Sextic centrifugal distortion constants of axially symmetric molecules

Expressions in terms of molecular parameters are obtained for the sextic centrifugal distortion constants H_J, H_JK, H_KJ, and H_K of axially symmetric molecules. A numerical example for the NH₃ molecule is given which shows that the sextic distortion of NH₃ is normal. It is emphasized that the observed sextic distortion constants provide additional information on the cubic anharmonic potential coefficients.