Masayuki Nasu

Bond energy oscillation in the cluster ion NO+(NO)n

The gas‐phase equilibria of the clustering reaction of NO+ with NO and F− with NO were measured with a pulsed electron‐beam high‐pressure mass spectrometer. Van’t Hoff plots of equilibrium constants lead to the determination of the thermochemical stabilities for NO+(NO)n with n=1–10 and F−(NO)n with n=1–3. The equilibrium constants Kn−1,n for the former reaction with n=4, 6, 8, and 10 were found to be larger than Kn−2,n−1 with (n−1)=3, 5, 7, and 9, respectively. That is, the cluster ions NO+(NO)n with even n are thermochemically more stable than the smaller ones, NO+(NO)n−1, under the present experimental conditions. The measured enthalpy (−ΔH 0n−1,n) and entropy changes (−ΔS0n−1,n) show odd–even oscillation. This is due to the electron‐spin pairing effect, i.e., dimer pair formation in the cluster ions. The sudden decrease in the bond energies for the cluster NO+(NO)n between n=2 and 3 suggests that the core in the cluster NO+(NO)n is NO+(NO)2. The bond energy oscillation is also likely for the negative ...

The small binding energies of the negative cluster ions: SF5−(SF6)1, SF6−(SF6)1 and F−(SF6)n (n = 1 and 2), in the gas phase

Abstract The gas-phase clustering reactions of SF 5 − , SF 6 − and F − ions with SF 6 were measured using a high-pressure mass spectrometer. The bond energies of SF 5 − (SF 6 ) 1 and SF 6 − (SF 6 ) 1 were estimated to be ⩽2.3 kcal/mol. The bond energies of F − (SF 6 ) n with n = 1 and 2 were determined to be 5.4 and ≈4.1 kcal/mol, respectively. This result indicates that the SF 6 molecule is the weakest electrophilic reagent toward F − ever reported. This is mainly due to the exchange repulsion between the negative ions and fluorine atoms surrounding the reactive S atom. The structure of the cluster ion F − (SF 6 ) 1 whose binding energy was surrrisingly small was investigated by ab initio MO calculations.

Gas-phase thermochemical stabilities of cluster ions [(N2)m(Ar)n]+ with (m+n)=1–5

Thermochemical stabilities of nitrogen-argon cluster ions [(N2)m(Ar)n]+ were measured using a pulsed-electron beam mass spectrometer. The thermochemical data obtained for the exchange reactions N4++Ar=N2Ar++N2 (ΔH0=−1.0 kcal/mol) and Ar2++N2=N2Ar++Ar (ΔH0=−2.1 kcal/mol) lead to the determination of the bond dissociation energies (D), D(N2Ar+→N2++Ar)=26.8, D(N2Ar+→Ar++N2)=30.5, and D(Ar2+→Ar++Ar)=28.4 kcal/mol. For the mixed cluster ions [(N2)m(Ar)n]+, the irregular decreases in bond dissociation energies are found with (m+n)=3→4. This fall-off indicates that the core ions in the mixed cluster ions are trimer cations, [(N2)m(Ar)n]+ with (m+n)=3 in agreement with the experimental results by Magnera and co-workers [Chem. Phys. Lett. 192, 99 (1992); J. Chem. Soc. Faraday Trans. 86, 2427 (1990)]. The most stable cluster ions of [(N2)m(Ar)n]+ are found to be those composed of the core ion N2Ar+N2 solvated by further N2 ligands. The rate of exchange reaction (N2)m++Ar=N2Ar+N2(N2)m−3+N2 was found to become slower...

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.

FORMATION OF THE TRIMER ION CORE IN THE HETEROGENEOUS RARE GAS CLUSTER IONS

Thermochemical stabilities of the cluster ions composed of mixed rare gases were measured using a pulsed-electron beam high pressure mass spectrometer. The formation of trimer ion cores, i.e., A+(B)2 and A2+(B)1, was found when A and B are next to each other in the Periodic Table. This trend is similar to that for the pure rare gas cluster ions, i.e., the formation of ion core Rg3+ for Rg=He, Ne, Ar, Kr, and Xe. Although Ar and Xe are not next to each other in the Periodic Table, the formation of the trimer ion core is found for Xe+(Ar)2. This may be due to the fact that the ionization potentials of Ar and Xe are close to each other. The bond energies of larger cluster ions A+(B)2(B)n−2 and A2+(B)1(B)n−1 were found to be similar to those of homogeneous cluster ions (B)3+(B)n−3. The experimental bond energies were confirmed by ab initio calculations with a modified G2(MP2) method [e.g., 0.2 kcal/mol (expt) and 0.3 kcal/mol (theory) for Ar2+⋅He].

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.