Fluoromethyl Cations and Group XIV Congeners AH_nF_{3 – n}⁺ (A = Si, Ge, Sn, Pb; n = 0–2): From Covalent Structures to Ion‐Molecule Complexes

Abstract

Ab initio calculations at the MP2 and CCSD(T) level of theory (triple‐zeta quality basis sets for Si and Ge and effective core potentials plus DZ valence basis sets for tin and lead) have been performed to investigate the geometries, harmonic frequencies and relative stabilities of the (Si,H,F)+, (Ge,H,F)+, (Si,F3)+, (Ge,F3)+ and (A,Hn,F3 – n)+ (A = Si, Ge, Sn, Pb; n = 1 and 2) isomeric ions. While the H–Si–F+, F–SiH2+, H–SiF2+ and SiF3+ covalent structures are invariably predicted as the most stable isomers, the Ge+–(HF), FA+–(H2) and FA+–(HF) complexes (A = Ge, Sn, Pb) are the global minima on the potential energy surfaces, more stable than the corresponding covalent structures by up to some tens of kilocalories per mole. For tin and lead, the HA+–(HF) isomeric ions are also significantly more stable than F–AH2+ (A = Sn and Pb). Compared with the parent group XIV hydrides (A,H3)+, investigated so far by Schleyer and co‐workers (J. Am. Chem. Soc. 1996, 118, 12154–12158) and found to possess, for A = Sn and Pb, energetically favoured HA+–(H2) connectivities, the structural switch from covalent structures to ion‐dipole complexes occurs, therefore, even earlier. This suggests that the “regular” covalent connectivity of even the simplest AH2X+ or AHX2+ cations of group XIV cannot be taken for granted, and, especially for germanium, tin and lead, the role of ion‐molecule complexes must be carefully investigated. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2006)

Halogenated silanes, radicals, and cations: Theoretical predictions on ionization energies, structures and potential energy surfaces of cations, proton affinities, and enthalpies of formation

“…For SiHF, Ignacio and Schlegel [29] obtained IE a = 9.48 eV at MP4/6-31G(d,p) level, which agrees with our G3(CC) value of 9.47 eV. Antoniotti et al [22] found that SiHF + is more stable than Si + -FH by 96.7 kJ/mol at CCSD(T,Full)/aug-ccpVTZ (AVTZ) level, agreeing excellently with our G3(CC) value of 96.9 kJ/mol.…”

“…The large error in IE a (SiCl) by Bosser Ground states of silylene radicals are singlet, even though RHFwavefunction instability is found for these radicals except for SiF 2 . Ion complex structures Si + -XH, being located here at B3LYP level, agree closely with the previous ones at MP2(Full)/6-311G(d,p) and CCSD(T,Full)/cc-pVTZ levels [22]. Unlike [GeH 2 ] + , [GeHF] + and [GeHCl] + , where the most stable structures are ion complexes as Ge + -H 2 , Ge + -FH, and Ge + -ClH [23], the most stable silylene cations have their normal divalent structures, due primarily to the stronger Si-H bonds than Ge-H ones.…”

“…B3LYP method also tends to underestimate the vibrational frequencies for Si-F/Cl/Br stretching modes and to overestimate the ones for Si-H stretching modes, e.g., the B3LYP values of 2126, 993 and 651 cm −1 , vs the experimental ones of 2041.8, 1050.7, and 678.2 cm −1 for SiH + [71], SiF + [68], and SiCl + [69], respectively. The B3LYP classical and non-classical structures of [SiHF] + , [SiH 2 F] + , and [SiHF 2 ] + are also in good agreement with the MP2(Full)/6-311G(d,p) and CCSD(T,Full)/cc-pVTZ ones [22], even though the B3LYP bond lengths are again longer than the MP2 and CCSD(T) ones. (Fig.…”

“…structural chemistry, where theoretical studies [15][16][17][18][19][20][21][22][23] on XH 4 + (X = C, Si, Ge, Sn), SiH m F n + , GeH m F n + and GeH m Cl n + have revealed large structural distortions to their neutrals. In additional to their occurrence in RF and plasma processes, the ion chemistry is an important method in measuring the bond energies of the neutral species [24].…”

Cationic germanium fluorides

“…On the other hand, the germanium-containing isomer 3 is less stable than the FGe + -OH 2 complex 1 by 36.6 kcal mol −1 , and quite close in energy to the separated GeF + and H 2 O. This extremely large stability difference between intermediates 3 and 6 reflects the increased tendency, down group XIV, to lower oxidation numbers [63] and is in particular consistent with our recent theoretical evidence [64] for the larger stability of ion-molecule complexes such as FGe + -(H 2 ) and FGe + -(HF) with respect to their corresponding covalent structures F-GeH 2 + and H-GeF 2 + . To aid the discussion of the detailed mechanism of reaction (2) (vide infra), we have also located the transition structure TS56 which connects the silicon-containing isomers 5 and 6.…”