CCL:G: Gaussian/PCM/chemical shifts

 Sent to CCL by: david.giesen:_:kodak.com
 Grigoriy -
 You may find the data and discussion in the paper below to be useful.  It
 demonstrates that most QM methods (HF/DFT/MP2) even with small basis sets
 give NMR shifts that correlate very well with experiment.  Correction
 factors are given in the paper for many methods that should allow you to
 get good quantitative agreement with experiment.  Your chosen method
 (B3LYP/6-311G**) is unfortunately not included, but many similar methods
 are.
 Using the method in this paper, very fast B3LYP/MIDI! calculations give
 13C NMR shift results that agree as well or better with experiment than
 many more computationally expensive calculations.
 Giesen and Zumbulyadis, Phys. Chem. Chem. Phys., 2002, 4, 5498-5507
 Dave Giesen
 PS - Unfortunately, the labels on Table 2 didn't make it into the final
 copy - the columns indicate the geometry optimization method and the rows
 indicate the method used to calculate the chemical shift.
 "Grigoriy Zhurko reg_zhurko ~~ chemcraftprog.com"
 <owner-chemistry||ccl.net>
 Sent by: owner-chemistry||ccl.net
 06/04/2007 11:58 PM
 Please respond to
 "CCL Subscribers" <chemistry||ccl.net>
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 "Giesen, David J " <david.giesen||kodak.com>
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 Subject
 CCL: Gaussian/PCM/chemical shifts
 Sent to CCL by: Grigoriy Zhurko [reg_zhurko]![chemcraftprog.com]
   I suppose, using the calculated TMS carbon shielding is not correct
 enough, because the TMS standard used in the experiment is not in the gas
 phase. I tried to calculate the TMS shielding at same level of theory as
 investigated molecules, but their chemical shifts with it were still
 overestimated.
 > Dear Grigoriy Zhurko,
 > in [1] we discuss the importance of the TMS 13C reference values.
 > Please find a paragraph which might be relevant to your message
 > cited below:
 > [..]
 > Particular attention should be paid to the case of
 > TMS. For this reference compound in proton and
 > carbon-13 NMR, two structures of Td symmetry exist,
 > as illustrated in ¨gure 1. The one in ¨gure 1 (a) is the
 > global minimum, and the structure of ¨gure 1 (b) is a
 > fourth-order saddle point. The energy of the global
 > minimum (¨gure 1 (a)) , taken from the B3LYP calcula-
 > tion, is 27. 3 kJ molÁ1 lower than the one for structure
 > (b). The corresponding absolute shielding value for 13 C
 > is 200.3 ppm. We chose this value as the reference chemi-
 > cal shift as it corresponds to a true minimum structure.
 > Cheeseman et al. [49] report a 13 C chemical shielding of
 > 195.1 ppm for TMS, whereas our calculation for the
 > non-minimum structure of ¨gure 1 (b) reproduced a
 > value of 195.4 ppm.
 > [..]
 > There is a little more discussion in the paper.
 > Regards,
 > Marc Baaden
 > [1]  M. Baaden, P. Granger and A. Strich; "Dependence of isotropic
 >      shift averages and nuclear shielding tensors on the internal
 >      rotation of the functional group X about the C-X bond in seven
 >      simple vinylic derivatives H2C=CH-X.", Mol. Phys., 98, 2000,
 >      329-342.
 >>>> "Grigoriy Zhurko reg_zhurko(!)chemcraftprog.com"
 said:
  >>>
  >>> Sent to CCL by: Grigoriy Zhurko [reg_zhurko]*[chemcraftprog.com]
  >>>   This problem is interesting for me too. I have found that
 computed
 gas-ph
 >      ase 13C chemical shifts (GIAO B3LYP/6-311G(D,P)) correlate very
 well with
 >      the experiment (in CCl4), but the computed values of chemical
 shifts are s
 >      ignificantly overestimated. Maybe the problem is in the reference
 value fo
 >      r converting isotropic shielding into chemical shifts (e.g. with
 TMS). Wha
 >      t value of the TMS shielding on carbon should be used?
   Sincerely,
   Grigoriy Zhurko.http://www.ccl.net/cgi-bin/ccl/send_ccl_messagehttp-:-//www.ccl.net/chemistry/sub_unsub.shtmlhttp-:-//www.ccl.net/spammers.txt