VITAMIN-C AT 120 K - EXPERIMENTAL AND THEORETICAL-STUDY OF THE CHARGE-DENSITY

Vitamin C is an extremely interesting molecule, not only for its impor tant biological properties, but also for its crystal structure in whic h the formation of several hydrogen bonds plays an important role. We have collected X-ray diffraction data at -153 degrees C up to sin thet a/lambda = 1.17 Angstrom(-1), The crystals do not undergo any phase tr ansformation and the space group remains P2(1) with two independent mo lecules in the asymmetric unit. In the final difference Fourier map no n-spherical electron density in the bonding regions was clearly indica ted. A study by means of a multipole expansion of the electron density has been performed and the experimental electrostatic potential maps obtained. We have also computed the electron density by means of the c rystal-orbital approach as coded in the program CRYSTAL(95) [R. Dovesi et all, CRYSTAL(95) User's Manual, University of Torino]. A complete ab initio conformational analysis of the isolated molecule has also be en carried out at HF-SCF and density functional level, with reasonably large basis sets. The results of this analysis are: (i) vitamin C mol ecules are tightly bound in the crystal by a network of eight intermol ecular H-bonds per molecule. The relative strength of these bonds is w ell accounted for by the experimental electrostatic potential. No rele vant intramolecular interactions are present; (ii) CRYSTAL(95) calcula tions give a value of 44 kcal mol(-1) as the sublimation energy per mo le of molecules, computed at correlated level with 6-21G(d,p) and Perd ew 91 potential [J.P. Perdew and Y. Wang, Phys. Rev. B, 45 (1992) 1324 4; M. Causa and A, Zupan, Chem. Phys, Lett., 220 (1994) 145]; (iii) th e CRYSTAL(95) electron density maps are in good agreement with the exp erimental ones; (iv) the calculations on the isolated molecule reveal that the most stable conformers are different from the X-ray structure and are characterized by intramolecular H-bonds; the maximum energy d ifference among the 11 conformers taken into account is less than 6 kc al mol(-1); (v) the NMR analysis in water reveals that the least popul ated side-chain conformation around bond C5-C6 is that found in our co mputed gas-phase conformers with the most folded-up side chain, which are indeed less prone to be solvated, (C) 1997 Elsevier Science B.V.