Index of content:
Volume 45, Issue 2, 15 July 1966
45(1966); http://dx.doi.org/10.1063/1.1727580View Description Hide Description
Measurements of pure rotational transitions ranging from J = 0 — 1 up to J = 8 — 9 have been carried out for the series of deuterated methyl halides listed below in a frequency range from 12 to 152 GHz. Resulting values of the rotational and centrifugal distortion constants B 0, DJJ , and DJK , respectively, are: for CD3 35Cl, 10 841.91 MHz, 10.84 kHz, and 102.7 kHz; for CD3 37Cl, 10 658.47 MHz, 10.56 kHz, and 99.46 kHz; for CD3 79Br, 7714.63 MHz, 5.83 kHz, and 63.56 kHz; for CD3 81Br, 7681.23 MHz, 5.86 kHz, and 63.27 kHz; for CD3 127I, 6040.31 MHz, 3.59 kHz, and 48.32 kHz. The quadrupole coupling constant eqQ for the halide nucleus in each of these molecules is, respectively: −74.6, −58.8, 575.7, 480.9, and −1929 MHz. Comparison of the above centrifugal distortion constants with those calculated from known force constants is made.
45(1966); http://dx.doi.org/10.1063/1.1727581View Description Hide Description
A phenomenological treatment of dielectric relaxation for symmetric‐top gases and binary gas mixtures is developed. It is shown that the orientational Z number is a monotonously increasing function of a collision parameter F=2(I A+I B)/mb 0 2. Long‐range attractive forces contribute substantially to the cross sections and give rise to a temperature dependence. A formula is derived which permits the prediction of the cross sections from kinetic‐theory data for the molecules. A graphical presentation of the formula illustrates various features of the process. The predictions of the model are shown to be in agreement with dielectric relaxation data for 25 gas mixtures.
45(1966); http://dx.doi.org/10.1063/1.1727582View Description Hide Description
The infrared spectra of gaseous CH3NF2 and CD3NF2, and of solid CH3NF2 have been recorded. They are discussed in terms of the symmetry of Point Group Cs . The unsymmetric methyl vibrations, which are degenerate under the local C 3v symmetry of the group, were found to undergo splitting of smaller magnitude than in analogous compounds of Cs over‐all symmetry. The two nearly degenerate components of each of these motions appear to be coupled through rotation.
45(1966); http://dx.doi.org/10.1063/1.1727583View Description Hide Description
The infrared spectrum of gaseous CF3NF2 has been recorded in the region of 250 to 2000 cm−1 and has been interpreted in terms of Cs symmetry. Thirteen bands have been identified with fundamental vibrations. A 14th fundamental has been derived from combination bands and from its first overtone.
45(1966); http://dx.doi.org/10.1063/1.1727584View Description Hide Description
Boyd's model for the calculation of the friction constant due to dielectric relaxation is extended to deal with solutions of finite concentrations. The first‐order effect of concentration on the friction constant is calculated using the Debye—Hückel potential for the moving ion. It is found that the concentration‐dependent ionic mobility due to dielectric relaxation contains no terms of order smaller than .
45(1966); http://dx.doi.org/10.1063/1.1727585View Description Hide Description
The homogeneous isotopic exchange reaction between hydrogen and deuterium in an excess of argon was studied in a 1‐in.‐diam single‐pulse shock tube.Reaction times behind the reflected shock were close to 1 msec and the temperature range was 1060°—1420°K. The hydrogen and deuterium were present in concentrations from 1%—10%, at total reaction densities of 1.18 to 5.31×10−2 mole/liter. Conversions were kept low in order to minimize the reverse reaction. From the average rates the reaction was found to be second order (total) and empirically represented by
This power‐rate law, with the surprising asymmetry in the H2—D2 dependence, can be rationalized by assuming that exchange occurs with high probability only during encounters between H2—D2 pairs, one of which is vibrationally excited to approximately 30 kcal/mole; that the probability for metathesis is low between molecules in their low‐lying vibrational states even when their relative kinetic energy along the line of centers plus vibrational excitation exceeds the activation energy. Support for this mechanism is provided by an approximate calculation of the pre‐exponential term of the exchange rate constant from vibrational relaxation data.
45(1966); http://dx.doi.org/10.1063/1.1727586View Description Hide Description
The rate of exchange of deuterium for hydrogen in the homogeneous gas‐phase reaction between D2 and CH4 (highly diluted in argon) was measured behind reflected shocks in a single‐pulse shock tube of the ``magic hole'' design. The studies covered the temperature range 1440°—1755°K, at CH4/D2 ratios from 0.1 to 3.0. The exchange rate was found to be approximately first order with respect to deuterium and one‐half order with respect to methane and argon. The empirical rate expression and the measured exchange rates cannot be accounted for either by a direct four‐center bimolecular exchange process or by a mechanism involving a dissociation followed by a chain of atom displacement steps. However, these results are readily accounted for by a vibrational excitation mechanism in which the exchange rate is limited by the rate of populating a critical vibrational level for D2 (approximately the fifth). The activation energy for exchange is 52.0±2.2 kcal/mole.
45(1966); http://dx.doi.org/10.1063/1.1727587View Description Hide Description
The electronic density distribution in lithium hydride has been studied, and compared to the two hypothetical situations of the free atoms and the free ions (Li+ and H−) at the equilibrium separation. It is found that these two models are very much the same in the case of LiH, because of the natures of the entities which are involved. It is proposed that the molecule can best be viewed as being made up of a lithium ion and a polarized hydrogen atom, with the fourth electron partly concentrated on the outer side of the lithium nucleus and partly spread out in a diffuse manner around and beyond the hydrogen nucleus.
45(1966); http://dx.doi.org/10.1063/1.1727588View Description Hide Description
Absorption spectra have been obtained of gaseous NdBr3 and NdI3 in the range 4000–25 000 cm−1 between 1000° and 1200°C. Spectra of solid and liquid NdI3 near the melting point have also been measured.
The most prominent feature of the gaseous spectra is a complex band located between 16 000–17 000 cm−1 with oscillator strengths of 3.3×10−4 and 5.3×10−4 for NdBr3 and NdI3, respectively. The band is assigned to the hypersensitive transition 4 I 9/2—4 G 5/2.
The oscillator strength of this transition in the vapor molecules is 10–50 times greater than in the solution systems studied by Carnall, Fields, and Wybourne. Various intensity mechanisms are discussed and it is concluded that the vibronic mechanism gives order‐of‐magnitude agreement with results. Judd's proposal that the parameter T 2 is particularly sensitive to environment is borne out by the results on the vapor spectra.
45(1966); http://dx.doi.org/10.1063/1.1727589View Description Hide Description
Heat capacities of argon adsorbed on a layer of xenon which had been preadsorbed on a graphitized carbon black were measured from 50°—80°K. The surface was characterized by determining adsorption isotherms and heats of adsorption. It was found that these data were equivalent to those obtained by other workers. A maximum in the heat capacity of one‐half monolayer of adsorbed argon was observed at 72°K. Since the analysis of the isotherm data indicates that this system should have a two‐dimensional critical point at a temperature close to the temperature of the maximum, it is concluded that the experimental curve corresponds to that for a two‐dimensional film at its critical density. The data are compared with theoretical calculations based on the lattice gas model and it is found that the experimental curve is not as sharply peaked as the theoretical. Possible reasons for this discrepancy are discussed.
45(1966); http://dx.doi.org/10.1063/1.1727590View Description Hide Description
Calculations are performed on the 2Σ g + and 2π u bound states of H2 + and the ground states of H2 and HeH+ using a basis set of nonintegral elliptical orbitals. The H2 + calculations are in the form of a single‐configuration wavefunction while both self‐consistent‐field and configuration‐interaction studies are carried out on H2 and HeH+.
Franck—Condon Factors in Ionization: Experimental Measurement Using Molecular Photoelectron Spectroscopy45(1966); http://dx.doi.org/10.1063/1.1727591View Description Hide Description
The photoelectron spectra excited by 21.21‐eV photons in argon, krypton, xenon, hydrogen, nitrogen, carbon monoxide, oxygen, and nitric oxide have been re‐examined using a 180° magnetic‐field electron velocity analyzer. In favorable cases peaks corresponding to vibrational levels in the resulting positive ion are completely resolved giving directly the Franck—Condon factors. Where the vibrational peaks are not fully resolved, the relative order of intensities is apparent and comparison is made with calculations of the Franck—Condon factors. Vibrational structure associated with the second level of nitric oxide is observed for the first time, and this allows an accurate value (15.65 eV) to be attached to the adiabatic ionization potential of this level.
45(1966); http://dx.doi.org/10.1063/1.1727592View Description Hide Description
The absorption spectra of large single crystals of Er2O3 observed at 10°, 80°, and 290°K were analyzed in the 2200–10 000‐Å wavelength region. Over 600 transitions from all eight crystalline Stark levels of the ground 4 I 15/2 state to over 30 excited Jmanifolds of the Er3+ ion in C 2 sites were assigned. In thick crystals, evidence of vibronic spectra of ions in C 2 and S 6 sites was observed for seven Jmanifolds. The observed spectrum is compared with a calculation which includes configuration‐interaction effects on all expected ``free‐ion'' energy levels of Er3+ between the ground state and 44 000 cm−1. The parameters E 1=6844, E 2=32.04, E 3=635, ζ=2354, α=12.2, β=−590, and Y(22, 1)=−4180 cm−1 yield an rms deviation of 73 cm−1 between observed and calculated energy levels. The significance of these parameters in comparison with those for Er3+ in other crystals is discussed.
45(1966); http://dx.doi.org/10.1063/1.1727593View Description Hide Description
Using a theta—theta diffractometer and quartz‐crystal monochromatized MoKα radiation, x‐ray scattering from liquidthallium has been measured as a function of temperature ranging from 350° to 700°C. The radial distribution functions (RDF) which have been obtained by Fourier inversion of the normalized intensity do not show any unusual feature and indicate a close‐packed structure for the liquidthallium. The modulation‐free part of the RDF in the region of small r where r is the radial distance suggests a high degree of reliability in the calculated value of atomic scattering factors of thallium and a considerable accuracy in the measured intensity. The interatomic distancer 1=3.30 Å of thallium does not change with temperature, and the coordination numbers change very little from 11.5 atoms at 350°C to 10.9 at 700°C. This decrease may be explained with excess‐ or free‐volume theory.
The resistivity and thermoelectric power have been determined using the x‐ray interference functions and theoretically calculated values of the pseudopotential elements. The predicted values of the resistivitiesR pred are little lower than the measured values R expt1 (e.g., R pred=63 μΩ·cm, R expt1=79 μΩ·cm at 500°C). Further, the temperature coefficient of the predicted resistivities is also smaller than that of the measuredresistivities. The observed discrepancy has been attributed to the inadequacy in pseudopotential elements.
45(1966); http://dx.doi.org/10.1063/1.1727594View Description Hide Description
A system of diffuse bands in the region 3700–4100 Å has been observed during the flash photolysis of allyl bromide and eight other allyl compounds. The bands are found to be shifted when a sample of allyl‐d 5 bromide is used. The new band system with the (0–0) band at 4083 Å is assigned to the allyl radical [Complex chemical formula]. The diffuseness of the bands is attributed to predissociation of the radical in the excited state and gives an upper limit of 70.0 kcal/mole (≡3.04 eV) for the dissociation energy of the radical.
45(1966); http://dx.doi.org/10.1063/1.1727595View Description Hide Description
A formal quantum theory for the unimolecular rate constant is presented. This theory is an elaboration on an earlier paper by Wilson and Thiele. The theory treats the unimolecular breakdown quantum mechanically, but retains two classical‐like assumptions concerning the collision mechanism and the criteria for reaction. A model calculation suggests that these assumptions can be used if ω≪kT/h. Some arguments are also presented which suggest this same limit of applicability for the general theory.
45(1966); http://dx.doi.org/10.1063/1.1727596View Description Hide Description
Perfluorocyclobutane has been re&examined in an attempt to determine more definitely the question of whether or not the ring is planar. The infrared spectrum of the gas has been obtained under higher resolution and has been extended to 33 cm−1. Two new fundamentals have been found, at 283 and 198 cm−1, and the observed fundamentals have been classified as perpendicular or parallel on the basis of band contours. Infrared and Raman spectra of the liquid have been obtained to facilitate identification of coincidences. The NMRspectrum has been measured from 25° to −102°C. An interpretation of the experimental results is given and the fundamental vibrational frequencies are tentatively reassigned, based on the convincing evidence for the nonplanar D 2d structure.
45(1966); http://dx.doi.org/10.1063/1.1727597View Description Hide Description
We have separated the quantum‐mechanical second virial coefficient for hard spheres B into two terms. The first represents the contribution of a Boltzmann gas, and the second is an exchange term embodying the effects of quantum statistics. Numerical computation of B to high precision then allows us to analyze the temperature dependence of the exchange term, which is found to decrease exponentially with temperature, and to determine the asymptotic expansion of the Boltzmann term at high temperatures.
45(1966); http://dx.doi.org/10.1063/1.1727598View Description Hide Description
An accurate potential‐energy curve for the B 1Σ u + state of H2 has been computed using variational wavefunction in the form of an expansion in elliptic coordinates and depending explicitly on the interelectronic distance. The theoretical clamped‐nuclei binding energy (De =28 896.3 cm−1) with diagonal corrections for nuclear motion (ΔD=−46.1 cm−1) gives the adiabatic binding energyDe =28 850.2 cm−1 in a good agreement with the experimental value De =28 852.8 cm−1. However, for small and large values of the internuclear distance, the theoretical potential‐energy curve very significantly differs from the corresponding RKR curve. The computed wavefunctions are analyzed in terms of some simple functions and it is shown that for 3<R<7 a.u. the B state is predominantly ionic in character.
45(1966); http://dx.doi.org/10.1063/1.1727599View Description Hide Description
The Schrödinger equation for nuclear motion in H2, HD, and D2 has been integrated numerically with accurate internuclear potential for numerous vibrational and rotational states. The theoreticaldissociation energies are larger than the experimental results by 3.9, 4.7, and 3.6 cm−1 for H2, HD, and D2, respectively. It is argued that this discrepancy exceeds the possible computational inaccuracies and is not due to the adiabatic approximation. The radiative corrections are also discussed and it is shown that the correction to the dissociation energy is definitely smaller in absolute value than 0.4 cm−1.
The computed vibrational quanta are larger than the observed ones by about 1 cm−1 while the theoretical rotational quanta are in a very good agreement with experiment.
The vibrational—rotational wavefunctions are used for averaging of some electronic expectation values. The computed quantities agree with experiment within the experimental errors.