Chemical Exchange Flashcards
what are the 3 kinds of dynamic changes that can be identifies in NMR
change of coordination environment
complexation
conformational change
what is shown in the spectrum during a slow symmetrical exchange
both peaks show in their original positions
what is shown in the spectrum during fast symmetrical exchange
1 peak at the average chemical shift position with the integral of both peaks
what is shown on the spectrum if the rate is of an intermediate speed
lines are very broad and hard to distinguish from the baseline
what is the formula for the rate of the exchange
k = 1/tau
for a slow symmetric exchange what is the formula of the line width
width = k/pi
what is the formula for line width for a fast symmetrical exchange
width = pi ^nu / 2R
what is the formula of R for intermediate exchange
R = pi^nu/root(2)
what formula can be used to extract rate constants by studying spectra at different temperatures
R = Ae^-Ea/RT
how does the NMR timescale compare to those of other spectroscopies
much slower
what does the length of the NMR timescale mean in a practical sense
shows average of multiple resonances
what will the position of an average peak in an asymmetric exchange
position is relative to the population of both of the peaks
what effect does exchange have on J values
change in geometry changes the NMR environment and therefore J couplings
how can rate constant be determined at the coalescence point
k = pi ^nu / root(2) k = 2.2 ^nu
