Density functional theory calculations were employed to study the relative contribution of resonance versus inductive effects toward the 37 kcal/mol enhanced gas-phase acidity (DeltaH degrees (acid)) of formic acid (1) over methanol (2). The gas-phase acidities of formic acid, methanol, vinyl alcohol (5), and their vinylogues (6, 8, and 9) were calculated at the B3LYP/6-31+G level of theory. Additionally, acidities were calculated for the formic acid and vinyl alcohol vinylogues in which the formyl group and the vinyl group, respectively, were perpendicular to the rest of the conjugated system. Comparisons among these calculated acidities suggest that inductive effects are the predominant effects responsible for the enhanced acidity of formic acid over methanol, accounting for between roughly 62% and 65% of the total enhanced acidity; the remaining 38% to 35% of the acidity enhancement appears to be due to resonance effects. Further comparisons suggest that resonance effects are between roughly 58% and 65% of the 26 kcal/mol calculated acidity enhancement of vinyl alcohol over methanol, and the remaining 42% to 35% are due to inductive effects.

in the potential range between 0 and 0.6 V. Impedance spectra were taken between 100 kHz and 50 MHz under sinusoidal potentials with an amplitude of 10 mV. To estimate the electrochemical parameters from the data, the whole system was represented by ideal elements of a simplified equivalent-circuit model composed of serial resistance, interface capacitance, and interface resistance. [4] and very limited experimental work has been reported. [5] Using DNA-based modules, we investigate here the compressive and adhesive mechanical properties of self-assembled polymer brushes, and we report the effects of surface anchor density, association strength, and monomer flexibility on those properties. Reversible polymer brush properties ( Fig. 1) were examined using atomic force microscopy (AFM). Patterned gold substrates were prepared using scanning electron microscopy (SEM), photolithography, and chemical vapor deposition. The patterning provided a background reference height and also allowed multiple samples to be introduced simultaneously into the AFM. Isolated gold dots (~1.4 lm in diameter) were spaced among quadrants of a ca. 1 cm 2 silicon surface. Each quadrant and its associated gold pattern were addressed independently with a solution of an oligonucleotide±thiol RP anchor 1, (5¢®3¢)GGTATACC±(CH 2 ) 3 ±SH, and a 6-mercapto-1-hexanol backlayer that resists non-specific adsorption of the RPs on the surface.[6] The relative concentration of the two components determined the surface density (0±1.2 10 12 cm ±2 )of the RP anchor within a self-assembled monolayer (SAM) on the pattern in each quadrant.[6] Using tapping-mode AFM in 1 M NaCl and phosphate buffer (pH 7.0), the heights of the gold dots in each quadrant were measured and found to be roughly equivalent at~40 nm (Table 1). All four quadrants were then immersed simultaneously into the RP solutions of interest and probed in rapid succession with a single gold-coated AFM tip that had also been treated with 1.The anchor 1 is complementary to the oligonucleotide RP monomer 2 ((5¢®3¢)GGTATACCGCTTAAGC). End-to-end duplex formation of 2 creates a linear, polymeric assembly that resembles larger duplex DNA, but in which the main chain is defined by the reversible base pairing.[7] The equilibrium RP solution structure of 2 has been thoroughly characterized, [8] and reversible polymers formed from oligonucleotide-based monomers have been found to be a useful model system for delineating molecule-to-material relationships in RPs. Polymer-brush synthesis is a facile, one-step self-assembly process. Immersing the 1-modified substrate into an 11 lM solution of 2 led, within minutes, to the formation of RP brushes, as revealed by changes in the AFM images (Table 1). The effective thickness of the brush is indicated by the difference in the height of the features in the two solutions, which increased with increasing surface density of anchors; the apparent brush thicknesses are 6 ± 2, 16 ± 2, and 23 ± 2 nm for surface anchor densities of 0.6 ± 0.1 10 11 , 1.1 ± 0.1 10 11 , ...

Gas-phase acidities (Delta H degrees (acid)) of lactones with ring sizes from four to seven have been measured on a Fourier transform ion cyclotron resonance mass spectrometer. Electron affinities (EAs) of the corresponding lactone enolate radicals were measured on a continuous-wave ion cyclotron resonance mass spectrometer, and the bond dissociation energies (BDEs) of the alpha C-H bonds were derived. In order of increasing ring size, Delta H degrees (acid) = 368.7 +/- 2., 369.4 +/- 2.2, 367.3 +/- 2.2, and 368.3 +/- 2.2 kcal/mol and BDE = 99.4 +/- 2.3, 94.8 +/- 2.3, 89.2 +/- 2.3, and 92.8 +/- 2.4 kcal/mol for beta-propiolactone, gamma-butyrolactone, delta-valerolactone, and epsilon-caprolactone, respectively. For their corresponding enolate radicals, EA = 44.1 +/- 0.3, 38.8 +/- 0.3, 35.3 +/- 0.3, and 37.9 +/- 0.6 kcal/mol. All of these lactones are considerably more acidic than methyl acetate, consistent with a dipole repulsion model. Both BDEs and EAs show a strong dependence on ring size, whereas Delta H degrees (acid) does not. These findings are discussed, taking into account differential electronic effects and differential strain between the reactant and product species in each reaction.

[reactions: see text] Resonance energies of the trimethylenemethane dication (1) and the butadienyl dication (4) were evaluated using two independent computational methodologies to provide insight into the validity of Y-aromaticity. One methodology employed density functional theory calculations and examined the resonance contribution of the C=C double bond toward the double hydride abstraction enthalpies of methylpropene (6) and 2-butene (8), yielding 1 and 4, respectively. These resonance contributions by the double bond were determined by calculating the double hydride abstraction enthalpies of both the parallel and perpendicular conformations of vinylogues of 6 and 8, in which n = 1-4 vinyl units were inserted between the central carbon-carbon double bond and each of the reaction centers. Extrapolation of the resonance contribution in each vinylogue to n = 0 yielded the resonance contribution in the respective parent molecules. The second methodology employed an orbital deletion procedure (ODP), which effectively allowed us to examine the energies of individual resonance structures. The resonance energy of each dication is computed as the difference between the most stable resonance structure and that of the delocalized species. The two methodologies are in agreement, suggesting that the resonance energy of the trimethylenemethane dication is substantially greater than that of the butadienyl dication. The origin of this difference in resonance stabilization is discussed.

Density functional theory was employed to calculate the acidities and hydride abstraction enthalpies of propene (3) and propane (4), along with their vinylogues (5 and 6, respectively). The same reaction enthalpies were calculated for the propene vinylogues in which the terminal vinyl group was rotated perpendicular to the rest of the conjugated system (7). The contribution by resonance and inductive effects toward the acidity and hydride abstraction enthalpy of each vinylogue of 5 (n = 1-3) was computed and extrapolated to n = 0 (the parent propene system). The resonance energies of the allyl cation and anion were determined to be about 20-22 and 17-18 kcal/mol, respectively. Comparisons are made to resonance energies calculated using other methodologies.