NH+-F hydrogen bonding in a fluorinated "proton sponge" derivative: integration of solution, solid-state, gas-phase, and computational studies

We report detailed studies on the characterization of an intramolecular NH-F hydrogen bond formed within a fluorinated "proton sponge" derivative. An ammonium ion, generated from 8-fluoro-N,N-dimethylnaphthalen-1-amine, serves as a charged hydrogen bond donor to a covalently bound fluorine appropriately positioned on the naphthalene skeleton. Potentiometric titrations of various N,N-dimethylnaphthalen-1-amines demonstrate a significant increase in basicity when hydrogen bonding is possible. X-ray crystallography reveals that NH-F hydrogen bonding in protonated 8-fluoro-N,N-dimethylnaphthalen-1-amine is heavily influenced by ion pairing in the solid state; bifurcated and trifurcated hydrogen bonds are formed depending on the counterion utilized. Compelling evidence of hydrogen bonding in the 8-fluoro-N,N-dimethylnaphthyl-1-ammonium cation is provided by gas-phase cryogenic vibrational photodissociation spectroscopy. Solution-phase infrared spectroscopy provides complementary results, and the frequencies of the N-H stretching mode in both phases are in excellent agreement with the computed vibrational spectra. NMR analysis of protonated 8-fluoro-N,N-dimethylnaphthalen-1-amine demonstrates significant H-F coupling between the N-H hydrogen and fluorine that cannot be attributed to long-range, through-bond interactions; the couplings correlate favorably with calculated values. The results obtained from these experiments are congruent with the formation of an NH-F hydrogen bond upon protonation of 8-fluoro-N,N-dimethylnaphthalen-1-amine.     

Characterization of highly unusual NH+-O hydrogen bonding to ester ether oxygen atoms through spectroscopic and computational studies

We characterize a highly unusual, charged NH-O hydrogen bond formed within esters of 8-(dimethylamino)naphthalen-1-ol in which an ammonium ion serves as an intramolecular hydrogen bond donor to spatially proximate ester ether oxygen atoms. Infrared spectroscopic analysis of the ester carbonyl frequencies demonstrates significant blue-shifting when ether hydrogen bonding is possible, in stark contrast to the more commonly observed red shift that occurs upon hydrogen bonding to the ester carbonyl oxygen. The intrinsic behavior of the linkage (i.e., in which counterions and solvent effects are eliminated) is provided by vibrational predissociation spectroscopy of the isolated gas-phase cations complexed with weakly bound D(2) molecules.     

Structural characterization of electron-induced proton transfer in the formic acid dimer anion, (HCOOH)2-, with vibrational and photoelectron spectroscopies

The (HCOOH)(2) anion, formed by electron attachment to the formic acid dimer (FA(2)), is an archetypal system for exploring the mechanics of the electron-induced proton transfer motif that is purported to occur when neutral nucleic acid base-pairs accommodate an excess electron [K. Aflatooni, G. A. Gallup, and P. D. Burrow, J. Phys. Chem. A 102, 6205 (1998); J. H. Hendricks, S. A. Lyapustina, H. L. de Clercq, J. T. Snodgrass, and K. H. Bowen, J. Chem Phys. 104, 7788 (1996); C. Desfrancois, H. Abdoul-Carime, and J. P. Schermann, ibid. 104, 7792 (1996)]. The FA(2) anion and several of its H∕D isotopologues were isolated in the gas phase and characterized using Ar-tagged vibrational predissociation and electron autodetachment spectroscopies. The photoelectron spectrum of the FA(2) anion was also recorded using velocity-map imaging. The resulting spectroscopic information verifies the equilibrium FA(2)(-) geometry predicted by theory which features a symmetrical, double H-bonded bridge effectively linking together constituents that most closely resemble the formate ion and a dihydroxymethyl radical. The spectroscopic signatures of this ion were analyzed with the aid of calculated anharmonic vibrational band patterns.     

Models for capacitated lot-sizing problem with backlogging,setup carryover and crossover

Setup operations are significant in some production environments. It is mandatory that their production plans consider some features, as setup state conservation across periods through setup carryover and crossover. The modelling of setup crossover allows more flexible decisions and is essential for problems with long setup times. This paper proposes two models for the capacitated lot-sizing problem with backlogging and setup carryover and crossover. The first is in line with other models from the literature, whereas the second considers a disaggregated setup variable, which tracks the starting and completion times of the setup operation. This innovative approach permits a more compact formulation. Computational results show that the proposed models have outperformed other state-of-the-art formulation.     

Stability and convergence of the spectral Lagrange-Galerkin method for mixed periodic/non-periodic convection-dominated diffusion problems

We present a convergence analysis of the spectral Lagrange-Galerkinmethod for mixed periodic/non-periodic convection-diffusionproblems. The scheme is unconditionally stable, independentof the diffusion coefficient, even in the case when numericalquadrature is used. The theoretical predictions are illustratedby a series of numerical experiments. For the periodic case,our results present a significant improvement on those givenby Süli & Ware (1991) SIAM J. Numer.Anal.28, 423-445).     

A valence bond study of three-center four-electron pi bonding: electronegativity vs electroneutrality

Three-center four-electron (3c4e) pi bonding systems analogous to that of the ozone molecule have been studied using modern valence bond theory. Molecules studied herein consist of combinations of first row atoms C, N, and O with the addition of H atoms where appropriate in order to preserve the 3c4e pi system. Breathing orbital valence bond (BOVB) calculations were preformed at the B3LYP/6-31G**-optimized geometries in order to determine structural weights, pi charge distributions, resonance energies, and pi bond energies. It is found that the most weighted VB structure depends on atomic electronegativity and charge distribution, with electronegativity as the dominant factor. By nature, these systems are delocalized, and therefore, resonance energy is the main contributor to pi bond energies. Molecules with a single dominant VB structure have low resonance energies and therefore low pi bond energies.