The stopped exchange region (170 K) of the dynamic 31P{1H} NMR spectrum of a
cis-Mo(CO)4{P(OR)3}2 (where R group is chiral) in dichloromethane-d2
consists of one doublet of a doublets (AX pattern) and a singlet. We have
assigned these phosphorus resonances to two unequal populations (3:2) of
enantiomeric pairs of two diastereomeric conformations of the complexes that
are undergoing chemical exchange.
Thus, both diastereomers undergo chemical exchange via ring_inversion at the
phosphorus and thus, at room temperature only one sharp 31P NMR resonance is
observed.
A(four line AX pattern)<=> B(singlet)
We have modeled this dynamic behaviour(over a 40 degree range from 170K )by
making use of the the populations(3:2) obtained at the stopped exchange
region while treating other parameters as variables for optimization by
iterative
procedure. (chemical shift did not vary more than 0.15 ppm over a 40 degree
range) i.e., we have kept the populations obtained at 170K constant for
the computation. However, when we treated populations as independent
parameter, it seemed they were too ill defined for iteration and were forced to
work with low-temperature values.
We have lately submitted this paper for publication and got an interesting
opinion from a referee who finds amusement with the methodology we have
used. He claims, "knowledge of both 'forward rate constant'(kf) and the
'equilibrium constant'(Keq) for the species at equilibrium is needed for the
computation of dynamic NMR line-shapes. Since, these two parameters can
only be independently obtained at low-temperatures (wherein separate
resonances are seen, ca. 170K); any line-shape computation using
low-temperature values of population is useless. Thus, he claims, this
limits the scope of the investigation and forms basis for rejection.
It is my understanding, that one assumption that is common to all of the
line-shapes theories is that the parameters which describe a spectrum in the
slow exchange limit can be used to calculate spectra at any rate. (I
understand, this assumption is difficult to avoid for intermediate and fast
rates, specifically for chemical shifts, which we have treated as
independent parameter in the computation)
Did I do any major mistake in line-shape calculation for paper to be rejected
for publication ?
In fact, I have come across recent publications from well known authors such
as "Eliel" who have treated similar exchange problems the very same way as
we have done. (i.e. assumed equilibrium constant is independent of
temperature for the temperature region of interest wherein computation is
performed).
Is it necessary to have "simultaneous knowledge of Keq and kf (forward rate
constant) at each and every temperature for all line-shape calculations ???
OR Is it possible to obtain them experimentally, given the fact that NMR lines
overlap and coalesce due to chemical exchange ??
OR How could one obtain a very reliable values(not some guestimates) of them ??
Well, It is my opinion using low-temperature values of populations is much
much better than some "guesstimates" obtained somehow; (Many publications in
the literature do not even mentioned how they have obtained "guestimates" of
populations and use lots of hand-waving)
What did I do wrong with my very first publication as a Graduate Student ???
Is treating populations obtained at the low-temperature spectrum for
line-shape analyses a fatal mistake ?? (I have performed integration of the
low-temperature spectrum over a 30 degree range (using deconvolution when
necessary) and it does not seem to be statistically different from each other)
OR, my manuscript happen to go some "idealist referee" at a wrong place at a
wrong time ??
I am frustrated and confused. All suggestions will be appreciated !!
Sincerely
Mahesh..
"Earth may have enough to cater to our needs, it surely will not put up with
our greed"
--Mahatma Gandhi