Gel-phase NMR spectroscopy

Despite the success with gel-phase NMR spectroscopy the spectra obtained are of limited value because of the broad peaks, and indeed the NMR spectra of unlabelled samples need relatively long acquisition times (because of the low abundance of C) especially with lightly loaded resins such as TentaGel and Argo-Gel. NMR [206, 207] and NMR [208, 209] spectroscopy of gel beads have also been shown to be a convenient NMR technique for monitoring reactions of fluorine or phosphorous-containing molecules, respectively, attached to solvent-swollen polymer supports. 

D. Stones, D. J. Miller, M. W. Beaton, T. J. Rutherford and D. Gani, A method for the quantification of resin loading using 19F gel phase NMR spectroscopy and a new method for benzyl ether linker cleavage in solid phase chemistry, Tet. Lett, 1998, 39, 4875 1878. 

Gel-phase NMR spectroscopy is a very convenient technique for the spectroscopic characterization of functionalized solid supports.Optimal conditions for acquiring standard gel-phase NMR spectra from a variety of resins are well-established optimal conditions, i.e. maximum swelling, result in reasonably narrow lines for the pendant groups. This method has the additional advantage of not requiring any special instrumentation besides a conventional FTNMR spectrometer and a conventional probe. In addition to NMR spectroscopy, NMR l and NMRt l spectroscopy can also be used in those cases where fluorine or phosphorous is present in the handle or in the first amino acid. 

Due to strong line-broadening, standard gel-phase NMR spectroscopy cannot be used for the analysis of protons. High resolution magic angle spinning (HRMAS) NMR spectroscopy has recently been developed to cover this gap.t l This method has the limitation that specialized HRMAS probes are required however, the method has been applied successfully to a variety of solid supports. Amazingly narrow lines are observed for the proton resonances and 2-D versions of HRMAS experiments can be very easily implemented. 

E Giralt, F Alberico, F Bardella, R Eritja, M Feliz, E Pedroso, M Pons, J Rizo. Gel-phase NMR spectroscopy as a useful tool in solid phase synthesis. Innov Persp Solid Phase Synthesis Collect. Pap., Int Symp. R. Epton (ed.) 111-120, 1990. 

The spectra were recorded at 25.2 MHz with the samples fully swollen in CDCI3 in 12 mm o.d. tubes. Extensive relaxation experiments and attempts to quantify relative peak areas demonstrated a number of artefacts to arise in the generation of these spectra (see later). Nevertheless the work set the scene for the expansion in the use of so-called gel-phase NMR spectroscopy applied to solid phase reactions on crosslinked polymer supports. 

C gel-phase NMR spectroscopy, because of smaller line widths and better chemical shift dispersion of the 13C lines, is a reliable technique to determine the success or failure of chemical transformations. However, as a consequence of the low sensitivity of the... 

GEL-PHASE NMR SPECTROSCOPY FOR REACTION MONITORING AND QUANTIFICATION OF RESIN LOADING... 

R 501 D. Sakellariou, C.A. Meriles and A. Pines, Advances in Ex-Situ Nuclear Magnetic Resonance , C.R.Phys., 2004,5,337 R 502 J.M. Salvino, F Gel-Phase NMR Spectroscopy for Reaction Monitoring and Quantification of Resin Loading , Chem.Anal., 2004,163,37 R 503 A. Schenck and G. Solt, Study of Multipolar Effects in Rare Earth and Actinide Intermetallic Compounds by pSR , J.Phys.-Con-dens.Mat. 2004,16,S4639... 

A frequent complication in the use of an insoluble polymeric support lies in the on-bead characterization of intermediates. Although techniques such as MAS NMR, gel-phase NMR, and single bead IR have had a tremendous effect on the rapid characterization of solid-phase intermediates [27-30], the inherent heterogeneity of solid-phase systems precludes the use of many traditional analytical methods. Liquid-phase synthesis does not suffer from this drawback and permits product characterization on soluble polymer supports by routine analytical methods including UV/visible, IR, and NMR spectroscopies as well as high resolution mass spectrometry. Even traditional synthetic methods such as TLC may be used to monitor reactions without requiring preliminary cleavage from the polymer support [10, 18, 19]. Moreover, aliquots taken for characterization may be returned to the reaction flask upon recovery from these nondestructive... 

Natural abundance gel-phase 13C NMR spectroscopy is used for assessing intermediate structures in solid-phase synthesis [20], but requires long acquisition times and so is not useful for following reactions. Affymax scientists [21] have pioneered the use of specifically 13C-labeled starting materials in conjunction with fast 13C-gel phase NMR to follow multi-step reactions [17,22,23],... 

Two-dimensional NMR spectroscopy has proven of great value for product analysis in solution phase chemistry and by its nature should provide a platform by which enhanced data quality and ease of interpretation is obtained. Geralt demonstrated that the use of 2D CH correlated NMR spectra (Fig. 6) enhances the utility of gel phase NMR even though the proton NMR spectrum was broad (36). Since MAS NMR spectra of the swollen resins are of higher quality than gel phase NMR, it is possible to treat these samples as if they... 

Typical characterization of the thermal conversion process for a given molecular precursor involves the use of thermogravimetric analysis (TGA) to obtain ceramic yields, and solution NMR spectroscopy to identify soluble decomposition products. Analyses of the volatile species given off during solid phase decompositions have also been employed. The thermal conversions of complexes containing M - 0Si(0 Bu)3 and M - 02P(0 Bu)2 moieties invariably proceed via ehmination of isobutylene and the formation of M - O - Si - OH and M - O - P - OH linkages that immediately imdergo condensation processes (via ehmination of H2O), with subsequent formation of insoluble multi-component oxide materials. For example, thermolysis of Zr[OSi(O Bu)3]4 in toluene at 413 K results in ehmination of 12 equiv of isobutylene and formation of a transparent gel [67,68]. 

Thus, the latter approach was clearly preferable and several NPG donors were coupled to 41 under these conditions in order to investigate trends in anomeric selectivity, as monitored by gel-phase 13C NMR spectroscopy (Table 5.1).66 As expected, neighboring-group active donors 44 and 45 produced solely the... 

Fourier-transform infrared (FT-IR) spectroscopy and small-angle X-ray (SAXS) and neutron (SANS) scattering studies have shown the existence of a further pressure induced gel phase in DPPC bilayers." " Pressure dependent NMR studies yielded complementary information on the pressure-induced gel phases. The gel state of DPPC bilayers shows a variation in lineshapes which depend on the particular pressure and temperature. The types of lineshapes seen in a high pressure investigation of dg2-DPPC by Jonas et al. are... 

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