ISSN:
1089-7690
Quelle:
AIP Digital Archive
Thema:
Physik
,
Chemie und Pharmazie
Notizen:
For a number of different structures and states of B3, B2N, BN2, and N3, optimum geometries and harmonic spectra were obtained at the HF/6-31G* level. The relative stability of the isomers was determined using full fourth-order Møller–Plesset theory, both with and without spin projection, as well as coupled cluster methods. Estimates for the dissociation energies are based on scaled CCD+ST(CCD) binding energies. Koopmans' vertical ionization potentials and Mulliken charge distributions, both at the UHF/6-31G* level, are quoted for the most stable isomers. B3 is found to be an equilateral triangle in its 2A'1 ground state. B2N has a symmetric linear arrangement in its 2Σ+u ground state with an extremely low bending frequency (73 cm−1), and an unusually low vertical ionization potential (6.75 eV). Its asymmetric stretching (2021 cm−1) is found to be extremely intense (8782 km mol−1). BN2 has four rather closely spaced states, of which an isosceles triangle is the absolute minimum (2A1 state). However, at high temperatures, an asymmetric linear arrangement (2Π state) is found to have equal importance, whereas a 4Σ− state plays a role there too. The same theoretical methods correctly predict for N3 a symmetric linear arrangement in the 2Πg ground state; the spectroscopic constants are found to be in reasonable agreement with experiment. Estimated dissociation energies (expected accuracy ±4 kcal mol−1) are: B3 197.9, B2N 265.0, BN2 224.9, N3 210.1 kcal mol−1. From a statistical thermodynamical analysis, B3 is stable against dissociation to B2 and B up to very high temperatures, B2N is extraordinarily stable, whereas BN2 and N3 dissociate spontaneously to B+N2 and N+N2 at all temperatures. From these results, the presence of B2N+ and B+3, the high abundance of B2N+, as well as the absence of BN+2 and N+3 in laser mass spectra of boron nitride is explained.
Materialart:
Digitale Medien
URL:
http://dx.doi.org/10.1063/1.456313
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