NMR Study of Water Structure in Super- and Subcritical Conditions

Abstract

The proton chemical shift of water is measured at temperatures up to 400 ± C and densities of 0.19, 0.41, 0.49, and 0.60 g͞cm 3 . The magnetic susceptibility correction is made in order to express the chemical shift relative to an isolated water molecule in dilute gas. Comparison of the chemical shifts of water in neat fluids at high temperatures to those in organic solvents at ambient conditions shows that the hydrogen bonding persists in supercritical water. At each density, the strength of the hydrogen bonding is found to reach a plateau value at high temperatures. [S0031-9007(97)02757-9] PACS numbers: 61.25. Em, 76.60.Cq, 82.30.Nr Water, which is an unusual solvent in ambient conditions, has recently been revealed to be a unique medium for chemical processes in super-and subcritical conditions [1]. In these extreme conditions, water loses its characteristics in ambient conditions and the solvation properties change drastically [1,2]. In order to understand and control the solvation properties on the molecular level, it is indispensable to characterize the microscopic structure of water in super-and subcritical conditions.Typically, the microscopic structure of water is described in terms of the correlation functions between the intermolecular pairs of atoms O-H, O-O, and H-H. In particular, the O-H pair correlation function provides information on the hydrogen bonding between a pair of water molecules. An important observation on the hydrogen bonding of water in super-and subcritical conditions was presented by Mountain [3] in his computer simulation studies. He showed, using the TIP4P model, that the hydrogen bonding persists in super-and subcritical water over a wide density range 0.1-1 g͞cm 3 . This observation is supported by most of the later simulation studies [4][5][6][7][8][9][10] with an exception of an ab initio molecular dynamics simulation by Fois et al. [11] A markedly different conclusion was obtained, however, from neutron scattering studies by Postorino et al. [12][13][14]. They showed with their O-H pair correlation function that at a supercritical temperature 400 ± C, the hydrogen bonding does not persist apparently even at a liquidlike density (0.66 g͞cm 3 ). Although there seems no established conclusion concerning the hydrogen bonding between a neighboring pair of water molecules in a supercritical state, it is concluded from all of the computer simulations [3][4][5][6][7][8][9][10][11], the neutron scattering studies [12][13][14], and the x-ray diffractometry [15] that the hydrogen bond network involving more than two water molecules is destroyed at high temperatures. Thus, it is desirable to study supercritical water with an experimental method which sensitively probes the short-range order of water.In this work, we study water in super-and subcritical conditions using proton NMR spectroscopy. The proton chemical shift is known to be sensitive to the hydrogen bonding of the observed proton with its environment, and we measure the proton chemical shifts of water up to a supercritical ...

Nuclear quantum effects and hydrogen bond fluctuations in water

“…This clearly indicates that the proton-transfer coordinate ν is a more natural structural parameter to use to characterize the electronic rearrangements associated with hydrogen bonding. This observation allows us to rationalize some of the qualitative experimental observations of Matubayasi et al (27,28). The 1 H chemical shift of the liquid was observed to decrease with increasing temperature, saturating to a near-constant value in the proximity of the critical temperature.…”

“…The 1 H NMR chemical shift of liquid water from room temperature to above the critical point has been measured using an isolated water molecule in a dilute gas at 473 K as an external reference (27,28). The observed variations of the chemical shift were linked to variations in the hydrogen bonding as a function of the thermodynamic state point.…”

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

“…Although for the ␦ data of refs. 21 and 38, the reference material was CH 4 , all measured values, after the proper correction, nicely fall onto a single master curve, whereby the reference system is a water molecule in a diluted gas in supercritical conditions (14). Fig.…”

“…If a water molecule in a dilute gas is taken to be an isolated-state reference, the chemical shift ␦ accounts for the change of the value of the magnetic shielding with respect to that of such a reference. Hence the chemical shift is related to the ''non-dilute'' or ''virial'' interaction of a water molecule with its surroundings, providing a picture of the intermolecular geometry (14)(15)(16)(17)(18)(19). Originally, it has been proposed, especially in the high temperature regime, that ␦ represents the number of hydrogen bonds (HB), N HB , with which a water molecule is involved at a certain temperature (20)(21)(22).…”

“…In the first interval HDL , in which the normal liquid region (273-353 K) and a region of moderate supercooling lie, ␦(T) increases as T decreases. Both the normal liquid and the supercritical regions have been considered from both the theoretical and experimental points of view to explain as the proton chemical shift reflects the properties of the local order (14,16,17) in regions in which there is a direct relation between ␦(T) and the average number of hydrogen bonds ͗N HB ͘, in which a water molecule is involved: ␦(T) ϰ ͗N HB ͘. On the basis of the thermal evolution of the LDL and HDL local structures (Fig.…”