Enthalpies of Formation of Gas-Phase N3, N3-, N5+, and N5- from Ab Initio Molecular Orbital Theory, Stability Predictions f
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- 作者：David A. Dixon ; David Feller ; Karl O. Christe ; William W. Wilson ; Ashwani Vij ; Vandana Vij ; H. Donald Brooke Jenkins ; Ryan M. Olson ; and Mark S. Gordon
- 刊名：Journal of the American Chemical Society
- 出版年：2004
- 出版时间：January 28, 2004
- 年：2004
- 卷：126
- 期：3
- 页码：834 - 843
- 全文大小：147K
- 年卷期：v.126,no.3(January 28, 2004)
- ISSN：1520-5126
文摘
Ab initio molecular orbital theory has been used to calculate accurate enthalpies of formationand adiabatic electron affinities or ionization potentials for N3, N3-, N5+, and N5- from total atomizationenergies. The calculated heats of formation of the gas-phase molecules/ions at 0 K are 
Hf(N3(2
)) =109.2, Hf(N3-(1
+)) = 47.4, Hf(N5-(1A1')) = 62.3, and Hf(N5+(1A1)) = 353.3 kcal/mol with an estimatederror bar of ±1 kcal/mol. For comparison purposes, the error in the calculated bond energy for N2 is 0.72kcal/mol. Born-Haber cycle calculations, using estimated lattice energies and the adiabatic ionizationpotentials of the anions and electron affinities of the cations, enable reliable stability predictions for thehypothetical N5+N3- and N5+N5- salts. The calculations show that neither salt can be stabilized and thatboth should decompose spontaneously into N3 radicals and N2. This conclusion was experimentallyconfirmed for the N5+N3- salt by low-temperature metathetical reactions between N5SbF6 and alkali metalazides in different solvents, resulting in violent reactions with spontaneous nitrogen evolution. It isemphasized that one needs to use adiabatic ionization potentials and electron affinities instead of verticalpotentials and affinities for salt stability predictions when the formed radicals are not vibrationally stable.This is the case for the N5 radicals where the energy difference between vertical and adiabatic potentialsamounts to about 100 kcal/mol per N5.
Hf(N3(2)) =109.2, Hf(N3-(1+)) = 47.4, Hf(N5-(1A1')) = 62.3, and Hf(N5+(1A1)) = 353.3 kcal/mol with an estimatederror bar of ±1 kcal/mol. For comparison purposes, the error in the calculated bond energy for N2 is 0.72kcal/mol. Born-Haber cycle calculations, using estimated lattice energies and the adiabatic ionizationpotentials of the anions and electron affinities of the cations, enable reliable stability predictions for thehypothetical N5+N3- and N5+N5- salts. The calculations show that neither salt can be stabilized and thatboth should decompose spontaneously into N3 radicals and N2. This conclusion was experimentallyconfirmed for the N5+N3- salt by low-temperature metathetical reactions between N5SbF6 and alkali metalazides in different solvents, resulting in violent reactions with spontaneous nitrogen evolution. It isemphasized that one needs to use adiabatic ionization potentials and electron affinities instead of verticalpotentials and affinities for salt stability predictions when the formed radicals are not vibrationally stable.This is the case for the N5 radicals where the energy difference between vertical and adiabatic potentialsamounts to about 100 kcal/mol per N5.