Binding energies of protonated betaine complexes: a probe of zwitterion structure in the gas phase

The dissociation kinetics of proton-bound dimers of betaine with molecules of comparable gas-phase basicity were investigated using blackbody infrared radiative dissociation (BIRD). Threshold dissociation energies were obtained from these data using master equation modeling. For bases that have comparable or higher gas-phase basicity, the binding energy of the protonated base.betaine complex is ~1.4 eV. For molecules that are ~2 kcal/mol or more less basic, the dissociation energy of the complexes is ~1.2 eV. The higher binding energy of the former is attributed to an ion-zwitterion structure which has a much larger ion-dipole interaction. The lower binding energy for molecules that are ~2 kcal/mol or more less basic indicates that an ion-molecule structure is more favored. Semiempirical calculations at both the AM1 and PM3 levels indicate the most stable ion-molecule structure is one in which the base interacts with the charged quaternary ammonium end of betaine. These results indicate that the measurement of binding energies of neutral molecules to biological ions could provide a useful probe for the presence of zwitterions and salt bridges in the gas phase. From the BIRD data, the gas-phase basicity of betaine obtained from the kinetic method is found to be 239.2 +/- 1.0 kcal/mol. This value is in excellent agreement with the value of 239.3 kcal/mol (298 K) from ab initio calculations at the MP2/6-31+g** level. The measured value is slightly higher than those reported previously. This difference is attributed to entropy effects. The lower ion internal energy and longer time frame of BIRD experiments should provide values closer to those at standard temperature.     

Coordination Chemistry of Acrylamide 4. Crystal Structures and IR Spectroscopic Properties of Acrylamide Complexes with CoII,NiII, and ZnII nitrates

Acrylamide complexes of metal nitrates: [M(O‐OC(NH₂)CHCH₂)_n(H₂O)_m][NO₃]₂ (M = Co( 1 ), Ni( 2 ) (n = 6 and m = 0) and Zn( 3 ) (n = 4 and m = 2)) have been determined by using single crystal X‐ray diffraction analysis. All complexes crystallize in the triclinic space group . The structures of 1 and 2 represent octahedral species [M(AAm)₆]²⁺ (AAm = O‐OC(NH₂)CHCH₂ and M = Co or Ni) and uncoordinated nitrate ions. The structure of 3 involves the octahedral cation [Zn(AAm)₄(H₂O)₂]²⁺ in which the Zn²⁺ environment includes oxygen atoms of four acrylamide and two water molecules that are stabilized using ionic nitrate ions. The observations of the solid‐state IR spectroscopic vibrational frequencies of these acrylamide complexes are in agreement with the crystal structures.     

Binding Energies of Proton-Bound Dimers of Imidazole and n-Acetylalanine Methyl Ester Obtained by Blackbody Infrared Radiative Dissociation

The dissociation kinetics of protonated n-acetyl-L-alanine methyl ester dimer (AcAlaME(d)), imidazole dimer, and their cross dimer were measured using blackbody infrared radiative dissociation (BIRD). Master equation modeling of these data was used to extract threshold dissociation energies (E(o)) for the dimers. Values of 1.18 +/- 0.06, 1.11 +/- 0.04, and 1.12 +/- 0.08 eV were obtained for AcAlaME(d), imidazole dimer, and the cross dimer, respectively. Assuming that the reverse activation barrier for dissociation of the ion-molecule complex is negligible, the value of E(o) can be compared to the dissociation enthalpy (DeltaH(d) degrees ) from HPMS data. The E(o) values obtained for the imidazole dimer and the cross dimer are in agreement with HPMS values; the value for AcAlaME(d) is somewhat lower. Radiative rate constants used in the master equation modeling were determined using transition dipole moments calculated at the semiempirical (AM1) level for all dimers and compared to ab initio (RHF/3-21G*) calculations where possible. To reproduce the experimentally measured dissociation rates using master equation modeling, it was necessary to multiply semiempirical transition dipole moments by a factor between 2 and 3. Values for transition dipole moments from the ab initio calculations could be used for two of the dimers but appear to be too low for AcAlaME(d). These results demonstrate that BIRD, in combination with master equation modeling, can be used to determine threshold dissociation energies for intermediate size ions that are in neither the truncated Boltzmann nor the rapid energy exchange limit.     

Kristallstrukturen von Säurehydraten und Oxoniumsalzen. XX. Die Oxoniumtetrafluoroborate H3OBF4, [H5O2]BF4 und [H(CH3OH)2]BF4

Crystal Structures of Acid Hydrates and Oxonium Salts. XX. Oxonium Tetrafluoroborates H₃OBF₄, [H₅O₂]BF₄, and [H(CH₃OH)₂]BF₄ The crystal structures of three oxonium tetrafluoroborates were determined. H₃OBF₄, oxonium tetrafluoroborate proper, is triclinic with space group P1 , Z = 2 and the unit cell dimensions a = 4.758, b = 6.047, c = 6.352 Å and α = 80.40, β = 79.48, γ = 88.25° at ?26°C. Cations H₃O⁺ and anions BF₄^? are linked by hydrogen bonds O? H…?F into ribbons of condensed rings. In [H₅O₂]BF₄ (diaquohydrogen tetrafluoroborate, monoclinic, P2₁/c, Z = 4, a = 6.584, b = 9.725, c = 7.084 Å, β = 95.15° at ?100°C) the hydrogen bond in the cation H₅O₂⁺ is 2.412 Å short, asymmetric and approximately centered and the linking of cations and anions three-dimensional. In [H(CH₃OH)₂]BF₄ (Bis(methanol)hydrogen tetrafluoroborate, monoclinic, P2₁/c, Z = 4, a = 5.197, b = 14.458, c = 9.318 Å, β = 94.61° at ?50°C) the cation [H(CH₃OH)₂]⁺ is characterized for the first time in a crystal structure with an again very short (2.394 Å), asymmetric and effectively centered hydrogen bond. By further hydrogen bonds cations and anions form only dimers of the formula unit of centrosymmetric cyclic structure.     

Three-component three-bond forming cascade via palladium photoredox catalysis

A highly modular radical cascade strategy based upon radical cyclisation/allylic substitution sequence between alkyl/aryl bromides, 1,3-dienes and nucleophiles ranging from sulfinates to amines, phenols and 1,3-dicarbonyls is described (>80 examples). Palladium phosphine complexes – which merge properties of photo- and cross coupling-catalysts – allow to forge three bonds with complete 1,4-selectivity and stereocontrol, delivering highly value added carbocyclic and heterocyclic motifs that can feature – inter alia – vicinal quaternary centers, free protic groups, gem-difluoro motifs and strained rings. Furthermore, a flow chemistry approach was for the first time applied in palladium–photocatalysed endeavors involving radicals.

Highly modular three-bond three-component cascade featuring palladium as dual photoredox/cross coupling catalyst.     

Manganese(I)-Catalyzed Regioselective C−H Allenylation: Direct Access to 2-Allenylindoles

A Mn^I-catalyzed regioselective C−H allenylation is reported that allows a broad range of 2-allenylindoles to be synthesized regioselectively on a gram scale under simple conditions. Notably, a highly efficient chirality transfer was observed (up to 93 % ee) in this transformation. This procedure was further found to allow, for the first time, the direct preparation of ketones by Mn^I-catalyzed C−H activation. Mechanistic investigations revealed that the precoordination of the oxygen atom to the manganese center as well as the congested tertiary carbon atom in the propargylic carbonates play a crucial role.     

Assemblies from metallic and semiconducting nanocrystals

Optical properties of nanomaterials such as semiconductor and metal quantum dots are important for sensors and photovoltaic applications. We report on optical, microscopic, and AFM investigations on bulk and single nanoobjects such as metal and semiconducting nanoparticles. Firstly, of special interest is the investigation of Ag metal nanoaggregates formed in zeolites. Here, the defined structure of the zeolite serves both as size directing and a stabilizing agent. The size selected Ag aggregates fluoresce in the zeolite cages even after storage under ambient conditions for almost one year. In addition, single Ag particles escape the cages and can be investigated by fluorescence microscopy also with respect to sensor applications. Secondly, with respect to photovoltaic applications, energy transfer among organic dye molecules and semiconductor quantum dots is of great importance. We report on the extension of the optical absorption of ZnSe quantum dots into the UV regime and investigate excitation energy transfer within self-assembled nanoaggregates of surface functionalized QDs and fluorescent styrylpyridine dyes.     

A new profile likelihood confidence interval for the mean probability of detection in collaborative studies of binary test methods

A confidence interval for the probability of detection across laboratories (LPOD) for qualitative methods, described in the AOAC INTERNATIONAL guidelines for validation of microbiological methods for food and environmental surfaces, is considered. It is demonstrated that under certain conditions, the observed confidence of this confidence interval can be rather low, so that statistical minimum requirements are not fulfilled. A new profile likelihood confidence interval based on a latent random laboratory effect approach is proposed. Observed confidence levels for this confidence interval demonstrate its applicability already for a small number of laboratories.