A solid-state NMR study of C70: A model molecule for amorphous carbons (original) (raw)
Related papers
A J Ambrozio, 2020
The 13 C NMR chemical shifts corresponding to different sites in 6 atomistic models of disordered carbons were computed at different H contents by 7 employing DFT calculations. Structural models were generated by molecular dynamics 8 simulations and validated by the pair distribution functions; further bonding analyses 9 were carried out to determine the amount of sp 3 and sp 2 carbons in the structures. 10 Specifically, the obtained results allow the distinction of the chemical shifts associated 11 with different types of carbon sites, with different hybridization states and bonded or 12 not to a hydrogen atom. The calculated NMR spectra show excellent agreement with 13 experimental data and are thus useful to identify local structural features of disordered 14 carbons. 15 ■ INTRODUCTION 16 Carbon materials have been extensively investigated in the past 17 years because of their unique properties at different allotropies, 18 for instance, graphite-like materialsgraphene, nanographites, 19 pyrocarbons, and disordered carbonscarbon blacks, amor-20 phous carbons, activated carbons, diamond-like materials, 21 fullerenes, carbon nanotubes, kerogens, and others. In many 22 of these studies, different structural models have been 23 employed for the description of the physical properties of 24 carbon materials. 1 Generally, the proposed models are 25 validated by confronting the predicted properties against 26 experimental results derived from diffraction techniques (e.g., 27 X-ray and neutron diffraction) as well as spectroscopic (e.g., 28 Raman and nuclear magnetic resonance spectroscopy) or 29 microscopic (e.g., transmission electron microscopy) meth-30 ods. 2−4 There is a wide range of computational techniques for 31 generating satisfactory structural models for carbon materials, 32 which are mainly based on molecular dynamics (MD), Monte 33 Carlo, or a combination of both. 5,6 In particular, MD has 34 proven to be a useful tool for generating appropriate models of 35 disordered carbon materials. 2−4,7 36 Among the important properties of carbon materials, the 37 nuclear magnetic resonance (NMR) chemical shielding is 38 particularly useful due its sensitivity to the local chemical 39 environment around the probe nuclei. Moreover, the 40 components of the NMR chemical shielding tensor can be 41 obtained from atomistic models and confronted directly with 42 experimental data; consequently, measurements and calcu-43 lations of the components of the chemical shielding tensor are 44 of high interest for both crystalline and disordered 45 materials. 8−10 Especially, the use of NMR to probe the local 46 bonding structure can be an adequate complement to 47 diffraction techniques (e.g, X-ray, electron or neutron 48 diffraction), which are more suited to probe the average 49 local environments or the medium-to long-range order in the 50 material. Recent theoretical reports have used first-principles 51 calculations based on the density functional theory (DFT) to 52 establish correlations between the shielding tensor and 53 structural features of carbon nanotubes, 11 graphite oxide, 12 54 graphene, and graphitic materials. 13,14 55 Amorphous hydrogenated carbons have been extensively 56 studied in the past decades due to their interesting mechanical 57 and tribological properties, which have promising applications. 58 These properties are dependent on chemical and structural 59 features such as the H content in the material and the amount 60 of atoms with sp and sp 3 hybridization. 15−17 Solid-state 13 C 61 NMR methods have been widely used in studies of amorphous 62 carbons, especially regarding the quantitative evaluation of the 63 sp 2 /sp 3 ratio. In fact, this ratio is a particularly important 64 property that can be easily obtained from 13 C NMR spectra, 65 considering the clear distinction in the chemical shift ranges 66 corresponding to sp 2 and sp 3 carbons. 18 As an example, Pan et 67 al. 19 estimated that just ca. 1.5% of the carbon content in an 68 amorphous carbon film could be detected in 13 C NMR spectra 69 obtained with 1 H-13 C cross-polarization (CP), 18 concluding
NMR investigation on molecular dynamics of C60 and C70 solids
Synthetic Metals, 1993
13C-NMR was employed to study C60 molecular dynamics in pure C60 crystals prepared by sublimation technique and by evaporation of CS2-solution of C60. The C6o molecular dynamics in the CS2-grown crystal with orthorhombic (OR) structure was significantly changed by heat-treatments. This behavior was assigned to a structural conversion of OR to FCC (face centered cubic) by heat-treatment and also to annealing effect of defects in the crystal. C70 molecular dynamics was also studied in pure C70 crystal. The C70 molecular rotation was found to be more restricted than those of C6O molecule in the crystals. It was suggested that an anisotropic molecular rotation is dominant below ~280K in the time scale of NMR line width (-20~ec).
Physical Chemistry Chemical Physics, 2013
The nuclear magnetic resonance (NMR) spectroscopy combined with theoretical calculations is an important tool for fullerene identification. However, the accuracy of available theoretical methods is often not adequate. Therefore, in this work, different computational aspects needed to simulate realistically chemical shifts in the C 70 molecule are investigated by density functional theory (DFT) calculations. The importance of the functional choice, basis set, solvent, and molecular motions was assessed. The solvent was simulated using the implicit conductor-like polarized continuum model. The molecular motions were included via anharmonic corrections and averaging of snapshots obtained from classical and first-principles molecular dynamics (MD) simulations. Comparison to experiment revealed that density functional calculations typically overestimate the 13 C NMR chemical shifts. Hybrid functionals, such as BHandH and BHandHLYP, and long-range corrected functionals, such as wB97xd and CAM-B3LYP, give the best results. While the solvent has a minor effect (chemical shift changes by B1 ppm), the vibrational and dynamical effects are surprisingly large, causing changes up to 9 ppm. Consideration of the latter was also necessary to explain the observed temperature dependence. While the dynamical corrections for MD performed in vacuum were overestimated, inclusion of the solvent in simulations provided more realistic results. The study thus points out the importance of an appropriate solvent model and a complex approach to the modelling, balancing the static, dynamic and environmental factors.
Solid State Communications, 2000
In this work, we present 13 C MAS NMR results of the rhombohedral (rh-2D) and tetragonal (tet-2D) two-dimensional polymerised C 60 , under high pressure, using the paramagnetic O 2 as a chemical shift and relaxation agent. The 13 C MAS NMR spectrum of the rh-2D polymer (under P O 2 2400 Torr) shows two resonances, a broad and intense line around 152.8 and a small one at 78.3 ppm. The spectrum of the tetragonal tet-2D polymer (under P O 2 2500 Torr), shows two isotropic lines around 150.6 and 77.9 ppm. For both polymers, the intense line around 150 ppm has several components, related to the deformation of the spherical shape of the C 60 molecules. We attribute the observed group of lines to the inequivalent sp 2 carbons on the C 60 molecules, and the small line around 78 ppm to the intermolecular sp 3 bonding carbons.
Characterization of amorphous carbon films by 13C CP MAS NMR spectroscopy
Journal of Polymer Science Part B: Polymer Physics, 1993
I3C CP MAS NMR spectroscopy was used to characterize a-C:H materials generated from methane and hydrogen mixtures using a microwave plasma. Dipolar dephasing experiments indicate a range of Td making quantification of quaternary "diamond"-like carbons difficult. Unconstrained lineshape analysis is not suitable for the deconvolution of the NMR spectra, but linewidth constrained analyses gave reasonable results.
Solid state NMR study of carbon bonding in amorphous hydrogenated carbon films
Applied Magnetic Resonance, 1991
Carbon bonding environments in hydrogenated amorphous carbon films (a-C:H) deposited from ah rf-biased methane plasma onto various substrates have been quantified by application of solid state 13C NMR. A family of films were prepared by systematically varying the substrate bias voltage. Quantitative data on carbon chemistry in these films is required for modeling the impact of structure on mechanical and optical properties. A va¡ of NMR acquisition pulse sequences have been investigated to determine the conditions under which quantitative 13C NMR data can be acquired in this system. The results indicate that data acquisition from this material requires different protocols than for the study of polymeric hydrocarbon films. With proper experimental design, NMR is ah excellent technique for structural studies of these materials.
Physical Chemistry Chemical Physics, 2013
The nuclear magnetic resonance (NMR) spectroscopy combined with theoretical calculations is an important tool for fullerene identification. However, the accuracy of available theoretical methods is often not adequate. Therefore, in this work, different computational aspects needed to simulate realistically chemical shifts in the C 70 molecule are investigated by density functional theory (DFT) calculations. The importance of the functional choice, basis set, solvent, and molecular motions was assessed. The solvent was simulated using the implicit conductor-like polarized continuum model. The molecular motions were included via anharmonic corrections and averaging of snapshots obtained from classical and first-principles molecular dynamics (MD) simulations. Comparison to experiment revealed that density functional calculations typically overestimate the 13 C NMR chemical shifts. Hybrid functionals, such as BHandH and BHandHLYP, and long-range corrected functionals, such as wB97xd and CAM-B3LYP, give the best results. While the solvent has a minor effect (chemical shift changes by B1 ppm), the vibrational and dynamical effects are surprisingly large, causing changes up to 9 ppm. Consideration of the latter was also necessary to explain the observed temperature dependence. While the dynamical corrections for MD performed in vacuum were overestimated, inclusion of the solvent in simulations provided more realistic results. The study thus points out the importance of an appropriate solvent model and a complex approach to the modelling, balancing the static, dynamic and environmental factors.
Non-empirical calculations of NMR indirect carbon-carbon coupling constants: 1. Three-membered rings
Magnetic Resonance in Chemistry, 2002
Carbon-carbon and carbon-hydrogen spin-spin coupling constants were calculated in the series of the first six monocycloalkanes using SOPPA and SOPPA(CCSD) methods, and very good agreement with the available experimental data was achieved, with the latter method showing slightly better results in most cases, at least in those involving calculations of J(C,C). Benchmark calculations of all possible 21 coupling constants J(C,C), J(C,H) and J(H,H) in chair cyclohexane revealed the importance of using the appropriate level of theory and adequate quality of the basis sets. Many unknown couplings in this series were predicted with high confidence and several interesting structural trends (hybridization effects, multipath coupling transmission mechanisms, hyperconjugative interactions) were elucidated and are discussed based on the present calculations of spin-spin couplings.
Canadian Journal of Chemistry, 1995
The carbon chemical shift tensors of the carbonyl and thiocarbonyl groups of acetamide, thioacetamide, thioacetanilide, 4'-methoxyacetanilide, and 4'-methoxythioacetanilide have been experimentally determined using dipolarchemical shift solid-state 13c NMR spectroscopy. The magnitudes of the three principal components of the carbon chemical shift tensors are found to exhibit marked variations between the carbonyl and thiocarbonyl functionalities. However, in contrast to the conclusions of an earlier comparative investigation involving benzophenone and thiobenzophenone, the orientations of the principal axis systems of these chemical shift tensors are found to be similar. These experimental results represent the first complete characterizations of the carbon chemical shift tensor in organic thiocarbonyls. The results of our ab initio GIAO and LORG calculations of carbon chemical shielding tensors in formaldehyde, thiofomaldehyde, formamide, and thioformamide as well as in acetamide and thioacetamide are in qualitative agreement with experiment. The findings of the present investigation provide conclusive evidence that the well-known isotropic deshielding of the carbon nucleus in the C=S group relative to C=O is primarily attributable to the decreased energy associated with the o tt T* excitation within the thiocarbonyl fragment. This result is in contrast with the conventional interpretation that the deshielding originates from a red shift in the C=S HOMO-LUMO n + T* transition.