A temperate Earth-sized planet with tidal heating transiting an M6 star.

Temperate Earth-sized exoplanets around late-M dwarfs offer a rare opportunity to explore under which conditions planets can develop hospitable climate conditions. The small stellar radius amplifies the atmospheric transit signature, making even compact secondary atmospheres dominated by N2 or CO2 amenable to characterization with existing instrumentation1. Yet, despite large planet search efforts2, detection of low-temperature Earth-sized planets around late-M dwarfs has remained rare and the TRAPPIST-1 system, a resonance chain of rocky planets with seemingly identical compositions, has not yet shown any evidence of volatiles in the system3. Here we report the discovery of a temperate Earth-sized planet orbiting the cool M6 dwarf LP 791-18. The newly discovered planet, LP 791-18d, has a radius of 1.03 ± 0.04 R⊕ and an equilibrium temperature of 300-400 K, with the permanent night side plausibly allowing for water condensation. LP 791-18d is part of a coplanar system4 and provides a so-far unique opportunity to investigate a temperate exo-Earth in a system with a sub-Neptune that retained its gas or volatile envelope. On the basis of observations of transit timing variations, we find a mass of 7.1 ± 0.7 M⊕ for the sub-Neptune LP 791-18c and a mass of [Formula: see text] for the exo-Earth LP 791-18d. The gravitational interaction with the sub-Neptune prevents the complete circularization of LP 791-18d's orbit, resulting in continued tidal heating of LP 791-18d's interior and probably strong volcanic activity at the surface5,6.

Halogen-Bond Mediated [2+2] Photodimerizations: À la Carte Access to Unsymmetrical Cyclobutanes in the Solid State.

The ditopic halogen-bond (X-bond) donors 1,2-, 1,3-, and 1,4-diiodotetrafluorobenzene (1,2-, 1,3-, and 1,4-di-I-tFb, respectively) form binary cocrystals with the unsymmetrical ditopic X-bond acceptor trans-1-(2-pyridyl)-2-(4-pyridyl)ethylene (2,4-bpe). The components of each cocrystal (1,2-di-I-tFb)·(2,4-bpe), (1,3-di-I-tFb)·(2,4-bpe), and (1,4-di-I-tFb)·(2,4-bpe) assemble via N···I X-bonds. For (1,2-di-I-tFb)·(2,4-bpe) and (1,3-di-I-tFb)·(2,4-bpe), the X-bond donor supports the C=C bonds of 2,4-bpe to undergo a topochemical [2+2] photodimerization in the solid state: UV-irradiation of each solid resulted in stereospecific, regiospecific, and quantitative photodimerization of 2,4-bpe to the corresponding head-to-tail (ht) or head-to-head (hh) cyclobutane photoproduct, respectively.

Supramolecular Sandwiches: Halogen-Bonded Coformers Direct [2+2] Photoreactivity in Two-Component Cocrystals.

The halogen-bond (X-bond) donors 1,3- and 1,4-diiodotetrafluorobenzene (1,3-di-I-tFb and 1,4-di-I-tFb, respectively) form cocrystals with trans-1,2-bis(2-pyridyl)ethylene (2,2'-bpe) assembled by N···I X-bonds. In each cocrystal, 2(1,3-di-I-tFb)·2(2,2'-bpe) and (1,4-di-I-tFb)·(2,2'-bpe), the donor molecules support the C=C bonds of 2,2'-bpe to undergo an intermolecular [2+2] photodimerization. UV irradiation of each cocrystal resulted in stereospecific and quantitative conversion of 2,2'-bpe to rctt-tetrakis(2-pyridyl)cyclobutane (2,2'-tpcb). In each case, the reactivity occurs via face-to-face π-stacked columns wherein nearest-neighbor pairs of 2,2'-bpe molecules lie sandwiched between X-bond donor molecules. Nearest-neighbor C=C bonds are stacked criss-crossed in both cocrystals. The reactivity was ascribed to the olefins undergoing pedal-like motion in the solid state. The stereochemistry of 2,2'-tpcb is confirmed in cocrystals 2(1,3-di-I-tFb)·(2,2'-tpcb) and (1,4-di-I-tFb)·(2,2'-tpcb).

X-ray crystal structure of trans-bis-(pyridin-3-yl)ethyl-ene: comparing the supra-molecular structural features among the symmetrical bis-(n-pyrid-yl)ethyl-enes (n = 2, 3, or 4) constitutional isomers.

The mol-ecular structure of trans-bis-(pyridin-3-yl)ethyl-ene (3,3'-bpe), C12H10N2, as determined by single-crystal X-ray diffraction is reported. The mol-ecule self-assembles into two dimensional arrays by a combination of C-H⋯N hydrogen bonds and edge-to-face C-H⋯π inter-actions that stack in a herringbone arrangement perpendicular to the crystallographic c-axis. The supra-molecular forces that direct the packing of 3,3'-bpe as well as its packing assembly within the crystal are also compared to those observed within the structures of the other symmetrical isomers trans-1,2-bis-(n-pyrid-yl)ethyl-ene ( n , n '-bpe, where n = n' = 2 or 4).

Halogen versus Hydrogen Bonding in Binary Cocrystals: Novel Conformation a Coformer with [2+2] Photoreactivity of Criss-Crossed C=C Bonds.

A series of cocrystals involving the hydrogen- and halogen-bond donor coformers catechol (cat) and 1,2-diiodotetrafluorobenzene (1,2-di-I-tFb), respectively, is reported. Each coformer forms a cocrystal with each of the three symmetric bipyridines trans-1,2-bis(n-pyridyl)ethylene (n,n'-bpe, where: n=n'=2, 3, 4). Four novel cocrystals (cat) ⋅ (3,3'-bpe), 2(1,2-di-I-tFb) ⋅ (2,2'-bpe), 2(1,2-di-I-tFb) ⋅ (3,3'-bpe), and (1,2-di-I-tFb) ⋅ (4,4'-bpe) comprise components that assemble by either O-H⋅⋅⋅N hydrogen bonds (cat) or N⋅⋅⋅I halogen bonds (1,2-di-I-tFb). In (cat) ⋅ (3,3'-bpe), cat acts as a template to support an intermolecular [2+2] photocycloaddition of 3,3'-bpe. The reactivity occurs via a one-dimensional (1D) hydrogen-bonded tape with stacked and criss-crossed olefins that react stereoselectively and quantitatively to form rctt-tetrakis(3-pyridyl)cyclobutane (3,3'-tpcb). The reactivity of the criss-crossed olefins is facilitated by a hitherto not reported cis-gauche conformation adopted by cat. The stereochemistry of 3,3'-tpcb is confirmed in the cocrystal 2(cat) ⋅ (3,3'-tpcb).

A Divergent Alkyne Diol Directs [2 + 2] Photoreactivity in the Solid State: Cocrystal, Supramolecular Catalysis, and Sublimation Effects.

2-butyne-1,4-diol (1,4-bd) is used as a divergent ditopic template that directs trans-1,2-bis (n-pyridyl) ethylene (n,n'-bpe, where n = n' = 3 or 4) to undergo an intermolecular [2 + 2] photodimerization in the solid state. The components of cocrystals [(1,4-bd)·(4,4'-bpe)]n and [(1,4-bd)·(3,3'-bpe)]n form 1D hydrogen-bonded polymers with n,n'-bpe assembled as infinite parallel stacks. The alkenes undergo [2 + 2] photocycloadditions to form rctt-tetrakis (n-pyridyl) cyclobutane (where n = 3 or 4). We demonstrate that the reactive solid involving 4,4'-bpe exhibits supramolecular catalysis.