Unusual Rearrangements of Radical Cations: The Role of Vibronic Coupling

Radicalcations often undergo very unexpected rearrangements. Three examples of such rearrangements are given, and it is shown how vibronic coupling between the ground and low-lying excited states may cause certain bonds that are quite solid in the neutral molecules to become so weak that they break spontaneously, even though the bond order does not change (or changes very little) on ionization. In radical cations where spin and charge are delocalized over two similar halves, vibronic coupling can lead to localization of spin and charge, which may greatly affect the reactivity. Finally, it is shown how vibronic coupling can lead to avoidance of state crossings, a feature that appears very frequently in rearrangements of radical cations

Properties of Organic Radical Ions in Rigid Media

This report describes the activities in the title field as carried out at the Institute of Physical Chemistry of the University of Fribourg over the past dozen years

Radiolysis of [1.1.1] propellane and of aziridines: fate of their molecular ions

Bei der Ionization von [1.1.1]Propellan (PRP+.) in Tieftemperaturmatrizen entsteht eine photolabile Spezies, die eine breite, schwache Bande im NIR Bereich (λmax = 1440 nm) und ein Triplett von 5.4 G eines Quintetts von 14.8 G im ESR Spektrum aufweist. Nach Bestrahlung mit NIR Licht wird diese Spezies in das Radikalkation von Vinylidencyclopropan (VCP+.) umgewandelt, welches unabhängig durch Ionisation von VCP nachgewiesen wurde. Quantenchemische Rechnungen zeigen, dass die spektroskopischen Eigenschaften des photolabilen Zwischenproduktes vergleichbar sind mit jenen des Radikalkations von Dimethylenallen (DIA+.). DIA+. wird auch bei der Ionisation von Dimethylencyclopropan (DMC) gebildet was darauf hinweist, dass DMC+. bei der Bildung von DIA+. aus PRP+. ein Intermediat darstellt. Die Potentialfläche, welche DIA+. und PRP+. verbindet, wurde untersucht und auf dem CCSD(T)/cc-pVDZ// B3LYP/6-31G* Niveau vollständig charakterisiert. Dabei wurde festgestellt, dass vibronische Wechselwirkungen beim Mechanismus des spontanen Zerfalls von ionisiertem PRP nach DIA+. eine wichtige Rolle spielen. Optische Spektren weisen auf die Bildung zusätzlicher Produkte bei Ionisation von PRP, DMC oder VCP in Freon- and Argonmatrizen hin; insbesondere jene von Vinylallen (VIA+.) , das ebenfalls durch Ionisation von VIA hergestellt und charakterisiert werden konnte. Experimente in Argonmatrizen zeigten ein weiteres Zwischenprodukt A, welches photochemisch zum Radikalkation von Cyclopentadien (CP+.) umgelagert werden konnte. A konnte nicht identifiziert werden, aber verschiedene Spezies, welche für die beobachteten Spektren verantwortlich sein könnten, wurden vorgeschlagen. Weiterführende Arbeiten werden nötig sein, um die unbekannten Intermediate, welche bei den erwähnten Umlagerungen auftreten, zu identifizieren. Der zweite Teil dieser Arbeit beschreibt die Bildung von Azomethinylid, Y, und des entsprechenden Radikalkations. Die experimentelle Untersuchung der Kraftkonstanten der C-N Bindungen in Y und der Vergleich mit jenen des Allylradikals sind von theoretischem Interessen. Durch das Hinzuziehen von Y-d1 und Y-d4 sollte es durch Skalierung des Kraftfeldes der drei Isotopomere möglich sein, diese Kraftkonstanten zu ermitteln. Weiter wird gezeigt, dass Substituenten - im besonderen Phenylgruppen - beim Verhalten der Aziridine unter Bedingungen von photoinduziertem Elektronentransfer (PET) eine wichtige Rolle spielen. N-Phenylringe stabilisieren das Aziridin-Radikalkation während C-Phenylringe bevorzugt das ringgeöffnete Azomethinylid Kation stabilisieren. Das Mass dieser Stabilisierung kann ermittelt werden indem der Energiebedarf für die Rotation der Phenylgruppe in eine senkrechte Position berechnet wird, in der die Resonanz zwischen der Phenylgruppe und dem Aziridinring bzw. dem Azomethinylid unterdrückt wird. Experimente und Berechnungen zeigten, dass das N-Phenylaziridin Radikalkation, NPA+, derart stark stabilisiert wird, dass es weder durch Ionisation noch durch anschliessende elektronische Anregung des Radikalkations zur Ringöffnung gebracht werden kann. Methylgruppen an den C-Atomen von NPA bewirken ein gewisses Absenken des Übergangszustandes der Ringöffnung, aber der Effekt ist zu gering, um die Überwindung der thermischen Barriere zu ermöglichen. Im Gegensatz dazu bewirkt C-Phenylsubstitution eine ausgeprägte Stabilisierung des Übergangszustandes der Ringöffnung relativ zum Edukt (und dem Produkt). Damit erleiden N-H- und N-Alkyl-C-Phenylaziridine unmittelbar nach Ionisierung Ringöffnung. Die Frage, welcher der beiden Effekte im Falle des C,N-Diphenylaziridins "gewinnt", konnte nicht befriedigend beantwortet werden. Jedoch genügte im Falle von ionisiertem Triphenylaziridin, TPA, eine kurze Photolyse, um eine Ringöffnung zu induzieren. Damit ist der Gebrauch von Aziridinen, die durch Ionisierung geöffnet werden, um mit A=B Dipolarophilen [3+2]Cycloadditionen einzugehen, auf jene Derivate beschränkt, bei denen die ring-geschlossene Form nicht zu stark von den Substituenten stabilisiert wird.On ionization of [1.1.1]propellane (PRP+.) in cryogenic matrices a photolabile species with a broad, weak NIR band (λmax = 1440 nm) and an ESR spectrum consisting of a 14.8 G quintet of 5.4 G triplets is formed. On NIR irradiation this species is converted into the radical cation of vinylidenecyclopropane (VCP+.), which was generated independently by ionization of VCP. Quantum chemical calculations show that the spectroscopic features of the photolabile intermediate are compatible with its assignment to the radical cation of dimethylene allene (DIA+.). DIA+. is also formed on ionization of dimethylenecyclopropane (DMC) which suggests that DMC+. is an intermediate in the formation of DIA+. from PRP+. The potential energy surface connecting DIA+. and PRP+. is explored and fully characterized at the CCSD(T)/cc-pVDZ//B3LYP/6- 31G* level. Thereby it is found that vibronic interactions play an important role in determining the mechanism for the spontaneous decay of ionized PRP to DIA+. Optical spectra indicate the formation of additional products on ionization of PRP, DMC, or VCP in Freon and Argon matrices, notably the radical cation of vinylallene (VIA+.) , which was also generated independently by ionization of VIA. Argon matrix experiments reveal also the presence of an intermediate A+. which can be transformed photochemically into the radical cation of cyclopentadiene (CP+). Intermediate A could not be identified, but several species which could be responsible for the corresponding spectroscopic manifestations are proposed. Further work will also be needed to shed more light on some other species which were observed during the above transformations. The second part of this work describes the generation of azomethine ylid, Y, and its radical cation. It is of theoretical interest to investigate experimentally the force constants for the symmetric and asymmetric stretching deformation of the C-N bonds of Y and to compare them to those of the allyl radical. By investigating also Y-d1 and the Y-d4 it should be possible to determine these force constants by fitting the force field of the three isotopomers. Furthermore we showed that substituents - particularly phenyl groups - play a very important role in determining the behavior of aziridines under photoinduced electron transfer (PET) conditions. N-phenyl rings stabilize the aziridine radical cation, whereas C-phenyl rings stabilize preferentially the ring-opened azomethine ylid cation. The extent of this stabilization can be assessed by calculating the energy increase on rotating the phenyl ring to a perpendicular position, so that resonance between the phenyl and the aziridine or the azomethine ylid moieties, respectively, is suppressed. Experiments and calculations showed that N-phenylaziridine radical cation, NPA+, is so strongly stabilized that it cannot undergo ring-opening on ionization (nor on subsequent electronic excitation of the radical cation). Methyl groups on the C atoms of NPA cause some lowering of the ring-opening transition state but the effect is too weak to allow a thermal crossing of the barrier. In contrast, C-phenyl substitution leads to a pronounced stabilization of the ring-opening TS, whereas the stabilization of the reactant and of the product are quite similar. Therefore N-H and N-alkyl-C-phenylaziridines undergo immediate ring-opening on ionization. The question which effect "wins" in ionized C,Ndiphenyl aziridine could not be answered satisfactorily. However for triphenylaziridine, TPA, short photolysis suffices to induce ring-opening. Thus, the use of aziridines, opened by ionization to perform [3+2]cycloadditions with dipolarophiles A=B, is limited to derivatives where substituents do not stabilize the ring-closed form too much

Electromers of the benzene dimer radical cation

The well-studied benzene dimer radical cation, which is prototypical for this class of species, has been reinvestigated computationally. Thereby it turned out that both the σ-hemibonded and the half-shifted sandwich structures of the benzene dimer cation, which had been independently proposed, represent stationary points on the B2PLYP-D potential energy surfaces. However, these structures belong to distinct electronic states, both of which are associated with potential surfaces that are very flat with regard to rotation of the two benzene rings in an opposite sense relative to each other. The surfaces of these two “electromers” of the benzene dimer cation are separated by only 3–4 kcal mol⁻¹ and do not intersect along the rotation coordinate, which represents a rather unique electronic structure situation. When moving on either of the two surfaces the title complex is an extremely fluxional species, in spite of its being bound by over 20 kcal mol⁻¹

The Primary steps in excited-state hydrogen transfer: the phototautomerization of o-nitrobenzyl derivatives

The quantum yield for the release of leaving groups from o-nitrobenzyl “caged” compounds varies greatly with the nature of these leaving groups, for reasons that have never been well understood. We found that the barriers for the primary hydrogen-atom transfer step and the efficient nonradiative processes on the excited singlet and triplet surfaces determine the quantum yields. The excited-state barriers decrease when the exothermicity of the photoreaction increases, in accord with Bell–Evans–Polanyi principle, a tool that has never been applied to a nonadiabatic photoreaction. We further introduce a simple ground-state predictor, the radical-stabilization energy, which correlates with the computed excited-state barriers and reaction energies, and that might be used to design new and more efficient photochemical processes

Simplified quantification of insulin, its synthetic analogs and C-peptide in human plasma by means of LC-HRMS.

The quantification of peptide hormones by means of liquid chromatography (LC) coupled to mass spectrometry (MS) or other techniques (e.g. immunoassays) has been a challenging task in modern analytical chemistry. Especially for insulin, its synthetic analogs, and C-peptide, reliable determinations are urgently needed due to their diagnostic value in the management of diabetes and insulin resistance and because of the illicit use of insulin as a performance-enhancing agent in professional sports or as an effective toxin in forensic toxicology. The concomitant measurement of C-peptide and insulin offers an established tool for the diagnostic workup of hypoglycemia (endogenous vs. exogenous hyperinsulinemia), characterizing hepatic insulin clearance, and the assessment of beta-cell function (insulin secretion). Thus, the present approach offers the possibility to determine human insulin and its synthetic analogs (lispro, glulisine, aspart, glargine metabolite, degludec, detemir, porcine, and bovine) and C-peptide simultaneously after sample preparation utilizing protein precipitation and a mixed-mode cation-exchange solid-phase extraction, and subsequent detection by LC-high resolution MS. The method was fully validated regarding the following parameters: specificity, limit of detection (0.2 ng/mL), limit of quantification (0.6 ng/mL), recovery (40-90%), accuracy (78-128%), linearity, precision (< 21%), carry over, robustness, and matrix effects. The proof-of-concept was shown by analyzing authentic plasma samples from adults with class II obesity and prediabetes collected in the course of an oral glucose tolerance test. All sample preparation steps were controlled by two stable isotope-labeled internal standards, namely [[2 H10 ] Leu B6, B11, B15, B17 ]-insulin, and [[13 C6 ] Leu 26, 30 ] C-peptide

Manual of VIKAASA: An application capable of computing and graphing viability kernels for simple viability problems

This manual introduces and provides usage details for an application we have developed called VIKAASA, as well as the library of functions underlying it. VIKAASA runs in GNU Octave or MATLAB®, using the numerical computing and graphing capabilities of those packages to approximate, visualise and test viability kernels for viability problems involving a differential inclusion of two or more dynamic variables, a rectangular constraint set and a single scalar control

Variations in rotational barriers of allyl and benzyl cations, anions, and radicals

High accuracy quantum chemical calculations show that the barriers to rotation of a CH2 group in the allyl cation, radical, and anion are 33, 14, and 21 kcal/mol, respectively. The benzyl cation, radical, and anion have barriers of 45, 11, and 24 kcal/mol, respectively. These barrier heights are related to the magnitude of the delocalization stabilization of each fully conjugated system. This paper addresses the question of why these rotational barriers, which at the Hückel level of theory are independent of the number of nonbonding electrons in allyl and benzyl, are in fact calculated to be factors that are of 2.4 and 4.1 higher in the cations and 1.5 and 1.9 higher in the anions than in the radicals. We also investigate why the barrier to rotation is higher for benzyl than for allyl in the cations and in the anions. Only in the radicals is the barrier for benzyl lower than that for allyl, as Hückel theory predicts should be the case. These fundamental questions in electronic structure theory, which have not been addressed previously, are related to differences in electron–electron repulsions in the conjugated and nonconjugated systems, which depend on the number of nonbonding electrons