Showing papers in "Molecular Physics in 1966"

Molecular multipole moments

Abstract: A summary and critical review of available information on molecular quadrupole and higher moments is presented. A theorem is also proved which shows that only one independent scalar quantity is required to determine a molecular electric multipole tensor of rank p for molecules with an n-fold axis of symmetry where p < n.

Proton spin-lattice relaxation in liquid water and liquid ammonia

Abstract: The proton spin-lattice relaxation time has been measured at 20·8 Mc/s for a series of solutions of water in heavy water and solutions of ammonia in heavy ammonia for the temperature range from the melting point to the liquid-vapour critical temperature. Measurements have also been made for water over limited temperature ranges at several fixed densities. The contributions to the spin-lattice relaxation time from direct dipolar and spin-rotation interactions have been separated. The spin-rotation interaction contribution appears to be the same for H2O as for HDO and also as between NH3, NH2D and NHD2 and this result is justified. The correlation times for molecular re-orientation, τd, and for molecular angular velocity, τsr, are derived from the results and in so doing some support for the Hubbard [12] relation betweent τsr and τd is adduced. It is found that at the critical temperature τsr≪τd which contrasts with other liquids for which it is usually found that τsr≪τd. The spin-rotation interaction cons...

Alternating linewidths. A novel relaxation process in the electron resonance of biradicals

Abstract: The factors affecting the electron resonance linewidths of a biradical, showing strong spin exchange, tumbling isotropically in solution are considered. For a biradioal with two equivalent magnetic nuclei the hyperfine lines should alternate in width provided the dominant relaxation mechanism is a modulation of the exchange interaction between the unpaired electrons. Certain nitroxide biradicals do indeed exhibit an alternating linewidth effect which we attribute to this mechanism.

The evaluation of the one-centre integrals in the semi-empirical molecular orbital theory

Abstract: An expression is given which fairly accurately provides valence-state energies. It is shown that one can formally attribute the meaning of the one-centre integrals to the coefficients in such an expression. In this way one also obtains the criteria of Pariser and Parr. Moreover, if one introduces such semi-empirical integrals in the expression which gives the vertical ionization potentials and electron affinities, one obtains the corresponding experimental values. On the basis of these facts usage of such semi-empirical integrals in the M.O.-L.C.A.O. theory is proposed. A discussion of the advantages and the limitations of its application is made.

Préionisation et prédissociation dans la dissociation des molécules triatomiques par impact électronique

Abstract: On montre que pour des raisons de symetrie electronique, les fragments OH+(3Σ−) ne peuvent pas etre obtenus directement a partir des ions H2O+ formes par arrachement d'un electron. Pourtant ces fragments sont observes en grande quantite dans les spectres de masse. On montre que ces ions sont formes par predissociation: les etats 2B2 et 2A1 de l'ion moleculaire sont predissocies par un etat 4A2, 4A″. La formation de S+ et SH+ dans H2S s'explique de la meme facon. De plus la preionisation est en competition avec des processus nonionisants dependant de la masse, ce qui provoque un effet isotopique sur les sections efficaces d'ionisation totale (effet Jesse-Platzman). Nous avons observe un effet de ce type dans D2S, qui est de 8 per cent. pour des electrons incidents de 70 eV et augmente quand l'energie des electrons diminue.

Nuclear magnetic resonance of organic free radicals

Abstract: The conditions under which N.M.R. spectra of organic free radicals can be observed are discussed. The N.M.R. spectra of several radicals are shown and compared with the E.S.R. spectra of the same radicals. The N.M.R. spectra are particularly useful to determine small coupling constants which are below the limit of resolvability of E.S.R. and ENDOR. Futhermore, N.M.R. renders the sign as well as the magnitude of the interaction between the unpaired electron and the nucleus investigated; it can contribute to the understanding of the mechanism of this interaction.

Deuteron spin lattice relaxation in benzene, bromobenzene, water and ammonia

Abstract: The deuteron spin-lattice relaxation times, T 1Q, have been measured for benzene, water and ammonia from the melting point to the liquid-vapour critical temperature and for bromobenzene over a smaller temperature range. The results are compared with previously measured proton spin-lattice relaxation times. It is shown that the deuteron T 1Q values agree with the proton intramolecular T 1 values for benzene, thus confirming the separation of the dipolar and spin-rotation contributions performed by Powles and Figgins [1]. A value of the electric quadrupole coupling constant, e 2 qQ/th, is deduced, which agrees with the value for the molecule in the solid. For bromobenzene a value of e 2 qQ/h=175±10 khz is obtained in a similar way. For water and ammonia a solution is found to the classic N.M.R. problem of separating the inter and intra-dipolar interaction contributions to the dipolar relaxation rate. The two contributions are about equal, in agreement with theory. The separation of the spin-rotation contrib...

The spectra and electronic structure of tetrahedral ions

Abstract: An S.C.F.-M.O.-L.C.A.O. calculation in the Roothaan-Popie scheme, using the Pariser-Parr approximations, has been carried out for the ions [CrO4]-- and [MnO4]-. The necessary integrals have been evaluated semi-empirically. For a more exact evaluation of the spectral transitions, a configuration interaction calculation, extended to the most important mono-excited configurations, has been performed. The order of levels previously suggested by Ballhausen and Liehr and other authors is confirmed by the results of the calculations. A new spectral assignment is presented, attributing to each observed absorption band two electronic transitions occurring between one of the occupied M.O.'s (t 1, 2t 2, 1t 2) and the two lowest empty M.O.'s (2e, 3t 2).

The theory of double bond fixation in conjugated hydrocarbons

Abstract: A general method is given for treating the problem of bond fixation in conjugated hydrocarbons. Two effects are considered separately: first-order bond fixation from which bond-length-bond-order relationships may be derived, but which cannot lower the symmetry of the molecule; and second-order bond fixation which can lower the symmetry. It is shown that the most favourable distortion in the latter case is given by the lowest eigenvalue of the bond-bond polarizability matrix. The critical value for this eigenvalue appears to be about 1·8β -1. For values whose modulus is greater than this a distortion will occur which overcomes the σ-bond compression energy, and may lower the symmetry of the molecule.

Molecular motion in liquid benzene by nuclear magnetic resonance

Abstract: The proton spin-lattice relaxation time, T 1, has been measured for a series of mixtures of benzene in perdeuterobenzene for the liquid in equilibrium with its vapour over the temperature range from below the normal freezing point up to the critical temperature. The two contributions to T 1 due to interactions within the molecule (T 1 intra) and between molecules (T 1 inter) have been separated and are found to be very different in magnitude and in variation with temperature. The variation and magnitude of T 1 inter correlates well with other translational motion dependent properties such as self diffusion and viscosity. The correlation of T 1 intra with other re-orientation dependent properties such as deuteron T 1 and Rayleigh scattering is poor. The observed variation in T 1 intra and in particular the broad maximum at higher temperatures is then interpreted as due to a combination of dipolar and spin-rotation effects. This interpretation results in good agreement between the activation energies for re...

Nuclear magnetic resonance studies of alkali metal halide solutions

Abstract: Chemical shifts of the nuclear resonances of 23Na, 39K, 87Rb and 133Cs in aqueous solutions of alkali halides and nitrates from concentrations of 0·2 molal up to saturation are recorded. The magnitudes of the shifts increase considerably with increasing atomic number of the cation, and shifts to both higher and lower fields relative to the cation at infinite dilution are observed. At concentrations below 4 molal the shift of the potassium, rubidium and caesium resonances varles linearly with the mean activity of the salt and the shifts have been expressed as activity shifts in p.p.m/activity. Anions show a definite series of shielding effects that is found for all cations, the order of increasing shielding is I−

Electron spin resonance of an X-ray irradiated single crystal of potassium dihydrogen arsenate, KH2AsO4

Abstract: The electron spin resonance of an x-ray irradiated single crystal of potassium dihydrogen arsenate has been examined and a radical which is thought to be AsO4 4- identified Analysis of the spectra for various orientations of the crystal with respect to the external magnetic field leads to an axially symmetric hyperfine interaction tensor which is in agreement with what would be expected for the AsO4 4- radical Semi-empirical LCAO molecular orbital calculations for the AsO4 4- radical with symmetry D 2d trapped in the tetragonal KH2AsO4 crystals leads to the conclusion that the odd electron enters a molecular orbital with A 1 symmetry The isotropic part of the hyperfine interaction tensor is calculated from this theory to be 2098 Mc/s which must be compared with the experimental value of 3038·6 Mc/s, The anisotropic part of the hyperfine interaction calculated from the theory, however, is much too low when compared with the value estimated from the experimental data It is possible that part of this dif

The electron spin resonance of irradiated glycine at 77°K

Abstract: The electron spin resonance spectrum of the radical ĊH(NH3 +)CO2 -, I, trapped in irradiated glycine has been re-examined at 77°k and the hyperfine couplings obtained when the -NH3 + ceases to rotate. The individual hydrogens are clearly identified by reference to the x-ray structure. Improved values are obtained for the room temperature couplings of this radical. There is also, after irradiation at 77°k, a spectrum consisting of two broad lines tentatively attributed to one of the molecular ions; the species giving rise to this spectrum changes irreversibly into I on warming above 77°k.

The nitrogen atom orbital populations in the azabenzenes as determined by their 14N nuclear quadrupole coupling constants

Abstract: An analysis is presented of the 14N nuclear quadrupole coupling constants of pyridine, γ-picoline, pyrazine, tetramethylpyrazine, phenazine and s-triazine in terms of the populations of the sigma and pi bonding orbitals around the nitrogen atom. The limitations inherent in the interpretation of the quadrupole coupling data are discussed and it is shown that it is best used to determine the difference between the sigma and pi orbital populations. By assuming that the sigma orbital population remains constant for all these compounds values of the pi orbital populations are obtained and compared with the predictions of molecular orbital theories of various degrees of sophistication.

The electron resonance of ground state triplets in liquid crystal solutions

Abstract: The electron resonance spectra of two ground state triplets have been measured in the isotropic and nematic mesophase of p-azoxyanisole. The extra transitions observed when the biradicals are in the nematic mesophase are interpreted in terms of an unaveraged zero-field splitting. The nitrogen hyperfine splitting is also found to decrease in the nematic mesophase. The observation of these extra lines in the spectra provides excellent quantitative confirmation for the triplet character of these radicals. The changes in the spectra are satisfactorily accounted for using the swarm model of nematic liquid crystals.

A Nuclear Magnetic Resonance Study of Mercury Dimethyl

Abstract: The 1H, 199Hg and 13C spectra of the more common of the isotopic species occurring in natural abundance in mercury dimethyl have been studied. The proton spectra were observed directly whilst the 199Hg and 13C spectra were measured indirectly by a double resonance method involving observation in the proton region. A hitherto unreported long range coupling of magnitude 0·44±0·02 c/s was observed between the methyl protons and new values were obtained for the directly bonded 13C-H coupling, ± 129·6±0·1 c/s, and for the geminal 199Hg-12C-H coupling, ± 101·9±0·1 c/s. The relative signs of these couplings, together with the sign and magnitude of the directly bonded 199Hg-13C coupling, ±689±2 c/s, were determined by experiments on the 0·33 per cent abundant species H3 13C199Hg-12CH3. The magnitude of the 199Hg-13C coupling is discussed in terms of the correlation between reduced coupling constants and atomic number. The vicinal 13C-Hg-12CH3 coupling appears to be zero.

Shock compression of liquid hydrogen

Abstract: High explosives have been used to shock liquid hydrogen to 39 500 bars, 5·2 cm3/g and an estimated temperature of 1100°k from an initial state near the normal boiling point, 20·5°k and 14·1 cm3/g. How far the melting curve and the equation of state can be pursued by shock wave experiments is discussed. The derived intermolecular potential between two hydrogen molecules is shown to be inadequately represented by the previously postulated Lennard-Jones 6–12 potential in the repulsive region between 2·0 and 2·7 A. The power of the inverse intermolecular distance is shown instead to be 8·5 in that region if a purely repulsive potential is used.

Classification of the states of non-rigid molecules

Abstract: An examination is made of the use of the symmetry group for non-rigid molecules introduced by Longuet-Higgins A detailed consideration of the energy eigenvectors based on an analysis of intramolecular tunnelling justifies the use of this symmetry group for the purpose of determining the statistical weights and the selection rules for radiative transitions

Tautomerie von 2-Picolylketonen

Abstract: Es wurden die hochaufgelosten N.M.R.-Spektren der tautomeren Gemische von 2-Picolylisopropyl- und 2-Picolylphenylketon exakt analysiert. Auser den Linien der Keto- und Enolformen konnten keine zusatzlichen Linien, die einer weiteren tautomeren Form zuzuschreiben sind, beobachtet werden.

Virial expansion for the radial distribution function of a fluid using the 6 : 12 potential

Abstract: The radial distribution function can be expressed in a virial expansion. Using the 6 : 12 potential the second-order density coefficient, g 2(r), is numerically calculated for a wide range of temperatures and intermolecular separations. These results are used to calculate the second-order density coefficient, c 2(r), in the expansion of the direct correlation function and to calculate the fourth virial coefficient, B 4. In addition, approximate results for g 2(r), c 2(r), and B 4 are calculated on the basis of the Percus-Yevick, hypernetted chain, and the self-consistent approximations of Hurst and Rowlinson. These approximate results are compared with the exact results. The Percus-Yevick theory is in good agreement with the exact results at high temperatures but is unsatisfactory at low temperatures. The hyper-netted-chain approximation is in fair agreement with the exact results at high temperatures, is in poor agreement at intermediate temperatures, but is in good agreement at low temperatures. The sel...

Forces in the benzene crystal

Abstract: The total lattice energy of benzene is evaluated by summation of pair interactions in the crystal The relative contributions of the σ and π electrons to the attractive part of the energy are separated out explicitly: the π electrons contribute 40 per cent of this energy (although they represent only 20 per cent of the valence electrons) However the three component σ-σ, σ-π, and π-π dispersion energies are of the same order of magnitude The effect of σ-π and π-π electron correlation is also investigated The total lattice energy agrees well with the available experimental heat of sublimation data

The structure of Coppinger's radical and its orientation within a cluster of a liquid crystal

Abstract: The electron resonance spectrum of Coppinger's radical has been measured throughout the nematic range of p-azoxyanisole and above the nematic-isotropic transition point. Because the proton hyperfine tensors are not cylindrically symmetric about an axis perpendicular to the molecular plane it is possible to deduce not only the mole fraction of radical in the cluster but also its probable orientation within the cluster. Although the results have been interpreted within the framework of the swarm theory we could equally well have used the distortion hypothesis.

A proton magnetic resonance investigation of biphenyl

Abstract: The proton magnetic resonance spectrum of biphenyl has been observed in dilute solution with d-chloroform and tetramethylsilane as the solvents. Theoretical analyses of these spectra were carried out with the aid of additional data provided by several deuterated biphenyls. A limited amount of data was also obtained for p-terphenyl and fluorene to assist in the interpretation. Estimates of the additional anisotropy contributions at each position from the second ring were calculated as a function of the dihedral angle of biphenyl. The results obtained are interpreted assuming a freely rotating model with weak interaction between phenyl rings.