Susumu Fujimaki

Stability and structure of benzene dimer cation (C6H6)+2 in the gas phase

The gas‐phase equilibria of the clustering reaction (C6H6)+n−1+C6H6 =(C6H6)+n were studied with a pulsed electron‐beam high‐pressure mass spectrometer. The enthalpy changes −ΔH0n−1,n for the reaction with n=2–4 were determined to be 20.6±1.0, 7.8±0.5, and ∼7 kcal/mol, respectively. The structure of dimer cation (C6H6)+2 was determined by ab initio spin‐restricted open‐shell Hartree–Fock molecular orbital (ROHF MO) calculations. Two isomers, ‘‘axial C2 ’’ and ‘‘parallel C2v, ’’ are calculated to be located at the potential‐energy minima. The axial C2 is 3.0 kcal/mol more stable than the parallel C2v by ROHF/6‐31G**//ROHF/3‐21G.

Weak ion-molecule complexes of F−(CF4)n and CF3−(CF4)n

Abstract Gas-phase thermochemical stabilities and structures of cluster ions F − (CF 4 ) n and CF 3 − (CF 4 ) n have been studied with a high-pressure mass spectrometer and ab initio calculations. The F − … CF 4 bonding energy is found to be only 6.4 kcal/mol. Its geometry is of an ion-dipole complex along an S N 2 reaction path. The high-symmetry (D 3h ) CF 5 − species is computed to be a transition state of the S N 2 path and not an energy minimum. The CF 3 − …CF 4 bonding energy is only 3.6 kcal/mol with F 2 CF − …CF 4 coordination. The F … F exchange repulsion hinders the F … C attraction in these species. Th bonding energies are in good agreement with measured ones.

Gas-phase stability of cluster ions SFm+ (SF6)n with m = 0–5 and n = 1–3

The gas-phase stabilities of cluster ions SFm+ (SF6)n with m = 0–5 were determined by using a high pressure mass spectrometer. The bond energies of SFm+ (SF6)1 were found to be less than 10 kcal/mol and to decrease with m = 0 → 5. There appear to be rather large gaps in the bond energies between n = 1 and 2 for the clusters SFm+ (SF6)n with m = 0–4. The structures of SF5+, SF+ (SF6)1 SF3+ (SF6)1 and SF5+ (SF6)1, were investigated by ab initio molecular orbital calculations. For SF5+, the D3h geometry is found to be most stable and C4v is a transition state of the Berry pseudorotation. For the ion-molecule complexes, the “on-top hat” models were found to be the most stable structures.

Comparative study of the gas-phase bond strengths of CO2 and N2O with the halide ions

Thermodynamic data, ΔHn-1, no and ΔSn-1, no, for clustering reactions of halide ions X−(X = F, Cl, Br, and I) with N2Owere measured with a pulsed electron beam high-pressure mass spectrometer. In contrast to the fact that CO2 forms a covalent bond with the fluoride ion to yield the fluoroformate ion, FCO2−, the interaction between F− and N2O is mainly electrostatic. It was found that the cluster ions F− (N2O)n complete the first shell at n = 6, thus forming an octahedral structure. The difference between F—CO2− and F− ... N2O is discussed in terms of Coulombic, exchange, and charge-transfer interactions. The X− (N2O)2 clusters (X = Cl, Br and I) are found to be of C2h symmetry, while F− (N2O)2 is of a twisted form and is slightly asymmetric due to a slight participation of covalency (charge transfer) in the core ion F− ... N2O.