固体核磁共振弛豫在高分子研究中的应用

本文综述了近年来固体核磁弛豫方法在高分子研究中的应用，共分５个部分加以介绍：（１）自旋－晶格弛豫过程；（２）在旋转坐标系中的＾１３Ｃ自旋－晶格弛豫过程；（３）交叉极化速率和旋转坐标系中的＾１Ｈ自旋－晶格弛豫过程；（４）自旋－自旋弛豫过程；（５）动态结构导致的线形变化。本文主要讨论磁性核的各种弛豫过程以及它们与分子结构和分子运动的关系。     

聚合物异核交叉弛豫及二维NOE谱

Kover和Batta首次采用选择性质子π脉冲来测定~(13)C自旋-晶格弛豫速率并与常规反转回复法测定的自旋-晶格弛豫速率相比较求出了交叉弛豫速率,并以此来计算异核间的距离,但他们的计算方法仅适用于满足极端窄化条件的刚性球状分子。二维异核NOE谱(HOESY)是1983年才发展起来的二维NMR技术,它可用来研究异核间的交叉弛豫作用及研究分子间相互作用。Kover和Batta在此方面作了许多     

固体氨基酸中甲基碳自旋晶格弛豫的瞬态NOE效应

本文讨论了固体氨基酸中^1^3C-{^1H}的瞬态核Overhauser效应(NOE),指出当^1^3C自旋晶格弛豫时间T~1^C小于质子的自旋晶格弛豫时间T~1^H时,^1^3C自旋晶格弛豫过程将出现明显的瞬态NOE效应。固体L-异亮氨酸有四个不等价的甲基,其中一个基团满足上述条件,实验上观测到了这种明显的瞬态NOE效应,本文对这一事实作了讨论。     

几种典型固体聚合物体系的^13C自旋—晶格弛豫特性

用固体高分辨ＮＭＲ系统地研究了几种典型的均聚物，共聚物，聚合物共混物以及用接枝共聚物增容的不相容聚合物共混体系的＾１３Ｃ自旋－晶格弛豫特性。研究结果表明：＾１３Ｃ自旋－晶格弛豫时间是表征固体聚合物体系的很有用参数，它能提供有关本体聚合物微观形态结构的信息，并要望建立聚合物的微观怀宏观性能的关系，它不仅能准确无误地反映共混体系中可能存在的各种相互作用，而且能定性地给出相互作用的大小和准确地指明相互     

1,2-聚丁二烯~(13)C-NMR弛豫的溶剂效应

本文测定了一系列具有不同微观结构的1,2-聚丁二烯样品在四种溶剂的50.3MHz ~(13)C自旋-晶格弛豫时间(T_1)和核Overhauser效应(NOE)值。并用Schaefer logx~2相关时间分布模型对实验数据进行了拟合。研究了1,2-聚丁二烯在溶液中的~(13)C-NMR弛豫的溶剂效应及其与结构的关系。发现聚合物与溶剂的溶解度参数之差△δ越大,聚合物在溶液中协同链段运动趋向越明显,~(13)C自旋-晶格弛豫速率(1/T_1)越大;1,2-链节较少,分子链较柔顺时,~(13)C-NMR弛豫受溶剂影响较显著。NMR弛豫参数对结构变化的反应在良溶剂中比在不良溶剂中敏感。     

几种典型国产活性炭吸附各种烃类的NMR研究——Ⅱ.苯、环己烷、正戊烷和正己烷吸附的~1H NMR研究

活性炭吸附苯、环己烷、正戊烷和正已烷的测量表明,在同一种活性炭中各烃的饱和吸附体积相同,四种烃的吸附量与其摩尔体积成反比。同一种被吸附物在不同活性炭中的吸附量与各活性炭的比孔容和孔径分布有关。活性炭的比孔容大,且孔径小于30的微孔比例大时,其吸附和解吸容量也大。这些体系的~1H NMR研究进一步证实了孔径小于30的微孔中发生毛细管凝聚,导致吸附和解吸的容量增大。被吸附分子的自旋晶格弛豫时间几乎不随被吸附物的种类而异。他们与活性炭表面上酸性基团总量有较好的曲线关系。     

几种典型国产活性炭吸附各种烃类的NMR研究——Ⅱ. 苯、环己烷、正戊烷和正己烷吸附的1H NMR研究

活性炭吸附苯、环己烷、正戊烷和正已烷的测量表明, 在同一种活性炭中各烃的饱和吸附体积相同, 四种烃的吸附量与其摩尔体积成反比。同一种被吸附物在不同活性炭中的吸附量与各活性炭的比孔容和孔径分布有关。活性炭的比孔容大, 且孔径小于3.0 nm的微孔比例大时, 其吸附和解吸容量也大。这些体系的~1H NMR研究进一步证实了孔径小于3.0 nm的微孔中发生毛细管凝聚, 导致吸附和解吸的容量增大。被吸附分子的自旋晶格弛豫时间几乎不随被吸附物的种类而异。他们与活性炭表面上酸性基团总量有较好的曲线关系。     

核磁共振中核的奥氏效应在结构有机化学中的应用

核磁共振谱目前已成为有机化学工作者的日常工具,过去从高分辨的核磁共振谱获得的有关分子结构的资料,大多是来自化学位移、偶合常数和相对积分强度。从考虑分子内的自旋——晶格弛豫过程,核的奥氏效应也可以获得一些关于分子立体化学的重要资料。本文仅述及质子——质子核的奥氏效应用于核磁共振谱线的指定和测定分子的结构、构型与构象。     

各种孔结构活性炭吸附苯、环己烷、正戊烷和正己烷的~1H和~(13)C NMR研究

~1H和~(13)C NMR研究证明被吸附在不同孔结构活性炭中的烃类以毛细管凝聚和吸附在固体表面两种状态存在。链状烷烃平铺地吸附在固体表面。被吸附烃与活性炭表面酸性基团的质子交换在弛豫过程中起着重要作用。     

吸附在活性炭上的二氧六环类固相和类液相性质的NMR研究

物理吸附在活性炭上的有机分子的性质,一直是人们感兴趣的问题。近年来,人们开始用NMR梯度场和核弛豫的方法,研究活性炭上吸附分子的扩散运动和分子交换运动.由于吸附分子在活性炭上处于不同的状态,其核弛豫表现出快、慢两个过程。通过拟合磁化矢量随时间变化的曲线,可以得到不同状态分子的百分含量。本文采用核弛豫的方法,研究活性炭的表面酸性基团及孔径对被吸附的二氧六环(DO)类固相和类液相形成的影响.并对不同吸附量下,DO在活性炭上的吸附行为进行讨论。     

吸附在活性炭上的二氧六环类固相和类液相性质的NMR研究

In this paper a study of the spin lattice relaxation times of dioxane(DO) adsorbed on various charcoals is presented. Results show that the magnetziation of DO decays biexponentially (Table 1) as observed in the case of adsorbed hydrocarbons~[6]. The fast relaxing molecules are considered to be those adsorbed in micropores with radii less than 1.0 nm whose motion is restricted and slowed down, thus they form a solid like phase. However, the slow relaxing ones are those condensed in the micropores, they are free to move and form the liquid like phase. Molecules condensed in the interior of the micropores exchange rapidly with those situated in the vicinity of the pore wall according to Eq.(2).Comparison of data in Table 1 may lead to a conclusion that charcoal with greater density of surface acidic groups and greater percentage of pore capacity with radius<1.0 nm (V_(10)/V_(30)) has a greater percentage of solid like phase (P_f). Although V_(10)/V_(30) of density of surface acidic groups. It is apparent that surface acidic groups in micropores play an important role in the formation of solid like phase of DO on charcoals. It seems that DO form hydrogen bond with the surface acidic groups. It is this hydrogen bond and the hinderance effect of pores restrict the DO from moving freely, which results in the formation of the so called solid like phase. It can be found from small (0.10 ml/gC). This implies that DO vapour may first condense in the micropores having more acidic groups and higher activation energy, and then fill in other pores step by step as the adsorption process proceeds.     

Changes in commercial wood charcoals by thermal treatments

As a continuation of a recent study of commercial wood charcoals as far as their potential production of carbon adsorbents is concerned, we have studied the influence of the final heating temperature (T) as carbonization variable in the range 250–1000 °C on the yield and on the characteristics of granular chars prepared from two very different charcoals: a holm-oak charcoal manufactured by partial combustion in a charcoal kiln and an eucalyptus charcoal industrially manufactured in a continuous furnace. Our study also includes the changes produced in both charcoals heated at 250 °C in air for 24 h, and their influences on the adsorption of water vapour at 25 °C. The samples were characterized by thermogravimetry, chemical analyses, Fourier transform infrared spectroscopy, densimetric measurements and mercury porosimetry. T affects the char yield, the chemical composition and the porosity of each char series differently. In particular, the total open pore volumes (to helium) of the starting charcoals, 0.475 and 1.044 cm³ g⁻¹, increase to 0.872 and 1.293 cm³ g⁻¹ heating up to 1000 and 500 °C, respectively. The changes by carbonization are mainly due to devolatilization; moreover, a moderate structural shrinkage occurs heating the eucalyptus charcoal at T > 500 °C. Concerning the air treatments, the yields do not present a significant difference; carbonyl groups are formed and the resulting pore structures depend on the starting charcoals. Water adsorption is larger for the eucalyptus carbons (approximately type V isotherms) than for the holm-oak carbons (type II isotherms).     

Exciton spin relaxation in colloidal CdSe quantum dots at room temperature

Size dependence of spin dynamics in colloidal CdSe quantum dots (QDs) are investigated with circularly polarized pump-probe transmission spectroscopy at room temperature. The excitation energy is tuned to resonance with the lowest exciton (1S(h)1S(e)) energy of the CdSe QDs. The exciton spin dynamics of CdSe QD with the diameter of 5.2 nm shows monoexponential decay with a typical time constant of about 1-3 ps depending on the excitation energy. For the cases of CdSe QDs with smaller size (with the diameter of 4.0 and 2.4 nm), the exciton spin relaxation shows biexponential decay, a fast component with time constant of several ps and a slow one with time constant of hundreds of ps to nanosecond time scale. The fast spin relaxation arises from the bright-dark transition, i.e., J = ±1 ? -/+2 transition. This process is dominated by the hole spin flips, while the electron spin conserves. The slow spin relaxation is attributed to the intralevel exciton transitions (J = ±1 ? -/+1 transition), which is relevant to the electron spin flip. Our results indicate that the exciton spin relaxation pathways in CdSe QD are controllable by monitoring the particle size, and polarized pump-probe spectroscopy is proved to be a sensitive method to probe the exciton transition among the fine structures.     

Specific Features of the Adsorption and Nuclear Magnetic Relaxation of the Water Molecules in Active Carbons: 2. The State of Water in Active Carbon with Relatively Large Pores According to the NMR Relaxation Data

The state of water upon adsorption on FAS-3 active carbon with relatively large micropores is studied by the NMR relaxation method. The dependences of the times of spin–lattice (T ₁) and spin–spin (T ₂) NMR relaxation of adsorbed water molecules on the adsorption value are established. The character of the dependences of T ₁ and T ₂ on the number of adsorbed water molecules per primary adsorption site reflects the specific features of the volume filling of micropores and the formation of a continuous adsorption layer on the mesopore surface due to cluster coalescence on the one wall of a pore. The results obtained are compared with the data for typical microporous active carbons, as well as with the data obtained by the adsorption method.     

Pulsed field gradient NMR study of phenol binding and exchange in dispersions of hollow polyelectrolyte capsules

The distribution and exchange dynamics of phenol molecules in colloidal dispersions of submicron hollow polymeric capsules is investigated by pulsed field gradient NMR (PFG-NMR). The capsules are prepared by layer-by-layer assembly of polyelectrolyte multilayers on silica particles, followed by dissolution of the silica core. In capsule dispersion, (1)H PFG echo decays of phenol are single exponentials, implying fast exchange of phenol between a free site and a capsule-bound site. However, apparent diffusion coefficients extracted from the echo decays depend on the diffusion time, which is typically not the case for the fast exchange limit. We attribute this to a particular regime, where apparent diffusion coefficients are observed, which arise from the signal of free phenol only but are influenced by exchange with molecules bound to the capsule, which exhibit a very fast spin relaxation. Indeed, relaxation rates of phenol are strongly enhanced in the presence of capsules, indicating binding to the capsule wall rather than encapsulation in the interior. We present a quantitative analysis in terms of a combined diffusion-relaxation model, where exchange times can be determined from diffusion and spin relaxation experiments even in this particular regime, where the bound site acts as a relaxation sink. The result of the analysis yields exchange times between free phenol and phenol bound to the capsule wall, which are on the order of 30 ms and thus slower than the diffusion controlled limit. From bound and free fractions an adsorption isotherm of phenol to the capsule wall is extracted. The binding mechanism and the exchange mechanism are discussed. The introduction of the global analysis of diffusion as well as relaxation echo decays presented here is of large relevance for adsorption dynamics in colloidal systems or other systems, where the standard diffusion echo decay analysis is complicated by rapidly relaxing boundary conditions.     

氢在多壁碳纳米管上吸附行为研究

根据热力学平衡原理推导了通用吸附等温方程.通过比较氢在碳纳米管和炭狭缝孔上的高阶维里吸附系数，分析了77～297 K温度区间，温度、管径(孔宽)对碳纳米管、炭狭缝孔吸附空间储氢容量的影响，并由氢在石墨平面上的最大吸附容量计算了本次试验多壁碳纳米管(MWCNTs)在各平衡温度时的最大氢吸附容量.运用确定参数后的吸附等温方程，线性回归分析了氢在本次试验MWCNTs上的吸附数据.结果表明，在160～180 K温度区间，管内被吸附氢分子之间由于吸附受压产生的排斥能出现极大值；随着温度升高，氢分子之间以吸引力为主，提高氢气压力后才发生明显吸附.     

An adsorption study of Sr2+ from saline sources by coconut shell charcoal

The adsorption potential of charcoal for the removal of heavy metal ions is well documented in the literature. However, its exploration for uptake of technologically valuable metal ions such as Sr²⁺ is poorly known. In this work, the batch adsorption study of Sr²⁺ ion from aqueous solution as well as from saline matrix (>3% of NaCl) onto charcoal has been carried out. The experiments were conducted with two charcoals, i.e., our prepared charcoal (coconut shell charcoal) and commercial charcoal. Strontium adsorption has been investigated as a function of its initial concentration, contact time, and varied mass of adsorbent. Equilibrium adsorption data were evaluated for Langmuir and Freundlich isotherm models. The adsorption capacities (mg/g) of Sr²⁺ present in the salt matrix onto coconut shell charcoal and commercial charcoal was found to be 18.4 and 22.2, respectively. Uptake of Sr²⁺ from subsoil brine onto coconut shell charcoal has been successfully demonstrated in this work.