Infrared spectra of tropolone(OH) and tropolone(OD) obtained from vapor phase, solvated, and rare gas matrix-isolated samples, and from fluorescence dip infrared spectroscopy experiments by Frost et al. on jet-cooled samples, are analyzed with the guidance of high level ab initio molecular orbital (MO) computations. It is found that the anharmonicity of the double minimum global potential energy surface of S0 tropolone is manifested by multistate local resonance networks coupling fundamental vibrations to nearby overtone and combination states. These resonance networks pervade the IR spectrum of tropolone above 500 cm−1, and the absorbances are much more strongly perturbed from harmonic level predictions than the frequencies. Some of the IR absorbances are also sensitive to intermolecular interactions. At maximum spectral resolutions reaching ∼0.2 cm−1 only the v1 and v22 (OH stretching and nascent skeletal tunneling) vibrations show resolved vibrational state-specific tunneling doublets. The tunneling behavior of tropolone is analyzed in the accompanying article.

Several hydrogen peroxide infrared absorption bands have been obtained under 0.2 cm-1 resolution. They demonstrate both doubling due to internal rotation and effects due to the inertial asymmetry. The groundstate rotational constants are found to be: A"=10.068 cm-" B"=0.8740, C"=0.8384. These parameters indicate a wide dihedral angle for H20 2, as proposed in the following structure: 00 bond=1.475 A, OR bond=0.950 A, dihedral angle = 119.8°, and OOH angle = 94.8°.The constants are applied to the microwave data for H20 2 and the deuterated species. The ground-state splitting is found to be 11.44 cm-I , and this, used with the recently reported torsion at 317 cm-I , yields the barriers hindering internal rotation. They are Voi.= 1300 cm-I and V',an,=300 em-I. A simple correlation of the infrared doublings is attempted.

Infrared spectra of monomeric acetic acids [CH3COOH(D) and CD3COOH(D)] isolated in Ar and N2 matrices near 4°K are reported. A large anharmonic potential-energy contribution is evident from the observed isotopic frequency shifts. It is proposed that this arises primarily from the double-minimum potential-energy curve for tunneling of the H atom between O atoms via the COH angle bending coordinate. In all cases, important coupling between the COH angle bending and C–O stretching coordinates is observed, with a strong addition of CD3 (umbrella) angle deformation in the case of CD3COOH. Exceptional matrix effects are observed for the coupled vibrations of the –OH molecules. A complete assignment is proposed for these acetic acids on the basis of the new data. As additional supporting evidence, the spectrum of matrix-isolated CH3COF and of acetic acid vapor at low pressure and long path length are reported.

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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 C2v 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 v1 (OH stretching) energies in quantum states N1 are strongly dependent on the skeletal geometry and, adiabatically separated from the slow v22 vibration, they contribute to markedly different 1D effective potential energy functions V22eff[N1] for v22. V22eff[N1=0] is a normal equal double minimum PEF while V22eff[N1≠0] have more complex shapes. Expressed as a function of the v22 skeletal displacement ΔS, the v1 states show a nonadiabatic curve crossing E1(1)→E1(2) contributing to the V22eff[N1=1→2] effective PEF for v22 vibration in the lowest excited OH stretching state. This function, rather than V22eff[N1=1], is strongly supported by the IR observations on v1. 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 C2v skeletal geometry. Overall, the independent computational model predicts the major spectroscopic features observed for S0 tropolone(OH) and tropolone(OD): (a) similar IR tunneling doublets with ∼10 cm−1 splittings for the v22 skeletal vibration; (b) weak v1 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.