Ion Chemistry in XH4/Allene (X = Ge, Si) Gaseous Mixtures – Formation of X–C Bonds

Gas-phase reactions of SiH(n)+ (n = 1,2) with NF3: a computational investigation on the detailed mechanistic aspects

The mechanism of the gas-phase reactions of SiH(n)(+) (n = 1,2) with NF(3) were investigated by ab initio calculations at the MP2 and CAS-MCSCF level of theory. In the reaction of SiH(+), the kinetically relevant intermediates are the two isomeric forms of fluorine-coordinated intermediate HSi-F-NF(2)(+). These species arise from the exoergic attack of SiH(+) to one of the F atoms of NF(3) and undergo two competitive processes, namely an isomerization and subsequent dissociation into SiF(+) + HNF(2) , and a singlet-triplet crossing so to form the spin-forbidden products HSiF(+) + NF(2). The reaction of SiH(2)(+) with NF(3) involves instead the concomitant formation of the nitrogen-coordinated complex H(2)Si-NF(3)(+) and of the fluorine-coordinated complex H(2)Si-F-NF(2)(+). The latter isomer directly dissociates into NF(2)(+) + H(2)SiF, whereas the former species preferably undergoes the passage through a conical intersection point so to form a H(2) SiF-NF(2)(+) isomer, which eventually dissociates into H(2)SiF(+) and NF(2).

Gas-phase reactions of XH3(+) (X = C, Si, Ge) with NF3: a comparative investigation on the detailed mechanistic aspects

The gas-phase reaction of CH(3)(+) with NF(3) was investigated by ion trap mass spectrometry (ITMS). The observed products include NF(2)(+) and CH(2)F(+). Under the same experimental conditions, SiH(3)(+) reacts with NF(3) and forms up to six ionic products, namely (in order of decreasing efficiency) NF(2)(+), SiH(2)F(+), SiHF(2)(+), SiF(+), SiHF(+), and NHF(+). The GeH(3)(+) cation is instead totally unreactive toward NF(3). The different reactivity of XH(3)(+) (X = C, Si, Ge) toward NF(3) has been rationalized by ab initio calculations performed at the MP2 and coupled cluster level of theory. In the reaction of both CH(3)(+) and SiH(3)(+), the kinetically relevant intermediate is the fluorine-coordinated isomer H(3)X-F-NF(2)(+) (X = C, Si). This species forms from the exoergic attack of XH(3)(+) to one of the F atoms of NF(3) and undergoes dissociation and isomerization processes which eventually result in the experimentally observed products. The nitrogen-coordinated isomers H(3)X-NF(3)(+) (X = C, Si) were located as minimum-energy structures but do not play an active role in the reaction mechanism. The inertness of GeH(3)(+) toward NF(3) is also explained by the endoergic character of the dissociation processes involving the H(3)Ge-F-NF(2)(+) isomer.

Ion chemistry in germane/fluorocompounds gaseous mixtures: a mass spectrometric and theoretical study

The ion-molecule reactions occurring in GeH(4)/NF(3), GeH(4)/SF(6), and GeH(4)/SiF(4) gaseous mixtures have been investigated by ion trap mass spectrometry and ab initio calculations. While the NF(x)(+) (x=1-3) react with GeH(4) mainly by the exothermic charge transfer, the open-shell Ge(+) and GeH(2)(+) undergo the efficient F-atom abstraction from NF(3) and form GeF(+) and F-GeH(2)(+) as the only ionic products. The mechanisms of these two processes are quite similar and involve the formation of the fluorine-coordinated complexes Ge-F-NF(2)(+) and H(2)Ge-F-NF(2)(+), their subsequent crossing to the significantly more stable isomers FGe-NF(2)(+) and F-GeH(2)-NF(2)(+), and the eventual dissociation of these ions into GeF(+) (or F-GeH(2)(+)) and NF(2). The closed-shell GeH(+) and GeH(3)(+) are instead much less reactive towards NF(3), and the only observed process is the less efficient formation of GeF(+) from GeH(+). The theoretical investigation of this unusual H/F exchange reaction suggests the involvement of vibrationally-hot GeH(+). Passing from NF(3) to SF(6) and SiF(4), the average strength of the M-F bond increases from 70 to 79 and 142 kcal mol(-1), and in fact the only process observed by reacting GeH(n)(+) (n=0-3) with SF(6) and SiF(4) is the little efficient F-atom abstraction from SF(6) by Ge(+). Irrespective of the experimental conditions, we did not observe any ionic product of Ge-N, Ge-S, or Ge-Si connectivity. This is in line with the previously observed exclusive formation of GeF(+) from the reaction between Ge(+) and C-F compounds such as CH(3)F. Additionally observed processes include in particular the conceivable formation of the elusive thiohypofluorous acid FSH from the reaction between SF(+) and GeH(4).

Gas-phase ion chemistry of GeH(4)/B(2)H(6) mixtures

The gas phase ion-molecule reactions in positively and negatively ionized germane/diborane mixtures have been studied by ion trap mass spectrometry. Reaction sequences and rate constants for the most interesting processes have been determined. In positive ionization, formation of Ge-B bonds exclusively occurs through condensation reactions of B(n)H(m)(+) ions with germane, followed by H(2) or BH(3) loss. No reactions of ions from germane with B(2)H(6) were observed under the experimental conditions used here. In negative ionization, the Ge(n)H(m)(-) (n = 1, 2) ion families react with diborane to yield the Ge(n)B(p)H(q)(-) (p = 1, 2) ions, again via dehydrogenation and BH(3) loss, while diborane anions proved to be unreactive. In both positive and negative ionization, Ge-B ions reach appreciable abundances. The present results afford fundamental information about the intrinsic reactivity of gas-phase ions and provide valuable indications about the first nucleation steps ultimately leading to amorphous Ge and B-doped semiconductor materials by chemical vapor deposition methods.

Gas-phase ion chemistry in organometallic systems

This review essentially deals with positive ion/molecule reactions occurring in gas-phase organometallic systems, and encompasses a period of time of approximately 7 years, going from 1997 to early 2004. Following the example of the excellent review by Eller & Schwarz (1991; Chem Rev 91:1121-1177), in the first part, results of reaction of naked ions are presented by grouping them according to the neutral substrate, while in the second part, ligated ions are grouped according to the different ligands. Whenever possible, comparison among similar studies is attempted, and general trends of reactivities are evidenced.

Gas-phase ion chemistry in XH4--C3H4--ZH3 (X==Si, Ge; Z==N, P) mixtures

The gas-phase ion chemistry of silane-allene-ammonia, germane-allene (or propyne)-ammonia (or phosphine) systems was studied by ion trap mass spectrometry. Reaction sequences were determined and rate constants were measured for the main processes observed. The mixture containing silane displays higher reactivity with respect to that with germane. Comparison with analogous systems provides useful information about the reactivity of different hydrocarbon molecules and the different affinities of silicon and germanium towards nitrogen and phosphorus. The most interesting product ions observed are those containing Si (or Ge), C and N (or P) elements together, as these ion species may be considered precursors of doped amorphous carbides, which are widely used in semiconductor devices.

Quadrupole ion trap studies of fundamental organic reactions

Over the past several decades, quadrupole ion traps have become important instruments in the study of gas phase ion/molecule reactions. As a part of this work, they have been employed in a number of studies focused on fundamental organic reaction mechanisms. In this review, the general features and limitations of quadrupole ion traps are discussed followed by a description of representative ion trap studies of organic reaction mechanisms such as substitution, elimination, and acyl transfer reactions.

Gas-phase ion chemistry in the ternary SiH(4)-C(3)H(6)-PH(3) system

Propene-phosphine and the silane-propene-phosphine gaseous mixtures were studied by ion trap mass spectrometry. For the binary mixture the variation of ion abundances under different partial pressures and the mechanisms of ion-molecule reactions are reported. Moreover, the rate constants of the main processes were measured and compared with the collisional rate constants to determine the reaction efficiencies. In the ternary silane-propene-phosphine mixture the mechanisms of formation of Si(m)C(n)P(p)H(+)(s) ion clusters were elucidated, but the complexity of the system and the low abundances of the ions usually isolated by successive steps prevented the determination of rate constants. The hydrogenated ternary ions are mainly formed by reactions of Si(r)P(s)H(+)(t) ions with propene, whereas a minor contribution comes from reactions of Si(m)C(n)H(+)(p) ions with phosphine. The C(v)P(w)H(+)(z) ions show very low reactivity with silane. The formation processes of these species are discussed in relation to their possible role as precursors of amorphous silicon carbides doped with phosphorus obtained by deposition from properly activated silane-propene-phosphine mixtures.