Peaks splitting

Al Ihe beginning of Ihis seclion we noted lhal an NMR speclrum provides slruc lural mformalion based on chemical shift Ihe number of peaks Iheir relative areas and Ihe mulhphcily or splitting of Ihe peaks We have discussed Ihe lirsl Ihree of Ihese fea lures of H NMR speclroscopy Lei s now torn our attention to peak splitting to see whal kind of information il offers... 

The physical basis for peak splitting mil dichloroethane can be explained with the aid of Figure 13 13 which examines how the chemical shift of the methyl protons IS affected by the spin of the methme proton There are two magnetic environments for... 

At the beginning of this section we noted that an NMR spectrum provides structural infoiination based on chemical shift, the number of peaks, their relative areas, and the multiplicity, or splitting, of the peaks. We have discussed the first three of these features of H NMR spectroscopy. Let s now turn our attention to peak splitting to see what kind of infonnation it offers. 

P-F 153 pm). However, the F nmr spectrum, as recorded down to — 100°C, shows only a single fluorine resonance peak (split into a doublet by P- F coupling) implying that on this longer time scale (milliseconds, as distinct from instantaneous for electron diffraction) all 5 F atoms are equivalent. This can be explained if the axial and equatorial F atoms interchange their positions more rapidly than this, a process termed pseudorotation by R. S. Berry (1960) indeed, PF5 was the first compound to show this effect. The proposed mechanism is illustrated in Fig. 12.13 and is discussed more fully in ref. 91 the barrier to notation has been calculated as 16 2kJmol". ( ... 

Barium and strontium salts of polystyrene with two active end-groups per chain were prepared by Francois et al.82). Direct electron transfer from tiny metal particles deposited on a filter through which a THF solution of the monomer was percolated yields the required polymers 82). The A.max of the resulting solution depends on the DPn of the formed oligomers, being identical with that of the salt of polymers with one active end-group per chain for DPn > 10, but is red-shifted at lower DPn. Moreover, for low DPn, (<5), the absorption peak splits due to chromophor-chromophor interaction caused by the vicinity of the reactive benzyl type anions. 

If we examined the NMR spectrum of the starting mixture, we would find only one peak (split into a doublet by the C—H) for the Me protons, since enantiomers give... 

The electrochemical behavior of the C70 solvent-cast films was similar to that of the C60 films, in that four reduction waves were observed, but some significant differences were also evident. The peak splitting for the first reduction/oxidation cycle was larger, and only abont 25% of the C70 was rednced on the first cycle. The prolate spheroidal shape of C70 is manifested in the II-A isotherm of C70 monolayers. Two transitions were observed that gave limiting radii consistent with a transition upon compression from a state with the long molecnlar axes parallel to the water snrface to a state with the long molecnlar axes per-pendicnlar to the water surface. 

Monolayers of l-tert-bntyl-l,9-dihydrofullerene-60 on hydrophobized ITO glass exhibited three well-defined rednction waves at -0.55 V, -0.94 V, and -1.37 V (vs. satn-rated calomel electrode, SCE), with the first two stable to cycling [283]. Improved transfer ratios near nnity were reported. The peak splitting for the first two waves was 65-70 mV, mnch less than reported for the pnre C60-modified electrodes. The rednction and oxidation peak cnrrents were equal however, the peak currents were observed to be proportional to the sqnare root of the scan rate instead of being linear with the scan rate as normally expected for snrface-confined redox species. 

Figure 3.6 Different peak distortion problems due to band broadening in time and band broadening in space observed during hot splitless injection. Band broadening in space is characterized by a broadening which grows proportionally with retention time and may result in peak splitting that is poorly reproducible. Band broadening in time is characterized by a constant broadening of all peaks. Partial solvent trapping results in characteristic chair and stool shaped peaks. (Adapted with permission from ref.

A partial solution to this problem can be obtained by so-called interrupted LSV, which means that on passing the first peak [see Fig. 3.35(a)] the voltage ramp is held sufficiently long at the value concerned before being continued, and so on with the same procedure for subsequent peaks the peak splitting in the interrupted technique is impressive [see Fig. 3.35(b)] in contrast to the normal technique. 

The structure of PF5 is also a trigonal bipyramid with axial bonds of 158 pm and equatorial bonds of 152 pm. In 19F NMR studies, a single peak split into a doublet by coupling with 31P is observed. Therefore, it appears that all five F atoms are equivalent, indicating that there is rapid exchange between the axial and equatorial positions. An explanation of this phenomenon was provided by... 

Evidence of a more ordered chalcopyrite structure can be shown in Fig. 6.26, where the (101) and (211) diffractions at 17.9° and 37.3°, respectively, can be clearly shown in the annealed film. Another characteristic of the chalcopyrite structure not shown in the sphalerite structure is the peak splitting in... 

Figure 6.26. XRD pattern from (A) as-deposited films and (B) annealed film. Inset shows expanded scale emphasizing peak splitting.

An air pocket in the pump can cause low or no pressure or flow, erratic pressure, and changes in retention time data. It may be necessary to bleed air from the pump or prime the pump according to system startup procedures. Air pockets in the column will mean decreased contact with the stationary phase and thus shorter retention times and decreased resolution. Tailing and peak splitting on the chromatogram may also occur due to air in the column. Air bubbles in the detector flow cell are usually manifested on... 

D 1H-13C CT-HSQC/HMQC identifies proton-carbon spin pairs in both sugar and base. The constant time evolution removes the peak splitting due to 13C-13C coupling. The proton and carbon resonances are subsequently connected by... 

The NMR spectrum also provides information about the number of nuclei within each distinct environment. This is given by the area under each resonant peak representing the relative number of nuclei of each type. Furthermore, the surrounding nuclei also cause a splitting pattern. For example, one H surrounded by n other H neighbors will have its resonance peak split into n + 1 peaks. 

A number of studies were done in order to determine which of these various factors contribute to the large peak separations observed here. First, it is well known that the effects of resistive elements can be obviated by applying positive feedback [132]. When positive feedback was applied to a thin-film confrol elecfrode similar fo fhaf described in Fig. 27, the peak separation decreased from 0.8 to —0.35 V (Fig. 28). These data show that resistance does, indeed, contribute to the large AEp values observed here. However, the fact that —0.35 V of this peak splitting cannot be removed by applying positive feedback clearly indicates that slow electron transfer kinetics also contribute to AEp. ... 

Adding these various contributions to the peak splitting together, we... 

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