Isotopomeric Elucidation of the Mechanism of Temperature Sensitivity in 59 Co NMR Molecular Thermometers.

Understanding the mechanisms governing temperature-dependent magnetic resonance properties is essential for enabling thermometry via magnetic resonance imaging. Herein we harness a new molecular design strategy for thermometry─that of effective mass engineering via deuteration in the first coordination shell─to reveal the mechanistic origin of 59 Co chemical shift thermometry. Exposure of [Co(en) 3 ] 3+ ( 1 ; en = ethylenediamine) and [Co(diNOsar)] 3+ ( 2 ; diNOsar = dinitrosarcophagine) to mixtures of H 2 O and D 2 O produces distributions of [Co(en) 3 ] 3+ - d n ( n = 0-12) and [Co(diNOsar)] 3+ - d n ( n = 0-6) isotopomers all resolvable by 59 Co NMR. Variable-temperature 59 Co NMR analyses reveal a temperature dependence of the 59 Co chemical shift, Δδ/Δ T , on deuteration of the N-donor atoms. For 1 , deuteration amplifies Δδ/Δ T by 0.07 ppm/°C. Increasing degrees of deuteration yield an opposing influence on 2 , diminishing Δδ/Δ T by -0.07 ppm/°C. Solution-phase Raman spectroscopy in the low-frequency 200-600 cm -1 regime reveals a red shift of Raman-active Co-N 6 vibrational modes by deuteration. Analysis of the normal vibrational modes shows that Raman modes produce the largest variation in 59 Co δ. Finally, partition function analysis of the Raman-active modes shows that increased populations of Raman modes predict greater Δδ/Δ T , representing new experimental insight into the thermometry mechanism.