The (13)C NMR chemical shifts for alpha-D-lyxofuranose, alpha-D-lyxopyranose (1)C(4), alpha-D-lyxopyranose (4)C(1), alpha-D-glucopyranose (4)C(1), and alpha-D-glucofuranose have been studied at ab initio and density-functional theory levels using TZVP quality basis set. The methods were tested by calculating the nuclear magnetic shieldings for tetramethylsilane (TMS) at different levels of theory using large basis sets. Test calculations on the monosaccharides showed B3LYP(TZVP) and BP86(TZVP) to be cost-efficient levels of theory for calculation of NMR chemical shifts of carbohydrates. The accuracy of the molecular structures and chemical shifts calculated at the B3LYP(TZVP) level is comparable to those obtained at the MP2(TZVP) level. Solvent effects were considered by surrounding the saccharides by water molecules and also by employing a continuum solvent model. None of the applied methods to consider solvent effects was successful. The B3LYP(TZVP) and MP2(TZVP)(13)C NMR chemical shift calculations yielded without solvent and rovibrational corrections an average deviation of 5.4 ppm and 5.0 ppm between calculated and measured shifts. A closer agreement between calculated and measured chemical shifts can be obtained by using a reference compound that is structurally reminiscent of saccharides such as neat methanol. An accurate shielding reference for carbohydrates can be constructed by adding an empirical constant shift to the calculated chemical shifts, deduced from comparisons of B3LYP(TZVP) or BP86(TZVP) and measured chemical shifts of monosaccharides. The systematic deviation of about 3 ppm for O(1)H chemical shifts can be designed to hydrogen bonding, whereas solvent effects on the (1)H NMR chemical shifts of C(1)H were found to be small. At the B3LYP(TZVP) level, the barrier for the torsional motion of the hydroxyl group at C(6) in alpha-D-glucofuranose was calculated to 7.5 kcal mol(-1). The torsional displacement was found to introduce large changes of up to 10 ppm to the (13)C NMR chemical shifts yielding uncertainties of about +/-2 ppm in the chemical shifts.

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The molecular structures of bonellin-dimethylester isomers and transition states, for the hydrogen migration of the inner hydrogens, have been optimized at density functional level by the use of split-valence basis sets augmented with polarization functions. Accurate values for the relative energies of the six isomers and the eight transition states have been obtained by performing second-order Mùller ± Plesset calculations. The isomer energies obtained at density functional level are 2 ± 5 kcal mol À1 smaller than second-order Mùller ± Plesset values, while the isomer energies calculated at Hartree ± Fock level are typically 1 kcal mol À1 too small compared with second-order Mùl-ler ± Plesset values, except for one isomer whose Hartree ± Fock energy is 7 kcal mol À1 larger. Compared with the second-order Mùller ± Plesset values, the energy barriers for the transition between the isomers calculated at density functional level are 3 ± 7 kcal mol À1 too small, whereas those obtained at Hartree ± Fock level are 6 ± 8 kcal mol À1 too large. Nuclear magnetic shielding constants calculated at Hartree ± Fock level are also reported. The calculated nuclear magnetic shieldings are used for analyzing the aromaticity and the aromatic pathway of the porphyrin nucleus.

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