Quantum Monte Carlo calculations of the dissociation energy of the water dimer

We report diffusion quantum Monte Carlo (DMC) calculations of the equilibrium dissociation energy D(e) of the water dimer. The dissociation energy measured experimentally, D(0), can be estimated from D(e) by adding a correction for vibrational effects. Using the measured dissociation energy and the modern value of the vibrational energy Mas et al., [J. Chem. Phys. 113, 6687 (2000)] leads to D(e)=5.00+/-0.7 kcal mol(-1), although the result Curtiss et al., [J. Chem. Phys. 71, 2703 (1979)] D(e)=5.44+/-0.7 kcal mol(-1), which uses an earlier estimate of the vibrational energy, has been widely quoted. High-level coupled cluster calculations Klopper et al., [Phys. Chem. Chem. Phys. 2, 2227 (2000)] have yielded D(e)=5.02+/-0.05 kcal mol(-1). In an attempt to shed new light on this old problem, we have performed all-electron DMC calculations on the water monomer and dimer using Slater-Jastrow wave functions with both Hartree-Fock approximation (HF) and B3LYP density functional theory single-particle orbitals. We obtain equilibrium dissociation energies for the dimer of 5.02+/-0.18 kcal mol(-1) (HF orbitals) and 5.21+/-0.18 kcal mol(-1) (B3LYP orbitals), in good agreement with the coupled cluster results.     

Identification of isomeric flavonoid glucuronides in urine and plasma by metal complexation and LC-ESI-MS/MS

Noncovalent complexes were used for structural determination and isomer differentiation of flavonoid glucuronides. Several flavonoid glucuronides including naringenin-7-O-glucuronide, synthesized here for the first time, were used as test compounds. Electrospray ionization quadrupole ion trap mass spectrometry with collision-induced dissociation (CID) was used to analyze complexes of the form [Co(II) (L-H) (Aux)]+ and [Co(II) (L-H) (Aux)2]+, in which L is the flavonoid glucuronide and Aux is a phenanthroline-based ligand. These complexes yielded characteristic fragmentation patterns that facilitated assignment of the substitution position of the glucuronides. The methods were adapted to liquid chromatography/tandem mass spectrometry (LC-MS/MS) with postcolumn cobalt complexation and were tested on extracts from biological fluids. The metabolites naringenin-7-O-glucuronide and naringenin-4'-O-glucuronide were detected in human urine following the consumption of grapefruit juice. Isomeric quercetin glucuronides were identified and differentiated after spiking rat plasma at the 1 microM level, proving that the new methods are effective at biologically relevant concentrations.