Attosecond physics with neutrons and electrons: Ultrafast entanglement and decoherence phenomena involving protons in condensed matter and molecules

Nuclei and electrons in condensed matter and/or molecules are usually entangled, due to the prevailing electromagnetic interactions. Usually, the “environment” of a microscopic scattering system (e.g., a proton) causes an ultrafast decoherence, thus making atomic and/or nuclear entanglement effects not directly accessible to experiments. However, neutron Compton scattering (NCS) and electron Compton scattering represent ultrafast techniques operating in the sub-femtosecond timescale, thus opening a way for investigation of such dehoherence and short-lived entanglement phenomena of atoms in molecules and condensed matter. The experimental context of NCS and a new striking scattering effect from protons (H-atoms) in several condensed systems and molecules are described. In short, one observes an “anomalous” decrease of scattering intensity from protons, which seem to become partially “invisible” to the neutrons. The experiments apply large energy (several electronvolts) and momentum (10–200 Å^?1 transfers, and the collisional (or scattering) time between the neutron and a struck proton is only 100–1000 attoseconds long. Similar results are also obtained with electron-atom Compton scattering at large momentum transfers. As an example, we present new NCS experimental results from a single crystal, which also provide new physical insights into the attosecond quantum dynamics of protons in molecules and condensed matter. Theoretical discussions and models are presented which show that the effect under consideration is caused by the non-unitary time evolution (due to decoherence) of open quantum systems during the ultrashort, but finite, time-window of the neutron-proton scattering process. The conceptual connection with the well known Quantum Zeno Effect is pointed out. The experimental results, together with their qualitative interpretation “from first principles,” show that epithermal neutrons being available at spallation sources, and electron spectrometers providing large momentum transfers, may represent novel tools for investigation of thus far unknown physical and chemical attosecond phenomena.     

季铵盐型双子表面活性剂16-4-16聚集状态的NMR研究

核磁共振弛豫，自扩散以及2D NOESY谱研究结果表明：双子表面活性剂16-4-16溶液在形成胶束的过程中，联结基团及其邻近的碳氢链质子形成胶束的壳层，而距离离子头较远的疏水质子位于胶束的内部. 与对应的单链的表面活性剂CTAB相比，其分子运动更受限制. 2D NOESY谱显示联接基团及临近的碳氢链的质子间有较强的交叉峰，表明形成胶束时，分子在联结基团附近堆积的较为紧密. 由2D NOESY谱计算得到的质子间距与HYPERCHEM模拟值有偏差，表明这些强交叉峰是分子间相互作用的结果，并且对应质子对在双子表面活性剂16-4-16分子中位于邻近的区域. 因此我们推测，双子表面活性剂16-4-16分子在球形胶束中形成特殊的排列方式.     

Charge Separation at the Evaporation (Condensation) Front of Water and Ice. Charging of Spherical Droplets

Charge separation at evaporation (condensation) front of water and ice is analyzed. Relatively low distribution coefficient of protons and hydroxide ions between vapor and condensed phase that is less than the distribution coefficient of water molecules leads to accumulation of protons and hydroxide ions at the phase front upon evaporation and a decrease in the amount of such species upon condensation. Interphase charge separation is caused by the subsequent diffusion of nonequilibrium protons and hydroxide ions. The charge separation is also affected by the double electric layer generated by orientation defects at the water and ice surfaces. Dependences of electric field at a plane surface of water and ice on the rate of phase transformation are calculated. Electric charges of spherical water droplets are estimated at different field strengths and droplet radii.     

A Comparative Multinuclear Relaxation Study of Protein-DMSO and Protein-Water Interactions

A comparative multinuclear relaxation study of DMSO-protein and water-protein interactions has been undertaken to discover whether cross relaxation between water and protein protons proceeds mainly by proton-exchange or direct dipolar interactions. The analysis suggests that the proton-exchange mechanism dominates the cross relaxation from water. In contrast, the high efficiency of dipolar cross relaxation in DMSO-protein gels, which lack exchangeable protons, arises from an unusually long lifetime of DMSO molecules at the protein interface. These results are important for understanding the origin of image contrast in clinical nuclear magnetic resonance imaging and the nature of protein-solvent interactions in nonaqueous systems.     

Protein hydration and location of water molecules in oxidized horse heart cytochrome c by (1)H NMR

The hydration properties of the oxidized form of horse heart cytochrome c have been studied by (1)H NMR spectroscopy. Two-dimensional, homonuclear ePHOGSY-NOESY experiments are used to map water-protein interactions. The detected NOEs reveal interactions between nonexchangeable protein protons and both water protons and labile protein protons which exchange with water protons. Among the many water molecules apparent in the X-ray structure, three have been identified with a residence time longer than 300 ps. One of them is located inside the distal heme cavity, in the deepest part of a hydration pathway extending toward the surface. The identification of hydrophilic regions and detection of three long-lived water molecules settles some ambiguities and provides a better representation of the water-protein interactions in oxidized cytochrome c.     

Features of the Formation of Correlated Coherent States and Nuclear Fusion Induced by the Interaction of Slow Particles with Crystals and Free Molecules

The results of the experiments, where the intense generation of alpha particles with an energy of about 8.6 MeV was observed at the interaction of 400–600-eV slow protons with a thin lithium foil or lithium vapors, have been analyzed. It has been shown that the high efficiency of these reactions at such a low energy cannot be explained within standard models of accelerator nuclear fusion. A model has been proposed and considered for the initiation of these reactions by correlated coherent states of protons, which are formed at the electrostatic interaction of these moving particles with the fields of surface lithium atoms in a solid target, as well as with atoms in molecules of lithium vapor. It has been shown that the formation of such states is accompanied by a sharp increase in short-term fluctuations of the energy of protons, which are due to the Schrödinger–Robertson uncertainty relation, to an amplitude of 30–80 keV. The calculation has shown that the optimal energy of the initial motion of protons, at which these processes occur and the probability of the (Li, p) reactions involving these protons is large, is near 500 eV in complete agreement with the experimental data.     

Measurement of one-bond heteronuclear dipolar coupling contributions for amine and diastereotopic methylene protons

One-bond heteronuclear and two-bond homonuclear residual dipolar couplings measured at methylene or amine sites can be utilized as long-range constraints in structure determination of molecules as well as to facilitate characterization of local conformation by stereospecific assignment of diastereotopic protons. We present two J-modulated HMQC type experiments to measure the one-bond heteronuclear dipolar coupling contributions of geminal protons individually. In addition two-bond homonuclear residual dipolar couplings between the diastereotopic protons are also obtained.     

癸基萘磺酸钠胶束中分子动态的质子核磁共振研究

在298、303和313 K下对浓度为0.82 和2.59 mmol/L的癸基萘磺酸钠的重水溶液中质子的自旋-自旋弛豫（T₂）时间进行了测量. 浓度为0.82 mmol/L 的溶液(临界胶束浓度CMC以下)中的所有质子的T₂值均随温度上升而增长，说明整个分子运动变的更自由. 然而，浓度在CMC以上(2.59 mmol/L)时，分子中一部分参与形成栅栏层的质子的T₂值却表现出相反的效应，它们的运动随温度上升而变慢，说明在高温下为防止水分子进入疏水胶束壳内，这些栅栏层的质子堆积的更紧密. 同时分子中其他质子的T₂值随温度上升而增长，表现出正常的分子运动的温度效应.     

Untersuchungen der protonenresonanz von adsorbiertem wasser an zeolithen des faujasit-typs mit verschiedenen silicium-aluminium-verhältnissen

The line-widths of the proton magnetic resonance have been studied over the possible range of water contents of faujasite-type zeolites for ten samples with different silicium/ aluminium ratio between 1.18 and 2.5. The results show that the mobility of the sorbed water molecules at low coverages is restricted with increasing Si/Al ratio. This effect can be described by a picture of many flat potential wells for low Si/Al ratios and a few deep potential wells at high Si/Al ratios at the surface of the large cavities of the zeolite. The sorbed molecules at intermediate and high water contents rapidly exchange their protons with the sorbed molecules or possibly with OH groups at the walls of the cavities over hydrogen bonds. The line-widths at higher water contents therefore can be explained by an averaging process of the protons of the mobile water molecules with the molecules sorbed at the surface.     

o-Semiquinonato and o-iminosemiquinonato rhodium complexes. EPR study of the reactions in coordination sphere of rhodium

Chemical exchange saturation transfer (CEST) processes in aqueous systems are quantified by evaluation of z-spectra, which are obtained by acquisition of the water proton signal after selective RF presaturation at different frequencies. When saturation experiments are performed in vivo, three effects are contributing: CEST, direct water saturation (spillover), and magnetization transfer (MT) mediated by protons bound to macromolecules and bulk water molecules. To analyze the combined saturation a new analytical model is introduced which is based on the weak-saturation-pulse (WSP) approximation. The model combines three single WSP approaches to a general model function. Simulations demonstrated the benefits and constraints of the model, in particular the capability of the model to reproduce the ideal proton transfer rate (PTR) and the conventional MT rate for moderate spillover effects (up to 50% direct saturation at CEST-resonant irradiation). The method offers access to PTR from z-spectra data without further knowledge of the system, but requires precise measurements with dense saturation frequency sampling of z-spectra. PTR is related to physical parameters such as concentration, transfer rates and thereby pH or temperature of tissue, using either exogenous contrast agents (PARACEST, DIACEST) or endogenous agents such as amide protons and -OH protons of small metabolites.     

Effects of thermal denaturation on the longitudinal relaxation time (T1) of water protons in protein solutions: study of the factors determining the T1 of water protons

The factors determining the longitudinal relaxation time (T1) of water protons in protein solutions were investigated by analyzing the effects of thermal denaturation on the T1 of the water protons. We treated the water protons and the protein protons "on a protein surface" as a dipole-dipole coupled two-spin system where relative translational diffusion is the dominant mechanism, and measured the change in the time development of the nuclear Overhauser effect (NOE) factors of the water protons. The T1 of the water protons was shortened markedly when the proteins were thermally denatured. Our analysis indicates that this relaxation enhancement is due to an increase in the value of the translational correlation time as well as the fraction of hydration water molecules, though the influence of "proton exchange" between the water protons and the labile protein protons cannot be completely neglected.     

A proton NMR study of water in hydrated sodium β-alumina

Proton nmr study of hydrated sodium β-alumina reveals a Pake doublet spectrum due to stationary water molecules. As the temperature is increased from -196C to 192C, linewidth changes indicate that the stationary protons sense the increasingly rapid motion of nearby sodium nuclei and other protons. The interproton spacing computed from the spectra was found to be 1.7A.     

表面活性剂及水溶性功能高分子的NMR研究

核磁共振（NMR）技术是研究表面活性剂在溶液中聚集状态的一种非常有用的工具,本文运用多种NMR技术研究了几种不同类型表面活性剂及水溶性功能高分子在水溶液中的聚集行为： 1. 季铵盐型双子表面活性剂16-4-16的聚集行为季铵盐型双子表面活性剂N,N′-双(十六烷基二甲基)-α,ω-丁烷溴化铵(16-4-16)分子中联接基团及靠近离子头的质子位于胶束的壳层, 运动受到一定限制. 而距离离子头较远的烷烃链位于胶束的内部,运动相对自由. 与对应的单链表面活性剂十六烷基三甲基溴化铵(CTAB)相比,16-4-16形成的胶束堆积更为紧密. 通过NOESY谱中交叉峰强度的定量计算,认为16-4-16在胶束中分子以上下交错排列的方式形成球形聚集体.  2. 脱氧胆酸钠与十六烷基三甲基溴化铵的相互作用在脱氧胆酸钠(NaDC)溶液中,NaDC质子H3与其他质子不同,其横向弛豫时间(T₂)表现为双指数衰减,表明此质子可能存在两种不同的状态. 实验证明,其它胆酸盐的H3的横向弛豫也呈现双指数衰减. 因此推测在胆酸盐的稀溶液中,3-OH质子和羰基氧之间有可能存在氢键作用,形成了头尾相连的分子对结构.  在NaDC和CTAB的混合溶液中,两者形成1∶1的混合胶束. 用NOESY和ROESY研究混合胶束的结构,显示CTAB的离子头位于NaDC的羧酸基团附近. 这可能是正负离子之间的静电性相互作用的结果.  3. 丙烯酰胺/丙烯酸模板共聚物的微结构研究了不同pH值条件下,丙烯酰胺和丙烯酸共聚物分子在水溶液中的聚集形态. 在酸性溶液中,分子内的氢键致使聚集体形成较为紧密的堆积,侧链的苯氧基团运动受阻;随着溶液pH值的增大,丙烯酸电离产生的阴离子使得分子间的静电斥力增大,分子链变得伸展,分子间的氢键作用导致了聚集体体积变大. 当溶液呈强碱性,丙烯酸完全电离,氢键作用力被破坏,分子呈现自由伸展的状态,侧链的苯氧基团运动相对自由.     

Molecular ordering of mixed surfactants in mesoporous silicas: a solid-state NMR study

The use of mixed surfactants in the synthesis of mesoporous silica nanoparticles (MSNs) is of importance in the context of adjusting pore structures, sizes and morphologies. In the present study, the arrangement of molecules in micelles produced from a mixture of two surfactants, cetyltrimethylammonium bromide (CTAB) and cetylpyridinium bromide (CPB) was detailed by solid-state NMR spectroscopy. Proximities of methyl protons in the trimethylammonium headgroup of CTAB and protons in the pyridinium headgroup of CPB were observed under fast magic angle spinning (MAS) by (1)H-(1)H double quantum (DQ) MAS NMR and NOESY. This result suggested that CTAB and CPB co-exist in the pores without forming significant monocomponent domain structures. (1)H-(29)Si heteronuclear correlation (HETCOR) NMR showed that protons in the headgroups of CTAB are in closer proximity to the silica surface than those in the CPB headgroups. The structural information obtained in this investigation leads to better understanding of the mechanisms of self-assembly and their role in determining the structure and morphology of mesoporous materials.     

Dissociation of alkane ionized molecules

The subject of investigation is the fragmentation of variously charged molecular ions arising in col-lisions of several kiloelectronvolt H⁺, He²⁺, and Ar⁶⁺ ions with molecules of the simplest alkanes (from methane to butane). Using the method of time-of-flight mass spectrometry, the formation cross sections of dissociation-induced fragment ions are measured. The dissociation takes place when an incident ion captures an electron from a methane, ethane, or propane molecule. The role of additional ionization of the molecule, which accompanies the electron capture by the incident ion, is elucidated. The kinetic energy spectrum for protons resulting from the fragmentation of multiply charged alkane ions is determined. The most plausible kinetic energies of protons depending on the degree of ionization and molecule size fall into the range 1–25 eV. It is shown that, when the molecule loses several electrons, the kinetic energies of protons are governed by Coulomb interaction between all fragment ions and are determined by their flying apart from the relative spatial arrangement of corresponding atoms in a parent molecule.     

基于CEST机制的pH成像方法、原理和应用

化学交换饱和转移（Chemical Exchange Saturation Transfer,CEST）技术作为一种新型的磁共振成像（Magnetic Resonance Imaging,MRI）技术.它的原理为溶质池中被激发饱和的质子与游离水中未被饱和的质子间的化学交换,能够引起水质子磁共振信号的下降,从而获得组织内生物分子的相关信息.由于质子间的交换速率k_ex与组织微环境的pH值之间存在直接联系,因而可以通过溶质质子的CEST信号对活体组织进行pH成像.目前用于pH成像的溶质分子既包括内源性游离的蛋白质、多肽分子,还包括一系列的外源性小分子和金属螯合物.通过不同类型的比率法、内源性胺和酰胺浓度-独立检测（Amine and Amide Concentration-independent Detection,AACID）等成像方法,能够获得肾脏、中风脑组织以及肿瘤组织的pH图谱.本文详细总结了2000年以来利用CEST技术进行pH成像方面的研究进展,包括对比剂、成像方法和相关应用,展望了活体组织pH成像的发展趋势和应用前景.     

Hydrocarbon ions with triplet ground states

The N.M.R. spectra of meta dichloro and meta dibromo benzenes are analysed in the nematic phase of 4-methoxy benzylidene-4-amino-α-methyl cinnamic acid-n-propyl ester at room temperature.

The direct couplings are found to be of negative sign. The molecules orient preferentially with the aromatic plane along the direction of the magnetic field and the C ₂-symmetry axis perpendicular to it.

The relations between the various direct couplings and the inter-proton distances are reported. The ratio of the distance between the meta protons along the axis perpendicular to the C ₂ symmetry and the ortho protons is determined precisely. The other ratios could not be determined to any reasonable precision. The reasons are discussed.

The angles between the C-H bonds of the equivalent protons and the C-C bonds in both the molecules are determined on the assumption of the C-H bond lengths and the ring geometry of benzene.     

Recoupled long-range C-H dipolar dephasing in solid-state NMR,and its use for spectral selection of fused aromatic rings

This work introduces a simple new solid-state 13C NMR method for distinguishing various types of aromatic residues, e.g. those of lignin from fused rings of charcoal. It is based on long-range dipolar dephasing, which is achieved by recoupling of long-range C-H dipolar interactions, using two 1H 180 degrees pulses per rotation period. This speeds up dephasing of unprotonated carbon signals approximately threefold compared to standard dipolar dephasing without recoupling and thus provides much more efficient differential dephasing. It also reduces the effects of spinning-speed dependent effective proton-proton dipolar couplings on the heteronuclear dephasing. Signals of unprotonated carbons with two or more protons at a two-bond distance dephase to <3% within less than 0.9 ms, significantly faster than those of aromatic sites separated from the nearest proton by three or more bonds. Differential dephasing among different unprotonated carbons is demonstrated in a substituted anthraquinone and 3-methoxy benzamide. The data yield a calibration curve for converting the dephasing rates into estimated distances from the carbon to the nearest protons. This can be used for peak assignment in heavily substituted or fused aromatic molecules. Compared to lignin, slow dephasing is observed for the aromatic carbons in wood charcoal, and even slower for inorganic carbonate. Direct 13C polarization is used on these structurally complex samples to prevent loss of the signals of interest, which by design originate from carbons that are distant from protons and therefore crosspolarize poorly. In natural organic matter such as humic acids, this combination of recoupled dipolar dephasing and direct polarization at 7-kHz MAS enables selective observation of signals from fused rings that are characteristic of charcoal.     

Conformational analysis of the chemical shifts for molecules containing diastereotopic methylene protons

Quantum chemistry SCF/GIAO calculations were carried out on a set of compounds containing diastereotopic protons. Five molecules, including recently synthesized 1,3-di(2,3-epoxypropoxy)benzene, containing the chiral or pro-chiral center and the neighboring methylene group, were chosen. The rotational averages (i.e. normalized averages with respect to the rotation about the torsional angle τ with the exponential energy weight at temperature T) calculated individually for each of the methylene protons in 1,3-di(2,3-epoxypropoxy)benzene differ by ca. 0.6 ppm, which is significantly less than the value calculated for the lowest energy conformer. This value turned out to be low enough to guarantee the proper ordering of theoretical chemical shifts, supporting the interpretation of the (1)H NMR spectrum of this important compound. The rotational averages of chemical shifts for methylene protons for a given type of conformer are shown to be essentially equal to the Boltzmann averages (here, the population-weighted averages for the individual conformers representing minima on the E(τ) cross-section). The calculated Boltzmann averages in the representative conformational space may exhibit completely different ordering as compared to the chemical shifts calculated for the lowest-energy conformer. This is especially true in the case of molecules, for which no significant steric effects are present. In this case, only Boltzmann averages account for the experimental pattern of proton signals. In addition, better overall agreement with experiment (lower value of the root-mean-square deviation between calculated and measured chemical shifts) is typically obtained when Boltzmann averages are used.     

Water-proton-spin-lattice-relaxation dispersion of paramagnetic protein solutions

The paramagnetic contributions to water-proton-spin-lattice relaxation rate constants in protein systems spin-labeled with nitroxide radicals were re-examined. As noted by others, the strength of the dipolar coupling between water protons and the protein-bound nitroxide radical often appears to be larger than physically reasonable when the relaxation is assumed to be controlled by 3-dimensional diffusive processes in the vicinity of the spin label. We examine the effects of the surface in biasing the diffusive exploration of the radical region and derive a relaxation model that incorporates 2-dimensional dynamics at the interfacial layer. However, we find that the local 2-dimensional dynamics changes the shape of the relaxation dispersion profile but does not necessarily reproduce the low-field relaxation efficiency found by experiment. We examine the contributions of long-range dipolar couplings between the paramagnetic center and protein-bound-water molecules and find that the contributions from these several long range couplings may be competitive with translational contributions because the correlation time for global rotation of the protein is approximately 1000 times longer than that for the diffusive motion of water at the interfacial region. As a result the electron-proton dipolar coupling to rare protein-bound-water-molecule protons may be significant for protein systems that accommodate long-lived-water molecules. Although the estimate of local diffusion coefficients is not seriously compromised because it derives from the Larmor frequency dependence, these several contributions confound efforts to fit relaxation data quantitatively with unique models.