2D NMR

The general task is to trace the evolution of the third order polarization of the material created by each of the above 12 Raman field operators. For brevity, we choose to select only the subset of eight that is based on two colours only—a situation that is connnon to almost all of the Raman spectroscopies. Tliree-coloiir Raman studies are rather rare, but are most interesting, as demonstrated at both third and fifth order by the work in Wright s laboratory [21, 22, 23 and 24]- That work anticipates variations that include infrared resonances and the birth of doubly resonant vibrational spectroscopy (DOVE) and its two-dimensional Fourier transfomi representations analogous to 2D NMR [25]. 

More generally, note that the applieation of almost any multiple pulse sequenee, where at least two pulses are separated by a time eomparable to the reeiproeal of the eoupling eonstants present, will lead to exehanges of intensity between multiplets. These exehanges are the physieal method by whieh eoupled spins are eorrelated in 2D NMR methods sueh as eorrelation speetroseopy (COSY) [21]. 

Figure Bl.11.12. H- H COSY-45 2D NMR spectrum of methyl-a-gliicopyranose (structure shown). The coupling links and the approximate couplings can be deduced by inspection.

Morris K F and Johnson C S Jr 1993 Resolution of discrete and continuous molecular size distributions by means of diffusion-ordered 2D NMR spectroscopy J. Am. Chem. See. 115 4291-9... 

For quadnipolar nuclei, the dependence of the pulse response on Vq/v has led to the development of quadnipolar nutation, which is a two-dimensional (2D) NMR experiment. The principle of 2D experiments is that a series of FIDs are acquired as a fimction of a second time parameter (e.g. here the pulse lengdi applied). A double Fourier transfomiation can then be carried out to give a 2D data set (FI, F2). For quadnipolar nuclei while the pulse is on the experiment is effectively being carried out at low field with the spin states detemiined by the quadnipolar interaction. In the limits Vq v the pulse response lies at v and... 

Borgias B A, Gochin M, KenA/ood D J and James T L 1990 Relaxation matrix analysis of 2D NMR data Prog. NMR Spectrosc. 22 83-100... 

An extensive presentation of the modern NMR theory, standard text for 2D NMR. Abragam A 1961 Principies of Nuciear Magnetism (Oxford Clarendon)... 

Orrell K G, Sik V and Stephenson D 1990 Quantitative investigations of moieouiar stereodynamios by ID and 2D NMR methods Prog. Nucl. Magn. Reson. Specfrosc. 22 141-208... 

Wall M R, Dieckmann T, Feigon J and Neuhauser D 1998 Two-dimensional filter-diagonalization spectral inversion of 2D NMR time-correlation signals including degeneracies Chem. Phys. Lett. 291 465... 

Robust implementations can currently propose correct structures from spectroscopic data, especially when the molecular formula and C NMR spectrum are available, or from 2D NMR spectra. 

The theoretical aspects in eluding pulse sequences that underlie 2D NMR are dis cussed in the May 1990 issue of the Journal of Chemical Education pp A125-A137... 

The more information you can extract from an NMR spectrum the better your chances at arriving at a unique structure Like spm-spm splitting which complicates the appear ance of a H NMR spectrum but provides additional information 2D NMR looks more complicated than it is while making structure determination easier... 

The key dimension m NMR is the frequency axis All of the spectra we have seen so far are ID spectra because they have only one frequency axis In 2D NMR a stan dard pulse sequence adds a second frequency axis Only pulsed FT NMR spectrometers are capable of carrying out 2D experiments... 

One kind of 2D NMR is called COSY, which stands for correlated spectroscopy With a COSY spectrum you can determine by inspection which signals correspond to spin coupled protons Identifying coupling relationships is a valuable aid to establishing a molecule s connectivity... 

A second 2D NMR method called HETCOR (heteronuclear chemical shift correlation) is a type of COSY in which the two frequency axes are the chemical shifts for different nuclei usually H and With HETCOR it is possible to relate a peak m a C spectrum to the H signal of the protons attached to that carbon As we did with COSY we 11 use 2 hexanone to illustrate the technique... 

A number of 2D NMR techniques are available for a variety of purposes They are especially valuable when attempting to determine the structure of complicated natural products and the conformations of biomolecules... 

Section 13 19 2D NMR techniques are enhancements that are sometimes useful m gam mg additional structural information A H H COSY spectrum reveals which protons are spin coupled to other protons which helps m deter mining connectivity A HETCOR spectrum shows the C—H connections by correlating C and H chemical shifts... 

COSY (Section 13 19) A 2D NMR technique that correlates the chemical shifts of spin coupled nuclei COSY stands for... 

HETCOR (Section 13 19) A 2D NMR technique that correlates the H chemical shift of a proton to the chemical shift of the carbon to which it is attached HETCOR stands for heteronuclear chemical shift correlation Heteroatom (Section 1 7) An atom in an organic molecule that IS neither carbon nor hydrogen Heterocyclic compound (Section 3 15) Cyclic compound in which one or more of the atoms in the nng are elements other than carbon Heterocyclic compounds may or may not be aromatic... 

Chapter 13 Spectroscopy has been supple mented by an expanded discussion of H and C chemical shifts and a new section on 2D NMR A new box Spectra by the Thousands points the way to websites that feature libraries of spectra and spectroscopic problems of every range of difficulty... 

You can often use experimental data, such as Nuclear Overhauser Effect (NOE) signals from 2D NMR studies, as restraints. NOE signals give distances between pairs of hydrogens in a molecule. Use these distances to limit distances during a molecular mechanics geometry optimization or molecular dynamics calculation. Information on dihedral angles, deduced from NMR, can also limit a conformational search. 

In S-N compounds containing H bonded directly to nitrogen, inverse detection 2D NMR spectroscopy can be applied to give N... 

Tile ID and 2D NMR experiments for 5-aza-7-deaza-2 -deoxyguanosine concluded that the oxo-amino tautomer 59a is preferable in DMSO-dg whereas the oxo-imino form 59b dominates in D2O (87JOC5136). Evidence for the hydroxy-imino tautomer 59c was not found. Poor solubility of the parent compound, 5-aza-7-deazaguanine (60) did not allow study of its tautomerism, but the pK values of protonation and deprotonation for 60 are identical with those for 59. 

The relative stereochemistry of hyperaspine (1) was determined by 2D NMR and MS methods. It has a m-fused bicyclic conformation 82 (01TL4621). The tram-fused one is disfavored by an axial pentyl group at C(8) and by a destabilising dipole-dipole interaction between the N and O atoms, which does not exist in the alternative cis conformation. 

Reaction of hydrazine 165 with a number of monosaccharides give hydrazones 166 (92BCJ546). Their acetylations gave peracetyl derivatives 167. The same reactions were extended to the /V-methyl and the 7-methyl derivatives of 165 (93 JPROO). That compounds 166 exist as open-chain structures was based on a 2D NMR study (93MI1) (Scheme 35). 

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