Study of Nα-benzoyl-L-argininate ethyl ester chloride, a model compound for poly(ester amide) precursors: X-ray diffraction, infrared and Raman spectroscopies, and quantum chemistry calculations

Abstract

Poly(ester amide)s (PEAs) are lacking in structural and spectroscopic information. This paper reports a structural and spectroscopic characterization of Nα-benzoyl-L-argininate ethyl ester chloride (BAEEH+·Cl−), an important amino acid derivative and an adequate PEAs’ model compound. Crystals of BAEEH+·Cl− obtained by slow evaporation in an ethanol/water mixture were studied by different complementary techniques. X-ray analysis shows that BAEEH+·Cl− crystallizes in the chiral space group P21. There are two symmetry independent cations (and anions) in the unit cell. The two cations have different conformations: in one of them, the angle between the least-squares planes of the phenyl ring and the guanidyl group is 5.1(2)º, and in the other the corresponding angle is 13.3(2)º. There is an extensive network of H-bonds that assembles the ions in layers parallel to the ab plane. Experimental FT-IR and Raman spectra of BAEEH+·Cl− were recorded at room temperature in the 3750–600 cm−1 and 3380–100 cm−1 regions, respectively, and fully assigned. Both structural and spectroscopic analysis were supported by quantum chemistry calculations based on different models (in vacuo and solid-state DFT simulations).