T, = 4867/(209 - Ôppm) CI C3 C2 85K
For H PLC GC TLC andJgnd.,
c3
C(,C2
c 4 ,c 4 · 98 Κ
c4 C· 128 Κ
C3
c,
C
c4 C
CHROMAboNcf
141 Κ
c3 C
300
200
100
c4-
0
298 Κ
-100
ppm
Figure 6. Variable-temperature 13C CP/MAS NMR spectra of anhydrous copper(ll) n-butyrate, a solid compound exhibiting antiferromagnetic exchange coupling. Ci refers to carboxylate carbons, C4 and C4' refer to methyl carbons, and C2 and C3 are the intermediate methylene carbons.
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(2)
This equation is useful from 77 Κ to approximately 270 K; at higher tem peratures, the change in chemical shift with temperature is too small for accu rate measurements. We have applied the samarium acetate tetrahydrate thermometer in a detailed study (14) of rf heating effects, thermal equilibra tion times, temperature gradients, and other factors affecting temperature measurement and control in VT MAS experiments. These studies have led to the development of the temperaturesensing arrangement in Figure 2, which works well provided that rf heating is minimal. Discovering improved solidstate chemical-shift thermometers that either replace or supplement samarium acetate tetrahydrate would be a useful contribution to the field. The application of VT MAS NMR to a somewhat more complicated magnet ic phenomenon, antiferromagnetic ex change coupling, is illustrated in Fig ure 6, which shows 13C data for anhy drous Cu(II) n-butyrate. Solid copper (II) carboxylates have interacting pairs of metal centers. The electronic ground state has all electron spins paired and is, therefore, a singlet. There is, howev er, a triplet state a few hundred c m - 1 above the ground state. Thermal popu lation of the upper state balanced with thermal randomization of electronspin orientation gives rise to a maxi mum in the susceptibility curve near room temperature. Starting with basic magnetic theory and assuming a con tact shift mechanism, it can be shown that the resonance frequencies of nu clear spins in d9-d9 dimers should have the following temperature dependence
(7 C /27r)/eT
spectra of a paramagnetic solid, samarium acetate tetrahydrate, the chemical shifts of which obey the Curie law (i.e., they are linear with 1/T) over a wide temperature range (22). The chelatingonly carbonyl signal (the more intense signal in Figure 5) is the more temperature-dependent resonance, and it has been applied by our group as the first MAS NMR chemical-shift thermometer. Determining temperature is one of the most basic physical measurements. In MAS NMR, however, it is not a trivial operation. The sample temperature (Ts) can differ from that of the surrounding gas streams (especially if the sample is subject to rf heating), but the sample itself is inaccessible to thermocouples or other sensing devices. Determining the chemical shift of the chelating-only carbonyl resonance of samarium acetate tetrahydrate allows the sample temperature to be calculated using Equation 2:
(3) where — 2J is the singlet-triplet energy separation, A is the hyperfine coupling constant for a given nucleus, and the other terms have their usual meanings in magnetic resonance theory. We have found that Equation 3 accurately pre dicts the temperature dependence of the 13C chemical shifts of all of the an hydrous Cu(II) carboxylates that we have examined to date. Statistical analysis of δ vs. Τ data allows values of —2J, A, and 5dia to be determined for these complexes. This information sheds light on the mechanism of anti ferromagnetic exchange coupling and may be of use in the development of magneto-structural correlations. Synthetic chemists commonly use solution-state NMR to obtain kinetic and mechanistic data for chemical re actions occurring in the NMR tube, and it is reasonable to expect that sol id-state and surface reactions can be studied with VT MAS NMR. In one
