Correlated spectroscopy cross-peak intensity

Hou et al. have recently described a family of experiments for R2 Driven Spin Diffusion (RDSD) spectroscopy suitable for homonuclear correlation experiments under fast MAS conditions. In these RDSD experiments, since the broadened second-order rotational resonance conditions are dominated by the rf field strength and the phase shifts, as well as the size of reintroduced dipolar couplings, the different R2 sequences display unique polarization transfer behaviours and different recoupling frequency bandwidths. In the subsequent report, Hou et al. present a series of modified R2/ sequences, dubbed combined R2 -Driven (CORD), that yield broadband homonuclear dipolar recoupling and give rise to uniform distribution of cross peak intensities... 

At 500 MHz, moderate-sized (more than six residues) oligosaccharides lie within the spin-diffusion limit. However, for smaller molecules, as the value of the function (OqT (where (Oq is the Larmor frequency of protons, and is the correlation time of the molecule) approaches 1 then the value of the NOE tends towards 0. Cross-peak intensities of NOESY spectra of smaller oligosaccharides (2-5 residues) may thus become too small to measure accurately. In such cases, the rotating frame Overhauser effect spectroscopy (ROESY, originally referred to as CAMELSPIN) experiment is commonly used to measure NOE values. To reduce the appearance of HOHAHA-like cross-peaks, a low power spin-lock field should be used, and the transmitter carrier offset to the low-field end of spectrum. The offset dependency of cross-peak intensities should also be removed by 90° pulses at either end of spin-lock period. 

A related experiment TOCSY (Total Correlation Spectroscopy) gives similar information and is relatively more sensitive than the REIAY. On the other hand, intensity of cross peak in a NOESY spectrum with a short mixing time is a measure of internuclear distance (less than 4A). It depends on the correlation time and varies as . It is positive for small molecules with short correlation time (o r <<1) and is negative for macromolecules with long correlation time (wr >>l) and goes through zero for molecules with 1 Relaxation effects should be taken into consideration for quantitative interpretation of NOE intensities, however. 

For a ToF-SIMS investigation of the surface oxidative degradation of low-density polyethylene (LDPE), the polymer was exposed to 1802 rather than 1602 in order to be able to readily discriminate oxygen introduced by the ageing process from that in the polymer prior to ageing [102], Figure 36 shows an example series of ToF-SIMS spectra from this investigation, which shows the clear separation of the lsO species from the lsO species. In the study, close correlation was observed between the intensity of the lsO carbonyl species determined by mid-infrared spectroscopy with the ToF-SIMS 180- peak intensity as a function of 1802 exposure time. ToF-SIMS spectra obtained from microtomed cross-sections showed no... 

In the H- H COSY (correlation spectroscopy) experiment (41), magnetization is transferred via the J-coupling and shows correlations between protons three bonds apart. The cross peak usually shows a characteristic antiphase-square pattern, but it may be split even more by additional passive couplings that lead to spectral crowding and loss of intensity. For small molecules. 

Unlike correlation spectroscopy based on spin diffusion, the adiabatic version enables, in principle, almost full exchange of magnetization between the two spins. As a result, the entire signal intensity will reside in the cross-peaks. Violation of the adiabaticity is characterized by the appearance of a diagonal peak and can be expected to occur if the rotation sweep is too fast compared to the interaction between spins. While numerical simulations indicate possible linear dependencies of the polarization transfer coefficient on spin coupling and the rate of the sweep over a range of practical values, the validity of this assumption remains to be tested. Here we present a semi-quantitative example of a relayed polarization transfer process. 

Figure 5.61. Pulse sequences for homonuclear 2D-shift correlated NMR spectroscopy (A) COS 7-90 pulse sequence (B) SECSY pulse sequence. The second 90° mixing pulse causes an exchange of magnetization between the coupled nuclei. If the second pulse in (A) is a 45° pulse, then a simplified COS 7-45 spectrum is obtained with reduced intensities of cross peaks near the diagonal.

---