Ab initio study of the influence of trimer formation on one- and two-bond spin-spin coupling constants across an X-H-Y hydrogen bond: AH:XH:YH3 complexes for A, X = 19F, 35Cl and Y = 15N, 31P

Ab initio equation-of-motion coupled-cluster singles and doubles (EOM-CCSD) calculations have been carried out to investigate the effect of a third polar near-neighbor on one-bond ((1)J(X)(-)(H) and (1h)J(H)(-)(Y)) and two-bond ((2h)J(X)(-)(Y)) spin-spin coupling constants in AH:XH:YH(3) complexes, where A and X are (19)F and (35)Cl and Y is either (15)N or (31)P. The changes in both one- and two-bond spin-spin coupling constants upon trimer formation indicate that the presence of a third molecule promotes proton transfer across the X-H-Y hydrogen bond. The proton-shared character of the X-H-Y hydrogen bond increases in the order XH:YH(3) < ClH:XH:YH(3) < FH:XH:YH(3). This order is also the order of decreasing shielding of the hydrogen-bonded proton and decreasing X-Y distance, and is consistent with the greater hydrogen-bonding ability of HF compared to HCl as the third molecule. For all complexes, the reduced X-H and X-Y spin-spin coupling constants ((1)K(X)(-)(H) and (2h)K(X)(-)(Y)) are positive, consistent with previous studies of complexes in which X and Y are second-period elements in hydrogen-bonded dimers. (1h)K(H)(-)(Y) is, as expected, negative in these complexes which have traditional hydrogen bonds, except for ClH:FH:NH(3) and FH:FH:NH(3). In these two complexes, the F-H-N hydrogen bond has sufficient proton-shared character to induce a change of sign in (1h)K(H)(-)(Y). The effects of trimer formation on spin-spin coupling constants are markedly greater in complexes in which NH(3) rather than PH(3) is the proton acceptor.     

Photochemistry of A1E, a retinoid with a conjugated pyridinium moiety: competition between pericyclic photooxygenation and pericyclization

The photochemistry of the retinoid analogue A1E shows an oxygen and solvent dependence. Irradiation of A1E with visible light (lambda(irr) = 425 nm) in methanol solutions resulted in pericyclization to form pyridinium terpenoids. Although the quantum yield for this cyclization is low (approximately 10(-4)), nevertheless the photochemical transformation occurs with quantitative chemical yield with remarkable chemoselectivity and diastereoselectivity. Conversely, irradiation of A1E under the same irradiation conditions in air-saturated carbon tetrachloride or deuterated chloroform produced a cyclic 5,8-peroxide as the major product. Deuterium solvent effects, experiments utilizing endoperoxide, phosphorescence, and chemiluminescence quenching studies strongly support the involvement of singlet oxygen in the endoperoxide formation. It is proposed that, upon irradiation, in the presence of oxygen, A1E acts as a sensitizer for generation of singlet oxygen from triplet oxygen present in the solution; the singlet oxygen produced reacts with A1E to produce cyclic peroxide. Thus, the photochemistry of A1E is characterized by two competing reactions, cyclization and peroxide formation. The dominant reaction is determined by the concentration of oxygen, the concentration of A1E, and the lifetime of singlet oxygen in the solvent employed. If the lifetime of singlet oxygen in a given solvent is long enough, then oxidation (peroxide formation) is the major reaction. If the singlet oxygen produced is quenched by the protonated solvent molecules faster than singlet oxygen reacts with A1E, then cyclization dominates.     

(4‐Nitrophenylsulfanylmethyl)triphenylstannane and (4‐nitrophenylsulfonylmethyl)triphenylstannane: R(X) rings (X is 10, 18 or 24) and C—H⋯π interactions

In the crystal structures of (4‐nitro­phenyl­sulfanyl­methyl)­tri­phenyl­stannane, [Sn(C₆H₅)₃(C₇H₆NO₂S)], (I), and (4‐nitro­phenyl­sulfonyl­methyl)­tri­phenyl­stannane, [Sn(C₆H₅)₃­(C₇H₆NO₄S)], (II), the mol­ecules are linked by paired C—H?O hydrogen bonds into centrosymmetric dimers which combine to form sheets. In (I), two such dimers form to give R(10) and R(24) rings. In (II), similar dimers form, here with R(10) and R(18) rings, but with an additional dimer due to the presence of the sulfone group, giving R(10) rings. In both structures, C—H?π interactions lead to a doubling of the width of the sheets.     

Predicted signs of reduced two-bond spin-spin coupling constants (2hKX-Y) across X-H-Y hydrogen bonds

The reduced two-bond Fermi-contact terms and the reduced spin-spin coupling constants (2h)K(X-Y) across X-H-Y hydrogen bonds for complexes stabilized by C-H-N, N-H-N, O-H-N, F-H-N, C-H-O, O-H-O, F-H-O and C-H-F hydrogen bonds are positive. The NMR Triplet Wavefunction Model (NMRTWM) indicates that the signs of the reduced FC terms and (2h)K(X-Y) are determined by excited triplet states that have an odd number of nodes intersecting the X-Y axis between X and Y, thereby leading to an antiparallel alignment of the nuclear magnetic moments of atoms X and Y.     

Effect of a random real on\kappa \to \left( {\kappa ,{\text{ }}\left( {_{\omega _1 }^\alpha } \right)} \right)^2

It is consistent that $\kappa \to (\kappa ,{\text{ }}\left( {_{\omega _1 }^\alpha } \right))^2 $ holds in the random extension.     

The Search for the Least-Coordinating Anion

Abstract

The brominated carborane, Br₆-CB₁₁ H₆- (structure 1) is the most recent candidate for the least coordinating anion. It has wide application to the chemistry of reactive cations. The closest approach to date to the long sought silylium ion, R₃Si⁺, utilized this anion in the structurally characterized compound (i-Pr₃Si^δ+)(Br₆-CB₁₁H₆ ^δ?)1 (structure 2).     

Asymptotically (in)dependent multivariate maxima of moving maxima processes

Smith and Weissman introduced a M4 class of processes which are very flexible models for temporally dependent multivariate extreme value processes. However all variables in these M4 models are asymptotically dependent and what this paper does is to extend this M4 class in a number of ways to produce classes of models which are also asymptotically independent. We shall study properties of the proposed models. In particular, asymptotic dependence indexes, coefficients of tail dependence, and extremal indexes are derived for each case.     

Hydro­gen bonding in C-methyl­ated nitro­anilines: a room-temperature monoclinic polymorph of 4-methyl-3-nitro­aniline with Z′ = 2

The title compound, C₇H₈N₂O₂, is monoclinic (space group P2₁/n) at 295 (2) K with Z′ = 2. The two types of mol­ecule form independent C(7) chains, and the structure is related to that of the low-temperature triclinic polymorph, where Z′ = 4 in P, by a simple displacive transformation.     

The Stool Volatile Metabolome of Pre-Term Babies

The fecal metabolome in early life has seldom been studied. We investigated its evolution in pre-term babies during their first weeks of life. Multiple (n = 152) stool samples were studied from 51 babies, all <32 weeks gestation. Volatile organic compounds (VOCs) were analyzed by headspace solid phase microextraction gas chromatography mass spectrometry. Data were interpreted using Automated Mass Spectral Deconvolution System (AMDIS) with the National Institute of Standards and Technology (NIST) reference library. Statistical analysis was based on linear mixed modelling, the number of VOCs increased over time; a rise was mainly observed between day 5 and day 10. The shift at day 5 was associated with products of branched-chain fatty acids. Prior to this, the metabolome was dominated by aldehydes and acetic acid. Caesarean delivery showed a modest association with molecules of fungal origin. This study shows how the metabolome changes in early life in pre-term babies. The shift in the metabolome 5 days after delivery coincides with the establishment of enteral feeding and the transition from meconium to feces. Great diversity of metabolites was associated with being fed greater volumes of milk.