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Volume 38, Issue 12, 15 June 1963
38(1963); http://dx.doi.org/10.1063/1.1733604View Description Hide Description
An electron‐density distribution is determined for the ammonia molecule in its equilibrium configuration by requiring the distribution to exert forces on the nuclei equal and opposite to the nuclear forces of repulsion. The density distribution so obtained is compared with those determined by SCF calculations. All of the density functions are tested by comparing the calculated and observed values for the following molecular properties: (i) the forces on the nuclei, (ii) the dipole moment, (iii) the electric‐field gradient at the nitrogen nucleus, (iv) the diamagnetic susceptibility, and (v) the diamagnetic contribution to the nuclear shielding constant.
38(1963); http://dx.doi.org/10.1063/1.1733605View Description Hide Description
A molecular orbital theory for carbon‐13 chemical shifts in conjugated molecules is formulated. Consideration of the various possible contributions to the shift shows that the local paramagnetic term σ p AA is expected to dominate. By a detailed LCAO analysis of σ p AA, its variation with the salient features of the electron environment is determined. It is demonstrated that there is a significant local‐charge dependence which agrees in sign and order of magnitude with an experimentally established correlation. In addition, the shielding is shown to be a function of the free valence of the atom under consideration and of the polarity of its sigma bonds. A linearized equation for the carbon‐13 chemical shift (with respect to benzene) is presented and compared with the available experimental data.
38(1963); http://dx.doi.org/10.1063/1.1733606View Description Hide Description
A detailed three‐dimensional refinement of the structure of hexagonal silver iodide from room‐temperature photographic data showed very little deviation from the ideal wurtzite‐type structure. Using anisotropic temperature factors a final R value of 6.6% was obtained. The space group is P63 mc, a=4.592, c=7.510, c/a=1.635, μ=0.3747. A general linear relation of the position parameter μ to the c/a axial ratio was found to apply to compounds having this structure type.
38(1963); http://dx.doi.org/10.1063/1.1733607View Description Hide Description
The near‐ultraviolet spectrum of the cyclic 1,3‐dione, tetramethyl‐1,3‐cyclobutanedione, has maxima near both 3400 and 3000 Å. On the basis of absorption intensity (low), solvent shift (transition energy increases with Z ), and position (with respect to cyclobutanone), the two transitions can be explained as n→π1 *+π2 * (lower transition energy) and n→π1 *—π2 * (higher transition energy). The assignment is similar to that used by Sidman and McClure for the 1,2‐dione, diacetyl. It is pointed out that s‐cis‐1,2‐diones should have low‐energy transitions at longer wavelengths than s‐trans‐1,2‐diones, and further, that a cyclic peroxy structure isomeric with the s‐cis‐1,2‐dione might possess sufficient stability for isolation. Evidence from early German literature is cited to show that 1,2‐benzoquinone does indeed exist in two isomeric forms; one, a red quinonoid form and a second, a colorless unstable form.
38(1963); http://dx.doi.org/10.1063/1.1733608View Description Hide Description
Infrared spectra of solid ketene and ketene in argon reveal a new fundamental at lower frequency (CH2CO, 438 cm—1; CD2CO, 371 cm—1; CHDCO, 398 cm—1) than any previously reported. These spectra and additional gas‐phase spectra provide a basis for a reassignment of the vibrational spectrum. The vibrational potential function, centrifugal distortion constants, Coriolis coupling constants, and thermodynamic properties of ketene have been calculated. The out‐of‐plane hydrogen bending force constant is found to be surprisingly low, about one‐third of that for ethylene. In addition, the analysis of the rotational structure of several perpendicular bands yields improved estimates of the A moment of inertia and hence of the HCH angle (122.3°) and the C–H bond length (1.079 Å) of ketene.
38(1963); http://dx.doi.org/10.1063/1.1733609View Description Hide Description
Measurements of the soundvelocity in nitrogen, argon, and krypton, and the derived heat capacities, are reported for a pressure range up to 12 atm and at two temperatures: the ice point and 303.7°K. It is believed that the data for nitrogen are more consistent than those reported by other investigators, while comparison data for argon and krypton are practically nonexistent.
38(1963); http://dx.doi.org/10.1063/1.1733610View Description Hide Description
The matrix elements of the spin—orbit operator between the zero‐order (spin‐free) (n, π*) states of nitrogen heterocyclics are examined. It is found that generally, to the first order, there is no spin—orbit coupling between singlet and triplet states of the same configuration. The coupling between states of different configurations due to differences in the occupancy of the π* orbitals, but having the same nonbonding orbital, is found to have matrix elements similar to those resulting from the coupling between (π, π*) states, which McClure has shown to be unimportant. For the diazines (or polyazines), the coupling between different configurations due to a different occupancy of the nonbonding orbitals is found to have matrix elements which cancel one another. The general conclusion is thus reached that spin—orbit (s.o.) coupling between singlet and triplet (n, π*) states is unimportant to the first order. Using the observed polarization and lifetime data, it is estimated that the ratiohas an upper value of 10—3. These results as well as vibrational overlap considerations suggest that the enhancement of the efficiency of the intersystem crossing in nitrogen heterocyclics with the lowest singlet state of the (n, π*) type might be due to an S (n, π*)→T (π, π*) rather than an S (n, π*)→T (n, π*) radiationless process. An alternative explanation to the one previously given for the observation of fluorescence of 9, 10‐diazaphenanthrene and s‐tetrazine is thus suggested. In these molecules, the strong interaction between the (n, π*) singlet levels might cause the lowest S (n, π*) excited level to have a lower energy value than the (π, π*) triplet level. This would therefore lead to a retardation of the intersystem crossing process and allow the fluorescence to be observed. The characteristics of the observed weak phosphorescence of 9,10‐diazaphenanthrene give spectroscopic support for this proposal.
For nitrogen heterocyclic molecules with the lowest singlet and lowest triplet states of the (π, π*) types, an examination of the quinoline total emission in comparison with that of the parent hydrocarbon, nath‐thalene, suggests an increase in the efficiency of the intersystem crossing process in quinoline. This is suggested to be due to the efficient S 1(π, π*)→T 2(n, π*)→T 1(π, π*) radiationless process. A probable enhancement of the internal conversion processes in these molecules is suggested to be due to an increase in the density of states and the efficiency of the S (π, π*)↔S (n, π*) radiationless process due to a large vibrational overlap integral.
38(1963); http://dx.doi.org/10.1063/1.1733611View Description Hide Description
The orientation distribution function of polar rigid macromolecules in dielectric media is obtained by solving the rotary diffusion equation in the steady state, when the solution flows with a constant velocity gradient and is acted upon by an electrostatic field perpendicular to the stream line. The optical properties, such as birefringence of the solution, are calculated by averaging the optical anisotropy of each macromolecule over all molecular orientations, which are weighted according to the orientation distribution function. The extinction angle and the amount of birefringence are expressed as functions of the velocity gradient and the electric field strength. The behavior of these properties is specifically dependent on the shape and the electric parameters of macromolecule. This method can provide new information about the macromolecular structure.
38(1963); http://dx.doi.org/10.1063/1.1733612View Description Hide Description
The infrared spectrum of crystalline potassium azide has been investigated over the region from 4000 to 550 wavenumbers by using a grating spectrometer. Selection rules for internal and lattice vibrations and their combinations were calculated by using the ``unit‐cell'' method of Bhagavantam and Venkatarayudu. The spectrum was characterized by the splitting of fundamental absorptions and the occurrence of numerous lattice combinations symmetrically spaced about active and inactive fundamentals. From observed lattice combinations, tentative frequency assignments were made for the lattice modes. Polarized spectra indicate that most of these combinations are oriented in the xy plane of the crystal. The spectrum shows good agreement with the accepted crystal structure as described by the space group D 4h 18—I4/mcm.
38(1963); http://dx.doi.org/10.1063/1.1733613View Description Hide Description
Carbon monoxide gas (1 atm pressure) has been vibrationally excited by absorption of infrared light emitted by rich CH4–O2flames. The CO issued from a 286°K porous plate with a laminar flow velocity of 10 cm/sec. It was contained in an annular flow of argon of matched velocity. Resonancefluorescence was observed in the fundamental vibration—rotation band of CO at 2143 cm—1 when impurities were reduced below the ppm level. Intensity measurements showed that a vibrational temperature of 993°K had been reached by light absorption. Under the same conditions the rotational temperature remained at 286°K as shown by the intensity distribution of the rotational lines. Trapping of the resonance radiation permitted observation of the vibrational excitation for 0.2 sec after removal of the exciting light. The rate of the quenching reaction CO (v=1)+M→CO (v=0)+M has been determined from Stern—Volmer quenching curves for the collision partners H2, HD, D2, He, Ne, and O2. An extreme dependence of collisional vibrational relaxation time upon collision mass has been observed, as predicted by theory. Oxygen is anomalously efficient as a collision partner by several orders of magnitude.
Monte Carlo Procedure for Statistical Mechanical Calculations in a Grand Canonical Ensemble of Lattice Systems38(1963); http://dx.doi.org/10.1063/1.1733614View Description Hide Description
The Monte Carlo method of estimating statistical mechanical averages in the petite canonical ensemble, described by Rosenbluth et al., Wood and Parker (for fluid systems), and Salsburg et al. (for lattice models), has been extended to a general multicomponent lattice model in a restricted grand canonical ensemble. The procedure is applied to the two‐dimensional traingular lattice gas with periodic boundary conditions at a supercritical temperature (βε= —ln2), and numerical results are presented for the energy, specific heat, density, isothermal compressibility, thermal‐expansion coefficient, and grand partition function (pressure) at Δ=0.1, 0.0, —0.1, —0.2, —0.3, —0.4, —0.6, —0.8 (with B=100); at B=16, 25, 36, 49, 64, 100, 196 (with Δ=0.0); where Δ=βμ—3βε; where ε is the nearest‐neighbor interaction; β=(kT)—1, where μ is the chemical potential; and B is the number of sites. These are compared (where possible) with previous (Salsburg et al.) and additional petite‐ensemble Monte Carlo results. This comparison emphasizes the different B dependence of intensive properties in these two ensembles and is supplemented by an asymptotic analysis of this difference. Properties in both ensembles display the type of irregular B dependence predicted by Lebowitz and Percus for very small systems. For the larger lattices, properties in the petite ensemble show a stronger B dependence than in the grand ensemble, which is in quantitative agreement with the leading term in the asymptotic analysis. A comparison with the exact analytical results (B= ∞, Δ=0) indicates that the accuracy of the Monte Carlo procedure for the grand ensemble can be reliably estimated by a statistical analysis of partial averages over the Markov chain.
38(1963); http://dx.doi.org/10.1063/1.1733615View Description Hide Description
The concept of the one‐, two‐, and three‐liquid approximations to the theory of nonelectrolyte solutions, introduced by Scott in terms of averaged pair potential functions, is extended by defining suitably averaged Boltzmann weighting factors which are composition dependent. These approximations are introduced and treated by means of a perturbation theory similar to that used by Nosonow to obtain a unified presentation of the averaged potential n‐liquid approximations.
38(1963); http://dx.doi.org/10.1063/1.1733616View Description Hide Description
The infrared spectra of gaseous CH3GeH3, CD3GeH3, and CH3GeD3 were observed in the range 4000–400 cm—1. The 12 fundamental frequencies for each molecule are assigned to normal modes and the spectra are discussed in some detail. Teller—Redlich product‐rule ratios support the assignments. The height of the barrier hindering internal rotation was found from measurements on twelve combination bands involving the torsional modes to have an average value of 1270 cal/mole. The torsional force constant was estimated to be 0.31×10—12 erg/rad. The corresponding barrier height and torsional force constant for methylsilane were estimated to be 1715 cal/mole and 0.44×10—12 erg/rad, respectively, from published data.
Analyses of the rotational structures of ν7–10 and of ν12 and 2ν12 in the spectrum of CH3GeH3 and of ν7–10 and ν12 in the spectrum of CD3GeH3 afforded values for the band origins, certain rotational constants and ζ7–12 for both molecules.
38(1963); http://dx.doi.org/10.1063/1.1733617View Description Hide Description
The electron spin exchange frequency of potassium peroxylamine disulfonate as a function of concentration has been studied by electron spin resonance techniques. The exchange frequencies have been calculated from measurements of the linewidths and the line separations using the modified Bloch equations. The frequencies thus obtained are in agreement with each other. The functional dependence of exchange frequency upon concentration can be expressed as ω=bCm, where m is of the order of unity. The effect of molecular oxygen upon the spin exchange measurements is described. The resonance line shape has been found to deviate from Lorentzian at low concentrations.
38(1963); http://dx.doi.org/10.1063/1.1733618View Description Hide Description
Calculations have been performed to determine the Stark energy levels of rigid symmetric‐top molecules. The continued fraction expression for the eigenvalues of the energy matrix is presented and techniques of evaluation described. Tables of reduced energy levels as a function of electric field are given for all rotational states through J=4. Graphs of these values and the effective dipole moments are included.
38(1963); http://dx.doi.org/10.1063/1.1733619View Description Hide Description
A quadrupole coupling criterion is proposed to evaluate the bonding in the xenon fluorides. Localized valence bonds will give a much higher quadrupole coupling constant for Xe131 than the delocalized linear combinations of p functions used in the polyhalides.
38(1963); http://dx.doi.org/10.1063/1.1733620View Description Hide Description
Zero‐field electron magnetic resonance (EMR) spectra have been observed for several dilute paramagnetic systems. Among these are peroxylamine disulfonate ions in aqueous solutions, Cr3+ in single crystals of MgO, free radicals in x‐irradiated powders of malonic acid,potassium hydrogen malonate, dipotassium malonate, succinic acid, and glutaconic acid. Zero‐field spectra were observed by means of a marginal oscillator—detector similar to that of Benedek and Kushida. The zero‐field splittings in the systems studied were due exclusively to the hyperfine couplings between electronic and nuclear spins. Results obtained were in good agreement with high‐field EMR measurements on these systems. In several cases the hyperfine couplings were obtained with greater precision than at high field. In addition, the radical —OOC–ĊH–COO— was identified, for the first time, in irradiated dipotassium malonate by means of its zero‐field spectrum. No zero‐field lines could be detected in electron‐irradiated polyethylene. Advantages and disadvantages of the zero‐field technique are discussed.
38(1963); http://dx.doi.org/10.1063/1.1733621View Description Hide Description
The photolysis of mixtures of diethyl ketone and carbon tetrachloride has been investigated at room temperature. The following reaction products were observed: CO, C2H6, C2H4, C4H10, CHCl3, C2H5CCl3, C2Cl6, C2H5 (CH3) CHCOC2H5, and CH3 (CCl3) CHCOC2H5. Primary radicals produced by photolysis of diethyl ketone and carbon tetrachloride are C2H5, CCl3, and Cl. Chlorine atoms react with diethyl ketone to form HCl and CH3CHCOC2H5 radicals. Disproportionation and recombination reactions involving CCl3, C2H5, and CH3CHCOC2H5 radicals are largely influential in the formation of many of the products. The ratios of disproportionation over recombination for CCl3 and C2H5 radicals and for CCl3 and CH3CHCOC2H5 radicals were obtained together with various ratios of other rate constants.
Formation of Fragment Ions from CH3Te125 and C2H5Te125 Following the Nuclear Decays of CH3I125 and C2H5I12538(1963); http://dx.doi.org/10.1063/1.1733622View Description Hide Description
The relative abundances of the fragment ions that result respectively from the nuclear decays of CH3I125 and C2H5I125 (I125 undergoes electron capture to give Te125) have been measured with a specially designed mass spectrometer. The data from both studies are similar and give evidence as follows of the highly destructive nature of the decay: (1) Only about 1% of the Te—hydrocarbon ions appear to remain intact in spite of the fact that these ions would be expected to be collected with high efficiency; (2) In both studies a large number of tellurium ions is found with charges as high as 18 and with an average charge of about 9; (3) Finally, in both studies, the singly, doubly, and triply charged carbon ions are found in greater abundance than the hydrocarbon ions. The cause of this extensive ionization and fragmentation is ascribed primarily to a series of Auger processes that occur subsequently to the formation of inner orbital vacancies in Te125 as the result of electron capture and internal conversion in the decay of I125.
38(1963); http://dx.doi.org/10.1063/1.1733623View Description Hide Description
Nitrogen dioxide was irradiated at 3660 and 4047 Å at various pressures of NO2, with and without CO2, NO, and N2 as added gases. The data indicate a primary dissociation at 3660 Å and shorter wavelengths corresponding to energies greater than the ON–O bond energy. At 4047 Å, the data are explained by an excited‐molecule mechanism. Isotopic oxygen scrambling experiments at 4047 Å indicate the probability of reactions to produce oxygen atoms at that wavelength, but photolysis of NO2 at trace concentrations and high inert‐gas pressures indicates that these atoms are not derived from the unimolecular decomposition of the photoactivated molecule. The inhibition of the quantum yield by NO addition was pressure‐dependent, lending further support to the premise that oxygen atoms are important in the mechanism at 4047 Å. It is postulated that the reaction NO2 *+NO2→N2O3+O is the source of atomic oxygen.