The structure of subcritical and supercritical methanol by neutron diffraction, empirical potential structure refinement, and spherical harmonic analysis

Pulsed neutron diffraction with isotope substitution on the hydroxyl hydrog ens (H) is used to study the structure of methanol in two supercritical con ditions (253 degrees C, 117.7 MPa, 0.700 g cm(-3) and 253 degrees C, 14.3 M Pa, 0.453 g cm(-3)) as well as a subcritical state (202 degrees C, 73.7 MPa , 0.700 g cm(-3)) (T-c=240 degrees C, P-c=8.1 MPa, rho(c)=0.272 g cm(-3) fo r methanol). From three experiments on CD3OD, CD3OH, and a CD3O(H-0.28+D-0. 72) mixture at the three thermodynamic states, the composite partial struct ure factors and pair correlation functions, XX, XH, and HH, are derived, wh ere X represents a weighted sum of correlations from carbon (C), oxygen (O) , and methyl hydrogen (M) atoms on the methanol molecule. The data are used in an empirical potential structure refinement (EPSR) computer simulation of methanol at the three thermodynamics states. Model distributions of mole cules consistent with these data are used to estimate the individual site-s ite radial distribution functions, the coefficients of the spherical harmon ic expansion of the orientational pair correlation function, the details of hydrogen bonding, and the three-dimensional structure of clusters formed i n subcritical and supercritical methanol. In both subcritical and supercrit ical states of moderate density, the hydrogen bonds remain, with the averag e number of hydrogen bonds of 1.6 +/- 0.1 per molecule and the average chai n-length of 3.1 +/- 0.4 molecules, which are less than the 1.77 +/- 0.07 pe r molecule and 5.5 +/- 1.0 molecules, respectively, found under ambient con ditions; however, in the subcritical and supercritical methanol at moderate density the hydrogen bonds are mostly associated with clusters of 3-5 meth anol molecules, in contrast with ambient methanol in which methanol molecul es are involved in a nonlinear hydrogen bonded chain structure. In the low- density supercritical methanol, the large clusters are broken to generate m onomers or small oligomers; the average number of hydrogen bonds per molecu le decreases to 1.0 +/- 0.1, and the chain-length is 1.8 +/- 0.2 with a max imum length up to 7. The present results are compared with those recently o btained by molecular dynamics simulations, NMR, and Raman scattering of sup ercritical methanol. (C) 2000 American Institute of Physics. [S0021-9606(00 )50819-8].