Application of Fourier transform ion cyclotron resonance mass spectrometry to the study of dicoordinated boron cations in the gas phase

Thilini Damayanthi Ranatunga, Purdue University

Abstract

The intrinsic gas-phase reactivity of simple dicoordinated boron cations toward various oxygen containing compounds was investigated by using Fourier-transform ion cyclotron resonance mass spectrometry. The product distributions, reaction mechanisms and thermochemistry of these reactions were studied by using various experimental approaches. The dicoordinated boron cations CH$\sb3$BCH$\sb3\sp{+}$, CH$\sb3$OBOCH$\sb3\sp{+}$ and CH$\sb3$(CH$\sb2$)$\sb2$OBOH$\sp{+}$ react at near collision rate with simple alcohols and ethers. The reaction is dominated by abstraction of water or a OH group by the ion. The efficiency of the OH abstraction reaction depends primarily on the stability of the alkyl cation formed. These two reactions likely occur via the same proton bound intermediate. The mechanism is analogous to the dissociation of protonated ethanol to ethylene and H$\sb3$O$\sp{+}$. The ion CH$\sb3$OBOCH$\sb3\sp{+}$ rapidly abstracts OH from cis- and trans-1,2-cyclopentanediols, and cis-(diexo and diendo)- and trans-trinorbornanediols. In addition, CH$\sb3$OBOCH$\sb3\sp{+}$ reacts with the cis-diols by an intramolecular displacement of CH$\sb3$OH in the initially formed adduct. CH$\sb3$BCH$\sb3\sp{+}$ and CH$\sb3$OBOCH$\sb3\sp{+}$ react with aldehydes and ketones by abstraction of an OH group and abstraction of part of the aldehyde or ketone as a small aldehyde. The reaction occurs via 1,2-hydride shifts and 1,5-hydride shifts in the initially formed adduct. Finally, the reactivity of five different boron cations, CH$\sb3$BCH$\sb3\sp{+}$, CH$\sb3$CH$\sb2$BH$\sp{+}$, CH$\sb3$OBOCH$\sb3\sp{+}$, CH$\sb3$(CH$\sb2$)$\sb2$OBOH$\sp{+}$ and (CH$\sb3$)$\sb2$NBN(CH$\sb3$)$\sb2\sp{+}$ was investigated toward diethyl ether, 2-propanol and 2-pentanone. The reactivity of these ions depends on the substituents on the boron. The most reactive ions are CH$\sb3$BCH$\sb3\sp{+}$ and CH$\sb3$CH$\sb2$BH$\sp{+}$ while the least reactive ion is (CH$\sb3$)$\sb2$NBN(CH$\sb3$)$\sb2\sp{+}$.

Degree

Ph.D.

Advisors

Kenttamaa, Purdue University.

Subject Area

Chemistry|Analytical chemistry|Organic chemistry

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