Tunneling Motion Research Articles

Water Docking Bias in [4]Helicene.

We report on the one- and two-water clusters of [4]helicene, the smallest polycyclic aromatic hydrocarbon with a helical sense, which were captured in the gas phase using high-resolution rotational spectroscopy. The structures of the complexes are unambiguously revealed using microwave spectra of isotopically enriched species. In the one-water cluster, the apparent splitting pattern is consistent with a tunneling motion that encompasses an exchange of strongly and weakly bonded water hydrogens. This motion is "locked" in the two-water cluster. The relevant intermolecular contacts, symmetry, and aromaticity effects are unveiled for the microsolvated chiral topologies. These observations entail the first glance at the structures and internal dynamics of the water binding motifs of a chiral polycyclic aromatic hydrocarbon.

Rotational Characterization of the Elusive gauche-Isoprene.

Isoprene (2-methyl-1,3-butadiene) is highly abundant in the atmosphere, second only to methane in hydrocarbon emissions. In contrast to the most stable trans rotamer, structural characterization of gauche-isoprene has proven challenging: it is weakly polar, present at the level of only a few percent at room temperature, and structurally complex due to both torsional and methyl tunneling motions. gauche-Isoprene has been observed by two distinct but complementary experimental approaches: chirped-pulse Fourier transform microwave (CP-FTMW) spectroscopy coupled with cryogenic buffer gas cooling, and cavity-enhanced FTMW spectroscopy with a pulsed discharge source. Thermal enhancement of the gauche population (from 1.7% to 10.3%) was observed in the cryogenic buffer gas cell when the sample was preheated from 300 to 450 K, demonstrating that high-energy rotamers can be efficiently isolated under our experimental conditions. Rotational parameters for the inversion states (0+/0-) have been determined for the first time, aided by calculations at increasing levels of theoretical sophistication. From this combined analysis, the inversion splitting Δ E and the Fbc Coriolis coupling constant between the two inversion states have been derived.

The Shape of the Simplest Non-proteinogenic Amino Acid α-Aminoisobutyric Acid (Aib).

The simplest non-proteinogenic amino acid α-aminoisobutyric acid (Aib), an analogue of glycine and alanine, has been vaporized by laser ablation and probed by high-resolution Fourier transform microwave spectroscopic techniques. Comparison of the experimental rotational and 14 N nuclear quadrupole constants with that predicted ab initio has allowed the identification of three conformers of Aib exhibiting three types of hydrogen-bond interactions I (NH⋅⋅⋅O=C, cis-COOH), II (OH⋅⋅⋅N, trans-COOH), and III (N-H⋅⋅⋅O-H, cis-COOH) within the amino acid backbone. The observation of conformer III, not detected previously for related proteinogenic amino acids with a nonpolar side chain in a supersonic expansion, indicates that the presence of the methyl groups should restrict the conformational relaxation from conformer Aib-III to Aib-I. For conformer Aib-II, the rotational spectra of the 13 C isotopomers reveal a tunneling motion arising from the two equivalent methyl groups in the molecule. The observation of a single spectrum at the midpoint between those predicted for the two 13 C of the methyl groups has been explained by considering a double-minimum potential function with a low-energy interconversion barrier for a large amplitude internal motion. This singular fact has been corroborated by the anomalous centrifugal distortion effects determined in conformer Aib-II.

Modeling the internal rotation tunnelling in benzyl alcohol by ring fluorination: The rotational spectrum of 3,5-difluorobenzyl alcohol

The rotational spectra of the OH and OD isotopologues of 3,5-difluorobenzyl alcohol have been measured by pulsed jet Fourier transform microwave spectroscopy. The main structural evidence is that the OCα-C1C2 dihedral angle, approximately 20°, is ca. 40° smaller than in benzyl alcohol. This difference is reflected in the features of the rotational spectrum: μc-type (instead of μb-) transitions are split into two evenly spaced component lines (by 14.367(1) × 2 MHz) showing that the CH2OH group undergoes a tunnelling motion which connects two equivalent minima above and below the aromatic ring. The barrier for the interconversion of the two mirror form has been estimated from the experimental tunnelling splitting to be 380 cm−1.

Microwave spectrum of the complex of 3,3,3-trifluoro-2-(trifluoromethyl)propanoic acid and formic acid

ABSTRACTWe report the first high resolution spectroscopic observation of the complex of 3,3,3-trifluoro-2-(trifluoromethyl)propanoic acid (TTPA) and formic acid. The rotational spectra were measured using a broadband chirped pulse and a narrow band cavity-based Fourier transform microwave spectrometer in the 8–10 GHz range. The conformational landscape of the TTPA-formic acid complex was explored at several levels of theory. The two most stable TTPA-formic acid conformers are of similar stability and feature the usual cyclic carboxylic double hydrogen bonded ring. Based on the broadband spectra obtained, only one stable heterodimer conformer was observed. We explain the absence of the second conformer to be a result of a double hydrogen tunnelling motion which converts the less stable heterodimer to the one observed in the jet expansion. Further CCSD(T) relative energy calculations confirm that the heterodimer conformer detected contains the most stable TTPA monomeric subunit. It is interesting to note that hydrogen bonding with formic acid offers a new path to effectively convert the less stable TTPA subunit to the most stable one in the jet expansion, while both TTPA monomeric conformers were detected in the same experiment.

Terahertz VRT Spectroscopy of the Water Hexamer-h12 Cage: Dramatic Libration-Induced Enhancement of Hydrogen Bond Tunneling Dynamics.

We report the assignment and analysis of 176 transitions belonging to a librational band of the (H2O)6 cage isomer near 525 cm-1(15 THz). From a fit of the transitions to an asymmetric top model, we observe both dramatic changes in the rotational constants relative to the ground state, indicating significant nonrigidity, and striking enhancement in the tunneling motions that break and reform the hydrogen bonds in the cluster. This is the fifth water cluster system to display such an enhancement in the 15 THz librational region, the details of which may help to elucidate the hydrogen bond dynamics occurring in bulk liquid water.

Feasibility Study of Submerged Floating Tunnels Moored by an Inclined Tendon System

Concepts of submerged floating tunnels (SFTs) for land connection have been continuously suggested and developed by several researchers and institutes. To maintain their predefined positions under various dynamic environmental loading conditions, the submerged floating tunnels should be effectively moored by reasonable mooring systems. With rational mooring systems, the design of SFTs should be confirmed to satisfy the structural safety, fatigue, and operability design criteria related to tunnel motion, internal forces, structural stresses, and the fatigue life of the main structural members. This paper presents a feasibility study of a submerged floating tunnel moored by an inclined tendon system. The basic structural concept was developed based on the concept of conventional cable-stayed bridges to minimize the seabed excavation, penetration, and anchoring work by applying tower-inclined tendon systems instead of conventional tendons with individual seabed anchors. To evaluate the structural performance of the new type of SFT, a hydrodynamic analysis was performed in the time domain using the commercial nonlinear finite element code ABAQUS–AQUA. For the main dynamic environmental loading condition, an irregular wave load was examined. A JONSWAP wave spectrum was used to generate a time-series wave-induced hydrodynamic load considering the specific significant wave height and peak period for predetermined wave conditions. By performing a time-domain hydrodynamic analysis on the submerged floating structure under irregular waves, the motional characteristics, structural stresses, and fatigue damage of the floating tunnel and mooring members were analyzed to evaluate the structural safety and fatigue performance. According to the analytical study, the suggested conceptual model for SFTs shows very good hydrodynamic structural performance. It can be concluded that the concept can be considered as a reasonable structural type of SFT.

Investigating nuclear quantum effects of water

The mystery of water mainly arises from the intermolecular hydrogen-bonding (H-bonding) interaction. It is well known that H bonds have a strong classic component coming from electrostatics. However, quantum behaviors of hydrogen nuclei (protons), including tunneling and zero-point motion have significant effects on the macroscopic properties, structure, and dynamics of water even at room temperature or higher. Despite enormous theoretical efforts toward pursuing proper treatment of the nuclear motion at a quantum mechanical level, accurate and quantitative description of NQEs on the H-bonding interaction has proven to be experimentally challenging. This review highlights the recent advances in investigating nuclear quantum effects (NQEs) of water, with focus on the studies of surface water at atomic scale. We first introduce the conventional spectroscopic techniques and the emerging scanning tunneling microscopy/spectroscopy (STM/S) and non-contact atomic force microscopy (nc-AFM), which allow the access to the quantum degree of freedom of protons at atomic scale. Real-space imaging of interfacial water with submolecular resolution has been achieved, enabling discrimination of the orientation of the single water molecules and the H-bonding directionality of water clusters at surfaces. In addition, the limit of vibrational spectroscopy of water has been pushed down to the single-bond level using tip-enhanced inelastic electron tunneling spectroscopy (IETS). We then discuss how those techniques are used to investigate the NQEs of surface water, such as proton tunneling and the impact of zero point motion on the H-bonding interaction. Proton tunneling has been observed in water/ice under various conditions, such as surface water, high-pressure ice and confined water. Interestingly, proton tunneling in H-bonded networks tends to involve many H bonds simultaneously, leading to concerted many-body tunneling, which has been visualized in the experiment as well. NQEs, in terms of zero-point motion, could influence the H-bonding interactions and consequently the structure of H-bonded network. A general physical picture for the NQEs of H bonds has been unraveled, that is, the anharmonic quantum fluctuations of hydrogen nuclei weaken the weak H bonds and strengthen the strong ones due to the competing effect between O-H stretching and H-bond bending. For completeness, we also briefly review the NQEs of bulk water/ice and confined water using the macroscopic spectroscopic techniques. Those findings may completely renovate our understanding of water and provide answers to many weirdness and the macroscopic isotope effects of water from a quantum mechanical view. In the end, we present an outlook on the current challenges and further opportunities in this field. There is an urgent need to develop new techniques to study the NQEs of water at ambient condition with atomic precision. It would be also interesting to extend the NQEs studies of water to other H-rich materials and, more generally, to light-element materials, such as Li, He, C, N, B.

Proton‐Coupled Electron Transfer in Soybean Lipoxygenase: Hydrogen Tunneling, Electrostatics, and Conformational Motions

The enzyme soybean lipoxygenase (SLO) serves as a prototype for hydrogen tunneling reactions because of its unusually large deuterium kinetic isotope effect (KIE) of 80 for the wild‐type enzyme and over 500 for a double mutant. The proton‐coupled electron transfer (PCET) reaction catalyzed by SLO has been studied with a vibronically nonadiabatic PCET theory that includes the quantum mechanical effects of the electrons and the transferring proton, as well as the proton donor‐acceptor and solvent motions. The calculations reproduced the experimentally observed magnitude and temperature dependence of the KIE for wild‐type SLO and a series of single mutants. The large KIE was found to arise from the small overlap between the reactant and product proton vibrational wave functions. Theoretical modeling of the double mutant illustrated that the colossal KIE of over 500 arises from a combination of a non‐optimal active site conformation corresponding to longer equilibrium donor‐acceptor distances and reduced sampling of shorter donor‐acceptor distances. Recent quantum mechanical/molecular mechanical (QM/MM) free energy simulations using a finite temperature string method combined with umbrella sampling have provided additional insights about the conformational motions, as well as the significance of electrostatics in this enzymatic reaction. These calculations highlight the critical roles of hydrogen tunneling, electrostatics, and protein conformational motions in enzyme catalysis.Support or Funding InformationNational Institutes of Health Grant GM056207This abstract is from the Experimental Biology 2018 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.

Crawling Experience Relates to Postural and Emotional Reactions to Optic Flow in a Virtual Moving Room

Infants show a dramatic shift in postural and emotional responsiveness to peripheral lamellar optic flow (PLOF) following crawling onset. The present study used a novel virtual moving room to assess postural compensation of the shoulders backward and upward and heart rate acceleration to PLOF specifying a sudden horizontal forward translation and a sudden descent down a steep slope in an infinitely long virtual tunnel. No motion control conditions were also included. Participants were 53 8.5-month-old infants: 25 prelocomotors and 28 hands-and-knees crawlers. The primary findings were that crawling infants showed directionally appropriate postural compensation in the two tunnel motion conditions, whereas prelocomotor infants were minimally responsive in both conditions. Similarly, prelocomotor infants showed nonsignificant changes in heart rate acceleration in the tunnel motion conditions, whereas crawling infants showed significantly higher heart rate acceleration in the descent condition than in the descent control condition, and in the descent condition than in the horizontal translation condition. These findings highlight the important role played by locomotor experience in the development of the visual control of posture and in emotional reactions to a sudden optically specified drop. The virtual moving room is a promising paradigm for exploring the development of perception–action coupling.

New insights onto dissociation of state-selected O2+ ions investigated by double imaging photoelectron photoion coincidence: The superimposed 32Πu and c4Σu- inner-valence states.

The double imaging photoelectron photoion coincidence technique has been applied to investigate the dissociation of state-selected O2+(32Πu, c4Σu-) ions at hv = 25.4 eV. It is shown that the O2+ ions prepared in the 32Πu inner-valence state can dissociate toward two channels where a distinct changeover from the third dissociation limit O(3P) + O+(2D) to the fifth limit O(3P) + O+(2P) is observed at a binding energy of ∼23.8 eV. Up to four dissociation limits have been identified in the dissociation of the v = 0 vibrational level of the c4Σu- electronic state, and in particular, the products toward the third limit with a very weak intensity are definitely identified and assigned. In addition, the individual contributions of the two superimposed 32Πu and c4Σu- states to the fifth dissociation limit are clearly separated and determined for the first time. The anisotropy parameters of the O+ fragment ion toward different dissociation limits have also been measured and exhibit specific state-selected characteristics. The dissociation mechanisms of the 32Πu and c4Σu- states including predissociation and tunneling motion are proposed based on the present experimental results and the existing theoretical potential energy curves.

Terahertz VRT spectroscopy of the water hexamer-d12 prism: Dramatic enhancement of bifurcation tunneling upon librational excitation

Using diode laser vibration-rotation-tunneling spectroscopy near 15 Thz (500 cm−1), we have measured and assigned 142 transitions to three a-type librational subbands of the water hexamer-d12 prism. These subbands reveal dramatically enhanced (ca. 1000×) tunneling splittings relative to the ground state. This enhancement is in agreement with that observed for the water dimer, trimer, and pentamer in this same frequency region. The water prism tunneling motion has been predicted to potentially describe the motions of water in interfacial and confined environments; hence, the results presented here indicate that excitation of librational vibrations has a significant impact on the hydrogen bond dynamics in these macroscopic environments.

Comparison of tunnel widening between interference screw and suture disc fixation after ACL reconstruction using CT scan

<p class="abstract"><strong>Background:</strong> <span lang="EN-IN">It is important to study the cause of tunnel widening which occurs after anterior cruciate ligament (ACL) reconstruction as it may affect tendon to bone healing. Amount of tunnel enlargement that happens after different fixation methods like interference screw or suture discs needs to be compared.</span>The objective of the study was <span lang="EN-IN">to test the hypothesis that aperture fixation (interference screw) reduces tunnel enlargement compared to suspensory fixation (suture disc) due to reduced graft tunnel motion.</span></p><p class="abstract"><strong>Methods:</strong> <span lang="EN-IN">24 bone tunnel diameters in 12 patients were evaluated by CT scan postoperatively after ACL reconstruction to measure tunnel widening. Two groups were formed, one consisting of 14 tunnels fixed by interference screws (IFS) and other consisting of 10 tunnels fixed with suture disc (SD). The difference between the two groups was compared by unpaired student’s t test</span>.<strong></strong></p><p class="abstract"><strong>Results:</strong> <span lang="EN-IN">The mean tunnel widening in IFS group was 0.414mm while that in the SD group was 1.23mm. The difference between the means of the two groups was statistically significant (p<0.001). </span></p><p class="abstract"><strong>Conclusions:</strong> <span lang="EN-IN">Tunnel widening phenomenon was significantly less with anatomic IFS fixation as compared to suture disc fixation probably due to reduced motion of graft within the walls of bony tunnel and consequently better graft healing.</span></p>

Guided modes in silicene-based waveguides

Silicene is a new Dirac-type electron system similar to graphene. A monolayer silicene sheet forms a quantum well induced by an electrostatic potential, which acts as an electron waveguide. The guided modes in the silicene waveguide have been investigated. Electron waves can propagate in the silicene-based waveguide in the cases of Klein tunneling and classical motion. The behavior of the wave function depends on the spin and valley indices. The amplitude of the electron wave function in the silicene waveguide can be controlled by the external electric field. These phenomena may be helpful for the potential applications of silicene-based electronic devices.

Rotational Spectroscopy of p-Toluic Acid and Its 1:1 Complex with Water.

The structure and internal dynamics of p-toluic acid and its 1:1 complex with water were investigated in the gas phase using chirped-pulse and cavity-based Fourier transform microwave spectroscopy. One conformer and one isomer were identified for the monomer and monohydrate, respectively. In the monohydrate, water acts as both a hydrogen bond donor and acceptor, participating in a six-membered intermolecular ring with the carboxyl group. Both a- and b-type transitions were observed for the monomer; only a-type transitions were observed for the monohydrate. Rotational transitions of both species show splittings originating from methyl internal rotation, for which the potentials include 3- and 6-fold symmetry terms of similar amplitude. For the monomer, a few b-type transitions are missing, and their intensities were found to be transferred to c-type transitions with common energy levels, which are otherwise forbidden. No splittings attributable to a water internal tunneling motion were observed for the monohydrate. Furthermore, the absence of c-type transitions in the spectrum of the monohydrate, despite the large μc dipole moment component of the equilibrium geometry obtained by ab initio calculations, is consistent with a barrierless wagging motion of the free hydrogen of water, leading to an average μc dipole moment component of zero. These results provide insights regarding the interactions between atmospheric p-toluic acid and water in prenucleation complexes and at the air-water interface of aqueous aerosols, where p-toluic acid may act as a surfactant.

Anomalous Raman peaks of intra-molecular hydrogen bond material 5-bromo-9-hydroxyphenalenone under various pressure and temperature conditions

ABSTRACTStudies on the Raman peaks appear at 593, 625, 651 and 1198 cm−1 in the intra-molecule hydrogen bond material 5-bromo-9-hydroxyphenalenone were performed under various pressure and temperature conditions. These peaks show two characteristic results: (1) in high pressure study, their pressure coefficients are approximately 4–6 times larger than those of other Raman peaks of this material, (2) in the latter experiment, they decrease in intensity with temperature and vanish around room temperature. These results support a model that these Raman peaks are generated by intra-molecule vibrations coupled with a proton tunneling motion.

Molecular structure and ring tunneling of phenyl formate as observed by microwave spectroscopy and quantum chemistry

Phenyl formate has been investigated by molecular jet Fourier-transform microwave spectroscopy in the frequency range from 2 to 26.5GHz. Quantum chemical calculations at the MP2/6-311++G(d,p) level of theory indicate that this molecule does not have a plane of symmetry at equilibrium, and that the phenyl ring performs a large amplitude tunneling motion from one side of the Cs configuration to the other. The tilt angle of the ring out of the H(CO)O plane is ±70° and the calculated tunneling barrier is only 28cm–1. The two lowest torsional states vt=0 and 1 are assigned in the experimental spectrum and fitted using the program SPFIT/SPCAT. The Coriolis splitting ΔE between these states is 46.2231(25) GHz, very close to the value of 48.24GHz calculated using a simple two-top torsional Hamiltonian of the formate group and the phenyl ring.

(Invited) Dielectric Response and Quantum Motion of Lithium Cations Trapped inside Carbon Cages

Endohedral fullerenes trapping atoms and molecules with magnetic and electric moments have a potential to be used for quantum computing. A C60 fullerene trapping a Li+ cation has an electric monopole moment, and forms a rock-salt-type cubic crystal, [Li@C60](PF6). The dielectric response and quantum motion of the Li+ cations in the crystal at low temperature were revealed by the dielectric and X-ray diffraction measurements [1]. The Li+ cations are moving inside the C60 cages by the hopping and tunneling motion between the potential wells under 20 hexagons above 100 K. The Li+ motion is suppressed to be localized at two equivalent positions on the threefold inversion axis with decreasing temperature below 100 K. The simultaneous occupation of the Li+ cation at the two positions is mainly achieved by the tunneling motion. The hopping motion is slowing down and hindered by the energy barriers between the wells. The tunneling motion is also hindered below T C = 24 K by the antiferroelectric interactions among local electric dipole moments formed between the Li+ cations inside and the PF6 − anions outside the C60 cages. The tunneling motion and intermolecular interaction of the Li+ cation found in [Li@C60](PF6) show that Li+@C60 has a potential to be used as a qubit in a quantum computer using electric dipole moments. [1] S. Aoyagi, A. Tokumitu, K. Sugimoto, H. Okada, N. Hoshino, and T. Akutagawa, J. Phys. Soc. Jpn. 85, 094605 (2016)

Quantum Phenomena Emerging Near a Ferroelectric Critical Point in a Donor–Acceptor Organic Charge-Transfer Complex

When a second-order transition point is decreased to zero temperature, a continuous quantum phase transition between different ground states is realized at a quantum critical point (QCP). A recently synthesized organic charge-transfer complex, TTF-2,5-QBr 2 I 2 , provides a platform for the exploration of the quantum phenomena that accompany a ferroelectric QCP. Here, we summarize the recent results showing the quantum phenomena associated with the ferroelectric QCP in TTF-2,5-QBr 2 I 2 . Whereas the enhanced quantum fluctuations lead to quantitative changes in the critical exponents of the critical phenomena, they qualitatively change the nature of the domain-wall kinetics from thermally activated motion to temperature-independent tunneling motion. The present findings highlight the great influence of quantum fluctuations on the low-temperature physical properties and suggest that TTF-2,5-QBr 2 I 2 is a model system for the uniaxial ferroelectric QCP.

Convergence of theory and experiment on the role of preorganization, quantum tunneling and enzyme motions into flavoenzyme-catalyzed hydride transfer.

Hydride transfer is one of the most common reactions catalyzed by enzymatic systems and it has become an object of study due to possible significant quantum tunneling effects. In the present work, we provide a combination of theoretical QM/MM simulations and experimental measurements of the rate constants and kinetic isotopic effects (KIEs) for the hydride transfer reaction catalyzed by morphinone reductase, MR. Quantum mechanical tunneling coefficients, computed in the framework of variational transition-state theory, play a significant role in this reaction, reaching values of 23.8 ± 5.5 for the lightest isotopologue; one of the largest values reported for enzymatic systems. This prediction is supported by the agreement between the theoretically predicted rate constants and the corresponding experimental values. Simulations indicate that the role of protein motions can be satisfactorily described as equilibrium fluctuations along the reaction coordinate, in line with a high degree of preorganization displayed by this enzyme.