In this communication we describe a simple approach to compensate for the effects of unstable static fields that can mask the temperature dependence of 79Br

isotropic chemical shifts. Since KBr has only one isotropic 79Br resonance line flanked by a family of spinning sidebands, a single spectrum cannot provide a conclusive Nivolumab manufacturer proof that the observed shift is purely induced by temperature. To overcome this problem, we used 13C resonance signals from adamantane mixed with KBr to monitor any change of the external magnetic field B0. Adamantane molecules freely rotate in a cubic phase between 208 and 543 K and the two 13C chemical shifts appear to be insensitive

to temperature, at least over the range probed in this work. Both KBr and adamantane in natural abundance provide strong signals and the difference between the 79Br and 13C resonance frequencies is only about 0.4%. Thus one can record both resonances in two consecutive single-pulse experiments within a few seconds without the need to retune the NMR probe. The experiments find more were conducted at two static fields using a Bruker 800 MHz wide-bore spectrometer equipped with a 3.2 mm E-free MAS probe and a Bruker 400 MHz wide-bore spectrometer equipped with 1.3, 2.5 and 4.0 mm MAS probes. The 79Br and 13C spectra were acquired using four scans each with

a recovery interval of 1.0 and 4.0 s, respectively. No decoupling was applied for recording 79Br spectra of KBr while low-power PISSARRO decoupling [16], [17] and [18] was used during the acquisition of 13C spectra of adamantane. Fig. 1 shows the temperature dependence of the observed 79Br and 13C chemical shifts recorded at two static fields science B0 = 9.4 T (99.8818 MHz for 79Br, 100.2455 MHz for 13C) and B0 = 18.8 T (200.4446 MHz for 79Br, 201.1682 MHz for 13C) using 4.0 and 3.2 mm probes, respectively, and setting both 79Br and 13C chemical shifts arbitrarily to zero at 296 K, referring to [15]. In each case, for decreasing temperatures, the single-pulse experiments were started only when the temperature reading of the temperature controller had been stable for at least 20 min. A roughly linear down-field shift of the 79Br signal is observed initially in both magnets when decreasing the temperature. The 13C lines of adamantane reveal small but significant up-field shifts at B0 = 9.4, and down-field shifts at 18.8 T. Quite unexpectedly however, a striking reversal of the trends of both 79Br and 13C chemical shifts was observed at 18.8 T below 290 K. This, at first glance puzzling, apparent reversal of the direction of the 79Br chemical shift is in fact due to the change of the static field.