Development of methods for structural characterization of radical cations and neutral molecules by Fourier transform ion spectrometry and characterization of the performance of a small low-field Fourier transform mass spectrometer

Diane Tutko Leeck, Purdue University

Abstract

Structural characterization of radical cations has been an area of interest in our laboratory. The structures of long-lived low-energy molecular ions of simple carbon esters were examined by using ion/molecule reactions with selected neutral reagents (i.e., cyclohexanone, $d\sb8$-isopropanol and triethylamine) in an FT/ICR mass spectrometer. Reference ions with known enol or distonic structures were synthesized and their reactivity toward the neutral reagents was examined. By comparing the reactivity of the ester molecular ions to that of the reference ions, the ester radical cations were concluded to isomerize to their lowest energy isomer. Reactions of distonic dimethylene ketene radical cation were examined with several disulfide compounds as a potential method to identify the presence of disulfide bonds in neutral compounds. This ion primarily reacts by efficient thioalkyl abstraction from the disulfides. Alkyl disulfide abstraction reaction was also observed for disulfides that could yield stable tertiary or secondary alkyl radicals. From the observed reactivity, ionized dimethylene ketene was concluded to be useful for identifying disulfide bonds in neutral molecules. The performance of a small FT/ICR mass spectrometer was investigated using unscreened and screened cell designs. The use of the screened cell improves the mass range, mass accuracy and dynamic range but reduces sensitivity and trapping efficiency. Ion/molecule reactions of CH$\sb3$OB$\sp+$OCH$\sb3$, a fragment of ionized trimethyl borate, were examined as a potentially informative method for structural identification of neutral molecules. These reactions were examined in a low-field FT/ICR mass spectrometer. CH$\sb3$OB$\sp+$OCH$\sb3$ was determined to be a promising reagent to distinguish stereoisomeric diols. The cis-isomers of 1,2-cyclopentanediol and 2,3-trinorbornanediol react with CH$\sb3$OB$\sp+$OCH$\sb3$ to give an adduct ion that was not observed for the corresponding trans-isomers. The product, i.e., stable acylium ion, generated in the reactions of CH$\sb3$OB$\sp+$OCH$\sb3$ with various long-chain esters provided valuable information about the chain length of the neutral esters.

Degree

Ph.D.

Advisors

Kenttamaa, Purdue University.

Subject Area

Analytical chemistry

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