NMR

This properties keyword predicts NMR shielding tensors and magnetic susceptibilities using the Hartree-Fock method, all DFT methods and the MP2 method [Gauss92, Gauss93, Gauss95, Cheeseman96].

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.

Spin-spin coupling constants may also be computed during an NMR job [Helgaker00, Sychrovsky00, Barone02, Peralta03, Deng06], via the SpinSpin option.

SpinSpin

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.

Mixed

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.

ReadAtoms

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:

atoms=list [notatoms=list]

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:

Bare integers without a keyword are interpreted as atom numbers:

CSGT

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).

GIAO

Compute NMR properties using the GIAO method only. This is the default.

IGAIM

Use atomic centers as gauge origins.

SingleOrigin

Use a single gauge origin. This method is provided for comparison purposes but is not generally recommended.

All

Compute properties with all three of the SingleOrigin, IGAIM, and CSGT methods.

PrintEigenvectors

Display the eigenvectors of the shielding tensor for each atom.

FCOnly

Compute only the Fermi contact spin-spin terms.

ReadFC

Read the Fermi contact spin-spin terms from the checkpoint file and then compute the other spin-spin coupling terms.

Susceptibility

Compute the magnetic susceptibility as well as the shielding.

SCF, DFT and MP2 methods. NMR may be combined with SCRF. NMR and Freq can now both be on the same route for HF and DFT.

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).