Kristopher J. Harris

Metal–organic frameworks with dynamic interlocked components

The dynamics of mechanically interlocked molecules such as rotaxanes and catenanes have been studied in solution as examples of rudimentary molecular switches and machines, but in this medium, the molecules are randomly dispersed and their motion incoherent. As a strategy for achieving a higher level of molecular organization, we have constructed a metal-organic framework material using a [2]rotaxane as the organic linker and binuclear Cu(II) units as the nodes. Activation of the as-synthesized material creates a void space inside the rigid framework that allows the soft macrocyclic ring of the [2]rotaxane to rotate rapidly, unimpeded by neighbouring molecular components. Variable-temperature (13)C and (2)H solid-state NMR experiments are used to characterize the nature and rate of the dynamic processes occurring inside this unique material. These results provide a blueprint for the future creation of solid-state molecular switches and molecular machines based on mechanically interlocked molecules.

Probing the structural origins of vapochromism of a triarylboron-functionalized platinum(II) acetylide by optical and multinuclear solid-state NMR spectroscopy.

A vapoluminescent triarylboron-functionalized platinum(II) complex that displays a mechanism of vapochromism differing from all previously reported platinum(II) compounds has been synthesized. The luminescence color of 1 switches in response to many volatile organic compounds in the solid state, including hexanes, CH(2)Cl(2), benzene, and methanol. While vapochromism due to changes in Pt-Pt or π-π stacking interactions has been commonly observed, absorption and luminescence studies and single-crystal and powder X-ray diffraction data as well as multinuclear solid-state NMR experiments ((195)Pt, (13)C, (11)B, (2)H, and (1)H) revealed that the vapochromic response of 1 is instead due to changes in the excited-state energy levels resulting from local interactions of solvent molecules with the complex. Furthermore, these interactions result in inversion of the lowest-energy excited states of the complex in some cases, the first observation of this phenomenon in the solid state.

Interaction tensors and local dynamics in common structural motifs of nitrogen: a solid-state 14N NMR and DFT study.

(14)N solid-state NMR powder patterns have been obtained at high field (21.1 T) using broadband, frequency-swept pulses and a piecewise acquisition method. This approach allowed the electric field gradient (EFG) tensor parameters to be obtained from model organic and inorganic systems featuring spherically asymmetric nitrogen environments (C(Q) values of up to ca. 4 MHz). The advantages and limitations of this experimental approach are discussed, and the observation of (14)N T(2) relaxation anisotropy in certain systems is also reported, which can shed light on dynamic processes, allowing motional geometries and jump rates to be probed. In particular, we show that observable effects of dynamics on (14)N spectra can be mediated by modulation of either the EFG tensor or heteronuclear dipolar couplings. It is demonstrated that the QCPMG protocol can be used to selectively enhance certain types of nitrogen environments on the basis of differences in T(2). We also present the results of extensive density functional theory calculations on these systems, which show remarkably good correlation with the experimental results and allow the prediction of tensor orientations, assignment of parameters to crystallographic sites, and a rationalization of the origin of the EFG tensors in terms of contributions from individual molecular orbitals. This work demonstrates that ultra-wideline (14)N solid-state NMR can, under favorable circumstances, be a straightforward, useful, and informative probe of molecular structure and dynamics.

Broadband adiabatic inversion pulses for cross polarization in wideline solid-state NMR spectroscopy.

Efficient acquisition of ultra-wideline solid-state NMR powder patterns is a continuing challenge. In particular, when the breadth of the powder pattern is much larger than the cross-polarization (CP) excitation bandwidth, transfer efficiencies suffer and experimental times are greatly increased. Presented herein is a CP pulse sequence with an excitation bandwidth that is up to ten times greater than that available from a conventional spin-locked CP pulse sequence. The pulse sequence, broadband adiabatic inversion CP (BRAIN-CP), makes use of the broad, uniformly large frequency profiles of chirped inversion pulses, to provide these same characteristics to the polarization transfer process. A detailed theoretical analysis is given, providing insight into the polarization transfer process involved in BRAIN-CP. Experiments on spin-1/2 nuclei including (119)Sn, (199)Hg and (195)Pt nuclei are presented, and the large bandwidth improvements possible with BRAIN-CP are demonstrated. Furthermore, it is shown that BRAIN-CP can be combined with broadband frequency-swept versions of the Carr-Purcell-Meiboom-Gill experiment (for instance with WURST-CPMG, or WCPMG for brevity); the combined BRAIN-CP/WCPMG experiment then provides multiplicative signal enhancements of both CP and multiple-echo acquisition over a broad frequency region.

Mechanically Interlocked Linkers inside Metal-Organic Frameworks: Effect of Ring Size on Rotational Dynamics.

A series of metal-organic framework (MOF) materials has been prepared, each containing a mechanically interlocked molecule (MIM) as the linker and a copper(II) paddlewheel as the secondary building unit (SBU). The MIM linkers are [2]rotaxanes with varying sizes of crown ether macrocycles ([22]crown-6, 22C6; [24]crown-6, 24C6; [26]crown-6, 26C6; benzo[24]crown-6, B24C6) and an anilinium-based axle containing four carboxylate donor groups. Herein, the X-ray structures of MOFs UWCM-1 (no crown) and UWDM-1(22) are compared and demonstrate the effect of including a macrocycle around the axle of the linker. The rotaxane linkers are linear and result in nbo-type MOFs with void space that allows for motion of the interlocked macrocycle inside the MOF pores, while the macrocycle-free linker is bent and yields a MOF with a novel 12-connected bcc structure. Variable temperature (2)H solid-state nuclear magnetic resonance showed that the macrocycles in UWDM-1(22), UWDM-1(24), and UWDM-1(B24) undergo different degrees and rates of rotation depending on the size and shape of the macrocycle.

Rapid Acquisition of 14N Solid-State NMR Spectra with Broadband Cross Polarization

Nitrogen is an element of utmost importance in chemistry, biology and materials science. Of its two NMR-active isotopes, (14)N and (15)N, solid-state NMR (SSNMR) experiments are rarely conducted upon the former, due to its low gyromagnetic ratio (γ) and broad powder patterns arising from first-order quadrupolar interactions. In this work, we propose a methodology for the rapid acquisition of high quality (14)N SSNMR spectra that is easy to implement, and can be used for a variety of nitrogen-containing systems. We demonstrate that it is possible to dramatically enhance (14)N NMR signals in spectra of stationary, polycrystalline samples (i.e., amino acids and active pharmaceutical ingredients) by means of broadband cross polarization (CP) from abundant nuclei (e.g., (1)H). The BRoadband Adiabatic INversion Cross-Polarization (BRAIN-CP) pulse sequence is combined with other elements for efficient acquisition of ultra-wideline SSNMR spectra, including Wideband Uniform-Rate Smooth-Truncation (WURST) pulses for broadband refocusing, Carr-Purcell Meiboom-Gill (CPMG) echo trains for T2-driven S/N enhancement, and frequency-stepped acquisitions. The feasibility of utilizing the BRAIN-CP/WURST-CPMG sequence is tested for (14)N, with special consideration given to (i) spin-locking integer spin nuclei and maintaining adiabatic polarization transfer, and (ii) the effects of broadband polarization transfer on the overlapping satellite transition patterns. The BRAIN-CP experiments are shown to provide increases in signal-to-noise ranging from four to ten times and reductions of experimental times from one to two orders of magnitude compared to analogous experiments where (14)N nuclei are directly excited. Furthermore, patterns acquired with this method are generally more uniform than those acquired with direct excitation methods. We also discuss the proposed method and its potential for probing a variety of chemically distinct nitrogen environments.

Detection of Electrochemical Reaction Products from the Sodium–Oxygen Cell with Solid-State 23Na NMR Spectroscopy

23Na MAS NMR spectra of sodium-oxygen (Na-O2) cathodes reveals a combination of degradation species: newly observed sodium fluoride (NaF) and the expected sodium carbonate (Na2CO3), as well as the desired reaction product sodium peroxide (Na2O2). The initial reaction product, sodium superoxide (NaO2), is not present in a measurable quantity in the 23Na NMR spectra of the cycled electrodes. The reactivity of solid NaO2 is probed further, and NaF is found to be formed through a reaction between the electrochemically generated NaO2 and the electrode binder, polyvinylidene fluoride (PVDF). The instability of cell components in the presence of desired electrochemical reaction products is clearly problematic and bears further investigation.

Ultra-wideline 14N solid-state NMR as a method for differentiating polymorphs: glycine as a case study

Nitrogen-14 solid-state NMR (SSNMR) is utilized to differentiate three polymorphic forms and a hydrochloride (HCl) salt of the amino acid glycine. Frequency-swept Wideband, Uniform Rate, Smooth Truncated (WURST) pulses were used in conjunction with Carr-Purcell Meiboom-Gill refocusing, in the form of the WURST-CPMG pulse sequence, for all spectral acquisitions. The 14N quadrupolar interaction is shown to be very sensitive to variations in the local electric field gradients (EFGs) about the 14N nucleus; hence, differentiation of the samples is accomplished through determination of the quadrupolar parameters CQ and ηQ, which are obtained from analytical simulations of the 14N SSNMR powder patterns of stationary samples (i.e., static NMR spectra). Additionally, differentiation of the polymorphs is also possible via the measurement of 14N effective transverse relaxation time constants, Teff2(14N). Plane-wave density functional theory (DFT) calculations, which exploit the periodicity of crystal lattices, are utilized to confirm the experimentally determined quadrupolar parameters as well as to determine the orientation of the 14N EFG tensors in the molecular frames. Several signal-enhancement techniques are also discussed to help improve the sensitivity of the 14N SSNMR acquisition method, including the use of selective deuteration, the application of the BRoadband Adiabatic INversion Cross-Polarization (BRAIN-CP) technique, and the use of variable-temperature (VT) experiments. Finally, we examine several cases where 14N VT experiments employing Carr-Purcell-Meiboom-Gill (CPMG) refocusing are used to approximate the rotational energy barriers for RNH3+ groups.

Spatially resolved surface valence gradient and structural transformation of lithium transition metal oxides in lithium-ion batteries

Layered lithium transition metal oxides are one of the most important types of cathode materials in lithium-ion batteries (LIBs) that possess high capacity and relatively low cost. Nevertheless, these layered cathode materials suffer structural changes during electrochemical cycling that could adversely affect the battery performance. Clear explanations of the cathode degradation process and its initiation, however, are still under debate and not yet fully understood. We herein systematically investigate the chemical evolution and structural transformation of the LiNixMnyCo1-x-yO2 (NMC) cathode material in order to understand the battery performance deterioration driven by the cathode degradation upon cycling. Using high-resolution electron energy loss spectroscopy (HR-EELS) we clarify the role of transition metals in the charge compensation mechanism, particularly the controversial Ni2+ (active) and Co3+ (stable) ions, at different states-of-charge (SOC) under 4.6 V operation voltage. The cathode evolution is studied in detail from the first-charge to long-term cycling using complementary diagnostic tools. With the bulk sensitive 7Li nuclear magnetic resonance (NMR) measurements, we show that the local ordering of transition metal and Li layers (R3[combining macron]m structure) is well retained in the bulk material upon cycling. In complement to the bulk measurements, we locally probe the valence state distribution of cations and the surface structure of NMC particles using EELS and scanning transmission electron microscopy (STEM). The results reveal that the surface evolution of NMC is initiated in the first-charging step with a surface reduction layer formed at the particle surface. The NMC surface undergoes phase transformation from the layered structure to a poor electronic and ionic conducting transition-metal oxide rock-salt phase (R3[combining macron]m → Fm3[combining macron]m), accompanied by irreversible lithium and oxygen loss. In addition to the electrochemical cycling effect, electrolyte exposure also shows non-negligible influence on cathode surface degradation. These chemical and structural changes of the NMC cathode could contribute to the first-cycle coulombic inefficiency, restrict the charge transfer characteristics and ultimately impact the cell capacity.

(14) N solid-state NMR spectroscopy of amino acids

14 N ultra-wideline solid-state NMR (SSNMR) spectra were obtained for 16 naturally occurring amino acids and four related derivatives by using the WURST-CPMG (wideband, uniform rate, and smooth truncation Carr-Purcell-Meiboom-Gill) pulse sequence and frequency-stepped techniques. The 14 N quadrupolar parameters were measured for the sp3 nitrogen moieties (quadrupolar coupling constant, CQ , values ranged from 0.8 to 1.5 MHz). With the aid of plane-wave DFT calculations of the 14 N electric-field gradient tensor parameters and orientations, the moieties were grouped into three categories according to the values of the quadrupolar asymmetry parameter, ηQ : low (≤0.3), intermediate (0.31-0.7), and high (≥0.71). For RNH3+ moieties, greater variation in N-H bond lengths was observed for systems with intermediate ηQ values than for those with low ηQ values (this variation arose from different intermolecular hydrogen-bonding arrangements). Strategies for increasing the efficiency of 14 N SSNMR spectroscopy experiments were discussed, including the use of sample deuteration, high-power 1 H decoupling, processing strategies, high magnetic fields, and broadband cross-polarization (BRAIN-CP). The temperature-dependent rotations of the NH3 groups and their influence on 14 N transverse relaxation rates were examined. Finally, 14 N SSNMR spectroscopy was used to differentiate two polymorphs of l-histidine through their quadrupolar parameters and transverse relaxation time constants. The strategies outlined herein permitted the rapid acquisition of directly detected 14 N SSNMR spectra that to date was not matched by other proposed methods.