Noble gas anions of general formula FNgBN- (Ng = He-Xe) have been investigated by MP2, coupled-cluster, and multireference-CI calculations with correlation-consistent basis sets. These species reside in deep wells on the singlet potential energy surface and are thermodynamically stable with respect to the loss of F, F-, BN, and BN-. They are unstable with respect to Ng + FBN-, but at least for Ng = Ar, Kr, and Xe, the involved energy barriers are high enough to suggest their conceivable existence as metastable species. The stability of FNgBN- arises from the strong F--stabilization of the elusive NgBN. The character of the boron-noble gas bond passes from purely ionic for FHeBN- and FNeBN- to covalent for FXeBN-.

The structure and stability of as yet unreported compounds with the general formula RNBeNg (Ng = He, Ne, Ar) have been theoretically investigated at various ab initio and B3LYP density functional (DFT) levels of theory. Exemplary species include the parent HNBeNg and saturated and unsatured compounds such as HONBeNg, FNBeNg, X−CH2−NBeNg, X−C(O)−NBeNg (X = H, OH, F), and C6H5−NBeNg. The thermochemical stability of these molecules, invariably characterized as true energy minima on the singlet potential energy surface, depends on two factors, namely, the energy of dissociation ΔE into singlet RNBe and Ng and the energy difference between the singlet and triplet states of RNBe. The values of ΔE are essentially independent of the nature of the substituent R and are around 6.5 kcal mol-1 for Ng = He, 8.5 kcal mol-1 for Ng = Ne, and 11.0 kcal mol-1 for Ng = Ar. In addition, for most of the investigated RNBeNg compounds, we have found that the singlet state of RNBe is more stable than the triplet state or for RNBe, with a triplet ground state, that the singlet−triplet gap is lower than the ΔE value given above. Therefore, our calculations support the prediction that this class of thermochemically stable RNBeNg compounds could actually be very large.

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Searching for novel compounds of the noble gases still remains a fascinating experimental and theoretical challenge. The trifluorogermylxenon cation F3Ge−Xe+, a stable xenon−germanium molecular species, is obtained in the gas phase by the nucleophilic displacement of HF from protonated GeF4 by Xe. The alternative isomers F2Ge−Xe−F+ and FGe−F−Xe-F+, theoretically less stable than F3Ge−Xe+ by ca. 80−90 kcal mol−1, are not attainable under the employed ion trap mass spectrometric conditions. The observation of F3Ge−Xe+ enlarges the evidence concerning the conceivable binding partners of xenon.

Results of gas-phase experiments and theoretical investigations are reported for ionic reactions in silane/ethene systems with the main interest in the formation and growth of species containing both silicon and carbon atoms. Ion/molecule reactions in different SiH4/C2H4 mixtures have been studied with an ion trap mass spectrometer, determining variation of ion abundances with reaction time, reaction paths starting from primary ions of both reagents and reaction rate constants of the main processes. The best yield in formation of new Si−C bonds occurs in mixtures with an excess of silane, through processes of silicon-containing ions with ethene molecules. Since reactions of SiH2 + with ethene have been observed to play a major role in this system, they have been investigated by high-level ab initio methods. Structures and energies of intermediates (SiC2H6 •+) and products (SiC2H5 +, SiC2H4 •+, SiCH3 +), as well as energy profiles of the pathways observed experimentally, have been determined. The initial step is formation of a SiC2H6 •+ adduct at −44 kcal mol-1 with respect to the reactants, followed by isomerization reactions to four different structures through viable paths. Hydrogen atom loss to give SiC2H5 + occurs through homolytic cleavage of a Si−H or C−H bond without energy barriers for the inverse process. Four different structures have been computed for SiC2H4 •+ ion species, but only three of them are attainable by H2 elimination from SiC2H6 •+ or by isomerization. Formation of SiCH3 + involves three isomerization steps of the SiC2H6 •+ adduct before the cleavage of a Si−C bond. Enthalpies of formation of all the structures have also been computed, and a good agreement with previously reported experimental data is generally observed for the most stable isomers.