A comparison of hydrogen release kinetics from 5- and 6-membered 1,2-BN-cycloalkanes

The reaction order and Arrhenius activation parameters for spontaneous hydrogen release from cyclic amine boranes, i.e., BN-cycloalkanes, were determined for 1,2-BN-cyclohexane (1) and 3-methyl-1,2-BN-cyclopentane (2) in tetraglyme. Computational analysis identified a mechanism involving catalytic substrate activation by a ring-opened form of 1 or 2 as being consistent with experimental observations.

The reaction order and Arrhenius activation parameters for spontaneous hydrogen release from cyclic amine boranes, i.e., BN-cycloalkanes, were determined for 1,2-BN-cyclohexane (1) and 3-methyl-1,2-BN-cyclopentane (2) in tetraglyme.

Amine boranes have long been targeted as a promising class of potential hydrogen storage materials.¹ The simplest amine borane, ammonia borane (H₃NBH₃), has attracted significant attention by virtue of its particularly high gravimetric hydrogen density (19.6 wt% H₂).² Thermal decomposition of ammonia borane to release H₂, however, frequently generates mixtures of oligomeric and polymeric products, which in turn can complicate efforts to regenerate the fully saturated starting material for reuse.¹ Extensive studies of variously substituted amine boranes have thus been conducted in the interest of identifying a system that releases H₂ more selectively.^3,4Our approach in this regard has been to incorporate the amine borane H₂NBH₂ unit into carbocyclic structures to form saturated carbon–boron–nitrogen (CBN) heterocycles.⁵ We have previously discovered that two of these compounds, 1,2-BN-cyclohexane (1)⁶ and its constitutional isomer 3-methyl-1,2-BN-cyclopentane (2)⁷ undergo full, thermally-induced BN-dehydrogenation to afford only the trimeric products 3 and 4 through the intermediacy of monomeric BN-“cycloalkene” species 1–H₂ (ref. 8) and 2–H₂, respectively (Scheme 1).Open in a separate windowScheme 1Selective thermal decomposition of 1 and 2 to trimers 3 and 4, respectively.While 1 and 2 appeared to exhibit the same general dehydrogenation selectivity, in subsequent experiments using neat material, we observed an intriguing difference in the thermal stability of these compounds. Specifically, thermo-gravimetric analysis-mass spectrometry (TGA-MS) revealed significant loss of H₂ from 2 initiated at ∼50 °C,⁹ but an analogous measure of decomposition of 1 occurred only upon heating to ∼70 °C (Fig. S2; see also Fig. S3^† for the temperature-programmed desorption-mass spectrometry (TPD-MS) of 1). Thus, of the two materials, only 1 would meet the minimum requirement for operational stability set by the US Department of Energy''s Hydrogen and Fuel Cells Program for on-board vehicular applications.¹⁰A mechanistic investigation of the origin of this dichotomy of dehydrogenation reactivity would aid in the design of better amine borane-based hydrogen storage materials and add to our fundamental understanding of the reactivity of amine boranes. Herein, we provide a solution-phase kinetic analysis of initial H₂ release from 1 and 2 using ReactIR. The kinetic data establish a second-order decomposition pathway that is in agreement with a computationally-derived mechanistic model. We also provide evidence that the ring strain associated with the 5-membered heterocycle 2 is ultimately responsible for its faster decomposition rates.^11,12The reaction order for the first step of dehydrogenation of 1 and 2 was determined using the initial rates method. Both 1 and 2 exhibit a characteristic IR frequency at 1600 cm⁻¹ (attributed to an N–H bending mode); the disappearance of starting material can thus be readily monitored in situ by ReactIR.¹³ The initial concentrations ([ ]₀) of either 1 or 2 in a tetraglyme solution at 140 °C were varied between 0.560 M and 1.283 M, and initial rates (r_i) were estimated based on linear regression of the respective portions of the substrate disappearance trends representing up to 20% conversion. As shown in Fig. 1, a linear fit of ln(r_i) vs. ln([ ]₀) yielded a slope of 2 for both 1 and 2, indicating the rate of initial H₂ loss from each follows a second-order concentration dependence.¹⁴Open in a separate windowFig. 1Abbreviated substrate disappearance trends measured at 140 °C for various initial concentrations of 1 (A) and 2 (C), and reaction order determination by the initial rates method for 1 (B) and 2 (D).A subsequent Arrhenius analysis was enabled by maintaining a constant initial concentration of 0.742 M, and varying the reaction temperature from 120–160 °C for 1 and from 100–160 °C for 2. As shown in Fig. 2). (Extrapolation to lower temperatures, e.g., 50 °C, reveals an even wider gap of approximately three orders of magnitude.) It thus appears that enthalpic factors, as represented by E_a values,¹⁵ are primarily responsible for the faster kinetics of H₂ release from 2 relative to those of 1.Experimentally determined activation energies (E_a) and pre-exponential factors (A) for 1 and 2 based on Arrhenius analyses

Trace carbonyl analysis in water samples by integrating magnetic molecular imprinting and capillary electrophoresis

In order to obtain high derivatization efficiency, the overuse of derivative agent 2,4-dinitrophenylhydrazine (2,4-DNPH) is necessary for carbonyl detection. But, the 2,4-DNPH residue will cause background interferences and limit the pre-concentration factor of the target analytes. In order to overcome the bottleneck problems, the magnetic molecularly imprinted polymer based solid-phase extraction (MMIPs-SPE) method was developed with 2,4-dinitroaniline (2,4-DNAN) as the dummy template. The characteristics and selectivity of the MMIPs were investigated. Under the optimized conditions, the enrichment of carbonyls-DNPH derivatives with simultaneous removal of the surplus 2,4-DNPH was achieved. By coupling with capillary electrophoresis (CE), a satisfactory analytical performance was obtained with the detection limit ranging from 1.2 to 8.7 μg L⁻¹ for 8 carbonyls. The MMIPs-SPE-CE method was applied successfully for the carbonyl assessment in stream water, tap water and bottled water. In addition, the migration of carbonyls in bottled drinking water was investigated under UV irradiation and heating.

By integrating MMIPs-SPE method and CE, the enrichment of carbonyls-DNPH derivatives with simultaneous removal of the surplus derivative agent 2,4-DNPH can be achieved.     

成人麻疹的流行现状及控制策略

近年来成人麻疹的大量增加是控制麻疹过程中出现的新的流行病学特征,是导致麻疹发病增加的一个重要影响因素。在美国等国开始重视成人计划免疫,如对高危成人接种1剂或2剂麻疹-腮腺炎-风疹联合疫苗(MMR)等手段预防麻疹时,我国尚缺乏控制成人麻疹的手段,且未受到足够的重视。此文主要对我国成人麻疹的流行状况以及如何控制成人麻疹的策略作了探讨。     

孕前尿路结石预处理的临床价值

目的探讨对孕前尿路结石进行预处理的适应证与临床价值。方法76例孕前获诊尿路结石患者按随机与自愿原则分为预处理组35例和待处理组41例。其中预处理组肾结石16例18侧、输尿管结石19例23侧,属孤立肾1例,根据结石类型、部位在孕前采用体外震波碎石(ESWL)、经皮肾镜或输尿管镜取石(PCNL);待处理组肾结石18例21侧、输尿管结石23例27侧,孕前定期复查。监测两组受孕及孕后继发尿路积水、肾绞痛和肾功能情况。结果预处理组孕前均排尽结石。33例(94%)受孕后6~9个月发生尿路积水7例(21%)、肾绞痛9例(27%)和肾功能不全1例(3%),无新增结石及妊娠期高血压病例。3例(9%)行双J管引流,其余保守治疗缓解,均顺利足月分娩;待处理组34例(82%)受孕后3~6个月继发尿路积水21例(61%)、肾绞痛25例(73%)、肾功能损害8例(22%)及肾性高血压5例(14%)。22例(64%)保守治疗无效需配合双J管置入、肾造痿或腔内碎石等处理,2例术后出现异常宫缩。1例因持续尿路感染及肾功能不良终止妊娠,其余病例安全渡过围产期。两组孕期与尿路梗阻有关的并发症发生率比较差异有显著性意义(p<0.01)。结论对孕前尿路结石尤多发性、孤立肾或双侧肾输尿管结石进行预处理可大大降低孕期尿路积水、肾绞痛及肾功能损害的发生机率和处理风险,提高妊娠期母婴的安全性。     

浙江省首例人附红细胞体病报告、随访及文献复习

目的附红细胞体病,临床少见,本文报告一例,以引起临床医生的注意。方法对一例附红细胞体病从临床症状、实验室检查、治疗等进行综合分析。结果附红细胞体病的临床特点:(1)间歇发热、乏力、出汗、纳差、体重减轻等中毒症状;(2)贫血、皮疹、黄疸、肝脾肿大、颌下淋巴结大;(3)肝功能检查显示谷丙转氨酶、谷草转氨酶、胆红素升高、球蛋白高于白蛋白;(4)外周血涂片附红细胞体阳性;(5)CT示肝脾肿大,右下肺炎;(6)强力霉素治疗效果满意。结论附红细胞体病临床上少见,详细询问病史和外周血检查附红细胞体,可以作出早期诊断,强力霉素治疗有效。     

Budd-Chiari综合征:肝脏病理学改变

目的探讨布-加综合征(BCS)患者肝脏的病理学改变。方法30例BCS患者,其中肝静脉型14例,下腔静脉型5例,混合型11例。介入治疗过程中行肝穿刺活检,所取标本行HE染色,光镜下观察。结果30例患者共行31次肝穿刺活检,其中12例使用DFBN活检针,19例采用弹枪式活检针,29次穿刺取出1～4cm长肝组织条,符合组织学诊断要求。病理显示为肝小叶中央静脉、肝血窦扩张淤血,部分可见窦周隙出血;肝细胞水肿,甚至气球样变;纤维组织增生,炎细胞浸润。结论BCS患者肝脏的病理学改变具有一定的特征。     

石辛含片治疗复发性口疮(胃火上炎证)的随机双盲安慰剂对照多中心临床试验研究

目的:评价石辛含片治疗复发性口疮(胃火上炎证)的有效性与安全性.方法:采用优效性检验、进行随机双盲单模拟、安慰剂平行对照、多中心临床试验设计.试验组和对照组分别给予石辛含片和安慰剂,疗程为5 d.主要疗效评价指标为疼痛初始缓解时间,次要疗效评价指标为全部溃疡愈合率、目标溃疡愈合率、目标溃疡存在时间、溃疡总数、疼痛指数、...     

白细胞介素17A在银屑病发病及治疗中的研究进展

【摘要】 近十余年来的基础和临床研究发现并确立了Th17/白细胞介素17A（IL-17A）在银屑病发病中的核心地位。IL-17A不仅能影响角质形成细胞的增殖活性和细胞功能,对银屑病免疫病理环境中的免疫细胞和相关细胞因子也有很重要的调控作用。近年来针对IL-17A通路的单抗如司库奇尤单抗、ixekizumab、brodalumab等在国内外陆续上市,在临床应用中展现出了显著的疗效。本文介绍IL-17A在银屑病发病机制中的作用以及靶向IL-17A及其受体IL-17RA生物治疗的最新进展。