Harrald V. Linnert

Benzyne-related mechanisms in the gas phase ion/molecule reactions of haloarenes

Abstract The low pressure ion/molecule reactions of NH−2, OHt- and MeO− with bromobenzene have been studied by Fourier transform ion cyclotron resonance to elucidate gas phase benzyne mechanisms. For OHt- and MeO−, the main reaction consists of benzyne elimination initiated by abstraction of an ortho proton and the subsequent formation of a solvated halide ion. Experiments with monodeuterated bromobenzenes suggest that reactions with OHt- are the result of long-lived complexes with extensive scrambling of hydrogen and deuterium. For NH−2, formation of all the isomeric bromophenide ions occurs without hydrogen-deuterium scrambling, revealing weak building in the collision complexes. The o-bromophenide ions are shown to react with other substrates by bromide transfer rather than by proton transfer, and evidence is presented that the meta and para isomers undergo isomerization to the ortho upon reaction with substrates of similar acidities. The ΔH0acid of bromobenzene is estimated to be 384.4 kcal mol− from bracketing experiments. An extension of these reactions to 1- and 2-bromonaphthalene provides an estimate for the heats of formation of 1,2-dehydronaphthalene (122 kcal mol−) and the 2,3-dehydronaphthalene (126 kcal mol−).

Gas-phase nucleophilic reactions in SO2F2: experiment and theory

Abstract The gas-phase ion-molecule reactions of simple anions (HO − , CH 3 O − , NH 2 − ) with SO 2 F 2 proceed with rate constants close to the collision limit. The energy surface for the OH − /SO 2 F 2 reaction has been characterized by ab initio calculations using basis functions adapted for a pseudopotential and corrected for anionic systems by the generator coordinate method (GCM) at the QCISD(T)/(ECP/TZV/GCM) level. The calculations indicate that reaction occurs by initial addition of the anion to SO 2 F 2 to form a hypervalent sulfur species. The high efficiency of the reaction is associated with a low energy barrier separating the initial adduct from the product side ion-neutral complex.

The gas-phase C6H4X- ion (X- = Br-, I-) : a halide-benzyne complex

The gas-phase C6H4Br– and C6H4I– ions react with more acidic substrates than either bromo- or iodo-benzene by halide transfer rather than by proton abstraction, a behaviour consistent with an ion structure corresponding to a halide–benzyne complex.

Configuration and stability of naturally occurring all-cis-tetrahydrofuran lignans from Piper solmsianum

The study of variability of the tetrahydrofuran lignan (−)-grandisin in leaves of Piper solmsianum (Piperaceae) revealed two unknown compounds, that were isolated and determined to be the all-cis tetrahydrofuran lignans 1a [rel-(7R,8S,7′S,8′R)-3,4,5,3′,4′,5′-hexamethoxy-7,7′-epoxylignan] and 1b [rel-(7R,8S,7′S,8R′)-3′,4′-methylenedioxy-3,4,5,5′-tetramethoxy-7,7′-epoxylignan]. Their structures were determined by spectroscopic analysis including 1D and 2D-NMR while their configurations were determined by ECD associated to the density functional theory (DFT) at the B3LYP/6-31G(d,p) level. The hydrogen bonds between methoxy groups in trimethoxyphenyl rings stabilizes the all-trans tetrahydrofuran lignan grandisin by 6.5 kcal mol−1 as compared to the corresponding all-cis isomer of grandisin. The occurrence of all-cis tetrahydrofuran lignans as natural products is a very rare event.

Dependência das vias de dissociação unimolecular com a energia interna da molécula de éter dimetílico

Internal energy dependence of the competitive unimolecular dissociation channels of dimethyl ether were studied with the statistical RRKM formalism. The C-O and C-H fission reactions and the 1,2-H and 1,3-H shifts, and 1,1-H2 and 1,3-H2 molecular eliminations are discussed as a function of energy dependence of ka(E*), the microcanonical rate constant for production of transition states. C-O fission is the dominant process while reaction channels involving C-H fission, 1,1-H2 and 1,3-H2 elimination and production of MeOH should be competitive at energies around 400 kJ mol-1. The less favorable process is the channel of CH4 formation.

Usando um website para explicar a espectrometria de ressonância ciclotrônica de íons por transformada de Fourier

This article shows the usefulness of a website to explain the concepts, operational events, vacuum system, applications and an experimental sequence of the Fourier Transform Ion Ciclotron Resonance Mass Spectrometry technique (http://143.107.46.113/icr/icrj.html).