NMR shielding tensors may be computed with the Continuous Set of Gauge Transformations (CSGT) method [Keith92, Keith93, Cheeseman96] and the Gauge-Independent Atomic Orbital (GIAO) method [London37, McWeeny62, Ditchfield74, Wolinski90, Cheeseman96]. Magnetic susceptibilities may also be computed with both GIAOs [Ruud93] and CGST. Gaussian also supports the IGAIM method [Keith92, Keith93] (a slight variation on the CSGT method) and the Single Origin method, for both shielding tensor and magnetic susceptibilities.
Structures used for NMR calculations should have been optimized at a good level of theory. Note that CSGT calculations require large basis sets to achieve accurate results.
Compute spin-spin coupling constants in addition to the usual NMR properties. Be aware that this calculation type has a computational cost of about twice that of computing vibrational frequencies alone. It is available only for Hartree-Fock and DFT methods.
Requests a two-step spin-spin coupling calculation [Deng06]. This option causes two job steps to be run. In the first, the basis set specified by the user is modified to be appropriate for the Fermi Contact term, by uncontracting the basis and adding tight polarization functions for the core. In the second step, the other three terms in the spin-spin coupling are calculated with the unmodified basis set specified in the route section. The final results reported at the end of the second job step include the Fermi Contact contribution from the first step. This significantly improves the accuracy of spin-spin coupling constants, especially when done with typical valence-oriented basis sets such as 6-311G+(d,p), aug-CC-pVDZ or aug-CC-pVTZ. This approach is also faster than computing all four terms using a modified basis set incorporating tight polarization functions.
Calculate spin-spin coupling constants only for selected atoms. The atom list is specified in a separate input section (terminated by a blank line). The list is initially empty.
The input section uses the following format:
where each list is a comma or space-separated list of atom numbers, atom number ranges and/or atom types. Keywords are applied in succession. Here are some examples:
|atoms=3-6,17 notatoms=5||Adds atoms 3, 4, 6 and 17 to the atom list. Removes 5 if present.|
|atoms=3 C 18-30 notatoms=H||Adds all C atoms and all non-H among atoms 3, 18-30.|
|atoms=C N notatoms=5||Adds all C and N atoms except atom 5.|
|atoms=1-5 notatoms=H atoms=8-10||Adds atoms 8-10 and non-hydrogens among atoms 1-5.|
Bare integers without a keyword are interpreted as atom numbers:
|1,3,5 7||Adds atoms 1, 3, 5 and 7.|
Compute NMR properties using the CSGT method only. The data file for the ACID program can be generated with NMR=CSGT IOp(10/93=1).
Compute NMR properties using the GIAO method only. This is the default.
Use atomic centers as gauge origins.
Use a single gauge origin. This method is provided for comparison purposes but is not generally recommended.
Compute properties with all three of the SingleOrigin, IGAIM, and CSGT methods.
Display the eigenvectors of the shielding tensor for each atom.
Compute only the Fermi contact spin-spin terms.
Read the Fermi contact spin-spin terms from the checkpoint file and then compute the other spin-spin coupling terms.
Here is an example of the default output from NMR:
Magnetic properties (GIAO method) Magnetic shielding (ppm): 1 C Isotropic = 57.7345 Anisotropy = 194.4092 XX= 48.4143 YX= .0000 ZX= .0000 XY= .0000 YY= -62.5514 ZY= .0000 XZ= .0000 YZ= .0000 ZZ= 187.3406 2 H Isotropic = 23.9397 Anisotropy = 5.2745 XX= 27.3287 YX= .0000 ZX= .0000 XY= .0000 YY= 24.0670 ZY= .0000 XZ= .0000 YZ= .0000 ZZ= 20.4233
For this molecular system, the values for all of the atoms of a given type are equal, so we have truncated the output after the first two atoms.
The additional output from spin-spin coupling computations appears as follows:
Total nuclear spin-spin coupling K (Hz): 1 2 1 0.000000D+00 2 0.147308D+02 0.000000D+00 Total nuclear spin-spin coupling J (Hz): 1 2 1 0.000000D+00 2 0.432614D+03 0.000000D+00
The various components of the coupling constants precede this section in the output file. It displays the matrix of isotropic spin-spin coupling between pairs of atoms in lower triangular form. The K matrix gives the values which are isotope-independent, and the J matrix gives the values taking the job’s specific isotopes into account (whether explicitly specified or the default isotopes).
Last updated on: 05 January 2017. [G16 Rev. C.01]