Thermodynamic properties of clathrates: I. The heat capacity and entropy of argon in the argon quinol clathrates

Measurements have been made of the heat capacity C_p from ～ 13°k to ～ 273°k of five clathrates of argon and β-quinol. The argon content ranged from ～ 20 per cent to ～ 80 per cent of the maximum possible amount. Over much of the temperature range studied, C_p proved to be a linear function of the argon content, but from 13°k to 20°k, and from 50°k to 100°k the relation between C_p and argon content is obscure, and may, in fact, be non-linear. Estimates have been made of the contribution to C_p made by a mole of argon in the temperature region where C_p is a linear function of composition, and these experimental values have been compared with those calculated according to the theory of J. H. van der Waals, which is based on the cell model of Lennard-Jones and Devonshire. The agreement between theory and experiment is satisfactory.     

Intramolecular excitation transfer. The lowest n→π* transitions in pyrazine

In the pyrazine molecule, which has filled non-bonding orbitals para to each other, two orbitally degenerate n→π* transitions are expected, one symmetry forbidden and one symmetry allowed. Interelectronic interactions remove this degeneracy. The lowest pair of (n, π*) triplet states and the corresponding pair of singlet states are studied with a view towards determining the magnitude of such interactions and the ordering of the forbidden and allowed components. Absorption spectra are obtained of pyrazine in crystalline hydrogen and rare gases at 4·2°k, of pure crystalline pyrazine at 4·2°k, and of 42 metres of pyrazine vapour at various pressures. The phosphorescence spectrum of pyrazine in crystalline rare gases at 4·2°k also is studied. The splitting between the two singlet components is found to be approximately 435 cm^?1 with the forbidden component lying lowest. The forbidden singlet-singlet transition gains some of its intensity through vibronic mixing with a (π, π*) state, but vibronic coupling between the two (n, π*) states also may be present. The strongest part of the singlet-triplet absorption spectrum is found to involve the same upper state as the phosphorescence spectrum, and the transition is shown to be symmetry allowed. It is strongly suggested that the ordering of the allowed and forbidden components of the triplet state is inverted from that of the singlet, or the two states may lie very close together. Using an ‘independent systems’ model, a calculation of the splitting, with the inclusion of exchange is made. The theory indicates that the singlet states are indeed split by a coulomb term having roughly the expected magnitude with the forbidden component lying lowest; in addition, there is a very small exchange term with the opposite sign. The splitting between the triplets is shown to involve only the small exchange term, and the order of the allowed and forbidden components becomes reversed. There is no indication of pyrazine fluorescence at 4·2°k in any of the solids used. This fact illustrates the high efficiency of the singlet-triplet radiationless process even under these conditions where the rate might be expected to be a minimum for condensed phases. An interesting alternation of spacing has been discovered in the first few quantum levels of the 600 cm^?1 a _1g ring-bending vibration of the ground electronic state of the molecule.     

The anomalous temperature dependence of the sound velocity in water

The quantum yields of naphthalene vapour fluorescence at 225°c and of phenanthrene vapour fluorescence at 365°c excited by the Hg 313 μ line are independent of concentration up to 0·014 and 0·007 moles/l. respectively; this is attributed to fast dissociation relaxation of the excimer at these temperatures.

The fluorescence of naphthacene vapour excited at temperatures of 355–435°c by the group of Hg lines at 365 μ decreases with increasing pressure at pressures below that at which absorption of the incident radiation is virtually complete, and is attributed to a combination of self-quenching and reabsorption of fluorescence. An analysis of the data for the limiting cases of complete and negligible fluorescence reabsorption provides upper and lower limits for self-quenching constant which are consistent with unit collisional quenching efficiency and a lifetime of 5·3 ± 2·2 + 10^-9 sec at 355°c.     

Paramagnetic resonance in phosphorescent aromatic hydrocarbons

Electron resonance spectra have been recorded of the lowest triplet states of benzene, toluene, triptycene and tribenzotriptycene. The experiments were carried out by ultra-violet irradiation of these molecules in rigid glass solutions both at 77°k and 20°k. From an analysis of the line shape of the spectra, which prove to be strongly temperature dependent, the following conclusions could be drawn.

Benzene in its first triplet state has a configuration of lowest energy which is not a regular hexagon. This finding, supported by the results for toluene, was substantiated by SCF-MO calculations; it is in agreement with a recent analysis by Liehr and Moffitt. Tunnelling between equivalent conformations of benzene is thought to occur at a rate of the order of 10⁹–10¹⁰ sec^-1.

The spectra of the phosphorescent states of triptycene and tribenzotriptycene are dominated by intramolecular excitation transfer between the benzene or naphthalene sub-systems. In triptycene at 77°k transfer occurs at a rate ? 10¹⁰ sec^-1, so that one observes the resonance signal of a triplet exciton. In tribenzotriptycene the transfer is somewhat slower. The rate of excitation transfer decreases with temperature and at 20°k most of the excitation in tribenzotriptycene is effectively trapped in the naphthalene sub-systems.     

Improved one-centre wave functions for the hydrogen and methane molecules

Corrections are made to the Joy-Parr hydrogen and the Saturno-Parr methane one-centre wave functions and the functions are accurately reminimized with respect to the energy. The electronic wave functions and total molecular energies are obtained for various internuclear distances and the equilibrium internuclear distance and the breathing force constant determined. Three methods for calculating the force constant are given and compared. The equilibrium bond length for hydrogen is found to be 1·38 a.u. (experimental, 1·40 a.u.) and the force constant 6·33 md/å (experimental, 5·75 md/å). For methane the equilibrium C-H distance is found to be 2·014 a.u. (experimental, 2·05 a.u. and the force constant 25·8 md/å (experimental, 23·5 md/å). The total computed molecular energies for the equilibrium configurations of hydrogen and methane are -1·1605 a.u. (experimental, -1·175 a.u.) and -39·8444 a.u. (experimental, -40·51 a.u.) respectively.     

Application of Brønsted's principle of congruence to n-alkane mixtures

The second virial coefficient of normal methane, ¹²CH₄, has been compared with that of the corresponding deutero compound ¹²CD₄, as well as with the other isotopic molecule, ¹³CH₄. These measurements were performed by means of a differential apparatus over a range of temperature extending from 110°k up to 300°k.     

Potential energy surfaces for the Walden inversion reaction

A semi-empirical valence bond calculation of potential surfaces for the gas phase reaction between methyl bromide and chloride ion is described. The system is treated in a four participant electron approximation with precautions to include the effects of orthogonalization to the core electrons not involved in bond formation during reaction. The core is taken to include the methyl group as a single fragment whose valence state energy is determined by hybridization at the carbon atom. The qualitative features of the surface are confirmed by a less extensive CNDO calculation.

The lowest potential surface is found to have a central dip, the minimum energy being 0·98 ev below the energy of the reaction products when the C-Cl distance is 3·60 a.u., the C-Br distance 4·11 a.u., and the H-C-Br angle 86°. Analogy with classical two-body scattering suggests that at incident energies of less than 2 ev, the reaction cross section is of the order of 10(1 + 1/E) å ², where E, the initial energy of relative motion, is in ev.     

An attempt to find a connection between the atomic distribution in liquid and solid metals

The distance of nearest neighbours r ₁, as well as the ‘reduced’ densities extrapolated to 0°k of liquid and solid metals are compared. Some liquids are close-packed at 0°k. With increasing temperature the degree of atomic misfit increases while r ₁ stays constant or even decreases, contrary to the behaviour in the solid phase.     

Rotational band contours in the 5000 Å 1 B 1g -1 Ag electronic system of p-benzoquinone

Using the technique of computer simulation of rotational band contours the 1–0 band in v ₇, band E, in the 5000 Å ¹ B _1g -¹ A_g system of p-benzoquinone has been rotationally analysed. It is a type A band and the excited state rotational constants are:

The excited state inertial defect determined from these constants is -0·8 ± 0·2 uÅ ². This value is almost certainly due not to non-planarity of the excited state but to a Coriolis interaction between v ₇ and perhaps the b _1u vibration v ₁₃. Such an interaction, if it were weak, would affect only the A′ rotational constant.

Previous assignments [2] of other type A bands and type B bands in the spectrum are reviewed where possible with the new evidence of the computed contours and the assignments remain largely unaltered.     

Free radicals in pyrimidines: E.S.R. of a γ-irradiated single crystal of 5-nitrouracil

Free radicals are observed in γ-irradiated single crystals of 5-nitrouracil with the unpaired electron showing hyperfine interaction with one nitrogen atom. The principal values of hyperfine coupling are A_x = 22·5 g, A_y = 25·2 g, and A_z = 40·0 g, and the principal values of the spectroscopic splitting factor are g_u = 2·0117, g_v = 2·0064 and g_w = 2·0027. The relationship of the directions of the corresponding principal axes to the molecular orientations show that the unpaired electron must be located in an sp ² orbital on either N₍₁₎ or N₍₅₎. Considerations of the mechanism of radical formation and comparison to radiation damage in other molecules make the N₍₁₎ location seem more probable. The π interaction of the nitro group on C₍₅₎ evidently prevents the formulation of free radicals with the unpaired electron on C₍₅₎. That carbon atom is the most common location of unpaired electron density in other pyrimidine free radicals.     

Ab initio calculation of force constants and equilibrium geometries in polyatomic molecules

The force constants and equilibrium geometry of water were calculated ab initio from Hartree-Fock self-consistent field wavefunctions using the force method, and a 53/31, a 73/3 + 1 and a 95/41 + 2 gaussian lobe basis set. The force method proved to be very economic and numerically accurate.

The calculated values agree well with the experimental ones. Especially good is the agreement for the interaction constant F_rα . Calculated force constants for the 95/41 + 2 basis are: F_r = 9·158 mdyn/å, F_α = 0·8513 mdyn/å, F_rα = +0·3007, F_rr = -0·1724; the experimental values from isotope frequencies are: F_r = 8·456, F_α = 0·762, F_rα = +0·246, F_rr = -0·100 mdyn/å. The agreement in the geometry is also good for the 95/41 + 2 basis r _e = 0·9518 å, α = 107·49°, and the experimental r _e = 0·9572 å, α = 104·52°.

Force constants were found to be quite insensitive to variation in the basis functions, all three sets giving almost the same result. On the other hand, the dipole moment derivative is very sensitive.     

50 years at the forefront of Physics

Mathemtical Techniques and Physical Applications. By J. Killingbeck and G. H. A. Cole. (Amdemic Press, 1971.) [Pp. xiv + 715.] £7£00. Scope: Textbook; reference. Level: Postgraduate; undergraduate.

Group Theory in Solid-state Physics. By H. W Streitwolf (Macdonald, 1971.) [Pp. 248.] £5·00. Scope: Textbook. Level: Postgraduate.

Statistical Physics. By Ya. P. Terletskii. (North-Holland, 1971.) [Pp xv+279.] $6·30. Scope: Textbook. Level: Postgraduate; undergraduate.

Ubsersichtsbeiträge zur Gasdynamik. Edited by E. Leiter and J. Zierep. (Springer-Verlag, 1971.) [Pp. 386.1 $40·70. Scope: Treatise. Level: Specialist/postgraduate.

Classical Theory of Particles and Melds. Vol. I and Vol. II. By K. H. Ruei. (University Press, Taiwan, Republic of China, 1971.) [Pp.: Vol. I, xxiii + 359; Vol. II, xxvi + 424.] Scope: Textbook. Level: Postgraduate; undergraduate.

Topics in Mathematical Physics. Vol. II: Spectral Theory and Wave Processes. By M. Sh. Birman. (Consultants Bureau, 1971.) [Pp. v + 121.] $14·00. Scope: Treatise. Level: For specialists only.

An Introduction to the Theory of Electromagnetic Waves. By A. C. Hewson. (Longman, 1971.) [Pp. viii+ 115.] £ 1·60. Scope: Textbook. Level: Undergraduate.

Modern Physics and Quantum Mechanics. By Elmer E. Anderson. (W. B. Saunders Co. Ltd., 1971.) [Pp. xi+430.] £6·20. Scope: Textbook. Level: Undergraduate.

Polarized Light and Optical Measurement. By Clarke and Grainqer. (Pergamon Press, 1971.) [Pp. viii+ 187.] £3·50 ($9·50.). Scope: Treatise. Level: Specialist/postgraduate.

Phenomena in Ionized Gases 1971. Edited by R. N. Franklin. (Donald Parsons, 1971.) [Pp. xl+450.] £12·00.

Technology Today. Edited by Edward de Bono. (Routledge &; Kegan Paul Press, 1971.) [Pp. xii+ 144.] Cloth edition £1·75; Paperback 50p. Scope: Survey. Level: General reader.

Diffusion Data. No. 1. Edited by DR. F. H. Wohlbier. (Trans Tech Publications, 1971.) [Pp. 289.] Scope: Library (reference). Level: Specialist/postgraduate.     

Reactions of excited atoms and molecules with atoms and molecules

Ionizing collisions of long lived excited particles with atoms and molecules are studied by a cross beam technique. For the first time reactions of atoms in high Rydberg states are included in the investigation. In this paper we report relative cross sections for the production of the ions RH⁺, RH ₂ ⁺ , and H ₂ ⁺ by collisions of excited rare gas atoms R^* with H₂. With HD as the target molecule the isotope effect for the production of RD⁺ and RH⁺ has been determined. In the case of argon and krypton, ions are produced only by the high Rydberg states, whereas in the case of helium and neon only the metastable states contribute to a measurable extent. The data indicate, that the reaction mechanism is different in principle for metastable and highly excited atoms. Simple models are proposed to explain the experimental results.     

Spectrophotometric Studies of the Energy Consumed During Oxidative Degradation of D-Fructose

ABSTRACT

Mechanistic investigation of the oxidative degradation of d-fructose (D-Fruc) has been studied by spectrophotometric technique. Molecular mechanics (MM +) calculations suggest that the potential energy (PE/kcal mol^?1) of the d-fruc (opening structure) is at least three (3.71) times more stable than the PE of the cycling structure of the same matrix. The oxidation constant (K _ox) of the anionic form of the d-Fruc (Fruc-NaOH) is about seven times greater than that of the protonated form (Fruc-H₂SO₄). Therefore, the anionic form is more highly oxidizable than is the cationic form of this matrix. The limit of detection can be as low as 18 ppm (mg L^?1) of d-Fruc. This is about 60 times lower than the blood sugar level (BSL) or 100 times lower than that reported previously. The proposed procedure was applied successfully for the oxidation of D-Fruc in uni-fructose powder. The anionic form of D-Fruc (Fruc-NaOH) has the ability to store energy about 744.72 kJ g^?1 h at 608 nm in a condensed lightweight form. Kinetic parameters of the oxidative degradation of the anionic form of D-Fruc at different concentration were deduced. A number of models were used to evaluate the kinetic parameters. The mechanism of the degradation of D-Fruc is explained on the basis of kinetic parameters.     

Assignment of a perturbation in the FT-ICLAS spectrum of 12C2H2 around 12 709.5 cm−1

The vibrational state perturbing the J = 17 and 18 rotational states of the zero-order v ₁ + 3v ₃ state of ¹²C₂H₂ is assigned to the state with vibrational energy predicted at G _ν = 12 685.1 cm^?1 using the cluster model (El Idrissi, M. I., Liévin, J., Campargue, A. and Herman, M., 1999, J. chem. Phys., 110, 2074). The assignment is discussed also in terms of the very special pressure shift behaviour demonstrated previously for absorption lines reaching these levels (Herregodts, F., Hepp, M., Hurtmans, D., Vander Auwera, J. and Herman, M., 1999, J. chem. Phys., 111, 7961). The experimental information arising from a set-up newly running at ULB, called FT-ICLAS brings decisive information in the assignment process. This set-up is described briefly.     

The absorption spectrum of 12C2D2 in the 10000–12500 cm−1 spectral region

The absorption spectrum of dideuteroacetylene has been recorded by intracavity laser absorption spectroscopy (ICLAS) in the 10 200–12 500cm^?1 spectral region. Among 25 absorption bands of ¹²C₂D₂ rotationally analysed in this spectral region, 17 are newly observed. They include one II_u-Σ⁺ _g and thirteen Σ⁺ _u-Σ⁺ _g bands starting from the vibrational ground state and eleven hot bands from the V ₄ = 1 and V ₅ = 1 lower states. The rotational structure of two excited levels is affected by a strongly J-dependent interaction with a perturber which induces intensity transfer to extra lines. The coupling is identified as a I-resonance interaction with δ_u dark states and the vibrational assignment of the perturbers is discussed. Two Σ-Σ bands of the ¹²C¹³ CD₂ species, present in natural abundance in the sample, could also be identified and rotationally analysed. Most of the corresponding excited vibrational levels of ¹²C₂D₂ were unambiguously assigned using the polyad model [Herman, M., el idrissi, M. I., Pisarchik, A., Campargue, A., Gaillot, A.-C., Biennier, L., di lonardo, G. and Fusina, L., 1998, J. chem. Phys., 108, 1377] which allows vibrational energies and B _V rotational constants to be predicted. In particular the previously highlighted 1/244 anharmonic resonance is confirmed by energy and intensity features in several {(V ₁, V ₂, V ₃, V ₄ = 0, V ₅ = 0),(V ₁ ?1, V ₂ + 1, V ₃ V ₄ = 2, V ₅ = 0)} dyads. Significant deviations between predicted and experimental energy levels are observed for a few levels and discussed.     

A re-examination of the I.R. spectrum of dinitrogen trioxide

Assuming a planar geometry of C _s symmetry and using a Urey-Bradley potential field, the force constants of dinitrogen trioxide have been re-determined within the formalism of Wilson's G-F matrix method. The outcome of the present investigation suggests a new set of force constants some of which are drastically (～ 50 per cent) different than those previously reported. Furthermore, since the i.r. frequencies used in the present investigation were obtained both in the solid state and at low temperatures, the mean amplitudes of thermal vibrations of both ¹⁴N₂O₃ and ¹⁵N₂O₃ are given at 77°k. Finally, it is suggested that more isotopic i.r. data is necessary to ascertain unequivocably the geometry of dinitrogen trioxide.     

Reflections on Ernest Roy Davidson: the UW years 1962–1984

In the following research acetylation as an unexplored factor in the anomeric effect in carbohydrate chemistry has been examined. Crystallographic data for methyl glycosides and their acetates have been compared and discussed. Some of the methyl glycosides form hydrogen bonding with the participation of acetal oxygen atoms. This seems to have the most significant influence on the structural diagnostic parameters for anomeric effect.

Abbreviations: Me-α-Glc: methyl α-D-glucopyranoside; Me-β-Glc: methyl β-D-glucopyranoside; Me-α-Gal: methyl α-D-galactopyranoside; Me-β-Gal: methyl β-D-galactopyranoside; Me-α-Man: methyl α-D-mannopyranoside; Me-β-Man: methyl β-D-mannopyranoside; Ac-Me-α-Glc: methyl 2,3,4,6-tetra-O-acetyl-α-D-glucopyranoside; Ac-Me-β-Glc: methyl 2,3,4,6-tetra-O-acetyl-β-D-glucopyranoside; Ac-Me-α-Gal: methyl 2,3,4,6-tetra-O-acetyl-α-D-galactopyranoside; Ac-Me-β-Gal: methyl 2,3,4,6-tetra-O-acetyl-β-D-galactopyranoside; Ac-Me-α-Man: methyl 2,3,4,6-tetra-O-acetyl-α-D-mannopyranoside; Ac-Me-β-Man: methyl 2,3,4,6-tetra-O-acetyl-β-D-mannopyranoside; GIPAW (Gauge Including Projector Augmented Waves) calculations: a DFT based method used for calculating nuclear magnetic resonance parameters; CP/MAS NMR: cross-polarisation (CP) magic angle spinning (MAS) NMR spectroscopy; δ_ss: chemical shift in ¹³C CP/MAS NMR spectrum; δ_t: theoretical chemical shift: as derived from GIPAW DFT; dis: distorted multiplet in ¹H NMR spectrum.     

Structure of the methyl radical

Carbon-13 hyperfine splittings equal to 41±3 gauss have been observed in the paramagnetic resonance of a mixture of C¹²H₃ and C¹³H₃ radicals produced by x-irradiation of CH₃I at 77°k. The observed splitting provides strong evidence that CH₃ is a planar molecule.     

The correlation of molecular orientation in liquid water by neutron and X-ray scattering

The neutron diffraction structure factor for heavy water at 22°c has been measured for momentum transfers, ?Q, up to a Q of 13 å ^-1. The results have been interpreted by abstracting the molecular structure effects as far as possible and using models for the correlation of orientation of pairs of molecules ranging from completely uncorrelated orientation to that for adjacent molecules in ice I. Neither these nor any of the popular models for the structure of water, nor a non-hydrogen bonded model, fit the neutron diffraction data over the whole range of Q. Nevertheless, information about the molecular-centres structure factor is obtained, especially for Q up to about 3 å ^-1.

The method of analysis is generalized and applied to x-ray diffraction data and in particular to x-ray data for liquid water. It is shown that this method of analysis has some advantages over the conventional one. The effect of correlation of molecular orientation is less than that of molecular structure on x-ray scattering by water and rather more extensive information is obtained for the molecular-centres structure factor than from neutron data—if the conventional assumption of atomic electron clouds is accepted.

A comparison of the x-ray and neutron scattering data for liquid heavy water shows that the two sets of data are reasonably consistent for any of the usual models for water, except for the region of Q between 3 and 5 å ^-1. The molecular-centres structure factor of water has a weak double humped first maximum with peaks at 2·0 å ^-1 and 2·9å ^-1, probably followed by a damped oscillation. This contrasts with atomic liquids but is similar to that found in liquid carbon tetrachloride. The structure of liquid water is not, therefore, necessarily unique or unusual.