From NMR Spectra to Molecular Structures and Conformation - ACS

Chapter 10

From NMR Spectra to Molecular Structures and Conformation Downloaded by UNIV OF FLORIDA on November 16, 2017 | http://pubs.acs.org Publication Date (Web): November 2, 2017 | doi: 10.1021/bk-2017-1258.ch010

Alan E. Tonelli* Fiber & Polymer Science Program College of Textiles, North Carolina State University, Campus Box 8301, 1020 Main Campus Drive, Raleigh, North Carolina 27606-8301, United States *E-mail: [email protected]

Chemists are interested in establishing the connections between the structures of molecules and the properties of materials made from them. Nearly 30 years ago I wrote a book, “NMR Spectroscopy and Polymer Microstructure, which was subtitled “The Conformational Connection”. Its purpose was to describe how 13C-NMR spectra of polymers, currently the most sensitive NMR probe, can be assigned to their polymer microstructures. At that time and now, even the most advanced quantum mechanical methods cannot estimate 13C resonance frequencies accurately enough to delineate the detailed molecular structures that produced them. Instead empirical nuclear shielding effects produced by substituents α, β, and γ to a carbon atom were successfully used to make the connections between polymer 13C-NMR spectra and their microstructures. Principal among these are the nuclear shieldings produced by γ substituents, which were demonstrated to have a conformational origin, i.e., a γ substituent could only shield a 13C nucleus if the central bond between them produced a proximal arrangement by adopting a gauche or cis conformation. In this review, honoring the memory of the late Prof. Ernest Eliel, we update this approach by presenting a few examples of more recent attempts to utilize the conformational characteristics of flexible polymers to characterize their microstructures, as well as the conformations of rigid solid polymers, using 13C-NMR.

© 2017 American Chemical Society Cheng et al.; Stereochemistry and Global Connectivity: The Legacy of Ernest L. Eliel Volume 2 ACS Symposium Series; American Chemical Society: Washington, DC, 2017.

Downloaded by UNIV OF FLORIDA on November 16, 2017 | http://pubs.acs.org Publication Date (Web): November 2, 2017 | doi: 10.1021/bk-2017-1258.ch010

Introduction The purpose of this selective review is to assess whether or not significant advances have been made, since the publication of ref. (1), in the use of 13C-NMR spectroscopy, currently the method of choice, to characterize the microstructures and conformations of polymers and other flexible molecules (1). The short answer is NO! On the theoretical side of this query, even though Quantum Mechanical theory and calculation methods have advanced over this period, it is still not possible to predict NMR resonance frequencies accurately enough to completely characterize their microstructures (See refs. (2–9)). This is particularly true for flexible molecules like polymers, because the magnetic shielding of nuclei must not only be predicted for particular microstructures, but also must be averaged over all the conformations appropriate to each microstructure. Instead the nuclear shielding effects produced by substituents α, β, and γ to and separated by one, two, and three bonds from, respectively, a carbon atom were successfully used to make the connections between 13C-NMR spectra and the contributing microstructures (1). Principal among these substituent effects were the nuclear shieldings produced by γ substituents, which were demonstrated to have a conformational origin, i.e., a γ substituent could only shield a 13C nucleus if the central bond between them produced their proximal arrangement by adopting a gauche or cis conformation. α-, β-, and especially the conformationally sensitive γ-effects were used to assign the NMR spectra and determine the microstructures of polymers in solutions and melts, where they are conformationally flexible, and to characterize their rigid conformations in solid samples. For example, in the exceptional case of polypropylene (PP), its resonances observed in solution by high resolution 13C-NMR exhibit a sensitivity to stereosequences at the undecad level. This means that eleven repeat unit fragments of PP different only in whether their terminal diads are meso (m) or racemic (r) evidence distinct resonance frequencies for the methyl carbons in their central (6th) repeat unit. [Busico and Cipullo (10), using high-field 13C-NMR (150 MHz for 13C), have in fact detected a 0.03 ppm difference in the resonance frequencies of the methyl carbons in the central repeat units of mmmmmmmrmr and mmmmmmmrmm PP undecads.] Though the sensitivity of 13C-NMR to the long-range stereochemistry of PPs is exceptional, more typically 13C-NMR is usually only sensitive to tetrad and pentad stereosequences in homopolymers and triad comonomer sequences in copolymers. The structural sensitivity of 13C-NMR presents a challenge to the chemist, i.e., how can the multitude of observed 13C resonances be assigned to the specific microstructures that generated them? As an example, the 0.03 ppm difference in resonance frequencies observed (10) between the methyl carbons in the central repeat units of mmmmmmmrmr and mmmmmmmrmm PP undecads mentioned above is reproduced by the small differences in the local populations of the central repeat unit conformations in these two PP undecads and their resultant γ-gauche shieldings (11). Here we update this approach by presenting a few examples of more recent attempts to utilize the conformational characteristics of flexible polymers to assign their observed 13C-NMR resonance frequencies and determine their microstructures and to characterize their rigid solid conformations, as well. 162 Cheng et al.; Stereochemistry and Global Connectivity: The Legacy of Ernest L. Eliel Volume 2 ACS Symposium Series; American Chemical Society: Washington, DC, 2017.


NMR Resonance Frequencies (Chemical Shifts) The numbers and types of atoms and groups of atoms attached to or near a magnetically active nucleus determines the degree of shielding it experiences from the static applied magnetic field Bo. This is because they affect the electron cloud surrounding each nucleus, which produce orbital currents accompanied by small local magnetic fields proportional to but in the opposite direction (diamagnetic) of Bo. Thus the local magnetic field actually experienced by the nucleus is Bloc = Bo(1-σ), where σ is the electronic screening constant, which is normally a tensor due to the typically anisotropic distribution of electrons that shield nuclei. It is this dependence of σ upon molecular structure that lies at the heart of NMR’s utility as a probe of molecular structure. Of the common NMR active spin ½ nuclei most abundantly present in organic molecules, 1H and 13C, the latter has an ~ order of magnitude greater sensitivity to molecular structure (σ), compared with 1H, and is unaffected by direct through bond scalar homonuclear coupling, which for 1H nuclei produce multiple resonance for identical microstructures. For these two reasons 13C-NMR remains the most effective experimental probe of molecular structure despite the greater natural abundance of 1H nuciei. However, as mentioned above, sufficiently accurate calculations of the dependence of σ upon the molecular structure surrounding magnetically active nuclei are currently not feasible (2–9). Instead we rely on the empirical effects of directly bonded nearest neighbor (α) and next nearest neighbor (β) substituent effects, and the conformationally dependent effects of γ-substituents separated by three intervening bonds from 13C nuclei (γ-gauche shielding). 13C NMR studies of paraffinic hydrocarbons (12–17) have led to the following substituent effect rules. Carbon substituents attached at α, β, and γ positions to an observed carbon produce approximate deshielding of +9 ppm downfield , deshielding of +9 ppm downfield, and a shielding of – (2 to 3) ppm upfield, respectively, compared to an unsubstituted carbon. For example, when these substituent effects are applied to the CH3, CH2, and CH carbons in each polypropylene (PP) repeat unit their calculated resonance frequencies closely follow the order observed in their 13C-NMR spectra and shown in Figure 1. In atactic PP the extensive splitting of resonances belonging to the same carbon type must be produced by the presence of different stereosequences, because the numbers of α, β, and γ substituents possessed by each carbon type are independent of stereosequence. However, we know the local conformations in vinyl polymers, such as atactic PP, are sensitive to stereosequence (18). The local magnetic field Bloc(i) experienced by a carbon nucleus i must be dependent upon the local conformation in its vicinity, Thus,

We need to know the dependence of the local magnetic field Bloc(i) on the local conformation before the connection between polymer microstructures and resonance frequencies (δ13Cis) can be made. The source of the dependence of the local magnetic field Bloc(i) on the local conformation turns out to be the effect of γ-substituents. An observed carbon Co and its γ-substituent Cγ are separated by 163 Cheng et al.; Stereochemistry and Global Connectivity: The Legacy of Ernest L. Eliel Volume 2 ACS Symposium Series; American Chemical Society: Washington, DC, 2017.


three intervening bonds (-Co–C-↻φ-C-Cγ-), and, depending on the conformation (φ) of the central bond, their mutual distance and orientation maybe varied. For example, changing the conformation from trans (φ = 0º) to gauche± (φ = ±120º) reduces their separation from 4 to 3Å. We have suggested that for a γ-substituent to shield a carbon nucleus they must be in a gauche arrangement. The methyl carbons of butane and the higher n-alkanes have a single γ-substituent, unlike the methyl carbons in propane, but they have the same number and kinds of α- and β-substituents as propane. The methyl carbons in liquid butane and higher n-alkanes resonate at ~13 ppm, while in liquid propane the methyls resonate at ~15 ppm (17). Because n-alkanes crystallize in their lowest energy, fully extended, all trans conformation, the methyl carbons of solid butane and higher n–alkanes are not gauche to their γ-methyl or methylene carbon substituents. We therefore expect that δCH3(solid CnH2n+2, n ≥ 4) = δCH3(liquid propane), which has in fact been observed (19).

Figure 1. 25 MHz 13C NMR spectra of (a) isotactic, (b) atactic, and (c) syndiotactic PPs.

164 Cheng et al.; Stereochemistry and Global Connectivity: The Legacy of Ernest L. Eliel Volume 2 ACS Symposium Series; American Chemical Society: Washington, DC, 2017.


We know how much gauche character, Pg, the –C-C- bonds in n-alkanes have (Pg = fractional population of Φ = ±120º conformations) (1, 18), so we can estimate the γ-gauche shielding (γC-C) produced at the methyl carbons in butane, for example. When the observed shielding ΔδCH3 = δCH3(butane) δCH3(propane) = 13.2 – 15.6 = -2.4 ppm is divided by the gauche character of the intervening bond (Pg = 0.46), γC-C = ΔδCH3/Pg = -2.4/0.46 = -5.2 ppm, as shown in Figure 2. When this procedure is similarly applied to n-butane,1-propanol, and 1-chloropropane, the following γ-gauche shielding effects are derived: γC-C = -5.2 ppm, γC-O = -7.2 ppm, and γC-Cl = -6.8 ppm (1). Along with the Rotational Isomeric States (RIS) conformational descriptions (18) determined for polymers containing all carbon backbones and side-chains containing C, O, and/or Cl atoms, these γ-gauche shielding effects can be used to determine their microstructure, as we will now demonstrate.

Figure 2. Derivation of the γ-gauche shielding on 13C nuclei produced by the γ substituents, C, OH, and Cl (See text).


Polymer Microstructures


E-VAc Copolymers Both poly(viny1acetate) (PVAc) and ethylene-vinyl acetate (E-VAc) copolymers have commercial significance. Combination of solution 13C-NMR observations made at high magnetic fields with comparisons to the spectra recorded for an extensive series of E-VAc model compounds (mono- and diacetates of aliphatic alcohols and diols, etc.), have served to establish their microstructures i.e., their comonomer and stereosequence distributions. (See refs. (18, 20–24) and refs. therein). In addition, a conformational description (RIS model) of E-VAc copolymers was developed (22) by merging the RIS models of the constituent homopolymers PE (23) and PVAc (24). This permitted evaluation of the conformationally sensitive γ-gauche shielding effects for E-VAc comonomer and stereosequences. In addition, comparison of the observed spectra to the 13C chemical shifts calculated via the γ-gauche effect method using the RIS model developed for E-VAc copolymers provided an assessment of its the validity. This comparison is shown below in Figures 3 and 4, where the generally close agreement between observed and calculated 13C chemical shifts of the backbone methylene and methine carbons validates the RIS conformational model developed for E-VAc copolymers (22).

Figure 3. Comparison of observed and calculated 13C chemical shifts for the methylene carbons in E-VAc copolymers.



Figure 4. Comparison of observed and calculated 13C chemical shifts for the methylene carbons in E-VAc copolymers.

A similar comparison of observed and calculated 13C chemical shifts of the backbone methylene and methine carbons for atactic poly(vinyl acetate) (PVAc) are presented in Figures 5 and 6. The RIS conformational model of PVAc developed by Sundararajan (24) was used to estimate the γ-gauche contributions to the calculated 13C chemical shifts. Note in Figure 5 that both the observed and calculated order of resonances is rxr, mxr (rxm), mxm from low to high field in the methylene carbon regions. The m-centered tetrads in the spectrum are, however, observed upfield from the corresponding r-centered tetrads, while the calculated methylene carbon chemical shifts evidence the opposite behavior. The calculated tetrad methylene carbon chemical shifts are averaged over all hexads containing common tetrad stereosequences. So it may not be too surprising that the tetrad methylene carbon chemical shifts are more sensitive to the terminal diad stereochemistry than the m or r character of the central diad. This is because it is the terminal diads that contain the backbone bonds which govern their gauche:trans conformational ratios and therefore should be highly sensitive to the terminal diad stereochemistry.



Figure 5. Comparison of observed and calculated 13C chemical shifts for the methylene carbons in atactic PVAc.

Figure 6 compares the observed and calculated backbone methine carbon chemical shifts, which appear to generally agree. The mrmr, rrrr, and mrrm stereosequence pentads were unable to be unambiguously assigned by Sung and Noggle (21), so we took the liberty to assign them in this order with increasing field strength to achieve greater consistency with our calculated methine carbon chemical shifts. The assignment of vinyl polymer spectra, particularly when they are stereochemically random like the atactic PVAc sample observed by Sung and Nogglel (21) (Pm = 0.481), and the resonances of all stereosequences appear with nearly equal intensities in the spectrum, are substantially aided by the ability to calculate the stereosequence dependent13C chemical shifts in vinyl polymers.



Figure 6. Comparison of observed and calculated 13C chemical shifts for the methine carbons in atactic PVAc. The generally good overall agreement presented here between the 13C-NMR spectra observed for E-VAc copolymers and their calculated 13C chemical shifts lends strong support to the conformational description/RIS model employed for the E-VAc copolymers (22). Our ability to calculate the microstructurally sensitive 13C- NMR chemical shifts for E-VAc copolymers not only aided the assignment of their spectra, but illustrated the potential for testing and/or deriving conformational RIS models for vinyl homo- and copolymers by comparing their observed 13C-NMR spectra with 13C chemical shifts calculated via the conformationally sensitive γ-gauche effect method (Also see ref. (25)). Similar success using substituent effects to assign 13C-NMR spectra of polymers in solution utilizing, in particular the conformationally averaged γ-gauche shielding effects, were achieved for additional polymers (26–28). However, in the case of poly (methyl methacrylate)s (PMMAs) we observed the “Failure of the γ-Gauche Effect Method To Predict the Stereosequence-Dependent 13C NMR Spectrum of the Disubstituted Vinyl Polymer Atactic Poly(methy1 methacrylate)” (29), even though PMMA is similar in structure to PVAc. Since 169 Cheng et al.; Stereochemistry and Global Connectivity: The Legacy of Ernest L. Eliel Volume 2 ACS Symposium Series; American Chemical Society: Washington, DC, 2017.

this study a revised RIS conformational model has been developed for PMMA (30). For the first time, the conformations (χ) of the side chains and their effect on backbone conformations were considered. This revised conformational model may eventually lead to improvement between the observed 13C-NMR spectra of PMMAs and the 13C chemical shifts estimated with γ-gauche shielding effects conformationally averaged using it.


Polydiacetylenes Because their solid-state polymerization from crystallized monomers can produce macroscopic single crystals of polymer, polydiacetylenes (PDAs) are an unusual class of polymers. 1,6-addition polymerization results in the conjugated backbones shown in Figure 7a. Along the backbone, extensive delocalization of π-electrons reduces absorption of light in the visible range and also gives PDAs interesting optical properties (31–34), which can be influenced by mechanical or thermal stresses through alteration of the amount of π-electron delocalization. As a result, PDAs are well known for their characteristic chromism.

Figure 7. (a) Schematic representation of the solid-state synthesis of polydiacetylenes. (b) Chemical structure of poly(4BCMU).



Most PDAs are insoluble, but several form solutions in common organic solvents. One such soluble PDA was first synthesized in 1978 (35, 36) from 5,7-dodecadiyne-1,12-diol bis[ (( butoxy-carbony1)methyl) urethane] and is commonly called poly(4BCMU) (See Figure 7b). Poly(4BCMU)’s long side chains apparently contribute enough conformational entropy to make it soluble in solvents like chloroform, nitromethane, and toluene (31). Poly(4BCMU) crystals and solution-cast films undergo chromic transitions, as well its solutions in organic solvents. The absorption spectrum and color of the as-polymerized polymer change from blue to red at ca. 110° C, and beyond 120° C its red crystals melt to a yellow isotropic liquid. When poly(4BCMU) is dissolved in a thermodynamically good solvent, like chloroform, it forms a yellow solution, but its solution becomes red upon addition of a nonsolvent like hexane (35). However, molecular mechanisms responsible for the chromism observed in poly(4BCMU) are not well understood (37–41). In an attempt to determine the mechanism(s) for its thermo- and solvato-chromism, we undertook a 13C-NMR examination of poly(4BCMU) (42). Tables 1 and 2 present the 13C-NMR resonance frequecies observed for poly(4BCMU) in its yellow chloroform solution and red toluene gel. Based on their γ-gauche shielding analyses we were able to conclude that similar mechan-isms are operating for the solid-state and solution chromism observed for poly(4BCMU). Its backbone is transformed to an increasingly nonplanar conformation, with a greater degree of π-electron localization, leading to a color change from blue to red in the solid as the temperature is raised and from red to yellow in solution when a good solvent is changed to a poor one.

Table 1. Chemical Shifts of Poly(4BCMU) in CDCl3a



Table 2. Chemical Shifts of Poly(4BCMU) in Toluene-d8a

In addition to changes in the backbone conformation with temperature and/or solvent, it was concluded that the first 3 CH2–CH2 side chain bonds separating the α, β, γ, and δ methylene groups (See Figure 7b) are g+, t, and g- in the blue and red low temperature solid and poor solution states, respectively, and transform to the extended ttt conformation at high temperature in the solid, or in solution with a poor solvent. Thermal energy drives the transition in the solid, while in solution the solvent quality produces the transition.

Solid State Polymer Conformations Here we will present some examples of analyzing the conformations of solid polymers via comparison of their high resolution solid-state 13C-NMR spectra with the 13C resonance frequencies obtained from γ-gauche shieldings expected from their rigid conformations. It has been observed many times in the high resolution solid-state 13C-CPMAS-DD-NMR spectra (43) of polymer samples that their resonance frequencies are generally a consequence of and dominated by their rigid chain conformations and are not very sensitive to their inter-chain packing. Poly(phenylene sulphide) and Its Solid Model Compounds Poly(p-phenylene sulphide) (PPS) is a high-performance polymer often used as a matrix for fiber-filled composites. Based on X-ray diffraction observed from stretched fibers, Tabor et al. (44) reported a crystal structure for PPS, which was more recently confirmed by electron diffraction from single PPS crystals obtained from solution and thin molten films (45). The PPS crystalline conform-ation is illustrated in Figure 8, where sulphur atoms are in the all-trans arrangement and located in the same plane, while successive phenyl rings are inclined at +45° with respect to the backbone plane. 172 Cheng et al.; Stereochemistry and Global Connectivity: The Legacy of Ernest L. Eliel Volume 2 ACS Symposium Series; American Chemical Society: Washington, DC, 2017.


Figure 8. Schematic drawing of the crystalline conformation of PPS.

The structurally related polymers, poly(p-phenylene oxide) (PPO) and poly(2,6-dimethyl-1,4-phenylene oxide) (PDMPO), adopt the same crystalline conformation (46, 47). Schaefer and Stejskal (43) noted that two resonances were observed for the protonated carbons (P) in the the high resolution 13C-CPMAS-DD-NMR spectrum of solid PDMPO. They suggested that their nonlinear C-O-C bonds produced non-equivalent magnetic environments for the protonated carbons, which become equivalent only when the phenyl rings rotate rapidly or are fixed at 90° out of the plane of the oxygen atoms. Doubling of P resonances in PDMPO crystals results because neither occurs. Because PPS adopts the same crystalline conformation as PDMPO, we expected, but in fact did not observe (48, 49) a doubling of P resonances in PPS (See Figure 9). The CP resonances corresponding to θ1 = 45° and θ2 = 135 ° in crystalline PPS likely remain unresolved due to the ~2 ppm line width observed in the 13C-CPMAS /DP-NMR spectrum in Figure 9, because Shaefer and Stejskal (43) were able to resolve the splitting of CP resonances in solid PDMPO, which assumes the same crystalline conformation as PPS. This implies that the short C-O bonds (~1.4Å) in PDMPO lead to a greater conformational sensitivity of 13C chemical shifts than do the longer C-S bonds (~1.8 Å) in PPS. 173 Cheng et al.; Stereochemistry and Global Connectivity: The Legacy of Ernest L. Eliel Volume 2 ACS Symposium Series; American Chemical Society: Washington, DC, 2017.


Figure 9. (a) CPMAS/DD 13C n.m.r, spectrum of PPS recorded at room temperature, with CQ downfield and CP upfield (b) Same as (a) except with a short 100 μs delay without spin-locking in the 1H channel after the Hartmann-Hahn match (49), causing the absence of the CP resonance.

The cyclic pentameter of PPS [c-(PS)5] crystallizes in a conformation (See Figure 10b) with the relative orientations of all five phenyl ring pairs bonded to common sulphur atoms significantly different from each other and also different from the orientation of phenyl rings in crystalline PPS (50–52) . Table 3 presents the dihedral angles θPQ between P and Q phenyl ring carbons connected on either side of the same S. Figure 11 presents The 13C-CPMAS/DD-NMR spectra recorded at room temperature for c-(PS)5 without and with dipolar dephasing. Both carbon types show at least six distinct resonances with overall spreads of 8 and 18 ppm for the Q and P resonances. The dihedral angles between CP and CQ carbons observed in the crystal structure of c(PS)5 (See Figure 10b) span the full range θPQ = 0 to 180 °, from the cis (0°) to trans(180°) arrangements of CP and CQ across the S atom. If different arrangements of phenyl rings about the C-S bonds was the principal source of the multiple resonances observed for the Cp and CQ carbons in crystalline c-(PS)5, then the different shielding between a cis (θPQ = 0 °) and a trans (θPQ = 180 °) arrangement of Cp and CQ carbons must be 8 and 18 ppm, respectively. Relative to their trans arrangement, a cis arrangement of the methyl and protonated ring carbons in solid di- and tri-methoxy benzenes produces (53–56) a 6 ppm 174 Cheng et al.; Stereochemistry and Global Connectivity: The Legacy of Ernest L. Eliel Volume 2 ACS Symposium Series; American Chemical Society: Washington, DC, 2017.


shielding, strongly suggesting that the 18 ppm spread in observed CP chemical shifts observed for c-(PS)5 cannot be solely a consequence of its crystalline conformation. It was hoped that observation of the13C-CPMAS/DD-NMR spectra of diphenyl sulphide (DPS) shown in Figure 10c and their comparison to the spectra of PPS and c-(PS)5 would suggest a possible crystalline conformation for DPS, which is presently unknown. In Figure 12 the liquid state 13C-NMR spectrum of DPS recorded at room temperature is displayed, while in Figure 13 the solid-state 13C-NMR spectra observed for DPS at -60°C are presented. To produce a quantitative spectrum, the spectrum in Figure 13a was recorded without CP and with a 420 s delay between decoupling pulses. The CPMAS/DD spectrum of solid DPS in Figure 13b was recorded with dipolar dephasing, and only shows CQ resonances.

Figure 10. a. Projection along the CQ–S bond in the crystallineconformation of PPS. b. Crystalline conformation of c-(PS)5. c. Crystalline conformation of DPS suggested by solid-state 13C-NMR. 175 Cheng et al.; Stereochemistry and Global Connectivity: The Legacy of Ernest L. Eliel Volume 2 ACS Symposium Series; American Chemical Society: Washington, DC, 2017.


Table 3. Dihedral angles (θPQ) between the P and Q ring carbons in crystalline c-(PS)5 (52)

Figure 11. (a) 13C-CPMAS/DD-NMR spectrum recorded for c-(PS)5 at room temperature. (b) CPMASS/DD ~3C n.m.r, spectrum recorded at room temperature with a 100 μs delay (without spinlocking) in the 1H channel after the Hartmann-Hahn match (49).



Figure 12.

13C-MAS-NMRspectrum

of liquid DPS recorded at room temperature.

Because the crystalline conformation of DPS is not expected to significant-ly influence their chemical shifts the m and p resonances are singlets. The o-CP carbon, on the other hand, shows a resonance doublet centred at 135.5 ppm with a 2:1 ratio of intensities, and a singlet resonance ~6ppm upfield at 129.9 ppm corresponding in intensity to a single o-CP carbon. The crystalline conformation of DPS, which has yet to be determined by X-ray diffraction, may be estimated from the pattern of o-CP resonances. A DPS conformation consistent with the observed pattern of CQ and o-CP resonances is drawn in Figure 8c, where one phenyl ring is nearly coplanar with CQ-S-CQ (~φ1 =0°), while the other phenyl ring is rotated (φ2 = 30 - 40°) out of this plane. The dihedral angles between o-CP and CQ are θ1 = 0°, θ2 = 180° and θ1 = 30-40°, θ2 = 140-150°, respectively, for these two distinct phenyl ring orientations. The least shielded resonances at 136.1 ppm would correspond to the 2 o-CP carbons with θ2 = 140-150 and 180°. The resonance at 135.1 ppm would correspond to the single o-CP carbon with θ1 = 30-40°, and the most shielded resonance at 129.9 ppm could be assigned to the o-CP carbon in the cis arrangement (θ1 =0 °) with CQ. Also consistent with θ1 = 30-40 and 0°, as proposed here for crystalline DPS, is the resonance doublet for the CQ carbons and their ~4 ppm separation.



Figure 13. (a) 13C-MAS-NMR spectrum of DPS recorded at - 60°C with a 420 s delay between decoupling pulses. (b) 13C-CPMAS-NMR spectrum recorded at -60° C with a100 μs delay (without spin-locking) in the 1H channel after the Hartmann-Hahn match (49). (c) Their difference spectrum showing only CP carbon resonances.

X-ray diffraction studies have been reported for bis(4-mercaptophenyl) sulphide (57) and 1,4-bis(phenylthio) benzene (58), which assume asymmetric conformations in their crystals. The asymmetric crystalline conformations of these mono- and disulphides do not differ appreciably from the asymmetric conformer proposed for crystalline DPS (See Figure 8c) based on solid-state 13C-NMR results. “State-of-the-art” quantum mechanical methods of that time were applied to estimate the 13C-NMR chemical shifts for crystalline c-(PS)5 (59). The calculated spectra were obtained by assuming all resonance peaks obtained from the calculated ab initio nuclear shieldings had Lorentzian line shapes with widths at half-height of 1 ppm. Note in Figure 14 that both the total spread and distribution of resonance peaks in the observed and calculated spectra of c-(PS)5 are reasonably to some-what similar. However, to achieve even the limited favorable comparison shown in Figure 14, the observed spectrum had to be uniformly shifted 10 ppm upfield.This example once again illustrates the present limitations of theoretically estimating the NMR resonance frequencies of molecules as a function of their molecular structures and conformations. 178 Cheng et al.; Stereochemistry and Global Connectivity: The Legacy of Ernest L. Eliel Volume 2 ACS Symposium Series; American Chemical Society: Washington, DC, 2017.


Figure 14. Comparison of observed (Exp) and calculated 13C NMR spectra of crystalline c-(PS)5 (59). Syndiotacic-Polystyrene and Its Solid Model Compounds Ishihara et al. (60) reported the synthesis of highly stereoregular, crystalline, syndiotactic polystyrene (s-PS), which Zambelli et a1. (61) confirmed. X-ray (60, 62) and electron diffraction (63) performed on oriented fibers and films indicated a chain axis repeat distance of 5.1Å consistent with s-PS chains adopting an alltrans,planar zigzag conformation (form I crystals). When cast from dilute chloroform or 1,2-dichlorobenzene solutions (64, 65) or when melt-quenched films or fibers were swollen in chloroform, dichloromethane, 1,2-dibromo- or dichloroethane, or cyclohexane, a different crystalline polymorph (form II) was found (62, 66). A fiber repeat of 7.5Å was 179 Cheng et al.; Stereochemistry and Global Connectivity: The Legacy of Ernest L. Eliel Volume 2 ACS Symposium Series; American Chemical Society: Washington, DC, 2017.


obtained by X-ray diffraction patterns observed (62) for oriented, swollen form II fibers, and is consistent with a ... ttggttgg ... chain conformation observed previously for syndiotactic polypropylene (s-PP) (67). Figure 15 presents the high resolution solid-state CPMAS/DD 13C NMR spectra of forms I and I1 s-PS. A single CH2 carbon resonance at 48.4 ppm is observed in the form I spectrum, while two CH2 resonances at 49.1 and 38.1 ppm are seen in the form I1 spectrum. These 13C chemical shifts are consistent with those expected (1) from γ-gauche shielding effects if the chains adopt the planar zigzag, ... tttt.. . and 21 helical, ... ttggttgg ... conformations, respectively. In the ... ttggttgg ... conformation half of the CH2 carbons are gauche to two γ substituents (CHs),while the remaining half are trans to both γ-CH substituents.

Figure 15. CPMAS/DD 13C NMR spectra of form I (a) and form I1 (b) s-PS. The form I1 sample of (b) was obtained by absorption of dichloro-methane into an amorphous, melt-quenched film of s-PS.



As was observed (68) in the CPMAS/DD 13C NMR spectrum of s-PP, we expected and see (69–72) two CH2 resonances separated by two γ-effects, or ca. (2)(-5 ppm) = -10 ppm. The separation observed in Figure 15 is 11 ppm. In form I crystals, the s-PS chains assume the all trans .... tttt ... conformation, so all methylene carbons are in the trans arrangement with their γ substituents (CH’s). As expected we do see a single CH2 at nearly the same chemical shift as the most downfield of the two CH2 resonances observed in the spectrum of the form II polymorph. The application of high resolution 13C-NMR (CPMASS/DD) to crystalline s-PS has clearly resulted in confirming the X-ray derived chain conformations adopted in the form 1 and 11 crystalline polymorphs.

Poly(tri-methylene terephthalate) and Solid Model Compounds

As we have attempted to indicate here, it has been repeatedly demonstrated (1) that the resonance frequencies observed for carbon nuclei in the high resolution solid-state 13C NMR spectra of organic polymers depend principally upon and can be analyzed by means of the conformationally sensitive γ-gauche effect (1). This whether polymers are constrained in their crystals to adopt single rigid conformations or are molten, mobile, and free to interconvert rapidly on the megahertz time scale between conformations (73). Though I hesitate to end this review chapter on a negative note, the failure to confirm the crystalline chain conformation of poly (trimethylene terephthalate) (PTT) by the γ-gauche analysis of its high resolution13C-NMR spectra should not be glossed over (74). In Figure 16 the crystalline conformations and resonance frequencies (in ppm vs TMS) of the central methylene carbons in the butylene glycol fragments of poly(butylene terephthalate) (PBT), along with those of several of its model compounds, are presented (75, 76). Because the model compounds were single crystal samples, their crystalline conformations are firmly established (76). Consistent with the γ-gauche effect, their observed 13C resonance frequencies have shown that the central methylene carbons in the butylene glycol fragment that are gauche to their γ-substituent ester oxygens are shielded (Figure 16) and resonate 3-4 ppm upfield from those that are in a trans arrangement (75). Since here want to compare the high-resolution solid-state13C NMR spectra and conformations of the closely related aromatic polyester PTT, whose structure is shown in Figure 17, the comparison to crystalline PBT is very relevant.



Figure 16. Conformations and 13C NMR resonances observed for the central methylene carbons in crystals of PBT and its model compounds (75, 76).

Figure 17. PTT repeat unit. The propylene glycol fragments in crystalline PTT are believed to adopt alternating tg±g±t (... tg+g+ttg-g-t ...) conformations, where all –O—CH2–and –CH2—O– bonds are trans (t) and –CH2—CH2– bonds are gauche (g±) (77). In the proposed crystalline conformation of PTT, the central methylene carbons in the propylene glycol fragments would be trans (φ1,4 = t) to both of their γ-substituent carbonyl carbons. In the absence of the requisite gauche shielding arrangement, the central methylene carbons would not be expected to be shielded by their neighboring carbonyl carbons, as seen below.

In solution, in the melt, and in the amorphous solid regions of PTT, on the other hand, the –O—CH2– and –CH2—O– bonds in PTT would adopt both trans and gauche conformations (φ1,4 = t and g±). There the central methylene 182 Cheng et al.; Stereochemistry and Global Connectivity: The Legacy of Ernest L. Eliel Volume 2 ACS Symposium Series; American Chemical Society: Washington, DC, 2017.


carbons (CH2)o would be expected to experience significant shielding from their immediately adjacent C=O’s [~5 ppm for each γ-(C=O) in a gauche arrangement] causing them to resonate upfield from their crystalline counterparts. In the high resolution solid-state 13C-NMR spectra of amorphous and semicrystalline PTTs (See Figure 18) the central CH2 carbons (CH2)o, to the contrary, show resonances at δ(13C) = 28.8(amorphous) and 26.1 ppm (crystalline). In other words, the (CH2)o methylene carbons in the crystalline regions of PTT are shielded in comparison to those in the amorphous regions and so resonate upfield. This in direct opposition to expectation based on the crystalline conformation proposed for PTT, with φ1,4 = t –O—CH2– and –CH2—O–bonds and therefore no γ-(C=O)s would be in a gauche shielding arrangement.

Figure 18. 13C-CP/MAS-NMRspectra of PTT films: (A) amorphous, (B) film (A) annealed at 160°C for 30 min, and (C) film (B) drawn to a DR= 3. 183 Cheng et al.; Stereochemistry and Global Connectivity: The Legacy of Ernest L. Eliel Volume 2 ACS Symposium Series; American Chemical Society: Washington, DC, 2017.


Single crystal X-ray diffraction of the PTT model compound trimethylene glycol dibenzoate (TMGDB) has revealed a crystalline conformation (78) essentially identical to that proposed for crystalline PTT. Though the spectra are not presented here, we have observed (74) the following resonance frequencies for the central methylene carbons (CH2)o inTMGDB: 30.0 ppm (melt, 60° C), 28.7 ppm (solution), 27.4 ppm (crystal).

In agreement with PTT, but again unexpectedly, the central methylene carbons in crystalline TMGDB resonate substantially (3 ppm) upfield from those in its melt or solution, as do the chemical shifts observed for the central methylene carbons in crystalline and amorphous regions of PTT. These solid-state 13C-NMR observations lead to the following conclusions: 1. 2.

As previously concluded by X-ray (77, 78), PTT and TMGDB have closely similar crystalline conformations. Both of their central methylene (CH2)o δ(13C) resonances seem not to be influenced by the nuclear shielding usually produced by gauche conformational arrangements between carbon nuclei and their γ-carbon substituents.

Conclusion 2. appears valid despite the fact that γ-gauche shielding successfully explains the 13C resonances observed in the crystals of the closely related aromatic polyester PBT and its model compounds (75, 76), in addition to those of many other crystalline polymers (1). Figure 19 illustrates the crystalline conformation of PTT. Unlike the related terephthalate polyesters PBT and poly (ethylene terephthalate), the PTT chain is not fully extended. Instead the PTT trimethylene glycol segments gently serpentine back and forth about the line connecting the centers of their phenyl rings. This prompted an investigation (74) of how ring currents generated by the π-electrons of the phenyl rings in PTT might possibly affect the resonance frequencies of their “central” (CH2)o carbons (79–81). Molecular modeling was used to evaluate the distances between the central methylene carbons and the centers of the phenyl rings in plane (ρ) and perpen-dicular to the plane (z) of the phenyl rings in their crystalline unit cells. These distances were obtained for all phenyl rings from the x, y, z coordinates obtained from the crystalline unit cells of PTT and TMGDB (77, 78). We repeated the distance calculations for a dynamic TMGDB molecule in vacuum (74) to account for its conformational flexibility in the melt or solution by obtaining conformationally weighted average distances. Because shielding from ring-currents vanish for ρ > 4Å or z > 3Å, Table 4 makes clear that all of the central methylenes (CH2)o in both PTT and TMGDB are too far removed from their nearest phenyl rings to be significantly shielded by their ring-currents. 184 Cheng et al.; Stereochemistry and Global Connectivity: The Legacy of Ernest L. Eliel Volume 2 ACS Symposium Series; American Chemical Society: Washington, DC, 2017.


Figure 19. PTT crystalline conformation (77).

The potential effects of the magnetic susceptibility anisotropy of neighboring carbonyl groups might have on the nuclear shielding of the central methylene carbons were also investigated using a similar calculation. In this case, (CH2)o in-plane and perpendicular to the plane distances of the C=O group were similarly determined for both rigid crystalline and flexible isolated TMGDB. 185 Cheng et al.; Stereochemistry and Global Connectivity: The Legacy of Ernest L. Eliel Volume 2 ACS Symposium Series; American Chemical Society: Washington, DC, 2017.


Table 4. Distances (Å) of “Central” CH2s to the Center of (ρ) and above or below (z) the Phenyl Rings in PTT and TMGDBa

The affects of neighboring carbonyl group anisotropies were estimated for TMGDB according to a previously implemented procedure (82), that adopts a point-dipole approximation (83, 84) for magnetically anisotropic groups, such as the C=O group. The known susceptibility value of the formaldehyde carbonyl group (85) was adopted. These calculations performed on both the rigid crystalline and conformationally averaged of the TMGDB model compound resulted in a maximum possible neighboring C=O group shielding contribution of 2.2 ppm. Remember that the central methylene carbons in TMGDB resonate at 30.0 ppm in the melt, at 28.7 ppm in solution), and at 27.4 ppm in the crystal. In other words, the flexible TMGDB resonances come 1.3 - 2.4 ppm downfield from rigid crystall-ine TMGDB, while analyses of γ-gauche conformational shielding suggests they be more shielded than crystalline TMGDB by 3 - 4 ppm. Our estimate of a maximum ~ 2.2 ppm δ(rigid crystal) - δ(flexible liquid) shielding of 186 Cheng et al.; Stereochemistry and Global Connectivity: The Legacy of Ernest L. Eliel Volume 2 ACS Symposium Series; American Chemical Society: Washington, DC, 2017.


“central” methylene carbons produced by the anisotropic magnetic susceptibility of neighboring C=O groups in TMGDB is far short of that necessary to overcome the larger 5 - 7 ppm downfield shift expected from the apparent lack of γ-gauche conformational shielding in the TMGDB crystal. Since PTT and TMGDB have closely similar crystalline conformations (77, 78), with all of their (CH2)o methylene carbons and their γ-substituent carbonyl carbons in a trans conformational arrangement, the resonances observed in their melts and solutions were expected to come several ppm upfield from the their crystalline resonances, and not several ppm downfield. This means that the discrepancies between observed resonance frequencies and those anticipated from consideration of conformationally sensitive γ-gauche effects are very large, in the range 5 - 7 ppm. Accounting for the potential effects of phenyl ring currents and anisotropic carbonyl group magnetic susceptibilities did not significantly reduce this large discrepancy, and so its cause currently remains perplexing.

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