Gas-Phase Ion Chemistry of Trimethylborate with Nucleotide Anions
399
three methanol molecules occurred for the heterodimers.
Thus, in each case, rather than undergoing a simple ‘lig-
and switching’ reaction, boron forms covalent bonds to
‘cross-link’ both partners of the hydrogen-bonded complex.
Unlikethemonomeranions, theratesofreactionofthedimers
with TMB, and numbers of methanols lost from the TMB
adduct, do not correlate with their previously determined
structures.[17]
the University of Melbourne for funds to purchase the
LCQ. A.K.V. acknowledges the award of a Science Faculty
Scholarship (Studentship).
References
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Experimental
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5.0 kV; capillary temperature 200◦C; nitrogen sheath pressure 40 psi;
and capillary voltage/tube lens offset 0–10V. CID of [M − H]− ions
were carried out by mass selecting the complexes with a 5–10Th
window and subjecting them to the following typical conditions: activa-
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100 ms.
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Ion–molecule reactions were carried out as previously described.[21]
Briefly, once a stable ESI ion signal was obtained, the TMB reagent was
introduced into the trap as a part of the helium bath gas. A constant flow
of the reagent (0.05–0.2 mL h−1) was established using a syringe pump
with the syringe needle directed into a measured flow of helium (850–
3420 mL min−1). The majority of the gas exits through a flowmeter,
whereas a small amount (∼1 mL min−1) is drawn into the trap.The LCQ
uses a constriction capillary to control the helium flow and is designed
to maintain 1.75 mTorr in the trap when 3 psi of He pressure is applied to
the capillary. In the stock system, the 3 psi is maintained by an internal
regulator that steps down the 40 psi of He that is delivered at the external
port. To avoid the dead volume in the internal regulator, we bypass it
and deliver the He mixture (3 psi) directly to the capillary. This greatly
decreases the lag time after changes in reagent concentration. Previous
studies by Gronert and coworkers on a nearly identically modified LCQ,
have shown that these systems give rate constants comparable to those
from flowing afterglow instruments.[22] Other work from Gronert’s lab-
oratory indicates that the ions in the trap are essentially at ambient
temperature (∼300 K).[23]
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Theory
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[22] S. Gronert, L. M. Pratt, S. Mogali, J. Am. Chem. Soc. 2001,
123, 3081.
Due to the size of these systems, we have restricted our calculations to
key structures and transition states at the PM3 and AM1 semiempirical
levels of theory using the GAMESS suite of programs.[24] Exhaustive
conformational searches were not carried out. Instead, the structures of a
few starting conformations in which the hydrogen-bonding interactions
were maximized were found. PM3 was used for those systems that con-
tained no boron, while ions that contained boron were optimized using
the AM1 level of theory.
Acknowledgments
We thank Professor Gronert for many stimulating discussions
and acknowledge his seminal contributions to the chemistry
of phosphate anions with trimethylborate. R.A.J.O. thanks
the ARC (grant A29930202) for financial support and
[23] S. Gronert, J. Am. Soc. Mass Spectrom. 1998, 9, 845.
[24] M. W. Schmidt, K. K. Baldridge, J. A. Boatz, S. T. Elbert,
M. S. Gordon, J. H. Jensen, S. Montgomery, GAMESS ver. 1
1998 (Iowa State University: Ames, IO).