Rotamers coupling constants

In this expression V x, V and V3 are partition functions for torsional vibration which can be temperature-dependent, 81 S2 and Ss are chemical shifts of the given proton in the three separate rotamers, Ex, Ez and E3 are the energies of the potential maxima from the zero point torsional levels in each separate rotamer. Coupling constants are averaged in a similar way by replacing S in equation (77) by J in order to obtain J averaging. 

Theoretical coupling constants for individual rotamers were obtained directly from MacroModel employing an empirical generalization of the Karplus Equation ( ). Calculated average couplings came from the expression ... 

In following the temperature dependence of AG°, Booth et al. [85JC-S(CC)467 87T4699]also determined A/f° and AS° for the conformational equilibria of 2-C1-, 2-OMe-, 2-OH-, and 2-NHMe-oxanes (see Table II) and discussed the results in terms of exo- or endo-anomeric effects (Section III,C,8). Employing the NOE and a number of H,H- and C,H-coupling constants as a means of analysis, the preferred rotamers of axial/equatorial-2-OMe-oxane were found to be in the conformations az and 2, respectively, as given in Scheme 1 (90T1525). 

M. Kraszni, Z. Szakacs, and B. Noszal, Determination of rotamer populations and related parameters from NMR coupling constants A critical review, Anal. Bioanal. Chem., 378 (2004) 1449-1463. 

Provided that the mutual size differences between H, M and L are significant, it is rather easy to predict the populations of the rotamers. Among the an/i-rotamers, anti-1, with antiperipla-nar hydrogens, is the only one devoid of the L1... L2 interaction. In the syn series, syn-2, with the hydrogens gauche, lacks the L1... L2 interaction. Therefore, it is expected that anti-1 and syn-2 dominate in their respective equilibrium, as would the respective coupling constants ... 

Using a similar basis, acyclic y-chloropropyl methyl ethers were discussed383. Again, steric 1,3-parallel interactions are invoked which reduce the number of conceivable rotamers vicinal H/H coupling constants are used as independent tools for configurational assignments383. 

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