Gaseous Fluorodiazonium Ions. Experimental and Theoretical Study on Formation and Structure of FN2+

Abstract

ion reaction mbar) and HN3 (p = 9.2 x NE+ + NF, NF,+ + NF,' (6) is responsible for the exponential decrease of NF'+ in Figure 3. The occurrence of the exothennic reaction (6), A?&' = 100.4 kJ mol-', has been confirmed by independent experiments involving isolated NF'+ ions in the presence of N F 3 alone. As Theoretical Results Whereas no experimental information is available on the structure and the stability of the various F2HNz+ isomers, nor on the thermochemistry of FN2+, the investigation of these aspects is well within the capabilities of routinely available theoretical methods,43 and several calculations concerning various FN2+ isomers and their interconnecting transition structures have been r e p ~ r t e d . ~ , ' ~ ~ ~ ~ ' ~ ~ ~ On the other hand, no theoretical data are available on the F2HNzf ions. Thus, ab initio calculations, up to the GAUSSIAN-1 level of theory, have been performed to get the necessary information for the discussion of the mass spectrometric experiments. At the MP2(FULL)/6-31G* level of theory, four distinct minima have been identified on the F2HN2+ potential energy surface, i.e. the FzN-NH+ ion, 1, the cisand trans-FNH-NF+ ions, 2 and 3, and the ion-dipole complex FNz-FH+, 4. The corresponding optimized geometries are shown in Figure 5 , the total energies, at the various computational levels are collected in Table 2, and the corrections for the evaluation of the GAUSSIAN-1 energies are listed in Table 3. From Figure 5 , the computed N-N, N-F, and N-H bond distances of ions 1-3 indicate the formation of covalently bound species, whereas the ion-dipole character of ion 4 clearly emerges from the large distance between the FN2+ and HF fragments. From Table 3, the corresponding interaction energy is derived as 51.9 kJ mol-' at the GAUSSIAN1 level of theory. At the same computational level, ion 4 is by far the most stable among the investigated F2HN2+ isomers. The computed energy difference with cisFNH-NF+ is as large as 127.6 kJ mol-], and the latter species is in turn more stable than the trans isomer by 13.4 kJ mol-'. Ion 1 is the less stable among the investigated isomers, an energy difference of 151.0 kJ mol-' being computed with respect to ion 4. From the GAUSSIAN-1 energies of Table 3, corrected for the thermal contributions at 300 K, the enthalpy change of (43) Hehre, W. J.; Radom, L.; Schleyer, P. v. R.; Pople, J. A. Ab Inifio Molecular Orbital Theory; Wiley: New York, 1986. 1330 Inorganic Chemistry, Vol. 34, No. 6, 1995 Cacace et al.