Cooperative interaction of H3O+ with 1,3-alternate tetrapropoxycalix[4]arene: NMR and theoretical study

Interaction of hydrated protons with octyl-phenyl-N,N-diisobutylcarbamoylmethyl phosphine oxide (CMPO): NMR and theoretical study

Interaction of octyl-phenyl-N,N-diisobutylcarbamoylmethylphosphine oxide (CMPO, the 'classical' rare metal extraction agent) with fully ionized hydrated protons (HP) was studied in acetonitrile-d(3) using (1)H, (13)C, (31)P NMR, PFG NMR and magnetic relaxation. The experimental results were confronted with high-precision ab initio DFT calculations. Relative chemical shifts of NMR signals of CMPO (0.01 mol/L) under the presence of HP in the molar ratio β = 0-2.0 mol/mol show binding between CMPO and HP. Self-diffusion measurements using (1)H PFG NMR demonstrate that larger complexes with higher content of CMPO are generally formed at β < 0.75. Analyzing the collective dependence of (13)C and (31)P NMR chemical shifts on β by the use of program LETAGROP, we obtained very good fitting for the assumed coexistence of two complexes (CMPO)(2)·HP (C(2)) and CMPO.HP (C(1)). The logarithms of the respective stabilization constants log K(i) were found to be 7.518 (C(2)) and 4.581 (C(1)). The system dynamics was studied by measuring the transverse (1)H NMR relaxation using CPMG sequence with varying delays t(p) between the π pulses in the mixtures with β = 0.4-0.8. The following exchange correlation times were obtained: τ(10) = 2.35 × 10(-5), τ(20) = 0.82 × 10(-4), τ(21) = 0.45 × 10(-3) s. The DFT calculations support the conclusion that the complexes C(1) and C(2) are the main species in the mixtures of CMPO with HP. They also agree with the NMR and FTIR observation that the main site to which H(3) O(+) is bound is the P=O group, whereas the amide group does not form a strong bond with the ion when excess water molecules are present.

Hydration modes of an amphiphilic molecule: NMR, FTIR, and theoretical study of the interactions in the water-lutidine system

Using (1)H and (13)C NMR spectra and relaxations, PFG NMR diffusion measurements, FTIR spectra, and quantum-chemical structure predictions and optimizations on the MP2/6-31G(d) level, we have studied interactions between water (W) and lutidine (2,6-dimethylpyridine, L) in a wide range of ratios. At low W content up to 35%, W was found to bind to L by an O-H***N hydrogen bond and form transient L-W aggregates containing two to four L molecules in cooperation with two to three other W molecules. At higher W content, these aggregates are gradually cleaved to single L molecules enwrapped by a hydration shell anchored in an O-H***N hydrogen bond. At all compositions of the mixture, the various hydrate forms are in fast mutual exchange with a correlation time on the order of 1 x 10(-5) s.

Lewis acid-base interactions enhance explosives sensing in silacycle polymers

The high sensitivity of silole- and silafluorene-containing polymers for detecting organic nitro, nitrate, and nitramine explosives cannot be solely attributed to favorable analyte-polymer hydrophobic interactions and amplified fluorescence quenching due to delocalization along the polymer chain. The Lewis acidity of silicon in conjugated poly(silafluorene-vinylene)s is shown to be important. This was established by examining the (29)Si NMR chemical shifts (Delta) for the model trimer fragment of the polymer CH(3)-silafluorene-(trans-C(2)H(2))-silafluorene-(trans-C(2)H(2))-silafluorene-CH(3). The peripheral and central silicon resonances are up-field from a TMS reference at -9.50 and -18.9 ppm, respectively. Both resonances shift down-field in the presence of donor analytes and the observed shifts (0 to 1 ppm) correlate with the basicity of a variety of added Lewis bases, including TNT. The most basic analyte studied was acetonitrile and an association constant (K(a)) of 0.12 M(-1) was calculated its binding to the peripheral silicon centers using the Scatchard method. Spin-lattice relaxation times (T(1)) of 5.86(3) and 4.83(4) s were measured for the methyl protons of acetonitrile in benzene-d(6) at 20 degrees C in the absence and presence of the silafluorene trimer, respectively. The significant change in T(1) values further supports a binding event between acetonitrile and the silafluorene trimer. These studies as well as significant changes and shifts observed in the characteristic UV-Vis absorption of the silafluorene group support an important role for the Lewis acid character of Si in polymer sensors that incorporate strained silacycles. The nitro groups of high explosives may act as weak Lewis-base donors to silacycles. This provides a donor-acceptor interaction that may be crucial for orienting the explosive analyte in the polymer film to provide an efficient pathway for inner-sphere electron transfer during the electron-transfer quenching process.

NMR and theoretical study of the cooperative interaction of hydrated proton with dibenzo-24-crown-8

Interaction of hydrated protons (HPs) with dibenzo-24-crown-8 (DBC in nitrobenzene-d(5) was studied by (1)H and (13)C NMR under assistance of ab initio-density functional theory (DFT) quantum calculations. HPs were afforded by hydrogen bis(1,2-dicarbollyl) cobaltate (HDCC) with 3.5 M excess of H(2)O. The forming of a complex between HP and DBC leads to marked and additive relative shifts of both (1)H and (13)C signals. This was utilized for the estimation of the stabilization constant K of the complex. Its value is at least 10(6) l/mol, which agrees with the result of independent extraction method (log K = 6.3). Using absolute integral intensities of the HP signal in a water-saturated system, it was shown that the form of HP present in the complex must be H(5)O(2)(+), in accord with formerly published structure of the complex in crystalline form. The investigation of the dynamics of exchange between bound and free DBC by transverse relaxation using variably spaced pulses in the Carr-Purcell-Meiboom-Gill (CPMG) sequence or on-resonance rotating-frame relaxation with variable spin-lock field intensity was partly hampered by the fast relaxation of some signals in the complex because of relative immobilization of its internal motions. In order to remove these effects, a pulse sequence dipolar interaction-free transverse relaxation (DIFTRE) for static DIFTRE was devised and the MLEV17 pulse sequence with high intensity of electromagnetic field was used in a separate series of experiments. Using the results of these latter experiments, the correlation time of exchange was established to be about 0.8 ms, which complied with the shape of the spectra. The accompanying ab initio DFT calculations showed that the apparent symmetry of the molecules of both DBC and its complex with H(5)O(2)(+) was probably the result of averaging of energetically close conformations (five for DBC and four for the complex). Both NMR and the calculations show that the basic mode of binding of the ion in the complex is analogous to that found in crystal but the most pronounced conformation is slightly different.