Subject: Ab initio prediction of time-averaged NMR shifts

From: "Jeremy R. Greenwood" <[email-masked]>
Date: Sun, 27 Apr 2003 13:53:46 +0200

Dear CCL,

Question: can someone provide a reference/suggestion/theory/method/example
for extending ab initio NMR chemical shift prediction to the reproduction 
of time-averaged shifts for systems in rapid equilibrium?

--

My problem:

I have characterised a potential energy surface (to basis set 
convergence with B3LYP) of a strained system as 
[1] -- TS1,2 -- [2] -- TS2,3 -- [3]
Where the transitions involve (et al.) ca. 60 degree 
torsional sweeps about RCH2--N-oxide(RR')--CH2R bonds.

The barriers are modest, and at normal temperatures the interchange 
is rapid, thus the NMR spectrum gives a sharp-line average.

I have calculated magnetic shielding tensors for minima and transition
states, and could in principle do it for points along the path. 
The 1H proton shifts vary dramatically over the energy surface as
the protons move in the shadow of the chiral N-O. I doubt a naive 
approach of linearly combining the shifts of the minima according 
to relative energy will tell the story of the time-averaged structure
and its magnetic properties.

Thanks in advance for any suggestions.

Jeremy
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Jeremy Greenwood                                  [email-masked]
Department of Medicinal Chemistry                      bh +45 35306117
The Danish University of Pharmaceutical Sciences       fx +45 35306040
Universitetsparken 2, DK-2100 Copenhagen, Denmark      ah +45 32598030
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