Photoinduced proton transfer in a pyridine based polymer gel.

We describe an experimental and theoretical consideration of photoexcited proton transfer in a poly(4-vinyl pyridine)/pyridine gel. Evidence was found for two states of a multiple state process analyzed by DFT modeling. According to the latter, following irradiation at 385 nm, the proton donor is the CH group of the polymer main chain and the proton acceptor is the nitrogen of the polymeric pyridine side chain.

Counteranion-dependent mechanisms of intramolecular proton transfer in aprotic solution.

Using the freon mixture CDF(3)/CDClF(2) as solvent we have been able to measure the (1)H and (15)N NMR spectra of the doubly (15)N labeled 2,2'-bipyridinium cation (BpyH(+)) at temperatures down to 115 K. The obtained NMR parameters strongly depend on the type of counteranions indicating the formation of ion pairs. In the case of the bulky poorly coordinating tetrakis[3,5-bis(trifluoromethyl)phenyl]-borate as the counteranion a strong intramolecular NHN hydrogen bond was observed in BpyH(+) exhibiting a degenerate intramolecular proton transfer which is of the order of 10(6) s(-1) even at 120 K.

Density functional study of the proton transfer effect on vibrations of strong (short) intermolecular O-H...N/O-...H-N+ hydrogen bonds in aprotic solvents.

The structure and spectroscopic properties of the 1:1 complexes of substituted pyridines with benzoic acid and phenol derivatives in aprotic solvents are studied using B3LYP functional combined with the polarizable continuum model approximation. Two extreme structures are investigated: the state without (HB) and with proton transfer (PT). In the presence of an external electric field the O.

H/D isotope effects on NMR chemical shifts of nuclei involved in a hydrogen bridge of hydrogen isocyanide complexes with fluoride anion.

(1)H, (2)H, (19)F and (15)N NMR spectra of a strongly hydrogen-bonded anionic cluster, CNHF(-), as an ion pair with a tetrabutylammonium cation dissolved in CDF(3)-CDF(2)Cl mixture were recorded in the slow exchange regime at temperatures down to 110 K. The fine structure due to spin-spin coupling of all nuclei involved in the hydrogen bridge was resolved. H/D isotope effects on the chemical shifts were measured.

A DFT and AIM analysis of the spin-spin couplings across the hydrogen bond in the 2-fluorobenzamide and related compounds.

In 1975 a large number of coupling constants were measured in 2-fluorobenzamide labeled with (15)N. Some of them were assigned to couplings through intramolecular N-H..

Solid-state NMR studies of aminocarboxylic salt bridges in L-lysine modified cellulose.

LysCel is a cellulose-based material in which l-lysine molecules are grafted with their amino side chains to the cellulose hydroxyl groups. This modification increases considerably the mechanical strength and resistance of cellulosic structures toward water. It has been attributed to the formation of double salt bridges between lysine aminocarboxyl groups in the zwitterionic state.

Synthesis and characterization of chiral benzylic ether-bridged periodic mesoporous organosilicas.

The first synthesis of a chiral periodic mesoporous organosilica (PMO) carrying benzylic ether bridging groups is reported. By hydrolysis and condensation of the new designed chiral organosilica precursor 1,4-bis(triethoxysilyl)-2-(1-methoxyethyl)benzene (BTEMEB) in the presence of the non-ionic oligomeric surfactant Brij 76 as supramolecular structure-directing agent under acidic conditions, an ordered mesoporous chiral benzylic ether-bridged hybrid material with a high specific surface area was obtained. The chiral PMO precursor was synthesized in a four-step reaction from 1,4-dibromobenzene as the starting compound.

NMR provides checklist of generic properties for atomic-scale models of periodic mesoporous silicas.

MCM-41 and SBA-15 silicas were studied by (29)Si solid-state NMR and (15)N NMR in the presence of (15)N-pyridine with the aim to formulate generic structural parameters that may be used as a checklist for atomic-scale structural models of this class of ordered mesoporous materials. High-quality MCM-41 silica constitutes quasi-ideal arrays of uniform-size pores with thin pore walls, while SBA-15 silica has thicker pore walls with framework and surface defects. The numbers of silanol (Q(3)) and silicate (Q(4)) groups were found to be in the ratio of about 1:3 for MCM-41 and about 1:4 for our SBA-15 materials.

Structural and dynamical properties of guest molecules confined in mesoporous silica materials revealed by NMR.

In the last fifteen years several novel porous silica materials, which are periodically structured on the mesoscopic length scale, have been synthesized. They are of broad interest for fundamental studies of surface-substrate interactions, for studies of the dynamics of guest molecules in confinement and for studies of the effect of confinement on the structural and thermophysical properties of fluids. Examples of such confinement effects include the change of the freezing and melting points or glass transitions of the confined liquids.

Nuclear magnetic resonance and ab initio studies of small complexes formed between water and pyridine derivatives in solid and liquid phases.

The structure and geometry of hydrogen-bonded complexes formed between heterocyclic bases, namely, pyridine and 2,4,6-trimethylpyridine (collidine), and water were experimentally studied by NMR spectroscopy in frozen phase and in highly polar aprotic liquefied freon mixtures and theoretically modeled for gas phase. Hydrogen-bonded species in frozen heterocycle-water mixtures were characterized experimentally using 15N NMR. When base was in excess, one water molecule was symmetrically bonded to two heterocyclic molecules.

Geometrical features of hydrogen bonded complexes involving sterically hindered pyridines.

The ability of strongly sterically hindered pyridines to form hydrogen bonded complexes was inspected using low-temperature 1H and 15N NMR spectroscopy in a liquefied Freon mixture. The proton acceptors were 2,6-di(tert-butyl)-4-methyl- and 2,6-di(tert-butyl)-4-diethylaminopyridine; the proton donors were hydrogen tetrafluoroborate, hydrogen chloride, and hydrogen fluoride. The presence of the tert-butyl groups in the ortho positions dramatically perturbed the geometry of the forming hydrogen bonds.

Hydrogen bonding of water confined in mesoporous silica MCM-41 and SBA-15 studied by 1H solid-state NMR.

The adsorption of water in two mesoporous silica materials with cylindrical pores of uniform diameter, MCM-41 and SBA-15, was studied by 1H MAS (MAS=magic angle spinning) and static solid-state NMR spectroscopy. All observed hydrogen atoms are either surface -SiOH groups or hydrogen-bonded water molecules. Unlike MCM-41, some strongly bound water molecules exist at the inner surfaces of SBA-15 that are assigned to surface defects.

Low-temperature NMR studies of the structure and dynamics of a novel series of acid-base complexes of HF with collidine exhibiting scalar couplings across hydrogen bonds.

The low-temperature (1)H, (19)F, and (15)N NMR spectra of mixtures of collidine-(15)N (2,4,6-trimethylpyridine-(15)N, Col) with HF have been measured using CDF(3)/CDF(2)Cl as a solvent in the temperature range 94-170 K. Below 140 K, the slow proton and hydrogen bond exchange regime is reached where four hydrogen-bonded complexes between collidine and HF with the compositions 1:1, 2:3, 1:2, and 1:3 could be observed and assigned. For these complexes, chemical shifts and scalar coupling constants across the (19)F(1)H(19)F and (19)F(1)H(15)N hydrogen bridges have been measured which allowed us to determine the chemical composition of the complexes.