NMR Spectroscopic Methods

The power of the HR-MAS method for on-resin analysis has been further underscored in the development of new linkers. Without this method, only indirect analytical data after removal from the resin was available. Direct assessment of the resin-bound linker greatly facilitated the introduction of a 4,5-dibromo octane- 1,8-diol linker that was converted into an octane-1,8-diol linker cleavable by olefin metathesis at the end of the synthesis.6 The disappearance and reappearance of the olefinic protons as well as the growing oligosaccharide chain was clearly visible in the H spectrum (Fig. 8.7).7 

Most of the methodology used so far in solid-phase oligosaccharide synthesis was developed before the analytical techniques outlined above had been adopted. Now that a closer insight into the reactions has become available, one may expect exciting 

Fundamentals of Fourier Transform Infrared Spectroscopy, CRC Press, Boca Raton, FL, 1996. 

Shoolery, J. N., Prog. Nucl. Magn. Reson. Spectrosc. 11, 79-93 (1977). 

Edited by Peter H. Seeberger Copyright 2001 John Wiley Sons, Inc. ISBNs 0-471-37828-3 (Hardback) 0-471-22044-2 (Electronic) 

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Hufford et al [13] used a 13C NMR spectroscopic method for the assignments of dissociation constants of primaquine. The first and second dissociation constants of primaquine were determined by titration with 0.1 N hydrochloric acid in acetonitrile-water mixtures and values were extrapolated to water by using linear regression analysis. The assignments of the dissociation constants were unambiguously achieved by studying the 13C NMR spectral data obtained with monohydrochloride, dihydrochloride, and trihydrochloride salts. 

The equilibrium constants, obtained by NMR spectroscopic methods, showed a strong dependence on both the macrocycles and the diorganozinc compounds. Polydentate nitrogen donor ligands, particularly triazacyclononanes, form much stronger complexes than cryptands and crown ethers. 

This reaction has been reexamined using optical, IR and NMR spectroscopic methods to probe NO reactions with Fe(TPP)(NO) and the more soluble Fe(TmTP)(NO) (92). These studies confirmed the formation of Fe(Por)(NO)2 in toluene-dg at low temperature (Eq. (43)). NMR line shape analysis was used to calculate K43 = 23 M-1 at 253 K (3100 M-1 at 179 K, AH° = —28kJmol 1) (92). The failure of the Fen(Por) complexes to promote NO disproportionation, in contrast to the behavior of the respective Ru(II) and Os(II) analogs, may find its origin partly in the relatively low stability of the dinitrosyl intermediate (K52 estimated to be 2.8 M-1 at 298 K) and unfavorable kinetics of subsequent reaction of this species with NO. 

Nitroanilines were extensively investigated by infrared, Raman, UV/VIS and H NMR spectroscopic methods. In some cases the comparison with similar spectroscopic data of the corresponding tV,/V-dimethylanilines provides simple and consistent conclusions involving geometrical properties of molecules caused by intramolecular hydrogen bonding. 

Further, [2.2]paracyclophane-4-acetic acid, a potential drug candidate tested for its anti-inflammatory activity (NSAID could be resolved on the 0-9-(tert-butylcarbamoyl)quinine-based CSP with a = 1.12 Rs = 2.6) and elution order (R)-(-)- before (5)-(- -)-enantiomer [125]. Samples that were assessed to be enan-tiomerically pure by an enantioselective H-NMR spectroscopic method contained 6% and 8% enantiomeric impurity in S- and/ -enantiomers, respectively. This clearly reveals that enantioselective HPLC is a more powerful technique than NMR-methods to assess stereoisomeric purity, in particular, if the enantiomeric impurity amounts to less than 10%. 

Nmr spectroscopic methods of determination of sites, of protonation may be divided into two types, depending on the kind of phenomena that are observed upon protonation. The first type is concerned with the spectral changes (chemical shifts, coupling constants) in the skeleton of the molecule Z caused by protonation the second is concerned with spectral changes at the protonation sites, X or Y (proton exchange phenomena, coupling to the captured proton, and the observation of the resonance of the captured proton itself). 

Studying the Catalytic Mechanism of Enantioseiective Hydrogenation with NMR Spectroscopic Methods... 

The resulting styrene/maleic acid copolymer is soluble in hot water, in contrast to the starting material the aqueous solution of the product gives a distinctly acid reaction. The disappearance of the anhydride moiety can be verified by IR or C-NMR spectroscopic methods.The IR spectra of polymers should be recorded from a film of the sample prepared on a KBr pellet (freshly made from KBr powder). For this, a drop of a solution of the polymer in a low-boiling solvent (e.g.,THF, methylene chloride) is placed on the pellet.The residual solvent can often be removed directly in the IR beam.The resulting spectra are characterized by their sharp bands. 

A general NMR spectroscopic method has been developed by Kyba19 using in situ complexes of diphosphines with (—)-bis(/r-chloro)bis[(7 )-dimethyl(a-methylbenzyl)aminato-C2,A]-dipalladium(II). 

The structure of alkenoyl cations (unsaturated acylium ions) was studied by Olah et al.658 by NMR spectroscopic methods. They found only a limited contribution from structure 343b and a substantial contribution of the delocalized ketene-like structure 343c, which is due to the ability of the Jt-electrons of the double bond to stabilize the positive charge. Substitution at the (3-carbon increases further the importance of 343c relative to 343a. Diprotonation of propenoyl and isopentenoyl cations studied theoretically [ab initio GIAO-CCSD(T) method]659 has been shown to result in the formation of dication 344 (tertiary carbenium-acylium dication) that is, the positive charges are localized primarily on CO and the (3-carbon. 

When assessing the purity of dendrimers it should also be noted that NMR spectroscopic methods approach their limit of detection at contamination levels of ca. 5%. Additional chromatographic method such as gel permeation chromatography (GPC, SEC Section 7.1.2) or mass spectrometric methods (MALDI-MS, ESI-MS), as presented in Section 7.4, should also be employed in verification of structural perfection and purity of dendrimers. 

Mohan et al. [8] used an NMR spectroscopic method to characterize impurity D in samples of clopidogrel bisulfate. The method entailed XH NMR (at 400.13 MHz) and 13C NMR (at 100.62 MHz), with a sample concentration of 1 mg/ml in DMSO-t/f, (this solvent also served as an internal chemical shift standard). It was found that the 1H NMR spectrum of impurity D exhibited one hydrogen band than did that of clopidogrel, while the 13C NMR and DEPT135 NMR spectra indicated the presence of one methyl carbon, two methylene carbons, eight methine carbons, and five quaternary carbons. This corresponded to a similar structure as for clopidogrel, but with one less methylene carbon and one more methine carbon. 

A particularly attractive aryl amide foldamer is talibiter (10.1) named after the mythological wyrm Ouroboros that Kekule claimed inspired his structure of benzene (Figure 10.5),6 which folds back on itself in a spiral conformation such that the pyrrole unit on one end is close to the other end of the molecule. The solution conformation of the foldamer was determined using NMR spectroscopic methods as described in Box 6.1. 

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