On the Role of Hydrogen Bonds in Photoinduced Electron‐Transfer Dynamics between 9‐Fluorenone and Amine Solvents

Using ultrafast fluorescence upconversion and mid‐infrared spectroscopy, we explore the role of hydrogen bonds in the photoinduced electron transfer (ET) between 9‐fluorenone (FLU) and the solvents trimethylamine (TEA) and dimethylamine (DEA). FLU shows hydrogen‐bond dynamics in the methanol solvent upon photoexcitation, and similar effects may be anticipated when using DEA, whereas no hydrogen bonds can occur in TEA. Photoexcitation of the electron‐acceptor dye molecule FLU with a 400 nm pump pulse induces ultrafast ET from the amine solvents, which is followed by 100 fs IR probe pulses as well as fluorescence upconversion, monitoring the time evolution of marker bands of the FLU S₁ state and the FLU radical anion, and an overtone band of the amine solvent, marking the transient generation of the amine radical cation. A comparison of the experimentally determined forward charge‐separation and backward charge‐recombination rates for the FLU‐TEA and FLU‐DEA reaction systems with the driving‐force dependencies calculated for the forward and backward ET rates reveals that additional degrees of freedom determine the ET reaction dynamics for the FLU‐DEA system. We suggest that hydrogen bonding between the DEA molecules plays a key role in this behaviour.     

The role of distonic ions in the formation of CH3NH 3 + and (CH3)2NH 2 + from the molecular ions of octopamine and synephrine

It is shown by mass-analyzed ion kinetic energy spectrometry that the metastably decomposing molecular ions of octopamine (p-HOC₆H₄CH(OH)CH₂NH₂) and synephrine (p-HOC₆H₄CH(OH)CH₂NHCH₃) yield only protonated methylamine and dimethylamine, respectively, as product ions. From deuterium labeling and variation of the internal energy of the molecular ions, experimental support has been obtained that these product ions are generated via the occurrence of a distonic ion-neutral complex. In the case of octopamine, this complex would consist of a nitrogen-protonated aminomethyl radical and p-hydroxylbenzaldehyde in which the former species abstracts the aldehydic or phenolic hydrogen atom from the latter to give protonated dimethylamine.     

Reactions induced by the γ-hydrogen shift in the molecular ion of 1-nitropropane

It is shown that the γ- or 1,5-hydrogen shift in the molecular ion of 1-nitropropane leads to three primary fragmentation reactions. They are the loss of a hydroxyl radical, a molecule of ethylene and a molecule of nitric oxide. The structures and chemistry of the resulting ions have been investigated by a series of experiments including deuterium labelling, spontaneous and collisionally induced dissociations and accurate mass measurements.     

On the 1,1-elimination of nitrous acid and the loss of hydroxyl from the molecular ion of the methyl ester of γ-nitrobutyric acid

It is shown by D labelling experiments that nitrous acid and hydroxyl, eliminated from the methyl ester of γ-nitrobutyric acid upon electron-impact, contain a γ-hydrogen atom to the extent of ～ 85% and ～ 100%, respectively. The loss of nitrous acid is the third case of a highly specific 1,1-elimination from a molecular ion^1a,1b reported hitherto and arguments are presented for the essential role played by carbomethoxy group in this reaction, i.e. it converts the nitro structure of the molecular ion to its acinitro structure. The results obtained further point to a stepwise character of the McLafferty rearrangement, at least for the present case.     

Phenolic proton transfer to the 1,2 double bond in the molecular ion of trans-1,2-tetrahydrocannabinol

It is known that the molecular ion of trans-1,6-tetrahydrocannabinol (1,6-THC) with m/e 314 decomposes via a retro Diels-Alder reaction to fragment m/e 246, which then loses a Me radical to give the ion m/e 231 (cf Scheme 1).¹A similar breakdown is found for trans-1,2-tetrahydrocannabinol (1,2-THC), suggesting a shift of the double bond from the 1,2 to the 1,6 position in its molecular ion.Methylation of the phenolic OH group in trans-1,2-tetrahydrocannabinol however, shows that the phenolic proton transfer to the 1,2 double bond (cf Scheme 2) is much more important (～ 35–80%) than simple double bond migration (～ 20%; Scheme 3) in the formation of fragment m/e 231.     

The role of large conformational changes in efficient ultrafast internal conversion: deviations from the energy gap law

Conversion of electronic excitation energy into vibrational energy was investigated for photochromic spiropyran molecules, using femtosecond UV-mid-IR pump-probe spectroscopy. We observe a weaker energy gap dependence than demanded by the "energy gap law". We demonstrate that large conformational changes accompanying the optical excitation can explain the observed time scale and energy gap dependence of ultrafast S(1) --> S(0) internal conversion processes. The possibility of dramatic deviations from standard energy gap law behavior is predicted. We conclude that controlling molecular conformations by rigid environments can have a substantial impact on photophysical and (bio)chemical processes.     

Mass spectrometry of aralkyl compounds with a functional group–X A reinvestigation of the structure of the [C8H9]+-ion from 1-phenylethylbromide and of the molecular ion [C8H8]+˙ from styrene by use of 13C labelling

Specific ¹³C-labelling in the side-chain of 1-phenylethylbromide and of styrene shows that it is not necessary to assume eight-membered ring structures for the [C₈H₉]⁺- and [C₈H₈]⁺⁺˙-ions to explain the almost complete randomization of all hydrogen atoms, as might be concluded from D-labelling data. It is now suggested that the eight-membered ring is predominantly present in [C₈H₉]⁺ and [C₈H₈]⁺˙ ions of low internal energy. In particular this appears to apply to styrene, which generates a cyclooctatetraene molecular ion with the original side-chain carbon atoms still linked together, as shown by ^13C-labelling.