and Alan C. Hopkinson

Benzyl, 9-fluorenyl and diphenylmethyl cations: structures and relative stabilities based on hydride transfer reactions

Abstract Ab initio molecular orbital calculations are reported at HF/6-31G(d,p) and MP2(fc)/6-31G(d) for the diphenylmethyl (I), 9-fluorenyl (II) and benzyl (III) cations, and for the hydrocarbons diphenylmethane (IV), fluorene (V) and toluene (VI). Structure optimisations have been performed at both levels of theory and all critical points were characterised by harmonic frequency calculations at the Hartree-Fock level. Based on hydride transfer reactions, one phenyl group is calculated to stabilise the methyl cation by 74.3 kcal mol−1, and a second phenyl adds a further stabilisation of 25.1 kcal mol−1. The 9-fluorenyl cation is intermediate in stability between the diphenylmethyl and benzyl cations, but is closer to the diphenylmethyl cation.

When does the Hard and Soft Acid Base principle apply in the gas phase

Abstract We have studied the silver ion affinities of several RCN ligands using density functional theory. It was found that they correlate linearly with experimental proton affinities, a contradiction to the HSAB principle as H + is a ‘hard’ acid, and Ag + is a ‘soft’ acid. This linear correlation between the Ag + and proton affinities diminishes as the number of RCN ligands attached to the Ag + ion increases from one to three. In the third addition step, the silver ion affinities nearly level off in agreement with the expectation that the electron donating or withdrawing properties of the R group become much less important than in the previous two steps, where the positive charge on the metal ion is not well delocalized. Delocalization of the charge of a metal ion occurs only when a sufficiently large number of ligands are attached to the metal ion. When this condition is satisfied, our data suggests that the HSAB principle may be applicable.

Destabilised carbocations: A comparison of the C2H4NS+ and C2H4NO+ potential energy surfaces

Abstract Ab initio molecular orbital calculations at the HF/6–31G(d,p) and MP2(full)/6–311G(d,p) levels are reported for the C2H4NO+ and C2H4NS+ potential energy surfaces. All structures have been subjected to geometry optimisations at both levels of theory and have been characterised by harmonic frequency calculations. On the C2H4NS+ surface the S-bridged thioformamidylmethyl cation, 3a, has the lowest energy while on the C2H4NO+ surface the ‘solvated ion’ H2CNH2+…CO, 8b, is lowest. On each surface the formamidylmethyl cation, 3, is in a deep well. On both surfaces there is an ion (structure 7) in which a three-membered ring contains both heteroatoms. On the C2H4NS+ surface 7a is 39.3 kcal mol−1 above 3 and there is a substantial barrier (25.8 kcal mol−1) for rearrangement of 7a to 3a. The oxygen analogue, 7b, is 62.6 kcal mol−1 above 3b. Dissociation of the α-aminothioacetyl cation, 6a, is endothermic (by 29.0 kcal mol−1) whereas dissociation of the α-aminoacetyl cation is exothermic by 12.1 kcal mol−1.

EXPERIMENTAL AND THEORETICAL STUDIES OF GAS-PHASE REACTIONS OF SIFX+ (X = 1-3) WITH AMMONIA : INTRAMOLECULAR H-ATOM TRANSFER REACTIONS WITH SIF3+ AND F2SI(NH2)+

Gas-phase ion-molecule reactions between the Lewis acids SiFx ( x 5 1‐3) and ammonia have been investigated both experimentally and theoretically. Experimental studies were performed with a selected ion flow tube (SIFT) apparatus with helium buffer gas at 294 6 3 K and at 0.35 6 0.01 Torr. The monofluorosilicon cation SiF 1 was found to be unreactive toward ammonia, while SiF21 was observed to undergo electron transfer with NH3. The trifluorosilyl cation SiF 3 reacted consecutively with two ammonia molecules by HF elimination to produce FSi(NH2)2 which subsequently reacted with three NH3 molecules in succession to form adduct ions, with no further HF eliminations. Molecular orbital calculations were performed on all ionic and neutral molecular species associated with the chemistry of SiF 3 . Gradient optimizations were performed on reactants, on transition structures, and on products, both at the Hartree-Fock (HF)/3-21G level of theory, and at the density functional Becke-Lee-Yang-Parr (B-LYP)/6-31G(d,p) level. Harmonic frequency calculations were performed on all optimized structures at critical points at the latter level, from which, also, zero-point vibrational energies (ZPE) were obtained. The results of molecular orbital investigations revealed mechanistic insight into the experimentally-observed HF elimination reactions; in particular, the occurrence of H-atom transfer on a double-minimum potential-energy hypersurface. Theory also confirmed the thermodynamic legitimacy of the observed reactions and the validity of the nonobservation of a HF elimination reaction between FSi(NH2)2 and ammonia under SIFT conditions, a process that was reported to have occurred in a previous Fourier transform ion cyclotron resonance (FTICR) study. Molecular orbital calculations also have shown that the lowest-energy isomer associated with the empirical formula of the second adduct ion, FSi(NH2)2(NH3)2 , is one which has a nearly tetrahedral geometry in the heavy atoms, and solvates an ammonia molecule by hydrogen bonding. Comparisons also were made between the results of the present study and those of an earlier SIFT investigation of the reactions of Lewis acids SiFx ( x 5 1‐3) with H2O. (Int J Mass Spectrom 185/186/187 (1999) 381‐392)

Reaction of Si+ (2P) with acetylene and with ethylene. Examination of the potential energy surfaces for adducts SiC2H2+ and SiC2H4+

Abstract Ab initio molecular orbital calculations have been performed at levels of theory up to, and including, QCISD(T)(full)/ 6–311 + + G(2df,p) on structural isomers associated with the adducts of Si + ( 2 P) with acetylene and of Si + ( 2 P) with ethylene. Critical points on each surface have been characterised by harmonic frequency calculations at the UHF/6–31G(d,p) level of theory. There are many similarities between the minima on the SiC 2 H 2 + surface and those on the analogous neutral surface, whereas significant topological differences were observed to exist between the SiC 2 H 4 + surface and its neutral analogue. The inclusion of electron correlation in post-Hartree-Fock calculations to second-order Moller-Plesset perturbational theory was observed to affect significantly the topologies of the two cationic surfaces. The global minimum on each of the cationic surfaces, as calculated at the MP2 level of theory, is a three-membered cyclic species ( 2 B 2 state), and represents the products of the symmetry-allowed additions of Si + ( 2 P), with its unpaired electron in 3p y , to acetylene and to ethylene respectively. By contrast, the formation of cyclic species with electronic states 2 A 1 and 2 B 1 , structures which are excited states of the respective global minima, from the addition of Si + ( 2 P) to the appropriate hydrocarbon, are both symmetry-forbidden processes. The existence of significant barriers (20 kcal mol −1 ) to the interconversion of three minima on the SiC 2 H 2 + and five minima on the SiC 2 H 4 + surfaces suggests the probable existence of several isomers of each formula as stable species in the gas phase. The formation of adducts from Si + + HC≡CH and from Si + + H 2 C=CH 2 is strongly exothermic; reactions which result in hydrogen elimination from these adducts are weakly exothermic or are thermoneutral.

Protonation of group 14 oxides: The proton affinity of SnO

Abstract Molecular orbital calculations are reported for the monoxides, XO, of group 14 elements ( X = C , Si , Ge and Sn ) and for both isomers, XOH + and HXO + , of the protonated monoxides. Structure optimisation has been carried out using the Density Functional Theory employing the B3LYP procedure and at both Hartree-Fock and MP2 (full) levels, all with a variety of medium-sized Gaussian basis sets. In all XO molecules the oxygen atom is the preferred site for protonation, except when X = C where HCO + is the lower energy isomer. Barriers to interconversion between the two isomers XOH + and HXO + are overestimated by the Hartree-Fock calculations, but with wave functions that include electron correlations they generally fall into the range 27–44 kcal mol −1 . Proton affinities increase as the atomic number of X increases, and values calculated by averaging over all wave functions that include electron correlation, give the following proton affinities: for CO, 141.5; for SiO, 189.3; for GeO, 196.1; and for SnO, 215.6 (all in kcal mol −1 ).