Solid-state deuteration

Quinoxalin-2-ones are in tautomeric equilibrium with 2-hydroxy-quinoxalines, but physical measurements indicate that both in solution and in the solid state they exist as cyclic amides rather than as hydroxy compounds. Thus quinoxalin-2-one and its A -methyl derivative show practically identical ultraviolet absorption and are bases of similar strength. In contrast, the ultraviolet spectra of quinoxalin-2-one and its 0-methyl derivative (2-methoxyquinoxaIine) are dissimilar. The methoxy compound is also a significantly stronger base (Table II). Similar relationships also exist between the ultraviolet absorption and ionization properties of 3-methylquinoxalin-2-one and its N- and 0-methyl derivatives. The infrared spectrum of 3- (p-methoxy-benzyl)quinoxalin-2-one (77) in methylene chloride shows bands at 3375 and 1565 cm" which are absent in the spectrum of the deuterated... 

It has recently been demonstrated that the analysis of MAS sidebands patterns can be used to study molecular dynamics in the solid state [85-88]. Indeed, the line narrowing effect of MAS can be partly offset, or completely eliminated, if the 2H quadrupole tensor is reoriented due to motion on a time scale comparable to (first-order quadrupolar broadening, such motion-induced effects should be less evident in the DQMAS spectrum, as has indeed been observed by Wimperis and colleagues in several deuterated solids [87, 88]. For example, the simulation of the SQ spectrum of tetrathionate dihydrate-cfi yielded the same reorientational rate constant as the previously described quadrupolar echo approach (Fig. 6). 

In this review we focus on the detailed side chain dynamic of PBLG and racemic PBG deuterated at several positions in a side chain by means of solid state 2H NMR. The brief description of the 2H NMR parameters used... 

As is obvious from the table, Tc is almost doubled upon deuteration. These isotope effects are one of the largest observed in any solid state system. The question arises about isotope effects in non-hydrogen-bonded ferro- and antiferroelectrics. As already mentioned in the Introduction, within a mean-field scheme and in a purely ionic model it was predicted that these systems should not exhibit any isotope effect in the classical limit, which has been verified experimentally. Correspondingly, there was not much effort to look for these effects here. However, using a nonlinear shell-model representation it was predicted that in the quantum limit an isotope effect should... 

The absence of an EPR signal in solution or in the solid state is indicative of a singlet ground state for the diradical ( BuBP Pr2)2- An indication of the radical character of this derivative is provided by a variety of facile oxidative addition reactions (Scheme 9.12). " For example, the treatment of ( BuBP Pr2)2 with diphenyl diselenide (or elemental selenium) produces a bicyclic compound in which a selenium atom bridges the two boron atoms. Trimethyl tin hydride reacts rapidly with ( BuBP Pr2)2 to give the trans adduct. Finally, ( BuBP Pr2)2 is slowly oxidised by deuterated chloroform to produce a R,R -dichloro adduct as a mixture of cis and trans isomers. 

Crystalline 2-methylimidazole exhibits different 13C (CPMAS) chemical shifts for C-4 and C-5 (125.0, 115.7 ppm). The average (120.3 ppm) is close to that reported for imidazole in deuterated DMSO (121.2 ppm). These results imply that solid state chemical shifts can be used instead of N-methyl models in tautomerism studies (87H(26)333). For imidazole the solid state l3C shifts are 137.6 (C-2), 129.3 (C-4), and 119.7 (C-5) (81JA6011). No proton exchange occurs in the solid, and the data support a structure resembling the crystal structure. Cooling imidazole solutions has not yet allowed the detection of individual tautomers, but by symmetry the compound exists in equal tautomeric forms, as does pyrazole (81CC1207). 

The solid-state deuterium NMR experiments were performed on surface-adsorbed material from triethoxyaminopropylsilane-rf, (DAPES) and triethoxy-aminobutylsilane-dj (DABES) prepared as described previously [9]. The samples were heated to 90°C at 10 mmHg for 12 h after adsorption. These deuterated coupling agents have been characterized by IR and NMR spectroscopy. The solid-state deuterium NMR spectroscopy was done on a Varian VXR-200 at 30.7 MHz. A quadrupole echo pulse sequence was used with a 2 s interval, 2 /is 90° pulse, 30 ps echo time, 2 MHz sweep width, and, typically, overnight accumulation. 

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