The role played by salts in the formation of SBA-15, an in situ small-angle X-ray scattering/diffraction study

The influence of salts (NaCl, NaBr, and NaI) on the formation of mesoporous silica SBA-15 was studied in situ by small-angle X-ray scattering and diffraction. Pluronic P104 was used as structure director. The micellar properties and the dynamics of formation were clearly dependent on the presence of salt. It was also shown that the kinetics of mesophase formation, the initial value of the cell parameters, and the extent of long-range order were all influenced by salt additions. The observations are explained to primarily originate from the influence of the anions on the ethylene oxide part of the polymer, i.e., the corona region of the Pluronic micelles. Two effects are identified: a general ion effect causing dehydration of the ethylene oxide part and consequently inducing micellar growth, and a specific ion effect that counterbalances this. The study provides the basis for understanding the means by which addition of simple Na-salts influence the formation of mesoscopically ordered silicas synthesized using nonionic surfactants as structure directors, hence advancing the knowledge base toward a more rational design of mesoporous materials.     

High-Resolution Laser Spectroscopy of YbCl: The AΠ–XΣ Transition

The A²Π–X²Σ⁺ transition of ¹⁷⁴Yb³⁵Cl and ¹⁷²Yb³⁵Cl has been rotationally analyzed for the first time. Doppler-limited laser excitation spectroscopy with selective detection of fluorescence was used to obtain spectra of the 0–0 and 1–0 bands with a measurement accuracy of approximately 0.0035 cm⁻¹. Resolved fluorescence was used to record the 0–1, 0–2, and 0–3 bands and to unequivocally assign the rotational numbering, N, to the laser excitation spectra. In total, over 1300 line positions have been measured and assigned for each of the two isotopomers and employed in least-squares fits of molecular parameters. The principal results for the A²Π state are A_e = 1491.494(2) cm⁻¹ and R_e = 2.4433(1) Å, and for the X²Σ⁺ state, R_e = 2.4883(2) Å and γ_e = 4.59(2) × 10⁻³ cm⁻¹. The interaction between the X²Σ⁺ and A²Π states has been investigated and is shown to be the main contributor to the spin–rotation splitting in the ground state.     

Curtin-Hammett and steric effects in HOBt acylation regiochemistry

While hydroxybenzotriazole is commonly used in a variety of bond-forming reactions, its acylation has been shown to produce a regiochemical (O vs N) mixture with complex kinetic behavior. Increased steric bulk on the electrophile favors formation of the oxygen-acylated product. Upon standing as a solid, the mixture can isomerize completely to the nitrogen adduct. An equilibrium ratio of regioisomers can be re-established in solution by adding either nucleophilic or electrophilic reagents, suggesting that the composition of the mixture is not significant to subsequent reactivity. Solvents can affect this regiochemical equilibrium through a Curtin-Hammett effect, where the shift in the tautomeric equilibrium of HOBt in polar solvents biases the reaction toward the oxygen adduct.     

A Computationally Efficient Oracle Estimator for Additive Nonparametric Regression with Bootstrap Confidence Intervals

Abstract

This article makes three contributions. First, we introduce a computationally efficient estimator for the component functions in additive nonparametric regression exploiting a different motivation from the marginal integration estimator of Linton and Nielsen. Our method provides a reduction in computation of order n which is highly significant in practice. Second, we define an efficient estimator of the additive components, by inserting the preliminary estimator into a backfitting˙ algorithm but taking one step only, and establish that it is equivalent, in various senses, to the oracle estimator based on knowing the other components. Our two-step estimator is minimax superior to that considered in Opsomer and Ruppert, due to its better bias. Third, we define a bootstrap algorithm for computing pointwise confidence intervals and show that it achieves the correct coverage.     

High-Resolution Laser Spectroscopy of YbCl: The B(2)Sigma(+)-X(2)Sigma(+) Transition

High-resolution laser excitation spectra have been obtained for the 0-0, 1-1, and 0-1 bands of the B(2)Sigma(+)-X(2)Sigma(+) transition of YbCl and a rotational analysis has been performed on the (174)Yb(35)Cl and (172)Yb(35)Cl isotopomers. Comparison of the spin-rotation constant, gamma, for the B(2)Sigma(+) state with the lambda-doubling constant of the A(2)Pi(1/2) state (1) shows that the two excited states form a unique perturber pair arising from the 6psigma and 6ppi orbitals centered on the Yb(+) ion. The principal results for the B(2)Sigma(+) state are B(e)=0.097552(5) cm(-1), R(e)=2.43623(6) ?, gamma(e)=-2.1655(6)x10(-4) cm(-1), and DeltaG(1/2)=313.111(2) cm(-1). Copyright 2001 Academic Press.