Intramolecular Hydrogen Bonding and Hydrogen Atom Abstraction in Gas-Phase Aliphatic Amine Radical Cations

The intramolecular hydrogen atom abstraction by the nitrogen atom in isolated aliphatic amine radical cations is examined experimentally and with composite high-level ab initio methods of the G3 family. The magnitude of the enthalpy barriers toward H-atom transfer varies with the shape and size of the cyclic transition state and with the degree of substitution at the nitrogen and carbon atoms involved. The lower barriers are found for 1,5- and 1,6-abstraction, for chairlike transition states, for abstraction reactions in ionized primary amines, and for abstraction of H from tertiary carbon atoms. In most cases, the internal energy required for 1,4-, 1,5-, and 1,6-hydrogen atom abstraction to occur is less than that required for gas-phase fragmentation by simple cleavage of C-C bonds, which explains why H-atom transfer can be reversible and result in extensive H/D exchange prior to the fragmentation of many low-energy deuterium labeled ionized amines. The H-atom transfer to nitrogen is exothermic for primary amine radical cations and endothermic for tertiary amines. It gives rise to a variety of distonic radical cations, and these may undergo further isomerization. The heat of formation of the gauche conformers of the gamma-, delta-, and epsilon-distonic isomers is up to 25 kJ mol(-1) lower than that of the corresponding trans forms, which is taken to reflect C-H-N hydrogen bonding between the protonated amino group and the alkyl radical site.

Competing Simple Cleavage Reactions: The Elimination of Alkyl Radicals from Amine Radical Cations

Two different alkyl radicals can be expelled when unsymmetrical aliphatic amine radical cations undergo C-C bond cleavage. The branching ratio is strongly dependent on the internal energy of the reactant, even when the competition involves loss of closely related alkyl radicals. In mass spectrometers, the rate of loss of the smaller radical (excepting methyl) always exceeds the rate of loss of the larger close to threshold. The preference is reversed for the more highly energized ions that react in the ion source, demonstrating that the rate of loss of the larger radical rises much more rapidly with increasing internal energy than does the rate of loss of the smaller radical. This result is not easily reconciled with a simple RRKM model, given the expected strong resemblance between the transition states involved, whereas it agrees well with a description based on variational transition state theory. The heats of formation of the products determined with the G3 composite ab initio method show that loss of the smaller radical is without exception the more favorable reaction. The relative rates of the competing C-C bond cleavage reactions of the metastable ions vary with the number of degrees of freedom of the reactant, with the critical energy of the reaction, and with the difference between the heats of formation of the products. The presence of intermediate energy barriers when cleavage occurs at a branching point can give rise to variations in relative rates that are not easily interpreted.

EPR Studies of Amine Radical Cations, Part 1: Thermal and Photoinduced Rearrangements ofn-Alkylamine Radical Cations to their Distonic Forms in Low-Temperature Freon Matrices

The thermal and photochemical transformations of primary amine radical cations (n-propyl 1.+, n-butyl 5.+) generated radiolytically in freon matrices have been investigated by using low-temperature EPR spectroscopy. Assignment of the spectra was facilitated by parallel studies on the corresponding N,N-dideuterioamines. The identifications were supported by quantum chemical calculations on the geometry, electronic structure, hyperfine splitting constants and energy levels of the observed transient radical species. The rapid generation of the primary species by a short exposure (1-2 min) to electron-beam irradiation at 77 K allowed the thermal rearrangement of 1.+ to be monitored kinetically as a first-order reaction at 125-140 K by the growth in the well-resolved EPR signal of the distonic radical cation .C(2CH2CH2NH3+. By comparison, the formation of the corresponding .CH2CH2CH2CH2NH3+ species from 5.+ is considerably more facile and already occurs within the short irradiation time. These results directly verify the intramolecular hydrogen-atom migration from carbon to nitrogen in these ionised amines, a reaction previously proposed to account for the fragmentation patterns observed in the mass spectrometry of these amines. The greater ease of the thermal rearrangement of 5.+ is in accordance with calculations on the barrier heights for these intramolecular 1,5- and 1,4-hydrogen shifts, the lower barrier for the former being associated with minimisation of the ring strain in a six-membered transition state. For 1.+, the 1,4-hydrogen shift is also brought about directly at 77 K by exposure to approximately 350 nm light, although there is also evidence for the 1,3-hydrogen shift requiring a higher energy. A more surprising result is the photochemical formation of the H2C=N. radical as a minor product under hard-matrix conditions in which diffusion is minimal. It is suggested that this occurs as a consequence of the beta-fragmentation of 1.+ to the ethyl radical and the CH2=NH2+ ion, followed by consecutive cage reactions of deprotonation and hydrogen transfer from the iminonium group. Additionally, secondary ion-molecule reactions were studied in CFCl2CF2Cl under matrix conditions that allow diffusion. The propane-1-iminyl radical CH3CH2CH=N. was detected at high concentrations of the n-propylamine substrate. Its formation is attributed to a modified reaction sequence in which 1.+ first undergoes a proton transfer within a cluster of amine molecules to yield the aminyl radical CH3CH2CH2N.H. A subsequent disproportionation of these radicals can then yield the propane-1-imine precursor CH3CH2CH=NH, which is known to easily undergo hydrogen abstraction from the nitrogen atom. The corresponding butane-1-iminyl radical was also observed.

Enzyme catalysis of 1,2-amino shifts: the cooperative action of B6, B12, and aminomutases

Ab initio molecular orbital theory is used to investigate 1,2-amino shifts catalyzed by aminomutases, coenzyme B12, and vitamin B6 (in the form of pyridoxal 5'-phosphate or PLP). Our calculations suggest essential catalytic roles for each of B12, B6, and the enzyme in aminomutase-catalyzed reactions. In the first place, coenzyme B12 provides a source of abstracting radicals, allowing the rearrangement reaction to take place on the radical surface. The involvement of radicals is supported by comparison of experimental and theoretical electron paramagnetic resonance parameters. Next, B6 allows the enzyme to lower the barrier height by introducing a double bond (allowing a low-energy intramolecular rearrangement pathway) and by providing a suitable site for partial protonation (preventing overstabilization of the reaction intermediate which could lead to enzyme inactivation). The PLP hydroxyl group is also identified as an important participant in these reactions. Finally, the enzyme holds the various reaction components in place and is the source of acidic functional groups that can provide partial protonation.