Theresa E. Redington

Tropolone monomer: Vibrational spectrum and proton tunneling

Abstract Infrared matrix-isolation spectra are reported for the tropolone monomers C 7 H 5 O 2 H and C 7 H 5 O 2 D. Tropolone possesses an intramolecular hydrogen bond and the possibility for proton tunneling from one oxygen atom to the other. Clearcut multiplets attributed to tunneling are observed for the OD stretching mode and for heavy atom modes of both C 7 H 5 O 2 D and C 7 H 5 O 2 H. The (mixed) carbonyl stretching mode particularly seems to facilitate tunneling from one conformation to the other. The tunneling phenomena suggests that tropolone monomer has nearly C 2 v symmetry. As a crude estimate, the tunneling potential energy barrier is calculated to be less than 5600 cm −1 in the ground electronic state of C 7 H 5 O 2 D. The barrier is lower in the Π ∗ electronic state than in the ground electronic state. A vibrational assignment that encompasses most of the 39 fundamental modes is proposed. The vibrations are classified using C 2 v symmetry species and a parallel vibrational analysis is presented for tropone, C 7 H 6 O, which is a true C 2 v molecule.

IR spectra of tropolone(OH) and tropolone(OD)

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 be...

Infrared matrix-isolation spectra of monomeric oxalic acid

Abstract Infrared spectra of monomeric oxalic acid-h2, -hd and -d2 have been observed using neon matrix-isolation spectroscopy. In addition, spectra of oxalic acid-h2, vapor were obtained using a heated absorption cell with a 10 m pathlength. All IR active fundamentals are assigned for oxalic acid-h2, and -d2, except the low frequency torsion. The spectra are interpreted in terms of the C2h intramolecularly hydrogen bonded model. Two vibrational assignments are discussed; one involves a very large intensity for the v8 + v11 (COH torsion) combination band of oxalic acid-h2. Tentative values for several Raman active fundamentals of oxalic acid-h2 are suggested using possible combination bands. Few fundamentals of oxalic acid-hd correlating with the Raman modes of the symmetric monomers could be observed. A force constant analysis of the by secular block is presented.

Implications of comparative spectral doublets observed for neon-isolated and gaseous tropolone(OH) and tropolone(OD)

Spectral doublet separations reported for gas phase and neon matrix-isolated samples of tropolone(OH) and tropolone(OD) are found to support recent work suggesting the possibility that tropolone has a slightly nonplanar geometry in the S1 (A 1B2) (pi*-pi) electronic state. Tautomerizations of gaseous tropolones in the S0 and S1 states are governed by equal double-minimum potential energy functions (PEFs), but interactions in the neon matrix environment transform the tautomerization PEFs of the slightly nonplanar S1 tropolones into unequal double-minimum PEFs. The spectral doublets reported for the zero-point S1-S0 transitions imply energy minima for the nonplanar S1 state in a neon matrix are offset by about 7 cm-1, and tunneling splittings in the symmetric double minimum PEFs of the gaseous molecules are damped about 2 cm-1 by the matrix environment. This means gas phase tunneling splittings smaller than 2 cm-1 are fully quenched in the neon matrix, and gas phase tunneling splittings near 20 cm-1 are damped by only 10%.

O18 effects on the infrared spectrum and skeletal tunneling of tropolone

Infrared-absorption profiles observed for vibrational transitions of gaseous tropolone often show sharp Q branch peaks, some of them ultranarrow spikes, indicative of the band origins for vibrational state-specific spectral tunneling doublets. In this work oxygen isotope effects for two CH wagging fundamentals, the COH torsion fundamental, and the skeletal contortion fundamental are reported. They allow considerations to be given: (1) oxygen isotope effects on the vibrational frequencies and state-specific tunneling splittings; (2) the asymmetry offset of the potential-energy minima for 16O and 18O tropolone; and (3) additional details concerning previously proposed high J rotation-contortion resonances in the contortional fundamental. The new results help to characterize the skeletal contortion fundamental and support the joint participation of skeletal tunneling with H tunneling in the vibrational state-specific tautomerization processes of tropolone in its ground electronic state.

Tunneling splittings for "O...O stretching" and other vibrations of tropolone isotopomers observed in the infrared spectrum below 800 cm(-1).

Fourier transform infrared absorption spectra containing evidence for about two dozen spectral tunneling doublets are reported for gaseous tropolone(OH), tropolone (OD), and 18O,18O-tropolone(OH) in the 800 to 300 cm-1 spectral range. No FTIR absorption was detected in the 300-150 cm-1 range. The known zero-point (ZP) tunneling splitting values Delta0 = 0.974 cm-1 for tropolone(OH) (Tanaka et al.) and 0.051 cm-1 for tropolone(OD) (Keske et al.) allow vibrational state-specific tunneling splittings Deltav to be estimated for fundamentals including three with strong O...O stretching displacements [cf. for tropolone(OH) nu13(a1) = 435.22 cm-1 with HDelta13 = 1.71 cm-1 = 1.76 HDelta0, and for tropolone(OD) nu13(a1) = 429.65 cm-1 with DDelta13 = 0.32 cm-1 = 6.27 DDelta0]. The majority of Deltav splittings in the sub-800 cm-1 range are dilated relative to the isotopomer Delta0 values. The FTIR spectra demonstrate the presence of dynamic couplings and potential function anharmonicity in addition to revealing Deltav splittings and many OH/D and 18O/16O isotope effects. Approximate values are obtained for the ZP splittings 88Delta0 and 86Delta0 of the doubly and singly 18O-labeled isotopomers of tropolone(OH). The diverse values of the observed Deltav/Delta0 splitting ratios underscore the inherent multidimensionality and corner-cutting activities entering the state-specific tunneling processes of the tropolone tautomerization reaction.

Infrared Absorption Spectra in the Hydroxyl Stretching Regions of Gaseous Tropolone OHO Isotopomers

Abstract Fourier transform infrared (FTIR) absorption spectra in the 2000 to 3500 cm–1 range are reported for the gaseous 16O,16O- and 18O,18O-isotopomers of tropolone[OH(OD)] at 25 oC. The spectral doublet component separations are near 20 and 19 cm–1 for 16O,16O- and 18O,18O-Tp(OH), respectively, and near 7 and 6.5 cm–1 for 16O,16O- and 18O,18O-Tp(OD). The spectra suggest the tautomerization tunneling mechanisms in these states are complex multidimensional processes including the participation of IVR. In general, the OHO isotope effects demonstrate a mixing of O atom displacement coordinates into the intramolecular dynamics for most of the vibrational states observed in the fundamental CH/OH(OD) stretching regions.

Excitation spectra of 2,5-dihydroxy-p-benzoquinone monomer and hydrates

The fluorescence excitation spectrum of the lowest allowed singlet–singlet transition of jet‐cooled 2,5‐dihydroxy‐p‐benzoquinone is reported. The transition is assigned as S3←S0, 1Bu←1Ag, π*←π and the 0–0 band origin is at 275.55 nm. Twenty vibrational levels, which include half of the Ag fundamentals, are assigned for the S3 state. The observed laser‐induced fluorescence transitions are structureless peaks with bandwidths that depend on the intensity of the excitation laser. No spectral multiplets attributable to intramolecular tunneling were observed. Deuterium isotope shifts of the 0–0 transition are +21 cm−1 per internal hydrogen bond. The 0–0 transitions of hydrate isotopomers with hydration shifts of only +11 cm−1 per water molecule are reported.