CCL: The use of NMR spectra computations for verification of computational method

[Nico Green <nicogreen6[a]gmail.com>]

Dear Grigoriy
 The values you obtain seems to reproduce correctly the experimental
 chemical shifts.
 However, may I suggest you try using the MSTD?
 Here are the references:
 for 13C: The Journal of Organic Chemistry 2009; 74(19):7254-60.
 for 1H: The Journal of Organic Chemistry 2012; 77(14):6059-65.
 On the other hand, in my humble experience mPW1PW91 is the best
 cost/accuracy basis set, depending on your structure the functional you'll
 need to use, but as a start a 6-31G(d) or 6-31G(d,p) should work fine.
 In GIAO NMR is very common to use one level for the optimization step and
 another for the NMR calculation. So you may use your geometries and just
 perform a single point NMR calculation with mPW1PW91/6-31G(d) and check.
 Hope this helps
 On Mon, Sep 21, 2015 at 6:28 PM Grigoriy Zhurko reg_zhurko^chemcraftprog.com
 <owner-chemistry() ccl.net> wrote:
 >
 > Sent to CCL by: Grigoriy Zhurko [reg_zhurko^chemcraftprog.com]
 > I have been computing NMR spectra of several organic compounds, comparing
 > them with the experiment. Usually these computations show big systematic
 > errors, but correlate well with the experiment (for  C13 spectra of organic
 > molecules, I got the correlation coefficient R about 0.9995 with
 > B3LYP/6-311G(D,P) method).
 > Now I have computed some NMR spectra of bilirubin molecule with different
 > DFT functionals, and I found that the correlation coefficient is not a good
 > criteria of computation accuracy in my case. This molecule has internal
 > hydrogen bonds, and different functionals (in particular, B3LYP and PBE)
 > give quite different O..H distance (the difference is about 0.1 A), while
 > other bond lengths in this molecule do not differ significantly (the
 > difference is 0.012 A or less). The PMR spectrum with B3LYP correlates with
 > the experimental one with R=0.997, and with PBE ? R=0.995. These values do
 > not differ very much. But the coefficient B in the equation Y=A+B*X (for
 > the linear approximation of experiment vs theory graph) is 1.02 for B3LYP,
 > and 1.14 for PBE. So, with PBE it is far less from 1. Does that mean that
 > PBE is much less appropriate method for this task?
 > I suppose, that the systematic error of absolute values of the NMR
 > chemical shifts is caused by unclear physical meaning of these chemical
 > shifts and shieldings (maybe the solvent gives some additional shielding in
 > experiment). So, my question is, whether the B coefficient in correlation
 > must be always equal 1. If yes, then instead of correlation coefficients R
 > I should use another criteria of computation accuracy ? the RMS of MAE
 > difference between the computed and experimental chemical shifts, if the
 > shielding of the standard (TMS) simply fitted for best agreement (not
 > computed quantum-chemically). Is that correct? In my case, these MAE
 > difference must be much bigger for PBE functional, than for the B3LYP
 > functional.
 >
 > Grigoriy Zhurko.>
 >
 >