This dissertation explores the coupling of internal rotation to the overall rotation ofmolecules and is divided into three distinct sections. The first section is a traditional torsion- rotation problem studying the Ka band spectrum of methyl tert-butyl ether, a gasoline additive. The molecule is extremely prolate (κ = −0.991) and has three possible large amplitude motions: methoxy methyl torsion, tert-butyl torsion, and geared methyl torsion. These three internal ro- tations were investigated by MP2/cc-pVDZ calculations, and the methoxy methyl torsion was predicted to be the most influential in the microwave spectrum. Using a recently constructed Ka band chirped-pulsed Fourier Transform Microwave Spectrometer, the torsional-rotational spectrum of this molecule was recorded. Due to the minimal molecular asymmetry, the A state spectrum resembled that of a symmetric top, while the torsion-rotation coupling in the E state spectrum broke the near symmetric pattern and was a more typical asymmetric top spectrum. A total of 405 transitions were recorded and fit using XIAM, and the methyl torsional barrier height was determined to be 495.648(720) cm−1 . The second section describes a new program, westerfit, for Cs molecules with internal rotation and spin angular momentum. The code implements a single diagonalization Rho Axis Method approach for the torsion-rotation alongside a complete treatment of nuclear quardupole interaction and spin-rotation coupling. Unlike other programs designed for internal rotation with spin effects, westerfit includes matrix elements off-diagonal in the rotational angular momentum quantum number, N , rather than the perturbative treatment of the spin-rotation and quadrupole interactions. This full combined approach allows fitting of all symmetrically allowed terms in both the spin-rotation and the quadrupole tensors as well as inclusion of any higher order terms coupling the large amplitude motion to the spin angular momentum. The program was benchmarked against other published programs and found to be capable of reproducing results from three different programs. Particular interest was paid to meta-chlorotoluene as this molecule has a low internal rotation barrier and a spin 3/2 nucleus which make it the exact type of molecule the program was designed to treat. Previous attempts to fit this molecule were complicated in part by XIAM’s limitations at very low barrier heights and its perturbative quadrupole treatment. While the new fit in westerfit does offer reduced RMS error and better precision on the spectroscopic parameters, transitions from excited torsional states will likely be necessary to accurately determine the barrier height. The final section details the theoretical interactions between the methyl rotation and a single unpaired electron’s spin angular momentum. A Hamiltonian for spin-torsion-rotation in the Rho Axis System was determined and the matrix element of the spin-torsion coupling operator was derived as well as possible definitions for calculating the associated parameter. The operator was implemented into the code described in the preceding chapter and was used for the theoretical investigations. The interaction of this operator with other second order operators was explored with the spin-torsion coupling being most sensitive to the torsional barrier height. Coupled cluster parameters for the example molecule, meta-methyl-phenoxyl, were calculated, and spin-torsion- rotation spectra were simulated with the spin-torsion term at zero and at a non-zero value. When the spin-torsion term was changed from zero, most of the simulated transitions underwent a subtle shift that was not uniform in magnitude or direction. The work presented here should provide a foundation for future work on the rotational spectra of radicals with internal rotation.