Bally and Rablen have followed up their important study of the appropriate basis sets and density functional needed to compute NMR chemical shifts1 (see this post) with this great examination of procedures for computing proton-proton coupling constants.2
They performed a comparison of 165 experimental coupling constants from 66 small, rigid molecules with computed proton-proton coupling constants. They use a variety of basis sets and functionals. They also test whether all four components that lead to nuclear-nuclear spin coupling constants are need, or if just the Fermi contact term would suffice.
The computationally most efficient procedure, one that still provides excellent agreement with the experimental coupling constants is the following:
- optimize the geometry at B3LYP/6-31G(d)
- Calculate only the proton-proton Fermi contact terms at B3LYP/6-31G(d,p)u+1s[H]. The basis set used for computing the Fermi contact terms is unusual. The basis set for hydrogen (denoted as “u+1s[H]”) uncontracts the core functions and adds one more very compact 1s function.
- Scale the Fermi contact terms by 0.9155 to obtain the proton-proton coupling constants.
This methodology provides coupling constants with a mean error of 0.51 Hz, and when applied to a probe set of 61 coupling constants in 37 different molecules (including a few that require a number of conformers and thus a Boltzmann-weighted averaging of the coupling constants) the mean error is only 0.56 Hz.
Bally and Rablen supply a set of scripts to automate the computation of the coupling constants according to this prescription; these scripts are available in the supporting materials and also on the Cheshire web site. It should also be noted that the procedure described above can be performed with Gaussian-09; no other software is needed. Thus, these computations are amenable to synthetic chemists with a basic understanding of quantum chemistry.
References
(1) Jain, R.; Bally, T.; Rablen, P. R., "Calculating Accurate Proton Chemical Shifts of Organic Molecules with Density Functional Methods and Modest Basis Sets," J. Org. Chem., 2009, 74, 4017-4023, DOI: 10.1021/jo900482q.
(2) Bally, T.; Rablen, P. R., "Quantum-Chemical Simulation of 1H NMR Spectra. 2. Comparison of DFT-Based Procedures for Computing Proton-Proton Coupling Constants in Organic Molecules," J. Org. Chem., 2011, 76, 4818-4830, DOI: 10.1021/jo200513q
Henry Rzepa responded on 15 Dec 2011 at 5:32 am #
I notice from manual, and particularly the NMR(spinspin,mixed) keyword, that these hybrid protocols are getting more common (in the sense of the original G1, G2, G3 series from Pople, in which extrapolations of the basis set and the Hamiltonian are used to achieve a FCI/CBS limit).
So not only do we have a functional Zoo, but we have a script zoo appearing!
It is going to get particularly challenging to be able to exactly reproduce any particular calculation (in the sense that these scripts are likely to be code specific, and perhaps even version specific). In this latter regard, the three revisions of eg Gaussian09 (A,B,C) differ income places in exactly how a protocol is implemented, and you may indeed get (slightly?) different results depending on which version of the code you use.
In a sense, we are now way beyond the “variational theorem” (one used to be able to rely on this theorem to reliably get the same answer from a variety of different quantum codes) and moving rapidly into the situation of molecular mechanics implementations of a few years ago, where the hidden parameters/boundary cut-offs and force field determined the precise result, and one dare not try a different code to double check an answer.
Steven Bachrach responded on 15 Dec 2011 at 9:01 am #
Henry’s point is quite valid. It speaks towards the importance of archiving computational results in a way that totally captures the approximations and assumptions that were made within the input and within the code itself. Thus the need for detailed XML reports from computational studies.
Holger Kruse responded on 16 Dec 2011 at 4:28 am #
“Thus the need for detailed XML reports from computational studies.”
AFAIK, the molpro program is the only code that prints as default additionally a xml-output file.
The geometry is in CML. But there is no standard for gradients, energies, dipoles, etc., or is it also included in CML ??
Dario Cambié responded on 17 Dec 2011 at 4:15 am #
First thanks for sharing these extremely interesting blog posts on the internet.
I’m writing here just to inform you that the RSS feed of the blog (/?feed=rss) is broken (because of extra newlines at the beginning).
For those looking for a feed I’ve mirrored and corrected it at http://ctf.netsons.org/varie/rss.php
Source:
Sorry for the noise.
Henry Rzepa responded on 19 Dec 2011 at 2:05 am #
Holger Kruse mentioned that there is no standard for gradients, energies et. The way this is handled in CML is via dictionaries; see here. In fact, Peter Knowles has contacted us about this, and so a solution might be on the horizon.
Computational Organic Chemistry » Computed C-C NMR coupling constants responded on 14 Nov 2012 at 3:14 pm #
[…] use of computed NMR coupling constants is starting to grow. In a previous post I discussed a general study by Rablen and Bally on methods for computing JHH coupling …. Now Williamson reports methods to experimentally obtain 1 JCC and 3JCC coupling constants.1 These […]