Gas phase fragmentation of protonated betaine and its clusters

Betaine [(CH3)3N+CH2COO−] is a methylated version of glycine and is a zwitterion in its neutral form. In this work, we have subjected protonated betaine, +(CH3)3NCH2COOH, to a range of fragmentation experiments which involve vibrational excitation, electronic excitation and electron capture. Low-energy (eV) collisions in combination with deuterium labelling reveal that the lowest energy dissociation pathway is the formation of N(CH3)3+˙ and ˙CH2COOH. The dominant channel after 50 keV collisions with molecular oxygen is the same as that after low-energy collisions; however, more fragmentation is seen which is most likely due to electronic excitation of the ions in the collision processes. Subsequent dissociation of the radical N(CH3)3+˙ was observed in agreement with the electron ionisation spectrum of N(CH3)3. Electron-induced dissociation by 22 eV electrons produced similar fragments to those formed after high-energy collision-induced dissociation. With caesium atoms as the target gas, protonated betaine captured electrons to give neutrals. These were reionised to cations a microsecond later in collisions with O2. The dominant dissociation channel of the betaine radical, [(CH3)3NCH2COOH]˙, involves formation of N(CH3)3 and ˙CH2COOH, as revealed from the presence of N(CH3)3+˙ radical cations. This channel is associated with a kinetic energy release of 0.1–0.2 eV. The ˙CH2COOH radical is unstable to dissociation into ˙CH3 and CO2 but in charge reversal experiments (two Cs collisions), CH2C(OH)O− anions were formed due to the short time between the collisions (nanoseconds). Density functional theory calculations support the spectral interpretations. Collision-induced dissociation of protonated betaine clusters resulted dominantly in loss of neutral betaines.