7
$\begingroup$

I am trying to calculate chemical shifts for Tellurium in an organotellurium compound with Gaussian16 but the calculated values do not match experimental values at all. I know that NMR calculation with heavier atoms can be problematic because of relativistic effects but there is another problem I can not make sense of. I calculated chemical shifts (B3LYP/def2-TZVP, GIAO) for a set of other organotellurium compounds and the values I obtain are nowhere around the calculated reference values, which were obtained using a similiar method. I know that to obtain a chemical shift from chemical shieldings, one has to calculate the difference between a reference compound (for Te-NMR: Dimethyltelluride, calculated isotropic shielding value: 387.4 ppm) and the compound of interest. But using this relation I obtain values that do not match any of the experimental or theoretical values I have seen so far. Did anyone do similiar calculations for Te compounds and is there something I am not getting right ?

Edit:

The references I compared my results to are:

https://pubs.acs.org/doi/10.1021/acs.jpca.0c05780

https://pubs.acs.org/doi/10.1021/acs.jpca.7b03198

The output with the tellurium chemical shieldings for the reference compound (Me2Te) looks like this:

SCF GIAO Magnetic shielding tensor (ppm):                    
     1  Te   Isotropic =   384.9991   Anisotropy =    39.7993
  XX=   411.5320   YX=    -0.0030   ZX=    -0.0004           
  XY=    -0.0014   YY=   371.8145   ZY=     0.0002           
  XZ=    -0.0003   YZ=     0.0002   ZZ=   371.6507           
  Eigenvalues:   371.6507   371.8145   411.5320

And for an organotellurium example (Tellurophene):

     4  Te   Isotropic =   379.3320   Anisotropy =    16.7719
  XX=   390.5132   YX=    -0.0006   ZX=     0.0000           
  XY=     0.0001   YY=   367.0601   ZY=     0.0000           
  XZ=     0.0000   YZ=    -0.0000   ZZ=   380.4225   
  Eigenvalues:   367.0601   380.4225   390.5132

Those results were obtained using Gaussian16 on the B3LYP/def2-TZVP level and the GIAO method. The structures were optimized on the same level.

Improve this question
$\endgroup$
2
  • 1
    $\begingroup$ +1 but why not show us the output file and experimental reference? $\endgroup$
    – Nike Dattani
    Dec 21, 2022 at 1:30
  • $\begingroup$ I have edited my question providing an excerpt from my output and my references $\endgroup$
    – C_Swann22
    Dec 21, 2022 at 2:07

1 Answer 1

Reset to default
7
$\begingroup$

def2-TZVP employs an effective core potential on Te, which describes the innermost 28 electrons only implicitly; this is almost certainly the cause of the problem. I am surprised the program even prints out shieldings for ECP containing centers!

You will probably need to switch to an all-electron approach to be able to describe NMR spectroscopies on Te compounds. I do not know how important relativistic effects are, but this should be discussed in the papers you linked. If non-relativistic calculations are okay, then you just need to switch to an all-electron basis set for Te. On a quick glance at the periodic table and the results of the paper, relativity should be pretty important for NMR on Te.

The two papers by Rusakova and coworkers you referenced above employed the DIRAC program for the relativistic calculations. DIRAC is nowadays open source, and is free to download and install on any system. As far as I know, DIRAC has the best selection of relativistic methods, and my suggestion would be to look into how to install DIRAC and perform the modeling with it. The program you appear to be using does have some support for scalar relativistic calculations with the Douglas-Kroll-Hess (DKH) Hamiltonian, but I don't know if it can do NMR in combination with DKH.

Improve this answer
$\endgroup$

You must log in to answer this question.

Not the answer you're looking for? Browse other questions tagged .