Knowledge Bank

We have written a computer program for use on a CDC 1604 computer which uses the internal axis method to calculate the rotational spectrum of molecules with hindered internal rotation. Our program differs from that of Dr. R. C. Woods in that we have used additional terms in the Fourier expansion of the internal rotation energy, have evaluated the torsional integrals by the harmonic oscillator approximation rather than setting them equal to their infinite barrier limits, and have diagonalized the entire $2J+1$ matrices numerically rather than converting to the Wang representation. The program is therefore applicable to molecules with lower reduced barriers than the $s\ge 20$ quoted by Woods. The free internal rotor method can be used for such intermediate barrier heights, but the size of the matrices limits its use to rather low values of J with the CDC 1604 computer. Some interesting and surprising comparisons of the eigenvalues and computer time have been made with the IAM program of Woods and with the free internal rotor method. The program has been used to determine the barrier height of nitrosomethane using data up to $J=28$.