Single-bond correlations

A] and A2 which serve to remove chemical shift evolution are omitted for simplicity). To produce the two-dimensional shift correlation experiment [54], the variable ti evolution period is added immediately after the initial proton excitation and prior to the polarisation transfer step so that the detected signal becomes modulated by the proton chemical shift as a function of ti (Fig. 6.31b). During this, H- H and coupling will also evolve, leading 

The resulting spectrum displays crosspeaks correlating carbon chemical shifts in f2 and proton shifts in f] which are further spread by homonuclear proton couplings in fi. Fig. 6.32 displays a part of the carbon-proton shift correlation spectrum of the palladium complex 6.11. Despite the extensive crowding in the aromatic region, the carbon shifts are sufficiently dispersed to resolve all correlations (note some resonances are broadened by restricted dynamic processes within the molecule and some are split by coupling to phosphorus). 

The scheme of Fig. 6.31b has been widely used to produce absolute-value shift correlation spectra, and is often referred to as HETCOR or hetero-COSY. Conversion to the preferred phase-sensitive equivalent (of which various forms have been investigated [55]) requires the reintroduction of the simultaneous 180°( H, C) pulses into the midpoints of both Ai and A2 to remove chemical shift evolution during these periods, exactly as in the full refocused INEPT. In addition, the incorporation of the States or TPPI phase cycling of the 90° proton pulse of the polarisation transfer step is required. Suppression of axial peaks is through the phase alternation of the final proton pulse together with the receiver 

Since polarisation transfer is employed, the experiment repetition rate is dictated by the recovery of the faster relaxing proton spins, and repetition times should be around 1.3 times the proton Tis. Resolution in the proton dimension can be quite low for routine applications since one does not usually wish to resolve the proton fine-structure and because the homonuclear coupling is in-phase there is no fear of signal cancellation fi digital resolution may therefore be as low as 10 Hz/pt or so, requiring rather few ti increments. The number of scans per increment should be set such that the carbon resonances of interest can just be observed in the spectrum of the first recorded FID (which is equivalent to the ID INEPT experiment). 

Numerous X-detected sequences have been presented over the years for establishing long-range H- C correlations [59] prior to the widespread adoption of the proton-detected counterparts. This section presents the most widely used sequence (COLOC) and briefly mentions more recent sequences that have superior performance in most instances. 

The scheme of Fig. 6.46b has been widely used to produce absolute-value shift correlation spectra and is often referred to as HETCOR or hetero-COSY. Conversion to the preferred phase-sensitive equivalent (of which various forms have been investigated [91]) 

COSY (HETCOR) spectoim (500 MHz) of die palladium complex 6.10. 2 K data points were collected for 256ti increments of 128 ttansients each for a 40 x 2.6-ppm window. Unshifted sine bells were applied in both dimensions and after zero filling once in each, die digital resolution was 2.5 and 5.0Hz/pt in/2 and/i, respectively. 

57ch produces a spectrum in which responses from CH2 groups are inverted. For routine application, selecting A2 = 1/37ch (typically 2.3 ms for provides positive intensities 

Numerous X-detected sequences have been presented over the years for establishing long-range correlations [95] prior to the widespread adoption of the proton-detected 

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D Proton-Carbon (Single Bond) Correlated Spectroscopy... 

If interpreting the single-bond correlation experiments is easy, the multiple bond experiment (HMBC) can be considerably less so... 

Figure 15.1. (A) COSY, (B) TOCSY, (C) 1H-1T HSQC or HMQC, (D) dl- Y HMBC, for 4-oxopentanal. For clarity, only key assignments have been given as an example. Note that the double-ended arrows indicate how to interpret the spectra. In the case of COSY and TOCSY the information is represented as cross-peaks that are symmetrically oriented with respect to the central diagonal. In the single-bond correlation (HSQC/HMQC) a cross-peak represents in one dimension the carbon chemical shift and in the other dimension the proton chemical shift. Note there is no diagonal in heteronuclear NMR experiments. In the HMBC, lines are drawn vertically to connect the cross-peaks. In HMBC 2-4 bonds, H-13C correlations are often observed. Note that the 4-bond correlation is less common in NMR but has been included here as an example, and 1-bond correlation is commonly filtered from the HMBC experiment to improve detection limits for the weaker 2-4 bond correlations.

The heteronuclear multiple-quantum correlation (HMQC) experiment is one of the two commonly employed proton-detected single-bond correlation experiments. Although this was suggested many years ago [1,2], the experiment lacked widespread use until a scheme was presented [3] that was able to overcome the technical difficulties associated with proton observation described above. Since then, and particularly since the advent of PFGs, the technique has come to dominate organic NMR spectroscopy. 

Besides the obvious D3 twisted bicyclo[2.2.2]octane framework, another characteristic of pentacyclic [m.n.o] triblattanes is the presence of a strained bicyclo [2.2.0] hexane moiety whose facile reductive cleavage of the central single bond correlates this class of triblattanes to the corresponding tetracyclic [m.n] triblattanes. Figure 21 illustrates the configurational correlation between these two classes of triblattanes the key liaison compound is the (+) <, 0-unsaturated ketone 172, whose absolute configuration had been related to that of the levorotatory carboxylic acid (-)-151 via (+)-methyl 3-(enrfo-2-norbornyl) propionate (179) (146). 

Figure 6.2. The 500-MHz HMQC single-bond correlation spectrum of menthol 6.1. The conventional ID proton and carbon spectra are also shown for reference.

The correlation signals of the INADEQUATE experiment directly build up the ring skeleton A of the compound. Elere characteristic C shifts (5c = 123.1, 137.6 148.9, 109.1) establish the existence and position of two double bonds and of one tetrahedral C-0 single bond (5c = 70.5). DEPT spectra for the analysis of the CH multiplicities become unnecessary, because the INADEQUATE plot itself gives the number of CC bonds that radiate from each C atom. 

Azulene does have an appreciable dipole moment (0.8 The essentially single-bond nature of the shared bond indicates, however, that the conjugation is principally around the periphery of the molecule. Several MO calculations have been applied to azulene. At the MNDO and STO-3G levels, structures with considerable bond alternation are found as the minimum-energy structures. Calculations which include electron correlation effects give a delocalized n system as the minimum-energy structure. ... 

Naively it may be expected that the correlation between pairs of electrons belonging to the same spatial MO would be the major part of the electron correlation. However, as the size of the molecule increases, the number of electron pairs belonging to different spatial MOs grows faster than those belonging to the same MO. Consider for example the valence orbitals for CH4. There are four intraorbital electron pairs of opposite spin, but there are 12 interorbital pairs of opposite spin, and 12 interorbital pairs of the same spin. A typical value for the intraorbital pair correlation of a single bond is 20kcal/ mol, while that of an interorbital pair (where the two MO are spatially close, as in CH4) is 1 kcal/mol. The interpair correlation is therefore often comparable to the intrapair contribution. 

This correction of i i for ligancy, together with the bond-number equation and a set of values of the new single-bond metallic radii, provides an improved system of correlating interatomic distances not only in metals and alloys but also in other crystals and molecules. 

The most powerful techniques of all are undoubtedly the 2-D proton-carbon experiments (Hetero-nuclear Multiple Quantum Coherence///eteronuclear Single Quantum Coherence, or HMQC/HSQC and //ctcronuclcar Multiple Bond Correlation, or HMBC) as they provide an opportunity to dovetail proton and carbon NMR data directly. 

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