Solid state deuteron NMR studies of polyamidoamine dendrimer salts. 1. Structure and hydrogen bonding

Temperature dependent deuteron quadrupole echo line shapes are reported for integer generations, G = 1, 2, 3, 5, 7, 9, of polyamidoammonium chloride salts. The spectra are characteristic of amorphous materials undergoing broad glass transitions between 25 and 65 degrees C. Adequate fits of room-temperature line shapes were obtained by assuming Gaussian distributions of hydrogen bond lengths for interior R2ND ... O and R3ND+... Cl- deuterons in the dendrimer spacers and Gaussian distributions of librational cone angles for motion of the C-3v axes of terminal RND3+ groups. The estimated average hydrogen bond lengths at both types of interior site are 2.2 +/- 0.15 Angstrom, independent of generation number. For G = 2 the spectra obtained upon anion substitution of Cl- by Br- demonstrate that both types of hydrogen bonding are counterion dependent. In the temperature range from -30 to 60 degrees C the decreases in quadrupole coupling constants and increases in observed asymmetry parameters at the interior sites are ascribed to effects of planar libration. In the dendrimer interior the average amplitude of planar libration increases with temperature and decreases with increasing generation. Thermal energy is dissipated in counterion mobility for low generations and in mobility of the dendrimer spacers for high generations. In addition to 3-fold rotation, asymmetric cone libration is required to explain the observed temperature dependent asymmetry parameters of terminal ND3+ groups. For both low- and high-generation dendrimers the terminal groups buried in the interior exhibit smaller average Librational amplitudes than those of generation 3.