A 13C NMR and density functional theory study of critical behaviour of binary water/2,6-lutidine solution

Temperature dependences of C-13 NMR shifts have been measured in binary water/2,6-lutidine mixture close to the lower critical solution point at T-CL = 306.0 +/- 0.5 K. In order to evaluate hydrogen bonding and solvent effect contributions to the measured chemical shifts, C-13 magnetic shielding tensors of non-bonded 2,6-lutidine molecule, as well as water/2,6-lutidine H-bond complex in vacuo and in various solvents (acetonitrile, water) have been calculated using the density functional theory (DFT) with the modified hybrid functional of Perdew, Burke, and Ernzerhof (PBEIPBE). The solvent reaction field effect has been taken into account using the polarizable continuum model (PCM). C-13 NMR shifts 'order parameters' (Delta delta = vertical bar delta(+) - delta(-)vertical bar) and 'diameters' (phi delta = vertical bar(delta(+) + delta(-))/2 - delta(C)vertical bar, where delta(+), delta(-), and delta(C) are the chemical shifts of coexisting phases and at the critical point respectively) have been calculated for each C-13 signal close to T-CL and processed using linear regression analysis of Delta delta similar to vertical bar T - T-CL vertical bar and phi delta similar to vertical bar T - T-CL vertical bar in the log-log plot. It has been shown that the critical index beta can be determined most correctly using temperature dependences of the C-13 NMR signals of C4 and C3,5 carbons of 2,6-lutidine. An evaluation of critical index of 'diameter' is rather imprecise because of problems of referencing of delta(C). The obtained phi delta slope from C4 data (0.65 +/- 0.08) are closer to 2 beta than to 1 - alpha value. The results are discussed in the light of DFIF data and within the concept of complete scaling.