DIAZASILINE (SINN) - IS THERE A CONFLICT BETWEEN EXPERIMENT AND THEORY

The molecule SiNN was investigated with the highly correlated coupled cluster method with single and double excitations and corrections for the connected triples [CCSD(T)], and with the multireference single an d double excitations configuration interaction approach (MRCI). A seri es of basis sets ranging from the simple 6-31G, 6-311G*, and Dunning' s double-zeta (DZ) plus polarization basis sets to the mole extended c orrelated consistent cc-pVTZ and cc-pVQZ ones were employed to check t he dependence of the geometry optimization and of the frequency evalua tion on the basis sets. Our highest level result of 1859 cm(-1) for th e harmonic stretching frequency of the NN bond differs considerably fr om a previous CCSD(T)/DZ prediction of 1726 cm(-1). In fact, most of t he results analyzed in this study point to a frequency value greater t han 1810 cm(-1). Reexamining the scant experimental evidence, we estim ate the harmonic frequency to be very close to 1830 cm(-1). Surprising ly, an extended internally contracted MRCI calculation with the cc-pVT Z basis predicts a frequency 144 cm(-1) higher than the corresponding CCSD(T)/cc-pVTZ result. Comparisons with existing density functional s tudies and with a previous MRCI calculation are also carried out. At t he CCSD(T)/cc-pVTZ level, other isomers of SiN2, are further investiga ted for the first time. One symmetric linear ((3) Sigma(g)(-)), and on e symmetric bent ((1)A(1)) structure are found to lie relatively high in energy: 84.60 and 102.23 kcal/mol, respectively, relative to SiNN ( (3) Sigma(-)). However, one cyclic (1)A(1) and the singlet asymmetric isomer SiNN ((1) Sigma(+)) are only 6.09 and 15.81 kcal/mol above the global minimum. Although the higher frequencies of the former isomers do not fall in the region of relevance to the experimental assignment discussed in this work, that of the lowest lying (1)A(1) state (1850 c m(-1)) is practically identical to the CCSD(T)/cc-pVTZ frequency compu ted for the (3) Sigma(-) state (C) 1997 American Institute of Physics. .