Volume 36, Issue 9, 01 May 1962
Index of content:
36(1962); http://dx.doi.org/10.1063/1.1732871View Description Hide Description
The chemiluminescent flame produced when sodium vapor diffuses into hydrogen coming from a discharge has been photographed and the ``intensity profile'' has been analyzed in detail according to productemitter flame theory. The analysis shows that sodium is being consumed. Attempts to account for this consumption on the basis of ternary reactions, lead to rate constants 104—105 times larger than those reported in the literature for reactions of this type. It appears that sodium is being consumed by a binary reaction with some species formed in or by the discharge. Since reaction with electronically excited metastable H2 * can be excluded, it is proposed that the binary reaction is Na+H2 † (v≥6)→NaH+H, where H2 † is vibrationally excited hydrogen in its ground electronic state.
Studies on the Alternant Molecular Orbital Method. I. General Energy Expression for an Alternant System with Closed‐Shell Structure36(1962); http://dx.doi.org/10.1063/1.1732872View Description Hide Description
The alternant molecular orbital method is used to derive a general energy expression for an arbitrary alternant system with closed‐shell structure. This expression exhibits a simple dependence on a single mixing parameter, minimization with respect to which yields the optimum energy. An analysis reveals the effectiveness of the method to depend on the relative magnitude of two quantities, one involving the one‐electron operator energies and the other related to electron interaction integrals which connect the bonding with the antibonding orbitals from the simple ASMO scheme. Some comments on the method, in the light of the results obtained, are added.
36(1962); http://dx.doi.org/10.1063/1.1732873View Description Hide Description
Using the general energy expression for an alternant system with closed‐shell structure, obtained in our previous paper on the alternant molecular orbital (AMO) method, a numerical analysis is presented of cyclic systems consisting of 2n=4v+2 electrons moving in the field of 2n[Complex chemical formula]centers. Special attention is paid to the case of large n, when we are essentially dealing with a linear chain with Born‐von Karman periodicity condition. The most important result obtained is that the energy depression per electron (a measure for the effectiveness of the method to take into account electron correlation) decreases only slowly with increasing n and approaches a limit in the neighborhood of 0.4 ev. Hence the AMO method is useful even in the case of large systems and may provide a powerful tool for tackling certain problems in solid‐state theory. A detailed discussion is given of the various approximations incorporated in our analysis and a critical comparison is made with some related results recently obtained by other investigators.
36(1962); http://dx.doi.org/10.1063/1.1732874View Description Hide Description
The proton spin‐lattice relaxation timeT 1 has been measured in normal hydrogen and in mixtures with 12 other gases as a function of density and composition at room temperature. Cross sections for mJ transitions of the H2 molecule deduced from the Bloembergen—Schwinger formula are interpreted qualitatively on the basis of electric multipole moments as measures of the anisotropy of the intermolecular forces, using the transient approximation. This approximation is justified in the case of H2–CO2 mixtures on the basis of the temperature dependence of T 1. The assumptions of the Bloemberger—Schwinger formula are discussed, and an experiment is proposed to determine the connection between two correlation times which appear in the theory.
36(1962); http://dx.doi.org/10.1063/1.1732875View Description Hide Description
The unimolecular isomerization of isopropyl radicals was investigated by studying the thermal decomposition of pure CD3CH2CH3 from 472° to 553°C. Isomerization of CD3CHCH3, was detected by measuring CD2CHD formed from the isomerized radical. The net result of isomerization is reaction (14).
It is found that isomerization of isopropyl up to 500°C proceeds at a rate, relative to decomposition via reaction (15), which can be described by k 14/k 15≤0.07. A mechanism which produces methyl and ethylene directly without intermediate isomerization gives k 14/k 15≤0.04 at 500°C. Contrary to other published estimates of k 14/k 15, the latter shows no downward trend with increasing temperature.
36(1962); http://dx.doi.org/10.1063/1.1732876View Description Hide Description
A variation‐perturbation formulation is presented for the calculation of the electronic contribution to the rotational energy and magnetic moment of a 1Σ molecule. An application of the method to a one‐center single‐configuration wave function for hydrogen fluoride is given and compared with a two‐center function result. The calculated magnetic moment is found to be 0.9276 nuclear magnetons for the one‐center function and 0.6354 nuclear magnetons for the two‐center function in comparison with the experimental value of 0.7392 nuclear magnetons. Analysis of the results shows that the nuclear charges make the dominant contribution and that the electronic ``slippage'' is almost complete. Furthermore, the convergence of the variational expansion indicates that a significant improvement in the theoretical value probably requires refinements in the ground‐state wave function.
36(1962); http://dx.doi.org/10.1063/1.1732877View Description Hide Description
The vibration‐rotation structure of the infrared absorption bands due to ν2 1+ν3, 2ν2 0+ν3, and ν1+ν3 have been observed and measured for Br79CN and Br81CN. The average value of α3 for these two isotopic molecules was found to be +0.000677 cm—1. The Fermi resonance constant for the resonance diad was calculated to be W 12=35.41 cm—1 for Br79CN and W 12=35.48 cm—1 for Br81CN. The positions of the Q branches of a number of other bands were also measured.
36(1962); http://dx.doi.org/10.1063/1.1732878View Description Hide Description
The energy of the first vibrational excited state of a molecular crystal is derived in terms of excitontheory, for the special case of an exciton state derived from a doubly degenerate molecular excited state. We also treat the case where the degeneracy is lost due to the anisotropic environment of the molecule in the crystal. From the several allowed values of such energies, the correlation field splittings in such bands are derived in terms of three‐dimensional ``chain sums.'' The origin of such splittings in certain cubic crystals is traced to the intermolecular exchange of the sense of vibrational angular momentum. An expression is also derived for ``site splittings,'' observable in solid solutions of molecules in their isotopically substituted modifications. A new interpretation of such splittings is offered. Finally, the effects of isotopic substitution in such crystals are discussed and several rules of isotopic invariance are derived. An appendix is included which treats as an example the vibrational fundamentals of crystal benzene derived from the e 1u molecular modes.
36(1962); http://dx.doi.org/10.1063/1.1732879View Description Hide Description
In the first of a series of papers on the structure of fn electron configurations the calculation of the energy matrices and the construction of the energy level schemes are discussed together with the approximations that such calculations involve. Several new calculations are reported.
36(1962); http://dx.doi.org/10.1063/1.1732880View Description Hide Description
The complete energy matrices of the f 5 configuration have been calculated using the methods of Racah. Energy level schemes have been constructed for the trivalent samarium,dysprosium, and plutonium ions. A preliminary analysis of the observed spectra of these ions is made.
36(1962); http://dx.doi.org/10.1063/1.1732881View Description Hide Description
The crystal structure of triethylscarphane, the compound formed upon addition of CS2 to P (C2H5)3, has been determined from three‐dimensional x‐ray diffraction data. The structure is monoclinic, space group P21/n, with a=7.50±0.02, b=11.97±0.02, and c=11.58±0.05 A; β=90.2°, Z=4. The compound is not a loose molecular complex but rather a zwitterion of a quaternary phosphonium derivative of dithioformate, (CH3CH2)3P+–CSS—. The phosphorus atom is bound nearly tetrahedrally to four carbon atoms (three from the ethyl groups and one from the CS2) with bond lengths averaging 1.80 A. The carbon‐sulfur bonds average 1.69 A and the S–C–S bond angle is 128.0°. Uncertainties are about 0.03 A for bond lengths and about 2° for bond angles.
36(1962); http://dx.doi.org/10.1063/1.1732882View Description Hide Description
The near‐ultraviolet absorptionspectrum of tetrolaldehyde vapor has been examined. The weak absorption appearing in the 300–400‐mμ region has been assigned as the n→π* transition, and a vibrational analysis places the 0–0 transition at 26 620 cm—1 and yields a value for the C=O stretch frequency in the π* state of 1036 cm—1.
Two other regions of absorption are observed in the 200–300‐mμ range, and one of these with center at 47 333 cm—1 is tentatively assigned as the N→V transition. The second of these appears as weak structure on the long wavelength edge of the first and may arise from a space forbidden π→π* transition.
36(1962); http://dx.doi.org/10.1063/1.1732883View Description Hide Description
Termination processes in the pyrolysis of dimethyl ether studied by trace product analysis using gas—liquid chromatography revealed the presence of methyl ethyl ether, 1,2‐dimethoxyethane, and methanol. These ethers had been predicted in the work of Benson and Jain by including CH3OCH2 as a chain terminator, and their identification is evidence for the reaction CH3OCH2→CH3+CH2O being an energy transfer process in its second‐order region. Methanol was also predicted from the reactions of CH3O or CH2OH. Quantitative measurement of the trace termination products were made to obtain yields for MeOEt, Me2O, and C2H6, which could be used to calculate the rate constant ratio k 2′/k 2 for the attack by CH3 on MeOEt compared to Me2O. When the steady state is approached from both sides, k 2′/k 2 ranges from 4–12 and is greater than unity, as expected. A reasonable difference in activation energy of about 2–4 kcal is estimated for these reactions. However, the MeOEt gas chromatography peak was broad when resolved from Me2O resulting in loss of precision and together with the required estimates in the steady‐state equation (7) severely limit the usefulness of trace termination product analysis in determining new rate constants. Ethane was measured and compared to a calculated steady‐state value. Because of attack on the ethane and the disappearance of radicals by other termination processes the ratio of the Me2O decomposed to the C2H6 found is not as good a measure of the chain length as those calculated from the rate law.
36(1962); http://dx.doi.org/10.1063/1.1732884View Description Hide Description
Ultraviolet Spectra of Stilbene, p‐Monohalogen Stilbenes, and Azobenzene and the trans to cis Photoisomerization Process36(1962); http://dx.doi.org/10.1063/1.1732885View Description Hide Description
In order to improve our understanding of the electronic states and photochemical reactions of stilbene, we have carried out a spectroscopic and photochemical investigation of stilbene, some substituted stilbenes, and azobenzene. High resolution absorption and fluorescence spectra of the singlet—singlet transition in dilute mixed crystal have been analyzed, and from them it is estimated that the potential barrier to trans‐cisisomerization in the first excited singlet state is about 40 kcal/mole. The absorptionspectrum of the first singlet—triplet transition has been observed by enhancement with a heavy atom solvent and is interpreted as showing that the central bond in the lowest triplet state has a very substantial barrier to rotation. An electronic energy level scheme for stilbene has been constructed by treating the molecule as one ethylene molecule interacting with two toluene molecules. This treatment suggests that as many as four triplet states may be of lower energy than the first excited singlet and would then be possible pathways for photoisomerization.
A study of the deactivation processes of photoexcited stilbene has included the temperature dependences of fluorescence and of isomerization efficiencies. The first showed an activated quenching process with a frequency factor of 1012, which makes it a less forbidden process than is common for singlet—triplet crossings. The second showed that the activated process has the major isomerization yield. In p‐bromostilbene an unactivated process has the major isomerization yield, indicating that there is an atomic number effect on the rate of the unactivated process.
Thus the direct isomerization in the first excited singlet or first excited triplet states has been ruled out; two other paths, one with a small activation energy and the other apparently a singlet to excited triplet crossing, have been found.
36(1962); http://dx.doi.org/10.1063/1.1732886View Description Hide Description
The 60 Mc/sec high‐resolution NMR spectrum of the protons of the five‐membered ring of indene[Complex chemical formula]has been studied with the aid of the audio sideband phase detection proton—proton spin decoupling technique. The chemical shifts and the spin—spin coupling constants have been measured. The relative sign of the spin coupling constant J AX was determined to be different than that of J AK and J KX by double irradiation experiments. Certain long‐range couplings of the aromatic protons with the five‐membered ring protons were detected.
36(1962); http://dx.doi.org/10.1063/1.1732887View Description Hide Description
Pauling's semiclassical model, in which the diamagnetic susceptibility of an aromatic molecule is attributed to ring currents, is re‐examined. The ring currents in a family of hexagonal polycyclic systems are inferred from quantum‐mechanical calculations and are compared with the semiclassical estimates. The Pauling model is found to be unsatisfactory for systems containing more than two or three rings, giving currents which are much too large.
36(1962); http://dx.doi.org/10.1063/1.1732888View Description Hide Description
Chlorpromazine (10‐dimethylaminopropyl phenothiazine) in the form of slabs cast from the melt is a semiconductor. The activation energy of conductivity has an average value of E=2.1 ev, referring to the equation σ=σ0 exp(—E/2kT), above a transition temperature of 32°C. There is no significant change of E upon fusion, its value remaining constant up to 80°C. The transition has been confirmed by thermal analysis,dielectricmeasurements, and by infrared spectroscopy. The spectra indicate that the side chain as well as the ring system are affected by the transition, which is believed to be due to an unfolding of the sidechain above a critical temperature, this causing a change in the micellar association of the substance. Values are reported, as functions of temperature, for the refractive index, the permittivity at 1 Mc/sec and at 1592 cps, the parallel ac conductivity and the loss tangent at the latter, lower, frequency. The substance is diamagnetic.
36(1962); http://dx.doi.org/10.1063/1.1732889View Description Hide Description
A correlation of the structural data on 15 hydrates obtained by x‐ray diffraction,neutron diffraction, and protonmagnetic resonance reveals that when a water molecule is hydrogen bonded into a crystal structure and the angle subtended at the donor water oxygen by the acceptor atoms deviates from the vapor H–O–H angle, bent hydrogen bonds are formed in preference to distortion of the H–O–H angle. Theoretical justification for this result is obtained from energy considerations by calculating the energy of formation of bent hydrogen bonds on the basis of the Lippincott—Schroeder potential function model for the hydrogen bond and the energy of deformation of the H–O–H angle from spectroscopic force constants.
36(1962); http://dx.doi.org/10.1063/1.1732890View Description Hide Description
Potential energy curves for the a 1Σ g +, X 3II u , 3Σ g —, b 1II u , A 3II g , c 1II g , B 3II g , d 1Σ u +, and e 1Σ g + states of C2 have been calculated by the Rydberg—Klein—Rees method. These curves have been compared with some of the recent quantum calculations of Clementi and Pitzer on this same system. The recent identification of the ground state as the a 1Σ g + by Ballik and Ramsay and the determination of the dissociation energy by Brewer, Hicks, and Krikorian have been used to calculate tentative values for the dissociation energies of all of the observed excited electronic states.