Copolymers of carbon dioxide and nitrogen: an AM1 and ab initio computational study

Extending work by various groups on possible dimers, trimers, etc. of dinit rogen and of carbon dioxide, the authors have studied analogous copolymers of N-2 and CO2 computationally. Twelve cyclic structures were examined with the AM1, HF/321G, HF/6-3IG* and MP2(FC)/6-31G* methods, and the acyclic "m onomer" to "tetramer" HO(C(O)O-N=N-)(n)H, n = 1-4, were studied at the AM1 and HF/3-21G levels, the cyclic species included 2-oxa-3,4-diazacyclobut-3- ene-1-one, 2-oxa-3,4,5,6-tetraazacyclohexa-3,5-diene-1-one, and various aza /oxa bicyclo[2.2.0] and bicyclo[2.2.2] systems. For the cyclic species, it was concluded that only the MP2(FC)/6-31G* results, which differ considerab ly from those at the other three levels, are likely to be reliable. These M P2 calculations indicate that only seven of the 12 cyclic structures studie d are stationary points tone is a transition structure), and none of them i s kinetically stable at room temperature. Although some have high energy de nsities (ca. 7-10 kJ g(-1), their expected low kinetic stabilities seems to make this of little practical value. The acyclic "copolymers" were all rel ative minima at the AM1 and HF/3-21G levels; unlike the cyclic species, the ir kinetic stabilities were not investigated directly by comparing the ener gies of reactants and decomposition transition states. The energy density o f the infinite acyclic polymer was found by extrapolation to be 5.1 (AM1) o r 5.6 (3-21G) kJ g(-1). The calculated vibrational spectra of the MP2 stati onary points and of the acyclic molecules gave some indication of instabili ty by the presence of low-frequency modes leading in the limit to decomposi tion. (C) 1999 Elsevier Science B.V. All rights reserved.