The $^{113}Cd$ NMR spectroscopy has been used to probe the cadmium ion binding sites in humic substances with the aid of variation of the $^{113}Cd$ chemical shift that are highly sensitive to the donor atoms, coordination number and geometry. The elucidation of metal ions binding sites on humic substances is complicated by two factors, namely macromolecular nature (conformational change) and heterogeneity (polyfunctionality). Therefore, the contribution of each of these factors that may cause $^{113}Cd$ chemical shift changes was investigated separately using synthetic polymers and model ligands including carboxylates, hydroxycarboxylates, and carboxylate possessing the nitrogen moiety.
A variety of complexes of cadmium with mono- and di-carboxylic acids used as model ligands give a single averaged chemical shift even at reduced temperature, demonstrating that the cadmium-monocarboxylate complexes are undergoing rapid exchange in solution. Using the known stability constants, the individual chemical shifts were calculated and were found to be in the range -22 to -24ppm for $CdL^+$ and -39 to -40ppm for $CdL_2$ (L = carboxylate) species: Carboxylates with higher basicity tend to increase the shielding of the cadmium in the complexes. With dicarboxylic acid ligands $HO_2C(CH_2)_nCO_2H$ (n = 0-3), the $^{113}Cd$ nucleus showed a more shielding trend as n increased: Cd(oxalate) +11.0ppm < Cd(malonate) -19.1ppm < Cd(succinate) -35.3ppm~Cd(glutarate) -35.9ppm. Only oxalate and malonate seem to behave as chelate ligands to form ring with cadmium. However the steric effect seem to hinder chelate ring formation in dialkylsubstituted malonate complexes of cadmium.
The $^{113}Cd$ NMR studies were extended to single crystals of cadmium(II)-malonate complexes formed in aqueous solution. Polymeric tetraaquobis(malonato)dicadmium(II) 1 was formed initially. But two other complexes, namely triaquo(malonato)-cadmium(Ⅱ) monohydrate 9 and diaquo(malonato)cadmium(II) 13 were obtained from the original compound 1. The $^{113}Cd$ NMR spectrum of 1 is characterized by double resonances at 18 and -108ppm. Its crystal structure contains two cadmium sites in distinct chemical environments; the six-coordinate and eight-coordinate sites formed by oxygen donors are correlated with 18 and -107ppm respectively. The chemical shift value corresponding to the eight-coordinate site lies within the isotropic chemical shift range observed in the metalloprotein such as concanavalin A and parvalbumin. The $^{113}Cd$ chemical shift value of the compound 9 is -47ppm. Its crystal structure shows a seven-coordinate Cd site formed by oxygen donors. The $^{113}Cd$ NMR spectrum of the compound 13 shows double resonances of -47 and -53ppm. Because the compound 9 and 13 are formed from the original compound 1, and these compounds are interconvertable where the water molecules, temperature and light may play important roles.
The major species formed in the cadmium(Ⅱ)-PAA complexes is $CdL_2$ (1:2 complex) and becoming more dominant as the [PAA]/[cd(Ⅱ)] ratio are raised. However, in cases of PMAA and PVBA the $CdL^+$ species is the major species at high [polymer]/[Cd(II)] ratio and this component is progressively converted to $CdL_2$ species with decreasing the [polymer]/[Cd(II)] ratio. It is suggested that the difference of chemical shift change patterns may due to a cooperative conformational transition from a cylindrical form to an sphere form of PMAA and PVBA.
The chemical shift of cadmium bound on FA was observed in the upfield shift region compared to that of synthetic polymers. This upfield shift of cadmium ions bound on FA may be explained by 1) conformational change effects observed in cases of PMAA and PVBA and 2) polyfunctionality of FA. The chemical shift value of -25ppm obtained by Cd(II)-FA, -sHA and coHA precipitates is similar to those of $CdL^+$ (1:1 complex) and of Cd(II)-hydroxy carboxylate complexes suggesting that the cadmium ions may be mainly bound by one carboxylate or hydroxy carboxylate-like moieties.
카드뮴-113NMR 의 민감한 화학적 이동값의 특징을 이용하여 휴믹 물질의 카드뮴 이온에 대한 결합 자리의 특성을 살펴보았다. 휴믹 물질의 결합자리의 특징은 이들의 고분자적 성질 (구조 변화)과 다작용기적 성질 때문에 매우 복잡하다. 그러므로 이들 각각의 성질이 카드뮴의 화학적 이동값에 기여하는 정도를 합성 고분자와 카르복실산, 하이드록시카르복실산 및 질소 성분을 갖는 카르복실산을 모델 리간드로 사용하여 분리하여 연구하였다.
모델 리간드로 사용된 모노카르복실산과 디카르복실산의 카드뮴 착물은 수용액속에 존재하는 각각의 화학종이 빠른 평형상태에 있으며, 관측된 화학적이동값은 이들 화학종의 평균에 의해서 주어진다. 평균화학적 이동값과 평형상수값을 이용하여 계산된 개개 화학종의 화학적 이동값은 1:1 착물인 $CdL^+$에 대해서 -22에서 -24ppm 이고 1:2 착물인 $CdL_2$ 에 대해서는 -39에서 -40ppm 이었다: 리간드의 염기도가 클수록 카드뮴의 가림효과도 증가하였다. 디카르복실산 리간드 $HO_2C(CH_n)_2CO_2H$ 에서 n이 증가할수록 카드뮴의 가림 효과도 증가하였다: Cd(oxalate)+11.0ppm
