title: A robust approach to correct for pronounced errors in temperature measurements by controlling radiation damping feedback fields in solution NMR.
abstract: Accurate temperature measurement is a requisite for obtaining reliable thermodynamic and kinetic information in all NMR experiments. A widely used method to calibrate sample temperature depends on a secondary standard with temperature-dependent chemical shifts to report the true sample temperature, such as the hydroxyl proton in neat methanol or neat ethylene glycol. The temperature-dependent chemical shift of the hydroxyl protons arises from the sensitivity of the hydrogen-bond network to small changes in temperature. The frequency separation between the alkyl and the hydroxyl protons are then converted to sample temperature. Temperature measurements by this method, however, have been reported to be inconsistent and incorrect in modern NMR, particularly for spectrometers equipped with cryogenically-cooled probes. Such errors make it difficult or even impossible to study chemical exchange and molecular dynamics or to compare data acquired on different instruments, as is frequently done in biomolecular NMR. In this work, we identify the physical origins for such errors to be unequal amount of dynamical frequency shifts on the alkyl and the hydroxyl protons induced by strong radiation damping (RD) feedback fields. Common methods used to circumvent RD may not suppress such errors. A simple, easy-to-implement solution was demonstrated that neutralizes the RD effect on the frequency separation by a "selective crushing recovery" pulse sequence to equalize the transverse magnetization of both spin species. Experiments using cryoprobes at 500 MHz and 800 MHz demonstrated that this approach can effectively reduce the errors in temperature measurements from about ±4.0 K to within ±0.4 K in general.
authors.affiliation: Department of Chemistry and Biochemistry, University of California at Los Angeles, Los Angeles, CA 90095, United States.
authors.forename: Stephanie M
authors.initials: SM
authors.lastname: Wolahan
authors.affiliation: Department of Chemistry and Biochemistry, University of California at Los Angeles, Los Angeles, CA 90095, United States.
authors.forename: Zhao
authors.initials: Z
authors.lastname: Li
authors.affiliation: Molecular Science and Technology Program, Taiwan International Graduate Program, Institute of Atomic and Molecular Sciences, Academia Sinica, Taipei 10617, Taiwan; Department of Chemistry, National Tsing Hua University, Hsinchu 30013, Taiwan.
authors.forename: Chao-Hsiung
authors.initials: CH
authors.lastname: Hsu
authors.affiliation: Instrumentation Center, National Taiwan University, Taipei 10617, Taiwan.
authors.forename: Shing-Jong
authors.initials: SJ
authors.lastname: Huang
authors.affiliation: Department of Chemistry and Biochemistry, University of California at Los Angeles, Los Angeles, CA 90095, United States.
authors.forename: Robert
authors.initials: R
authors.lastname: Clubb
authors.affiliation: Department of Chemistry, National Taiwan University, Taipei 10617, Taiwan; Institute of Atomic and Molecular Sciences, Academia Sinica, Taipei 10617, Taiwan.
authors.forename: Lian-Pin
authors.initials: LP
authors.lastname: Hwang
authors.affiliation: Department of Chemistry and Biochemistry, University of California at Los Angeles, Los Angeles, CA 90095, United States. Electronic address: yylin@chem.ucla.edu.
authors.forename: Yung-Ya
authors.initials: YY
authors.lastname: Lin
mesh_terms: D000465:Algorithms; D016477:Artifacts; D019855:Ethylene Glycol / Q000737:chemistry*; D005246:Feedback; D060526:Magnetic Fields; D009682:Magnetic Resonance Spectroscopy; D000432:Methanol / Q000737:chemistry*; D015203:Reproducibility of Results; D012680:Sensitivity and Specificity; D013696:Temperature; D013817:Thermography / Q000379:methods*
chemical_list: D019855:Ethylene Glycol; D000432:Methanol
keywords: Dynamic frequency shifts; Nonlinear NMR; Radiation damping; Temperature measurements