Knowledge Bank

Free cumulence carbenes $H2Cn(n\ge 3)$, are of substantial current interest, mainly due to their importance in interstcllar cloud $chemistry1$. Following our previous work on $H2CCC2$ we have carried out large-scale CCSD(T) $calculations,3$ using a basis set of 220 contracted Gaussian-type orbitals and correlating all electrons. The equilibrium geometry is: $rc(CH)=1.0810$ {\AA}, $\alpha e(HCH)=119.5e,R1e(C(1)C(2))=1.3062$ {\AA}, $R2c(C(2)C(3))=1.2902$ {\AA}, and $R3c(C(3)C(4))=1.2866$ {\AA}. The harmonic vibrational wavenumbers for totally symmetric modes are (in $cm-1$); $\omega 1=3164,\omega 2=2117,\omega 3=1709,\omega 1=1393$, and $\omega 3=905$.