Acid-base chemistry at the ice surface: reverse correlation between intrinsic basicity and proton-transfer efficiency to ammonia and methyl amines.
Proton transfer from the hydronium ion to NH(3), CH3NH2, and (CH3)2NH is examined at the surface of ice films at 60 K. The reactants and products are quantitatively monitored by the techniques of Cs+ reactive-ion scattering and low-energy sputtering. The proton-transfer reactions at the ice surface proceed only to a limited extent. The proton-transfer efficiency exhibits the order NH3>(CH3)NH2=(CH3)2NH, which opposes the basicity order of the amines in the gas phase or aqueous solution. Thermochemical analysis suggests that the energetics of the proton-transfer reaction is greatly altered at the ice surface from that in liquid water due to limited hydration. Water molecules constrained at the ice surface amplify the methyl substitution effect on the hydration efficiency of the amines and reverse the order of their proton-accepting abilities.
Park
Seong-Chan
SC
Department of Chemistry, Seoul National University, Gwanak-gu, Seoul 151-747, Republic of Korea.
Kim
Joon-Ki
JK
Lee
Chang-Woo
CW
Moon
Eui-Seong
ES
Kang
Heon
H
D000143:Acids / Q000737:chemistry*; D000588:Amines / Q000737:chemistry*; D000641:Ammonia / Q000737:chemistry*; D006863:Hydrogen-Ion Concentration; D007053:Ice; D013058:Mass Spectrometry; D008745:Methylation; D011522:Protons; D013499:Surface Properties; D013816:Thermodynamics
D000143:Acids; D000588:Amines; D007053:Ice; D011522:Protons; D000641:Ammonia