We have probed under high pressure the C-H hydrogen bonds formed by N,N(')-disubstituted imidazolium ions having PF(6) (-) and Br(-) counterions. High-pressure infrared spectral profiles, x-ray crystallographic analysis, and ab initio calculations allow us to make a vibrational assignment of these compounds. The appearance of a signal for the free-NH unit (or weakly bonded N-H...F unit) in the infrared spectrum of the PF(6) (-) salt indicates that conventional N-H...O and N-H...N hydrogen bonds do not fully dominate the packing. It is likely that the charge-enhanced C(2)-H...F interactions, combined with other weak hydrogen bonds, disturb the formation of N-H hydrogen bonds in the PF(6) (-) salt. This finding is consistent with the pressure-dependent results, which reveal that the C(2)-H...F interaction is enhanced upon increasing the pressure. In contrast to the PF(6) (-) salt, the imidazolium C-H bonds of the Br(-) salt have low sensitivity to high pressure. This finding suggests that the hydrogen bonding patterns are determined by the relative hydrogen bond acceptor strengths of the Br(-) and PF(6) (-) ions.

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We have investigated pressure-enhanced C−H···O interactions in tert-butyl alcohol (TBA)/D2O mixtures. On the basis of its responses to changes in pressure and concentration, TBA appears to be the ideal candidate to study the variations in structural and dynamical properties of C−H···O interactions. For dilute aqueous TBA, the pressure dependence of the C−H bands yielded blue frequency shifts at pressures below 0.3 GPa, but an increase in pressure leads to a red frequency shift at pressures above this value. This discontinuity in frequency shift is related to enhanced C−H···O interactions. The frequency of the C−H stretching modes that characterize C−H···O hydrogen bonding undergoes a blue shift with pressure. This behavior is in contrast with the general trend of red shifts observed in strongly hydrogen-bonded systems that occur through O−H···O and CO···H interactions. We discuss the results of density functional theory calculations that predict the frequency shift of the C−H stretching vibrations.

Luminescence D 6540Synthesis of YAG:Ce Phosphor via Different Aluminum Sources and Precipitation Processes. -Y3Al5O12:Ce 3+ phosphors are synthesized by four different precipitation processes using either Al(NO3)3 or NH4Al(SO4)2 as the aluminum source. A phase-pure phosphor is obtained by the normal strike precipitation process from Al(NO3)3 as aluminum source and NH4HCO3 as precipitation agent at 850°C. This sample has the highest emission intensity at 535 nm when excited at 470 nm. -(CHIANG*, C. C.; TSAI, M. S.; HSIAO, C. S.; HON, M. H.; J.

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