Ab initio study of proton chemical shift in supercritical methanol using gas-phase approximation

Chemical shifts of the OH proton in supercritical methanol referenced to the methyl proton of methanol monomer have been estimated theoretically using the ab initio molecular orbital (MO) method. The degree of dissociation from hydrogen-bonded methanol clusters to monomers calculated using the CCSD(T)/6-31+G(d)//MP2(frozen-core)/6-31+G(d) level of theory indicates that supercritical methanol is comprised of 89% monomer and 10% cyclic tetramer plus similar to1% dimer at the critical point (T-c = 512.6 K; P-c = 8.09 MPa). The predominant existence of the cyclic tetramers rather than the dimers in supercritical methanol is in contrast to previous theoretical results for supercritical water that indicate the composition of, except for 80% monomer, 20% dimer with little existence of a larger size of clusters at the critical point (T-c = 647.1 K; P-c = 22.06 MPa). It is also found that a significant fluctuation of the composition of methanol should be caused by a greater change in the degree of dissociation of the cyclic tetramer near the critical point. On the basis of the above supercritical methanol composition, the chemical shift of the OH proton is determined to be -2.00 ppm at the MP2(frozen-core)/6-31+G(d)//MP2(frozen-core)/6-31+G(d) level of theory, which excellently reproduces the recent NMR experimental results.