Relaxation agent for speed up 13C NMR spectra for the Kinetic Isotope Effect measurements

• Grant number: 22/13038-5
• Support Opportunities: Scholarships in Brazil - Scientific Initiation
• Start date: November 01, 2022
• End date: October 31, 2023
• Field of knowledge: Physical Sciences and Mathematics - Chemistry - Organic Chemistry
• Principal Investigator: Claudio Francisco Tormena
• Grantee: Leonardo Claudio
• Host Institution: Instituto de Química (IQ). Universidade Estadual de Campinas (UNICAMP). Campinas , SP, Brazil
• Associated research grant: 20/10246-0 - Nuclear magnetic resonance spectroscopy: from pulse sequences to structural assignments, AP.TEM

Abstract

The measuring Kinetic Isotope Effect (KIE) at the natural isotopic abundance of 13C NMR is a time-consuming task, as it requires very high signal-to-noise ratio quantitative 13C NMR spectra, necessary to reduce systematic errors in the integration procedure. Typically, each quantitative 13C NMR experiment takes at least 30 hours, for a highly concentrated sample and considering that quintuplicate measurements are necessary, it is possible to estimate around seven days to acquire a single KIE measurement. This long experiment duration is essential to guaranty that the net magnetization is restored to the equilibrium condition (relaxation time T1). The T1 of 13C signals are usually between 5 to 60 seconds, depending on the carbon substitution in the molecular framework. For quantitative measurements, setting a recycle delay (i.e. the period between scans, d1) equals to 5 times T1 is mandatory (for the longest T1), resulting often in 300 seconds between scans. For concentrate sample (from 30 to 80 mg per mL) considering a small molecule (i.e. MW up to 500 Daltons), an adequate signal-to-noise ratio can be achieved using at least 512 scans, resulting in an experiment of 40 hours, or ten days, for single KIE data. This time-consuming experiment is impracticable for many NMR laboratories since a dedicated NMR spectrometer is necessary.