Theoretical prediction of proton chemical shift in supercritical water using gas-phase approximation

Chemical shifts of the OH proton in supercritical water referenced to the b enzene proton have been estimated theoretically using the ah initio molecul ar orbital (MO) method. The degree of dissociation from hydrogen-bonded wat er clusters to monomers calculated using the CCSD (T)/6-31+G(d)//MP2 (froze n core)/6-31+G(d) level of theory indicates that supercritical water is com prised of 80% monomer and 20% dimer at the critical point (T-c = 647.1 K, P -c = 22.06 MPa). On the basis of this supercritical water composition, the chemical shift of the OH proton is determined to be -6.19 ppm at the MP2 (f rozen core)/6-31+G(d)// MP2 (frozen core)/6-31+G(d) level of theory, which reproduces the recent NMR experimental results well. (C) 2001 Elsevier Scie nce B.V. All rights reserved.