Ilja G. Shenderovich

Pyridine-15N: A mobile NMR sensor for surface acidity and surface defects of mesoporous silica

The hydrogen bond interaction of pyridine with the silanol groups of the inner surfaces of MCM-41 and SBA-15 ordered mesoporous silica has been studied by a combination of solid-state NMR techniques. The pore diameters were varied between 3 and 4 nm for MCM-41 and between 7 and 9 nm for SBA-15. 1 H MAS experiments performed under magic angle spinning (MAS) conditions in the absence and the presence of pyridine-d 5 reveal that the large majority of silanol groups are located in the inner surfaces, isolated from each other but able to form hydrogen bonds with pyridine. On the other hand, low- and room-temperature 1 5 N CPMAS and MAS experiments (CP ≡ cross-polarization) performed on pyridine- 1 5 N show that at low concentrations all pyridine molecules are involved in hydrogen bonds with the surface silanol groups. In the presence of an excess of pyridine, a non-hydrogen-bonded pyridine phase is observed at 120 K in the slow hydrogen bond exchange regime and associates with an inner core phase. From these measurements, the number of pyridine molecules bound to the inner surfaces corresponding to the number of silanol groups could be determined to be n O H 3 nm - 2 for MCM-41 and 3.7 nm - 2 for SBA-15. At room temperature and low concentrations, the pyridine molecules jump rapidly between the hydrogen-bonded sites. In the presence of an excess of pyridine, the hydrogen-bonded binding sites are depleted as compared to low temperatures, leading to smaller apparent numbers n O H . Using a correlation established previously between the 1 5 N and 1 H chemical shifts and the NHO hydrogen bond geometries, as well as with the acidity of the proton donors, the distances in the pyridine-hydroxyl pairs were found to be about r H N = 1.68 A, r O H = 1.01 A, and r O N = 2.69 A. This geometry corresponds in the organic solid state to acids exhibiting in water a pK a of about 4. Roomtemperature 1 5 N experiments on static samples of pyridine- 1 5 N in MCM-41 at low coverage show a residual 1 5 N chemical shift anisotropy, indicating that the jumps of pyridine between different different silanol hydrogen bond sites is accompanied by an anisotropic reorientational diffusion. A quantitative analysis reveals that in this regime the rotation of pyridine around the molecular C 2 axis is suppressed even at room temperature, and that the angle between the Si-O axes and the OH axes of the isolated silanol groups is about 47°. These results are corroborated by 2 H NMR experiments performed on pyridine-4-d 1 . In contrast, in the case of SBA-15 with the larger pore diameters, the hydrogen bond jumps of pyridine are associated with an isotropic rotational diffusion, indicating a high degree of roughness of the inner surfaces. This finding is correlated with the finding by 2 9 Si CPMAS of a substantial amount of Si(OH) 2 groups in SBA-15. in contrast to the MCM-41 materials. The Si(OH) 2 groups are associated with surface defects, exhibiting not only silanol groups pointing into the pore center but also silanol groups pointing into other directions of space including the pore axes, leading to the isotropic surface diffusion. All results are used to develop molecular models for the inner surface structure of mesoporous silica which may be a basis for future simulations of the surfaces of mesoporous silica.

NUCLEAR SCALAR SPIN - SPIN COUPLING REVEALS NOVEL PROPERTIES OF LOW-BARRIER HYDROGEN BONDS IN A POLAR ENVIRONMENT

The structure of the hydrogen bridge 19 F· ·· 1 H· ·· 15 N in the acid - base complex A ··· H ··· B formed by HF and ( 15 N)2,4,6-trimethylpyridine in CDF3/ CDF2Cl has been studied between 112 K and 200 K by low-temperature, multinuclear NMR spectroscopy. For the first time scalar spin - spin coupling between all three nuclei of a hydrogen bridge is observed. This bridge exhibits a two-bond coupling constant 2 J19F15N of about 96 Hz, which is larger than the one-bond coupling constants 1 J1H15N and 1 J19F1H. The latter are strongly dependent on temperature. The function 1 J1H15Na f( 1 J19F1H) cannot be described in terms of a conventional equilibrium between the molecular and the zwitterionic form, but only with the intermediate forma- tion of very strongly hydrogen-bonded complexes of the type A dˇ ··· H· ·· B da that exhibit a vanishing or very small barrier for the proton motion. Here, the difference between the covalent bond and the hydrogen bond disappears even in the case of a polar solvents, as indicated by the large value of 2 J19F15N. Implications for the mechanism of pro- ton transfer and of acid - base catalyzed enzyme reactions in a locally aprotic but polar environment are discussed.

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. The evidence for the presence of the chiral units in the organosilica precursor as well as inside the PMO material is provided by optical activity measurements.

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...N distance is contracted and the PT state does appear. The PT state of both the pyridine-benzoic and the pyridine-phenol complexes displays the only IR-active band in the 2800-1800 frequency region, which is located around 2000 cm(-1). However, the nature of the band is different for these two complexes. In the pyridine-benzoic acid complex it is practically a pure stretching vibration of the HN(+) group, while in the pyridine-phenol complex it is the mixed vibration of the bridging proton. A specific feature of the PT state in the pyridine-phenol complex is an IR-intensive band near 600 cm(-1), associated with the asymmetric stretching vibrations of the O(-)...HN(+) fragment. Its intensity is reciprocally proportional to the O...N distance. The appearance of this band provides an efficient criterion to differentiate between the HB and PT states of the 1:1 complexes of phenols with pyridines in aprotic solvents.

Hydrogen Bonding of Water Confined in Controlled-Pore Glass 10-75 Studied by 1 H-Solid State NMR

The adsorption of water in the mesoporous silica material with cylindrical pores of uniform diameter, Controlled Pore Glass 10-75 (CPG), was studied by 1H-MAS solid state NMR spectroscopy. From the NMR spectra it is evident that inside the mesopores of the silica different water environments exist, which are characterized by their individual chemical shift. All observed hydrogen atoms are either surface –SiOH groups or hydrogen bonded water molecules. It is found that there exist some stronger bound water molecules on the surface which are not removable even by heating at a vacuum pump. As a tentative assignment these water molecules are attributed to surface defects or inaccessible cavities in the CPG 10-75. At intermediate water filling levels, the principal signal is a single NMR line with continuously varying chemical shift. This finding is interpreted as the result of a radial water filling mechanism. That is, the filling of the pore grows from the pore surface towards the pore axis. Finally it is shown that water is a sensor for surface and structural inhomogeneity and that a coexistence of inner pore and outer bulk water exists in the system.

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 15N. Some of them were assigned to couplings through intramolecular Nuf8ffH···F hydrogen bonds (HBs). These couplings change dramatically when CDCl3 is replaced by DMSO‐d6. In this theoretical paper we provide density functional theory (DFT) calculations that justify the existence of a weak HB in the absence of solvent, while solvents that act as HB acceptors break down the intramolecular hydrogen bond (IMHB) of 2‐fluorobenzamide. Atoms in molecules (AIM) analyses and Steiner‐Limbach plots were used to analyze the structure of the compounds. Copyright

Interpretation of Hydrogen/Deuterium Isotope Effects on NMR Chemical Shifts of (FHF) − Ion Based on Calculations of Nuclear Magnetic Shielding Tensor Surface

Abstract Using ab initio calculations of 1H, 19F magnetic shielding tensors of hydrogen difluoride ion as the functions of three coordinates of symmetry, an attempt is made to estimate the contributions of different vibrational isotope effects to H/D NMR isotope shifts referred in the literature. It is shown that the contributions of the amplitudes of proton stretching and bending vibrations dominate whereas the contribution of the totally symmetric vibration can be neglected. Different signs of the H/D isotope effects on hydron and fluorine chemical shifts are caused by very strong angle dependence of the fluorine magnetic shielding. The agreement of the calculated and measured values is nearly quantitative. An unusually strong paramagnetic deshielding of hydrogen for the equilibrium geometry of the [FHF]− ion is noted.

2H-solid state NMR and DSC study of isobutyric acid in mesoporous silica materials

Solid state deuterium NMR has been used to study the molecular motion of d(6)-isobutyric acid (d(6)-iBA) in the pure (unconfined) state and confined in the cylindrical pores of two periodic mesoporous silica materials (MCM-41, pore size 3.3 nm and SBA-15, pore size 8 nm), and in a controlled pore glass (CPG-10-75, pore size ca. 10 nm). The line shape analysis of the spectra at different temperatures revealed three rotational states of the iBA molecules: liquid (fast anisotropic reorientation of the molecule), solid I (rotation of the methyl group) and solid II (no rotational motion on the time scale of the experiment). Transition temperatures between these states were determined from the temperature dependence of the fraction of molecules in these states. Whereas the solid I-solid II transition temperature is not affected by confinement, a significant lowering of the liquid-solid I transition temperature in the pores relative to the bulk acid was found for the three matrix materials, exhibiting an unusual dependence on pore size and pore morphology. Complementary DSC measurements on the same systems show that the rotational melting (solid I-liquid) of d(6)-iBA in the pores occurs at a temperature 20-45 K below the thermodynamic melting point. This finding indicated that the decoupling of rotational and translational degrees of freedom in phase transitions in confined systems previously found for benzene is not restricted to molecules with non-specific interactions, but represents a more general phenomenon.

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. In order to characterize this unusual structure, we have performed high-resolution solid-state (15)N and (13)C CPMAS NMR experiments on LysCel samples labeled with (15)N in the alpha-position or epsilon-position. Furthermore, (13)C-(15)N REDOR experiments were performed on LysCel where half of the aminocarboxyl groups were labeled in 1-position with 13C and the other half in alpha-position with (15)N. The comparison with the 13C and 15N chemical shifts of l-leucine lyophilized at different pH shows that the aminocarboxyl groups of LysCel are indeed zwitterionic. The REDOR experiments indicate distances of about 3.5 A between the carboxyl carbon and the nitrogen atoms of different aminocarboxyl groups, indicating that the latter are in close contact with each other. However, the data are not compatible with isolated aminocarboxyl dimers but indicate the assembly of zwitterionic aminocarboxyl dimers either in a flat ribbon or as tetramers, exhibiting similar intra- and interdimer (13)C...(15)N distances. This interaction of several aminocarboxyl groups is responsible for the zwitterionic state, in contrast to the gas phase, where amino acid dimers exhibiting two OHN hydrogen bonds are neutral.

NMR Study of Blue-Shifting Hydrogen Bonds Formed by Fluoroform in Solution

Abstract Low-temperature (193 K) 1H, 13C and 15N NMR spectra of blue- and red-shifting H-bonded complexes formed by fluoroform with various proton acceptors were measured. Experimental NMR parameters were plotted versus the ab initio calculated H-bond strength (MP2/6–31+G(d, p); interaction energy varies from ~5 up to 25 kJ · mol−1 in the series). We show that experimental 1H and 15N shieldings, as well as the H/D isotope effect on 13C shielding change monotonously with the calculated H-bond strengthening. The 13C chemical shift and the CH scalar coupling change non-monotonously and the extremum points are situated approximately in the region of transformation from blue- to red-shifting H-bonds. The most informative NMR feature is the H/D isotope effect on 15N shielding which changes its sign upon transformation from blue- to red-shifting H-bonds. To rationalize these observations, ab initio calculations of 13C and 15N shieldings as functions of C···H and C···N distances were performed for complexes of CHF3 with acetonitrile (blue-shifting) and pyridine (red-shifting). The coupling of the vibrations of the covalent and hydrogen bonds has been accounted for by direct computation of the distance C···N = f(C···H) dependence. We demonstrate that the unusual sign of the H/D isotope effect on 15N chemical shift across a blue-shifting H-bond can be explained as a result of the inversion of the dynamic coupling of two vibrations.