J.N. Murrell

Symmetries of activated complexes

Previous work has indicated that in calculating rates using activated-complex theory one should omit symmetry numbers from the rotational partition functions and multiply by a statistical factor; this factor is the number of equivalent forms of the activated complex that can arise from the reactants. This procedure can lead to error if the activated complex is chosen to have such a high degree of symmetry that it can form more than one form of the reactants or products. It is shown from a consideration of multi-dimensional potential-energy surfaces that a single activated complex cannot exist at the intersection of two valleys; three valleys, for example, will give rise to three separate activated states, each of which represents the lowest pass between a pair of valleys. The implications of this conclusion are considered with reference to several reactions, including that between hydrogen and iodine.

Intermolecular Forces in the Region of Small Orbital Overlap

The perturbation method of Yaris for the case where the unperturbed wavefunction is not an eigenfunction of the unperturbed Hamiltonian is developed to give an expression for intermolecular energies in the region of small orbital overlap. The resulting expressions are the same as these obtained earlier using a more traditional aproach, but the derivation does not suffer the lack or rigor in this earlier treatment.

Calculation of the intensities of the vibrational components of the ammonia ultra-violet absorption bands

The intensities of the vibrational components of the A ← X and B ← X band systems of NH3 have been calculated from vibrational functions appropriate to the best one-dimensional potential curves for the inversion mode. For the A ← X system the experimental data is fitted by an expression: The Franck-Condon approximation in which the Q 2 matrix element is neglected gives a poor fit to the data. We deduce that the electronic transition moment increases as the planar molecule is bent. For the B ← X system, which is a forbidden electronic transition, the experimental data are fitted well by the matrix element 2 which arises from the leading term in the Taylor expansion of the electronic transition moment. Using the same potential functions for ND3, and a reduced mass based on a pure bending mode, gives good agreement for the intensities of the corresponding ND3 bands. New experimental values for the extinction coefficients of the A ← X system have been obtained.

Frequency optimized potential energy functions for the ground-state surfaces of HCN and HCP

Abstract Potential energy functions for the ground states of the linear triatomics HCN, HNC ( X 1 Σ + ) and HCP ( X 1 Σ + ) were derived, by minimizing the difference between the observed vibrational frequencies and those calculated from the potentials by a variational method. Good agreement is obtained between the observed and calculated spectra for HCN and HNC. For the HCP system, a secondary minimum is predicted to lie on the ground-state surface (linear HPC) having dimensions R HP = 1.43 A , R CP = 1.56 A , and an energy of 3.8 eV above the HCP ( X 1 Σ + ) minimum.

Theoretical study of the O(1D) + H2(1Σ g +) reactive quenching process

Classical trajectory calculations have been made for the system O(1 D) + H 2(1Σ g +) →OH(2Π) + H(2 S) using an analytical singlet ground state surface for H 2O. A rate constant of 1·73 × 10-10 cm3 molecule-1s-1 at 300 K has been obtained. The distribution of energy in the products is approximately 30 per cent in translation, 45 per cent in vibration and 25 per cent in rotation. Because of the preponderance of vibrationally long-lived trajectories, statistical theories gave a good interpretation of the gross features of the reaction.

The Intensities of the Symmetry-forbidden Electronic Bands of Benzene

The molecular orbital method is used to calculate the intensities of the symmetry-forbidden bands of benzene. Both the bands at 2650 A and 2000 A derive intensity by interaction with the upper state of the 1800 A band. The states of symmetry B1u and B2u are made accessible by e2g vibrations of the benzene framework at 606 cm-1 and 1595 cm-1. The 606 cm-1 vibration is most important in making the transition to the B2u state (at 2650 A) allowed, whereas that at 1595 cm-1 is most important for the A1g → B1u transition. The alternative assignments B1u and E2g are considered for the 2000 A band, but the calculated total intensities cannot differentiate between them.

Effect of Overlap on Dispersion Energies near the van der Waals Minimum

It is shown that a single excited configuration with optimized ζ can give accurate values of the dispersion‐energy coefficients of H2 and He2. The resulting wavefunctions are then used to obtain the effect of overlap on dispersion energies. It appears that this correction is small in the region of the van der Waals minimum.

Importance of Doubly Excited Configurations in the Interpretation of Electronic Spectra

An investigation has been carried out to determine the importance of doubly excited configurations in calculating the energies of the electronic states of an aromatic hydrocarbon. It is found that the calculated low energy states of benzene are largely unaffected by consideration of doubly excited states, except for the appearance of an E2g state with energy comparable to that of the E1u states. In addition, inductive and mesomeric shifts in the lowest energy (α) transition have been calculated for all the distinct positions of substitution in benzene, in terms of two parameters which depend on the substituent. It is seen that the inclusion of doubly excited states in the calculation is necessary in order to predict these effects reliably.

Analytical potentials for triatomic molecules

Analytical potential energy functions which are valid at all dissociation limits have been derived for the ground states of SO2 and O3. The procedure involves minimizing the errors between the observed vibrational spectra and spectra calculated by a variational procedure. Good agreement is obtained between the observed and calculated spectra for both molecules. Comparisons are made between anharmonic force fields, previously determined from the spectral data, and the force fields obtained by differentiating the derived analytical functions at the equilibrium configurations.

The photoelectron spectra of the halomethanes

The photoelectron bands associated with ionization from the formally non-bonding p orbitals of the halogen atoms in the halomethanes have been interpreted in terms of a pseudo one-electron hamiltonian. Account has been taken of interactions between the halogen atoms, of interactions between the halogen p orbitals and the σ-bonding orbitals, and of spin-orbit coupling. This model leads to a systematic assignment of all the bands of the chloro and bromomethanes, and gives a satisfactory account of the spin-orbit splittings in the bromomethanes.