Applications and evaluation of IGAIM ¹³C and ¹H chemical shift calculations for unsaturated hydrocarbons and organolithium compounds

Abstract

(4)), carbanions (prop-2-en-1-yl (allyl) (S), bicyclo[3.2.1]octa-3,6-dien-2-yl (a)), and lithium complexes (prop-2-en-lyllithium (6) and its dimer 7, bicyclo[3.2.1]octa-3,6-dien-2-yllithium (9)). The theoretical isotropic I3C NMR chemical shiftsof the neutral molecules, relative to the calculated value for TMS(tetramethylsilane), are in excellent agreement with the experimental values, with differences between the sets of data ranging from +4.9 to -7.1 ppm. For the same group of compounds the theoretical 'H shifts are lower than the experimental values by increments ranging between 0.4 and 1.29 ppm. For allyllithium, which exists as an unsymmetrical fluxional dimer, the theoretical averaged I3C shifts are larger, 2.6 pprn for the terminal carbons and 16.7 pprn for the central carbon, than the experimental values. In the case of 8, originally considered to be a bishomoaromatic species, the theoretical I3c chemical shifts of its Li' complex 9 differ from the experimental ones for THFsolvated 9 by values that range from +6.2 to -15.0 ppm. Yet, the relative theoretical chemical shifts -of special importance is the fact that the carbons of the vinylene bridge of this compound are unusually shielded relative to the parent diene 1 -correlate with the experimental data. The 'H chemical shifts calculated for the hydrocarbons 1, 2,3,4 and the lithium complexes 7 and 9 range from 0.08 to 1.38 pprn less than the experimental values. To gain information on whether variations in charge density play a significant role in determining the magnitudes of the chemical shifts, we used AIMPAC calculations to obtain the atom electron populations of diene 1,5,6, dimer 7,8, and 9. We find no obvious correlation between the charges on the carbon atoms and the (9)). Les dtplacements chimiques thCoriques isotropes en RMN du 13c des moltcules neutres, relatifs aux valeurs calculCes pour le TMS (tCtramCthylsilane), sont en bon accord avec les valeurs expCrimentales; les difftrences entre les ensembles de donnCes varient de +4,9 B -7,I ppm. Pour le m&me groupe de composCs, les dkplacements thtoriques des 'H sont plus faibles que les valeurs experimentales par des incrkments variant de 0,4 B 1,29 ppm. Pour l'allylithium qui existe sous la forme d'un dimkre en fluxion asymCtrique, les dkplacements thCoriques moyens du I3c sont plus ClevCs (2,6 pprn pour les carbones terminaux et 16,7 pprn pour l'atome de carbone central) que les valeurs exptrimentales. Dans le cas du produit 8, qui avait originalement Ctt considCrC comme une espkce bishomoaromatique, les dCplacements chimiques thCoriques du 13C de son complexe avec le lithium, 9, different des valeurs expCrimentales pour le compost 9 solvat6 par du THF par des valeurs allant de +6,2 B -15,O ppm. MalgrC tout, les dtplacements chimiques thCoriques relatifs -ce qui prCsente le plus d'importance est le fait que les carbones du pont vinylkne de ce composC sont inhabituellement blind& par rapport au dikne parent, 1 -prCsentent une corrklation avec les donntes expCrimentales. Les dtplacements chimiq...

Influence of the basis set on the calculation of the absolute 13C shieldings of methyl derivatives (CH3X with X = CH3, CN, NH2, NO2, OH, F) with special emphasis in the cases of X = Cl, Br, SH, SeH, and PH2

“…Three experimental absolute shieldings of CH 3 X compounds were summarized previously [31]. CH 4 195.1 ppm [58,63] (this value has been often used as reference in theoretical papers [64][65][66][67]), CH 3 NO 2 126.9 ppm [59] (see also [68]), and TMS 188.1 ppm [58] (see also [64,[69][70][71][72][73][74][75][76]), the method that best reproduce these three values is MPW1PW91/6-311? ?G(d,p): 193.64, 123.68, and 188.33 ppm, respectively (see Table 1).…”

Regioselectivity of the Reactions of Heteroatom-Stabilized Allyl Anions with Electrophiles

“…The rotational barrier for the allyl anion , was predicted to be larger than the 10.7 kcal/mol observed for allyllithium. Additional studies of the positions of the hydrogen atoms in allyllithium have been reported 430f. A comparative ab initio study by Clark, Rhode, and Schleyer reveals that allyllithium and -sodium prefer symmetrically bridged structures.…”

“…In 1976 Palmieri et al430c estimated the energy difference between the most stable bridged and the syn structure of allyllithium to be about 8 kcal/mol by means of RHF/STO-3G ab initio calculations, obviously not performing full geometry optimization. Clark, Jemmis, and Schleyer 430d confirmed these qualitative results by choosing better basis sets (RHF/STO-3G for the geometry optimizations, RHF/4-31G and RHF/6-31G*, single point calculations to estimate relative energies).…”

“…Allylmagnesium hydride should exist with an asymmetric geometry, the barrier of the 1,3 MgH shift should be quite low . More recently, these investigations have been continued for the allyl alkali metals (Li to Cs) 430j…”

“…The application of the IGAIM method by Keith and Bader 441 for the 13 C and 1 H chemical shift calculations at the 6-31+G* level of theory are summarized by Westiuk and Ma 430k. The authors conclude that IGAIM will generally be useful to calculate shifts of unsaturated organolithium compounds.…”