Composites of silver particles of diameters in the range 16.4-33.3 nm and polyacrylamide were prepared by a chemical method. Positron lifetime and Doppler broadening measurements of these samples were carried out. The positron lifetime spectra of all the samples could be decomposed into three components having lifetimes around 200, 500, and 1800 ps. These are believed to arise due to vacancy clusters on the grain surfaces, the open spaces between the grain surface and the surrounding polymer layer and the annihilation of orthopositronium at the free-volume defects, respectively. The lifetime of positrons trapped at the grain surface defects and the grain-polymer interface is found to decrease as the grain size is increased. Doppler broadening measurements were carried out from 13 K to 300 K on a silver-polyacrylamide nanocomposite containing silver particles of diameters in the range 2-20 nm. The line-shape parameter S is decomposed by a mixture rule to obtain the contribution of electrons from the nanoparticles. This shows a sharp increase at around 80 K that is adduced as evidence for the splitting of the electron energy levels in the nanosized silver particles leading to a semiconductorlike behavior. ͓S0163-1829͑98͒03102-6͔

Blue-shifting C-H...O hydrogen bonded complexes between chloroform and three small cyclic ketones (cyclohexanone, cyclopentanone, and cyclobutanone) have been identified by use of FTIR spectroscopy in CCl(4) solution at room temperature. The shifts of the C-H stretching fundamental of chloroform (nu(C-H)) in the said three complexes are +1, +2, and +5 cm(-1), respectively, and the complexation results in enhancement of the nu(C-H) transition intensity in all three cases. The 1:1 stoichiometry of the complexes is suggested by identifying distinct isosbestic points between the carbonyl stretching (nu(C=O)) fundamentals of the monomers and corresponding complexes for spectra measured with different chloroform to ketone concentrations. The nu(C=O) bands in the three complexes are red-shifted by 8, 19, and 6 cm(-1), and apparently have no correlation with the respective blue shifts of the nu(C-H) bands. Spectral analysis reveals that the complex with cyclohexanone is most stable, and the stability decreases with the ring size of the cyclic ketones. A qualitative explanation of the relative stabilities of the complexes is presented by correlating the hydrogen bond acceptor abilities of the carbonyl groups with the ring size of the cyclic ketones. Quantum mechanical calculations at the DFT/B3LYP/6-311++G(d,p) and MP2/6-31+G(d) levels were performed for predictions of the shapes of the complexes, electronic structure parameters of C-H (donor) and C=O (acceptor) groups, intermolecular interaction energies, spectral shifts, and evolution of those properties when the hydrogen bond distance between the donor-acceptor moieties is scanned. The results show that the binding energies of the complexes are correlated with the dipole moments, proton affinity, and n(O) --> sigma*(C-H) hyperconjugative charge transfer abilities of the three ketones. NBO analysis reveals that the blue shifting of the nu(C-H) transition in a complex is the net effect of hyperconjugation and repolarization/rehybridization of the bond under the influence of the electric field of carbonyl oxygen.

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