium—oxygen system has been established by experimental measurements of the change in composition of the solid residue accompanying vaporization as determined by x‐ray diffraction, chemical analysis, and color of sample. The solid phases studied were Ti2O, TiO, Ti2O3, Ti3O5, Ti4O7, Ti5O9, Ti10O19, and TiO2. A congruently vaporizing composition was found in the Ti3O5 phase region. These results are compared with previous investigations and with thermochemical data available for solid and gaseous species.
Dissociation Energy of TiO(g) and the High‐Temperature Vaporization and Thermodynamics of the Titanium Oxides. II. Trititanium Pentoxide46(1967); http://dx.doi.org/10.1063/1.1841071View Description Hide Description
Vapor‐pressure and mass‐spectrometric measurements on Ti3O5(s) contained in tungsten crucibles show that for the congruent vaporization,The vapor‐pressure measurements were obtained in the temperature range 1837°—2040°K and are used with auxiliary values to obtain ΔH 298° = 713.85±5.25 from the second‐law treatment, ΔH 298° = 717.42±3.10 kcal/mole from the third‐law treatment, ΔS 298° = 212.16±2.71 (second‐law), and ΔS 298° = 213.98 eu (third‐law). Derived values based on the third‐law result and on numerous auxiliary values are:Earlier work and discrepancies are discussed, and suggestions are made for future efforts.
46(1967); http://dx.doi.org/10.1063/1.1841072View Description Hide Description
A classical expression is developed for the partition function of the coupled bending and rotational degrees of freedom of a linear triatomic molecule. The usual rigid‐rotator harmonic‐oscillator partition function can be derived as an approximation to our result. Analytical approximations to both the bending‐rotator and the rigid‐rotator partition functions for a molecule with a quadratic—quartic potential are considered and the accuracy of these approximations is assessed by numerical calculations. Values for the thermodynamic functions for C3 are calculated using the best available spectroscopic data. It is shown that for the bending and rotational degrees of freedom, quantum corrections are negligible at temperatures above a few hundred degrees while anharmonicity and vibration—rotation coupling are quite important. For example, at 2400°K, a quartic term in the potential of C3 lowers the calculated value of S° by 2.8 eu, and the vibration—rotation coupling lowers it by another 1.1 eu to a computed value of 81.4 eu. A drastic steepening of the potential gives a lower limit, more or less consistent with the spectroscopic data, of 79.8 eu. The best experimental values for S°2400 are 76 or 77 eu.
46(1967); http://dx.doi.org/10.1063/1.1841073View Description Hide Description
A general theory is developed for the interaction of exctions with molecular vibrations in pure and impure molecular crystals. A study is made of a model Hamiltonian which describes the interaction of Frenkel excitons with harmonic oscillators, with particular attention paid to the case in which exciton bandwidth is fairly narrow. Equations of motion for molecular excitons are obtained in the form of Dyson equations using the method of double‐time Green's function. By solving the Dyson equations, it is shown that the vibrational energies of excitons in pure and impure molecular crystals, which can be classified into zero‐phonon and one‐phonon energies, are very different for different combinations of the energies of molecular vibrations and the exciton bandwidth. For example, a series of vibronic exciton bands is obtained for a pure crystal when the energies of phonons are larger than the exciton bandwidth while for an impure crystal, under certain circumstances, molecular vibrations can induce a delocalization of exciton impurity states in the opposite case. A study is then made of the role of phonons in the excitonenergy transfer in a pure molecular crystal. It is shown that the effect of exciton—phonon interactions is to reduce the exciton bandwidth from free exciton value while there can exist a phonon‐assisted energy transfer with emission or absorption of a single phonon, which has a long‐range oscillatoryinteraction form similar to the Ruderman—Kittel interaction. A brief discussion is also given of the transfer of energy associated with an exciton impurity state to host molecules.
46(1967); http://dx.doi.org/10.1063/1.1841074View Description Hide Description
In this paper it is shown that the Lothe and Pound correction to nucleation theory based upon the introduction of translational and rotational partition functions for individual drops does not arise. The error originates in the improper assumption that the free energy assigned to a drop, in the conventional theory of nucleation, corresponds to that for a drop whose center of mass is stationary.
46(1967); http://dx.doi.org/10.1063/1.1841075View Description Hide Description
The reaction of O(1 D) with CH4 has been studied by photolysis (2537 Å) of O3–CH4 mixtures dissolved in liquid argon at 87°K. The quantum yields of O3 decomposition and the product yields based on O3 decomposed were measured. The products found were CH3OH, CH2O, and H2. Added O2 was found to lower the quantum yields of O3 decomposition and to increase the product yields. Experiments with added 18O2 were carried out to trace the origin of the products. At 6000 Å, which can only give O (3 P), no photo‐reaction could be detected.
The results indicate that the O(1 D)—CH4reaction has essentially zero activation energy and proceeds by three paths:
The effect of added O2 is attributed to scavenging of methyl radicals which otherwise react with O3. The reaction of CH3 with O3 has zero Ea and occurs at a rate approximately equal to the reaction of CH3 with O2. The magnitude of the scavenging effect shows that at least two O3 molecules are decomposed by each CH3. By contrast, OH radicals do not react with O3 in these experiments, indicating a nonzero activation energy for the reaction OH+O3→HO2+O2.
46(1967); http://dx.doi.org/10.1063/1.1841076View Description Hide Description
Analytical and stochastic methods are used to investigate hot‐atom reactions for relatively low‐energy hot atoms such as those produced by photochemical processes. Collision densities for hot atoms are presented for several reactive systems in which the reaction probability and the threshold energy are varied. Solutions for the collision density obtained by analytical methods and independently by stochastic methods were in satisfactory agreement. The results show that even small reaction probabilities can have a marked influence on the collision density distribution, an effect attributed to highly anisotropic scattering collisions. The temperature dependence of hot‐atom reaction rates for a system containing two kinds of reactive molecules with different threshold energies was also investigated.
46(1967); http://dx.doi.org/10.1063/1.1841078View Description Hide Description
The electron spin resonance of x‐irradiated single crystals of hydroxyurea have been observed and analyzed for different orientations of the crystal in the magnetic field. The free radical responsible for the resonance has been identified as NH2CONH, and the measurements indicate that 41% of the unpaired electron is localized in the 2p orbital of the nitrogen atom. Principal values for the hyperfine coupling constants in gauss are −21.2, −13.5, and −1.5 for H, and 22.5, 1.5, and 1.2 for N. On the basis of these measurements the induced isotropic hyperfine constants for an electron entirely localized in a 2p orbital of the nitrogen atom in the R–N–H radical would be 20.2 G for nitrogen and −31 G for hydrogen. The principal g values are 2.0027, 2.0062, and 2.0108. The results indicate that the trapped radical retains the approximate orientation of the parent molecule in the lattice and that the radical is planar.
46(1967); http://dx.doi.org/10.1063/1.1841079View Description Hide Description
The ultraviolet, visible, and near‐infrared spectra of single‐crystal Nd2O3 and La2O3:Nd3+ observed at 10°, 80°, and 290°K, were analyzed. Over 300 absorption lines were assigned from the five crystalline Stark levels of the ground state to over 20 excited Jmanifolds in each crystal. In thick crystals, transitions to many excited levels from the higher Stark levels of the state were observed. The spectral assignments are compared with calculations of ``free‐ion'' energy levels and with the assignments of Y2O3:Nd3+. The parameters E 1, E 2, E 3, and ξ are calculated for the three oxide crystals, and comparisons between observed and calculated energy levels are made. The significance of these parameters in comparison with those for Nd3+ in other crystals is discussed, and the effects of configuration interaction are pointed out.
46(1967); http://dx.doi.org/10.1063/1.1841080View Description Hide Description
The infrared spectrum of the solid benzene—Br2 complex has been measured to give the changes in frequency and an estimate of the changes in intensity from the pure benzene and pure Br2 crystals. The complex was usually prepared by condensing a 1:1 mixture of benzene and bromine vapor onto a cold plate, but other techniques were also tried, and partial studies are reported. These spectra show definitely that neither the benzene nor bromine molecules in our samples are located on a site possessing a center of symmetry. From the analysis of the spectrum of the complexed benzene, a site symmetry of C 3v is deduced. This apparent contradiction of the results of the x‐ray studies of Strømme and Hassel is discussed and reconciled. The frequency shifts and intensity changes on complex formation are discussed in some detail.
46(1967); http://dx.doi.org/10.1063/1.1841081View Description Hide Description
The absorptionspectrum of crystalline horse‐heart ferricytochrome‐c was investigated. Measurements were made on single crystals at room temperature from 3700 to 7600 Å using a microspectrophotometer. Molar extinction coefficients for light parallel and perpendicular to the fourfold c crystallographic axis are given.
The Soret band (4100 Å) and main visible band (5300 Å) were found to have the same polarization, while the weaker band at 5650 Å is polarized differently. These transitions are discussed in terms of the molecular orbital theory for simple metalloporphyrin complexes. Assuming the Soret and main visible bands correspond to transitions to an Eu state, the angle that the normal to the porphyrin plane makes with the c axis is calculated from the polarization ratios to be 72°. Using this angle the isotropic extinction coefficients calculated from the crystal data agree well with the extinction coefficients for these bands in solution.
The weak band at 6950 Å is polarized out of the porphyrin plane and is tentatively ascribed to the promotion of an electron from the porphyrina 2u (π) orbital to the irona 1g (d z2) orbital. The intensity of this band has been shown to be dependent on the conformation of the polypeptide. The proposed mechanism for this transition indicates two different ligand‐heme configurations in ferricytochrome‐c which are consequent upon the two ``conformational isomers'' postulated by Schejter and George.
It is concluded that hemoproteins provide a near perfect example of an oriented gas of chromophores.
46(1967); http://dx.doi.org/10.1063/1.1841082View Description Hide Description
The structures of the (111), (100), and (110) crystal faces of platinum were studied as a function of temperature in the range 300°—1769°C (melting point). The (111) and (100) substrates are stable while the (110) face shows faceting above 600°C. Several surface structures were found to exist on the stable platinum substrates. These have well‐defined temperature ranges of stability. There are two types of surface structures, ordered and disordered. Both types appear to be the property of the clean platinum substrates. The ordered structures appear during annealing, after ion bombardment at lower temperatures (<900°C). They exhibit long‐range order and their stability ranges overlap on a given substrate. These structures are believed to be due to ordered arrays of vacancies in the substrate plane which, under proper conditions, show remarkable stability. A high, nonequilibrium concentration of defects at the surface may be necessary to induce their formation.
The disordered surface structures appear at high temperatures, above the stability range of most of the ordered structures. They are characterized by ringlike diffraction features which develop gradually as a function of increased heating time or temperature. The formation of these structures is irreversible and they can only be removed by ion bombardment. Near the melting point, the ring patterns remain the only diffraction features of the presumably greatly disordered surfaces.
The ratio of lattice parameters assigned to the diffraction rings on each substrate indicate that they can be due to domains of (111) surface structures. These hexagonal structures appear on all of the platinumsurfaces, are freely rotated in the substrate plane, and show an ∼11% contraction with respect to the inter‐planar spacing in the ordered (111) face. The disordered close‐packed‐hexagonal structure seems to be the stable high‐temperature surface phase of platinum.
46(1967); http://dx.doi.org/10.1063/1.1841083View Description Hide Description
A model for classical fluids given previously has been extended in two ways. An approximate Helmholtz free energy is obtained by using the equation of state of hard spheres to continue a virial series truncated after N+1 terms into the whole density—temperature plane. The model given earlier corresponded to N=1. A modified version of the model is given, which contains an additional function of the excess entropy. It is shown that the two versions of the model constitute truncation after one and two terms of an expansion of the thermodynamic function which transforms from the model free energy to the exact free energy. The expansion is made around the line of zero excess pressure, suggesting that the modified model is appropriate for liquids. It is formulated entirely in terms of experimentally accessible thermodynamic quantities. Comparison with experimental data for argon favors the modified version.
46(1967); http://dx.doi.org/10.1063/1.1841084View Description Hide Description
A complete, finite expansion is given for an arbitrary state of a three‐electron atom. This expansion separates the simple dependence of the wavefunction on the spin and Euler‐angle variables from the complicated and in general unknown dependence on the relative positions of the particles.
46(1967); http://dx.doi.org/10.1063/1.1841085View Description Hide Description
Absolute rate constants have been determined by the pulse radiolysis method for the electron transferreaction from a series of aromatic anion radicals in isopropanol solution to a variety of different aromatic molecules:Nine donor—acceptor pairs were investigated in which the donor anion was diphenylide, p‐terphenylide, m‐terphenylide, and o‐terphenylide, and the acceptor molecule was naphthalene, phenanthrene, p‐terphenyl, pyrene, and anthracene. Several, but not all, of the reactions appear to be diffusion controlled with electron transferrate constants ranging from 2.6×108 to 6.4×109 M −1·sec−1 at 25°C.
46(1967); http://dx.doi.org/10.1063/1.1841086View Description Hide Description
The fluorescence of Eu3+‐activated lanthanide oxyhalides LnOX (Ln=Y, La, Gd, and X=Cl, Br) is reported. Although all host lattices under consideration are isomorphous, the spectral energy distribution of the Eu3+ emission depends strongly on the choice of X and to a lesser extent on that of Ln. This variation is accounted for qualitatively by assuming a strongly varying linear component of the crystal field. The excitation spectra of the Eu3+fluorescence and the diffuse reflection spectra are also dependent on the choice of Ln and X. The quantum efficiency of GdOCl: Eu and YOCl: Eu is high (about 60%—70%).
46(1967); http://dx.doi.org/10.1063/1.1841087View Description Hide Description
The fluorescence quenching of the two phosphors and Gd1−x Eu x Al3B4O12 is reported. The shortest distance between the rare‐earth sites in these materials is large and nearly equal, viz., 6.0 and 5.9 Å, respectively. The quenching behavior is different: In the materials Ba2Gd1−x Eu x Me5+O6, concentration quenching occurs at x≃02, in the materials Gd1−x Eu x Al3B4O12, no quenching was found under uv excitation. These results are discussed in relation to the Van Uitert—Johnson model for concentration quenching of Eu3+fluorescence.
46(1967); http://dx.doi.org/10.1063/1.1841088View Description Hide Description
Viscosity measurements have been made on nine pure hydrocarbon liquids at six temperatures ranging from 15.56° to 135°C and at pressures as high as 4000 bars. The samples included rigid bicyclic compounds of relatively high symmetry and three n‐alkanes, n‐C12, n‐C15, and n‐C18. These data were analyzed using the Eyring significant‐structure theory, the Cohen—Turnbull free‐volume model, and the empirical Doolittle equations. All of these equations produced essentially identical fits to the data at atmospheric pressure. The hypothesis that the constants v 0 and vs in these equations represent the specific volume of a ``solid'' or condensed phase was tested by comparing best‐fit values of these constants with experimental values for the solid‐phase specific volume.
The Cohen—Turnbull and Doolittle equations were modified for use at elevated pressures with the result that the values of v 0 necessary to satisfy the equations at high pressures were shown to be analogous to the specific volumes of a glassy state at high pressures. Further, the change in v 0 as a function of pressure was compared with the change in the experimental values of the solid‐phase volume at the melting point as a function of pressure and a definite correlation between the two was established.