Accurate equilibrium structures for small polyatomic molecules,radicals and carbenes

The paper reports on recent efforts to establish accurate equilibrium structures for small polyatomic molecules, radicals and carbenes most of which are of relevance to interstellar cloud chemistry, combustion processes or atmospheric chemistry. Among these are the well–known interstellar cyanopolyynes HC₃N and HC₅N, members of the series NC_{2 n} N, HC_{2 n ?+?1}P and NC_{2 n} P, linear silicon carbides of type SiC_{2 n} and pure carbon chains up to C₁₄, the radicals HC₃O, HOCO and CH₂CHO as well as cumulene carbenes of type H₂C _n . Whenever possible, a mixed experimental/theoretical procedure is employed which makes use of accurate experimental values for the ground–state rotational constants of a sufficiently large number of different isotopomers plus the results of coupled cluster calculations (variant CCSD(T)). In less favourable cases, the transfer of well–established correction parameters as developed from related smaller molecules appears to be the method of choice.