NMR Crystallography of Campho[2,3-c]pyrazole (<i>Z</i>′ = 6): Combining High-Resolution ¹H-¹³C Solid-State MAS NMR Spectroscopy and GIPAW Chemical-Shift Calculations

1H-13C two-dimensional magic-angle spinning (MAS) solid-state NMR correlation spectra, recorded with the MAS-J-HMQC experiment, are presented for campho[2,3-c]pyrazole. For each 13C moiety, there are six resonances associated with the six distinct molecules in the asymmetric unit cell (Z′ = 6). The one-bond C−H correlations observed in the 2D 1H-13C MAS-J-HMQC spectra allow the experimental determination of the 1H and 13C chemical shifts associated with the separate CH, CH2, and CH3 groups. 1H and 13C chemical shifts calculated by using the GIPAW (Gauge Including Projector Augmented Waves) plane-wave pseudopotential approach are presented. Calculations for the whole unit cell (12 × 29 = 348 atoms, with geometry optimization of all atoms) allow the assignment of the experimental 1H and 13C chemical shifts to the six distinct molecules. The calculated chemical shifts for the full crystal structure are compared with those for isolated molecules as extracted from the geometry-optimized crystal structure. In this way, the effect of intermolecular interactions on the observed chemical shifts is quantified. In particular, the calculations are sufficiently precise to differentiate the small (1H chemical shifts of the six resonances associated with each distinct CH or CH2 moiety

Self-Assembled Oligoanilinic Nanosheets: Molecular Structure Revealed by Solid-State NMR Spectroscopy

The products obtained during the early stages of the oxidative polymerization of aniline in the “falling pH” reaction were investigated using multinuclear solid-state magic-angle spinning (MAS) NMR combined with first-principles NMR chemical shielding calculations using the GIPAW (gauge-including projector augmented wave) method. A sample was synthesized starting from a 50:50 mixture of U–¹³C aniline and ¹⁵N-labeled aniline; two-dimensional ¹³C refocused INADEQUATE, ¹⁵N–¹³C double CP, ¹⁵N PDSD, and ¹H–¹³C/¹⁵N refocused INEPT MAS NMR spectra revealed the presence of quinoneimine structural units. Structural models that are consistent with the connectivities revealed by the ¹³C refocused INADEQUATE and ¹⁵N–¹³C Double CP spectra are proposed. GIPAW chemical shift calculations are performed for model structures based on the proposed oligomeric structures; noting that the model structures do not take into account intermolecular hydrogen bonding and CH−π interactions, agreement and discrepancy to experiment are discussed

NMR Crystallography of Campho[2,3-c]pyrazole (<i>Z</i>′ = 6): Combining High-Resolution ¹H-¹³C Solid-State MAS NMR Spectroscopy and GIPAW Chemical-Shift Calculations

1H-13C two-dimensional magic-angle spinning (MAS) solid-state NMR correlation spectra, recorded with the MAS-J-HMQC experiment, are presented for campho[2,3-c]pyrazole. For each 13C moiety, there are six resonances associated with the six distinct molecules in the asymmetric unit cell (Z′ = 6). The one-bond C−H correlations observed in the 2D 1H-13C MAS-J-HMQC spectra allow the experimental determination of the 1H and 13C chemical shifts associated with the separate CH, CH2, and CH3 groups. 1H and 13C chemical shifts calculated by using the GIPAW (Gauge Including Projector Augmented Waves) plane-wave pseudopotential approach are presented. Calculations for the whole unit cell (12 × 29 = 348 atoms, with geometry optimization of all atoms) allow the assignment of the experimental 1H and 13C chemical shifts to the six distinct molecules. The calculated chemical shifts for the full crystal structure are compared with those for isolated molecules as extracted from the geometry-optimized crystal structure. In this way, the effect of intermolecular interactions on the observed chemical shifts is quantified. In particular, the calculations are sufficiently precise to differentiate the small (1H chemical shifts of the six resonances associated with each distinct CH or CH2 moiety

Role of Aniline Oligomeric Nanosheets in the Formation of Polyaniline Nanotubes

We report the phenomenon of nanosheet rolling during typical aqueous polymerization and study its implications for the self-assembly of polyaniline nanotubes. Specifically, this is done through a detailed morphological and structural characterization of products obtained after 20 min, 1 h in falling pH experiments, and after 20 h at constant pH 2.5 during the oxidative polymerization of aniline with ammonium persulfate in the presence of alanine. The chemical structure has been investigated by FTIR, UV−vis, solid-state 13C and 15N NMR, liquid NMR, and XRD, whereas the morphology was imaged using scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The presence of self-assembled nanoflakes with different thicknesses ranging from tens to hundreds of nanometers is confirmed through SEM. TEM revealed the presence of very thin layers: nanosheets with sharp and well-defined edges. The presence of hydrogen bonds is confirmed by FTIR and is consistent with XRD results. The stacking of nanosheets and the formation of thicker flakes based on π−π electron interactions have been proposed on the basis of XRD experiments, where self-assembled layers made of cross-linked oxidized aniline structures stack on each other and are stabilized by hydrogen bonds and π−π interactions. In this way, hydrophobic cross-linked oligomers (formed at the beginning of the synthesis at higher pH) minimize their surface energy, self-assembling into well-ordered macromolecular structures. On the basis of the SEM and TEM images and the information obtained from other analytical techniques applied here, the presence of PANI nanotubes formed in the reaction carried out at constant pH of 2.5 is confirmed. The role of the nanosheets in the formation of nanotubes is proposed

Identifying Guanosine Self Assembly at Natural Isotopic Abundance by High-Resolution ¹H and ¹³C Solid-State NMR Spectroscopy

By means of the ¹H chemical shifts and the proton–proton proximities as identified in ¹H double-quantum (DQ) combined rotation and multiple-pulse spectroscopy (CRAMPS) solid-state NMR correlation spectra, ribbon-like and quartet-like self-assembly can be identified for guanosine derivatives without isotopic labeling for which it was not possible to obtain single crystals suitable for diffraction. Specifically, characteristic spectral fingerprints are observed for dG(C10)₂ and dG(C3)₂ derivatives, for which quartet-like and ribbon-like self-assembly has been unambiguously identified by ¹⁵N refocused INADEQUATE spectra in a previous study of ¹⁵N-labeled derivatives (Pham, T. N.; et al. <i>J. Am. Chem. Soc.</i> <b>2005</b>, <i>127</i>, 16018). The NH ¹H chemical shift is observed to be higher (13–15 ppm) for ribbon-like self-assembly as compared to 10–11 ppm for a quartet-like arrangement, corresponding to a change from NH···N to NH···O intermolecular hydrogen bonding. The order of the two NH₂ ¹H chemical shifts is also inverted, with the NH₂ proton closest in space to the NH proton having a higher or lower ¹H chemical shift than that of the other NH₂ proton for ribbon-like as opposed to quartet-like self-assembly. For the dG(C3)₂ derivative for which a single-crystal diffraction structure is available, the distinct resonances and DQ peaks are assigned by means of gauge-including projector-augmented wave (GIPAW) chemical shift calculations. In addition, ¹⁴N–¹H correlation spectra obtained at 850 MHz under fast (60 kHz) magic-angle spinning (MAS) confirm the assignment of the NH and NH₂ chemical shifts for the dG(C3)₂ derivative and allow longer range through-space N···H proximities to be identified, notably to the N7 nitrogens on the opposite hydrogen-bonding face

Identifying Guanosine Self Assembly at Natural Isotopic Abundance by High-Resolution ¹H and ¹³C Solid-State NMR Spectroscopy

By means of the ¹H chemical shifts and the proton–proton proximities as identified in ¹H double-quantum (DQ) combined rotation and multiple-pulse spectroscopy (CRAMPS) solid-state NMR correlation spectra, ribbon-like and quartet-like self-assembly can be identified for guanosine derivatives without isotopic labeling for which it was not possible to obtain single crystals suitable for diffraction. Specifically, characteristic spectral fingerprints are observed for dG(C10)₂ and dG(C3)₂ derivatives, for which quartet-like and ribbon-like self-assembly has been unambiguously identified by ¹⁵N refocused INADEQUATE spectra in a previous study of ¹⁵N-labeled derivatives (Pham, T. N.; et al. <i>J. Am. Chem. Soc.</i> <b>2005</b>, <i>127</i>, 16018). The NH ¹H chemical shift is observed to be higher (13–15 ppm) for ribbon-like self-assembly as compared to 10–11 ppm for a quartet-like arrangement, corresponding to a change from NH···N to NH···O intermolecular hydrogen bonding. The order of the two NH₂ ¹H chemical shifts is also inverted, with the NH₂ proton closest in space to the NH proton having a higher or lower ¹H chemical shift than that of the other NH₂ proton for ribbon-like as opposed to quartet-like self-assembly. For the dG(C3)₂ derivative for which a single-crystal diffraction structure is available, the distinct resonances and DQ peaks are assigned by means of gauge-including projector-augmented wave (GIPAW) chemical shift calculations. In addition, ¹⁴N–¹H correlation spectra obtained at 850 MHz under fast (60 kHz) magic-angle spinning (MAS) confirm the assignment of the NH and NH₂ chemical shifts for the dG(C3)₂ derivative and allow longer range through-space N···H proximities to be identified, notably to the N7 nitrogens on the opposite hydrogen-bonding face

Probing Heteronuclear ¹⁵N−¹⁷O and ¹³C−¹⁷O Connectivities and Proximities by Solid-State NMR Spectroscopy

Heteronuclear solid-state magic-angle spinning (MAS) NMR experiments for probing 15N−17O dipolar and J couplings are presented for [2H(NH3),1-13C,15N,17O2]glycine·2HCl and [15N2,17O2]uracil. Two-dimensional 15N−17O correlation spectra are obtained using the R3-HMQC experiment; for glycine·2HCl, the intensity of the resolved peaks for the CO and C−O2H 17O resonances corresponds to the relative magnitude of the respective 15N−17O dipolar couplings. 17O−15N REDOR curves are presented for glycine·2HCl; fits of the initial buildup (ΔS/S 15N−17O REAPDOR curves for the 15N resonances in glycine·2HCl and uracil fit well to the universal curve presented by Goldbourt et al. (J. Am. Chem. Soc. 2003, 125, 11194). Heteronuclear 13C−17O and 15N−17O J couplings were experimentally determined from fits of the quotient of the integrated intensity obtained in a heteronuclear and a homonuclear spin−echo experiment, SQ(τ) = SHET(τ)/SHOM(τ). For glycine·2HCl, 1JCO was determined as 24.7 ± 0.2 and 25.3 ± 0.3 Hz for the CO and C−O2H resonances, respectively, while for uracil, the average of the two NH···O hydrogen-bond-mediated J couplings was determined as 5.1 ± 0.6 Hz. In addition, two-bond intramolecular J couplings, 2JOO = 8.8 ± 0.9 Hz and 2JN1,N3 = 2.7 ± 0.1 Hz, were determined for glycine·2HCl and uracil, respectively. Excellent agreement was found with J couplings calculated using the CASTEP code using geometrically optimized crystal structures for glycine·HCl [1JCO(CO) = 24.9 Hz, 1JCO(COH) = 27.5 Hz, 2JOO = 7.9 Hz] and uracil [2hJN1,O4 = 6.1 Hz, 2hJN3,O4 = 4.6 Hz, 2JN1,N3 = 2.7 Hz]

Probing Heteronuclear ¹⁵N−¹⁷O and ¹³C−¹⁷O Connectivities and Proximities by Solid-State NMR Spectroscopy

Heteronuclear solid-state magic-angle spinning (MAS) NMR experiments for probing 15N−17O dipolar and J couplings are presented for [2H(NH3),1-13C,15N,17O2]glycine·2HCl and [15N2,17O2]uracil. Two-dimensional 15N−17O correlation spectra are obtained using the R3-HMQC experiment; for glycine·2HCl, the intensity of the resolved peaks for the CO and C−O2H 17O resonances corresponds to the relative magnitude of the respective 15N−17O dipolar couplings. 17O−15N REDOR curves are presented for glycine·2HCl; fits of the initial buildup (ΔS/S 15N−17O REAPDOR curves for the 15N resonances in glycine·2HCl and uracil fit well to the universal curve presented by Goldbourt et al. (J. Am. Chem. Soc. 2003, 125, 11194). Heteronuclear 13C−17O and 15N−17O J couplings were experimentally determined from fits of the quotient of the integrated intensity obtained in a heteronuclear and a homonuclear spin−echo experiment, SQ(τ) = SHET(τ)/SHOM(τ). For glycine·2HCl, 1JCO was determined as 24.7 ± 0.2 and 25.3 ± 0.3 Hz for the CO and C−O2H resonances, respectively, while for uracil, the average of the two NH···O hydrogen-bond-mediated J couplings was determined as 5.1 ± 0.6 Hz. In addition, two-bond intramolecular J couplings, 2JOO = 8.8 ± 0.9 Hz and 2JN1,N3 = 2.7 ± 0.1 Hz, were determined for glycine·2HCl and uracil, respectively. Excellent agreement was found with J couplings calculated using the CASTEP code using geometrically optimized crystal structures for glycine·HCl [1JCO(CO) = 24.9 Hz, 1JCO(COH) = 27.5 Hz, 2JOO = 7.9 Hz] and uracil [2hJN1,O4 = 6.1 Hz, 2hJN3,O4 = 4.6 Hz, 2JN1,N3 = 2.7 Hz]

Quantifying Weak Hydrogen Bonding in Uracil and 4-Cyano-4‘-ethynylbiphenyl: A Combined Computational and Experimental Investigation of NMR Chemical Shifts in the Solid State

Weak hydrogen bonding in uracil and 4-cyano-4‘-ethynylbiphenyl, for which single-crystal diffraction structures reveal close CH···OC and C⋮CH···N⋮C distances, is investigated in a study that combines the experimental determination of 1H, 13C, and 15N chemical shifts by magic-angle spinning (MAS) solid-state NMR with first-principles calculations using plane-wave basis sets. An optimized synthetic route, including the isolation and characterization of intermediates, to 4-cyano-4‘-ethynylbiphenyl at natural abundance and with 13C⋮13CH and 15N⋮C labeling is described. The difference in chemical shifts calculated, on the one hand, for the full crystal structure and, on the other hand, for an isolated molecule depends on both intermolecular hydrogen bonding interactions and aromatic ring current effects. In this study, the two effects are separated computationally by, first, determining the difference in chemical shift between that calculated for a plane (uracil) or an isolated chain (4-cyano-4‘-ethynylbiphenyl) and that calculated for an isolated molecule and by, second, calculating intraplane or intrachain nucleus-independent chemical shifts that quantify the ring current effects caused by neighboring molecules. For uracil, isolated molecule to plane changes in the 1H chemical shift of 2.0 and 2.2 ppm are determined for the CH protons involved in CH···O weak hydrogen bonding; this compares to changes of 5.1 and 5.4 ppm for the NH protons involved in conventional NH···O hydrogen bonding. A comparison of CH bond lengths for geometrically relaxed uracil molecules in the crystal structure and for geometrically relaxed isolated molecules reveals differences of no more than 0.002 Å, which corresponds to changes in the calculated 1H chemical shifts of at most 0.1 ppm. For the C⋮CH···N⋮C weak hydrogen bonds in 4-cyano-4‘-ethynylbiphenyl, the calculated molecule to chain changes are of similar magnitude but opposite sign for the donor 13C and acceptor 15N nuclei. In uracil and 4-cyano-4‘-ethynylbiphenyl, the CH hydrogen-bonding donors are sp2 and sp hybridized, respectively; a comparison of the calculated changes in 1H chemical shift with those for the sp3 hybridized CH donors in maltose (Yates et al. J. Am. Chem. Soc. 2005, 127, 10216) reveals no marked dependence on hybridization for weak hydrogen-bonding strength

Quantifying Weak Hydrogen Bonding in Uracil and 4-Cyano-4‘-ethynylbiphenyl: A Combined Computational and Experimental Investigation of NMR Chemical Shifts in the Solid State

Weak hydrogen bonding in uracil and 4-cyano-4‘-ethynylbiphenyl, for which single-crystal diffraction structures reveal close CH···OC and C⋮CH···N⋮C distances, is investigated in a study that combines the experimental determination of 1H, 13C, and 15N chemical shifts by magic-angle spinning (MAS) solid-state NMR with first-principles calculations using plane-wave basis sets. An optimized synthetic route, including the isolation and characterization of intermediates, to 4-cyano-4‘-ethynylbiphenyl at natural abundance and with 13C⋮13CH and 15N⋮C labeling is described. The difference in chemical shifts calculated, on the one hand, for the full crystal structure and, on the other hand, for an isolated molecule depends on both intermolecular hydrogen bonding interactions and aromatic ring current effects. In this study, the two effects are separated computationally by, first, determining the difference in chemical shift between that calculated for a plane (uracil) or an isolated chain (4-cyano-4‘-ethynylbiphenyl) and that calculated for an isolated molecule and by, second, calculating intraplane or intrachain nucleus-independent chemical shifts that quantify the ring current effects caused by neighboring molecules. For uracil, isolated molecule to plane changes in the 1H chemical shift of 2.0 and 2.2 ppm are determined for the CH protons involved in CH···O weak hydrogen bonding; this compares to changes of 5.1 and 5.4 ppm for the NH protons involved in conventional NH···O hydrogen bonding. A comparison of CH bond lengths for geometrically relaxed uracil molecules in the crystal structure and for geometrically relaxed isolated molecules reveals differences of no more than 0.002 Å, which corresponds to changes in the calculated 1H chemical shifts of at most 0.1 ppm. For the C⋮CH···N⋮C weak hydrogen bonds in 4-cyano-4‘-ethynylbiphenyl, the calculated molecule to chain changes are of similar magnitude but opposite sign for the donor 13C and acceptor 15N nuclei. In uracil and 4-cyano-4‘-ethynylbiphenyl, the CH hydrogen-bonding donors are sp2 and sp hybridized, respectively; a comparison of the calculated changes in 1H chemical shift with those for the sp3 hybridized CH donors in maltose (Yates et al. J. Am. Chem. Soc. 2005, 127, 10216) reveals no marked dependence on hybridization for weak hydrogen-bonding strength