Volume 47, Issue 1, 01 July 1967
Index of content:
47(1967); http://dx.doi.org/10.1063/1.1711846View Description Hide Description
Mean‐square fluctuations about a correlation function in its approach to equilibrium are defined and studied. For the case of a correlation function for molecular reorientation, it is shown that this time‐dependent fluctuation can be evaluated by combining experimental infrared and Raman band shapes. Examples are given for liquid CO and CH4. The experiments suggest that nonequilibrium fluctuations approach their equilibrium values faster than do the corresponding average correlations.
47(1967); http://dx.doi.org/10.1063/1.1711887View Description Hide Description
Measurements of dynamic shear impedance at 40 MHz and room temperature are reported for two polystyrene‐solvent systems as a function of concentration. These measurements were made by an ultrasonic reflectance technique. The polymers had sharp molecular‐weight distributions, M̄w/M̄n ∼1.08, and molecular weights of 2.39×105 and 2.67×105. The concentrations ranged from about 1% to 20% and 3% to 30% in di‐n‐butyl phthalate, DBP (a near theta‐solvent) and in toluene (a good solvent) for the two polymers, respectively. Results are reported in terms of the in‐phase, G′, and out‐of‐phase, G″—ωη s , components of the dynamic shear modulus of the polymer and in terms of the reduced dynamic viscosity (η′—η s )/(η—η s ), where η′ is the dynamic viscosity, η s is the solventviscosity, and η is the steady‐flow solutionviscosity. Results on the polystyrene—toluene system indicate that the system undergoes a change from Zimm‐like to Rouse‐like behavior with increasing concentration and, hence, with reduced frequency. No evidence of a reduced high‐frequency limiting viscosity is observed and the value for the lowest concentration falls on a line extrapolated from earlier data of Lamb and Matheson. Results on the polystyrene DBP system indicate that the slope of logG′ vs log volume fraction of polymer increases at about 5% concentration, the predicted concentration for the onset of entanglement coupling. As concentration increases, a reduced high‐frequency limiting viscosity is present up to the entanglement concentration, but beyond this point the reduced dynamic viscosity decreases and indicates a trend toward Rouse‐like behavior at 20% concentration. Within experimental uncertainty neither solvent exhibits viscoelastic behavior.
47(1967); http://dx.doi.org/10.1063/1.1711833View Description Hide Description
The electron paramagnetic resonance spectra of copper monochloroacetate 2.5 hydrate, Cu(O2CCH2Cl)2·2.5H2O, [Cu(ClAc)2], and zinc‐doped Cu(ClAc)2 have been observed at both K‐band and X‐band frequencies. The EPRspectrum of the pure material was characteristic of a species with a spin of one, while the spectrum from zinc‐doped Cu(ClAc)2 revealed an additional spectrum due to a species with a spin of ½. The g values for both the copper—copper pairs and the copper—zinc pairs were identical with values gz =2.38 and gx =gy =2.07. The hyperfine coupling constant for the copper—copper pairs was about 84×10−4 cm−1, while in the copper—zinc spectrum it was about 158×10−4 cm−1. These results are consistent with the assumption of a weakly coupled interaction. The temperature variation of the doubly integrated intensity of the derivative spectrum indicated that the triplet state lies some 230 cm−1 above the singlet ground state. In the optical absorptionspectrum of Cu(ClAc)2 three bands were observed. A very intense band was found at 14 000 cm−1, while two other bands were centered at 11 000 cm−1 and 26 000 cm−1.
Microwave Studies of Collision‐Induced Transitions between Rotational Levels. II. Observations in H2CO, HCN, and H2CCO47(1967); http://dx.doi.org/10.1063/1.1711838View Description Hide Description
The technique of high‐power microwave double resonance is applied to study collision‐induced transitions between rotational levels in H2CO, HDCO, HCN, DCN, and H2CCO. The K‐type doublets (or l‐type doublets) of these molecules for selected J levels have been ``pumped'' by high‐power microwave radiation and the decreases in the intensities of the absorption lines of K‐type doublets (or l‐type doublets) of other J levels have been observed. The ``preferred'' collision‐induced transitions previously observed for ethylene oxide have been more strikingly confirmed. Also multiple transitions with ΔJ≥2 have been observed. An approach by the use of steady‐state equations is developed and used for the analysis of the observed results. Relative values of the rate constants for various collision processes (such as the ΔJ=1 collision‐induced transitions versus the ΔJ=0 collision‐induced transitions or collision‐induced transitions with dipole selection rules vs collision‐induced transitions with quadrupole‐type selection rules) have been determined. The analysis has led to the following conclusions.
(1) The collision‐induced transitions follow selection rules. For the molecules studied in this paper, the dipole selection rules (ΔJ=0, ±1, parity +↔—) are dominant although not always by a very large factor [see Conclusion (3) below].
(2) The rate constants for the collision‐induced transitions with ΔJ=1 are of the same order of magnitude as those with ΔJ=0, although, molecules make transitions between more widely spaced energy levels in the former case. This result is evidence which supports the idea of ``rotational resonance'' introduced by Anderson for explaining the pressure broadening of ammonia.
(3) For HCN, DCN, and H2CCO, rather large rate constants have been obtained for the ΔJ=2 transitions. This suggests that the collision‐induced transitions with quadrupole‐type selection rules or possibly those with even higher multipole‐type selection rules also play a role in these molecules.
47(1967); http://dx.doi.org/10.1063/1.1711860View Description Hide Description
Three types of dipole selection rules are proved for k≠0 in a restricted Frenkel exciton limit. They are: (1) Δk=0; (2) general selection rules based on interchange symmetry; and (3) the g↔u selection rule for centrosymmetric crystals. Even though the latter two selection rules are generally valid only for special values of k, and in particular for k=0, they can be shown to be valid for all k in crystals where restrictions are placed on the magnitude of interactions between certain translationally equivalent molecules. For benzene and naphthalene, and probably for many other organic crystals, the translationally equivalent interactions that ruin the selection rules are nonnearest neighbors. Evidence suggests these in some cases are small, in which event the selection rules are excellent approximations. It is also shown that the transition matrix element in this restricted Frenkel limit is k‐independent, a fact that is important in analyzing the intensity distribution of band←band transitions in terms of the density‐of‐states function. Inability to detect a u←u, band←band transition in crystalline benzene is consistent with the theoretical results.
47(1967); http://dx.doi.org/10.1063/1.1711865View Description Hide Description
An experimental study of the ratio of intensities of the central line, in the molecular scattering of light, to the Brillouin components has been made in water over a temperature range from 0° to 50°C. A 75‐mW He–Ne gas laser was used as a light source and the scattered radiation was analyzed by a pressure‐scanned Fabry—Perot interferometer in conjunction with a dry‐ice‐cooled phototubedetector with recorder output. The ultrahigh‐frequency sound velocity is compared with values obtained by using ultrasonic techniques. There is no evidence of relaxation effects in the frequency range studied. An analysis of the Landau—Placzek ratio is made for water considered as an ideal fluid. It is shown that the usual approximations used in deriving the ratio (κ T —κ S )/κ S are not valid for water. The actual behavior of the Landau—Placzek ratio for water as a function of temperature can be explained in part when an exact solution is used.
Thermodynamic Properties of Octafluoropropane from 14°K to Its Normal Boiling Point. An Estimate of the Barrier to Internal Rotation from the Entropy and Heat Capacity of the Gas47(1967); http://dx.doi.org/10.1063/1.1711886View Description Hide Description
The heat capacity of octafluoropropane has been measured from 14°K to its normal boiling point of 236.42°K for a sample of greater than 99.99 mole % purity. The following heats of transition and transition temperatures were found: Crystal I—Crystal II, 849.8 cal/mole, 99.37°K; Crystal I—liquid, 114.1 cal/mole, 125.46°K; and liquid—vapor (1 atm), 4723 cal/mole, 236.42°K. The vapor pressure up to 1 atm pressure is represented by the equationand the density of the liquid between 197° and 234°K by the equation
The value of the entropy of the ideal gas at 1 atm pressure at the normal boiling point calculated from the calorimetric data is 88.75 cal/ (mole·°K).
An average potential barrier of 3300 cal/mole restricting the rotation of the trifluoromethyl groups is estimated from the entropy computed from molecular data. A reasonable but not an entirely satisfactory interpretation of the heat capacity of the gas is also obtained with a barrier of the above order of magnitude.
Thermodynamic Properties of Hexafluoroacetone from 12°K to Its Normal Boiling Point. An Estimate of the Barrier to Internal Rotation from the Entropy of the Gas47(1967); http://dx.doi.org/10.1063/1.1711888View Description Hide Description
The heat capacity of hexafluoroacetone has been measured between 12°K and the normal boiling point of 245.87°K for a sample of greater then 99.99 mole % purity. The molar heat of fusion at 147.70°K was found to be 2003.5 cal/mole. The molar heat of vaporization at 245.87°K was found to be 5166 cal/mole. The vapor pressure of the liquid to 1 atm pressure is represented by the equationand the density of the liquid between 199° and 243°K byThe entropy for the ideal gas at the normal boiling point was found to be 89.63 cal/ (mole·°K) from the calorimetric data. The calorimetric entropy is shown to be reasonably interpreted in terms of existing spectroscopic and molecular data and a potential barrier of 1470 cal/mole restricting the rotation of the trifluoromethyl groups.
47(1967); http://dx.doi.org/10.1063/1.1711889View Description Hide Description
Accurate two‐center calculations using a basis set of elliptical orbitals are made on the lowest‐lying singlet and triplet‐Π states of HeH+. The calculated dissociation energies of these states are 0.234 eV for the singlet and 0.147 eV for the triplet. Single‐center wavefunctions for these two states are also computed at the equilibrium internuclear separation predicted by the two‐center results (8.05 bohrs for the singlet and 7.67 bohrs for the triplet). The differences in the energies of the one‐ and two‐center calculations are 144 cal and 94 cal for the singlet and triplet, respectively. Expectation values of several operators are given along with calculated values of the spectroscopic constants.
Kinetics of Hydrogen Halides in Shock Waves. II. A New Measurement of the Hydrogen Dissociation Rate47(1967); http://dx.doi.org/10.1063/1.1711890View Description Hide Description
Decomposition rates for H2, diluted in Ar, were studied behind incident shock waves over the temperature range 2900° to 4700°K. HCl and the infrared emission from this molecule were used in a manner to trace the course of decomposition of the H2. In terms of recombination rate constants, we found (cc, moles, sec units) k −3=1018 T −1, k −4=2.5k −3, and k −5=20k −3, where the subscripts 3, 4, and 5 refer respectively to Ar, H2, and H as third bodies.
47(1967); http://dx.doi.org/10.1063/1.1711891View Description Hide Description
A detailed study has been made of the mechanism of electronic energy transfer between a metastable argon atom and a nitrogen molecule. The transition involves the excitation of nitrogen to the C 3Π state. The metastable energy of argon is sufficient to excite the v′ = 2 level of the C 3Π state if the nitrogen is originally in its v″ = 0 vibrational level of the X 1Σ state. It was found that (1) the v′ = 3 level was not excited, (2) the population of the v′ = 0 level was greatly enhanced with the excess energy going into rotational energy of the N2 molecule, (3) the relative intensities of the v′ = 0, 1, and 2 levels could not be predicted on the basis of the Franck—Condon factors, and (4) the collision cross section for the observed energy transfer is about 100 times greater than the argon—argon de‐excitation cross section. It is shown that the enhanced rotational structure of the v′ = 0 level of the C 3Π state can be explained from the viewpoint of a minimum amount of initial internal energy transferring to relative kinetic energy of the particles. The v′ = 1 and 2 levels did not show enhanced rotational structure. From a consideration of the carbon monoxide—argon system, it is shown that the Wigner spin‐conservation rule governs the efficiency of the electronic energy transfer during collision. This condition applies regardless of whether or not the transitions are optically allowed.
47(1967); http://dx.doi.org/10.1063/1.1711892View Description Hide Description
A theory for unipolar ``diffusion charging'' of small aerosol particles is developed for the free‐molecule and transition regions. The free‐molecule theory is, of course, exact within the restrictions imposed on the physical system. The theory for the transition region is based on the Knudsen iteration solution of a relaxation model of the Boltzmann equation. Insufficient experimental data are available to permit detailed comparison with the developed theories.
47(1967); http://dx.doi.org/10.1063/1.1711893View Description Hide Description
Calculations have been made of the electric field gradient at the proton in H2 + and H2 with single‐center wavefunctions, where the expansion center is at one of the protons, or at the midpoint. In all cases, the electric field gradients are in good agreement with experiment; although, for a given amount of computational effort, the midpoint expansions appear superior. Apparently this is due, in part at least, to the fact that a large contribution to the electric field gradient comes from the electrons in the bonding region.
47(1967); http://dx.doi.org/10.1063/1.1711894View Description Hide Description
The adsorption of oxygen has been studied on a polycrystallinetungsten ribbon at temperatures from 300° to 850°K. Two types of tungstensurface structure are distinguishable by their adsorption and desorption behavior. At 300°K, adsorption on either type of surface occurs into a single phase, but, at higher temperatures, to a maximum of ∼765°K, additional adsorption occurs, with an energy of activation of ∼0.25 eV. This process probably involves a reconstruction of the tungsten surface rather than the adsorption into a pre‐existing set of sites. Limited measurements at temperatures between 800° and 1000°K are consistent with the model of a solution process restricted to the surface layers of the tungsten. The removal of adsorbed oxygen, in a high‐temperature flash, occurs only by oxide evaporation below ∼1650°K and by evaporation of both an oxide and oxygen, above this temperature.
47(1967); http://dx.doi.org/10.1063/1.1711895View Description Hide Description
A procedure is presented for the direct purification of untreated, stockbottle KCl by zone refining in a dilute halogen atmosphere. The distribution of Na and Br at parts‐per‐million concentrations is followed by neutron‐activation analyses. Computer‐analyzed distributions in refined ingots yield k * Na=0.53±0.03 and k * Br=0.72±0.02.
47(1967); http://dx.doi.org/10.1063/1.1711896View Description Hide Description
The near‐visible absorptionspectrum of a single‐domain crystal of cesium uranyl nitrate at a temperature of 20°K is interpreted. Selection rules are given for a trigonally coordinated uranyl complex in a crystal having two molecules per unit cell. Lower lying levels, whose excitation gives rise to the fluorescent and magnetic series, are assigned as the A 2 and E species of the D 3 point group. Given these assignments, absorption between levels exhibiting the strongest fluorescence occurs only by allowed quadrupole transitions; a vibronic interaction is suggested to account for the radiative decay rates. Certain higher levels are assigned alternately, in pairs, to the A 2g and A 2u species of the D 3d point group as a consequence of the intermolecular interaction in the unit cell. Estimates of the strengths of the various interactions considered are required and, as a first step, the singlet—triplet intervals for two uranyl models are calculated and found to be comparable with, or less than, the spin—orbit splitting.
47(1967); http://dx.doi.org/10.1063/1.1711897View Description Hide Description
The lifetime, τ=1/k, of the first excited singlet state of vapor‐phase phenathrene has been measured as a function of pressure and excitation wavelength. Excitation in the first singlet absorption band (3400–3100 Å) resulted in τ=50±2 nsec and excitation in the second singlet band resulted in τ=45±2 nsec. The singlet‐to‐triplet intersystem‐crossing efficiency, Φ ST =kST/k, was measured and found to be Φ ST =0.88±0.05. From the measured absorption and emission spectra, the fluorescence efficiency, Φ F =kF/k, was calculated to be Φ F =0.07. Internal conversion from the lowest excited singlet state to the ground state was shown to be very inefficient (Φ IC <0.05). All parameters were found to be independent of phenanthrene pressure. It is also shown that triplet—triplet annihilation to give delayed fluorescence is at least 20 times more efficient than any other second‐order triplet process. The first‐order triplet decay constant β was found to be β=3.3×102 sec−1.
47(1967); http://dx.doi.org/10.1063/1.1711831View Description Hide Description
The fluorescence intensity and the yield of the NO β bands, originating from photodissociation of N2O, was measured as a function of wavelength of incident light in the vacuum ultraviolet region. Photon energies required for the occurrence of the fluorescence are far less than the threshold energy to produce NO(A 2 Σ+, B 2Π r ) directly from N2O, indicating that the emission is due to secondary processes. The fluorescence intensity curve follows closely the absorption curve of N2O indicating that excited species responsible for the emission are produced from dissociation of electronically excited N2O rather than from the direct dissociation to these species. A mechanism of fluorescence is discussed on the basis of the threshold energy above which a specific photochemical process is possible. Reactions responsible for the β emission in the absorption region 1400 to 1550 Å of N2O are production of N(2 D) and O(1 S) followed by and . In the region below 1400 Å, N(2 P) and N2(B 3Π g , B′ 3Σ u −) also can contribute to the emission. When NO was added to N2O, the fluorescence intensity increased considerably and the emission changed from β to γ bands. Reactions which produce the emission in the absorption region 1400 to 1550 Å arefollowed by . Below 1350 Å additional excited molecules N2(B 3Π g , B′ 3Σ u −) can contribute to the emission in similar reactions.
Vibrational and Rotational Effects on the Nuclear Quadrupole Coupling Constants in Hydrogen, Deuterium, and Tritium Halides47(1967); http://dx.doi.org/10.1063/1.1711832View Description Hide Description
The differences of the nuclear quadrupole coupling constants in hydrogen, deuterium, and tritium halides have been studied by considering effects of vibration and rotation on the field gradient. The variation of the coupling constant can be explained by the change in size of the molecule arising from the substitution of hydrogen by deuterium or tritium.
47(1967); http://dx.doi.org/10.1063/1.1711834View Description Hide Description
Integrated Raman intensities of the spectral contour arising from the intermolecular librational motions of pure water have been obtained in the temperature range of ∼10°—95°C. In addition, integrated intensities of nearly symmetric librational components centered near ∼475 and ∼710 cm−1 were obtained from manual contour analysis according to two components. However, contour analysis was also accomplished by means of a special‐purpose analog computer, and three Gaussian librational components having average frequencies of 439, 538, and 717 cm−1 were thus revealed. The total contour intensity, the manually determined component intensities, and the Gaussian component intensities were found to have the same temperature dependence, and that dependence was found to be in excellent quantitative agreement with the previously reported temperature dependence of the hydrogen‐bond‐stretching intensity [J. Chem. Phys. 44, 1546 (1966)]. Integrated Raman intensities of pure water were also obtained in the temperature range of 10°—90°C for the intramolecular valence and deformation contours in the spectral region of ∼2800–3900 cm−1, and near 1645 cm−1, respectively. The integrated intensity of the deformation contour was found to be nearly independent of temperature, but the total integrated intensity of the intramolecular valence contour was found to decrease with increasing temperature. However, heights of the high‐frequency portion of the intramolecular valence contour were observed to increase, whereas heights of the low‐frequency portion were observed to decrease at nearly the same rate, with increasing temperature. An isosbestic point was also found at approximately 3460 cm−1. Further, computeranalysis revealed the existence of four Gaussian components having opposite temperature dependences in pairs—two intense valence components at ∼3247 and ∼3435 cm−1 were found to decrease in intensity with increasing temperature, and two weak components at ∼3535 and ∼3622 cm−1 were found to increase in intensity. Computeranalysis of infrared absorbance spectra also revealed four Gaussian components at approximately 3240, 3435, 3540, and 3620 cm−1. The quantitative agreements involving temperature dependences of the intermolecular hydrogen‐bond‐stretching and librational intensities, as well as the intramolecular valence data, would appear to preclude models of water structure involving consecutive hydrogen‐bond breakage. Continuum models of water structure are also precluded by the inter‐ and intramolecular intensity dependences, and particularly by the isosbestic point in the intramolecular valence region, but a model involving an equilibrium between two forms of water is consistent with all of the data. The two forms refer to water molecules which have or have not surmounted a barrier arising from a partially covalent hydrogen‐bond potential of C 2v symmetry, and they may be described as nonhydrogen‐bonded monomeric water, and as lattice water, respectively. Polarized argon‐ion‐laser—Raman spectra were also obtained in the intermolecular frequency region of the water spectrum, and the depolarization ratios of the intermolecular Raman bands were found to be in complete agreement with predictions from intermolecular C 2v symmetry. Studies of the intramolecular valence region were also made with polarized mercury excitation, and the spectra were analyzed by the analog method. Short‐lived CS intramolecular perturbations were indicated by the observed depolarization ratios of the four Gaussian valence components. Accordingly, CS intramolecular valence perturbations occur in the lattice water, as well as in the nonhydrogen‐bonded water, but the perturbations are of little importance on the intermolecular time scale.