An electrospray ionization flow tube study of the protonated betaine/ammonia complex

Abstract

Reactions of protonated betaine, BetH +, with ammonia and deuterated ammonia were studied under thermal conditions with a combination of an electrospray ionization (ESI) source and a flow tube reactor. Reaction with NH 3 in a flow tube pressure range of 0.1–0.45 Torr leads, with a low reaction efficiency of 0.034 ± 0.011, to formation of a collisionally stabilized protonated betaine/ammonia complex, which can have the ion/dipole BetH +.NH 3 structure and/or the ammonium ion stabilized salt–bridge complex structure, Bet.NH 4 +. Reaction with ND 3 leads to a highly efficient H/D exchange of the labile hydrogen of BetH + presumably via the excited, chemically activated, salt-bridge complex, [Bet.ND 3H +]∗. Ions selected from the ESI source upstream in the flow tube, or generated by the reactions with ammonia midstream and then collisionally stabilized and thermalized to 292 ± 3 K by collisions with He or N 2 carrier gas species at 0.23 ± 0.01 Torr were subjected to multi-collision-induced dissociation (CID) with He or N 2 just before sampling. CID is achieved by raising the potential of the sampling nose cone from 0 to −20 V. The threshold voltage required for multi-collision-induced dissociation of the protonated betaine/ammonia complex to BetH + and NH 3 is considerably lower than the threshold voltage for dissociation of the protonated betaine homodimer, (Bet) 2H + in accord with the lower binding energy of the former.