Modelling of the alkyl environment effects on the ¹³C chemical shift

Abstract

Using a topological parameter, A,, to characterize the effects of a variable mono-or polyalkyl environment, R, on widely varied types of "C sites gives rise to a homogeneous description of their chemical shifts over the entire range of 613C variation (220 ppm/TMS). The proposed model, 613C= w,A,+q, expresses the chemical shift in terms of this parameter together with the sensitivity, o , , of the perturbed 13C site, bonded to a constant environment in a given family, and with qc as the reference shift. In the case of polyalkyl environments, R=Zi=, Ri (nc4); since the use of A, presupposes that groups R, coexist in R without interacting, it is expressed as the sum of the intrinsic effects of each of these groups. Thus, this model demonstrates the general character of most of the common alkyl environment effects and the specific behaviour of the various types of 13C atoms. The homogeneity of this description relies on an assessment of the Ri contribution by the DARC/PELCO topological analysis of alkyl sites in a series of reference alkynes. The 613C values calculated for 138 compounds, belonging to the alkane, alkene, allene, alcohol, amine, epoxide, ester and ketone families, are highly satisfactory (F=0.979, S=0.98).

Carbon-13 nuclear magnetic resonance spectral interpretation by a computerized substituent chemical shift method

“…25 Dubois has used a topological parameter to model the alkyl environment. 26 Levy and Nelson27 and Ejchart28,29 have proposed substitution methods whereby the 13C shifts are first estimated for the hydrocarbons, and heteroatoms are substituted later. Although these rules have varying accuracy, they serve as good starting points for spectral interpretation, especially when simple analogues cannot be located in the spectral libraries.…”

A set of alkyl substituent constants for correlating chemical shifts of a variety of nuclei

“…Number of points used in correlations. The alkyl groups used in the correlations are given in parentheses after the series number (numbering of alkyl groups corresponds to that in Table 1); 1 (1)(2)(3)(4)(5)(6)(7)(8)(9)(10)(11)(12)(13)(14)(15)(16)(18)(19)(20); 2 (for R = 1,l-9,16,18,19; for R = 2,2, 3,5,9,16,18; for R = 3,9,16; for R = 5,5,9); 3 (for R = R' = 1,l-3,6-8,10,11,13,14 The points for R = i-Bu and s-Bu deviate considerably from the correlation line and are not included. The reason for the deviation is not certain.…”

“…for R = R' = 2,2-4; for R = 1, R' = 2,Z-4,6-8,ll; for R = 1, R' = 4,3,4,6,8); 4 (1-4,6,8); 5 (for R = 1,l-4,6,8,11; for R = 2,Z-4,7,8; for R = 4,4,7,8); 6(1)(2)(3)(4)6,8); 7 (for R = 1,l-4,6-8,11,21; for R = 2,2,4,7,8; for R = 4.8,4); 8(1)(2)(3)7,9,13,23-28,321; 9 (1-4,7-9,11-13,26,32); 10(1,2,4,8,9,29-31); 11-12 (1-8,10, 11,13); 13-14 (1-6); 15-16 (for R = R' = 1,1-8,10,13; for R = 1, R' = R"= 2-6; for series 16 an additional substituent is R = R' = 1, R"= 11); 17 (1-7,9); 18 (1-4,8,11); 19 (1-4,6,8,11,13); 20 (R = R ' = 1-5; for R = 1,2,5,8; for R =4,8); 21 (1-5'7); 22 (1,2,4,7); 23 (1,2,4,5,8); 24 (1-5,8,9,16); 25 (for R = 1,l-12; 17; for R = 2,2-9,11,12; for R = 3,3,4; for R = 4,4,8; R = R' = 5,7,8,10); 26 (1-1 3,15,17,22,33,34); 27 (1-5,8,12); 28 (for R = 1,R'= R"=2,3,9,16; for R = 2 , R ' = R'=2,3,4,9,16); 29(for R = R ' = R"= 1,2,3,5,9,18; for R = R ' = R"= 2,1,2,5,7; R = R ' = ~"~3 , R'"= 2; R = R ' = 1, R"= R"= 2); 30 (R = R ' = R"= 2,3,5,6,9,16,19; R = 1, R' = R" = 3,5,9,16; R = 2, R' = R'= 3,4,6,16); 31 (R = R' = R" = R"= 1-3,5; for R=R'=R"=2,1,3,5; for R=R'=R"=3,2,5; for R=R'=R"=4,2); 32(1-6); 33(1-7); 34(1-8,11,13); 35(1-4,8); 36-38(1,2,4,8); 39(for R = l , 1,2,8; R=R'=2,4); 40(1-6); 41 (1,2,4); 42(1-5). 24(14); 25(15); 26(16); 27(17); 28-30(15); 31 (18); 32-34(19); 35(17); 36-39(20); 40(21); 41 (22); Correlation coefficient.…”

Elucidation by progressive intersection of ordered substructures from carbon-13 nuclear magnetic resonance

“…The principle of structural elucidation by EPIOS relies on continuous analysis of structure/13C relationships. Originally devoted to accurate modeling of the behavior of various 13C atoms (20), this analysis has guided the construction of the EKB (17). The rules within this knowledge base are thus derived from the correlations that associate the chemical shifts of connected carbon atoms.…”