Vitamin C at 120 K: experimental and theoretical study ofthe
charge density1
M. Milanesioa, R. Bianchib, P. Ugliengoa, C. Roettia, D.
Viterboa,*aDipartimento di Chimica IFM, Universita di Torino, Via
P. Giuria 7, I-10125 Torino, Italy
bCentro di Studio per le Relazioni tra Struttura e Reattivita`
Chimica, CNR, Via C. Golgi 19, 20133 Milano, Italy
Received 1 November 1996; accepted 12 February 1997
Abstract
Vitamin C is an extremely interesting molecule, not only for its
important biological properties, but also for its crystalstructure
in which the formation of several hydrogen bonds plays an important
role. We have collected X-ray diffraction data at- 1538C up to
sinv/l = 1.17 A- 1. The crystals do not undergo any phase
transformation and the space group remainsP21 withtwo independent
molecules in the asymmetric unit. In the final difference Fourier
map non-spherical electron density in thebonding regions was
clearly indicated. A study by means of a multipole expansion of the
electron density has been performedand the experimental
electrostatic potential maps obtained. We have also computed the
electron density by means of the crystal-orbital approach as coded
in the programcrystal95 [R. Dovesi et al.,crystal95 Users Manual,
University of Torino]. Acomplete ab initio conformational analysis
of the isolated molecule has also been carried out at HF-SCF and
density functionallevel, with reasonably large basis sets. The
results of this analysis are: (i) vitamin C molecules are tightly
bound in the crystal bya network of eight intermolecular H-bonds
per molecule. The relative strength of these bonds is well
accounted for by theexperimental electrostatic potential. No
relevant intramolecular interactions are present; (ii)crystal95
calculations give avalue of 44 kcal mol- 1 as the sublimation
energy per mole of molecules, computed at correlated level with
6-21G(d,p) andPerdew 91 potential [J.P. Perdew and Y. Wang, Phys.
Rev. B, 45 (1992) 13244; M. Causa` and A. Zupan, Chem. Phys. Lett.,
220(1994) 145]; (iii) thecrystal95 electron density maps are in
good agreement with the experimental ones; (iv) the calculationson
the isolated molecule reveal that the most stable conformers are
different from the X-ray structure and are characterized
byintramolecular H-bonds; the maximum energy difference among the
11 conformers taken into account is less than 6 kcal mol- 1;(v) the
NMR analysis in water reveals that the least populated side-chain
conformation around bond C5C6 is that found in ourcomputed
gas-phase conformers with the most folded-up side chain, which are
indeed less prone to be solvated.q 1997Elsevier Science B.V.
1. Introduction
Vitamin C was first isolated by the Hungarianscientist Albert
Szent-Gyorgyi from the adrenal cortexof cattle, and later from
orange juice and cabbage water.
The molecular formula C6H8O6 was also establishedand the new
molecule was named huxuronic acid [1].
Journal of Molecular Structure (Theochem) 419 (1997) 139154
THEOCH 5284
0166-1280/97/$17.00q 1997 Elsevier Science B.V. All rights
reserved.PII S0166-1280(97)00252-2
* Corresponding author. E-mail: [email protected] Dedicated
to Linus Pauling, a giant in chemistry and in life.
