GC–MS with photoionization of cold molecules in supersonic molecular beams—Approaching the softest ionization method

A new type of photoionization ion source was developed for the ionization of cold molecules in supersonic molecular beams (named Cold PI). The system was based on a GC–MS with supersonic molecular beams and its fly‐through EI of cold molecules ion source (Cold EI) plus quadrupole mass analyzer. A continuously operated deuterium VUV photoionization lamp was added and placed above and between the supersonic nozzle and skimmer whereas the Cold EI ion source served only as a portion of the ion transfer ion optics. The supersonic nozzle and skimmer were voltage biased and the VUV light crossed the supersonic expansion about 10 mm from the nozzle. We obtained over three orders of magnitude enhancement in the relative abundance of the molecular ion of squalane in Cold PI versus in photoionization of this compound as a thermal compound. Accordingly, we also proved that standard photoionization is not as soft ionization method as previously perceived for large compounds. We found that Cold PI is as soft as and possibly softer than field ionization; thus, it could be the softest known ionization method. The ionization yield was about 200–300 times weaker than with Cold EI yet our limit of detection was about 200 femtogram in SIM mode for cholesterol and pyrene which is reasonable. Practically, all hydrocarbons gave only molecular ions with rather uniform response whereas alcohols gave some molecular ions plus major fragment ions particularly with a loss of water (similarly to field ionization). We tested Cold PI in the GC–MS analysis of diesel fuels and analyzed the time averaged data for group type information. We also found that we can analyze the diesel fuels by fast under 20‐s flow injection analysis in which the generated averaged mass spectrum of molecular ions only could serve for the characterization of fuels.     

Gas phase photoisomerization of urocanic acid – a theoretical study

The photochemistry of urocanic acid is investigated theoretically by means of time-dependent density functional theory. The topology of the potential energy surface along the isomerization pathway and close to the Franck–Condon region is investigated and consequences for the photoisomerization reactions are outlined. A recently published supersonic jet spectroscopy study is reinterpreted in the light of these and earlier theoretical results to give a clear picture of the gas phase photochemistry. It is found that the photochemistry of the two isomers is fundamentally different, in contrast to the situation in solution.     

Carbohydrates Exhibit a Distinct Preferential Solvation Pattern in Binary Aqueous Solvent Mixtures

Molecular dynamics simulations reveal the entire solvation shell around a model disaccharide dissolved in the binary 1:3 molar mixture of dimethyl sulfoxide and water becomes distinctly structured (see drawing). Such preferential solvation is due to the large number of hydroxyl groups and the rich network of hydrogen bonds of a disaccharide formed with the solvent.     

Hydrogen abstraction from deoxyribose by a neighbouring uracil-5-yl radical

Hydrogen abstraction from the C1' and C2' positions of deoxyadenosine by a neighbouring uracil-5-yl radical in the 5'-AU*-3' DNA sequence is explored using DFT. This hydrogen abstraction is the first step in a sequence leading to single or double strand break in DNA. The uracil-5-yl radical can be the result of photolysis or low-energy electron (LEE) attachment. If the radical is produced by photolysis the neighbouring adenine will become a cation radical and if it is produced by LEE the adenine will remain neutral. The hydrogen abstraction reactions for both cases were investigated. It is concluded that it is possible for the uracil-5-yl to abstract hydrogen from C1' and C2'. When adenine is neutral there is a preference for the C1' site and when the adenine is a radical cation the C2' site is the preferred. If adenine is positively charged, the rate-limiting step when abstracting hydrogen from C1' is the formation of an intermediate crosslink between uracil and adenine. This crosslink might be avoided in dsDNA, making C1' the preferred site for abstraction.     

Photoswitching electron transport properties of an azobenzene containing thiol-SAM

The influence of conformational and electrical properties of azobenzene molecules on the electron transfer barrier properties of their SAMs was studied by SECM and ellipsometry.     

Relativistically parameterized extended Hückel calculations. VII. Nuclear spin—spin coupling tensors and densities of states for cluster models of CdTe,HgTe and PbTe

Relativistically parameterized extended Hückel (REX) calculations of spin—spin coupling tensors ¹J(MTe) for cluster models of CdTe, HgTe and PbTe are reported. The relativistic equivalent of Ramsey's theory is used. Electronic densities of states are obtained as a by-product. Two assignments of the experimental NMR data are shown to be possible. The REX results support the original Nolle one for PbTe and the alternative ones for CdTe and HgTe. The increase of ΔJ is attributed to relativistic effects, arising from the M(ns)—Te(5p$\text{1}\text{2}$) AO combination, recently discussed by Pyykkö and Wiesenfeld. A principle is proposed that both K and ΔK are positive for systems with dominant bonding-to-antibonding excitations, a case likely for half-filled valence shells. Nearly empty or nearly full valence shells with dominant bonding-to-bonding or antibonding-to-antibonding excitations should lead to negative K and ΔK. Double-zeta REX radial parameters are reported for ZnHg and OPo.     

Ion permeability of SAMs on nanoparticle surfaces

This article reports on the ion permeability of self-assembled monolayers (SAMs) formed on the surface of charged alkanethiol-protected gold nanoparticles, so-called monolayer-protected clusters (MPCs). The capacitance and thus the charging energy required to add/remove an electron from the metal core are extremely sensitive to ions entering the monolayer, and the extent of ion penetration can be tuned by the charge and size of the ions and the permittivity of the solvent. Experimentally, this effect is comparable to ion association with conventional redox molecules, indicating that MPCs despite their large size and the fundamentally differing nature of the electron transfer process can be treated analogously to redox molecules.