Molecular motion of the main chain for a series of Poly(alkyl l-glutamate)s as studied by 2H NMR

²H NMR measurements were carried out for a series of poly(alkyl l-glutamate)s (PALG) in which the ¹H of the amide group in the main chain is replaced by ²H in order to investigate the mobility and motional mode of the main chain. At low temperature, the ²H spectra were typical powder patterns, which have three principal values. The temperature dependencies for the ²H NMR spectra varied with the side chain length. For PALG with a short side chain length, Δv₁, Δv₂, and Δv₃ are almost constant in all temperature ranges. As the side chain length increases, the difference between the peaks and shoulders decreased with temperature. For PG-12-N-D, the peaks and shoulders are fused at high temperature in a liquid crystalline state. The mobility and molecular motion of the main chain is discussed based on the obtained ²H NMR spectra.     

Microstructure of poly(methyl methacrylate-co-isopropyl acrylate) studied by 13C NMR

The distribution of configurational–compositional sequences of poly(methyl methacrylate-co-isopropyl acrylate) (PMMA/iPrA) has been determined from the carbonyl and β-CH₂ signals in the 100?MHz ¹³C NMR spectra of the copolymer. The carbonyl signal provided information on configurational–compositional sequences up to heptads, whereas β-CH₂ signals offered complementary information on even sequences up to hexads. The assignment of the sequences to the respective signals was based on a comparison with the spectra of respective homopolymers, that is, PMMA and PiPrA followed by a computer simulation applying an incremental calculation of chemical shifts of the individual sequences.     

Conformation of drawn poly(trimethylene terephthalate) studied by solid-state 13C NMR

The change in the conformation of the flexible O-CH2-CH2-CH2-O segment of poly(trimethylene terephthalate) (PTT) monofilament caused by drawing was investigated by means of the gamma-gauche effect on the 13C solid-state NMR chemical shift of the internal methylene carbon, combined with the NMR relaxations. The conformation around the O-CH2 and CH2-O bonds for as-spun fiber was trans/trans. On drawing, followed by heat treatment, the conformation changed to gauche/gauche. The ratio of gauche/gauche to trans/trans for the drawn PTT fiber was determined quantitatively.     

Conformational equilibria in isomeric methylcyclohexanols studied by 13C NMR spectroscopy

The conformational equilibrium constants for isomeric methylcyclohexanols (cis- and trans-1,2-, 1,3- and 1,4-methylcyclohexanols) have been determined from peak area measurements in the completely proton decoupled low temperature ¹³C NMR spectra of the individual conformers. The ¹³C chemical shifts are discussed in terms of the additive model.     

A technique for in situ monitoring of crystallization from solution by solid-state 13C CPMAS NMR spectroscopy

We report a technique for carrying out in situ solid-state NMR studies of crystallization from solution, allowing the evolution of different solid state structures (polymorphs) produced during the crystallization process to be identified. The technique exploits selectivity in NMR properties (specifically, the efficiency of cross-polarization from (1)H to (13)C) between molecules in the solid and solution states, such that the first solid particles produced during the crystallization process are observed selectively, without detecting any signal from dissolved solute (or solvent) molecules. The application of the technique is demonstrated to reveal new insights concerning an isotope effect on the polymorphic outcome of crystallization of glycine from water. As revealed by this example, the in situ solid-state NMR approach reported here creates significant new opportunities for probing and understanding details of the evolution of solid state structures produced during crystallization from solution.     

Protolytic properties of polyamine wasp toxin analogues studied by 13C NMR spectroscopy

Acid-base properties of the natural polyamine wasp toxin PhTX-433 (1) and seven synthetic analogues [PhTX-343 (2), PhTX-334 (3), PhTX-443 (4), PhTX-434 (5), PhTX-344 (6), PhTX-444 (7), and PhTX-333 (8)], each having four protolytic sites, were characterized by 13C NMR spectroscopy. Nonlinear, multiparameter, simultaneous fit of all chemical shift data obtained from the NMR titration curves yielded macroscopic pKa values as well as intrinsic chemical shift data of all differently protonated macrospecies. Analyses of the chemical shift data demonstrated strong interactions between all four sites and provided information about complex relationships between chemical shift values and protonation state. Deprotonation of fully protonated forms starts at the central amino group of the polyamine moiety, and the extent of this trend depends on the distance to the flanking, protonated amino groups. The pKa1 values of 1-8 are in the range 8.2-9.4. Hence, some of the toxins are incompletely protonated at the pH and ionic strength conditions used for assessment of their interactions with ionotropic glutamate and nicotinic acetylcholine receptors, and the degree of protonation is expected to have pharmacological importance in the ion-channel binding event.     

Polymer electrolyte poly(ethylene oxide)/LiCF3SO3 studied by solid-state 13C NMR spectroscopy and ab initio calculations

Solid-state ¹³C NMR spectra and ab initio calculations of ¹³C NMR chemical shifts show that poly(ethylene oxide) (PEO) carbons in complex with LiCF₃SO₃ (both in crystalline and amorphous phase) are more shielded in comparison with neat PEO, due to the coordination to the Li⁺ cation. The results obtained from ¹³C NMR cross-polarization dynamics are in agreement with the published X-ray crystal structure of the PEO/LiCF₃SO₃ complex. The mobility of PEO in the crystalline complex is lower than in neat crystalline PEO.     

Structure of polymer networks in liquid crystals studied by 13C NMR

We report on 13C NMR measurements above and below the clearing temperature of the liquid crystal 4-n-hexyloxyphenyl 4-methoxybenzoate constrained to an oriented, low concentration polymer network. The network is obtained by UV-irradiation of the reactive monomer 1,4-di-[4-(6-acryloyloxyhexyloxy)benzoyloxy]-2-methylbenzene which is in admixture with the liquid crystal. The characterization of orientational order of the mixtures before UV-radiation, and hence before polymerization, reveals the high order of the components at the polymerization temperature. The chosen geometry explains the LC director orientation only by the aligned network. Above the nematic-isotropic transition a strong pretransitional order is detected. Fast molecular translational diffusion averages the order over dimensions smaller than 1 mum. The Landau-de Gennes theory predicts a relation between pretransitional order and the lateral dimension of the LC regions. The experimental data are successfully explained by pore diameters of 35 and 98 nm for concentrations of 20 and 8 mol% of monomer, respectively. The results support the model of nearly cylindrical shaped liquid crystal domains surrounded by thin walls of crosslinked network.     

Conformations and 13C NMR chemical shifts of some polyamides in the solid state as studied by high-resolution 13C NMR spectroscopy

High-resolution ¹³Carbon nuclear magnetic resonance (NMR) spectra of Nylons 4, 6, and 66 in the solid state were measured over a wide range of temperature. From the results, it was found that resonance lines of crystalline and noncrystalline components were separable and their chemical shifts were determined. The ¹³C chemical shift behavior is closely related to their conformation. The origin of the conformational effects on the chemical shifts is discussed.     

Molecular weight effect on the complexation of poly(methacrylic acid) and poly(ethylene oxide) as studied by high-resolution solid-state C NMR spectroscopy

Complexes of poly(methacrylic acid) (PMAA) and poly(ethylene oxide) (PEO) with different PEO molecular weight were studied by solid-state high-resolution ¹³C NMR spectroscopy, with the emphasis on the PEO molecular weight effect on inter-polymer interaction, morphology and molecular motion. It is found that the crystalline phase of PEO is completely destroyed in the complex. The results of ¹H transverse relaxation times and ¹³C spin-lattice relaxation times indicate that the chain mobility of both PEO and PMAA are greatly restricted by inter-molecular hydrogen-bonding interactions, especially when the molecular weight of PEO is 1500. The bulk structures of the complexes are found to be closely dependent on the molecular weight of PEO. The fraction of “free” PEO segments without forming hydrogen-bonds with PMAA increases with increasing PEO molecular weight.     

Stereoregularity of poly(α-methylvinyl alkyl ether)s determined by 1H- and 13C-NMR spectroscopy

α-Methylvinyl methyl ether, ethyl ether, and isobutyl ether were polymerized under various polymerization conditions and the structure of the polymers was determined by ¹H- and ¹³C-NMR spectroscopy. α-Methyl and β-methylene carbon spectra of poly(α-methylvinyl isobutyl ether) showed splitting and were analyzed by triad and tetrad sequences. β-Methylene carbon spectra of poly(α-methylvinyl ethyl ether) also showed splitting. When Eu(fod)₃ was added, α-methyl and methoxy proton spectra in benzene of poly(α-methylvinyl methyl ether) showed splitting assigned to triad tacticities. All the polymers obtained in polar solvents exhibited an increase in syndiotacticity. The polymerization mechanism is discussed.     

Sequence analysis of poly (ether sulfone) copolymers by 13C NMR

Random and block copolymers of poly (ether sulfone) (PES) and poly (ether ether sulfone) (PEES) were synthesized by the nucleophilic polycondensation of 4,4′‐dichlorodiphenyl sulfone (DCDPS) with 4,4′‐dihydroxydiphenyl sulfone (DHDPS) and hydroquinone (HQ). Chemical structures of these copolymers were characterized by ¹³C NMR. The monomer molar fraction, sequential distribution, and degree of randomness of the copolymers were determined through analyses of the resonances of quaternary carbons in the DCDPS unit. Experimental results show that the molar fractions of the comonomer determined by ¹³C NMR analyses are close to the charged values in the synthetic step. Moreover, these copolymers, which were prepared by different polymerization methods, revealed different number‐average sequential length and degree of randomness. © 2005 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 43: 1624–1630, 2005     

Topological structure and molecular mobility of linear and branched poly(meth)acrylates studied by NMR relaxation

The method for the determination of fractions of terminal chains and chains bonded by physical or chemical knots from free induction decays (FID) and line shape was proposed on the basis of the earlier developed theory of NMR spectra in linear and cross-linked polymers. The density and topological structure of the effective polymer network and the characteristic correlation times of segmental motion of chains were determined for linear and branched poly(meth)acrylates at 130 °C from the coincidence of the theoretical and experimental FID curves and the line shape. A quantitative estimation of chain rigidity in the branched polymer with an increase in the brancher content was performed for the first time.     

Structure of ultrahigh-molecular-weight polyethylene in the solid state as studied by variable-temperature 13C CP/MAS NMR spectroscopy

In order to obtain some insight into the structure of an ultrahigh-molecular-weight polyethylene sample, ¹³C CP/MAS NMR experiments have been carried out at temperatures from 23 to ?108°C. The peak for the crystalline component moves upfield with a decrease in temperature, which is contrary to what was reported previously for a melt-crystallized polyethylene sample. On the basis of x-ray diffraction results and quantum-chemical calculations, it is suggested that the methylene carbons are in the distorted orthorhombic form at low temperatures.     

Investigations by (13)C NMR spectroscopy of ethene-initiated catalytic CO hydrogenation

13C NMR spectroscopy shows that the n-alkene and n-alkane products from the catalytic hydrogenation of CO in the presence of (13)C(2)H(4) probes over Ru/150 degrees C, Co/180 degrees C, Fe/220 degrees C, or Rh/190 degrees C (1 atm, CO:H(2) 1:1, "mild conditions") contain terminal (13)CH(3)(13)CH(2)- units. This is consistent with their formation by a regiospecific polymerization of C(1) species derived from CO and initiated by (13)C(2)H(4). Although the activities toward individual products differed somewhat, similar distributions and similar product labeling patterns were obtained over all the four catalysts. 1-Butene and the higher 1-n-alkenes from all the catalysts were largely (13)CH(3)(13)CH(2)(CH(2))(n)()CH=CH(2) (n = 0-3), propene formed over Ru or Co was (13)CH(3)(13)CH=CH(2), while both (13)CH(3)(13)CH=CH(2) and (13)CH(2)=(13)CHCH(3) were formed over Fe or Rh. Comparison of the conclusions from these probe experiments with those from isotope transient experiments by other workers indicates that the ethene initiator does not significantly modify the course of the CO hydrogenation. The reaction products are largely kinetically determined, and the primary products are mainly linear 1-n-alkenes, while the n-alkanes and 2-n-alkenes largely arise via secondary processes. Since the distribution of products and the labeling in them is so similar, it is concluded that one basic primary mechanism applies over all the four metals. Several different reaction paths involving a polymerization of surface methylene, [CH(2(ad))], have been proposed. Although the predictions based on several of these mechanisms agree with many of the results, the alkenyl + [CH(2(ad))] mechanism, initiated by a surface vinyl [CH(2)=CH((ad))], most easily accommodates the experimental evidence. An alternative path involving sequential addition of surface methylidyne and hydride either to a growing alkylidene chain (alkylidene + [CH(ad) + H(ad)]) or to an alkyl chain (alkyl + [CH((ad)) + H(ad)]) has recently been proposed by van Santen and Ciobica. The [CH(2(ad))] mechanism offers an easier explanation for the formation of the various alkenes, the distribution of products, and of the initiation, while the [CH(ad) + H(ad)] mechanism can explain any n-alkanes formed as primary products and not derived from alkenes. At higher reaction temperatures over Ru and Co, considerable (13)C(1) incorporation (from natural abundance in the CO and from cleavage of the (13)C(2)H(4) probe) was found in all the hydrocarbons. Thus, at higher temperatures (13)C(1(ad)) in addition to (13)C(2(ad)) species participate in both chain growth and initiation. In summary, adsorbed CO is transformed very easily into surface C(1(ad)), probably [CH(2(ad))] in equilibrium with [CH((ad))+H(ad)], which act as the propagating species.     

Phase transition of L-Ser monohydrate crystal studied by (13)C solid-state NMR

We used gravimetric analysis (GA) and (13)C solid-state nuclear magnetic resonance (NMR) to study solid-phase transition from the transparent single crystal of L-serine (L-Ser) monohydrate to a turbid powder. We found that L-Ser monohydrate loses water molecules and transforms into an anhydrate, thus experimentally demonstrating Frey's assumption. Application of a handmade cross-polarization (CP) NMR probe with a saddle-type coil to the oriented crystal of the L-Ser monohydrate revealed the dehydration mechanism. Furthermore, the chemical shift tensor components of the carboxyl carbon in L-Ser monohydrate were determined. The difference in the tensor component of delta(22) between the monohydrate and anhydrate forms was more than 7 ppm, probably owing to differences in the hydrogen-bonding structure of each form.     

Microstructural study of poly(methyl methacrylate-co-n-propyl acrylate) by 13C NMR

Analysis of carbonyl and β-CH₂ signals in the 100?MHz ¹³C NMR spectra of poly(methyl methacrylate-co-n-propyl acrylate) (PMMA/nPrA), provided distribution of configurational-compositional sequences for a series of the copolymer samples of different composition at pentad level for carbonyl signal and hexad level for the backbone methylene carbons. Computer simulation of the spectra based on incremental calculation of the chemical shifts for individual sequences provided very good agreement with the experimental spectra.     

Characterization of blends of poly(vinyl chloride) and poly(N‐vinyl pyrrolidone) by FTIR and 13C CP/MAS NMR spectroscopy

Blends of poly(vinyl chloride) (PVC) with Poly(N‐vinyl pyrrolidone) (PVP) were investigated by Fourier infrared spectroscopy (FTIR) and high‐resolution solid‐state ¹³C cross‐polarization/magic angle spinning (CP/MAS) nuclear magnetic resonance (NMR) spectroscopy. The intermolecular interactions between PVP and PVC are weaker than the self‐association of PVP and the inclusion of the miscible PVC results in the decreased self‐association of PVP chains, which was evidenced by the observation of high‐frequency shift of amide stretching vibration bands of PVP with inclusion of PVC. This result was further substantiated by the study of ¹³C CP/MAS spectra, in which the chemical shift of carbonyl resonance of PVP was observed to shift to a high field with inclusion of PVC, indicating that the magnetic shielding of the carbonyl carbon nucleus is increased. The proton spin‐lattice relaxation time in the laboratory frame (T₁ (H)) and the proton spin‐lattice relaxation time in the rotating frame (T_1ρ(H)) were measured as a function of the blend composition to give the information about phase structure. It is concluded that the PVC and PVP chains are intimately mixed on the scale of 20–30Å. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 2412–2419, 1999     

Dimers from cyclic α-diketones. Confirmation of structure by solid-phase CPMAS 13C NMR spectroscopy

The solid-phase ¹³C CPMAS NMR spectra of the crystalline dimers from 10-, 12- and 14-membered cyclic α-diketones were found to be in full accord with the symmetrical tricyclic 2,5-dihydroxycyclohexane-1,4-dione structures previously proposed on the basis of molecular formulae and infrared spectroscopy.