INTERACTION ENERGIES IN GLYCINE CRYSTALS FROM A CHARGE DENSITY STUDY AT 23 K
R. Destro1,2 , M. Barzaghi1 , R. Soave1, and R. E. Marsh2
1 Dipartimento di Chimica Fisica ed
Elettrochimica, e Centro CNR, Universita' di Milano, Via Golgi
19, 20133 Milano, Italy
2 Beckman Institute, California Institute of
Technology, Pasadena, California 91125, USA
X-ray diffracted intensity measurements were recorded at T = 23 K for 10891 reflections, up to 2Mo = 109o, from a crystal of a-glycine ground to a spherical shape. After correction for scan-truncation errors [1], weighted averaging of the multiple measurements yielded 3822 independent values, of which 33 only were negative, and 2921 occurred above 0.7 Å-1 in sin/q.
A reasonable account of anisotropic extinction effects was obtained by adopting a type I model with a Lorentzian distribution of the mosaic. Analysis of the data was performed with the multipole formalism of Stewart [2], as implemented in the VALRAY [3] set of programs, up to the hexadecapole level for the C, N and O atoms, and to the quadrupole level for the H atoms. Anisotropic displacement parameters (adp's) for the latter nuclei were based on spectroscopic information and the adp's of the 5 non-H atoms.
Least-squares refinement of 216 variables led to final figures of merit R(F)=0.0129, Rw(F)=0.0145, R(F2)=0.0172 and Rw(F2)=0.0272, with a goodness-of-fit S=1.0410 (for the 697 reflections within the Cu sphere R(F) < 0.01).
Energies between pairs of molecules of a-glycine were calculated according to the model described by Spackman and coworkers [4], with the electrostatic interaction energy treated as a classical interaction between the charge distribution of one molecule (including nuclei) and the electrostatic potential of the other.
Results so obtained will be compared with those calculated by
ab initio methods with several basis sets.