Isotopomeric Elucidation of the Mechanism of Temperature Sensitivity in 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 Co chemical shift thermometry. Exposure of [Co(en)] (; en = ethylenediamine) and [Co(diNOsar)] (; diNOsar = dinitrosarcophagine) to mixtures of HO and DO produces distributions of [Co(en)]- ( = 0-12) and [Co(diNOsar)]- ( = 0-6) isotopomers all resolvable by Co NMR. Variable-temperature Co NMR analyses reveal a temperature dependence of the Co chemical shift, Δδ/Δ, on deuteration of the N-donor atoms. For , deuteration amplifies Δδ/Δ by 0.07 ppm/°C. Increasing degrees of deuteration yield an opposing influence on , diminishing Δδ/Δ by -0.07 ppm/°C. Solution-phase Raman spectroscopy in the low-frequency 200-600 cm regime reveals a red shift of Raman-active Co-N vibrational modes by deuteration. Analysis of the normal vibrational modes shows that Raman modes produce the largest variation in Co δ. Finally, partition function analysis of the Raman-active modes shows that increased populations of Raman modes predict greater Δδ/Δ, representing new experimental insight into the thermometry mechanism.