The minimum energy pathway leading between the tautomers of tropolone was calculated using molecular orbital (MO) methods. This, with various 1D and 2D cuts of the potential energy surface (PES) topography, reveals the {tunneling skeleton}/{tunneling H atom} mechanism for tautomerization. In the zero-point states the H atom is localized to one of the O atoms until the tropolone skeleton becomes sufficiently vibrationally displaced towards C-2v configurations that near-equal double-minimum potential energy functions (PEFs) arise for the H atom vibration. The resulting delocalization of the H atom between the two O atom sites allows the skeletal displacement to proceed through the barrier and the tautomerization process to be completed. The v(1) (OH stretching) energies in quantum states N-1 are strongly dependent on the skeletal geometry and, adiabatically separated from the slow v(22) vibration, they contribute to markedly different 1D effective potential energy functions V-22(eff)[N-1] for v(22). V-22(eff)[N-1=0] is a normal equal double minimum PEF while V-22(eff)[N(1)not equal 0] have more complex shapes. Expressed as a function of the v(22) skeletal displacement Delta S, the v(1) states show a nonadiabatic curve crossing E-1(1)--> E-1(2) contributing to the V-22(eff)[N-1=1 --> 2] effective PEF for v(22) vibration in the lowest excited OH stretching state. This function, rather than V-22(eff)[N-1=1], is strongly supported by the IR observations on v(1). The computed effective energy barriers on the "model" tunneling path for the zero point states are 4.97 kcal/mol for the skeletal motion, and 3.22 kcal/mol for the H atom vibration at C-2v skeletal geometry. Overall, the independent computational model predicts the major spectroscopic features observed for S-0 tropolone(OH) and tropolone(OD): (a) similar IR tunneling doublets with similar to 10 cm(-1) splittings for the v(22) skeletal vibration; (b) weak v(1) IR absorbance with 20 and 5 cm(-1) tunneling doublet separations for the isotopomers; (c) small tunneling splittings of the zero point states; and (d) unresolved vibrational state-specific IR tunneling doublets for all other fundamentals.