The equilibria H+(CH3OH)" = H+fCEfiOH),-, + CHsOH and H+(CH3OCH3)" = H+(CH3OCH3)"-! + CH3OCH3 were measured in the gas phase for = 2 to 8 (methanol) and = 2 to 3 (dimethyl ether). A pulsed electron beam high pressure ion source mass spectrometer was used. Temperature dependence of , -1 led to evaluation of AH".n-u AG°",.-i, and AS°".n-1. The present results were compared with earlier determinations for the proton hydrates H+(H20)". The initial interaction (2,1) is nearly equally strong for the three systems. For water and methanol the energies AH",n-1 and AG",n-1 decrease quite regulary with n. There is a dramatic drop for dimethyl ether between = 2 and 3. This must be due to blocking of H bonding past structure H+(CH3OCH3)2. Small discontinuities in values for water and methanol indicate somewhat more stable structures for H+(CH3OH)3 and H+(H20)4. The formation of (CH3)2OH+ in methanol and (CH3)30+ in dimethyl ether was observed at high temperatures.Several years ago a study1 of the gas phase equilibria ( , -1) was made in our laboratory. Determina-H+(H20)" = H+(H20)"-i + H20 ( , -1) tion of the temperature dependence of the equilibrium constants Kn,n_x permitted the evaluation of AG°r,n-1, AH°",n-u and for = 1 to 8. A later study,2 with improved apparatus, of the kinetics of the above reaction, confirmed the thermodynamic values for the reactions with > 4 but led to somewhat lower AH2li and AH¡ 2 and somewhat higher AG°4,3.The above determinations provided the first experimental values for the dissociation energies of the strongly hydrogen-bonded clusters H+(H20)". The results showed a large Z)(H30+ -H20) « AH2,x = 32 kcal/mol. The successive AHn,n-\ decreased initially rapidly and then more slowly leading to AHi:7 = 10.3 kcal for the last step that could be measured. The AH°n,n.i and AG°","_i plotted vs. n presented a rather smooth curve.This result was to a certain extent surprising since the symmetric Eigen structure3-4 H30+(H20)3 (see IVb, Figure 1) had been expected to be of high stability and thus lead to large discontinuity in the changes in the AG°",K_i and AH°n,n-1 values. No such large effect was evident, but a close examination of the data (Figure 5, ref 1) did show a small break between the AG°4,3 and AG°6,4, which could be due to a somewhat more stable configuration for the cluster n = 4, i.e., the cluster which could have the Eigen structure.
Collectively, man-made emissions of a few greenhouse gases may cause about the same amount of global warming as increasing carbon dioxide. Among the most potent of these non-CO2 greenhouse gases are the perfluorocarbons that have extraordinarily long atmospheric lifetimes of 10,000 to more than 50,000 yr. We report atmospheric concentrations over two decades, between 1978 and 1997, of the three most abundant perfluorocarbons--CF4, C2F6, and C3F8--and delineate the sources that account for the present abundances and trends. We show that C2F6 and C3F8 are present at only 2.9 and 0.2 pptv, respectively. CF4 is the most abundant perfluorocarbon at 74 pptv (in 1997) of which about 40 pptv are from natural emissions, 33 pptv from aluminum manufacturing, and 1 pptv from the semiconductor industry. The increasing trend of CF4 has slowed in recent years due to the major reductions in the emission rate per ton of aluminum produced. The effect of the falling emission factor is partially offset by increased production and increasing use by the semiconductor industry.
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