Routine Spectra

Carbon-13 operating frequency is determined by the spectrometer available (cf. Table 5.4.1). 

Spectral width, SW, should exceed the range of expected frequencies, which is about 240 ppm for lignins. 

Acquisition time, AT, is automatically fixed when SW and NP are selected since AT = NP/2SW. 

Relaxation delay, RD, between acquisitions can range from 0 to Is to improve observation of the quaternary carbons in carbonyl or carboxyl groups, which have longer relaxation times (T,) than other carbons. 

13c Spectral NP Acquisition Relaxation Digital Number of Recording 

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C and 15N nuclei undergo 7j dipole-dipole interactions with attached protons and, to a lesser extent, with other nearby protons. There are further complications with l5N nuclei. In routine spectra of 13C and 15N, large 7j values result in only partial recovery of the signal so that a delay interval must be inserted between the individual pulses (see Figure 4.2a). Thus, we see that 7j relaxation is intimately involved with peak intensity. 

Geminal coupling constants are usually negative numbers, but this can be ignored except for calculations. Note that geminal couplings are seen in routine spectra only when the methylene protons are diastereotopic. 

C peak intensities do not correlate with the number of carbon atoms in a given peak in routine spectra, due to longer 7) values and NOE. 

Unlike H NMR spectra, whose integration of signals represents the ratio of protons, integration of 13C peaks do not correlate with the ratio of carbon atoms in routine spectra. There are two major factors that account for the problem of peak intensities in 13C spectra ... 

In this chapter we explore some of the problems that you may encounter when running what are meant to be routine spectra. The increasing computer control found in modem spectrometers is a great benefit to the user, but can also lead to some subtle types of operator error. The problems discussed in this chapter are not the sort one encounters as a result of a failure in the system. Many of them appear when a parameter is wrongly set through forgetfulness or ignorance, or when a command is mistyped or misused. 

The total data acquisition time is usually long It is recommended that the acquired data be stored m successive blocks to save the data in case of an accidental power interruption Otherwise, except for the longer RD, acquisition parameters are the same as those considered in the recording of routine spectra (cf Table 5 4 2)... 

Adequate proton NMR spectra can be obtained by continuous wave measurements however, routine spectra could not be obtained until the development of Fourier transform spectrometers which allow one to accumulate a large number of spectra in a relatively short period of time and to average these spectra to increase the signal-to-noise ratio of the measurements. 

All DOR experiments were performed at room temperature on pulsed NMR spectrometers operating at either 104.23 MHz or 130.29 MHz, using home-built DOR probes that have been described elsewhere [15]. DOR spinning speeds of 5 kHz for the inner rotor and 800 Hz for the outer rotor were obtained routinely. Spectra were obtained using a 1-s delay between 45 pulses with 1000-2000 acquisitions. All isotropic Al shifts have been referenced to A1(N03)3 in aqueous solution. 

As mentioned earlier, initiator 2 has rotational isomers in which the alkylidene substituent is turned either towards the second multiple bond (5y ) or away from it anti)-, eqn. (11). In this case the equilibrium position is very much in favour of the syn rotamer K= 1450 in toluene at 25°C), making it difficult to detect the anti rotamer in routine spectra. However, the equilibrium can be displaced by UV irradiation (366 nm) of the solution for several hours at — 80°C to yield a mixture containing about 33% of the anti rotamer as determined from the resonances syn, 6 12.11,7ch = 120.3 Hz anti, 5 13.30, Jch= 153.3 Hz (Oskam 1992, 1993a). On adding 0.33 equivalents of 2,3-bis(trifluoromethyl)norbomadiene to this solution and running the spectrum again at — 80°C it is found that the anti rotamer has been completely consumed, giving the syn first-addition product, eqn. (12), while the syn rotamer has scarcely reacted at all. It is estimated that the... 

For routine spectra, a medium-sized spectral slit width is used, depending on the natural bandwidth, and the scan rate selected should not be too high. A medium time constant with a medium degree of amplification avoids noisy spectra. 

The solid state deuterium NMR spectra were recorded on a home-built spectrometer operating at 55.26 MHz for deuterium (360 MHz for protons). The spectrometer has been described previously (5). Routine spectra were obtained in quadrature using the quadrupolar echo pulse sequence (Figure 3)... 

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