²⁹Si–¹³C spin–spin couplings over Si–O–C_arom link

Abstract

29Si-13C couplings were measured in para substituted silylated phenols, X--C6H4--O--SiR1R2R3 (X = NO2, CF3, Cl, F, H, CH3, CH3O). The SiR1R2R3 silyl groups included trimethylsilyl (Si(CH3)3, TMS), tert-butyldimethylsilyl (Si(CH3)2C(CH3)3, TBDMS), dimethylsilyl (SiH(CH3)2, DMS), and tert- butyldiphenylsilyl (Si(C6H5)2C(CH3)3, TBDPS). Previously developed (Si,C,Si)gHMQC methods and narrow 29Si lines allowed the determination of coupling constants over up to five bonds. Besides the number of intervening bonds between the silicon and carbon atoms, all the measurable couplings depend also on the nature of the substituents on the silicon. The two- and three-bond couplings are not affected by ring substitution in the para position. These properties render the 29Si-13C couplings suitable for line assignment in the spectra of silylated polyphenols. The experimental results are in reasonable agreement with theoretical calculations. The calculations show, in agreement with the data reported in the literature for couplings between other nuclei, that the two-bond and three-bond couplings, which are of similar magnitudes, are of opposite signs. If the signs of these geminal and vicinal couplings could be determined experimentally, they would greatly facilitate the line assignment. The four- and five-bond couplings are affected by the substituent X in a nontrivial manner.

Relative signs of 29Si13C couplings

“…Also, some long-range couplings that have similar magnitude differ in sign, as demonstrated by 29 Si-NMR analysis of 2 J( 29 Si-O-13 C) and 3 J( 29 Si-O-C- 13 C) in trimethylsiloxybenzenes. [1] In general, the signs of very few 29 Si long-range couplings have been determined; practically all the information available is the work of McFarlane (e.g. Refs [2][3][4]) and his former associates, most notably Wrackmeyer.…”

Improved pulse sequences for measurement of small long‐range couplings between heteronuclei (29Si13C) at natural abundance

“…3 (1D spectra) 2 SiMe 2 , and attenuation losses are only due to one-bond 13 C-1 H couplings, the improvements brought by the modified sequence are small (Fig. 3).…”

“…1(a) and 2(a)), were developed [1] for measurement of precise absolute values of small silicon-carbon coupling constants (with typical magnitudes of approximately 2 Hz for couplings through two or more bonds) at natural abundances of all isotopes. These sequences [1] were employed for measurements of Si-C couplings in series of alkoxy- [2] and aryloxysilanes. [3] The sequences detect either carbon (INEPT-(Si,C)gCOSY) or silicon (INEPT-(Si,C,Si)gHMQC) coherences.…”

SQ—SQ experiment for determination of relative signs of small nJ(29Si,13C) couplings in a wide variety of silicon compounds

“…[34] Silicon-carbon couplings over more than three bonds were not measured because their magnitudes are too small, usually below 0.5 Hz. [33] …”