Muon capture on light nuclei

This work investigates the muon capture reactions 2H(\mu^-,\nu_\mu)nn and 3He(\mu^-,\nu_\mu)3H and the contribution to their total capture rates arising from the axial two-body currents obtained imposing the partially-conserved-axial-current (PCAC) hypothesis. The initial and final A=2 and 3 nuclear wave functions are obtained from the Argonne v_{18} two-nucleon potential, in combination with the Urbana IX three-nucleon potential in the case of A=3. The weak current consists of vector and axial components derived in chiral effective field theory. The low-energy constant entering the vector (axial) component is determined by reproducting the isovector combination of the trinucleon magnetic moment (Gamow-Teller matrix element of tritium beta-decay). The total capture rates are 393.1(8) s^{-1} for A=2 and 1488(9) s^{-1} for A=3, where the uncertainties arise from the adopted fitting procedure.Comment: 6 pages, submitted to Few-Body Sys

Electromagnetic structure of A=2 and 3 nuclei in chiral effective field theory

The objectives of the present work are twofold. The first is to address and resolve some of the differences present in independent, chiral-effective-field-theory (\chiEFT) derivations up to one loop, recently appeared in the literature, of the nuclear charge and current operators. The second objective is to provide a complete set of \chiEFT predictions for the structure functions and tensor polarization of the deuteron, for the charge and magnetic form factors of 3He and 3H, and for the charge and magnetic radii of these few-nucleon systems. The calculations use wave functions derived from high-order chiral two- and three-nucleon potentials and Monte Carlo methods to evaluate the relevant matrix elements. Predictions based on conventional potentials in combination with \chiEFT charge and current operators are also presented. There is excellent agreement between theory and experiment for all these observables for momentum transfers up to q< 2.0-2.5 (1/fm); for a subset of them, this agreement extends to momentum transfers as high as q~5-6 (1/fm). A complete analysis of the results is provided.Comment: 34 pages, Revte

Minimally non-local nucleon-nucleon potentials with chiral two-pion exchange including Δ\Delta's

We construct a coordinate-space chiral potential, including $\Delta$-isobar intermediate states in its two-pion-exchange component. The contact interactions entering at next-to-leading and next-to-next-to-next-to-leading orders ($Q^2$ and $Q^4$, respectively, $Q$ denoting generically the low momentum scale) are rearranged by Fierz transformations to yield terms at most quadratic in the relative momentum operator of the two nucleons. The low-energy constants multiplying these contact interactions are fitted to the 2013 Granada database, consisting of 2309 $pp$ and 2982 $np$ data (including, respectively, 148 and 218 normalizations) in the laboratory-energy range 0--300 MeV. For the total 5291 $pp$ and $np$ data in this range, we obtain a $\chi^2$/datum of roughly 1.3 for a set of three models characterized by long- and short-range cutoffs, $R_{\rm L}$ and $R_{\rm S}$ respectively, ranging from $(R_{\rm L},R_{\rm S})=(1.2,0.8)$ fm down to $(0.8,0.6)$ fm. The long-range (short-range) cutoff regularizes the one- and two-pion exchange (contact) part of the potential.Comment: 32 pages, 19 figures, accepted for publication as a Regular Article in Physical Review

A Library Approach to the Development of BenzaPhos, Highly Efficient Chiral Supramolecular Ligands for Asymmetric Hydrogenation

A library of chiral supramolecular ligands named BenzaPhos, of straightforward preparation (two steps from commercial or readily available starting materials) and modular structure, was designed and synthesized. The ligands were screened in the search of new rhodium catalysts for the enantioselective hydrogenation of several benchmark and industrially relevant substrates. Once a series of hits were identified, structural modifications were introduced on three of the best ligands and a small second-generation library was created. Members of the latter showed outstanding levels of activity and enantioselectivity in the hydrogenation of challenging olefins such as enamide S4 and beta-dehydroamino ester S5 (> 99% ee: best value ever reported in both cases). A series of control experiments were undertaken in order to clarify the role of hydrogen bonding in determining the catalytic properties of the new ligands. The results of these experiments, together with those of computational studies carried out on four dihydride complexes involved in the catalytic hydrogenation of substrate S4, strongly suggest that a substrate orientation takes place in the catalytic cycle by formation of a hydrogen bond between the ligand amide oxygen and the substrate amide NH

Muon capture on deuteron using local chiral potentials

The muon capture reaction $\mu^- + d \rightarrow n + n + \nu_{\mu}$ in the doublet hyperfine state is studied using nuclear potentials and consistent currents derived in chiral effective field theory, which are local and expressed in coordinate-space (the so-called Norfolk models). Only the largest contribution due to the $^1S_0$ $nn$ scattering state is considered. Particular attention is given to the estimate of the theoretical uncertainty, for which four sources have been identified: (i) the model dependence, (ii) the chiral order convergence for the weak nuclear current, (iii) the uncertainty in the single-nucleon axial form factor, and (iv) the numerical technique adopted to solve the bound and scattering $A=2$ systems. This last source of uncertainty has turned out essentially negligible. The $^1S_0$ doublet muon capture rate $\Gamma^D(^1S_0)$ has been found to be $\Gamma^D(^1S_0)=255.8(0.6)(4.4)(2.9)$ s$^{-1}$, where the three errors come from the first three sources of uncertainty. The value for $\Gamma^D(^1S_0)$ obtained within this local chiral framework is compared with previous calculations and found in very good agreement.Comment: 33 pages, 3 figure

Muon capture on deuteron and 3He

The muon capture reactions 2H(\mu^-,\nu_\mu)nn and 3He(\mu^-,\nu_\mu)3H are studied with conventional or chiral realistic potentials and consistent weak currents. The initial and final A=2 and 3 nuclear wave functions are obtained from the Argonne v18 or chiral N3LO two-nucleon potential, in combination with, respectively, the Urbana IX or chiral N2LO three-nucleon potential in the case of A=3. The weak current consists of polar- and axial-vector components. The former are related to the isovector piece of the electromagnetic current via the conserved-vector-current hypothesis. These and the axial currents are derived either in a meson-exchange or in a chiral effective field theory (chiEFT) framework. There is one parameter (either the N-to-\Delta axial coupling constant in the meson-exchange model, or the strength of a contact term in the chiEFT model) which is fixed by reproducing the Gamow-Teller matrix element in tritium beta-decay. The model dependence relative to the adopted interactions and currents (and cutoff sensitivity in the chiEFT currents) is weak, resulting in total rates of 392.0 +/- 2.3 Hz for A=2, and 1484 +/- 13 Hz for A=3, where the spread accounts for this model dependence.Comment: 15 pages, 1 figure, submitted to Phys. Rev.

Local chiral interactions and magnetic structure of few-nucleon systems

The magnetic form factors of $^2$H, $^3$H, and $^3$He, deuteron photodisintegration cross sections at low energies, and deuteron threshold electrodisintegration cross sections at backward angles in a wide range of momentum transfers, are calculated with the chiral two-nucleon (and three-nucleon) interactions including $\Delta$ intermediate states that have recently been constructed in configuration space. The $A\,$=$\,$3 wave functions are obtained from hyperspherical-harmonics solutions of the Schr\"odinger equation. The electromagnetic current includes one- and two-body terms, the latter induced by one- and two-pion exchange (OPE and TPE, respectively) mechanisms and contact interactions. The contributions associated with $\Delta$ intermediate states are only retained at the OPE level, and are neglected in TPE loop (tree-level) corrections to two-body (three-body) current operators. Expressions for these currents are derived and regularized in configuration space for consistency with the interactions. The low-energy constants that enter the contact few-nucleon systems. The predicted form factors and deuteron electrodisintegration cross section are in excellent agreement with experiment for momentum transfers up to 2--3 fm$^{-1}$. However, the experimental values for the deuteron photodisintegration cross section are consistently underestimated by theory, unless use is made of the Siegert form of the electric dipole transition operator. A complete analysis of the results is provided, including the clarification of the origin of the aforementioned discrepancy.Comment: 24 pages, 13 figure

Rhodium-catalyzed asymmetric hydrogenation of olefins with PhthalaPhos, a new class of chiral supramolecular ligands

A library of 19 binol-derived chiral monophosphites that contain a phthalic acid diamide group (Phthala- Phos) has been designed and synthesized in four steps. These new ligands were screened in the rhodium-catalyzed enantioselective hydrogenation of prochiral dehydroamino esters and enamides. Several members of the library showed excellent enantioselectivity with methyl 2-acetamido acrylate (6 ligands gave >97% ee), methyl (Z)-2- acetamido cinnamate (6 ligands gave >94% ee), and N-(1-phenylvinyl)acetamide (9 ligands gave >95% ee), whilst only a few representatives afforded high enantioselectivities for challenging and industrially relevant substrates N-(3,4-dihydronaphthalen-1- yl)-acetamide (96% ee in one case) and methyl (E)-2-(acetamidomethyl)-3- phenylacrylate (99% ee in one case). In most cases, the new ligands were more active and more stereoselective than their structurally related monodentate phosphites (which are devoid of functional groups that are capable of hydrogen-bonding interactions). Control experiments and kinetic studies were carried out that allowed us to demonstrate that hydrogen-bonding interactions involving the diamide group of the PhthalaPhos ligands strongly contribute to their outstanding catalytic properties. Computational studies carried out on a rhodium precatalyst and on a conceivable intermediate in the hydrogenation catalytic cycle shed some light on the role played by hydrogen bonding, which is likely to act in a substrate-orientation effect. \ua9 2012 Wiley-VCH Verlag GmbH&Co. KGaA, Weinheim

Solid-phase synthesis of peptides containing reverse-turn mimetic bicyclic lactams

The solid-phase synthesis and characterization of a series of peptides (4-15) containing reverse-turn mimetic bicyclic lactams is reported. The bicyclic lactams (1a, 1b) possess high structural similarity to the two central residues of a Pturn. Amino acid conjugates of these bicyclic lactams were synthesized on solid supports following a g-fluorenylmethoxycarbonyl (FMOC) protection strategy on WangMerrifield resin. Coupling between amino acids was accomplished by means of diisopropylcarbodiimide (DIC)/ hydroxyazabenzotriazole (HOAt). Coupling between amino acids and the mimics was performed with the potent Carpino's reagent O-(7-azabenzotriazol-1-yl)-N,N,N',N'-tetramelhyluronium hexafluorophosphate (HATU). The final compounds were cleaved from the resin and obtained as N-acetylated methyl esters or benzyl amides

Synthesis and Conformational Studies of Peptidomimetics Containing a New Bifunctional Diketopiperazine Scaffold Acting as a \u3b2-Hairpin Inducer

A practical synthesis of a new bifunctional diketopiperazine (DKP) scaffold 1, formally derived from the cyclization of L-aspartic acid and (S)-2,3-diaminopropionic acid, is reported. DKP-1 bears a carboxylic acid and an amino functionalities in a cis relationship, which have been used to grow peptide sequences. Tetra-, penta-, and hexapeptidomimetic sequences were prepared by solution-phase peptide synthesis (Boc strategy). Conformational analysis of these derivatives was carried out by a combination of 1H NMR spectroscopy, IR spectroscopy, CD spectroscopy, and computer modeling, and reveals the formation of beta-hairpin mimics involving 10-membered and 18-membered H-bonded rings and a reverse turn of the growing peptide chain