EXAFS investigations of temperature-dependent structure in cobalt-59 molecular NMR thermometers

Abstract

EXAFS spectroscopy was used to study the temperature-dependent molecular structures of a series of cobalt-59 NMR thermometers.

Ligand Control of 59Co Nuclear Spin Relaxation Thermometry

“…We instead propose that these changes in motion synergize with changes in the local symmetry of the 59 Co nucleus to produce the observed trends in ∆T 1 /∆T, especially in the series of D 3 structures. Previous studies of 3-6 revealed~0.007 Å changes in Co-N bond distances per • C over the 50 • C temperature range of our investigations here [27]. These changes in bond distances were also accompanied by changes in symmetry of the coordination geometry through changes in N-Co-N angles.…”

“…To assist in understanding the relaxation time data, we computed values of the quadrupolar coupling constant parameter (e 2 qQ) for the Co-N6 encapsulation series (2-6) at different temperatures within the 10-60 °C window. Predictions of e 2 qQ were completed from partially optimized, variabletemperature structures following analyses from extended X-ray absorption fine-structure (EXAFS) spectroscopy [27]. Values of e 2 qQ computed for these structures range from −1.861 to −1.910 MHz for As an additional method of comparing the variation in 59 Co nuclear spin properties of 1-6, we investigated the dephasing time, or T 2 *, a relaxation time analogous to T 2 above.…”

“…This effect may be rationalized by a hindered variation in the symmetry of the structure due to the relative interconnectivity of the individual N donor atoms. Indeed, EXAFS analyses suggest that 4 exhibits the greatest transition towards O h symmetry with increasing temperature when 3 , 5 , and 6 all deviate toward D 3 symmetry [ 27 ]. This subtle difference in temperature-dependent structure is likely an important point toward designing future 59 Co NMR thermometers.…”

“…Computational analyses were completed for the Co–N 6 encapsulation series ( 2 – 6 ) by structural optimizations over a range of temperatures. Temperature-specific optimizations were assisted by previous extended X-ray absorption fine-structure (EXAFS) characterization by fixing Co–N distances according to experimentally determined metal–ligand bond lengths to the three temperatures utilized in the EXAFS study, i.e., 13, 35, and 57 °C [ 27 ]. The remainder of the structure was allowed to optimize freely about the fixed Co–N 6 coordination sphere using the Gaussian 16 [ 28 ] electronic structure package.…”

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

“…More “flexible” ligands could be envisioned to produce species with greater temperature-dependent inner-coordination spheres, thereby producing higher Δδ/Δ T . However, recent studies by us − and others , suggest that bond-distance expansion alone is not the dominant feature. In a prior work, we posited a model wherein structural rigidity imparted by the ligand shell engenders long-lived vibrations caused by collisions of a given 59 Co NMR thermometer with the surrounding solvent (Figure ) and thus temperature-driven structures and Δδ/Δ T .…”