1H, 13C, and 73Ge NMR spectral analysis of substituted aryltrimethylgermanes

Adam J. Fleisher, Elizabethtown College
Charles D. Schaeffer, Elizabethtown College
Beth A. Buckwalter, Franklin and Marshall College, Lancaster
Claude H. Yoder, Franklin and Marshall College, Lancaster

Abstract

NMR chemical shifts of H, C, and Ge are reported for a series of monosubstituted aromatic trimethylgermanes of the type XC H Ge(CH ) ; X = p-N(CH ) , p-OCH , p-OC H , p-C(CH ) , p-Si(CH ) , p-Ge(CH ) , p-Sn(CH ) , p-CH , m-CH , -H, m-OCH , p-Cl, p-Br, m-F, m-CF , p-CF , o-OCH , and o-CH . The relatively narrow Ge resonances show a strong correlation with Hammett sigma constants, with a correlation coefficient of 0.976 and 0.876 for Ge chemical shifts in meta- and para-substituted derivatives, respectively. The C 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 XC H M(CH ) ; 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(CH ) group of +0.13 compares with the NMR-derived constants of -0.12 for p-C(CH ) , +0.14 for p-Si(CH ) , and -0.14 for p-Sn(CH ) . 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 π-acceptor interactions with the benzene ring. Copyright © 2005 John Wiley & Sons, Ltd. 1 13 73 73 73 13 6 4 3 3 3 2 3 2 5 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 6 4 3 3 3 3 3 3 3 3 3 3