A study of the transition state for the unimolecular decomposition of a triatomic molecule

A quantitative examination of the transition state model of unimolecular reactions has been made using an explicit three-dimensional potential. The results obtained using RRKM theory have been applied to the three principal experimental areas of unimolecular decay for the decomposition of water. The results of shock-tube studies of the decomposition of water imply that one of the rotational degrees of freedom is active in energy randomization. The consequences of assuming tight and loose transition states for long-lived complexes in molecular beam studies have been examined. The distribution of energy amongst the products of reaction in a mass spectrometric decay has been calculated and the importance of the initial rotational energy on this is emphasized.     

Computer study of the hydrogen atom recombination reaction under high pressure conditions

The hydrogen-atom recombination reaction has been simulated using a molecular dynamics technique recently formulated by the authors [1]. The rate of recombination has been calculated over a range of temperatures and inert gas concentrations (He and Ar) and agrees well with available experimental data. The calculations reproduce the negative activation energy characteristic of an atom recombination process. Over the range of conditions studied recombination was found to proceed via the energy transfer mechanism only, no evidence of bound HAr or HHe species was observed. Recombination was found to occur through an intermediate metastable diatomic molecule which is in equilibrium with its environment and from which there is a bottleneck to the formation of a stable molecule. The initial formation of a metastable species is sensitive to the hydrogen-inert gas potential, but relaxation of the total energy is primary determined by the mass of the third-body and the collision frequency.     

Exact statistical mechanical lattice model and classical Lindemann theory of melting of inert gas solids

A modified form of Lindemann’s model shows that the melting points of the heavy inert gases and other effectively spherical molecular species are proportional to the depths of their diatomic potential wells. The success of the model when compared with experiment seems to rely on the almost constant value of the ratio of the fractional volume and entropy changes during fusion. The Lindemann proposal can be incorporated into an exactly treated statistical mechanical lattice model utilising expandable clusters which reproduces the solid–liquid melting phenomenon for argon with a realistic volume change and melting line.     

The effect of solvent on the position of the first absorption band of nitrobenzene

The weak n → π* absorption band of nitrobenzene, which occurs in the region 25000–33000 cm^?1, has been measured in the gas phase, in perfluorohexane and in other solvents. The reported red-shift in this band in polar solvents is shown to be due to an increase in the underlying intensity of the π → π* bands whose first maximum is at 34750 cm^?1.     

Finite analytical expressions for two-centre exchange integrals between having the same exponents

Closed analytical expressions are derived for some two-centre exchange integrals between Slater orbitals. Integrals involving 1s, 2s and 2p orbitals are considered with the restriction that the two orbitals have the same exponent. An expansion formula accurate for large values ofR is also derived.

---

Zusammenfasung Für eine Reihe von Zweizentren-Austauschintegralen zwischen Slaterorbitalen werden geschlossene analytische Ausdrücke mitgeteilt, wobei allerdings nur ls-, 2s- und 2p-Orbitale mit gleichen Exponenten behandelt werden. Schließlich wird noch eine asymptotische Entwicklung für großeR angegeben.

---

Resumé Obtention d'expressions analytiques compactes pour certaines intégrales d'échange bicentriques entre orbitales de Slater. On considère des intégrales impliquant des orbitales ls, 2s et 2p avec comme restriction l'égalité des exposants orbitaux. Un développement valable pour les grandes valeurs deR est aussi obtenu.

---

---

On leave from the Institute Ruder Boskovi, Zagreb, Jugoslavia     

A comparative study of species N3 and P3

Calculations on linear and bent structures of N₃ and P₃ show that these species are quite different. N₃ is linear, P₃ is bent almost to a D_3h geometry. The symmetry of P₃ in D_3h is ²E″ but the Jahn-Teller distortion is very small, ≈4°. The many-body expansion of the energy of P_n clusters appears to be only slowly convergent.     

Pseudo-steady-state solutions for solidification in a wedge

Polubarinova-Kochina's analytical differential equation methodis used to determine the pseudo-steady-state solution to problemsinvolving the freezing (solidification) of wedges of liquidwhich are initially at their fusion temperature. In particular,we consider four distinct problems for wedges which are: freezingwith the same constant boundary temperature, freezing with thesame constant boundary heat fluxes, freezing with distinct constantboundary temperatures and freezing with distinct constant fluxesat the boundaries. For the last two problems, a Heun's differentialequation with an unknown singularity is derived, which in bothcases admits a particularly elegant simple solution for thespecial case when the wedge angle is . The moving boundariesobtained are shown pictorially.     

Quantum mechanical calculations on collinear reactive collisions over potential wells

Quantum mechanical calculations are reported for collinear reactive collisions on a number of surfaces with potential wells. The density and percentage of resonance states which were found have been related to the eigenvalues of the close-coupling equations in which only asymptotically closed channels were included. The resonant behaviour leads to a highly structured state-to-state transition probability but in no case does it dominate the reaction probability as would be required for a statistical theory to be applicable. This is probably due to the fact that the systems studied have only one internal degree of freedom.