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Separating shifts and couplings: J-resolved spectroscopy
Chapter 7 Separating shifts and couplings: J-resolved spectroscopy
7.1. INTRODUCTION Unlike the techniques encountered in the previous two chapters that exploit scalar (J) couplings to correlate the chemical shifts of interacting spins, ‘J-resolved’ experiments aim to separate (or resolve) chemical shifts from scalar couplings, thus allowing the chemist to examine one parameter without complications arising from the other. For example, the analysis of crowded proton spectra is often complicated by the overlap of neighbouring multiplets, making the extraction of coupling constants or the accurate measurement of chemical shifts difficult or even impossible. This overlap clearly results from the similarities in chemical shifts of the corresponding protons; so if the individual multiplets could in some way be displayed independently of their shifts the overlap would be removed and, in principle, the coupling patterns analysed. In practice, J-resolved methods are subject to a number of technical difficulties that may limit their effectiveness in this respect, and as such the methods have found far less use in routine structural work than the shift correlation experiments. That said, one of the major sources of complication when using J-resolved methods (strong coupling between spins) can be eliminated in many instances, or perhaps reduced to an acceptable level, by the use of higher magnetic field strengths. As these become more routinely available to the research chemist, the J-resolved methods are perhaps more likely to re-establish themselves as useful tools in the chemist’s armoury, rather than fall further into relative obscurity. The methods are all based on 2D spectroscopy in which the shift and coupling parameters are resolved by presenting chemical shifts in f2 and only spin couplings in f1, which may be heteronuclear or homonuclear couplings, depending on the details of the experiment. The principal techniques of this chapter are summarised in Table 7.1. These techniques were the most widely studied methods in the early development of 2D NMR spectroscopy and may be understood with reference to the vector model, being based on simple spin-echoes. As such, I would recommend familiarity with spin-echoes before proceeding (see Section 2.2).
7.2. HETERONUCLEAR J-RESOLVED SPECTROSCOPY In the heteronuclear version of the experiment, the chemical shift of the X spin is presented in f2 whilst couplings to a second nucleus, typically protons, are presented in f1. The f1 dimension therefore enables an analysis of resonance multiplicity as well as measurement of the heteronuclear coupling constants (JXH) themselves, as described further below. To reduce the f1 information content of the 2D spectrum to only couplings, it is necessary to make the detected FIDs insensitive to chemical shift evolution during the t1 period, which is readily achieved by the use of a spin-echo during t1 (Fig. 7.1a) [1, 2]. Thus, following initial X spin excitation, simultaneous 180 proton and carbon pulses are applied at the midpoint of t1, such that X spin chemical shifts will refocus but the heteronuclear coupling will continue to evolve. There is no ‘mixing’ step in J-resolved experiments because magnetisation or coherence transfer to other spins is not employed. The sequence Table 7.1. The principal applications of the main techniques described in this chapter Technique
Principal applications
Heteronuclear J-resolved
Separation of heteronuclear couplings (usually 1H–X) from chemical shifts. Used to determine the multiplicity of the heteroatom or to provide direct measurement of heteronuclear coupling constants.
Homonuclear J-resolved
Separation of homonuclear couplings (usually 1H–1H) from chemical shifts. Used to provide direct measurement of homonuclear coupling constants or to display resonance chemical shifts without homonuclear coupling fine structure (e.g. ‘proton-decoupled’ proton spectra).
‘Indirect’ homonuclear J-resolved
Separation of proton homonuclear couplings according to chemical shift of attached carbon centre. Used to provide direct measurement of homonuclear coupling constants.
7.1. INTRODUCTION Unlike the techniques encountered in the previous two chapters that exploit scalar (J) couplings to correlate the chemical shifts of interacting spins, ‘J-resolved’ experiments aim to separate (or resolve) chemical shifts from scalar couplings, thus allowing the chemist to examine one parameter without complications arising from the other. For example, the analysis of crowded proton spectra is often complicated by the overlap of neighbouring multiplets, making the extraction of coupling constants or the accurate measurement of chemical shifts difficult or even impossible. This overlap clearly results from the similarities in chemical shifts of the corresponding protons; so if the individual multiplets could in some way be displayed independently of their shifts the overlap would be removed and, in principle, the coupling patterns analysed. In practice, J-resolved methods are subject to a number of technical difficulties that may limit their effectiveness in this respect, and as such the methods have found far less use in routine structural work than the shift correlation experiments. That said, one of the major sources of complication when using J-resolved methods (strong coupling between spins) can be eliminated in many instances, or perhaps reduced to an acceptable level, by the use of higher magnetic field strengths. As these become more routinely available to the research chemist, the J-resolved methods are perhaps more likely to re-establish themselves as useful tools in the chemist’s armoury, rather than fall further into relative obscurity. The methods are all based on 2D spectroscopy in which the shift and coupling parameters are resolved by presenting chemical shifts in f2 and only spin couplings in f1, which may be heteronuclear or homonuclear couplings, depending on the details of the experiment. The principal techniques of this chapter are summarised in Table 7.1. These techniques were the most widely studied methods in the early development of 2D NMR spectroscopy and may be understood with reference to the vector model, being based on simple spin-echoes. As such, I would recommend familiarity with spin-echoes before proceeding (see Section 2.2).
7.2. HETERONUCLEAR J-RESOLVED SPECTROSCOPY In the heteronuclear version of the experiment, the chemical shift of the X spin is presented in f2 whilst couplings to a second nucleus, typically protons, are presented in f1. The f1 dimension therefore enables an analysis of resonance multiplicity as well as measurement of the heteronuclear coupling constants (JXH) themselves, as described further below. To reduce the f1 information content of the 2D spectrum to only couplings, it is necessary to make the detected FIDs insensitive to chemical shift evolution during the t1 period, which is readily achieved by the use of a spin-echo during t1 (Fig. 7.1a) [1, 2]. Thus, following initial X spin excitation, simultaneous 180 proton and carbon pulses are applied at the midpoint of t1, such that X spin chemical shifts will refocus but the heteronuclear coupling will continue to evolve. There is no ‘mixing’ step in J-resolved experiments because magnetisation or coherence transfer to other spins is not employed. The sequence Table 7.1. The principal applications of the main techniques described in this chapter Technique
Principal applications
Heteronuclear J-resolved
Separation of heteronuclear couplings (usually 1H–X) from chemical shifts. Used to determine the multiplicity of the heteroatom or to provide direct measurement of heteronuclear coupling constants.
Homonuclear J-resolved
Separation of homonuclear couplings (usually 1H–1H) from chemical shifts. Used to provide direct measurement of homonuclear coupling constants or to display resonance chemical shifts without homonuclear coupling fine structure (e.g. ‘proton-decoupled’ proton spectra).
‘Indirect’ homonuclear J-resolved
Separation of proton homonuclear couplings according to chemical shift of attached carbon centre. Used to provide direct measurement of homonuclear coupling constants.