Resonance coalescence

Their H NMR spectra feature one resonance due to the Al-R groups at ambient temperature, whereas at —70 °C two resonances of the Al-R groups in a relative intensity of 1 2 appear, obviously caused by the presence of (R3A1)2 dimers (terminal and bridging substituents) in solution. Both resonances coalesce between —30 and — 50 °C (Al-Me) and —25 and —40°C (Al-Et), respectively, as was previously described for (Me3Al)2 and (Et3Al)2. 

Fig. 6. Part of the spectrum of lysozyme in the presence of the inhibitor (G1cNAc)3. At the concentration used, two resonances from 1 proton of trp 63 can be observed at 6.25 ppm and 6.05 ppm. These correspond to the resonance of this protein in the unbound and inhibitor-bound protein, respectively. The shift indicates that a conformational change occurs in the protein on binding (G1cNAc)3. At low temperature (c) the rate of the conformational change is slow on the time scale of the nmr experiment, but at higher temperature (a) the two resonances coalesce as the rate of the conformational change increases. At 45°C (b) exchange broadening occurs. The rate of the conformational change was measured from these data, and at 45°C is 20 sec-1 (see Ref. 22).

The second article presents NMR spectra of ethanol-water mixtures. Weinberg and Zimmerman found that the alcohol O—H resonance is distinct from the water O—H resonance when the water concentration is below 20 percent by weight (2146). At 27 percent by weight water (corresponding to a 0.5 mole ratio) the two resonances coalesce, and remain so at higher water concentrations. These workers attribute the coalescence to the onset of proton exchange between water and alcohol with an exchange time of 0.05 seconds. 

Figure 1 shows the effect of progressively increased conformational motion as the applied ultrasonic intensity is increased. It also shows the NMR spectrum of the sample when heated to 44 °C. The sample had to be heated electrically to over 100 °C before an equivalent H CH3 resonance coalescence occurred. After the ultrasound experiment the final temperature was ca. 30 °C. 

Fast-exchange limit. The fast exchange limit is said to be reached when no further increase in the rate at which a dynamic process occurs will alter observed spectral features. Normally, we speak of resonance coalescence as occurring when the fast-exchange limit is reached. 

Rapid chemical exchange A chemical exchange process that occurs so rapidly that two or more resonances coalesce into a single resonance. 

The right panel of Figure 2.9 shows that the rate of level v = 8 reaches zero twice. This multiple occurrence of the zero-width phenomenon has been related to the possibility to produce several times a diabatic-adiabatic coincidence as the intensity increases [69]. This is so because the adiabatic (vibrational) levels goes up faster than the diabatic ones and therefore a given adiabatic level can cross several diabatic levels as the intensity increases. It is to be noted that resonance coalescence, i.e., the existence of an (EP), requires an appropriate choice of both frequency and intensity, while a ZWR would show up at some critical intensity(ies), irrespective of the choice of the wavelength, for all resonances of Feshbach type. One must keep in mind that the classification into shape and Feshbach depends strongly on the wavelength. 

R. Lefebvre, O. Atabek, M. Sindelka, N. Moiseyev, Resonance coalescence in molecular photodissociation Phys. Rev. Lett. 103 (2009) 123003. 

The complex involved is formed by photolysis of Cp2M2(CO)4 (M = Ru, Fe) in the presence of an acetylene. The fluxional process shown in reaction 1 occurs so rapidly that the two cyclopentadienyl resonances coalesce at 56°C, and at 90°C the resonances due to the ketonic and terminal carbonyl ligand are merged. 

The seven-co-ordinate [W(CO)3(L)a] (L=dimethyldithiocarbamate ion) shows three n.m.r. resonances at 163 K. On raising the temperature, two of these resonances coalesce independently of the third (AC = 8.1 kcal mol ) and at still higher temperatures all three resonances merge (AG =9.0kcal mol ). Both processes involve intramolecular isomerism. A series of seven-coordinate complexes of the type [MoCNjRXSaCNMea),] [R= aryl (1)] are also fluxional, and the activation parameters for the dynamic process which is observed are collected in Table 3. Simultaneous collapse of all of the methyl resonances is involved. A good correlation between the logarithm of the observed... 

---