The π-Donor Ability of the Halogens in Cations and Neutral Molecules. A Theoretical Study of AX₃⁺, AH₂X⁺, YX₃, and YH₂X (A = C, Si, Ge, Sn, Pb; Y = B, Al, Ga, In, Tl; X = F, Cl, Br, I)

Abstract

Quantum mechanical ab initio calculations at the MP2/VDZ+P level of theory (for some molecules at MP2/VTZ+D+P) are reported for the cations AX3 + and AH2X+ (A = C, Si, Ge, Sn, Pb; X = F, Cl, Br, I) and the isoelectronic neutral Lewis acids YX3 and YH2X (Y = B, Al, Ga, In, Tl; X = F, Cl, Br, I). The π-donor ability of the halogens given by the p(π) population at atom A increases for all cations with F < Cl < Br < I, which is the opposite to what has been reported in a recent study of Olah and co-workers (J. Am. Chem. Soc. 1996, 118, 3580). The same trend is found at the central atom Y of most neutral compounds, but the increase is less pronounced than for the cations. The calculated order of the energetic stabilization of the cations and neutral compounds by the halogens is less uniform, because the σ- and π-donation show different trends. The energies of isodesmic reactions and the X3A+−OH2 complexation energies predict for the thermodynamic stabilization of the Y-conjugated cations the same trend as the π-donor ability, i.e. F < Cl < Br < I. The energy differences are much less for the isodesmic reactions of the singly substituted AH2X+ than for AX3 +. Fluorine has a higher stabilizing effect in CH2F+ than in CF3 +, because the σ-withdrawing strength of F leads to a large positive charge at the carbon atom of CF3 +. The complexation energies of the boron halides BX3 show little differences for X3B−OH2 among the halogens, although the p(π) population at boron increases clearly from fluorine to iodine. The hydride affinity, however, increases strongly from BF3 to BI3 and from BH2F to BH2I. The heavy-atom homologues of C+, i.e. Si+, Ge+, Sn+, Pb+, and those of B, i.e. Al, Ga, In, Tl, show the same trends for the halogens, but the p(π) donation and the thermodynamic stabilization or destabilization is clearly less than for C+ and B, respectively.

Fluoromethyl Cations and Group XIV Congeners AHnF3 – n+ (A = Si, Ge, Sn, Pb; n = 0–2): From Covalent Structures to Ion‐Molecule Complexes

“…The geometries and harmonic frequencies of the FSiH 2 + isomer 3 and of the H-SiF 2 + isomer 6 of C 2v symmetry are also in good agreement with previous HF, [14][15][16] MP2 [10,17] and CASSCF [18] ab initio studies and are again typical of covalent structures. On the other hand, the bond lengths, harmonic frequencies and dissociation energies of the (Si,H 2 ,F) + isomers 4 and 5, and of the (Si,H,F 2 ) + isomers 7 and 8, point to ion-molecule complexes between ground-state SiF + ( 1 Σ + ) [19] and H 2 (4) or HF (7), and ground-state SiH + ( 1 Σ + ) [20] and HF (5) or F 2 (8).…”

“…[6,7] Concerning the structure of cations such as AHX + , AX 2 + , AH 2 X + , AHX 2 + and AX 3 + (A = C, Si, Ge, Sn, Pb; X = monovalent group), they are usually assumed to be "regular" covalent structures and the conceivable role of ion-molecule complexes such as A + -(HX), AH + -(HX) or AX + -(H 2 ) is still essentially unexplored. As a first step in this direction, stimulated by our continuing interest in the chemistry of fluorinated cations, [8] we report here an ab initio study on the heavier congeners of the fluoromethyl cations CH 2 F + , CHF 2 + and CF 3 + , which are known to possess covalent structures of C 2v or D 3h symmetry, [9][10][11][12] . Therefore, our calculations provide additional examples of only "seemingly familiar" [1] compounds of group XIV and confirm the crucial role of noncovalent isomers in the gas-phase ion chemistry of the heaviest elements: germanium, tin and lead.…”

Oxidation of CS2 by AsBr4+: The Unexpected Formation of the Simple CS2Br3+ Carbenium Ion

“…However, cations bearing an NR 2 group are better addressed as iminium salts. For the completely halogen-substituted CX 3 + ions it was predicted [1,2,16,17] that the stabilizing effect of the halogen substituent increases from F to I (in opposition to the earlier conclusions [18] ). The theoretical prediction was confirmed by the synthesis [17] and crystal structure of a room-temperature stable CI 3 + salt.…”

“…We attribute the small high-field shift of D(d 13 C) of 11-21 ppm in 1 to the presence of the heavy bromine atom at the carbenium center and the inverse halogen dependence of the shifts of compounds with a heavy halogen substituent. [16,27] Compound 1 is very sensitive to air and moisture, but is stable at room temperature in the solid state and for at least 20 h in CH 2 Cl 2 .…”

π-Donation and Stabilizing Effects of Pnicogens in Carbenium and Silicenium Ions: A Theoretical Study of [C(XH2)3]+ and [Si(XH2)3]+ (X=N, P, As, Sb, Bi)

“…Of the ions investigated in this study, only the silicenium ions that are stabilized by pnicogens from the third to Introduction Heteroatom-stabilized carbenium ions, in which the positively charged carbocation acts as a strong p(p)-acceptor that is stabilized by up to three main group elements that act as p(p)-donors of varying strengths, have been one of the most interesting and thoroughly examined systems recently. [1] In this respect, the structures and energies of monosubstituted and triply substituted carbenium ions of the general type [H 2 C-(XH n )] and [C(XH n ) 3 ] have been of particular interest, and complexes with pnicogens (n 2), [2,3] chalcogens (n 1), [3±5] or halogens (n 0) [3,6] as heteroatoms X were studied in more detail. It was concluded that the p(p)-donor ability of such substituents plays a crucial and at times controversial role in establishing trends in the stabilization exerted by a particular p(p)-donor within a group or a row of the periodic system.…”