Index of content:
Volume 44, Issue 3, 01 February 1966
Stark Effect and Hyperfine Splitting for Near‐Degenerate Levels of an Asymmetric Rotor. Application to NO2Cl and NOBr44(1966); http://dx.doi.org/10.1063/1.1726832View Description Hide Description
An approximate method is presented for calculating the Stark effect and hyperfine splitting of near‐degenerate rotational levels of an asymmetric‐top molecule. This method has been used to calculate the Stark effects for the 21→30 transition of NO2Cl and the 11→20 transition of NOBr. The results are compared with the measured spectra and are found to agree within experimental error. The dipole moments obtained are μ = 0.53 D for NO2Cl and μ a = 1.80 D for NOBr. The component μ b for NOBr cannot be determined from the spectrum.
44(1966); http://dx.doi.org/10.1063/1.1726833View Description Hide Description
The effect of impurities on Frenkel excitons in molecular crystals is studied with particular attention to impurity levels appearing outside exciton bands and virtual impurity levels lying in the continuum. A study is made of a specific case in which the transitiondipole moments of the impurities are parallel to that of the host molecules. A secular equation which determines the energies of impurity levels due to any number of impurities is obtained, and a resonance form of the inverse relaxation time of excitonsscattered by an impurity is derived. In the one‐impurity problem, a detailed discussion is made of the criterion for the appearance of impurity levels and virtual impurity levels. It is concluded that for excitons with fairly large effective masses these impurity states are very likely to occur. Numerical calculations are performed for a simple cubic lattice in which only interactions between nearest‐neighbor molecules are important. The results are then applied to a study of the interaction of impurity levels due to small but finite concentrations of impurities. It is shown that there can exist long‐range jumps of excitons from one impurity site to another due to the dipole—dipole interaction. An impurity‐induced energy transfer is suggested.
Microwave Spectra, Dipole Moments, Structure, and Ring Puckering Vibration of Cyclobutyl Chloride and Cyclobutyl Fluoride44(1966); http://dx.doi.org/10.1063/1.1726834View Description Hide Description
Microwave spectra of nine isotopic species of cyclobutyl chloride and of the normal species of cyclobutyl fluoride have been studied. From the ground state moments of inertia, the structure of cyclobutyl chloride has been determined. Some of the more important structural parameters are: r(Cα–Cβ) = 1.525±0.005 Å r(Cβ–Cγ) = 1.550±0.005 Å, r(Cα–Cl) = 1.775±0.005 Å, ∠Cl–(CβCαCβ) = 135°±1°, and ∠(CβCαCβ)– (CβCγCβ) = 20°±1°. The chlorine atom is found at the equatorial position of the puckered ring, and the axial form was not observed. Quadrupole coupling constants of the chlorine atom for CH2CH2CH2CH35Cl species were found to be χ aa = −56.58±0.10 Mc/sec and χ cc = 23.60±0.1 Mc/sec. The transformed coupling constant along the C–Cl bond is −65.98 Mc/sec (assuming the quadrupoletensor was axially symmetric about the C–Cl bond). The moments of inertia of cyclobutyl fluoride calculated using r(C–F) = 1.37 Å, ∠F–(CβCαCβ) = 132° and the remaining structural parameters of cyclobutyl chloride agreed with the observed moments of inertia. The dipole moments of cyclobutyl fluoride were determined by the Stark effect: μ a = 1.870±0.005 D, μ c = 0.52±0.02 D, and μ = 1.94±0.01 D.
44(1966); http://dx.doi.org/10.1063/1.1726835View Description Hide Description
The chemical shift of 19F in gaseous ClF, about 160 ppm above the estimated shift for the free ion F−, represents a striking discrepancy with simple fluorine chemical‐shift theories, which generally predict fluorine resonance shifts to be paramagnetic relative to F−. This result can be accounted for satisfactorily in terms of a simple p‐bonded model with the use of a relatively uncomplicated equation for shielding. In the second‐order or excitation term, it is essential (1) to consider separate contributions from different excitations, (2) to include terms on both atoms for some matrix elements, and (3) to recognize the role of the π‐bonding coefficient even though the π shell is filled. The anomaly for ClF arises from the valence shell π*→σ* excitation, which corresponds to electron circulation in opposite senses on the two atoms. No such effect occurs for F2, on account of symmetry restrictions, or for HF, because of the absence of π electrons on the H atom.
Studies of Lithium Hydride Systems. II. Solid—Liquid Equilibrium in the Sodium Chloride—Lithium Hydride System44(1966); http://dx.doi.org/10.1063/1.1726836View Description Hide Description
Thermal analysis has been used to determine the temperature—composition solid—liquid equilibrium diagram for the lithium hydride—sodium chloride reciprocal system. A eutectic composition of 55.2 mole % lithium hydride was observed, melting at 565.7°C. No evidence for the formation of solid solutions was observed. The values of chemical potential of each component have been calculated along the equilibrium curves where the solutions are saturated with respect to the pure solids. The limiting slopes of the saturation curves are consistent with the interpretation that sodium chloride exists primarily as an un‐ionized monomer when dissolved in lithium hydride and that lithium hydride also exists primarily as an un‐ionized monomer when dissolved in sodium chloride.
Studies of Lithium Hydride Systems. III. Solid—Liquid Equilibrium in the Lithium Bromide—Lithium Hydride and Lithium Iodide—Lithium Hydride Systems44(1966); http://dx.doi.org/10.1063/1.1726837View Description Hide Description
Thermal analysis has been used to determine the temperature—composition solid—liquid equilibrium diagrams for the two binary systems, LiBr–LiH and LiI–LiH. Both are simple eutectic systems with no experimental evidence for the formation of solid solutions. Eutectic compositions of 29.7 mole % lithium hydride, mp 453.3°C, and 23.5 mole % lithium hydride, mp 390.8°C, were observed for the bromide and iodide systems, respectively. The values of the chemical potential of each component have been calculated along the equilibrium curves where the solutions are saturated with respect to the pure solids.
The limiting slopes of the saturation curves are consistent with the interpretation that lithium bromide and lithium iodide both exist primarily as un‐ionized dimers when dissolved in lithium hydride and that lithium hydride exists primarily as an un‐ionized monomer when dissolved in either lithium halide.
44(1966); http://dx.doi.org/10.1063/1.1726838View Description Hide Description
Electron spin resonance(ESR) spectra were obtained for solutions of manganese(II) perchlorate in dimethylformamide (DMF), diethylformamide (DEF), dimethylsulfoxide (DMSO), and water. Variation of line shape with solvent and temperature was obtained and interpreted at low temperatures in terms of a model in which solvent fluctuations about the solvated ion modulate the ligand field and relax the electron spin through spin—orbit interaction. Relaxation times for Δms =1 transitions of the system were calculated using a semiclassical density‐matrix formalism. In DMF, DEF, and DMSO solutions another contribution to relaxation was observed at high temperatures which may be attributable to ligand exchange or to internal motion in the complex ion. The high‐temperature process was not observed in aqueous manganese(II) solutions.
44(1966); http://dx.doi.org/10.1063/1.1726839View Description Hide Description
ESRspectra of concentrated solutions of manganese(II) perchlorate (1–3.2M) do not indicate strong spin exchange and the dipole—dipole interaction is considered to be the predominant intermolecular‐relaxation process. A model of the ESR spectrum is constructed using 30 Lorentzian distribution functions positioned according to a relation developed by Hurd, Sachs, and Hershberger. Ratios of the second moment for the various electron‐magnetic transitions are used to approximate the linewidth ratios of the transitions and thereby reduce the number of linewidth parameters to one, ΔH D½, the dipolar‐linewidth contribution to the −½→½ absorption. ΔH D½ is calculated from the experimental spectra by trial and error. The variations of ΔH D½ with increasing temperature indicates that for concentrations less than 1.9M a simple rapid‐motion diffusion model is sufficient to explain the narrowing of the spectrum. Above 1.9M, ΔH D½ follows a modified exponential function, ΔH D½=AD—BD exp(—u/kT), in which AD varies linearly with concentration and BD and U are very nearly concentration independent. A weak exchange interaction is suggested for the more concentrated solutions.
44(1966); http://dx.doi.org/10.1063/1.1726840View Description Hide Description
The average structure of ethylene in the ground vibrational state has been calculated from the internuclear distances (r g ) obtained from electron diffraction by making corrections for the effect of atomic displacements perpendicular to the equilibrium bond directions and the centrifugal distortion, and from the rotational constantsA 0, B 0, and C 0 obtained from infrared and Raman spectra by making corrections for the interactions of vibration and rotation. Good agreement is observed between the diffraction and spectroscopic average structures, and the following average structure of C2H4 has been determined: r z (C=C) = 1.335±0.003 Å, r z (C–H) = 1.090±0.003 Å, and ∠ z C–C–H = 121.7°±0.4°.
44(1966); http://dx.doi.org/10.1063/1.1726841View Description Hide Description
State energies, transition moments, and reactivity indices for tropone and heptafulvene are calculated by the semiempirical SCF—MO method including all singly excited configuration interactions. The results of the calculations are listed in Table II, III, and IV. In heptafulvene there are three absorption bands. The band of longest wavelength and the band of next‐longest wavelength are transitions 1 A 1—1 B 1 and 1 A 1—1 A 1, respectively. In tropone these two transitions shift closely to each other and make one band. There are two absorption bands in tropone. Since π electrons of tropone and heptafulvene localize partially upon double bonds, the lowest triplet energy is lower than that of aromatic compounds which contain the same number of π electrons as tropone or heptafulvene.
Cross Sections for Ionization by Electrons. I. Absolute Ionization Cross Sections of Zn, Cd, and Te2. II. Comparison of Theoretical with Experimental Values for Atoms and Molecules44(1966); http://dx.doi.org/10.1063/1.1726842View Description Hide Description
Ionization cross sections for Zn, Cd, and Te2 vapors have been measured by collection of the total positive ion currents produced from electron impact by 50‐eV electrons in the specially designed ion source of a mass spectrometer. The results: σ i (Zn) = 5.03±0.45×10−16 cm2, σ i (Cd) = 8.54±0.33×10−16 cm2, σ i (Te2) = 17.46±0.48×10−16 cm2. These results are combined with available data to set up a compilation of absolute ionization cross sections for 27 atoms. Very limited agreement is found with theoretical ionization cross sections: It is shown that ionization cross sections of molecules cannot be calculated reliably by summation of the ionization cross sections of the constituent atoms.
44(1966); http://dx.doi.org/10.1063/1.1726843View Description Hide Description
The infrared spectra of trioxane and trioxane‐d 6 have been measured in the region from 4000 to 300 cm−1 on the gaseous, solution,liquid, and solid samples. Polarized spectra have been taken on the oriented thin layer of crystal grown between NaCl or KBr plates. Group‐theoretical analyses were made for a free molecule (point group C 3v ) and the crystal (site group C 3 and space group R3c—C 3v 6). The symmetry species of the observed bands were determined on the basis of the polarized spectra of the crystal. The normal vibrations of a free molecule were calculated according to Wilson's GFmatrix method by use of the Urey—Bradley potential field. The observed bands have been interpreted reasonably based on the calculated results. The band shapes of the gaseous spectra were found to be consistent with the dichroism of the polarized spectra of the crystal by taking into account the Coriolis interactions.
Properties of the Quartic Anharmonic Contribution to the Specific Heat of a Face‐Centered Cubic Lattice44(1966); http://dx.doi.org/10.1063/1.1726844View Description Hide Description
In this paper we study Maradudin's recent approximation to the complex many‐body expression for the quartic anharmonic contribution F 4 to the free energy of a face‐centered cubic crystal. It turns out that one can calculate C 4, the quartic anharmonic contribution to the specific heat corresponding to F 4, almost exactly at any arbitrary temperature by use of the vibration spectrum, thus necessitating only a single summation over the Brillouin zone. The machine‐calculated result is an accurately determined curve of C 4/C har, where C har is the harmonic specific heat, for a lattice model chosen to represent copper. This curve shows structural features in the form of wiggles and in the discussion it is argued that the cubic anharmonic contribution C 3/C har would not negate the structure of C 4/C har in the total form of the anharmonic contribution to the specific heat (C 3+C 4)/C har. Consequently, the theoretical results are compared with the experimental specific heat of copper expressed in the form CL [T, V(0°K)]/C har, where CL denotes the experimental lattice specific heat remaining after the necessary thermodynamic corrections have been made. Certain features of the experimental curve agree with the structural features of C 4/C har alone, despite the fact that C 3/C har is not presently taken into account.
44(1966); http://dx.doi.org/10.1063/1.1726845View Description Hide Description
The optically active vibration modes of iron group divalent fluorides have been investigated by reflection measurements on single crystals and by optical‐transmission studies on sinteredpowders containing fluorides diluted in polyethylene. The obtained spectra are compared with the theoretical predictions for optical absorption due to photon—phonon interaction. For some spectra Kramers—Krönig inversion was performed, allowing more precise measurements of the optically active normal‐modes frequencies and providing an evaluation of the oscillator parameters.
44(1966); http://dx.doi.org/10.1063/1.1726846View Description Hide Description
The emission properties of acetone and acetone‐d 6 have been studied in order to investigate the mechanisms for relaxation of electronic excitation. The results are summarized in Fig. 2 and Table I.
The phosphorescence lifetime of acetone at 77°K in ether—IPA glass was 4×10−4 sec and the quantum yield for phosphorescence was ΦP=0.03±0.01. Deuteration of the acetone increased the phosphorescence lifetime to 1×10−3 sec and the quantum yield to Φ P =0.08±0.02. The phosphorescence of acetone was found to be quenched by the addition of pentene−2 to the rigid solution, but the fluorescence was unaffected. From the quenching data it is concluded that the excited‐singlet→excited‐triplet intersystem‐crossing quantum yield is ΦIC=1.0±0.1, independent of temperature between 298° and 77°K. The intersystem‐crossing rate constant is k IC=4×107 sec−1 and is not affected by deuteration. The excited triplet state is populated with nearly unit efficiency upon excitation of acetone to its lowest singlet state, and therefore the low phosphorescence yield, even in acetone‐d 6, indicates that radiationless transitions from the triplet state to the ground state are primarily responsible for determining the lifetime of the lowest triplet state.
A medium effect on the acetone phosphorescence lifetime at 77°K was observed; it was found to vary from 3×10−4 in glycerine solutions to 1×10−3 sec in crystalline acetone. The triplet‐state lifetime in crystalline acetone decreased only slightly upon increasing the temperature to just below the melting point of crystalline acetone, but in the case of glycerine solutions, the triplet‐state lifetime decreased by at least a factor of 10 in raising the temperature from 77° to 200°K, even though the glycerine remained quite solid. The fluorescence quantum yield in liquid solutions was found to be independent of solvent at 25°C.
The lifetime of the lowest (n, π*) singlet state of acetone is primarily determined by intersystem crossing to the lower‐lying excited triplet state and is therefore τS=2.5×10−8 sec. The quantum yield for acetone fluorescence at room temperature was found to be ΦF=0.01±0.003. No deuterium effect on ΦF was observed and therefore it may be concluded that either there is no deuterium effect on the rate of internal conversion from the excited singlet state to the ground state, or, what is more likely, that internal conversion is much slower than intersystem crossing which is not affected by deuteration.
The fluorescence spectra of acetone and acetone‐d 6 were found to be identical under all conditions, as were the phosphorescencespectra of acetone and acetone‐d 6. The phosphorescence was found to be polarized perpendicular to the lowest singlet—singlet absorption, consistent with theoretical predictions.
44(1966); http://dx.doi.org/10.1063/1.1726847View Description Hide Description
Improved methods for the evaluation of the matrix elements in the usual energy expressions for l‐uncoupled electronic states of diatomic molecules are discussed. The proposed approximations yield molecular constants which greatly improve the quantitative agreement between theory and experiment. The specific examples of the v = 0 levels of the 3d complexes of BH and He2 are considered in detail. Modifications in the off‐diagonal (interaction) elements seem to be the major remaining considerations. Such modifications have only minor effects on the most important of the molecular constants (the electronic energies and B values), however, inclusion of such effects should substantially improve the determination of the higher‐order rotational constants.
44(1966); http://dx.doi.org/10.1063/1.1726848View Description Hide Description
Assuming that truncated strongly orthogonal geminals for a system are already known, the possibility of using r ij‐dependent correction functions in the geminals is discussed with an emphasis on the violation of the strong orthogonality conditions. It is shown that for given truncated geminals there always exists at least one optimally decomposed complete set of one‐electron functions, which makes it possible to pick out the configurations giving rise to strong orthogonality condition violating terms in the energy expression. The latter can be neglected, as compared with others which are of second order in the correction functions, when the system consists of spatially separated pairs in the sense that the one‐electron functions contributing appreciably to one pair do not have significant differential overlap with the starting basis functions of other pairs. In this case interorbital effects are also small. For systems consisting of weakly localizable pairs, when the strong orthogonality condition violating terms are all omitted, the energy expression is correct only up to first order in the correction functions. It is shown that the same difficulties arise in the many‐electron theory of Sinanoğlu.
44(1966); http://dx.doi.org/10.1063/1.1726849View Description Hide Description
Fluorine NMRspectra of hexafluorodisilane and hexafluorodisiloxane have been recorded and analyzed. The 19F–19F and 29Si–19F spin coupling constants were obtained from the satellite spectra arising from the 29Si‐substituted molecules present in natural abundance. The ``near'' and ``far'' 29Si–19F coupling constants in Si2F6 were found to be of opposite sign.
44(1966); http://dx.doi.org/10.1063/1.1726850View Description Hide Description
Ultrasonic shear relaxation data are reported for the associated liquid pentanediol−1,5 at frequencies between 13 and 470 Mc/sec. Reduction of the data indicates a relatively narrow distribution of relaxation times.Viscosity data are also reported on this liquid which show a slight but definite deviation from Arrhenius behavior (lnη s ∝1/T). The shear data are compared to that for two other diols: 2‐methyl pentanediol−2,4 and butanediol−1,3. In this comparison it is shown qualitatively that the width of the distribution of shear relaxation times increases as the temperature dependence of the average relaxation time deviates from the Arrhenius form. It is argued that the origin of both the non‐Arrhenius character of the average shear relaxation time and the distribution of these times lie in cooperative molecular motion.
44(1966); http://dx.doi.org/10.1063/1.1726851View Description Hide Description
The existence of the isomer 130Im (9.2 min) has been established by the growth of the gamma‐ray spectrum of the 130I ground state following neutron irradiation of 129I, and also by the incorporation of 130I into organic combination as a result of the isomeric transition of I2(130Im) dissolved in n‐hexane. The isomer decays to the ground state by a highly converted isomeric transition with a 0.77 probability and to the 0.54‐MeV level of 130Xe by beta emission with 0.23 probability. The cross‐section ratio, σ(130Im)/σ(130I), for production of the isomers from 129I by pile neutrons is about 2.0. The fraction of the isomeric transition events which result in organic incorporation of the 130I daughter in solutions of I2(130Im) in n‐hexane, or in solutions of RI(130Im) in ethyl iodide, is about 0.40.