Ion-molecule reaction studies of: Methyl cation affinities and hydrogen bonding in protonated ions

• Main Author: Nicol, Gordon Robert
• Format: Dissertation
• Language: English
• Published: ProQuest Dissertations & Theses 01-01-1988
• Subjects: Chemistry; Physical chemistry
• ISBN: 0315454547; 9780315454545

The extent of proton transfer from H$\sb3$O$\sp{+}$(H$\sb2$O)$\sb{\rm h}$ to compounds B was studied with a pulsed electron high pressure mass spectrometer. The sensitivity of analytes B in an atmospheric pressure ionization mass spectrometer is proportional to the extent of proton transfer reaction H$\sb3$O$\sp{+}$(H$\sb2$O)$\sb{\rm h}$+ B = BH$\sp{+}$(H$\sb2$O)$\sb{\rm b}$+(h-b+1)H$\sb2$O. The analytes can be divided into three groups: (1) bases with gas-phase basicities (GB) greater than 200 kcal/mol, mostly nitrogen bases, have high and relatively uniform sensitivities and the sensitivity is dependant on the kinetics of the proton transfer reaction. (2) bases with GB less than 200 kcal/mol, mostly oxygen bases, have sensitivities that increase with increasing GB. These bases were found to reach proton transfer equilibria with the water hydrates and their sensitivity was dependant on the equilibrium constant. (3) The third group consists mostly of carbon bases which show a very low sensitivity with respect to their GB. This is due mainly to the low stability of the BH$\sp{+}$(H$\sb2$O)$\sb{\rm b}$ hydrates. Symmetric hydrogen bond strengths were measured for HCl, HBr, MeCl and MeBr and are compared to hydrogen bond strengths of other heteroatoms. The trend of increasing hydrogen bond strength with electronegativity was observed. With poly functional compounds there is the possibility of internal hydrogen bonding. The thermodynamics of inter- as well as the intra-molecular hydrogen bvonding in $\alpha,\omega$-dialcohols were studied. The $\triangle$H$\sp\circ$ and $\triangle$S$\sp\circ$ for the formation of the proton bound dimer of the $\alpha,\omega$-dialcohols indicated the formation of a second ring. Methyl cation affinities (MCA) were measured for several bases B. The relative values were anchored to the value for the MCA(N$\sb2$). The MCA(B) follow in general the proton affinities. The rate constants for the methyl cation transfer were measured and found to be equal to the ADO collision rate constants at large exothermicity and decreased below collision rates at low exothermicity. The electron affinity (EA) of C$\sb6$F$\sb6$ was determined to be 14.8 $\pm$ 2.5 kcal/mol. The reaction between C$\sb6$F$\sb6\sp-$ and O$\sb2$ was also studied. The rate constant was found to be 8.7 $\times$ 10$\sp{-11}$ molec$\sp{-1}$ cm$\sp3$s$\sp{-1}$ at 70C. At temperatures greater than 150C electron detachment from C$\sb6$F$\sb6\sp-$ occurred. From the temperature dependance of the rate of electron detachment an EA of 15.6 $\pm$ 2.6 kcal/mol was estimated.