Relative signs of ²⁹Si¹³C couplings

Abstract

Two pulse sequences applicable to the determination of relative signs of coupling constants, gHSQC-RELAY(P) and gHSQC-RELAY(D), were developed and tested. These sequences are suitable for determination of relative signs of long-range coupling constants (<2 Hz) between two heteronuclei of low abundance (such as (29)Si and (13)C), and are applicable even to cases in which one of the heteronuclei ((29)Si) does not exhibit coupling with some of the detected protons ((1)H). The two sequences differ in the manner in which they suppress undesirable homonuclear coherence transfers. Each of the sequences can be combined with an isotope filter for better suppression of the centerlines arising from more abundant NMR-inactive isotopes. The sequences were tested on ethoxytrimethylsilane and (E)-(buta-1,3-dienyloxy)trimethylsilane, and we conclude that (2)J((29)Si-O-(13)C) is positive while (3)J((29)Si-O-C-(13)C) is negative in both compounds.

29Si13C spin–spin couplings over an SiOCsp3 link

“…In the meantime we have developed methods by which relative signs of such couplings can be determined even in compounds with an extensive proton coupling network, provided that there is at least one proton in the OR fragment that is coupled to the 29-silicon nucleus in question [21] (which is not the case of the silylated phenols studied earlier and compounds of the type (CH 3 ) 3 Si-O-CR 3 studied here). Using 1 J( 13 C-1 H) (>0) as the 'key coupling constant', [34] application of the method to (CH 3 ) 3 Si-O-CH 2 CH 3 [21] and (CH 3 ) 3 Si-O-CH(CH 3 ) 2 (studied here) resulted in determination of a negative sign for 3 J( 29 Si-O-C-13 C) couplings and a positive sign for 2 J( 29 Si-O-13 C) couplings. The sign of 2 J( 29 Si-O-13 C) was also confirmed as positive on the basis of measurements of (CH 3 ) 3 Si-O-CH(C 6 H 5 ) 2 (the sign of 3 J coupling could not be determined because of the absence of a proton on the β-carbon of this compound).…”

“…Since these signs are, to the best of our knowledge, not experimentally accessible in silylated phenols, we have chosen to study other types of compounds [Me 3 Si-O-(CH 2 ) n CH 3 (series I, n = 0-3) and Me 3 Si-O-CH 3−n R n (R = Me, n = 0-3, series II; R = Ph, n = 0-3, series III; R = Vi, n = 0-2, series IV)] which allow sign determinations for at least some of the members of the series by one of the recently described methods. [21] Three other compounds with closely related structures are also included in the study: Me 3 Si-O-CH(CH 3 )CH 2 CH 3 , Me 3 Si-O-CH 2 C(CH 3 ) 3 , and Me 3 Si-O-CH 2 C CH. The experimental results as well as the results of theoretical calculations on these compounds are presented here.…”

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

“…The signs of these couplings cannot be detected either by the usual E.COSY or TROSY type correlation experiments or by gHSQC-RELAY (P) or (D) experiments [27] , because there is no coupling connection between the silicon and any aromatic proton. The sequence was first tested on known couplings and signs of 1 (2)) is −1.7 Hz.…”

“…Of the many 29 Si- 13 3 J(Si,C(β)) couplings may be detected using gHSQC-RELAY (P) or gHSQC-RELAY (D) pulse sequences [27] , but only under assumption that both C(α) and C(β) have directly attached hydrogen(s). If C(α) bears no hydrogen atom (m = 0), none of the silicon-carbon couplings can be detected by this method.…”

SQSQh: 1H‐detected SQ–SQ experiment for determination of signed silicon–carbon coupling constants

“…)), n > 2, cannot be detected in this way. It was shown [7] that adding homonuclear polarization transfer H(α) → H(β,γ ,. .…”