Enthalpies and entropies of dimerization have been determined as functions of concentration for dibutyltin dipropoxide, dibutyltin dibutoxide, and dibutyltin diisopropoxide in toluene-d 8 and cyclohexane-d 12 solutions from the variation in 119 Sn NMR chemical shifts with temperature. The values of ΔH and ΔS obtained in toluene-d 8 for dibutyltin dibutoxide were -69.5 ( 3.0 kJ mol -1 and -197 ( 10 J mol -1 K -1 , respectively, and for dibutyltin diisopropoxide -67.1 ( 1.4 kJ mol -1 and -244 ( 9 J mol -1 K -1 , respectively. Enthalpies and entropies of activation for this process for dibutyltin diisopropoxide have been derived by simulation of the temperature-dependent broadening of the 119 Sn NMR spectra. The free energy of activation for dimerization was 33 kJ mol -1 . The same thermodynamic parameters for a greater range of dibutyltin dialkoxides were derived theoretically by using MP2 single-point calculations on B3LYP-optimized geometries using the LANL2DZdp basis set with diffuse and polarization functions and its effective core potential for tin and 6-311G(2d,p)// 6-31G(d,p) for other atoms. Quite good agreement with the experimental results was achieved with this level of theory. The present study confirms that steric effects dictate the degree of dimerization for dibutyltin dialkoxides, with monomers becoming favored at room temperature when the alkoxide groups are changed from primary to secondary.

The regio-isomeric distribution of the omega-3 polyunsaturated fatty acids (PUFA) 8,11,14,7,10,13,16, in the triacylglycerols (TAG) of anchovy/sardine fish oil was determined by 13 C nuclear magnetic resonance (NMR) analysis under quantitative conditions. From the measurements of sn-1,3 and sn-2 carbonyl peak areas it was established that EPA was mainly located in the sn-1,3 positions, whereas DHA primarily occupied the sn-2 position. Reconstituted TAG prepared by Candida antarctica lipase-B (CALB) glycerolysis of the ethyl ester (EE) or the free fatty acid (FFA) forms of anchovy/sardine fish oil, displayed a different pattern: EPA was equally distributed, while DHA was preferentially attached to the sn-1,3 positions. TAG concentrates of varying EPA and DHA molar fractions were prepared by CALB-catalyzed glycerolysis of the corresponding EE and FFA. 13 C-NMR analysis of the purified products revealed a lack of CALB regioselectivity for EPA and a slight sn-1,3 regioselectivity for DHA. Since this pattern was observed in all cases of this study, it was concluded that the lipase regioselectivity during TAG synthesis is independent of both the acyl donor type (carboxylic acid or ester) and the fatty acid content of the oil substrate.

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