Assessing weak intramolecular donor-acceptor interactions using 1H-117Sn J-HMQC spectroscopy.
It is demonstrated that long-range nJ(1H,117Sn) coupling constants down to 0.3 Hz, can be accurately quantified from non-linear fitting of the sine modulation of the associated 1H-117Sn correlation cross-peak intensities sampled as a function of the heteronuclear antiphase coherence preparation time in the 1H-117Sn J-HMQC pulse sequence. The contribution of additional, undesired modulations is illustrated and assessed using the product operator formalism, and is traced back to contributions that arise from miss-setting of the wandering 180 degrees pulse angle in the constant time period. The power of the method and its use in the characterization of weak intramolecular donor-acceptor interactions are illustrated by the determination of long-range nJ(1H,117Sn) coupling constants of bis[3-(dimethylamino)propyl]tin derivatives, [Me2N(CH2)3]2SnR2 (', R = Me; 3, R = Ph; 4, R = t-Bu). By comparing these with the values found for the corresponding bis(4-methylpentyl)tin derivatives, [Me2CH(CH2)3]2SnR2 (2', R = Me; 3', R = Ph), which lack such interactions, the use of long-range coupling constants to detect intramolecular donor-acceptor interactions is evaluated. It is concluded that nJ(1H,117Sn) couplings up to six bonds through an organic carbon chain can be quantified, whether donor-acceptor interactions are present or not. Furthermore, evidence is presented that, when two scalar coupling pathways co-exist, the pathway involving an intramolecular donor-acceptor interaction can have opposite sign, thus decreasing the overall coupling constant to a value smaller than that actually measured in the absence of a donor-acceptor interaction, where only one coupling pathway is active. There is nevertheless clear numerical value discrimination in the series of compounds investigated between long-range couplings in derivatives without weak intramolecular donor-acceptor interactions and those where such interactions can exist.
High Resolution NMR Centre, Vrije Universiteit Brussel, Pleinlaan 2, B-1050 Brussels, Belgium. mbiesema@vub.ac.be
Monique
M
Biesemans
José C
JC
Martins
Klaus
K
Jurkschat
Nicole
N
Pieper
Stefan
S
Seemeyer
Rudolph
R
Willem
D000465:Algorithms; D001665:Binding Sites; D003198:Computer Simulation; D007554:Isotopes; D009682:Magnetic Resonance Spectroscopy / Q000379:methods*; D008956:Models, Chemical; D008958:Models, Molecular; D008968:Molecular Conformation; D019906:Nuclear Magnetic Resonance, Biomolecular / Q000379:methods*; D010084:Oxidation-Reduction; D011522:Protons; D014001:Tin / Q000032:analysis / Q000737:chemistry*; D017971:Tin Compounds / Q000032:analysis / Q000737:chemistry*
D007554:Isotopes; D011522:Protons; D017971:Tin Compounds; D014001:Tin