We combined the PIETA (J. H. Baltisberger 2012) method for measuring J-couplings faster than traditional J-resolved (W.P. Aue 1976) experiments with INEPT (G.A. Morris 1979) to give significant enhancement of sensitivity and reduction in experimental time to study systems of linear siloxanes.
One example of the kinds of materials we looked at is a polysiloxane with three kinds of building blocks, specifically terminal M groups, (Me)3SiO1/2–, linear D groups, –O1/2Si(Me)2O1/2–, and linear DH groups, –O1/2Si(Me)2O1/2– which can be represented as M–DHx–Dy–M. These D and DH groups are randomly distributed along the chain producing many local microstructures. The 29Si NMR spectrum for this sort of molecule gives rise to peaks near 9 ppm, a group near –22 ppm and another near –35 ppm. Assigning the peaks near +9.0 ppm to the M species and an area of 2.00, we are provided with the relative amount of D (–22.0 ppm, y = 6.86 area) and DH (–34.0 ppm, x = 9.56 area). Analysis frequently breaks the D units into three “triads” assuming D–D–D, DH–D–D, and DH–D–DH. Statistical microstructure analysis supports this sort of assumption, but it is difficult to really go much farther into breaking up what is more than twenty each of D and DH peaks.
One way to help analyze this would be to use 2D-INADEQUATE which might provide the connectivities in a second double-quantum dimension. This is difficult though since the J-coupling constants for 2JSi–O–Si are around 1.3 Hz (M–D) and 2.6 Hz (D–D), meaning the anti-phase doublets in the standard experiment will mostly cancel out due to the J-coupling approaching the observed linewidth. Secondly, given that the J-couplings are so different, it is hard to see both M–D and D–D peaks in the same spectrum. Finally, the sensitivity of 2D-INADEQUATE is challenging given the long relaxation time of 29Si and relatively low abundance of 29Si–29Si pairs. We have thus exploited the 2D in 1D nature of the PIETA experiment to rapidly get a J-resolved spectrum that allows us to see in a second dimension the actual connections between sites. We have added the INEPT transfer before the PIETA to give us the faster relaxation time and signal enhancement from the 1H in the –CH3 sites attached to each Si. We also have modified the phase cycling from the original PIETA so that data may be collected without an extra phase dimension. Future work includes adding a double-quantum filter to the PIETA and performing some INADEQUATE-PIETA/CPMG experiments under development.