Asymmetric Double Minimum Potential Research Articles

Phonon-assisted proton tunneling in the hydrogen-bonded dimeric selenates of Cs3H(SeO4)2.

In phases III and IV of Cs3H(SeO4)2, the vibrational state and intrabond transfer of the proton in the dimeric selenates are systematically studied with a wide range of absorbance spectra, a spin-lattice relaxation rate of 1H-NMR (T1 -1), and DFT calculations. The OH stretching vibrations have extremely broad absorption at around 2350 (B band) and 3050 cm-1 (A band), which originate from the 0-1 and 0-2 transitions in the asymmetric double minimum potential, respectively. The anharmonic-coupling calculation makes clear that the A band couples not only to the libration but also to the OH bending band. The vibrational state (nano-second order) is observed as the response of the proton basically localized in either of the two equivalent sites. The intrabond transfer between those sites (pico-second order) yields the protonic fluctuation reflected in T1 -1. Together with the anomalous absorption [νp2 phonon, libration, tetrahedral deformation (δ440), and 610-cm-1 band], we have demonstrated that the intrabond transfer above 70 K is dominated by the thermal hopping that is collectively excited at 610 cm-1 and the phonon-assisted proton tunneling (PAPT) relevant to the tetrahedral deformation [PAPT(def)]. Below 70 K, T1 -1 is largely enhanced toward the antiferroelectric ordering and the distinct splitting emerges in the libration, which dynamically modulates the O(2)-O'(2) distance of the dimer. The PAPT(lib) associated with the libration is confirmed to be a driving force of the AF ordering.

Millimeter-wave spectroscopy of HDC=CH.

Rotational transitions of the mono(β)-deuterated vinyl radical, HDC=CH, produced in a supersonic jet expansion by the ArF excimer laser photolysis, were observed by millimeter-wave spectroscopy. The b-type rotational transitions together with the weak a-type transitions were observed only for the lower component of the tunneling doublet, and no tunneling-rotation transitions connecting the lower and upper components were observed, suggesting that state mixing between the two components is negligibly small. The derived molecular constants such as the A rotational constant, Fermi contact interaction constants, and magnetic dipolar interaction constants indicate that the carrier of the observed spectrum is the trans-form of HDC=CH isomers, where the α-proton is located on the opposite side of the β-deuteron. The present conclusion of the trans-form of HDC=CH was also supported by the ab initio calculation in the CCSD(T)/cc-pVTZ level since the trans-form is calculated to be located by 30.04 cm-1 lower than the cis-form due to the difference in the zero point energy. As a result, the tunneling components in the ground state of HDC=CH behave as two different isomers localized at the trans- and cis-wells of the asymmetric double minimum potential. Observed hyperfine constants for HDC=CH were compared with those for H2C=CH to be consistent with each other.

Structural Refinements and Thermal Properties of L(+)-Tartaric, D(–)-Tartaric, and Monohydrate Racemic Tartaric Acid

<p>Differential scanning calorimetry, thermogravimetric-differential thermal analysis, and X-ray diffraction measurements were performed on single crystals of L(+)-tartaric, D(–)-tartaric, and monohydrate racemic (MDL-) tartaric acid. The exact crystal structures of the three acids, including the positions of all hydrogen atoms, were determined at room temperature. It was pointed out that one of O–H–O hydrogen bonds in MDL-tartaric acid has an asymmetric double-minimum potential well along the coordinate of proton motion. The weight losses due to thermal decomposition of L- and D-tartaric acid were observed to occur at 443.0 and 443.2 K, respectively, and at 306.1 and 480.6 K for MDL-tartaric acid. The weight losses for L- and D-tartaric acid during decomposition were probably caused by the evolution of 3H<sub>2</sub>O and 3CO gases. By considering proton transfer between two possible sites in the hydrogen bond, we concluded that the weight losses at 306.1 and 480.6 K for MDL-tartaric acid were caused by the evaporation of half the bound water molecules in the sample, and by the evaporation of the remaining water molecules and the evolution of 3H<sub>2</sub>O and 3CO gases, respectively.</p>

Conformational analysis, tautomerization, IR, Raman, and NMR studies of 3-phenylazo-2,4-pentanedione

A complete conformational analysis of the azo-enol, oxo-azo, and hydrazo forms of 3-phenylazo-2,4-pentanedione (PAPD), a β-dicarbonyl compound, was carried out by ab initio calculations, at the density functional theory (DFT) level. In addition to nine stable azo-enol conformers, three hydrazo and two oxo-azo were also obtained. The considerably higher energy of the oxo-azo and azo-enol compared to that of the most stable hydrazo conformer makes the presence of oxo-azo and azo-enol form, at least in the gas phase, unlikely. Harmonic vibrational frequencies of this stable hydrazo form and its deuterated analog were also calculated and compared with the experimental data. According to the theoretical calculations, the intramolecularly hydrogen bonded proton in the most stable tautomer moves in an asymmetric double minimum potential with a hydrogen bond stronger than that of 4-amino-3-penten-2-one (APO). The calculated O· · ·N distance for the most stable conformer of PAPD is about 0.07–0.08 Ǻ shorter than that of APO.

Evidence for Two Forms, Double Hydrogen Tunneling, and Proximity of Excited States in Bridge-Substituted Porphycenes: Supersonic Jet Studies

Laser-induced fluorescence and dispersed fluorescence spectra measured in supersonic jets for 9,10,19,20-tetra-n-methylporphycene and 9,10,19,20-tetra-n-propylporphycene reveal, for both compounds, the presence of two different species which are assigned to trans and cis tautomeric forms. Doublet splitting of lines is observed, disappearing upon deuteration of the inner nitrogen atoms. This finding is interpreted as an indication of double hydrogen tunneling. The values of tunneling splitting are obtained for both ground and lowest singlet excited states. The splitting is similar for cis and trans forms, and the barrier for tautomerization is larger in the excited state. Due to the coupling of hydrogen motion with rotation of alkyl substituents, tautomerization occurs in an asymmetric double minimum potential, with the ordering of energy minima reversed upon excitation. The second singlet excited state is found to lie very close to S(1), thus facilitating an efficient radiationless depopulation.

Elastic neutron scattering study of water dynamics in ion-exchanged type-Azeolites

With the aim to investigate, by means of elastic neutron scattering, the effects produced by the cation substitution on the dynamics of water in zeolites, we measured, using a neutron backscattering spectrometer, the temperature dependence of mean-square atomic displacements [u2] derived from window integrated quasielastic spectra of fully and partially hydrated Na-A and Mg50-A zeolites. The results, collected in the 20-273 K temperature range, reveal that, at low temperature, the [u2] shows a harmonic trend independent of hydration and cation substitution, and, at higher temperatures, the onset of a non-Gaussian dynamics of the elastic intensity. This latter takes place at T approximately 200 K and approximately 150 K for fully and partially hydrated samples, respectively. This behavior has been interpreted in terms of reorientational jumps of H atoms described by two-site processes within an asymmetric double-minimum potential. In spite of its simplicity, the model seems to reproduce the rearrangement of the hydrogen bond network of zeolitic water. The fit results indicate a reduced proton mobility by diminishing the water content and by the induced Na+-->Mg2+ ion exchange, in agreement with previous incoherent quasielastic neutron scattering results at higher temperatures.

Proton mobilities in crambin and glutathione S-transferase

Using a neutron backscattering spectrometer, the temperature dependence of mean-square atomic displacements derived from window-integrated quasielastic spectra was measured for two D2O-hydrated proteins: crambin and glutathione S-transferase. Analyses show that the anharmonic dynamics observed around and above 200 K is consistent with a description in terms of proton/deuteron jumps within asymmetric double-minimum potentials. Also determined were activation energies along with estimates of effective masses and average oscillator energies. 2003 Elsevier Science B.V. All rights reserved.

X-Ray and spectroscopic re-investigation of urotropine–p-nitrophenol complex

The product of the reaction between urotropine and p-nitrophenol, reported as a 1:1 adduct that belongs to the triclinic P1 space group, is, in fact, a 1:2 [C6H12N4]·[p-HOC6H4NO2]2·[H2O] hydrate that crystallizes in the monoclinic C2 space group. In the crystal structure, one nitrogen atom of the urotropine moiety is linked by a hydrogen bond to the hydroxyl group of a p-nitrophenol molecule [N⋯O=2.655(7)Å]; the [C6H12N4]·[p-HOC6H4NO2] entities are linked into a linear one-dimensional chain through the lattice water molecule [O⋯N=2.871(8),O⋯O=2.878(7)Å]. The other p-nitrophenol moiety is disordered over two positions across the two-fold axis; its interaction with adjacent water molecules [O⋯O=2.53(1), 2.57(2)Å] holds neighboring chains together. The FTIR spectrum of the complex in the solid indicate the formation of OH⋯N intermolecular hydrogen bond described by an asymmetric double minimum potential and weak Zundel's polarizability. The spectroscopic measurements in solution demonstrate the dissociation of the complex observed in the solid and prove the presence of the 1:1 complexes urotropine–p-nitrophenol. In these complexes no proton transfer from p-nitrophenol to urotropine is observed.

OHO]− and +[NHO]− hydrogen bonds in the 2:1 adduct of pentachlorophenol with 7-methyl-1,5,7-triazabicyclo[4.4.0]dec-5-ene [(PCP)2·MTBD

The results of X-ray diffraction and IR spectroscopic studies for 2:1 pentachlorophenol-7-methyl-1,5,7-triazabicyclo[4.4.0]dec-5-ene [(PCP)2·MTBD] adduct are reported. The geometry of MTBD cations reflects the equal distribution of the positive charge among three nitrogen atoms. Short asymmetric [OHO]− hydrogen bonds with O⋯O distance of 2.508(2)Å and NH+⋯O− hydrogen bonds with N⋯O distance of 2.802(2)Å are formed showing broad IR absorption with two maxima located at ∼1200 and ∼2400cm−1. The second maximum is interpreted as due to the 0→2 transition between split levels in an asymmetric double minimum potential. One of the oxygen atoms forms an additional O⋯H–N+ hydrogen bonding with an MTBD cation. The situation is somewhat different in acetonitrile solution whose IR spectrum shows continuous absorption extended over whole the IR region. In acetonitrile, dissociation to free OHO− and +NH ions takes place and the OHO− bridges become dynamically symmetric. The broadening is interpreted in terms of a stochastic distribution of the geometry and the Zundel polarizability theory.

Deuterium NMR Study of Unstable Phenomena and Water Molecular Dynamics in Samarium Nitrate Hexahydrate Crystal

Deuterium NMR spectra, spin−lattice relaxation time T1, and differential thermal analysis (DTA) were measured for Sm(NO3)3·6D2O. The magnetization recovery can be divided into two components. From deuterium NMR spectra and T1, the short and the long components of T1 were found to be mainly dominated by the 180° flip of the crystallization and the coordinated water molecules, respectively. For the nonannealed sample, the reproducibility of T1 was observed. The influence of the instability was not seen in the long component of T1. The results of the long component of T1 gave the activation energy Ea = 20 kJ mol-1 and the correlation time at infinite temperature τc0 = 7.8 × 10-12 s for the 180° flip of the coordinated water molecule. The motion of the crystallization water at low temperatures can be explained by the 180° flip in the asymmetric double minimum potential. τc0 = 1.2 × 10-12 s, Ea = 16 kJ mol-1, and the energy difference in the potential wells ΔE = 2.0 kJ mol-1 were obtained for the 180° flip of ...

FT-IR Investigation of OH···N ⇌ O-···H+N Hydrogen Bonds with Large Proton Polarizability in Phosphinic Acid + N-Base Systems in the Middle and Far Infrared Region

Sixteen 1:1 dimethylphosphinic acid + N-base systems were studied in acetonitrile−chloroform (1:2) solutions as a function of the basicity of the N-bases. The complexes were measured in the middle infrared (MIR) and far-infrared (FIR) region at 20 °C and at −40 °C. The observed IR continua demonstrate that in the systems with the weaker bases an asymmetrical double minimum proton potential with the deeper well at the acid site is present. With increasing basicity the well at the base site becomes deeper and deeper and the proton potentials obtain a more symmetrical shape. The largest proton polarizability is attained with the system that shows the (on the average) most symmetrical proton potential, as indicated by the maximum bathochromic shift of the IR continuum. This shift toward lower wavenumbers is largest with the dimethylphosphinic acid + triallylamine complex. The most intense integrated absorbance of the IR continuum is also observed in this system. The characteristic intensity distribution at the symmetry point indicates medium-strong and relatively long OH···N ⇌ O-···H+N hydrogen bonds. With further increasing basicity the proton potential becomes asymmetrical again, but now with the deeper well at the base site. The proton transfer was studied considering the PO stretching vibration bands. The simultaneous observation of the acid as well as the acid anion PO bands reflect a proton-transfer equilibrium between the nonpolar and polar structure. The far-IR region demonstrates that in the most symmetrical cases the IR continua extend down to about 100 cm-1. Moreover, in contrast to the results with other families of systems, it was for the first time possible to distinguish the nonpolar and polar structure in the far-IR region. Furthermore, the position of the hydrogen bond vibration indicates a significant trend of the force constant. The precise evaluation shows that the system with the (on the average) most symmetrical hydrogen bonds is achieved with a little weaker base than triallylamine, showing the largest bathochromic shift of the IR continuum.

Proton transport in ordered hydrogen-bonded solids

The proton transfer is considered in hydrogenbonded chains with asymmetric double-minimum potentials for proteons. The transport is described by the kinksoliton propagation in an energetically asymmetric double-well potential for a proton. The asymmetry can be caused by the spontaneous polarization in hydrogenbonded ferroelectrics, medium polarization in nonferroelectric hydrogen-bonded solids, crystallographic structure, geometry of the proton neighbourhood and internal electric field in biological macromolecules. The soliton dynamics of the system is analytically studied including the influence of a constant external electric field and damping mechanism. The velocity, energy, momentum and mobility of kinks are calculated.

FTIR investigation of polarizable hydrogen bonds in carboxylic acid–pyridine complexes in the mid- and far-IR region

Twelve carboxylic acid–pyridine systems have been studied in chloroform solutions in the mid-IR and the far-IR regions as a function of the pKa of the carboxylic acids at 25 °C and at –40 °C. If the acidity of the carboxylic acid is small a strongly asymmetrical double-minimum proton potential is present. With increasing acidity the hydrogen bonds become stronger, i.e. the bond length shortens and the proton potentials become more symmetrical in character. In these cases the barrier of the potentials becomes very low and the situation may be compared with a broad, flat, asymmetrical single minimum. Therefore, the continua in these complexes are very similar to those in systems with broad, flat, single-minimum proton potentials. With increasing polarity of the complexes the hydrogen bonds lengthen again and the double minimum becomes still more symmetrical. The two proton-limiting structures OH⋯N ⇌ O–⋯H+N can be distinguished by different carbonyl bands; the band of the polar structure is found at relatively high wavenumbers indicating that the barrier is low. Almost symmetrical double-minimum proton potentials are now present. With the most acidic system at –40 °C, the polar structure has a larger weight.

Optical–optical double resonance study of the and &([a-z]+);(3A2) states of thiophosgene

The &([a-z]+);(3A2) and [Formula: see text] states of jet-cooled thiophosgene (Cl2CS) have been studied by optical–optical double resonance (OODR) spectroscopy. Two OODR schemes have been used to probe the [Formula: see text] state. One scheme uses selected vibronic levels of the &([a-z]+);(1A2) state as the intermediate state, while the other uses the vibrationless and 2131 levels of the &([a-z]+);(3A2) state. All of the vibronic levels in the 33 980−35 600 cm−1 region can be rationalized with the following origin band and fundamentals: 0° = 34 277 cm−1, v1 = 505 cm−1, v2 = 470 cm−1, v3 = 213 cm−1, v6 = 249 cm−1, 42 = 341 cm−1, 44 = 627 cm−1. The discrepancies among the various studies of the [Formula: see text] state will be discussed and reconciled. It is conjectured that the [Formula: see text] state potential along the C—S coordinate exhibits an asymmetric double-minimum potential resulting from the interaction of the 1A1 states arising from the [Formula: see text] configurations. The minimum corresponding to the [Formula: see text] configuration lies higher in energy and the principal decay mechanism for molecules pumped to its first few vibronic levels is fluorescence. On the other hand, molecules pumped to the minimum corresponding to the nominal [Formula: see text] configuration decay nonradiatively. The barrier height to inversion and the out-of-plane bending angle along the out-of-plane bending coordinate, v4, have been determined to be 945 cm−1 and 25°, respectively, by fitting quartic-quadratic and quadratic-Gaussian double-minimum potentials to the observed energy levels. The &([a-z]+);(3A2) state has been studied by a novel OODR scheme which uses the fluorescent vibrationless level of the [Formula: see text] state to monitor [Formula: see text]transitions. A vibronic analysis has been carried out and the following origin band and fundamentals derived for the &([a-z]+);(3A2) state: 0° = 17 499 cm−1, v1 = 923 cm−1, v2 = 474 cm−1, v3 = 247 cm−1, 42 = 297 cm−1, 44 = 560 cm−1, 46 = 741 cm−1. With the exception of a few corrections and additions, the results confirm the findings of previous studies, notably regarding the bent geometry and barrier height to inversion. An overall comparison of the data suggests that the wavenumber of v6 in theÃ(1A2) state is 279 cm−1 instead of 189 cm−1.

Barrier-crossing dynamics in an asymmetric double-minimum potential

On the basis of our previous study on barrier-crossing dynamics in a symmetric double-well potential, we investigate fast rate processes corresponding to barrier-crossing dynamics in an arbitrary asymmetric double-minimum potential. The evolution of the rate kernels is determined by a projected Fokker—Planck operator and its spectral properties are studied. This work provides a deeper insight into the nature of fast chemical interconversions.

Proton transfer dynamics in the hydrogen bond. Inelastic neutron scattering, infrared and Raman spectra of Na 3H(SO 4) 2, K 3H(SO 4) 2 and Rb 3H(SO 4) 2

Na 3H(SO 4) 2, K 3H(SO 4) 2 and Rb 3H(SO 4) 2 crystals are composed of (SO 4HSO 4) −3 dimers linked by rather strong hydrogen bonds ( R O…O=2.43 Å for Na 3H(SO 4) 2, R O…O=2.48 Å for Rb 3H(SO 4) 2 and R O…O=2.49 Å for K 3H(SO 4) 2). Crystallographic data of the salts at room temperature indicate either asymmetric (Na 3H(SO 4) 2) or symmetric (K 3H(SO 4) 2 and Rb 3H(SO 4) 2) hydrogen bonds. Inelastic neutron scattering (INS), infrared and Raman spectra of crystal powders at 20 K are reported for these three compounds. The OH bending modes, which give large INS intensities, appear only weakly in the infrared. The two bending modes are degenerate in Na 3H(SO 4) 2 which has the shortest hydrogen bond but are well separated in K 3H(SO 4) 2 and Rb 3H(SO 4) 2. The OH stretching band profiles in INS are also quite different from those in the infrared. Strong INS bands at 57 and 44 cm −1 for K 3H(SO 4) 2 and Rb 3H(SO 4) 2, respectively, are assigned to 0→1 transitions in quasi-symmetric double-minimum potentials for the OH stretching coordinates. For K 3H(SO 4) 2 the frequency is unaffected by temperature between 2 and 100 K. Potential functions are calculated and the dynamics of the proton transfer are discussed. Infrared spectra are thus dominated by OH stretching transitions in asymmetric double-minimum potentials with low barriers, with relative intensities indicating a large electrical anharmonicity.

On an Isomorphous Transition

A phase diagram and several physical quantities related to an isomorphous transition are studied on the basis of a Landau-type free energy function. Relationship with a two-sublattice model including an asymmetric double minimum potential is discussed.

Numerical calculation on ferroelectric tc's in a two-sublattice model with an asymmetric double minimum potential

Abstract Numerical calculation on a two-sublattice model was performed with special attention to the Tc -change in the mixed systems of dicalcium strontium propionate.

The dynamics of hydrogen atoms in the hydrogen bonds of carboxylic acid dimers

The techniques of pulsed NMR spectroscopy and inelastic neutron scattering (INS) have been employed to study the coordinated motion of hydrogen atoms in the hydrogen bonds of carboxylic acid dimers. The proton spin-lattice relaxation time T1 has been measured at two frequencies in diglycolic acid and suberic acid and the reorientation rate in diglycolic acid has been measured directly by quasi-elastic neutron scattering. Furthermore T1 data have been collected on benzoic acid, terephthalic acid and malonic acid at different spectrometer frequencies to those in earlier published data. The NMR and INS data have been successfully fitted to a new model proposed by Skinner and Trommsdorff (1988) in which the low temperature dynamics, under the influence of an asymmetric double-minimum potential, are dominated by phonon-assisted tunnelling while that at high temperature follows an Arrhenius rate law. Fitting the T1 data at two different frequencies simultaneously with the INS data restricts the possible values of the parameters in the theory to a narrow range. The values of the parameters, particularly the barrier heights and asymmetry, are discussed in terms of the known molecular structures. Further discussion centres upon some new NMR data of the proton dipolar relaxation time T1D in terephthalic acid and benzoic acid.

NMR study of environment modulated proton tautomerism in crystalline and amorphous phthalocyanine

Using high resolution 15N and 13C CPMAS NMR spectroscopy (CP - cross polarization, MAS - magic angle spinning) we have detected fast thermally activated proton transfer processes in solid 15N enriched phthalocyanine (Pc). The synthesis of the latter is described. NMR experiments were performed on the crystalline α- and β-modifications, as well as on a novel amorphous modification (am-Pc). In order to extract thermodynamic and kinetic data from the NMR spectra an appropriate lineshape theory of bistable molecules in ordered and disordered matrices is developed. Bistable molecules are subject to exchange between at least two molecular states. The lineshape theory includes the possibility of site dependent perturbations of the rate and equilibrium constants of state exchange, as well as of exchange between different sites. For disordered environments bi-Gaussian distributions of the reaction enthalpies of the state exchange and of the enthalpies of activation are proposed. Different possibilities, including Marcus theory, of reducing this two-dimensional site distribution function to a one-dimensional distribution are discussed. The analysis of the NMR spectra gave the following results. Whereas the proton potential in α-Pc is quasisymmetric, the degeneracy of the tautomers in the β modification is lifted because of a subtle interplay between inter- and intramolecular interactions. The amorphous modification is characterized by a broad distribution of differently perturbed asymmetric double-minimum potentials, as expected for a disordered environment. Rotation of Pc in either of these modifications can be excluded. Proton transfer in the β phase is faster than in the α phase due to smaller energy of activation. This finding is interpreted with a different geometric arrangement of the inner nitrogen atoms in both phases. In addition, the proton transfer in the β phase is characterized by a smaller pre-exponential factor than in the α phase. This effect indicates substantial but different tunnel contributions to the reaction rates in both phases. The implications of the environment modulated proton transfer processes in Pc for the mechanism of hydrogen transfer reactions in liquids is discussed.