A refined estimate of the bond length of methane

The equilibrium structure of methane is determined by two different procedures. The first is based on high level ab initio calculations using the CCSD(T) treatment of electron and basis sets containing up to 365 contracted Gaussian functions. Using an extrapolation procedure to account for residual inadequacies of the basis set and an estimate of higher order correlation effects, an equilibrium internuclear separation of 1.08595 ± 0.0003 Å is obtained. This value is in almost perfect agreement with that obtained by correcting measuring ground-state rotational constants of methane isotopomers for vibrational effects and using the resulting empirical equilibrium rotational constants to infer the equilibrium structure.