Mode-specific tunneling dynamics in the ground electronic state of tropolone

The mode specificity of proton-transfer dynamics in the ground electronic state (X (1)A(1)) of tropolone has been explored at near-rotational resolution by implementing a fully coherent variant of stimulated emission pumping within the framework of two-color resonant four-wave mixing spectroscopy. Three low-lying (E(vib) approximately 550-750 cm(-1)) vibrational features, assigned to nu(30)(a(1)), nu(32)(b(2)), and nu(31)nu(38)(a(1)), have been interrogated under ambient, bulk-gas conditions, with term energies determined for the symmetric and antisymmetric (tunneling) components of each enabling the attendant tunneling-induced bifurcations of 1.070(9), 0.61(3), and 0.07(2) cm(-1) to be extracted. The dependence of tunneling rate (or hydron migration efficiency) on vibrational motion is discussed in terms of corresponding atomic displacements and permutation-inversion symmetries for the tropolone skeleton.     

18O 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.     

A reassignment of the vibrational fundamentals of sulfuryl chloride fluoride

Low temperature Raman spectroscopy has led to a significant revision in the assignment of the vibrational fundamentals of sulfuryl chloride fluoride (SO₂FCl). The fundamentals in cm⁻¹ are: (a′) 1230, 826, 632, 503, 422, 295; (a″) 1469, 476, 303. All are from gas-phase i.r. spectra except 295 cm⁻¹, which is from the liquid-phase Raman spectrum. The revised assignments are consistent with the predictions of Pfeiffer's normal coordinate calculations [Z. phys. chem., Leipzig 240, 380 (1969)].     

Quantum tunneling effect in entanglement dynamics

Quantum tunneling effect in entanglement dynamics between two coupled particles with separable Gaussian initial state is investigated using entangled trajectory molecular dynamics method in terms of the reduced‐density linear entropy. It has been presented through showing distinguish contribution of single trajectory to linear entropy between classical trajectory and entangled trajectory with same initial state. We find that quantum tunneling effect makes single trajectory's contribution remarkably decrease under quantum dynamics compared to classical dynamics. The nonlocality of quantum entanglement is presented, and the energy transfer between two coupled particles through quantum correlations and classical ones is also discussed in the end. © 2015 Wiley Periodicals, Inc.     

Spectroscopic and theoretical study of vibrational spectra of hydrogen‐bonded tropolone

Theoretical simulation of the bandshape and fine structure of the ν_s stretching band is presented for tropolone‐H and tropolone‐D taking into account an adiabatic coupling between the high‐frequency O–H(D) stretching and the low‐frequency intra‐ and intermolecular OO stretching modes, and linear and quadratic distortions of the potential energies for the low‐frequency vibrations in the excited state of the O–H(D) stretching vibration. In order to determine the low‐frequency vibrations, the experimental spectra of the polycrystalline tropolone in the far‐infrared and the low‐frequency Raman range have been recorded for the first time. The experimental frequencies in the low‐frequency region are compared with the results of the HF/6‐31G** and Becke3LYP/6‐31G** calculations carried out for the tropolone dimer. ©1999 John Wiley & Sons, Inc. Int J Quant Chem 73: 275–282, 1999     

Inelastic electron tunneling spectroscopy and vibrational coupling

We discuss the relationship between the inelastic electron tunneling spectroscopy (IETS) and vibronic coupling constant within the Green's function formalism at a level of perturbation theory approximation. We also compare our results with experimental measurements. Our results can provide insights into the mechanism of active vibronic modes for IETS.     

Trajectory on-the-fly molecular dynamics approach to tunneling splitting in the electronic excited state: A case of tropolone

The semiclassical tunneling method is applied to evaluate the tunneling splitting of tropolone due to the intramolecular proton transfer in the electronic excited state, first time, in a framework of the trajectory on-the-fly molecular dynamics (TOF-MD) approach. To prevent unphysical zero-point vibrational energy transfer among the normal modes of vibration, quantum zero-point vibrational energies are assigned only to the vibrational modes related to intramolecular proton transfer, whereas the remaining modes are treated as bath modes. Practical ways to determine the tunnel-initiating points and tunneling path are introduced. It is shown that the tunneling splitting decreases as the bath-mode energy increases. The experimental tunneling splitting value is well reproduced by the present TOF-MD approach based on the Wentzel-Kramers-Brillouin (WKB) approximation.     

Multidimensional vibrational spectroscopy for tunneling processes in a dissipative environment

Simulating tunneling processes as well as their observation are challenging problems for many areas. In this study, we consider a double-well potential system coupled to a heat bath with a linear-linear (LL) and square-linear (SL) system-bath interactions. The LL interaction leads to longitudinal (T1) and transversal (T2) homogeneous relaxations, whereas the SL interaction leads to the inhomogeneous dephasing (T2*) relaxation in the white noise limit with a rotating wave approximation. We discuss the dynamics of the double-well system under infrared (IR) laser excitations from a Gaussian-Markovian quantum Fokker-Planck equation approach, which was developed by generalizing Kubo's stochastic Liouville equation. Analytical expression of the Green function is obtained for a case of two-state-jump modulation by performing the Fourier-Laplace transformation. We then calculate a two-dimensional infrared signal, which is defined by the four-body correlation function of optical dipole, for various noise correlation time, system-bath coupling parameters, and temperatures. It is shown that the bath-induced vibrational excitation and relaxation dynamics between the tunneling splitting levels can be detected as the isolated off-diagonal peaks in the third-order two-dimensional infrared (2D-IR) spectroscopy for a specific phase matching condition. Furthermore, this spectroscopy also allows us to directly evaluate the rate constants for tunneling reactions, which relates to the coherence between the splitting levels; it can be regarded as a novel technique for measuring chemical reaction rates. We depict the change of reaction rates as a function of system-bath coupling strength and a temperature through the 2D-IR signal.     

Quantum mechanical predictions of chiroptical vibrational properties

The developments in quantum mechanical calculations of vibrational circular dichroism, vibrational Raman optical activity, and vibrational contributions to optical rotation are summarized. Further developments needed in each of these areas are pointed out. © 2006 Wiley Periodicals, Inc. Int J Quantum Chem, 2006     

Quantum mechanical tunneling in synaptic and ephaptic transmission

Quantum mechanical tunneling theory is applied to the problem of synaptic vesicle release and to the problem of electric transmission at the ephaptic junction. In the synapse the tunneling produces conformational changes in macromolecules to open and close vesicle macrogates. Quantum mechanical tunneling as a basis for charge transfer and physical release of vesicles at junction membranes provides a unified concept of ephaptic and synaptic transmission. Details of this model are in agreement with experimental data for miniature endplate potential frequency and delay effects as a function of polarization, osmotic pressure, and temperature. The theory accounts for anatomical specializations at the synaptic cleft and the narrow junction observed for the ephapse.     

Quantum tunneling process for double well potential

Quantum tunneling effects of Gaussian wave packet in one‐ and two‐dimensional double well potentials are investigated using quantum Liouville equation for time evolution of Wigner distribution in phase space. It is shown that a trajectory‐based solution of this problem can be constructed by the entangled trajectory ensemble simulating the evolving quantum state. Quantum effects arise in this approach as a breakdown of the statistical independence of the trajectory ensemble. © 2014 Wiley Periodicals, Inc.     

Quantum tunneling of magnetization and related phenomena in molecular materials

Molecules comprising a large number of coupled paramagnetic centers are attracting much interest because they may show properties which are intermediate between those of simple paramagnets and classical bulk magnets and provide unambiguous evidence of quantum size effects in magnets. To date, two cluster families, usually referred to as Mn12 and Fe8, have been used to test theories. However, it is reasonable to predict that other classes of molecules will be discovered which have similar or superior properties. To do this it is necessary that synthetic chemists have a good understanding of the correlation between the structure and properties of the molecules, for this it is necessary that concepts such as quantum tunneling, quantum coherence, quantum oscillations are understood. The goal of this article is to review the fundamental concepts needed to understand quantum size effects in molecular magnets and to critically report what has been done in the field to date.     

Quantum control of vibrational excitations in a heteronuclear diatomic molecule

Optimal control theory is applied to obtain infrared laser pulses for selective vibrational excitation in a heteronuclear diatomic molecule. The problem of finding the optimized field is phrased as a maximization of a cost functional which depends on the laser field. A time dependent Gaussian factor is introduced in the field prior to evaluation of the cost functional for better field shape. Conjugate gradient method^21,24 is used for optimization of constructed cost functional. At each instant of time, the optimal electric field is calculated and used for the subsequent quantum dynamics, within the dipole approximation. The results are obtained using both Morse potential as well as potential energy obtained using ab initio calculations.     

Quantum equations for vibrational dynamics on metal surfaces

A first principles treatment of the vibrational dynamics of molecular chemisorbates on metal surfaces is presented. It is shown that the mean field quantum evolution of the vibrational position operator is determined by a quantum Langevin equation with an electronic friction. In the mean field limit, the quantum noise and friction are related by the quantum fluctuation-dissipation theorem. The classical limit of this model is shown to agree with previously proposed models. A criterion is presented to describe the validity of the weak-coupling approximation and equations of motion for the dynamics in the presence of strong nonadiabatic coupling to electron-hole pairs are presented.     

Quantum chemical calculation of force constants and vibrational spectra

As a result of the development of direct derivative methods and improved computational facilities, ab initio quantum chemical calculations have become an increasingly important source of information for the determination of molecular force constants. Within the Hartree-Fock (H-F) SCF model and using moderate size basis sets such calculations are now economically feasible for molecules of up to 2o–3o atoms. At this level of theory, harmonic diagonal force constants are overestimated by 1o–3o%, corresponding to 5–15% in the frequencies. However, the largely systematic errors can be accounted for by simple empirical corrections. The resulting SQM (Scaled Quantum Mechanical) force fields are probably the most reliable ones available at present for larger molecules. Calculated infrared intensities are semi-quantitatively correct. Beyond the H-F model, large scale calculations including electron correlation give great improvements in the force constants, but there are still residual errors of a few percent.     

Quantum tunneling dynamics in multidimensional systems: a matching-pursuit description

Rigorous simulations of quantum tunneling dynamics in model systems with up to 20 coupled degrees of freedom are reported. The simulations implement an extension of the recently developed matching-pursuit/split-operator Fourier-transform method to complex-valued coherent-state representations. The resulting method recursively applies the time-evolution operator, as defined by the Trotter expansion to second order accuracy, in dynamically adaptive coherent-state representations generated by an approach that combines the matching-pursuit algorithm with a gradient-based optimization method.     

Quantum entanglement between electronic and vibrational degrees of freedom in molecules

We consider the quantum entanglement of the electronic and vibrational degrees of freedom in molecules with tendencies towards double welled potentials. In these bipartite systems, the von Neumann entropy of the reduced density matrix is used to quantify the electron-vibration entanglement for the lowest two vibronic wavefunctions obtained from a model Hamiltonian based on coupled harmonic diabatic potential-energy surfaces. Significant entanglement is found only in the region in which the ground vibronic state contains a density profile that is bimodal (i.e., contains two separate local maxima). However, in this region two distinct types of density and entanglement profiles are found: one type arises purely from the degeneracy of energy levels in the two potential wells and is destroyed by slight asymmetry, while the other arises through strong interactions between the diabatic levels of each well and is relatively insensitive to asymmetry. These two distinct types are termed fragile degeneracy-induced entanglement and persistent entanglement, respectively. Six classic molecular systems describable by two diabatic states are considered: ammonia, benzene, BNB, pyridine excited triplet states, the Creutz-Taube ion, and the radical cation of the "special pair" of chlorophylls involved in photosynthesis. These chemically diverse systems are all treated using the same general formalism and the nature of the entanglement that they embody is elucidated.