NMR evidence of a sharp change in a measure of local order in deeply supercooled confined water

Using NMR, we measure the proton chemical shift delta, of supercooled nanoconfined water in the temperature range 195 K < T < 350 K. Because delta is directly connected to the magnetic shielding tensor, we discuss the data in terms of the local hydrogen bond geometry and order. We argue that the derivative -(partial derivative In delta/partial derivative T)(P) should behave roughly as the constant pressure specific heat C-P(T), and we confirm this argument by detailed comparisons with literature values of C-P(T) in the range 290-370 K. We find that -(partial derivative In delta/partial derivative T)(P) displays a pronounced maximum upon crossing the locus of maximum correlation length at approximate to 240 K, consistent with the liquid-liquid critical point hypothesis for water, which predicts that C-P(T) displays a maximum on crossing the Widom line.