ELECTRONIC JOURNAL OF THEORETICAL CHEMISTRY, VOL. 1, 18–25 (1995) Ab initio CHF-GIAO 13 C nuclear shielding calculations on carbohydrates PHILIP J. WILSON, DAVID M. DURRAN, BRENDAN J. HOWLIN AND GRAHAM A. WEBB Department of Chemistry, University of Surrey, Guildford, Surrey, GU2 5XH,UK SUMMARY The effect of basis set quality and glycosidic conformation on the calculated chemical shieldings of the C-1 and C-4 glycosidic carbon atoms of some naturally and commercially important model β-(1 → 4), α-(1 → 4) and α-(1 → 3) linkage systems have been investigated using ab initio molecular orbital coupled Hartree–Fock gauge included atomic orbital (CHF-GIAO) methods. The STO-3G, 3-21G, 4-31G and 6-31G ∗∗ basis sets have been tested in our calculations. KEY WORDS ab initio; basis sets; C-13 chemical shifts; carbohydrates. INTRODUCTION In view of the lack of previous ab initio shielding calculations on carbohydrate systems, this theoretical work aims to consolidate previous experimental studies by several groups, and extend the scope of recent shielding calculations by us. The various correlations observed between chemical shifts and glycosidic linkage conformations of a variety of crystalline α-(1 → 4) glucans obtained from solid state cross-polarization magic angle spinning (CPMAS) NMR studies have been reported by several groups listed in Reference 1. They in turn have been rationalized by Jarvis in terms of the anomeric effect [2]. In addition, Cros et al. [3] and DiNola et al. [4] determined the conformation of pectin disaccharides using a combination of 1 H/ 13 C NMR and molecular modelling/dynamics techniques. In 1978, Jeffrey et al. [5] performed ab initio restricted Hartree–Fock (RHF) calculations using 4-31G and 6-31G ∗ quality basis sets on the molecular structure of dimethoxymethane as a model disaccharide compound. Their results were found to be consistent with the predictions of the anomeric and exoanomeric effects. Recently, we have successfully generated a φ, ψ shielding surface of a model α-(1 → 4) glucan using a 3-21G basis set [1]. The surface reproduces the experimentally observed conformationally dependant chemical shift trends of the C-1 and C-4 glycosidic carbon atoms. In addition, a comparison of the results obtained at the 3-21G level with the larger 6-31G ∗∗ basis as a function of torsion angles showed almost identical trends. This present work contains the results of shielding calculations carried out using a greater range of basis sets. In addition, the influence of molecular structure and basis set quality on the calculated nuclear shieldings of the C-1 and C-4 glycosidic carbon atoms in the gellan polysaccharide will be investigated. It has the tetrasaccharide single chain repeat unit (Figure 1). The three β-(1 → 4) and α-(1 → 3) linkages will be modelled using glycosidic fragments where the number of atoms surrounding each of these linkages will be increased in three steps. CCC 1082–4928/95/010018–08 Received 16 June 1995 1995 by John Wiley & Sons, Ltd. Revised 22 June 1995