VIZSLA-Versatile Ice Zigzag Sublimation Setup for Laboratory Astrochemistry.

In this article, a new multi-functional high-vacuum astrophysical ice setup, VIZSLA (Versatile Ice Zigzag Sublimation Setup for Laboratory Astrochemistry), is introduced. The instrument allows for the investigation of astrophysical processes both in a low-temperature para-H2 matrix and in astrophysical analog ices. In the para-H2 matrix, the reaction of astrochemical molecules with H atoms and H+ ions can be studied effectively. For the investigation of astrophysical analog ices, the setup is equipped with various irradiation and particle sources: an electron gun for modeling cosmic rays, an H atom beam source, a microwave H atom lamp for generating H Lyman-α radiation, and a tunable (213-2800 nm) laser source. For analysis, an FT-IR (and a UV-visible) spectrometer and a quadrupole mass analyzer are available. The setup has two cryostats, offering novel features for analysis. Upon the so-called temperature-programmed desorption (TPD), the molecules, desorbing from the substrate of the first cryogenic head, can be mixed with Ar and can be deposited onto the substrate of the other cryogenic head. The efficiency of the redeposition was measured to be between 8% and 20% depending on the sample and the redeposition conditions. The well-resolved spectrum of the molecules isolated in an Ar matrix serves a unique opportunity to identify the desorbing products of a processed ice. Some examples are provided to show how the para-H2 matrix experiments and the TPD-matrix-isolation recondensation experiments can help understand astrophysically important chemical processes at low temperatures. It is also discussed how these experiments can complement the studies carried out by using similar astrophysical ice setups.

Selective conformational control by excitation of NH imino vibrational antennas.

An imino group was used for the first time as a vibrational antenna to manipulate molecular conformations. Imino-thiol isomers of thioacetamide were generated upon UV-irradiation of its amino-thione tautomer isolated in argon matrices at 11 K. Selective and reversible conformational isomerizations were induced by narrowband near-IR irradiation tuned at the frequencies of the 2ν(NH) first stretching overtone of each imino-thiol isomer. The conformational isomerization concerns the change in the orientation of a remote -SH group, while the orientation of the imino (C[double bond, length as m-dash]NH) group remains the same. Supported by quantum chemical anharmonic computations, this allowed for a reliable, isomer-selective vibrational assignment of the four imino-thiol isomers extending now over the full mid-IR and near-IR ranges. It was found that the experimental IR intensities of the 2ν(NH) first stretching overtones (computed 4-5 km mol-1) of the imino-thiol forms are comparable to those of the ν(NH) stretching fundamentals (computed 2-4 km mol-1). This is the first time such a phenomenon is reported for an imine molecule. The kinetics of conformational isomerization was monitored in situ, indicating that the irradiation-induced processes are significantly faster than the tunneling-driven spontaneous cis-trans rotamerization of the -SH group. Quantum yields for the rotamerizations of the -SH group resulting from the vibrational excitation of a remote -NH group were estimated and found to be comparable to those observed for matrix-isolated carboxylic acids and amino acids, where conformational changes of the -OH group were induced by the direct vibrational excitation of 2ν(OH) first stretching overtones.

Donor-acceptor properties of isonitriles studied by photoelectron spectroscopy and electron transmission spectroscopy.

Some alkyl and aryl isonitriles, considered as CO analogue sigma-donor and pi-acceptor ligands in transition metal chemistry, were studied by means of HeI photoelectron spectroscopy and electron transmission spectroscopy, in order to evaluate their donor-acceptor properties from the measured ionization energies (IE) and vertical electron attachment energies (VAE). The investigated molecules were 2-propyl, 1-butyl, tert-butyl, 1-pentyl, cyclohexyl, 2,6-dimethylphenyl, 4-methoxyphenyl and 4-chorophenyl isonitrile. By interpreting the spectra on the basis of literature data and quantum chemical calculations, the spectral features associated with the molecular orbitals mainly involved in coordination and back-donation were identified. The results show that the IE (10.62-10.95 eV) of the sigma electron pair (n(c)) responsible for the sigma-donor capability is substantially lower than that of CO. The VAEs of the empty pi* orbitals involved in the d/pi* back-donation indicate that aryl isonitriles are better acceptors (VAE <0.3 eV) than their aliphatic counterparts (VAE >2.7 eV). In the case of aryl derivatives, the pi-donor ability could also play some role in metal-ligand bonding (IE 8.74-9.34 eV). Isonitrile coordination characteristics are also compared with those of CO, N(2) and CH(3)CN.

Modeling of the gas-phase ion chemistry of protonated arginine.

Arginine is often involved at the C-terminus of peptides obtained from tryptic digests of proteins. The very basic guanidine group of the side-chain of arginine has a large effect on the backbone fragmentation of protonated peptides. Furthermore, arginine exhibits specific fragmentation reactions involving its side-chain. Various tautomerization states, conformers and side-chain dissociation channels of protonated arginine were studied using theoretical methods. The guanidine loss of protonated arginine is proved to be an S(N)2 substitution on the delta-carbon of the side-chain, starting from species containing the N(epsilon)H-C(+)(N(eta)H(2))(N(eta')H(2)) or -N(epsilon) (+)H(2)-C(N(eta)H)(N(eta')H(2)) moieties and leads to formation to either protonated guanidine or protonated proline. In the corresponding transition structures the proline moiety is protonated. Under low-energy collision conditions the extra proton transfers to the guanidine moiety, leading to the formation of C(+)(NH(2))(3). On the other hand, the lifetime of the fragmenting species under high-energy collision conditions is shorter, resulting in enhanced formation of protonated proline and its dissociation products. The first step of ammonia loss is the leaving of a preformed NH(3) from tautomers containing the -N(epsilon)H-C(N(eta)H(3) (+))(N(eta')H) or -N(epsilon)-C(N(eta)H(3) (+))(N(eta')H(2)) moieties. The resulting protonated carbodiimide group can be stabilized by intramolecular nucleophilic attack, leading to ring formation. Overall, reactions involved in the ammonia loss from protonated arginine can be considered as an S(N)1 substitution on the central zeta-carbon of the guanidine group.