Frequency corresponding to the centre of the
spectrum window for a given nucleus under observation and the nominal frequency used to generate the hard, full
spectrum pulses.
Source:
PAC, 2021, 93, 647. 'Glossary of methods and terms used in analytical spectroscopy (IUPAC Recommendations 2019)' on page 699 (https://doi.org/10.1515/pac-2019-0203)