Estimates of the energy of intramolecular hydrogen bonds

A method for the estimation of the energy of intramolecular hydrogen bonds in conjugated systems existing in a variety of conformations is presented. The method is applied to determine the intramolecular hydrogen bond energy in 3-aminopropenal and 3-aminopropenthial. According to the proposed estimation scheme, the intramolecular H-bond energies are found to be of the order of 5-7 kcal/mol. These results are compared with those obtained by using other estimation schemes as well as with the recent results by other authors. Also, the H-bond energies in dimers and trimers of the two molecules are calculated and compared with the corresponding data for internally hydrogen-bonded monomers. This comparison shows that the bond equalization effect is primarily due to proton donor-proton acceptor proximity. In comparison with intermolecular hydrogen bonds, the rigidity of the chelate skeleton enhances this proximity effect. The same effect can be seen in systems with intermolecular hydrogen bonds, although its magnitude is diminished because of the absence of additional forces which pull the proton donor and proton acceptor groups toward each other. No specific resonance-assisted origin of the intramolecular hydrogen bond energy seems to be needed to elucidate the energetics of these bonds.     

Attractive halogen–halogen interactions: F3CCl⋯FH and F3CCl⋯FCH3 dimers

The F₃CCl?FH and F₃CCl?FCH₃ dimers, which feature the halogen–halogen contacts, are investigated at MP2/6–311++G(d,p) and MP2/aug–cc–pVDZ levels of approximation. The binding energies of these complexes are found to be comparable to those of the weak hydrogen bonds. In both complexes the Cl?F are found to be significantly shorter than the sum of the corresponding van der Waals radii. The C–Cl?F contacts are also found to exhibit certain deviation from linearity. However, the energy differences between linear and bent structures are very small and primarily accounted for by electrostatic interactions between remote parts of the dimer. This indicates a high conformational flexibility of the halogen–halogen contacts and may help to explain the diversity of structural features in crystals formed by halogen-containing molecules. In both dimers the halogen–halogen interaction leads to certain shortening of the C–Cl electron accepting bond. This is accompanied by a small increase of the C–Cl stretching frequency. Hence, the two investigated dimers can possibly be classified as the blue-shifting halogen–halogen contacts.     

Transfer of electron density as a result of hydrogen bond formation

It has been found that the amount of charge transfer between donor and acceptor molecules in four sets of hydrogen‐bonded complexes may be adequately described as an exponential function of the equilibrium distance between the hydrogen atom and the nearest atom of the acceptor molecule. The exponential factors of the transfer are of the same order but somewhat larger than the factors found otherwise in the investigations of dynamic electron transfer. © 2008 Wiley Periodicals, Inc. Int J Quantum Chem, 2009     

Variable basis sets in perturbation theory: Numerical finite perturbation versus analytic approach

We consider the problem of determining variational, external-field-dependent corrections to nonoptimal zero-field nonlinear parameters. Both a direct analytic perturbation analysis and finite perturbation methods are described in a general way and in detail for the SCF approximation. The abstract theory is illustrated by reference to the results of several explicit calculations. Also, the sensitivity of the results to the choice of zero-field values is discussed.     

Stable photo-sensitive polymer layers for holography

Stable red-sensitive layers consisting of methylene blue, acrylamide, and a reducing agent have been prepared. The reducing agent can be a sodium salt of p-toluene-sulphinic acid or dimedone, dissolved in the polymers gelatin or polyvinyl alcohol. A simple method for characterizing the effect of light at 633 nm on the layers, in terms of refractive index changes and of surface relief formation, is described. The application of the layers in spatial filtering is proposed.     

On the improvement of the gauge invariance in finite basis set coupled hartree—fock calculations of molecular magnetic properties

Finite basis set coupled Hartree—Fock calculations of molecular magnetic susceptibilities and nuclear shielding constants show a very large gauge dependence (i.e., a dependence on the origin of the magnetic vector potential). The best agreement with experiment is expected for a gauge origin at the electronic centroid for which choice the so-called paramagnetic cotributions to the above properties are small. However, with a finite basis set agreement with experiment may be fortuitous and some estimate of the contribution of the unknown functions is required.A general scheme is presented for dealing with the above problems. The essence of the method is to apply an average energy approximation to the unknown functions and to determine this energy by enforcing the gauge invariance requirement. Consequently the technique can still be categorised as an ab initio method and a close approach to the Hartree—Fock limit is achieved without the need to employ very large basis sets with extensive optimisation. The method is applied to the hydrogen fluoride molecule which provides numerical justification of the proposed scheme.     

14N chemical shifts in the enamino ketone quaternary ammonium salts

The ¹⁴N chemical shifts were measured for a series of quaternary ammonium salts of enamino ketones. An upfield shift of the ¹⁴N resonance signals upon quaternization has been observed. The interpretation in terms of the approximate theory of nuclear magnetic shielding is given. The calculations for some model systems result in a general agreement with the experimental data.     

Microsolvation of HCl within cold NH3 clusters

The solid state solvation of HCl molecules with small ammonia clusters at an average temperature of 100 K was investigated by on-the-fly molecular dynamics methodology. Structures close to the proton jump from HCl molecule to the ammonia have been further checked with the MP2/aug-cc-pvDZ calculations. Ionization of HCl and/or sharing of the proton were found. Two Zundel-type ions were observedone with proton being shared between ammonium ion and Cl (-) anion (Cl (-)...H (+)...NH 3) in all complexes, and the second, between hydrogen chloride and Cl (-) anion in the HCl...Cl (-)...NH 4 (+)...(NH 3) 2 complex. However, in contrast to methanol clusters, ammonia clusters are not good for the proton wires since once the proton moves to ammonia, it is localized on the ammonium ion units.     

Blue-shifting intramolecular C-H...O interactions

Two model systems, 3-methylacroleine and 3-(difluoromethyl)acroleine, are investigated computationally with respect to the character of the C-H...O interaction in their chelate-type (ZZ) conformers. By selecting the appropriate reference conformers, the C-H...O interaction is shown to result in the increase of the C-H stretching frequency (i.e., in the blue shift of the C-H stretching band). This is accompanied by the shortening of the C-H bond distance as compared to its values in reference molecules. Parallel to calculations of the C-H bond distance and stretching frequency, the energy contribution of the C-H...O interaction to the total energy (i.e., the energy associated with the C-H...O contact) is evaluated by using the methods proposed recently for the estimation of the energies of intramolecular hydrogen bonds. It is found that the C-H...O contact in the chelate-type forms of 3-methylacroleine and 3-(difluoromethyl)acroleine corresponds to the negative energy contribution and is repulsive. It is concluded that, despite the stability of the ZZ conformers of the two molecules and their deceptive structural shape, no hydrogen bond in the usual sense is formed between the C-H bond and the lone pair donor. The results are interpreted in terms of the steric compression, which leads to the dominance of the valence repulsion contribution in the C-H...O contact. This mechanism suggests that blue-shifting intramolecular interactions should not be that uncommon, although their recognition requires a careful consideration of the reference system.