Pulsed microwave irradiation

One very recently published result must be mentioned. Petukhov et al. have reported the detection of the EPR spectrum of micron-sized crystals of Fe8 via their magnetization response as a function of apphed magnetic field, using a Hall-probe magnetometer under either continuous wave or pulsed microwave irradiation at 118 GHz and between 1.4 and 50 K [53]. Dips are observed in the magnetization vs. field curves corresponding to resonant absorption - that is, EPR transitions. This method offers potentially extraordinary sensitivity and, furthermore, manipulation of the magnetization data in the absence and presence of the microwave radiation allows determination of the spin temperature. 

Microwave processing has been used to process thermoset polymers and polymer composites, including polyesters, polyurethanes polyimides and epoxies, and in most studies it has been concluded that the curing speed is faster using microwave energy (Clark and Sutton, 1996). The effects of continuous and pulsed microwave irradiation on the polymerization rate and final properties also have been studied, and it was demonstrated that, for certain epoxies, a pulsed microwave cure resulted in improvements in mechanical properties, better temperature uniformity and a faster polymerization mte (Thuillier and Jullien, 1989). 

Emulsion polymerization of methyl methacrylate under the action of pulsed microwave irradiation was studied by Zhu et al. [11], The reactions were conducted in a self-designed single-mode microwave reaction apparatus with a frequency of 1250 MHz and a pulse width of 1.5 or 3.5 ps. The output peak pulse power, duty cycles, and mean output power were continuously adjustable within the ranges 20-350 kW, 0.1-0.2%, and 2-350 W, respectively. Temperature during microwave experiments was maintained by immersing the reaction flask in a thermostatted jacket with a thermostatic medium with little microwave absorption (for example tetrachloroethylene). In a typical experiment, 8.0 mL methyl methacrylate, 20 mL deionized water, and 0.2 g sodium dodecylsulfonate were transferred to a 100-mL reaction flask which was placed in the microwave cavity. When the temperature reached a preset temperature, 10 mL of an aqueous solution of the initiator (potassium persulfate) was added and the flask was exposed to microwave irradiation. 

In the early works, it was found that the pulse method could lead to the fastest heating of epoxy resins [98] and improve their mechanical properties [99]. For example, it was shown that a computer controlled pulsed microwave processing of epoxy systems that consisted of diglycidyl ether of bisphenol A (DER 332) and 4,4 -diaminodiphenyl sulfone (DOS) in a cavity operated in TM012 mode could be successfully applied to eliminate the exothermic temperature peak and maintain the same cure temperature at the end of the reaction [100]. The epoxy systems under pulsed microwave irradiation were cured faster, and it was possible to cure them at higher temperatures when compared with a continuous microwave or conventional thermal processing. 

Cheng et al. (2005) investigated the ATR polymerization of styrene under pulsed microwave irradiation. Controlled polymerization of styrene revealed that polymerization rates were three times larger than those for thermal heating. 

Scheme 2.6 Pulsed versus continuous microwave irradiation (bmimPF6 = l-butyl-3-methylimidazolium hexafluorophosphate).

No specific recommendations can be given about the optimum reaction time. As speeding up reactions is a key motive for employing microwave irradiation, the reaction should be expected to reach completion within a few minutes. On the other hand, a reaction should be run until full conversion of the substrates is achieved. In general, if a microwave reaction under sealed-vessel conditions is not completed within 60 min then it needs further reviewing of the reaction conditions (solvent, catalyst, molar ratios). The reported record for the longest microwave-mediated reaction is 22 h for a copper-catalyzed N-arylation (see Scheme 6.63). The shortest ever published microwave reaction requires a microwave pulse of 6 s to reach completion (ultra-fast carbonylation chemistry see Scheme 6.49). 

Microwave-induced, catalytic gas-phase reactions have primary been pursued by Wan [63, 64], Wan et al. [65] have used pulsed-microwave radiation (millisecond high-energy pulses) to study the reaction of methane in the absence of oxygen. The reaction was performed by use of a series of nickel catalysts. The structure of the products seemed to be function of both the catalyst and the power and frequency of microwave pulses. A Ni/Si02 catalyst has been reported to produce 93% ethyne, whereas under the same irradiation conditions a Ni powder catalyst produced 83% ethene and 8.5 % ethane, but no ethyne. 

Dimetlioxybenzene la (5 mmol), diethyl azodicarboxylate (5 mmol) and InCl3-Si02 (3 wt equiv. of arene) were admixed in an Erlenmeyer flask and exposed to microwave irradiation at 450 W using BPL, BMO-700 T focused micro-wave oven for 3 min (pulsed irradiation 1 min with 20 s interval). On completion, the reaction mixture was directly charged on a small silica gel column and eluted with a mixture of ethyl acetate-hexane (2 8) to afford pure hydrazine 2a in 88% yield as a pale yellow solid. 

Microwave irradiation considerably improved the reaction speed and yields of 2,4-disubstituted quinolines in a MCR of aldehydes, anilines and alkynes [111]. The cyclocondensation was catalyzed by montmorillonite clay doped with copper bromide and was completed within 3-5 minutes (pulsed irradiation technique—1 min with 20 s off interval), when performed in a household microwave oven. Oil-bath heating at 80 °C for 3-6 hours was necessary to achieve comparable yields of quinolines (71-90%) (Scheme 42). 

Pd(2-thpy)2 is dissolved in an n-octane matrix and is excited at low temperature (T < 2 K) by a c.w. source (non-pulsed, e.g. at A = 330 nm [61]). Additionally, microwave irradiation is applied and scanned in frequency. The microwave radiation can cause transitions between the triplet sublevels in the case of resonance, and thus, the previously different and non-thermalized steady state populations of the substates are usually altered. Under suitable conditions (see below) a change of the phosphorescence intensity will result due to microwave perturbation. Usually, this effect is very weak. Therefore, microwave pulse trains are applied, for example, with a repetition rate of 150 Hz. Thus, one can monitor the microwave-induced intensity changes by a phase-sensitive lock-in technique [90]. 

This method is not only limited to halide-based ionic liquids. Synthesis of ionic liquids containing a tetrafluoroborate anion and imidazolium cation has been performed successfully [16] by use of a modified domestic microwave oven with pulsed irradiation (5 x 30 s). Approximately 90% yields of the desired ionic liquids were usually obtained compared with 36% after the same reaction time using conventional heating. Microwave irradiation has been used in the synthesis of l-ethyl-3-methylimidazolium benzoate and dialkyl imidazolium tetrachloroaluminate ionic liquids [17, 18]. These reactions were again performed in a domestic microwave oven using pulsed irradiation. 

The Kabachnik-Fields reaction is an effective means of preparing biologically active a-amino phosphonates [64]. It involves the three component reaction of an aromatic aldehyde, an aniline, and diethylphosphite. The reaction has recently been performed using microwave irradiation with [BMIM]PF6, [BMIM]SbF6, [BMIM]BF4, and DMF as solvents and lanthanide triflates as catalysts (Scheme 7.18) [65]. The reactions were performed using a domestic microwave oven and pulsed irradiation. Catalyst activity in the ionic liquids was found to be higher than or comparable with that in DMF. It was also found that catalyst activity varied depending on the ionic liquid used. For example, Yb(OTf)3 was very active in [BMIM]BF4 but Sc(OTf)3 was more active in [BMIMjPFe. Excellent product yields were obtained. 

Integrated Solid Effect (ISE) was first introduced by Henstra et al. [17]. It can overcome the low efficiency of SE when the homogeneous width is much larger than the nuclear Larmor frequency (A (Uo v)> in which the polarization effect could be canceled by simultaneous saturation of the forbidden transitions at ct)o fflow-The ISE can preserve the polarization in the case of A coow by inverting a forbidden EPR transition prior to saturation of an allowed transition. This effect can be achieved by using a selective inversion pulse after the irradiation on resonance at cooe i (Oon, or applying CW microwave irradiation at a fix frequency... 

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