Gas-Phase Reactions of SiH n 1 (n 5 1,2) with NF 3 :A Computational Investigation on the Detailed Mechanistic Aspects Paola Antoniotti,* [a] Elena Bottizzo, [a] Stefano Borocci, [b] Maria Giordani, [b] and Felice Grandinetti [b] The mechanism of the gas-phase reactions of SiH n þ (n ¼ 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 þ , the kinetically relevant intermediates are the two isomeric forms of fluorine-coordinated intermediate HSi-F-NF 2 þ . These species arise from the exoergic attack of SiH þ to one of the F atoms of NF 3 and undergo two competitive processes, namely an isomerization and subsequent dissociation into SiF þ þ HNF 2 , and a singlet-triplet crossing so to form the spin-forbidden products HSiF þ þ NF 2 . The reaction of SiH 2 þ with NF 3 involves instead the concomitant formation of the nitrogen-coordinated complex H 2 Si-NF 3 þ and of the fluorine-coordinated complex H 2 Si-F-NF 2 þ . The latter isomer directly dissociates into NF 2 þ þ 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 þ isomer, which eventually dissociates into H 2 SiF þ and NF 2 . V C 2012 Wiley Periodicals, Inc. DOI: 10.1002/jcc.23023 Introduction The gas-phase reactions of the cationic silicon hydrides SiH n þ (n ¼ 0–3) are of interest for fundamental reasons, and also to appreciate the role of ionic processes in the chemical vapor deposition of electronic and optoelectronic materials from gas- eous mixtures containing SiH 4 . [1] Thus, over the years, the gas- phase ion chemistry of binary and ternary mixtures containing SiH 4 and hydrogen, [2] hydrocarbons, [3–16] water, [17] inorganic oxides, [18,19] ammonia, [20–24] phosphine, [25–29] and halocar- bons [30–32] was investigated by various experimental and theo- retical methods. More recently, ion trap mass spectrometry (ITMS) and ab initio calculations were used to study the reac- tions occurring in ionized mixtures of SiH 4 and NF 3 . [33–35] These systems are also of applied interest when used to deposit fluorinated electronic and optoelectronic materials by chemical vapor deposition techniques. [36–40] The SiH n þ (n ¼ 0-3) react with NF 3 so to form ionic and/or neutral products with SiAF bonds. No evidence was instead obtained for SiAN products, and this reflects the special strength of the silicon- fluorine bond. [41] More interesting was the observation that the nature of the observed processes reflects the spin multi- plicity of the cation. Thus, as shown in Table 1, the open-shell Si þ and SiH 2 þ promote exclusively the fission of a NAF bond, with formation of SiF þ , and H 2 SiF þ /NF 2 þ , respectively, but the closed-shell SiH þ and SiH 3 þ undergo the concomitant forma- tion of products, that involve the scrambling of F and H atoms. Theoretical calculations were also performed to investigate the mechanisms of the reactions of Si þ and SiH 3 þ with NF 3 . [33,35] Although the formation of SiF þ from the reaction between Si þ and NF 3 involves the intermediacy of the N-coor- dinated isomer Si-NF 3 þ , all the six ionic products observed from the reaction between SiH 3 þ and NF 3 arise from the disso- ciation and/or rearrangement of the F-coordinated isomer H 3 Si-F-NF 2 þ , and no evidence was obtained for the kinetic role of the N-coordinated complex H 3 Si-NF 3 þ . To further appreciate the influence of the electronic structure and the spin multiplic- ity of the cation on the reactions between SiH n þ (n ¼ 0-3) and NF 3 , we explored the mechanisms of the processes involving SiH þ and SiH 2 þ . We note in particular that one of the channels from the reaction between SiH þ and NF 3 (HSiF þ þ NF 2 ) involves a change of spin multiplicity from singlet to triplet, and that the concomitant formation of H 2 SiF þ and NF 2 þ from the reaction between SiH 2 þ and NF 3 requires to pass through a real crossing between different electronic states. Thus, both the reactions of SiH þ and SiH 2 þ with NF 3 involve transitions between electronic states of different and/or same spin multi- plicities, and processes like these are of relevance in gas-phase ion chemistry. [42,43] The results of our calculations are dis- cussed in the present article. [a] P. Antoniotti, E. Bottizzo Dipartimento di Chimica, Universit  a di Torino, C.so M. D’ Azeglio, 48, 10125 Torino, Italy E-mail: paola.antoniotti@unito.it [b] S. Borocci, M. Giordani, F. Grandinetti Dipartimento per la Innovazione nei sistemi Biologici, Agroalimentari e Forestali (DIBAF), Universit  a della Tuscia, L.go dell’Universit  a, s.n.c., 01100 Viterbo, Italy Contract/grant sponsors: Universit a di Torino, Universit a della Tuscia, Italian Ministero dell’Istruzione, dell’Universit a e della Ricerca (MIUR) (Cofinanziamento di Programmi di Ricerca di Rilevante Interesse Nazionale). V C 2012 Wiley Periodicals, Inc. Journal of Computational Chemistry 2012, 000, 000–000 1 WWW.C-CHEM.ORG FULL PAPER