The structure of liquid methanol revisited:: A neutron diffraction experiment at -80°C and +25°C

Pulsed neutron diffraction with isotope substitution on the hydroxyl hydrogens (H) is used to study the structure of pure liquid methanol at -80 degrees C and +25 degrees C. Although this liquid has been studied with neutrons several times in the past this is the first time that the composite partial structure factors, XX, XH and HH, are derived from the diffraction data. Here 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 the liquid at both temperatures. Model distributions of molecules consistent with these data are used to estimate the individual site-site radial distribution functions, the coefficients of the spherical harmonic expansion of the orientational pair correlation function, and the length of possible chains of methanol molecules formed in the liquid. Although the results are qualitatively similar to those of earlier computer simulation studies of this liquid, they do differ in detail from previous work. In particular, although most molecules in the liquid are found as part of chains up to 10 molecules long, the average chain length is only about 2.7 molecules at both temperatures, there are about 1.3 hydrogen bonds per molecule on average, and the chains are highly nonlinear on average.