The Evaporating Massive Embedded Stellar Cluster IRS 13 Close to Sgr A*. I. Detection of a rich population of dusty objects in the IRS 13 cluster

A detailed analysis of the Nuclear Stellar Cluster (NSC) concedes not only the existence of the Scluster with its fast-moving stars and the supermassive black hole (SMBH) Sgr A*. It also reveals an embedded region of gas and dust with an exceptionally high stellar density called IRS 13. The IRS 13 cluster can be divided into the northern and the eastern counterparts, called IRS 13N and IRS 13E, respectively. This work will focus on both regions and study their most prominent members using rich infrared and radio/submm data baselines. Applying a multiwavelength analysis enables us to determine a comprehensive photometric footprint of the investigated cluster sample. Using the raytracing-based radiative transfer model HYPERION, the spectral energy distribution of the IRS 13 members suggests a stellar nature of the dusty sources. These putative Young Stellar Objects (YSOs) have a comparable spectroscopic identification to the D and G sources in or near the S cluster. Furthermore, we report the existence of a population of dusty sources in IRS 13 that can be mostly identified in the H-, K-, and Lband. Together with the objects reported in literature, we propose that this population is the outcome of a recent star formation process. Furthermore, we report that these presumably young objects are arranged in a disk structure. Although it cannot be excluded that the intrinsic arrangement of IRS 13 does show a disk structure, we find indications that the investigated cluster sample might be related to the counterclockwise disk.Comment: 59 pages, 44 figures, accepted and published by the Ap

New horizons for fundamental physics with LISA

The Laser Interferometer Space Antenna (LISA) has the potential to reveal wonders about the fundamental theory of nature at play in the extreme gravity regime, where the gravitational interaction is both strong and dynamical. In this white paper, the Fundamental Physics Working Group of the LISA Consortium summarizes the current topics in fundamental physics where LISA observations of gravitational waves can be expected to provide key input. We provide the briefest of reviews to then delineate avenues for future research directions and to discuss connections between this working group, other working groups and the consortium work package teams. These connections must be developed for LISA to live up to its science potential in these areas

Molecules in a Hurry to Get Rid of Antiaromaticity

When light strikes an aromatic molecule, the electrons rearrange, and the compound can gain antiaromatic character, becoming especially reactive. From there, the drive to escape excited-state antiaromaticity can trigger all sort of photochemical reactions—the clock is ticking, and the molecule is in a hurry to get rid of antiaromaticity. For example, benzene is [4n+2] π-aromatic in the ground state, but [4n+2] π-antiaromatic in the excited state. To alleviate excited-state antiaromaticity, benzene quickly isomerizes to fulvene or even to the highly strained benzvalene. This dissertation focuses on demonstrating the important consequences of excited-state antiaromaticity in photoinduced electron and proton transfer reactions. Upon photoexcitation, o-salicylic acid converts to the “rare” keto tautomer by proton transfer, and this alleviates excited-state antiaromaticity of the π-ring. In the photoinduced electron transfer reaction of phenol, leading to O–H bond cleavage, a π-electron departs from the photoexcited π-ring and relieves antiaromaticity. The photoinduced proton-coupled electron transfer reaction of Watson–Crick DNA base pairs is another example of excited-state antiaromaticity relief. Transfer of an electron and a proton from the photoexcited purine to the pyrimidine significantly alters the π-system of the paired bases. The computational findings presented here provide valuable insights for understanding the photoreactions of many “aromatic compounds” of chemical and biological relevance

The sthrength of O-H-O intramolecular hydrogen bonding for some 1,3-disubstituted acyclid compouds: the effect of the alkyl group

Intramolecular hydrogen bonding is one of the most important intramolecular interactions, which is a critical element in deciding the molecular arrangement. Conformational analysis is the most powerful tool to evaluate the hydrogen bonding importance for a conformational preference. The term conformational analysis covers two aspects: determining molecular geometry and conformer energies, followed by studies to determine which steric and electronic interactions are responsible for the conformational stability. To investigate this influence in acyclic compound, the conformational preferences of 3-R-propanol [R = OH (1), OCH3 (2), OCH2CH3(3), OCH(CH3)2 (4) e OC(CH3)3(5)], 3-R-butanol [R = OH(6), OCH3(7)] e 3-metil- 3-R-butanol [R = OH (8), OCH3 (9)] are evaluated by means of theoretical calculations along with experimental infrared and nuclear magnetic resonance spectroscopies. Compounds 1, 2, 6, and 7 were purchased and the rest of remaining compound were synthesized. In fact, the most stable conformation of these compounds exhibit IAHB. Thermal population of hydrogenbonded conformers are 65, 66, 73, 69, 97, 92, 79, 99, and 99% for compound 1-9, respectively. Experimental infrared data show the red-shift value increase of 76, 87, 96, 100 to 112 cm-1 for compound 1-5, respectively, suggesting that the addition of alkyl groups to the IAHB proton acceptor atom increases the strength of this interaction and, although the steric repulsion increases along with increasing substituent volume, the increase in the IAHB strength is higher than the increase in the steric repulsion. Infrared data also show that the IAHB strength is greater for compound 2 ( = 87 cm-1) than for compound 6 ( = 77 cm-1), indicating that the increase in the IAHB strength is greater when alkyl groups are bonded directly to the oxygen proton acceptor than to the α-carbon. Experimental 3JH1H2 increases with the increase in solvent basicity for all compounds, indicating a change in the conformacional preference along with the increase in solvent basicity. An equation based on vicinal coupling constant is proposed to analyze 1,3-disubstituted acyclic compounds, allowing measurement of the experimental molar fraction (XHB) of conformers hydrogen-bonded in any solvent. The XHB values changed of 59, 53, 56, 57, 78, 88, and 74% in the CCl4 solvent to 22, 13, 15, 12, 16, 44, and 13% in pyridined5 as solvent. These results indicate that conformers hydrogen-bonded are predominant in nonbasic solvents, while the population of conformers non-hydrogen-bonded increases as solvent basicity increases.LFQOLigações de hidrogênio intramoleculares (LHI) estão entre as interações estabilizantes mais importantes que uma molécula pode apresentar, sendo muitas vezes a interação responsável por determinar o arranjo molecular. A avaliação da importância de ligações de hidrogênio para a estabilização de confôrmeros de uma molécula é realizada através de análise conformacional, que é o ramo da química orgânica que se atém a determinação da geometria molecular e da energia dos confôrmeros de uma molécula e de estudos para determinar quais as interações estéricas e eletrônicas que são responsáveis pela estabilização de cada confôrmero. Para investigar esta influência em compostos acíclicos 1,3-dissubstituídos, as preferências conformacionais dos compostos 3-R-propanol [R = OH (1), OCH3 (2), OCH2CH3(3), OCH(CH3)2 (4) e OC(CH3)3(5)], 3-R-butanol [R = OH(6), OCH3(7)] e 3-metil-3-R-butanol [R = OH (8), OCH3 (9)] foram avaliadas através de cálculos teóricos utilizando o funcional LC- ωPBE/6-311+G(d,p) atrelados a técnicas experimentais de espectroscopia no infravermelho e ressonância magnética nuclear. Os compostos 1, 2, 6 e 7 foram obtidos comercialmente, os demais foram sintetizados. Para todos os compostos o confôrmero mais estável foi o que apresenta LHI O-H...O. Através do estudo de população térmica observou-se que a somatória da população dos confôrmeros que fazem LHI sempre foi muito superior a população dos que não fazem LHI. A população térmica dos confôrmeros que fazem LHI foi de 65, 66, 73, 69, 97, 92, 79, 99 e 99 % para os compostos de 1 a 9, respectivamente. As análises de infravermelho mostraram que os valores de  aumentaram de 76, 87, 96, 100 e 112 cm-1 para os compostos de 1 a 5, respectivamente, indicando que a adição de grupos alquilas ao oxigênio aceptor de hidrogênio aumenta a força da LHI, apesar da repulsão estérica aumentar com o aumento do volume do substituinte. Os resultados de infravermelho mostram também que a força da LHI é maior para o composto 2 ( = 87 cm-1) do que para o composto 6 ( = 77 cm-1) indicando que a adição de grupos alquilas diretamente ao oxigênio aceptor de hidrogênio aumenta mais a força da LHI do que quando adicionados ao carbono α a este oxigênio. Os resultados de 3JHH obtidos pelo RMN de 1H mostraram que os valores de 3JHH são alterados com a mudança da basicidade do solvente. Os valores de 3JHH são menores em CCl4 e maiores em DMSO-d6 para todos os compostos. Uma equação foi proposta para mensurar a fração molar de confôrmeros com ligação de hidrogênio intramolecular através de constantes de acoplamento vicinais baseado em observações teóricas e experimentais. Os valores de fração molar dos confôrmeros que fazem LHI foram de 59, 53, 56, 57, 78, 88 e 74 % em CCl4 para 22, 13, 15, 12, 16, 44 e 13 % em piridina-d5, para os compostos de 1, 2, 3, 4, 5, 8 e 9, respectivamente. Estes resultados mostram que em solventes mais básicos, os confôrmeros que não fazem LHI são mais estáveis, consequentemente as rotações entre as ligações carbono-carbono são facilitadas e os valores de 3JHH são próximos ou maiores que 7,0 Hz, que é um valor médio para livre rotação

Barrier-Lowering Effects of Baird Antiaromaticity in Photoinduced Proton-Coupled Electron Transfer (PCET) Reactions

Baird antiaromaticity plays a central role in the photochemistry of proton-coupled electron transfer (PCET) reactions. We recognize that many popular organic chromophores that catalyze photoinduced PCET reactions are Hückel aromatic in the ground state, but gain significant Baird antiaromatic character in the lowest ππ* state, having important barrier-lowering effects for electron transfer. Two examples, 1) the photolytic O–H bond dissociation of phenol and 2) solar water splitting in the pyridine-water complex, are discussed. Contrary to an assumed homolytic O–H bond dissociation, both reactions proceed through loss (and gain) of an electron in the π-system (i.e., antiaromaticity relief), followed by heterolytic cleavage of the polar O–H bond near barrierlessly. Nucleus-independent chemical shifts (NICS), ionization energies (IE), electron affinities (EA), and excited-state PCET energy profiles of selected [4n] and [4n+2] π-systems are presented.</p

How DNA Base Pairs Escape From the Excited-State: Antiaromaticity Relief in the Picoseconds

Before the development of an ozone layer in the Archean atmosphere, the flux of UV radiation reaching Earth was suggested to be several orders of magnitude higher than it is today. For the emerging biomolecules, constant exposure to strong UV irradiation meant that useful molecules had to be resistant to UV damage and harmful photochemical reactions. From this prebiotic environment, the Watson–Crick structures of A·T and G·C base pairs survived to encode genetic information—and the photostability of these winning pairs in this specific arrangement is astonishing. Upon UV irradiation, the photoexcited canonical base pairs undergo proton-coupled electron transfer (PCET), followed by non-radiative decay, and convert internally to the electronic ground state within picoseconds. But the underlying reason why this process happens so efficiently has not been explained. Here we show that efficient photodeactivation in isolated base pairs are driven by antiaromaticity relief during PCET. According to computed nucleus independent chemical shifts, the A·T and G·C base pairs are aromatic in the electronic ground state, but the purines become highly antiaromatic in the first 1ππ* state, and PCET relieves this excited-state antiaromaticity. We found especially pronounced antiaromaticity relief for the major PCET pathway of isolated Watson–Crick A·T and G·C base pairs, when compared to alternative proton transfer routes or to PCET reactions in non-canonical pairs. Our findings suggest that excited-state deactivation of isolated base pairs are tied to sudden changes in aromaticity and antiaromaticity within the picoseconds that follow a strike of UV-light.</p

Excited-State Proton Transfer: Molecules in a Hurry to Get Rid of Antiaromaticity

Baird’s rule explains why and when excited-state proton transfer (ESPT) reactions happen in organic compounds. Bifunctional compounds that are [4n+2] π-aromatic in the ground state, become [4n+2] π-antiaromatic in the first 1ππ* states, and proton transfer (eitherinter-or intra-molecularly) helps relieve excited-state antiaromaticity. Computed nucleus independent chemical shifts (NICS) for several ESPT examples (including excited-state intramolecular proton transfers (ESIPT), biprotonic transfers, dynamic catalyzed transfers, and proton relay transfers) document the important role of excited-state antiaromaticity. o-Salicylic acid undergoes ESPT only in the “antiaromatic” S1(1ππ*) state, but not in the “aromatic” S2(1ππ*) state. Stokes’ shifts of structurally-related compounds (e.g., derivatives of 2-(2-hydroxyphenyl)benzoxazole and hydrogen-bonded complexes of 2-aminopyridine with pro tic substrates) vary depending on the antiaromaticity of the photoinduced tautomers. Remarkably, Baird’s rule predicts the effect of light on hydrogen bond strengths; hydrogen bonds that enhance (and reduce) excited-state antiaromaticity in compounds become weakened (and strengthened) upon photoexcitation.</p

Electron-driven proton transfer relieves excited-state antiaromaticity in photoexcited DNA base pairs

The Watson-Crick A.T and G.C base pairs are not only electronically complementary, but also photochemically complementary. Upon UV irradiation, DNA base pairs undergo efficient excited-state deactivation through electron driven proton transfer (EDPT), also known as proton-coupled electron transfer (PCET), at a rate too fast for other reactions to take place. Why this process occurs so efficiently is typically reasoned based on the oxidation and reduction potentials of the bases in their electronic ground states. Here, we show that the occurrence of EDPT can be traced to a reversal in the aromatic/antiaromatic character of the base upon photoexcitation. The Watson-Crick A.T and G.C base pairs are aromatic in the ground state, but the purines become highly antiaromatic and reactive in the first 1(pi pi)* state, and transferring an electron and a proton to the pyrimidine relieves this excited-state antiaromaticity. Even though proton transfer proceeds along the coordinate of breaking a N-H pi-bond, the chromophore is the pi-system of the base, and EDPT is driven by the strive to alleviate antiaromaticity in the pi-system of the photoexcited base. The presence and absence of alternative excited-state EDPT routes in base pairs also can be explained by sudden changes in their aromatic and antiaromatic character upon photoexcitation

Paleorecords from MD02-2575 (Figure 2)

Palaeoclimate records and numerical model simulations indicate that changes in tropical and subtropical sea surface temperatures and in the annual average position of the intertropical convergence zone are linked to high-latitude climate changes on millennial to glacial-interglacial timescales. It has recently been suggested that cooling in the high latitudes associated with abrupt climate-change events is evident primarily during the northern hemisphere winter, implying increased seasonality at these times. However, it is unclear whether such a seasonal bias also exists for the low latitudes. Here we analyse the Mg/Ca ratios of surface-dwelling foraminifera to reconstruct sea surface temperatures in the northeastern Gulf of Mexico for the past 300,000 years. We suggest that sea surface temperatures are controlled by the migration of the northern boundary of the Atlantic Warm Pool, and hence the position of the intertropical convergence zone during boreal summer, and are relatively insensitive to winter conditions. Our results suggest that summer Atlantic Warm Pool expansion is primarily affected by glacial-interglacial variability and low-latitude summer insolation. Because a clear signature of rapid climate-change events, such as the Younger Dryas cold event, is lacking in our record, we conclude that high-latitude events seem to influence only the winter Caribbean climate conditions, consistent with the hypothesis of extreme northern-hemisphere seasonality during abrupt cooling events

Cyclobenzoin Esters as Hosts for Thin Guests

Nitriles and terminal alkynes are important compounds in industrial and academic settings. Their supramolecular binding has been challenging without the intervention of metals, because of the small dimensions of their linear –C≡N and –C≡CH groups. Using a combination of crystallography and computation, we have shown that cyclotetrabenzoin esters can host terminal triple bonds of alkynes and nitriles in their electron-poor cavities. Within these cavities, π-clouds of triple bonds can establish favorable and virtually equidistant interactions with the four aromatic walls of the cyclotetrabenzoin skeleton. Binding is selective for aliphatic nitriles and terminal alkynes, with their aromatic counterparts residing outside of the cyclotetrabenzoin cavity. These findings are of relevance in the binding, separations, and activation of these and other linear molecular guests