A computational analysis of electronic effects of borazine and its monosubstituted derivatives

The effect of several subtituent groups on the borazine ring has been investigated by an ab initio molecular orbital (SCF) method at the 6-31G level. The optimized geometry was used to calculate the molecular electrostatic potential of these structures at the 6-31G level. The negative potentials generated near the B-H protons suggest that these protons are more easily substituted than the N-H protons in borazine. The magnitude of the negative potential near the B-H protons decreases in the order OH> F> NO₂, reflecting their varying electron-withdrawing ability. Both the methyl and amino groups tend to increase this negative potential indicating their electron-donating ability.     

Zundel‐like and Eigen‐like Hydrated Protons on a Platinum Surface

The nature of hydrated protons is an important topic in the fundamental study of electrode processes in acidic environment. For example, it is not yet clear whether hydrated protons are formed in the solution or on the electrode surface in the hydrogen evolution reaction on a Pt electrode. Using mass spectrometry and infrared spectroscopy, we show that hydrogen atoms are converted into hydrated protons directly on a Pt(111) surface coadsorbed with hydrogen and water in ultrahigh vacuum. The hydrated protons are preferentially stabilized as multiply hydrated species (H₅O₂⁺ and H₇O₃⁺) rather than as hydronium (H₃O⁺) ions. These surface‐bound hydrated protons may play an important role in the interconversion between adsorbed hydrogen atoms and solvated protons in solution.     

Location, acid strength, and mobility of the acidic protons in Keggin 12-H3PW12O40: a combined solid-state NMR spectroscopy and DFT quantum chemical calculation study

Solid-state 13C NMR experiments and quantum chemical Density Functional Theory (DFT) calculations of acetone adsorption were used to study the location of protons in anhydrous 12-tungstophosphoric acid (HPW), the mobility of the isolated and hydrated acidic protons, and the acid strength heterogeneity of the anhydrous hydroxyl groups. This study presents the first direct NMR experimental evidence that there are two types of isolated protons with different acid strengths in the anhydrous Keggin HPW. Rotational Echo DOuble Resonance (REDOR) NMR experiments combined with quantum chemical DFT calculations demonstrated that acidic protons in anhydrous HPW are localized on both bridging (Oc) and terminal (Od) atoms of the Keggin unit. The CP/MAS NMR experiments revealed that the isolated acidic protons are immobile, but hydrated acidic protons are highly mobile at room temperature. The isotropic chemical shift of the adsorbed acetone suggested that the acid strength of the H(H2O)n+ species in partially hydrated HPW is comparable to that of a zeolite, while the acidity of an isolated proton is much stronger than that of a zeolite. Isolated protons on the bridging oxygen atoms of anhydrous HPW are nearly superacidic.     

Nature of acidic protons in metallosilicate molecular sieves as studied by variable temperature 1H MAS NMR

The dynamic properties of protons in H-[Ga]-ZSM-5, H-[B]-ZSM-5 and H-[Al-B]-ZSM-5 were compared with that of protons in H-[Al]-ZSM-5 by temperature dependence of ¹H MAS NMR in the range of 298 k and 473 K. The temperature dependence of the line width of ¹H MAS NMR reveals that protons in H-[Ga]-ZSM-5 were more mobile than those in H-[Al]-ZSM-5 at temperature as low as 373 K. The protons in H-[B]-ZSM-5 were not mobile at 473 K and fixed in the zeolite frame work as the bridging hydroxyl groups, ≡B-OH-Si≡. The thermal motion of protons in ≡Al-OH-Si≡ was suppressed by introducing B³⁺ cations into the framework of H-[Al]-ZSM-5.     

Nuclear magnetic resonance studies of polymer solutions. I. Poly(methyl methacrylates)

The microscopic interactions of solvent with the diastereoisomeric units of isotactic and syndiotactic poly(methyl methacrylate) have been studied by high-resolution nuclear magnetic resonance. The changes in chemical shifts in various solvents were compared with those of low molecular weight analogs, methyl acetate, and methyl propionate. These changes are caused mainly by the ring-current effect, which has been found to be larger for the low molecular weight analogs than for the polymer. This is especially true when the protons on the polymer backbone are compared with the corresponding ones in the low molecular weight compounds. As one changes from a chloroform solvent to an aromatic solvent, the displacements of the chemical shifts of the polymer can be expressed as percentages of the corresponding shifts of methyl acetate. For syndiotactic poly(methyl methacrylate) in chlorobenzene, benzene, and α-chloronaphthalene, respectively, the percentages are 82, 93, 75 for ester protons; 35, 29, 17 for the backbone methylene protons; and 18, 6.7, 0 for the backbone α-methyl protons. For isotactic poly(methyl methacrylate) in chlorobenzene and benzene, respectively, the percentages are 71, 76 for the ester protons; 41, 38 for the backbone methylene protons; and 41, 32 for the backbone α-methyl protons. These results are discussed in terms of the local stereochemistry of the polymer systems. The exploitation of procedures of this sort in revealing details of polymer behavior in solution indicates dramatic possibilities for future investigations.     

Fragmentation of photomodified oligodeoxynucleotides adducted with metal ions in an electrospray-ionization ion-trap mass spectrometer

We report the effect of metal-ion adduction on the fragmentation of oligodeoxynucleotides (ODNs) bearing DNA photoproducts. When protons on backbone phosphates of ODNs are completely replaced with metal ions, cleavages occur readily within the photoproduct moiety, whereas those cleavages do not occur in photomodified ODNs in which the phosphates are associated with protons. For example, thymine/adenine (TA*) photoproducts revert to their undamaged precursors upon collisional activation, the pyrimidine(6-4)pyrimidone product and its Dewar valence isomer show a characteristic neutral loss of C4H3NO3, and dimeric adenine photoproducts show a distinctive loss of NH2CN from the adenine six-membered ring. The product-ion mass spectra of photodamaged ODNs that are adducted to metal ions are complementary in terms of structure information to those spectra of ODNs in which the phosphates are associated with protons. The results also demonstrate that the energy required for strand cleavages is higher for ODNs adducted with metal ions than that for ODNs bound with protons. Furthermore, the loss of a pyrimidine is more favorable than the loss of a purine in the fragmentation of ODNs associated with metal ions.     

Accurate prediction of proton chemical shifts. II. Peptide analogues

Proton chemical shifts of eight cyclic amide molecules were measured in DMSO and D2O solutions. The magnetic shieldings of the corresponding aliphatic, aromatic, and amide protons were calculated by Hartree-Fock and DFT, using the 6-311G**, 6-311++G**, and TZVP basis sets. For aliphatic protons, all of these methods reproduce the experimental values in DMSO solutions excellently after linear regression. The Hartree-Fock method tends to give slightly better agreement than DFT. The best performance is given by the HF/6-311G** method, with an rms deviation of 0.068 ppm. The deviations from experimental chemical shifts in D2O solutions are only slightly larger than those in DMSO solutions. This suggests that we can use the calculated gas phase proton chemical shifts directly to predict experimental data in various solvents, including water. For amide protons, which exchange with water and form hydrogen bonds with DMSO, only modest agreement is obtained, as expected. The present studies confirm that the GIAO approach can reach high accuracy for the relative chemical shifts of aliphatic and aromatic protons at a low cost. Such calculations may provide constraints for the conformational analysis of proteins and other macromolecules.     

Donor-Acceptor Complexes in Copolymerization. IV. Alternating Tendency in Free Radical Copolymerization. NMR Analyses of Alternating and Random Copolymers

An NMR investigation was carried out on variable composition, random and equimolar, alternating copolymers of acrylonitrile (A) with styrene (S), isoprene (I), and butadiene (B). The NMR spectra of the SA copolymers contained peaks at 3 τ (aromatic ring protons), 7.2-7.5 τ (CH protons of A), and 8.1 -8.5 τ (CH and CH₂ protons of S and CH₂ protons of A). All NM R peaks of the alternating SA copolymer were shifted to the higher field due to the shielding effect of S. The NMR spectra of the IA copolymers contained peaks at 4.72-4.91 τ (?CH protons of I), 7.27-7.4 τ (CH protons of A), 7.71-7.93 τ (CH₂ protons of I), and 8.35 τ (CH₃ protons of I and CH₂ protons of A). The peaks at 4.72 τ (?CH) and 7.72 τ (CH₂) were assigned to I in the I-A diad and the peaks at 4.91 τ (?CH) and 7.93 τ (CH₂) were assigned to I in the I-I diad. The NMR spectra of the BA copolymers contained peaks at 4.4-4.6 τ (?CH protons of B), 7.2-7.5 τ (CH protons of A), 7.71-7.97 T (CH₂ protons of B), and 8.0-8.4 τ (CH₂ protons of A). The peaks at 4.42 τ (?CH) and 7.71 τ (CH₂) were assigned to B in the B-A diad and the peaks at 4.6 τ (?CH) and 7.9 τ(CH₂) were assigned to B in the B-B diad. The alternating structure of the copolymers prepared through metal halide-activated complexes was confirmed by NMR analysis. The random copolymers prepared by free radical initiation contain a high concentration of alternating sequences, as anticipated from the values of r₁ and r₂ where r₁(S, I, and B) is 6-10 times higher than r₂ (A).     

Nitrogen-15 nuclear magnetic resonance spectroscopy. Differential rates of N?H proton-exchange reactions of hydrazine-carbothioamide and 4-methylhydrazine-carbothioamide in dimethyl sulfoxide

The proton-coupled nitrogen-15 NMR spectra of hydrazinecarbothioamide and 4-methylhydrazinecarbothioamide have been taken at the natural-abundance level in neutral, basic and acidic solutions at 25°C. The N? H proton-exchange reactions of the hydrazino-NH₂ groups in both compounds were found to be very rapid in the presence of acid, but quite slow in the presence of base. The hydrazino-NH protons of hydrazinecarbothioamide exhange six times and 200 times faster than the amide protons in the presence of either base or acid, respectively. Similarly, acid- and base-catalyzed N? H proton exchanges of the hydrazino-NH group of 4-methylhydrazinecarbothioamide were found to be two to three orders of magnitude faster than those of N-methylamido protons. These results can be rationalized by consideration of the effect of the lone pair on the hydrazino? NH₂ group on the reactivity of the adjacent ? NH? group.     

Chemische Verschiebungen von Methylprotonen in 1H-NMR-Spektren von Alkylcyclohexanen

The chemical shifts of methyl protons of 51 methyl and tert-butyl substituted cyclohexanes were determined. The resonance range of axial methyls extends from δ = 0,63 to 0,98 ppm and equatorial groups from δ = 0,81 to 1,02 ppm. the chemical shifts of axial methyl groups are more greatly influenced by neighbouring groups than those of equatorial methyls. The shift effects of alkyl groups on the chemical shifts of methylprotons and ring protons were compared.     

季铵盐型双子表面活性剂C14-s-C14·2Br的聚集行为

用1H NMR弛豫、2D NOESY研究了双子表面活性剂14-s-14胶束在重水溶液中的聚集行为. 1H NMR自旋-晶格、自旋-自旋弛豫实验表明, 联结基团质子及其邻近的亚甲基质子的运动受到严重限制, 表明这些质子形成胶束疏水核的壳层；而远离极性头的疏水质子的运动则相对自由, 说明其位于胶束的内部. 14-s-14系列(s=2, 3, 4)的弛豫实验结果还表明, 联结基团越短, 分子的碳氢链在疏水核中的排列越紧密. 与14-4-14的对应单链表面活性剂TTAB相比, 它们的分子运动更受限制. 14-4-14的2D NOESY 谱给出了与16-4-16类似的交叉峰信息, 这表明14-4-14同16-4-16一样, 形成了表面不平滑的球形胶束. 因此, 烷烃链的长短对胶束中分子的排列方式没有影响.     

小檗碱与牛血清白蛋白相互作用的核磁共振研究

采用核磁共振波谱技术(NMR)研究小檗碱与牛血清白蛋白(BSA)的相互结合作用.比较不同浓度小檗碱在一定浓度的BSA溶液中的化学位移、弛豫时间和扩散系数的变化情况.研究表明,小檗碱环上芳香质子与BSA的结合作用较强,其它烷基质子与BSA结合作用较弱;BSA的结合使得小檗碱各个质子的1HT1弛豫时间明显减少,质子在溶液中...     

Chemical shifts in methylcyclohexane

In order to determine the effect of a single methyl group on the chemical shifts of protons in a cyclohexane ring, methylcyclohexane-1,2,3,3,5,5,-d₆ has been synthesized. The protons in the 4-position and the 2,6-equatorial protons are not significantly different from those in ring-frozen cyclohexane. The 2,6-axial protons, however, experience an up-field shift of about 15 Hz from the position of the other axial protons. These observations are discussed in terms of the structure of methylcyclohexane.     

Effect of MeV protons on the phase behaviour and thermal stability of polytetrafluoroethylene

Thermomechanical spectroscopy analysis was used to study the influence of accelerated protons on the molecular-topological properties of polytetrafluoroethylene (PTFE). The study showed changes in a wide number of polymer parameters as a result of bombardment with 1, 2 and 4 MeV protons at fluences up to 2 × 10¹⁵ protons/cm². The basic topological process occurring under proton bombardment is amorphicity, as found for γ-irradiation of PTFE. The flow temperature of bombarded PTFE significantly decreases with increasing the fluxes and energy of the accelerated protons. The general process resulting from proton bombardment is cleavage of C-F bonds, leading to formation of “centered” radicals ～CF₂CF · CF₂～ and HF. The thermal stability of bombarded PTFE is below than that of virgin polymer. The rate of thermal destruction noticeably increases and the temperature of the initiation of effective thermal decomposition decreases after bombardment. The gaseous products generated during thermal destruction of the bombarded and virgin PTFE are similar.     

Surface Energy Characteristics of Polytetrafluoroethylene Bombarded with MeV Protons

Energy characteristics of the irradiated surface of a polytetrafluoroethylene film depend on the energy and fluence of bombarding MeV protons. Irradiation with 2–4 MeV protons leads to an increase in the surface free energy; 4 MeV protons at a fluence of 10¹⁵ proton/cm² increase the polarity of the polymer surface by 40 times due to the appearance of functional groups, the polarity enhancement being manifested in an increase in the acid–base component of the surface energy by more than a factor of 50. There is a correlation between the dispersion component of the surface energy and the degree of crystallinity of the near-surface layer of the polymer a period. They both grow symbatically in the case of bombardment with 1–2 MeV protons and decrease upon irradiation with 4 MeV protons. It has been found that dehydrofluorination results in carbonization of the irradiated surface, a decrease in the fluorine content, and an increase in the proportion of oxygen due to oxidation of the radicals generated by proton bombardment.     

Solvent effect on chemical shift non-equivalence of methylene protons in dibenzyl-, diphenacyl- and benzyl phenacyl sulphoxides

The methylene protons of dibenzyl sulphoxide are magnetically equivalent in low dielectric and non-equivalent in high dielectric constant solvents, while for diphenacyl sulphoxide this behaviour is reversed. In both cases, the variation in the magnitude of the non-equivalence reflects a greater downfield shift of one methylene proton than the other on passing from non-polar to polar solvents. By contrast, the chemical shift difference between the benzyl methylene protons or between the phenacyl methylene protons of benzyl phenacyl sulphoxide varies only slightly with solvent polarity.     

Proton shielding tensors in potassium hydrogen maleate

The proton NMR in single crystals of potassium hydrogen maleate has been sttudied by means of multiple-pulse line-narrowing techniques. The magnetic shielding tensors of all magnetically inequivalent protons in the unit cell could be determined independently. Two of these protons are carboxylic, forming hydrogen bonds. The orientations of the shift tensors are consistent with the position of the hydrogens at the midpoints of the 0–0 intervals. The range of anisotropy of 32 ppm, found for the shift tensor of the caboxylic hydrogen, is larger than that found for hydrogen bonds in acids and seems to be characteristics of acidic salts.The other protons in the unit cell are olefinic. Two features distinguish this type of protons from those studied so far: (1) The magnetic shielding tensor is not even approximately axially symmetric, the principal values being ?2.4, ?5.1, ?7.3 ± 05 ppm (from adamantane); and (2) the principal directions reflect all characteristic directions of the carboncarbon double bond (while the CH direction is of no importance). The principal value in the direction perpendicular to the sp² system is the least shielded one.     

Multi-spot paper chromatograms of sodium orthophosphate

The formation of two zones on paper chromatograms was investigated by chromatographing orthophosphoric acid and the primary, secondary and tertiary sodium salts, using pyridine-ethyl acetate/water as mobile phase. Movement of the species from the origin depends on washing the paper with acid and suggests an exchange of protons for sodium as a prerequisite condition. Washing with EDTA does not alter this requirement. Zone movement and multi-spot formation require that the forming solvent contain at least 5% of water (v/v). Between this value and saturation, increasing water content enhances zone disengagement. A solute spot of a sodium salt exchanges with the protons to produce a fast-migrating protonated phosphate species. If the solute sample has a sodium content less than the amount of exchangeable protons, a single fast spot is produced ; if the concentration exceeds this capacity, a second slower spot results.     

Multistep correlations via covariance processing of COSY/GCOSY spectra: opportunities and artifacts

Long-range homonuclear coupling pathways can be observed in COSY or GCOSY spectra by the acquisition of spectra with larger numbers of increments of the evolution period, t(1), than would normally be used. Alternatively, covariance processing of COSY-type spectra acquired with modest numbers of t(1) increments, allows the observation of multistage correlations. In this work results obtained from covariance-processed GCOSY spectra are fully analyzed and compared to normally processed COSY and 80 ms TOCSY spectra. Multistage or 'RCOSY-type' correlations are observed when remote protons both exhibit correlations to the same coupling partner e.g. A --> B and B --> C gives rise to an A --> C correlation. In the strict sense, RCOSY-type responses are artifacts albeit providing useful information. Nonbeneficial artifact correlations are observed when protons couple to other protons that overlap or partially overlap. The origin of artifact responses is also analyzed.     

NMR study on the photoresponsive DNA tethering an azobenzene. Assignment of the absolute configuration of two diastereomers and structure determination of their duplexes in the trans-form

Two diastereomers of a photoresponsive oligodeoxyribonucleotide tethering a trans-azobenzene, based on the chirality of the central carbon of a diol linker, were separated by reversed-phase HPLC. On the basis of 2D NMR analysis, absolute configurations of the diastereomers alpha and beta (tentatively designated from differences in their retention time) were determined as R- and S-forms, respectively. For both diastereomers, their NMR-determined duplex structure showed that trans-azobenzene intercalates between base pairs, because distinct NOEs were observed between the protons of azobenzene and those of the adjacent base pairs, such as with the imino protons and methyl protons of thymine. The melting temperatures of both duplexes were higher than that of the corresponding native duplex, which contained no azobenzene residue, due to the intercalated trans-azobenzene stabilizing the duplex by a stacking interaction. Between these two diastereomers, differences in T(m) were also found: the melting temperature of the R-form duplex (alpha-isomer) was higher than that of the S-form (beta-isomer). On the basis of the NMR-determined structure, this difference was attributed to the fact that the S-form (beta isomer) causes more stress forming the duplex than does the R-form (alpha isomer) due to disturbances of the right-hand helix.