Although the existence of Cα−H···OC hydrogen bonds in protein structures recently has been established, little is known about their strength and, therefore, the relative importance of these interactions. We have discovered that similar interactions occur in N,N-dimethylformamide dimers. High level ab initio calculations (MP2/aug-cc-pTZV) yield electronic association energies (D e) and association enthalpies (ΔH 298) for four dimer geometries. These data provide a lower limit of D e = −2.1 kcal mol-1 for the Cα−H···OC hydrogen bond. A linear correlation between C−H···O bond energies and gas-phase proton affinities is reported. The gas-phase anion proton affinity of a peptide Cα−H hydrogen was calculated (355 kcal mol-1) and used to estimate values of D e = −4.0 ± 0.5 kcal mol-1 and ΔH 298 = −3.0 ± 0.5 kcal mol-1 for the Cα−H···OC hydrogen bond. The magnitude of this interaction, roughly one-half the strength of the N−H···OC hydrogen bond, suggests that Cα−H···OC hydrogen bonding interactions represent a hitherto unrecognized, significant contribution in the determination of protein conformation.

The last member of the series of technetium(VII) oxide fluorides, TcOF5, has been prepared by oxidative fluorination of TcO2F3 with KrF2 in anhydrous HF. The pseudooctahedral (C 4 v ) structure of TcOF5 has been determined by 19F and 99Tc NMR, Raman, and infrared spectroscopies and by single-crystal X-ray diffraction. TcOF5 crystallizes in the orthorhombic crystal system, space group Pna21, with a = 9.235(3) Å, b = 4.939(2) Å, c = 8.502(3) Å, V = 387.7(2) Å3, and Z = 4 at −102 °C, R1 = 0.0256 and wR2 = 0.0730. TcOF5 behaves as a fluoride ion donor toward AsF5 and SbF5 in HF solvent, giving the Tc2O2F9 + cation, which has been characterized as the AsF6 - and Sb2F11 - salts by Raman spectroscopy and as the [Tc2O2F9][Sb2F11] salt by single-crystal X-ray diffraction. [Tc2O2F9][Sb2F11] crystallizes in the orthorhombic crystal system, space group Pbcm, with a = 6.2925(4) Å, b = 21.205(2) Å, c = 11.7040(8) Å, V = 1561.7(2) Å3, and Z = 8 at −90 °C, R1 = 0.0368 and wR2 = 0.0896. The Tc2O2F9 + cation consists of two fluorine-bridged square pyramidal TcOF4 groups in which the fluorine bridge is trans to the oxygens. Solution 19F and 99Tc NMR spectra of Tc2O2F9 + salts in HF and of TcOF5 dissolved in SbF5 are consistent with the formation of the TcOF4 + cation. Local density functional theory has been used to calculate the geometrical parameters, vibrational frequencies, and 19F and 99Tc NMR parameters of MOF5 (M = Tc, Re, Os) and Tc2O2F9 +, which are in good agreement with available experimental values. The results of ab initio calculations and normal coordinate analyses for MOF5 confirm the trans influence of oxygen, which leads to lengthening of the axial fluorine−metal bond length and a correspondingly lower stretching force constant relative to that of the shorter equatorial metal−fluorine bonds.

The potential energy surfaces at different levels of ab initio electronic structure theory with correlation effects included are reported for rotation about the C(sp 2 )-C(aryl) bond in N-methylbenzamide. The results reveal a minimum at a CdCsCdO dihedral angle of (28°with barrier heights (MP2/aug-cc-pVTZ//BLYP/DZVP2/ A2) of 0.48 kcal/mol at 0°and 2.80 kcal/mol at 90°. Fully optimized geometries are in good agreement with crystal structure data, and potential energy surfaces are consistent with the experimental dihedral angle distribution. The results are used to assign MM3 force field parameters to allow calculation on Nmethylbenzamide and other benzamide derivatives.

Zero valent Cu polyamidoamine (PAMAM) dendrimer nanocomposites were synthesized using UV irradiation starting from the aqueous Cu(II)/dendrimer system. The size of the nanoparticles is strongly dependent on the generation of the dendrimers and the pH of the solution. Larger nanoparticles are obtained with higher generation dendrimers, as well as in more basic solution. EPR spectra show that the sites bonded to the Cu(II) ion are significantly different at different pH values. Density functional theory (DFT) calculations have been used to predict the structures and EPR spectra of the Cu(II)-dendrimer complexes. At pH ) 3, the hydrated ion complexes Cu(H 2 O) 6 2+ or Cu(H 2 O) 5 2+ are present, as expected and reported previously. At pH ) 7.8, a chelating complex with two tertiary amine sites with or without two amide oxygen sites is present. At pH ) 11, the Cu(II) ion binds to either the primary amine and amide oxygen sites on a single branch or to two tertiary amines and four amide oxygen sites on all four branches. Our results show the importance of the amide sites in Cu(II)-dendrimer complexes in neutral and basic solutions.