Gas phase fragmentation of protonated betaine and its clusters

Abstract

Betaine [(CH(3))(3)N(+)CH(2)COO(-)] is a methylated version of glycine and is a zwitterion in its neutral form. In this work, we have subjected protonated betaine, (+)(CH(3))(3)NCH(2)COOH, 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(CH(3))(3)(+) and CH(2)COOH. 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(CH(3))(3)(+) was observed in agreement with the electron ionisation spectrum of N(CH(3))(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 O(2). The dominant dissociation channel of the betaine radical, [(CH(3))(3)NCH(2)COOH] , involves formation of N(CH(3))(3) and CH(2)COOH, as revealed from the presence of N(CH(3))(3)(+) radical cations. This channel is associated with a kinetic energy release of 0.1-0.2 eV. The CH(2)COOH radical is unstable to dissociation into CH(3) and CO(2) but in charge reversal experiments (two Cs collisions), CH(2)[double bond, length as m-dash]C(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.