Multiconfigurational quantum chemical methods (CASSCF/CASPT2) have been used to study the chemiionization reactions Ce + O → CeO⁺ + e^- and Ce + O₂ → CeO₂⁺ + e^-. Selected spectroscopic constants for CeO_n and CeO_n⁺ (n = 1, 2), as well as reaction enthalpies of the chemiionization reactions of interest, have been computed and compared with experimental values. In contrast to the lanthanum case, for both Ce + O₂(X³Î£_g^-) and Ce + O₂( a¹Î”_g), the Ce + O₂ → CeO₂⁺ + e^- reaction is shown to be exothermic, and thus, contributes to the experimental chemielectron spectra. The apparent discrepancy between the computed reaction enthalpies and the high kinetic energy offset values measured in the chemielectron spectra is rationalized by arguing that chemielectrons are produced mainly via two sequential reactions (Ce + O₂ → CeO + O, followed by Ce + O → CeO⁺ + e^-) as in the case of lanthanum. For Ce + O₂ (a¹Î”_g), a chemielectron band with higher kinetic energy than that recorded for Ce + O₂( X³Î£_g^-) is obtained. This is attributed to production of O( ¹D) from the reaction Ce + O₂( a¹Î”_g) → CeO + O( ¹D), followed by chemiionization via the reaction Ce + O( ¹D) → CeO⁺ + e^-. Accurate potential energy curves for the ground and a number of excited states of CeO and CeO⁺ have been computed, and a mechanism for the chemiionization reactions investigated experimentally was proposed.

The La + O and La + O₂ chemiionization reactions have been investigated with quantum chemical methods. For La + O₂(X³Î£_g) and La + O₂(a¹Î”_g), the chemiionization reaction La + O₂ → LaO₂⁺ + e^− has been shown to be endothermic and does not contribute to the experimental chemielectron spectra. For the La + O₂(X³Î£_g) reaction conditions, chemielectrons are produced by La + O₂ → LaO + O, followed by La + O → LaO⁺ + e^−. This is supported by the same chemielectron band, arising from La + O → LaO⁺ + e^−, being observed from both the La + O(³P) and La + O₂(X³Î£_g) reaction conditions. For La + O₂(a¹Î”_g), a chemielectron band with higher electron kinetic energy than that obtained from La + O₂(X³Î£_g) is observed. This is attributed to production of O(¹D) from the reaction La + O₂(a¹Î”_g) → LaO + O(¹D), followed by chemiionization via the reaction La + O(¹D) → LaO⁺ + e^−. Potential energy curves are computed for a number of states of LaO, LaO* and LaO⁺ to establish mechanisms for the observed La + O → LaO⁺ + e^− chemiionization reactions.

Matrix isolation infrared (IR) studies have been carried out on the vaporisation of the alkali-metal azides MN₃ (M = Na, K, Rb and Cs). The results show that under high vacuum conditions, molecular KN₃, RbN₃ and CsN₃ are present as stable high-temperature vapour species, together with variable amounts of nitrogen gas and the corresponding metal atoms. The characterisation of these molecular azides is supported by ab initio molecular orbital calculations and density functional theory (DFT) calculations, and for CsN₃ in particular, by the detection of the isotopomers Cs(¹⁴N¹⁵N¹⁴N) and Cs(¹⁵N¹⁴N¹⁴N). The IR spectra are assigned to a "side-on" (C_2v) structure by comparison with the spectral features predicted both by vibrational analysis and calculation. The most intense IR features for KN₃, RbN₃ and CsN₃ isolated in nitrogen matrices lie at 2005, 2004.4 and 2002.2 cm^-1, respectively, and correspond to the N₃ asymmetric stretch. The N₃ bending mode in CsN₃ is identified at 629 cm^-1. An additional feature routinely observed in these experiments occurred at approximately 2323 cm^-1 and is assigned to molecular N₂, perturbed by the close proximity of an alkali-metal atom. The position of this band appeared to show very little cation dependence, but its intensity correlated with the extent of sample thermal decomposition.

The U+O chemi-ionization reaction has been investigated by quantum chemical methods. Potential-energy curves have been calculated for several electronic states of UO and UO⁺. Comparison with the available spectroscopic and thermodynamic values for these species is reported and a mechanism for the chemi-ionization reaction U+O→UO⁺+e^− is proposed. The U+O and Sm+O chemi-ionization reactions are the first two metal-plus-oxidant chemi-ionization reactions to be studied theoretically in this way.

The fragmentation behaviour of the ion MeP(O)OMe⁺ has been investigated using quantum mechanical calculations at the B3LYP and MP2 levels to support experiments made with an Ion Trap Mass Spectrometer. Two mechanisms for the loss of CH₂O are found, one involving a 1,3-H migration to phosphorus and the other a 1,2-methyl migration to give P(OMe)₂⁺ followed by a 1,3-H migration. In each case an ion-dipole complex is formed that rapidly dissociates to yield CH₂O. The relative importance of each route has been previously determined experimentally via isotopic labelling experiments, and the theoretical results are found to be consistent with these experimental results. The mechanisms suggested in the earlier work involving a 1,4 H migration to O are shown to be energetically unfavourable.

The Sm+O chemiionization reaction has been investigated theoretically using a method that allows for correlation and relativistic effects. Potential energy curves have been calculated for several electronic states of SmO and SmO⁺. Comparison with available spectroscopic and thermodynamic values for these species is reported and a mechanism for the chemiionization reaction Sm+O is proposed. The importance of spin–orbit coupling in the excited states of SmO, in allowing this chemiionization reaction to take place, has been revealed by these calculations. This paper shows the metal-plus-oxidant chemiionization reaction.

The structure and vibrational frequencies of the UO₂ molecule have been determined using multiconfigurational wave functions (CASSCF/CASPT2), together with a newly developed method to treat spin−orbit coupling. The molecule has been found to have a (5fφ)(7s), ³Î¦_u, Ω = 2 ground state with a U−O bond distance of 1.77 Ã…. The computed antisymmetric stretching σ_u frequency is 923 cm^-1 with a 16/18 isotope ratio of 1.0525 which compares with the experimental values of 915 cm^-1 and 1.0526, respectively. Calculations of the first adiabatic ionization energy gave the value 6.17 eV, which is 0.7 eV larger than the currently accepted experimental result. Reasons for this difference are suggested.