Irradiation sequence length

The intense discoloration which developed rapidly upon UV exposure reveals the high photosensitivity of C-PVC that is even more pronounced than for PVC itself, as shown by the UV-visible absorption spectra of figure 5. After 15 minutes of irradiation, large amounts of polyenes have already accumulated in C-PVC, with sequence lengths up to 20 conjugated double bonds, while PVC is hardly affected after that short exposure. 

The measurement procedure is known as the pulse sequence, and always starts with a delay prior to switching on the irradiation pulse. The irradiation pulse only lasts a few microseconds, and its length determines its power. The NMR-active nuclei (here protons) absorb energy from the pulse, generating a signal. 

The general observation in the published work has been that for species such as hexamminechromium, thiocyanatopentammine, or the hexa-aquo ion where O18 exchange was looked at, irradiation produced a substitution reaction and nothing else. Moreover the reaction mode was independent of wave length, and the quantum yields did not change much. From a morphological point of view, there are essentially three types of explanations. First all excited states independently lead to the same chemical sequence, and we suppose that the primary act is simply a heterolytic bond fission. 

Equivalently, the second gradient can be of the same sign of the first, provided that a 180° pulse is applied in between the two. In the so-called Watergate sequence [14], selective non-excitation is achieved by tailoring the 180° pulse in such a way that all frequencies but the unwanted frequency are irradiated. The simplest scheme employed a 90° x (sel) 180° (non-sel)90° JC (sel) pulse sequence (Fig. 9.1G). The selectivity increases with the length of the 90° pulses. 

An alternative method for excitation of nuclei over a range of chemical shifts is by irradiation with a weak, noise-modulated radio-frequency, instead of with strong r.f. pulses. In one realization of this method, protons were irradiated with repetitive sequences of noise that was truly random,162 and, in another,163 fluorine nuclei were excited by pseudo-random noise generated by amplitude modulation of the r.f. with maximum-length sequences of pulses from a computer or shift register (a series of flip-flop devices connected by feedback loops). With the carrier wave suppressed, the latter process is equivalent to phase modulation of the r.f. by+7r/2 radians when the pulse is turned on, and by —ir/2 radians when it is turned off. This method is identical with that used in most broadband, heteronuclear, noise decouplers, except that greater power is required for decoupling. 

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