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A new quartic force field for the SO$_2$ $\tilde{\text{C}}$$^1$B$_2$ statehas been derived, based on high resolution data from S$^{16}$O$_2$ andS$^{18}$O$_2$. Included are eight $b_2$ symmetry vibrational levels ofS$^{16}$O$_2$ reported in the first paper of this series [G. B. Park,$\textit{et al.}$, J. Chem. Phys. $\textbf{144}$, 144311 (2016)]. Many of theexperimental observables not included in the fit, such as the Franck-Condonintensities and the Coriolis-perturbed effective $C$ rotational constants ofhighly anharmonic $\tilde{\text{C}}$ state vibrational levels, are wellreproduced using our force field. Because the two stretching modes of the$\tilde{\text{C}}$ state are strongly coupled via Fermi-133 interaction, thevibrational structure of the $\tilde{\text{C}}$ state is analyzed in aFermi-system basis set, constructed explicitly in this work via$\textit{partial}$ $\textit{diagonalization}$ of the vibrational Hamiltonian.The physical significance of the Fermi-system basis is discussed in terms ofsemiclassical dynamics, based on study of Fermi-resonance systems by Kellmanand coworkers [M. E. Kellman and L. Xiao, J. Chem. Phys. $\textbf{93}$, 5821(1990)]. By diagonalizing the vibrational Hamiltonian in the Fermi-systembasis, the vibrational characters of all vibrational levels can be determinedunambiguously. It is shown that the bending mode cannot be treated separatelyfrom the coupled stretching modes, particularly at vibrational energies of morethan 2000 cm$^{-1}$. Based on our force field, the structure of the Coriolisinteractions in the $\tilde{\text{C}}$ state of SO$_2$ is also discussed. Weidentify the origin of the alternating patterns in the effective $C$ rotationalconstants of levels in the vibrational progressions of the symmetry-breakingmode, $\nu_\beta$ (which correlates with the antisymmetric stretching mode inour assignment scheme).

The rotation-vibration structure of the SO$_2$ $\mathrm{\tilde{C}}^1\mathrm{B}_2$ state explained by a new internal coordinate force field