Infrared Spectroscopy of Jet-cooled "grandPAHs" in the 3-100 <i>μ</i>m Region

Although large polycyclic aromatic hydrocarbons (PAHs) are likely to be responsible for IR emission of gaseous and dusty regions, their neutral experimental high-resolution gas-phase IR spectra - needed to construct accurate astronomical models - have so far remained out of reach because of their nonvolatility. Applying laser desorption to overcome this problem, we report here the first IR spectra of the jet-cooled large PAHs coronene (C24H12), peropyrene (C26H14), ovalene (C32H14), and hexa(peri)benzocoronene (C42H18) in the 3-100 μm region. Apart from providing experimental spectra that can be compared directly to astronomical data, such IR spectra are crucial for assessing the accuracy of theoretically predicted spectra used to interpret interstellar IR emission. Here we use the experimental spectra to evaluate the performance of conventional calculations using the harmonic approximation, as well as calculations with an anharmonic (GVPT2) treatment. The harmonic prediction agrees well with the experiment between 100 and 1000 cm-1 (100 and 10 μm) but shows significant shortcomings in the combination band (1600-2000 cm-1, 6.25-5 μm) and CH-stretch (2950-3150 cm-1, 3.4-3.17 μm) regions. Especially the CH-stretch region is known to be dominated by the effects of anharmonicity, and we find that large PAHs are no exception. However, for the CH out-of-plane region (667-1000 cm-1, 15-10 μm) the anharmonic treatment that significantly improves the predicted spectra for small PAHs leads to large and unrealistic frequency shifts, and intensity changes for large PAHs, thereby rendering the default results unreliable. A detailed analysis of the results of the anharmonic treatment suggests a possible route for improvement, although the underlying cause for the large deviations remains a challenge for theory

Femtosecond coincidence imaging of multichannel multiphoton dynamics

The technique of femtosecond time-resolved photoelectron-photoion coincidence imaging was applied to unravel dissociative ionization processes in a polyatomic molecule. The femtosecond coincidence imaging of C

Competing C-4 and C-5-Acyl Stabilization of Uronic Acid Glycosyl Cations.

Uronic acids are carbohydrates carrying a terminal carboxylic acid and have a unique reactivity in stereoselective glycosylation reactions. Herein, the competing intramolecular stabilization of uronic acid cations by the C-5 carboxylic acid or the C-4 acetyl group was studied with infrared ion spectroscopy (IRIS). IRIS reveals that a mixture of bridged ions is formed, in which the mixture is driven towards the C-1,C-5 dioxolanium ion when the C-5,C-2-relationship is cis, and towards the formation of the C-1,C-4 dioxepanium ion when this relation is trans. Isomer-population analysis and interconversion barrier computations show that the two bridged structures are not in dynamic equilibrium and that their ratio parallels the density functional theory computed stability of the structures. These studies reveal how the intrinsic interplay of the different functional groups influences the formation of the different regioisomeric products.Bio-organic Synthesi

Characterization of glycosyl dioxolenium ions and their role in glycosylation reactions

Controlling the chemical glycosylation reaction remains the major challenge in the synthesis of oligosaccharides. Though 1,2-trans glycosidic linkages can be installed using neighboring group participation, the construction of 1,2-cis linkages is difficult and has no general solution. Long-range participation (LRP) by distal acyl groups may steer the stereoselectivity, but contradictory results have been reported on the role and strength of this stereoelectronic effect. It has been exceedingly difficult to study the bridging dioxolenium ion intermediates because of their high reactivity and fleeting nature. Here we report an integrated approach, using infrared ion spectroscopy, DFT computations, and a systematic series of glycosylation reactions to probe these ions in detail. Our study reveals how distal acyl groups can play a decisive role in shaping the stereochemical outcome of a glycosylation reaction, and opens new avenues to exploit these species in the assembly of oligosaccharides and glycoconjugates to fuel biological research.Bio-organic Synthesi

IR Spectroscopic Techniques to Study Isolated Biomolecules

The combination of mass spectrometry, infrared action spectroscopy and quantum-chemical calculations provides a variety of approaches to the study of the structure of biologically relevant molecules in vacuo. This chapter reviews some of the experimental methods that are currently in use, which can roughly be divided into two main categories: (1) low-temperature neutral molecules in a molecular beam environment, which can be investigated in a conformationally selective manner by the application of double-resonance laser spectroscopy and (2) ionized species which can conveniently be manipulated and selected by mass spectrometric methods and which can be investigated spectroscopically by wavelength-dependent photo-dissociation. Both approaches rely on the application of infrared tunable laser spectroscopy and the laser sources most commonly used in current studies are briefly reviewed in Sect. 3. Along with quantum-chemical calculations, reviewed in Chapter 3 of this book (Gaigeot and Spezia, Top Curr Chem doi:10.1007/128_2014_620), the experimental IR spectra reveal a wealth of information on the structural properties of the biological species