Abstract
The observation of signals in solution NMR requires nuclei with sufficiently large transverse relaxation times (T(2)). Otherwise, broad signals embedded in the baseline afford an invisible fraction of nuclei (IF). Based on the STD (saturation transfer difference) sequence, IF-STD is presented as a quick tool to unveil IF in the (1)H NMR spectra of polymers. The saturation of a polymer in a region of the NMR spectrum with IF (very short (1)H T(2)) results in an efficient propagation of the magnetization by spin diffusion through the network of protons to a visible-invisible interphase with larger (1)H T(2) (STD(on)). Subtracting this spectrum from one recorded without saturation (STD(off)) produces a difference spectrum (STD(off-on)), with the nuclei at the visible-invisible interphase, that confirms the presence of an IF. Analysis of a wide collection of polymers by IF-STD reveals IF more common than previously thought, with relevant IF figures when STD > 0.4% at 750 MHz. A fundamental property of the IF-STD experiment is that the signal is generated within a single state comprising polymer domains with different dynamics, as opposed to several states in exchange with different degrees of aggregation. Contrary to a reductionist visible-invisible dichotomy, our results confirm a continuous distribution of nuclei with diverse dynamics. Since nuclei observed (edited) by IF-STD at the visible-invisible interphase are in close spatial proximity to the IF (tunable with the saturation time), they emerge as a privileged platform from which gaining an insight into the IF itself.