Long-lived states involving a manifold of fluorine-19 spins in fluorinated aliphatic chains

<i>Long-lived states</i> (LLSs) have lifetimes <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow><mml:msub><mml:mi>T</mml:mi> <mml:mi>LLS</mml:mi></mml:msub> </mml:mrow> </mml:math> that exceed longitudinal spin-lattice relaxation times <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow><mml:msub><mml:mi>T</mml:mi> <mml:mn>1</mml:mn></mml:msub> </mml:mrow> </mml:math> . In this study, lifetimes <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow><mml:msub><mml:mi>T</mml:mi> <mml:mi>LLS</mml:mi></mml:msub> </mml:mrow> </mml:math> (¹⁹F) have been measured in three different achiral per- and polyfluoroalkyl substances (PFAS) containing two or three consecutive CF₂ groups. In a static magnetic field <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow><mml:msub><mml:mi>B</mml:mi> <mml:mn>0</mml:mn></mml:msub> <mml:mo>=</mml:mo> <mml:mn>11.7</mml:mn></mml:mrow> </mml:math> T, the lifetimes <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow><mml:msub><mml:mi>T</mml:mi> <mml:mi>LLS</mml:mi></mml:msub> </mml:mrow> </mml:math> (¹⁹F) exceed the longitudinal relaxation times <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow><mml:msub><mml:mi>T</mml:mi> <mml:mn>1</mml:mn></mml:msub> <mml:msup><mml:mo>(</mml:mo> <mml:mn>19</mml:mn></mml:msup> </mml:mrow> </mml:math> F) by about a factor of 2. The lifetimes <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow><mml:msub><mml:mi>T</mml:mi> <mml:mi>LLS</mml:mi></mml:msub> </mml:mrow> </mml:math> (¹⁹F) can be strongly affected by binding to macromolecules, a feature that can be exploited for the screening of fluorinated drugs. Both <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow><mml:msub><mml:mi>T</mml:mi> <mml:mi>LLS</mml:mi></mml:msub> </mml:mrow> </mml:math> (¹⁹F) and <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow><mml:msub><mml:mi>T</mml:mi> <mml:mn>1</mml:mn></mml:msub> </mml:mrow> </mml:math> (¹⁹F) should be longer at lower fields where relaxation due to the chemical shift anisotropy (CSA) of ¹⁹F is less effective, which is demonstrated here by running experiments at two fields of 11.7 and 7 T.