Fleisher Adam J, Schaeffer Charles D, Buckwalter Beth A, Yoder Claude H
Chemistry Department, Elizabethtown College, Elizabethtown, PA 17022-2298, USA.
Magn Reson Chem. 2006 Feb;44(2):191-4. doi: 10.1002/mrc.1746.
NMR chemical shifts of 1H, 13C, and 73Ge are reported for a series of monosubstituted aromatic trimethylgermanes of the type XC6H4Ge(CH3)3; X = p-N(CH3)2, p-OCH3, p-OC2H5, p-C(CH3)3, p-Si(CH3)3, p-Ge(CH3)3, p-Sn(CH3)3, p-CH3, m-CH3, -H, m-OCH3, p-Cl, p-Br, m-F, m-CF3, p-CF3, o-OCH3, and o-CH3. The relatively narrow 73Ge resonances show a strong correlation with Hammett sigma constants, with a correlation coefficient of 0.976 and 0.876 for 73Ge chemical shifts in meta- and para-substituted derivatives, respectively. The 13C chemical shifts of the methyl carbons bonded to germanium also display a relationship, with correlation coefficients of 0.904, 0.993, and 0.911 for para-, meta- and all derivatives, respectively. Comparisons of the Hammett plots for the homologous series XC6H4M(CH3)3; M = C, Si, Ge, Sn, show that, in general, correlation coefficients decrease while slopes increase significantly down the group, presumably reflecting the corresponding increase in chemical shift range of the group 14 atom. The Hammett constant derived for the p-Ge(CH3)3 group of +0.13 compares with the NMR-derived constants of -0.12 for p-C(CH3)3, +0.14 for p-Si(CH3)3, and -0.14 for p-Sn(CH3)3. The indication of electron release by carbon and tin can be rationalized through traditional hyperconjugative arguments for carbon and by the low electronegativity and consequent inductive effect of tin. The small electron attraction suggested by the positive constants for silicon and germanium can be simply, and perhaps naively, attributed to pi-acceptor interactions with the benzene ring.
报道了一系列XC6H4Ge(CH3)3型单取代芳基三甲基锗的1H、13C和73Ge的核磁共振化学位移；X = 对-N(CH3)2、对-OCH3、对-OC2H5、对-C(CH3)3、对-Si(CH3)3、对-Ge(CH3)3、对-Sn(CH3)3、对-CH3、间-CH3、-H、间-OCH3、对-Cl、对-Br、间-F、间-CF3、对-CF3、邻-OCH3和邻-CH3。相对较窄的73Ge共振峰与哈米特σ常数有很强的相关性，间位和对位取代衍生物中73Ge化学位移的相关系数分别为0.976和0.876。与锗相连的甲基碳的13C化学位移也显示出一种关系，对位、间位和所有衍生物的相关系数分别为0.904、0.993和0.911。对同系物系列XC6H4M(CH3)3；M = C、Si、Ge、Sn的哈米特图进行比较表明，一般来说，随着该族元素的向下，相关系数降低而斜率显著增加，这大概反映了第14族原子化学位移范围的相应增加。对-Ge(CH3)3基团得出的哈米特常数为+0.13，与之相比，对-C(CH3)3的核磁共振得出的常数为-0.12，对-Si(CH3)3为+0.14，对-Sn(CH3)3为-0.14。碳和锡的电子释放迹象可以通过碳的传统超共轭论点以及锡的低电负性和随之而来的诱导效应来解释。硅和锗的正常数所表明的小电子吸引可以简单地（也许是天真地）归因于与苯环的π-受体相互作用。
