Spectra of Diastereotopic Systems

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FIGURE 7.19 The structures of the enantiomers in a racemic mixture of ethyl 3-hydroxybutanoate. 

FIGURE 7.20 H NMR spectrum of racemic ethyl 3-hydroxybutanoate (500 MHz, CDCI3). 

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The configurational stability of germyllithium compounds has been studied by temperature variation of the proton NMR spectra. The selected system Ph(i-Pr)2GeLi possesses groups (methyl of the i-Pr groups) which are diastereotopic. The methyl non-equivalence was observed in diglyme up to 185 °C, which is the upper experimental limit. Assuming that rotation around the Ge—C bond was fast on the NMR time scale, the non-equivalence... 

Chapters 5 and 6 covered only the most essential elements of nuclear magnetic resonance (NMR) theory. Now we will consider appUcations of the basic concepts to more complicated situations. In this chapter, the emphasis is on the origin of coupling constants and what information can be deduced from them. Enantiotopic and diastereotopic systems will be covered as well as more advanced instances of spin-spin coupling, such as second-order spectra. 

ACD/H-NMR from Advanced Chemistry Development (ACD) Labs calculates H-NMR spectra under any basic frequency. The system uses 3D molecular structure minimization and Karplus relationships to predict proton-proton coupling constants. The software recognizes spectral differences among diastereotopic protons, cis-trans isomers of alkenes, syn-anti isomers of amides, oximes, hydrazones, and nitrosa-mines. The base data set includes more than 1,000,000 experimental chemical shifts and 250,000 experimental coupling constants. To quantify intramolecular interactions in new organic structures and to predict their chemical shifts, ACD/HNMR uses an algorithm based on intramolecular interaction parameters to quantify intramolecular interactions in new organic structures and to predict their chemical shifts. 

As shown by multinuclear NMR studies, compounds 3a/3b-5a/5b also exist in solution (3a-5a, 4b [DslDMSO 3b, 5b CDCI3). Due to the chiral nature of these zwitterions, their SiCH2N protons are diastereotopic (3a-5a ABX spin systems in the H-NMR spectra, with F as the X nucleus 3b-5b ABMX spin systems in the H-NMR spectra, with N// as the M nucleus and F as the X nucleus). 

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