《自然》（20201029出版）一周論文導讀

編譯 | 未玖

Nature,29 October 2020,VOL 586,ISSUE 7831

《自然》2020年10月29日，第586卷，7831期

天文學Astronomy

The Philae lander reveals low-strength primitive ice inside cometary boulders

菲萊著陸器揭示彗星巨石內部的低強度原始冰

▲ 作者：Laurence O』Rourke, Philip Heinisch, Jürgen Blum, Sonia Fornasier, Gianrico Filacchione, Hong Van Hoang, et al.

▲ 連結：

https://www.nature.com/articles/s41586-020-2834-3

▲ 摘要

2014年11月12日，「菲萊」著陸器降落在67P/丘留莫夫-格拉西緬科彗星附近，在彗星表面彈跳了兩次，然後到達阿拜多斯地區的一個懸崖下。這一著陸過程為人們了解彗星核的性質提供了線索。

研究組對此前未發現的第二次著陸點進行調查，菲萊在這兒花了近兩分鐘進行穿越彗星的旅程，在兩個相鄰的彗星巨石上產生了四次不同的表面接觸。當它穿過巨石之間的縫隙時，使石塊內部的原始水冰暴露了出來，即45億年前彗星形成以來的水冰。

研究組在19個月後進行了多儀器觀測，發現這種水冰混合了無處不在的富含深色有機物的材料，其局部塵/冰質量比為2.3 + 0.2-0.16：1，與之前在碰撞時新暴露的水冰和陰影中的水冰中觀測到的值一致。

在縫隙的末端，菲萊在巨石冰上留下了0.25米深的印記，提供了原位測量結果，確認原始冰的抗壓強度極低（小於12帕斯卡，比剛落下的小雪還軟），並可對冰內部的孔隙率（75±7％）進行關鍵評估。研究組的結果為彗星著陸器收集揮發性高的冰樣品提供了限制條件。

▲ Abstract

On 12 November 2014, the Philae lander descended towards comet 67P/Churyumov–Gerasimenko, bounced twice off the surface, then arrived under an overhanging cliff in the Abydos region. The landing process provided insights into the properties of a cometary nucleus. Here we report an investigation of the previously undiscovered site of the second touchdown, where Philae spent almost two minutes of its cross-comet journey, producing four distinct surface contacts on two adjoining cometary boulders. It exposed primitive water ice—that is, water ice from the time of the comet’s formation 4.5 billion years ago—in their interiors while travelling through a crevice between the boulders. Our multi-instrument observations made 19 months later found that this water ice, mixed with ubiquitous dark organic-rich material, has a local dust/ice mass ratio of 2.3+0.2−0.16:1, matching values previously observed in freshly exposed water ice from outbursts and water ice in shadow. At the end of the crevice, Philae made a 0.25-metre-deep impression in the boulder ice, providing in situ measurements confirming that primitive ice has a very low compressive strength (less than 12 pascals, softer than freshly fallen light snow) and allowing a key estimation to be made of the porosity (75 ± 7 per cent) of the boulders』 icy interiors. Our results provide constraints for cometary landers seeking access to a volatile-rich ice sample.

Diverse polarization angle swings from a repeating fast radio burst source

不同偏振角擺動的重複快速射電暴

▲ 作者：R. Luo, B. J. Wang, Y. P. Men, C. F. Zhang, J. C. Jiang, H. Xu, et al.

▲ 連結：

https://www.nature.com/articles/s41586-020-2827-2

▲ 摘要

快速射電暴（FRB）是持續時間為毫秒的未知來源的無線電瞬變。兩種可能從FRB產生極相干發射的機制為中子星磁層或遠離中心能源的相對論激波。詳細的偏振觀測可能有助於人們理解發射機制。

但是，現有的FRB偏振數據令人困惑，因為它們顯示了許多偏振特性，包括某些中繼器在每次爆發期間的恆定偏振角或在其他一些明顯的一次性事件中的可變偏振角。

研究組報告了來自FRB 180301的15次爆發的觀測結果，並發現其中有7次發生了不同的偏振角波動。這些爆發的偏振角特徵的多樣性與無線電發射的磁層起源一致，並不利於引用相對論激波的輻射模型。

▲ Abstract

Fast radio bursts (FRBs) are millisecond-duration radio transients of unknown origin. Two possible mechanisms that could generate extremely coherent emission from FRBs invoke neutron star magnetospheres or relativistic shocks far from the central energy source. Detailed polarization observations may help us to understand the emission mechanism. However, the available FRB polarization data have been perplexing, because they show a host of polarimetric properties, including either a constant polarization angle during each burst for some repeaters or variable polarization angles in some other apparently one-off events. Here we report observations of 15 bursts from FRB 180301 and find various polarization angle swings in seven of them. The diversity of the polarization angle features of these bursts is consistent with a magnetospheric origin of the radio emission, and disfavours the radiation models invoking relativistic shocks.

材料學Materials Science

High-throughput calculations of magnetic topological materials

磁性拓撲材料的高通量計算

▲ 作者：Yuanfeng Xu, Luis Elcoro, Zhi-Da Song, Benjamin J. Wieder, M. G. Vergniory, Nicolas Regnault, et al.

▲ 連結：

https://www.nature.com/articles/s41586-020-2837-0

▲ 摘要

本徵磁性拓撲材料的發現，包括具有大反常霍爾效應的半金屬和軸子絕緣體，指導了固態材料的基礎研究。拓撲量子化學使對順磁拓撲材料的理解和研究成為可能。使用磁拓撲量子化學（MTQC）獲得的磁拓撲指數，研究組基於第一性原理計算對磁拓撲材料進行高通量搜索。

研究組以畢爾巴鄂晶體學網站伺服器上的磁性材料資料庫作為起點，該資料庫包含超過549種具有通過中子散射實驗推導出磁性結構的磁性化合物，並確定了130種強化半金屬，以及拓撲絕緣子。

對於每種化合物，研究組執行完整的電子結構計算，包括使用不同哈伯德電位值的完整拓撲相圖。使用自定義代碼查找所有磁性空間組中所有波段的磁共表示，研究組生成數據饋入MTQC算法，以確定每種磁性材料的拓撲。其中一些材料顯示了以前未知的拓撲相，包括對稱指示的磁性半金屬、三維反常霍爾絕緣體和高階磁性半金屬。

研究組分析了各種交互作用下材料的拓撲趨勢：130種拓撲材料中有60％具有對交互作用敏感的拓撲，其他材料在各種交互作用下則具有穩定的拓撲。研究組為未來的實驗研究提供了一個材料資料庫和診斷磁性材料拓撲的開放原始碼。

▲ Abstract

The discoveries of intrinsically magnetic topological materials, including semimetals with a large anomalous Hall effect and axion insulators, have directed fundamental research in solid-state materials. Topological quantum chemistry has enabled the understanding of and the search for paramagnetic topological materials. Using magnetic topological indices obtained from magnetic topological quantum chemistry (MTQC), here we perform a high-throughput search for magnetic topological materials based on first-principles calculations. We use as our starting point the Magnetic Materials Database on the Bilbao Crystallographic Server, which contains more than 549 magnetic compounds with magnetic structures deduced from neutron-scattering experiments, and identify 130 enforced semimetals, and topological insulators. For each compound, we perform complete electronic structure calculations, which include complete topological phase diagrams using different values of the Hubbard potential. Using a custom code to find the magnetic co-representations of all bands in all magnetic space groups, we generate data to be fed into the algorithm of MTQC to determine the topology of each magnetic material. Several of these materials display previously unknown topological phases, including symmetry-indicated magnetic semimetals, three-dimensional anomalous Hall insulators and higher-order magnetic semimetals. We analyse topological trends in the materials under varying interactions: 60 per cent of the 130 topological materials have topologies sensitive to interactions, and the others have stable topologies under varying interactions. We provide a materials database for future experimental studies and open-source code for diagnosing topologies of magnetic materials.

化學Chemistry

Efficient epoxidation over dinuclear sites in titanium silicalite-1

鈦矽石-1中雙核位上的高效環氧化

▲ 作者：Christopher P. Gordon, Hauke Engler, Amadeus Samuel Tragl, Milivoj Plodinec, Thomas Lunkenbein, Albrecht Berkessel, et al.

▲ 連結：

https://www.nature.com/articles/s41586-020-2826-3

▲ 摘要

鈦矽石-1（TS-1）是一種具有MFI骨架結構的沸石材料，其中1-2％的矽原子被鈦原子取代。由於它能與過氧化氫（H2O2）催化烯烴環氧化，只留下水作為副產品，因此在工業上得到廣泛應用；使用該工藝每年可生產約一百萬噸環氧丙烷。TS-1的催化性能通常歸因於沸石骨架中分離的Ti（IV）位點的存在。

雖然進行了近40年的實驗和計算研究，但要充分表徵TS-1仍很困難，這些活性Ti（IV）位點的結構尚未得到證實。研究組結合光譜學和顯微技術，詳細描述了一系列具有良好分散鈦原子的高活性和選擇性TS-1丙烯環氧化催化劑。

研究組發現，與H217O2接觸時，所有樣品均表現出特徵性的固態17O核磁共振特徵，這表明在雙核鈦位點上形成了橋接過氧物質。此外，密度泛函理論計算表明，兩個鈦原子之間的協同作用使丙烯能夠通過低能反應途徑進行環氧化，其關鍵的氧轉移過渡態類似於過酸烯烴環氧化。

因此，研究組提出TS-1在H2O2丙烯環氧化反應中高效的原因，是雙核鈦位點，而非骨架中孤立的鈦原子。這一修正後的活性位點結構可以實現TS-1和工業環氧化工藝的進一步優化。

▲ Abstract

Titanium silicalite-1 (TS-1) is a zeolitic material with MFI framework structure, in which 1 to 2 per cent of the silicon atoms are substituted for titanium atoms. It is widely used in industry owing to its ability to catalytically epoxidize olefins with hydrogen peroxide (H2O2), leaving only water as a byproduct; around one million tonnes of propylene oxide are produced each year using this process. The catalytic properties of TS-1 are generally attributed to the presence of isolated Ti(IV) sites within the zeolite framework. However, despite almost 40 years of experimental and computational investigation, the structure of these active Ti(IV) sites is unconfirmed, owing to the challenges of fully characterizing TS-1. Here, using a combination of spectroscopy and microscopy, we characterize in detail a series of highly active and selective TS-1 propylene epoxidation catalysts with well dispersed titanium atoms. We find that, on contact with H217O2, all samples exhibit a characteristic solid-state 17O nuclear magnetic resonance signature that is indicative of the formation of bridging peroxo species on dinuclear titanium sites. Further, density functional theory calculations indicate that cooperativity between two titanium atoms enables propylene epoxidation via a low-energy reaction pathway with a key oxygen-transfer transition state similar to that of olefin epoxidation by peracids. We therefore propose that dinuclear titanium sites, rather than isolated titanium atoms in the framework, explain the high efficiency of TS-1 in propylene epoxidation with H2O2. This revised view of the active-site structure may enable further optimization of TS-1 and the industrial epoxidation process.

Metal-free photoinduced C(sp3)–H borylation of alkanes

無金屬光誘導的C(sp3) -H硼化烷烴

▲ 作者：Chao Shu, Adam Noble & Varinder K. Aggarwal

▲ 連結：

https://www.nature.com/articles/s41586-020-2831-6

▲ 摘要

硼酸及其衍生物是化學科學中最有用的試劑，其應用範圍涵蓋藥物、農用化學品和功能材料。催化C-H硼化是將這些和其他硼基團引入有機分子的一種有效方法，因為它可用於直接官能化原料化學品的C-H鍵，而無需進行底物預活化。這些反應傳統上依賴於貴金屬催化劑裂解C-H鍵，因此，對於芳香族C(sp2)-H鍵硼化的選擇性顯著高於脂肪烴的C(sp3)- H鍵。

研究組報導了一種不同機制的、使用氫原子轉移催化的無金屬硼化反應，其中C(sp3)- H鍵的均相裂解產生烷基自由基，通過與二硼試劑直接反應而被硼化。該反應通過N-烷氧基鄰苯二甲醯亞胺氧化劑與氯化物氫原子轉移催化劑之間的紫外光誘導電子轉移而進行。不同尋常的是，較強的甲基C-H鍵優先被硼化，而非較弱的仲、叔甚至苯基C-H鍵。

機理研究表明，甲基的高選擇性是由於氯自由基-硼「酯」配合物的形成，選擇性地裂解了空間不受阻礙的C-H鍵。通過使用光誘導的氫原子轉移策略，這種無金屬的C(sp3)-H硼化可以在溫和的條件下，將不活潑的烷烴轉化為有價值的有機硼試劑，其選擇性與現有的金屬催化方案相反。

▲ Abstract

Boronic acids and their derivatives are some of the most useful reagents in the chemical sciences, with applications spanning pharmaceuticals, agrochemicals and functional materials. Catalytic C–H borylation is a powerful method for introducing these and other boron groups into organic molecules because it can be used to directly functionalize C–H bonds of feedstock chemicals without the need for substrate pre-activation. These reactions have traditionally relied on precious-metal catalysts for C–H bond cleavage and, as a result, display high selectivity for borylation of aromatic C(sp2)–H bonds over aliphatic C(sp3)–H bonds. Here we report a mechanistically distinct, metal-free borylation using hydrogen atom transfer catalysis, in which homolytic cleavage of C(sp3)–H bonds produces alkyl radicals that are borylated by direct reaction with a diboron reagent. The reaction proceeds by violet-light photoinduced electron transfer between an N-alkoxyphthalimide-based oxidant and a chloride hydrogen atom transfer catalyst. Unusually, stronger methyl C–H bonds are borylated preferentially over weaker secondary, tertiary and even benzylic C–H bonds. Mechanistic studies indicate that the high methyl selectivity is a result of the formation of a chlorine radical–boron 『ate』 complex that selectively cleaves sterically unhindered C–H bonds. By using a photoinduced hydrogen atom transfer strategy, this metal-free C(sp3)–H borylation enables unreactive alkanes to be transformed into valuable organoboron reagents under mild conditions and with selectivities that contrast with those of established metal-catalysed protocols.

氣候學Climatology

Large Chinese land carbon sink estimated from atmospheric carbon dioxide data

根據大氣二氧化碳數據估算，中國陸地碳匯量很大

▲ 作者：Jing Wang, Liang Feng, Paul I. Palmer, Yi Liu, Shuangxi Fang, Hartmut Bösch, et al.

▲ 連結：

https://www.nature.com/articles/s41586-020-2849-9

▲ 摘要

限制全球平均溫度的上升取決於減少二氧化碳（CO2）的排放以及通過陸地碳匯去除。中國目前是全球最大的CO2排放國，2017年約佔全球化石燃料排放量的27％（每年26.7億噸）。對中國陸地生物圈通量的了解一直受到數據覆蓋較少的阻礙，導致對通量後驗估計的範圍很廣。

研究組提供了2009-2016年間從中國6個地區測得的大氣中CO2摩爾分數的最新可用數據。使用這些數據，研究組估計2010-2016年間，中國陸地生物圈的平均碳匯為每年-11.1億噸，相當於研究組估計的同期中國每年人為排放的45％。

該結果反映了此前被低估的中國西南地區（雲南、貴州和廣西省）全年的土地碳匯，以及中國東北地區（尤其是黑龍江和吉林省）整個夏季的土地碳匯。這些省份已經建立了逐漸擴大區域的快速造林模式，在過去的10-15年中，省級森林面積每年增加4萬至44萬公頃。

這些大規模的變化反映了快速增長的人工林的擴張，這為木材出口和國內紙張生產做出了貢獻。星載植被綠度的觀測顯示，在該研究期間，植被綠度隨時間大幅增加，支持了這些造林區域土地碳匯的時間和增加。

▲ Abstract

Limiting the rise in global mean temperatures relies on reducing carbon dioxide (CO2) emissions and on the removal of CO2 by land carbon sinks. China is currently the single largest emitter of CO2, responsible for approximately 27 per cent (2.67 petagrams of carbon per year) of global fossil fuel emissions in 2017. Understanding of Chinese land biosphere fluxes has been hampered by sparse data coverage, which has resulted in a wide range of a posteriori estimates of flux. Here we present recently available data on the atmospheric mole fraction of CO2, measured from six sites across China during 2009 to 2016. Using these data, we estimate a mean Chinese land biosphere sink of −1.11 ± 0.38 petagrams of carbon per year during 2010 to 2016, equivalent to about 45 per cent of our estimate of annual Chinese anthropogenic emissions over that period. Our estimate reflects a previously underestimated land carbon sink over southwest China (Yunnan, Guizhou and Guangxi provinces) throughout the year, and over northeast China (especially Heilongjiang and Jilin provinces) during summer months. These provinces have established a pattern of rapid afforestation of progressively larger regions, with provincial forest areas increasing by between 0.04 million and 0.44 million hectares per year over the past 10 to 15 years. These large-scale changes reflect the expansion of fast-growing plantation forests that contribute to timber exports and the domestic production of paper. Space-borne observations of vegetation greenness show a large increase with time over this study period, supporting the timing and increase in the land carbon sink over these afforestation regions.