Selenoamides modulate dipole-dipole interactions in hydrogen bonded supramolecular polymers of 1,3,5-substituted benzenes

We report the synthesis and self-assembly behavior of a chiral C3-symmetrical benzene-tricarboselenoamide. The introduction of the selenoamide moiety enhances the dipolar character of the supramolecular interaction and confers a remarkable thermal stability to the supramolecular polymers obtained

Observations of tetrel bonding between sp3-carbon and THF

We report the direct observation of tetrel bonding interactions between $sp^{3}$-carbons of the supramolecular synthon 3,3-dimethyl-tetracyanocyclopropane (1) and tetrahydrofuran in the gas and crystalline phase. The intermolecular contact is established via σ-holes and is driven mainly by electrostatic forces. The complex manifests distinct binding geometries when captured in the crystalline phase and in the gas phase. We elucidate these binding trends using complementary gas phase quantum chemical calculations and find a total binding energy of −11.2 kcal mol$^{−1}$ for the adduct. Our observations pave the way for novel strategies to engineer $sp^{3}$-C centred non-covalent bonding schemes for supramolecular chemistry

Photodissociation of aliphatic PAH derivatives under relevant astrophysical conditions

International audienceThe interaction of polycyclic aromatic hydrocarbons (PAHs) with vacuum ultraviolet (VUV) photons triggers the emission of the well-known aromatic infrared bands (AIBs) but other mechanisms such as fragmentation can be involved in this interaction. Fragmentation leads to selection effects that favor specific sizes and structures. We investigate the impact of aliphatic bonds on the VUV photo-stability of PAH cations under conditions applicable for photodissociation regions (PDRs). Cations of pyrene (C16H10) and coronene (C24H12) derivatives containing aliphatic bonds (methyl or ethyl sidegroups, superhydrogenation) are submitted to VUV (10.5 eV) photons over long timescales (~1000 s) in the cryogenic PIRENEA setup. The fragmentation cascades are analyzed with a simple kinetics model; fragmentation pathways, rates and branching ratios are derived. Aliphatic PAH derivatives are found to have a higher fragmentation rate and carbon loss compared to regular PAHs. The fragmentation of PAHs with alkylated sidegroups forms species with peripheral pentagonal cycles, which can be more stable than the bare PAH cations. This stability is quantified and the most stable species, for which there is an effective competition of the fragmentation with isomerization and radiative cooling are identified. This work supports a scenario in which the evaporation of nanograins with a mixed aliphatic and aromatic composition followed by VUV photoprocessing results in both the production of the carriers of the 3.4um AIB by methyl sidegroups and in an abundant source of small hydrocarbons at the border of PDRs. An additional side effect is the efficient formation of stable PAHs that contain some peripheral pentagonal rings. Our experiments also support the role of isomerization processes in PAH photofragmentation, including the H-migration process, which could lead to an additional contribution to the 3.4um AIB

Selenium in Proteins: Conformational Changes Induced by Se Substitution on Methionine, as Studied in Isolated Model Peptides by Optical Spectroscopy and Quantum Chemistry

International audienceThe side-chain of methionine residues is long enough to establish NH⋯S H-bonds with neighboring carbonyl groups of the backbone, giving rise to so-called intra-residue 6δ and inter-residue 7δ H-bonds. The aim of the present article is to document how the substitution of sulfur with a selenium atom affects the H-bonding of the Met system. This was investigated both experimentally and theoretically by conformation-resolved optical spectroscopy, following an isolated molecule approach. The present work emphasizes the similarities of the Met and Sem residues in terms of conformational structures, energetics, NH⋯Se/S H-bond strength and NH stretch spectral shifts, but also reveals subtle behavior differences between them. It provides evidence for the sensitivity of the H-bonding network with the folding type of the Sem/Met side-chains, where a simple flip of the terminal part of the side-chain can induce an extra 50 cm−1 spectral shift of the NH stretch engaged in a 7δ NH⋯S/Se bond

Impact of aliphatic bonds on the stability of VUV photo-processed PAHs under relevant astrophysical conditions

International audienceStudying the interaction of polycyclic aromatic hydrocarbons (PAHs) with VUV light is crucial for the understanding of the physical and chemical evolution of photodissociation regions (PDRs). In particular, this interaction induces the well-known aromatic infrared bands (AIB) emission [1] but other mechanisms such as fragmentation [2] can be involved, leading to selection effects favoring specific sizes and structures. In this context, despite their lower stability, PAHs containing aliphatic C-H bonds are considered as good candidates for the 3.4 μm AIB [3,4]. We have recently combined a 10.5 eV photon source with our dedicated cryogenic ion trap PIRENEA [5] in order to investigate the impact of aliphatic bonds on the VUV photo-stability of PAH cations with conditions applicable for PDRs [6]. The coronene cation (C24H12+) and its alkylated derivatives (with methyl or ethyl sidegroups) were VUV photo-processed over long timescales (~1000 s). Their fragmentation cascades were analyzed with a simple kinetics model which enabled to derive fragmentation pathways, rates and branching ratios.Alkylated coronene derivatives are found to have a higher fragmentation rate and carbon loss compared to the bare coronene. However, their VUV fragmentation cascade also leads to the formation of species carrying a peripheral pentagonal cycle, which is as stable as coronene. The stability of each detected species is quantified and the most stable ones, for which there is an effective competition of fragmentation with isomerization and radiative cooling, are identified. This work supports a scenario in which the evaporation of nanograins with mixed aliphatic and aromatic composition followed by VUV photo-processing results in both the production of the carriers of the 3.4µm AIB by methyl sidegroups and in an abundant source of smaller hydrocarbons at the border of PDRs. This process also results in the formation of PAHs containing pentagonal ring. Finally, our study supports the role of isomerization processes in PAH photofragmentation, including the H-migration process [7], which could lead to an additional contribution to the 3.4 µm AIB

Interaction of nanosecond laser pulse with tetramethyl silane (Si(CH3)4) clusters: Generation of multiply charged silicon and carbon ions

Present work reports significantly high levels of ionization, eventually leading to Coulomb explosion of Tetramethyl silane (TMS) clusters, on interaction with laser pulses of intensity ∼109 W/cm2. Tetramethyl silane clusters, prepared by supersonic expansion were photoionized at 266, 355 or 532 nm and the resultant ions were detected using time-of-flight mass spectrometer. It is observed that wavelength of irradiation and the size of the cluster are crucial parameters which drastically affect the nature of charge species generated upon photoionization of cluster. The results show that clusters absorb significantly higher energy from the laser field at longer wavelengths (532 nm) and generate multiply charged silicon and carbon ions which have large kinetic energies. Further, laser-cluster interaction at different wavelengths has been quantified and charge densities at 266, 355 and 532 nm are found to be 4x 1010, 5x 1010 and 5x 1011 charges/cm3 respectively. These unusual results have been rationalized based on dominance of secondary ionization processes at 532 nm ultimately leading to Coulomb explosion of clusters. In another set of experiments, multiply charged ions of Ar (up to +5 state) and Kr (up to +6 state) were observed when TMS doped inert gas clusters were photoionized at 532 and 355 nm. The extent of energy absorption at these two wavelengths is clearly manifested from the charge state of the atomic ions generated upon Coulomb disintegration of the doped cluster. These experiments thus demonstrate a novel method for generation of multiply charged atomic ions of inert gases at laser intensity of ∼ 109 W/cm2. The average size of the cluster exhibiting Coulomb explosion phenomena under giga watt intensity conditions has been estimated to be ∼ 6 nm. Experimental results obtained in the present work agree qualitatively with the model proposed earlier [D. Niu, H. Li, F. Liang, L. Wen, X. Luo, B. Wang, and H. Qu, J. Chem. Phys. 122, 151103(2005)] and point towards interaction of quasi-free electrons, generated during primary multiphoton ionization step, with a given wavelength in the presence of Coulombic field