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

Abstract

Using NMR, we measure the proton chemical shift δ, of supercooled nanoconfined water in the temperature range 195 K < T < 350 K. Because δ 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 −(∂ ln δ/∂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 −(∂ ln δ/∂T)_P displays a pronounced maximum upon crossing the locus of maximum correlation length at ≈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.