Theoretical study of hydrogen bonding and proton transfer in the ground and lowest excited singlet states of tropolone

Fourier transform microwave spectroscopy of the 13C- and 18O-substituted tropolone. Proton tunneling effect for the isotopic species with the asymmetric potential wells

Fourier-transform microwave spectroscopy has been applied for the 13C/18O-substituted tropolone to observe tunneling-rotation transitions as well as pure rotational transitions. The tunneling-rotation transitions were observed between the 13C-4 and -6 forms as well as between 13C-3 and -7, between 13C-1 and -2, and between 18O-8 and -9 (we denote these tunneling pairs as 13C-46, etc., below) although they have an asymmetric tunneling potential due to the difference in the zero point energy (ZPE). From the observed tunneling splittings ΔEij (0.9800-1.6824 cm-1), the differences in ZPE Δij for the 13C-46, -37, -12, and 18O-89 species are derived to be -0.1104, 0.5652, -1.3682, and 1.3897 cm-1 to agree well with the DFT calculation. The state mixing ratio of the tunneling states decreases drastically from (44%:56%) to (8.7%:91.3%) for 13C-46 and 18O-89 with an increase in the asymmetry Δij of the tunneling potential function. The observed tunneling-rotation interaction constants Fij decrease from 16.001 to 9.224 cm-1 as the differences in ZPE Δij increase, while the diagonal tunneling-rotation interaction constants Fu increase from 1.767 to 13.70 cm-1, explained well by the mixing ratio of the tunneling states.

Proton-Tunneling Dynamics along Low-Barrier Hydrogen Bonds: A Full-Dimensional Instanton Study of 6-Hydroxy-2-formylfulvene

Understanding the dynamics of proton transfer along low-barrier hydrogen bonds remains an outstanding challenge of great fundamental and practical interest, reflecting the central role of quantum effects in reactions of chemical and biological importance. Here, we combine ab initio calculations with the semiclassical ring-polymer instanton method to investigate tunneling processes on the ground electronic state of 6-hydroxy-2-formylfulvene (HFF), a prototypical neutral molecule supporting low-barrier hydrogen-bonding. The results emerging from a full-dimensional ab initio instanton analysis reveal that the tunneling path does not pass through the instantaneous transition-state geometry. Instead, the tunneling process involves a multidimensional reaction coordinate with concerted reorganization of the heavy-atom skeletal framework to substantially reduce the donor-acceptor distance and drive the ensuing intramolecular proton-transfer event. The predicted tunneling-induced splittings for HFF isotopologues are in good agreement with experimental findings, leading to percentage deviations of only 20-40%. Our full-dimensional results allow us to characterize vibrational contributions along the tunneling path, highlighting the intrinsically multidimensional nature of the attendant hydron-migration dynamics.

Wavepacket dynamical study of H-atom tunneling in catecholate monoanion: the role of intermode couplings and energy flow

We present a study of H-atom tunneling in catecholate monoanion through wavepacket dynamical simulations. In our earlier study of this symmetrical double-well system [Phys. Chem. Chem. Phys., 2022, 24, 10887], a limited number of transition state modes were identified as being important for the tunneling process. These include the imaginary frequency mode Q₁, the CO scissor mode Q₁₀, and the OHO bending mode Q₂₉. In this work, starting from non-stationary initial states prepared with excitations in these modes, we have carried out wavepacket dynamics in two and three dimensional spaces. We analyse the dynamical effects of the intermode couplings, in particular the role of energy flow between the studied modes on H-atom tunneling. We find that while Q₁₀ strongly modulates the donor-acceptor distance, it does not exchange energy with Q₁. However, excitation in Q₂₉ or Q₁ does lead to rapid energy exchange between these modes, which modifies the tunneling rate at early times.

Δ-Machine Learned Potential Energy Surfaces and Force Fields

There has been great progress in developing machine-learned potential energy surfaces (PESs) for molecules and clusters with more than 10 atoms. Unfortunately, this number of atoms generally limits the level of electronic structure theory to less than the "gold standard" CCSD(T) level. Indeed, for the well-known MD17 dataset for molecules with 9-20 atoms, all of the energies and forces were obtained with DFT calculations (PBE). This Perspective is focused on a Δ-machine learning method that we recently proposed and applied to bring DFT-based PESs to close to CCSD(T) accuracy. This is demonstrated for hydronium, N-methylacetamide, acetyl acetone, and ethanol. For 15-atom tropolone, it appears that special approaches (e.g., molecular tailoring, local CCSD(T)) are needed to obtain the CCSD(T) energies. A new aspect of this approach is the extension of Δ-machine learning to force fields. The approach is based on many-body corrections to polarizable force field potentials. This is examined in detail using the TTM2.1 water potential. The corrections make use of our recent CCSD(T) datasets for 2-b, 3-b, and 4-b interactions for water. These datasets were used to develop a new fully ab initio potential for water, termed q-AQUA.

Coherent destruction of tunnelling in a symmetrical double well driven by a series of time dependent δ functions

It is analytically and numerically shown that the coherent tunnelling between the individual wells of a symmetrical double well potential can be totally suppressed when it is driven by a periodic series of δ function in time, depending on the time period and strength of the δ function. We have applied time dependent perturbation theory to have an understanding over the process. In absence of any kind of perturbation, the average position of the particle makes a sinusoidal oscillation between two wells. With the application of a periodic δ function, the amplitude and the frequency of the oscillation both get modified. In this article we have explored how the frequency and strength of the applied perturbation controls the quantum dynamics of tunnelling and finally, how these parameters drive the system towards a complete stand still situation, which is described as coherent destruction of tunnelling.

Multidimensional H-atom tunneling in the catecholate monoanion

We present the catecholate monoanion as a new model system for the study of multidimensional tunneling. It has a symmetrical O-H double-well structure, and the H atom motion between the two wells is coupled to both low and high frequency modes with different strengths. With a view to studying mode-specific tunneling in the catecholate monoanion, we have developed a full (33) dimensional potential energy surface in transition state (TS) normal modes using a Distributed Gaussian Empirical Valence Bond (DGEVB) based approach. We have computed eigenstates in different subspaces using both unrelaxed and relaxed potentials based on the DGEVB model. With unrelaxed potentials, we present results up to 7D subspaces that include the imaginary frequency mode and six modes coupled to it. With relaxed potentials, we focus on the two most important coupling modes. The structures of the ground and vibrationally excited eigenstates are discussed for both approaches and mode-specific tunneling splitting and their trends are presented.

BEP-Like Correction of Nonequilibrium Thermodynamic Parameters of the Solvent-Assisted Reactions: The DFT and Ab Initio Study of Hydration, Peroxidation, and Enolization of Acetone and 1,1,1-Trifluoroacetone in Aqueous Solutions

The mechanisms of enolization and reactions of nucleophilic addition to carbonyl compounds were analyzed by density functional theory (DFT) (PBE1PBE) and ab initio (DLPNO-CCSD(T)) level of theory using the interaction of water and hydrogen peroxide with acetone and 1,1,1-trifluoroacetone (TFA) as the reference reactions. The transition states of the studied reactions were localized within the integrated approach that includes both the dielectric continuum theory (polarizable continuum model (PCM)) and the cyclic or two-cluster explicit solvation models. The considered models provide proton transfer in the enolization, hydration, and peroxidation reactions by the Grotthuss mechanism. It is shown that the calculated activation parameters at a sufficiently high level of theory and a sufficiently flexible solvation model can be additionally refined using the Bell-Evans-Polanyi (BEP)-like correction (in a form of the Bell-Evans-Polanyi equation), which is linear scaling of the model potential energy surface according to the equilibrium parameters of the reference reaction (experiment or high-level calculation). Quite good correspondence of the corrected and reference activation parameters and the lower sensitivity of the calculation results to the choice of the solvation model indicate the high reliability of the proposed BEP-like correction technique.

Substituent Effects on Excited-State Intramolecular Proton Transfer Reaction of 2-Aryloxazoline Derivatives

Different substituents and benzene ring numbers had significant effects on the fluorescence phenomenon of 2-aryloxazoline derivatives as observed in an experiment. Here, we select five 2-aryloxazoline derivatives with different substituents and benzene ring numbers (2u, 2ad, 2af, 2ai, and 2ah) to analyze the effects on the fluorescence phenomena. For 2ad, 2ah, and 2ai, first, the geometric structures are optimized based on the density functional theory and time-dependent density functional theory methods. The analysis of the obtained bond parameters reveals the variation of hydrogen bond interactions from S₀ to S₁ states. Second, the calculated absorption and emission spectra are consistent with the experimental values, which proves that the theoretical method is feasible. Finally, through the analysis of the infrared vibrational spectrum, reduced density gradient isosurfaces, frontier molecular orbitals, and potential energy curves, the strengthening mechanism of the hydrogen bond interaction and the ability of the excited-state intramolecular proton transfer (ESIPT) reaction to occur are further explained. Since the proton transfer reactions of 2u and 2af occur spontaneously under photoexcitation, they have no stable structures in the S₁ state. In conclusion, due to the different substituents, 2u is more prone to the proton transfer reaction than 2ad. For 2af, 2ai, and 2ah with different benzene ring numbers, the ESIPT reaction is more difficult to occur as the number of benzene rings increases. The ability of the ESIPT reaction to occur follows the order 2af → 2ah → 2ai. For 2-aryloxazoline derivatives with different substituents or different benzene ring numbers, the hydrogen bond strengthening mechanism has been authenticated, which promotes the occurrence of the ESIPT reactions.

Multiple ESIPT pathways originating from three-state conical intersections in tropolone

Internal conversion decay dynamics associated with the potential energy surfaces of three low-lying singlet excited electronic states, S₁ (ππ^*, A'), S₂ (ππ^*, A'), and S₃ (nπ^*, A″), of tropolone are investigated theoretically. Energetic and spatial aspects of conical intersections of these electronic states are explored with the aid of the linear vibronic coupling approach. Symmetry selection rules suggest that non-totally symmetric modes would act as coupling modes between S₁ and S₃ as well as between S₂ and S₃. We found that the S₁-S₂ interstate coupling via totally symmetric modes is very weak. A diabatic vibronic Hamiltonian consisting of 32 vibrational degrees of freedom is constructed to simulate the photoinduced dynamics of S₀ → S₁ and S₀ → S₂ transitions. We observe a direct nonadiabatic population transfer from S₁ to S₃, bypassing S₂, during the initial wavepacket propagation on S₁. On the other hand, the initial wavepacket evolving on S₂ would pass through the S₂-S₃ and S₁-S₃ conical intersections before reaching S₁. The presence of multiple proton transfer channels on the S₁-S₂-S₃ coupled potential energy surfaces of tropolone is analyzed. Our findings necessitate the treatment of proton tunneling dynamics of tropolone beyond the adiabatic symmetric double well potentials.

The theoretical study of excited-state intramolecular proton transfer of 2,5-bis(benzoxazol-2-yl)thiophene-3,4-diol

The symmetrical structures 2,5-bis(benzoxazol-2-yl)thiophene-3,4-diol (BBTD) can take shape two intramolecular hydrogen bonds in chloroform. In order to research the molecular dynamic behavior of BBTD upon photo-induced process, we utilize density functional theory (DFT) and time-dependent density functional theory (TDDFT) to complete theoretical calculation. Through the comparison of bond length, bond angle, IR spectra, and frontier molecular orbitals between ground state (S₀) and first excited state (S₁), it clearly indicates that photoexcitation have slightly influence for intensity of hydrogen bond. For the sake of understanding the mechanism of excited state intramolecular proton transfer (ESIPT) of BBTD in chloroform, potential energy surfaces have been scanned along with the orientation of O₁-H₂ and O₄-H₅ in S₀ and S₁ state, respectively. A intrigued hydrogen bond dynamic phenomenon has been found that ESIPT of BBTD is not a synergetic double proton transfer process, but a stepwise single proton transfer process BBTD→BBTD-S→BBTD-D. Moreover, the proton transfer process of BBTD-S→BBTD-D is easier to occur than that of BBTD→BBTD-S in S₁ state.

Ab initio and density functional theory study of keto-enol equilibria of deltic acid in gas and aqueous solution phase: a bimolecular proton transfer mechanism

Keto-enol tautomerism in deltic acid (2,3-dihydroxycycloprop-2-en-1-one) has been studied using ab initio methods and the B3LYP functional of density functional theory, as well as complete basis set (CBS-QB3 and CBS-APNO) and G4 methods. Relative and absolute energies were calculated with each of the methods, whereas computations of geometries and harmonic frequencies for dihydroxycyclopropenone and hydroxycyclopropanedione were computed in the gas phase but were limited to HF, MP2, and the B3LYP functional, in combination with the 6-31++G(3df,3pd) basis set. Using the MP2/6-31++G(3df,3pd) gas phase optimized structure, each species was then optimized fully in aqueous solution by using the polarizable continuum model (PCM) self-consistent reaction field approach, in which HF, MP2, and B3LYP levels of theory were utilized, with the same 6-31++G(3df,3pd) basis set. In both gas and aqueous solution phases, the keto form is higher in energy for all of the model chemistries considered. From the B3LYP/6-31++G(3df,3pd) Gibbs free energy, the keto-enol tautomeric equilibrium constant for 2,3-dihydroxycycloprop-2-en-1-one/3-hydroxy-1,2-cyclopropanedione is computed to be K(T)(gas) = 2.768 × 10(-12) and K(T)(aq) = 5.469 × 10(-14). It is concluded that the enol form is overwhelmingly predominant in both environments.

Computational study of the intramolecular proton transfer reactions of 3-hydroxytropolone (2,7-dihydroxycyclohepta-2,4,6-trien-1-one) and its dimers

The proton transfer reaction and dimerization processes of 3-hydroxytropolone (3-OHTRN) have been investigated using density functional theory (DFT) at the B3LYP/6-31+G** level. The influence of the solvent on the proton transfer reaction of 3-OHTRN was examined using the self-consistent isodensity polarized continuum model (SCI-PCM) with different dielectric constants (ε = 4.9, CHCI₃; ε = 32.63, CH₃OH; ε = 78.39, H₂O). The intramolecular proton transfer reaction occurs more readily in the gas phase than in solution. Results also show that the stability of 3-OHTRN dimers in the gas phase is directly affected by the hydrogen bond length in the dimer structure.

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

Fourier transform infrared (FTIR) absorption spectra in the 2000 to 3500 cm–1 range are reported for the gaseous 16 O, 16 O- and 18 O, 18 O-isotopomers of tropolone[OH(OD)] at 25 oC. The spectral doublet component separations are near 20 and 19 cm–1 for 16 O, 16 O- and 18 O, 18 O-Tp(OH), respectively, and near 7 and 6.5 cm–1 for 16 O, 16 O- and 18 O, 18 O-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.

Quantum Tunneling in the Midrange Vibrational Fundamentals of Tropolone

The Fourier transform infrared spectrum of tropolone(OH) vapor in the 1175-1700 cm(-1) region is reported at 0.0025 and 0.10 cm(-1) spectral resolutions. The 12 vibrational fundamentals in this region of rapidly rising vibrational state density are dominated by mixtures of the CC, CO, CCH, and COH internal coordinates. Estimates based on the measurement of sharp Q branch peaks are reported for 11 of the spectral doublet component separations DS(v) = |Delta(v) +/- Delta(0)|. Delta(0) = 0.974 cm(-1) is the known zero-point splitting, and three a(1) modes show tunneling splittings Delta(v) approximately Delta(0), four b(2) modes show splittings Delta(v) approximately 0.90Delta(0), and the remaining four modes show splittings Delta(v) falling 5-14% from Delta(0.) Significantly, the splitting for the nominal COH bending mode nu(8) (a(1)) is small, that is, 10% from Delta(0). Many of the vibrational excited states demonstrate strong anharmonic behavior, but there are only mild perturbations on the tautomerization mechanism driving Delta(0). The data suggest, especially for the higher frequency a(1) fundamentals, the onset of selective intramolecular vibrational energy redistribution processes that are fast on the time scale of the tautomerization process. These appear to delocalize and smooth out the topographical modifications of the zero-point potential energy surface that are anticipated to follow absorption of the nu(v) photon. Further, the spectra show the propensity for the Delta(v) splittings of b(2) and other complex vibrations to be damped relative to Delta(0).

An exploration of electronic structure and nuclear dynamics in tropolone. I. The X̃A11 ground state

The ground electronic state (X 1A1) of tropolone has been examined theoretically by exploiting extensive sets of basis functions [e.g., 6-311++G(d,p) and aug-cc-pVDZ] in conjunction with the high levels of electron correlation made possible by density functional (DFT/B3LYP), Moller-Plesset perturbation (MP2), and coupled-cluster [CCSD and CCSD(T)] methods. Unconstrained MP2 and CCSD optimization procedures performed with the reference 6-311++G(d,p) basis predict a slightly nonplanar equilibrium structure characterized by a small barrier to skeletal inversion (< or =10 cm(-1) magnitude). Complementary harmonic frequency analyses have shown this nonplanarity to be a computational artifact arising from adversely tuned carbon d-orbital exponents embodied in the standard definitions of several Pople-type basis sets. Correlation-consistent bases such as Dunning's aug-cc-pVDZ are less susceptible to these effects and were employed to confirm that the X 1A1 hypersurface supports a rigorously planar global minimum. The fully optimized geometries and vibrational force fields obtained by applying potent coupled-cluster schemes to the relaxed-equilibrium (Cs) and transition-state (C2v) conformers of tropolone afford a trenchant glimpse of the key features that mediate intramolecular hydron exchange in this model system. By incorporating perturbative triples corrections at the substantial CCSD(T) level of theory, an interoxygen distance of r(O...O)=2.528 A was determined for the minimum-energy configuration, with the accompanying proton-transfer reaction being hindered by a barrier of 2557.0 cm(-1) height. The potential energy landscape in tropolone, as well as the nature of the attendant hydron migration process, is discussed within the framework of the encompassing G4 molecular symmetry group.

High-resolution studies of tropolone in the S0 and S1 electronic states: Isotope driven dynamics in the zero-point energy levels

Rotationally resolved microwave (MW) and ultraviolet (UV) spectra of jet-cooled tropolone have been obtained in S(0) and S(1) electronic states using Fourier-transform microwave and UV-laser/molecular-beam spectrometers. In the ground electronic state, the MW spectra of all heavy-atom isotopomers including one (18)O and four (13)C isotopomers were observed in natural abundance. The OD isotopomer was obtained from isotopically enriched samples. The two lowest tunneling states of each isotopomer except (18)O have been assigned. The observed inversion splitting for the OD isotopomer is 1523.227(5) MHz. For the asymmetric (13)C structures, the magnitudes of tunneling-rotation interactions are found to diminish with decreasing distance between the heavy atom and the tunneling proton. In the limit of closest approach, the 0(+) state of (18)O was well fitted to an asymmetric rotor Hamiltonian, reflecting significant changes in the tautomerization dynamics. Comparisons of the substituted atom coordinates with theoretical predictions at the MP2/aug-cc-pVTZ level of theory suggest the localized 0(+) and 0(-) wave functions of the heavier isotopes favor the C-OH and C=O forms of tropolone, respectively. The only exception occurs for the (13)C-OH and (13)C[Double Bond]O structures which correlate to the 0(-) and 0(+) states, respectively. These preferences reflect kinetic isotope effects as quantitatively verified by the calculated zero-point energy differences between members of the asymmetric atom pairs. From rotationally resolved data of the 0(+) <--0(+) and 0(-) <--0(-) bands in S(1), line-shape fits have yielded Lorentzian linewidths that differ by 12.2(16) MHz over the 19.88(4) cm(-1) interval in S(1). The fluorescence decay rates together with previously reported quantum yield data give nonradiative decay rates of 7.7(5) x 10(8) and 8.5(5) x 10(8) s(-1) for the 0(+) and 0(-) levels of the S(1) state of tropolone.

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.

An Ab Initio and Density Functional Theory Study of Keto−Enol Equilibria of Hydroxycyclopropenone in Gas and Aqueous Solution Phase

Keto-enol tautomerism in hydroxycyclopropenone (2-hydroxy-2-cyclopropen-1-one) has been studied using ab initio methods, the B3LYP functional of density functional theory, as well as complete basis set (CBS-QB3 and CBS-APNO) and G3 methods. Absolute and relative energies were calculated with each of the methods, whereas computations of geometries and harmonic frequencies for hydroxycyclopropenone and 1,2-cyclopropanedione were computed in the gas phase but were limited to HF, MP2 and CCSD levels of theory, and the B3LYP functional, in combination with the 6-31++G** basis set. Using the MP2/6-31++G** gas phase optimized structure, each species was then optimized fully in aqueous solution by employing the polarizable continuum model (PCM) self-consistent reaction field approach, in which HF, MP2 and B3LYP levels of theory were utilized, with the same 6-31++G** basis set. In both gas and aqueous solution phases, the keto form is higher in energy for all of the model chemistries considered. The presence of the solvent, however, is found to have very little effect on the bond lengths, angles and harmonic frequencies. From the B3LYP/6-31++G** Gibbs free energy, the keto-enol tautomeric equilibrium constant for 2-hydroxy-2-cyclopropen-1-one <==> 1,2-cyclopropanedione is computed to be K(T)(gas) = 2.35 x 10(-6), K(T)(aq) = 5.61 x 10(-14). It is concluded that the enol form is overwhelmingly predominant in both environments, with the effect of the solvent shifting the direction of equilibrium even more strongly in the favor of hydroxycyclopropenone. The almost exclusive nature of this species is attributed to stabilization resulting from aromaticity. Confirmation is provided by comparison of the simulated vibrational spectra of hydroxycyclopropenone with the measured infrared spectrum in an argon matrix.

Dynamical hydrogen atom tunneling in dichlorotropolone: A combined quantum, semiclassical, and classical study

Based on the Cartesian Reaction Surface framework we construct a four-dimensional potential for the tropolone derivative 3,7-dichlorotropolone, a molecule with an intramolecular O-H...O hydrogen bond. The reduced configuration space involves the in-plane hydrogen atom coordinates, a symmetric O-O vibrational mode, and an antisymmetric mode related to deformations of the seven-membered ring. The system is characterized in terms of quantum mechanical computations of the low-lying eigenstates as well as a classical and semiclassical analysis of spectra obtained via Fourier transforming autocorrelation functions. For the semiclassical analysis we utilize the amplitude-free correlation function method [K. Hotta and K. Takatsuka, J. Phys. A 36, 4785 (2003)]. Our results demonstrate substantial anharmonic couplings leading to highly correlated wave functions even at moderate energies. Furthermore, the importance of dynamical tunneling in tropolone is suggested since many low-lying states--including the ground state--lie above the classical saddle point but nevertheless appear as split pairs.

Polarization spectroscopy of gaseous tropolone in a strong electric field

We report studies of polarization spectroscopy of gaseous tropolone in a strong electric field using resonantly enhanced multiphoton ionization. The electric field induces localization of the tunneling proton between the two equivalent oxygen atoms. As a result, the C2v symmetry of the molecular frame is broken, and the parity selection rule is violated. The field induced transitions are type A with transition dipoles perpendicular to those under field free conditions. The polarization ratios, i.e., the ratios of the overall excitation yield under different polarizations of the resonant laser, thus deviate from those of a pure type B transition. In a field of 60 kV/cm, the experimental polarization ratio implies an essentially equal mixture of type B and type A transitions. Moreover, the induced transitions overlap with the two field-free subbands, and the resulting intensity ratios between the two subbands demonstrate dependence on the applied electric field. These observations can be qualitatively modeled using a quantum mechanical approach by assuming a two level system. A puzzling result is the magnitude of the transition dipole of the induced transition, which is proven to be essentially linearly dependent on the applied electric field.

Rotation–tunneling analysis of the origin band in the tropolone π*←π absorption system

The tunneling-split origin band of the tropolone A (1)B(2)-X (1)A(1) (pi(*)<--pi) absorption system was interrogated under ambient, bulk-gas conditions by exploiting high-resolution degenerate four-wave mixing techniques. The inherent complexity of this spectral region was alleviated by performing polarization-resolved measurements, with judicious selection of transverse characteristics for the incident and detected electromagnetic fields enabling rovibronic transitions to be discriminated according to their attendant changes in rotational angular momentum, DeltaJ. Quantitative simulation of recorded data sets showed the vibrationless level of the electronically excited state to be bifurcated by Delta(0) (A)=19.846(25) cm(-1), representing a factor of 20 increase in proton-transfer efficiency over the corresponding level of the ground electronic state. Spectroscopic parameters extracted for the 0(+) and 0(-) manifolds of A (1)B(2) tropolone yield unexpectedly large values of the inertial defect, DeltaI(0(+) ) (A)=-0.802(86) amu A(2) and DeltaI(0(-) ) (A)=-0.882(89) amu A(2), strongly suggesting that a loss of molecular planarity accompanies the pi(*)<--pi electron promotion. These results, as well as complementary information deduced for interloping hot-band resonances, are discussed in terms of the unique structural and dynamical properties exhibited by tropolone and related proton-transfer species.

Determination of the proton tunneling splitting of the vinyl radical in the ground state by millimeter-wave spectroscopy combined with supersonic jet expansion and ultraviolet photolysis

The vinyl radical in the ground vibronic state produced in a supersonic jet expansion by 193 nm excimer laser photolysis of vinyl bromide was investigated by millimeter-wave spectroscopy. Due to the proton tunneling, the ground state is split into two components, of which the lower and higher ones are denoted as 0+ and 0-, respectively. Eight pure rotational transitions with Ka = 0 and 1 obeying a-type selection rules were observed for each of the 0+ and 0- states in the frequency region of 60-250 GHz. Tunneling-rotation transitions connecting the lower (0+) and upper (0-) components of the tunneling doublet, obeying b-type selection rules, were also observed in the frequency region of 190-310 GHz, including three R- and six Q-branch transitions. The observed frequencies of the pure rotational and tunneling-rotation transitions were analyzed by using an effective Hamiltonian in which the coupling between the 0+ and 0- states was taken into account. A set of precise molecular constants was obtained. Among others, the proton tunneling splitting in the ground state was determined to be DeltaE0 = 16,272(2) MHz. The potential barrier height was estimated to be 1580 cm(-1) from the proton tunneling splitting, by an analysis using a detailed one-dimensional model. The spin-rotation and hyperfine interaction constants were also determined for the 0+ and 0- states together with the off-diagonal interaction constants connecting the 0+ and 0- states, epsilonab + epsilonba for the spin-rotation interaction and Tab for the hyperfine interaction of the alpha (CH) proton. The hyperfine interaction constants, due to the alpha proton and the beta (CH2) protons, are consistent with those derived from electron spin resonance studies.