Quantitative adiabatic‐refocused INEPT (QA‐RINEPT) as a tool for fast and reliable characterization of polyols

Abstract

C nuclear magnetic resonance (NMR) is a powerful tool for the detailed characterization and structure elucidation of polymeric samples. The low natural abundance and sensitivity of the C isotope, however, lead to very long acquisition time, therefore limiting the use of such technique. We report here the implementation of a quantitative method, quantitative adiabatic-refocused insensitive nuclei enhanced by polarization transfer (QA-RINEPT), for the characterization of polyol samples. The method, based on the well-known insensitive nuclei enhanced by polarization transfer sequence, allows the boost in sensitivity of the carbon resonances without sacrificing the quantitative aspects of the analysis. This is achieved controlling the polarization transfer mechanism and introducing a response factor to calculate precisely the sensitivity gain of the different carbon signals. Compared with the standard single pulse quantitative experiment, the QA-RINEPT method produces up to 4.7× signals enhancement per unit of time. An in-depth statistical analysis was conducted to confirm the reliability of the QA-RINEPT method. We show that there is excellent agreement between the new and old C-NMR methods for the quantitative determination of several important polyol properties such as the comonomer and initiator content as well as the ratio of primary and secondary hydroxyl groups.

Combination of 13C and 2H SNIF‐NMR isotopic fingerprints of vanillin to control its precursors

“…It should be noted that other works have been published using such a sequence, but that the precision of the order of 1‰ was not addressed, so the capability to produce an isotopic 13 C profile is not known. [52][53][54] The drawback of the INEPT sequence is the absence of signals for the quaternary carbons, because they are not directly coupled to any proton. With the aim of optimizing the method by using the INEPT sequence, the information provided by the quaternary carbons, corresponding to sites 2, 3, and 4 of the vanillin molecule, is of importance.…”

“…a decrease of the acquisition time and/or the quantity of product required decreased to 30 mg for the analysis. It should be noted that other works have been published using such a sequence, but that the precision of the order of 1‰ was not addressed, so the capability to produce an isotopic 13 C profile is not known . The drawback of the INEPT sequence is the absence of signals for the quaternary carbons, because they are not directly coupled to any proton.…”

Sensitivity-Enhanced Spin Echo Nuclear Magnetic Resonance Pulse Sequence for Quantitative Polymer Characterizations with a Nuclear Magnetic Resonance Cryoprobe

“…The schematic drawing of the pulse sequence CPMG-INEPTRDG5 is shown in Figure (the pulse code in Bruker format is shown in the Supporting Information), and the green bullets represent the G5 pulses. The excitation bandwidth is shown in Figure , and the error is only 1% in a much wider 120 ppm range (Figure b), which is very important for analyzing polymers such as polyols as they need at least 65 ppm useful uniform excitation bandwidth …”

“…The excitation bandwidth is shown in Figure 6, and the error is only 1% in a much wider 120 ppm range (Figure 6b), which is very important for analyzing polymers such as polyols as they need at least 65 ppm useful uniform excitation bandwidth. 26 To demonstrate the sensitivity enhancement of the CPMG-INEPTRDG5 pulse sequence, a blend of "high melting" and "low melting" EO was analyzed with the CPMG-INEPTRDG5 pulse sequence (Figure 7). It should be noted that the sensitivity enhancement with INEPT is modulated by J CH coupling constant, delays before and after G5 pulses (shown in Figure 7), and the number of 1 H attached to 13 C, and theoretical equations are reported by Burum and Ernst.…”

“…To increase signal sensitivity, refocused insensitive nuclei enhancement by polarization transfer (INEPTRD/INEPTRDSP) , elements were added to the front of 90_h180 and 90_a180 spin echo pulse sequences . INEPTRD with 90_h180 was named INEPTRD-HSE (HSE, hard pulse spin echo; Figure S1); INEPTRDSP , with 90_a180 was called INEPTRDSP-SSE (SSE, shaped pulse spin echo; Figure S2) (the schematic drawings of the pulse sequences and codes in Bruker format are provided in the Supporting Information). Both new sequences were applied to a sample of EO in TCE- d 2 containing 0.025 M Cr­(acac) 3 .…”