2230
J . Phys. Chem. 1989, 93. 2230-2236
and the I17,2/95,1/5,1 bases of the IGLO paper, while the qso values reported by Tossell and L a ~ z e r e t t i ~for - ~ the CHFPT method were rather close to but sometimes larger and sometimes smaller than those of the 117,2/95,1/5,1 basis favored for the IGLO method. Since Kutzelnigg et a1.I' used a large basis set for intercomparing compounds and also employed gauge-invariant orbitals, the IGLO work is the best tested and will be used here as a comparison standard. For the compounds of Table I, the values of Adim = u,(GIAO method) - u,,(IGLO method), using the favored basis set for each, are found to be 40 and 38 for Si,H2,+2 with n = 1, 2 and 41, 38, 42 and 48 for SiH,_,F, with n = I , 2, 3, 4. The average of these six Auisovalues is 41 f 4. On the other hand, Auiso = uiso(CHFPT method) - ui,(IGLO method), each with its favored basis set, equals 2, 12, 22,40, 55, and 48. The average of these six Au, values is 30 f 21. Obviously our GIAO computations are quite uniformly consistent with the IGLO computations and changing the choice of shift reference standard will not lead to large differences between the two. The opposite is true for the comparison between the CHFPT and the IGLO methods. As would be expected from this analysis, our GIAO and the IGLO computed chemical shifts both agree rather well with the experimental data for these test compounds, as shown in Table I, while the CHFPT method does not. It is interesting
to note that a theoretical paper28predicted the superiority of the gauge-invariant method in 1977. This paper has shown that the kind of with-d calculation we have employed in this paper should be useful in predicting experimental 29Sichemical shifts of new and unknown compounds, with the best results obtained when the shift is measured with respect to a related molecular structure. In referencing an unknown to a related known molecule, obtain this chemical shift from the difference in the calculated isotropic total shieldings and then convert this shift to the TMS standard by using a table16J7 that contains the shift of the related molecule with respect to the usual reference standard. Registry No. H3SiSiH3, 1590-87-0; SiH3F, 13537-33-2;SiH2F2, 13824-36-7; SiHF3, 13465-71-9;SiF4, 7783-61-1;SiFt-, 17084-08-1; SO:-, 17 18 1-37-2: Si(CH3),, 75-76-3; SiH(CH3)3,993-07-7;SiH2(CH3)*, 1 I 1 1-74-6; SiH3(CH3),992-94-9; SiH4, 7803-62-5; SiH3C1, 13465-78-6;SiH3NS0, 57251-86-2;SiH3CF3,10112-1 1-5; H3SiCH2SiH3, 1759-88-2; H3SiNHSiH3,5702-11-4; H3SiOSiH3,13597-73-4; SiH
