In-plane and Out-of-plane Motion of Benzene Trapped in a Cd(py)2{Ag(CN)2}2 Host as Studied by Deuterium NMR

2H-NMR powder patterns of the deuterated benzene molecule trapped in a Cd(py)2{Ag(CN)2}2 host were measured in the temperature range of 123-403 K in order to obtain information about motional behavior of the guest benzene molecule undergoing an in-plane motion combined with an out-of-plane motion. The activation energy for the in-plane motion, which is a 60° jump among neighboring sites on the benzene ring, was derived to be 18.2(5) kJ mol-1 from the line-shape simulation of the 2H-NMR powder patterns. The jump frequency of the out-of-plane motion, which is a reorientation of the benzene ring between two sites separated with a 57.8° azimuthal angle, was in a fast motion region even at 123 K and the activation energy of the two-site jump is very low compared with that of the in-plane motion.     

Dynamics of chloromethanes in cryptophane-E inclusion complexes: a 2H solid-state NMR and X-ray diffraction study

In this paper, we present a variable temperature (2)H solid-state NMR investigation of cryptophane-E:chloroform and cryptophane-E:dichloromethane inclusion complexes. The (2)H line shapes and nuclear spin relaxation rates were analyzed in terms of the distribution of C-D bond orientations and the time scale of the guest dynamics. It was found that encaged chloroform produces broad (2)H spectra, and that its reorientation is relatively slow with a correlation time of approximately 0.17 mus at 292 K. In contrast, the (2)H line shapes of encaged dichloromethane are narrow and the motion of this guest molecule is fast with a correlation time of approximately 1.4 ps at 283 K. The (2)H NMR data were complemented by an X-ray diffraction study of the cryptophane-E:dichloromethane structure, which was utilized in the analysis of the NMR parameters.     

Motion of aromatic hydrocarbons in the microporous aluminum methylphosphonates AlMePO-alpha and AlMePO-beta

(2)H wide-line NMR has been used, in conjunction with molecular dynamics simulations where appropriate, to follow the reorientation of the monoaromatic compounds benzene, toluene, and p-xylene within the one-dimensional channels of the alpha- and beta-polymorphs of aluminum methylphosphonate, Al(2)(CH(3)PO(3))(3). Variable-temperature, static, (2)H NMR spectra of adsorbed d(6)-benzene, d(3)-, d(5)-, and d(8)-toluenes, and d(3),d(3)-p-xylene were matched by line shape simulation. The motion of p-xylene in both polymorphs is approximated by the long axis of the molecule describing a cone within the channels, the half-angle of which is greater for the slightly wider channels in AlMePO-beta (27-30 degrees cf. 18-19 degrees). The (2)H NMR of d(3)-toluene is simulated using a similar model, whereas the signal from aromatic deuterons in d(5)- and d(8)-toluenes is simulated by a ring undergoing 2pi/3 flips around the para axis. The reorientation of benzene shows the largest differences between the two pore structures. In AlMePO-beta it tumbles with little restriction, although at low temperatures the spectral details are better matched by allowing the molecule to spend a greater proportion of its time closer to the wall. In AlMePO-alpha the much broader line shape arises from constrained motion within the strongly triangular channels. Molecular dynamics simulations of benzene in the two structures confirm the differences. They support a model for benzene in AlMePO-alpha where its motion is restricted to rotations about its 6-fold axis and 2pi/3 jumps between symmetry-related sites in the pores, so that the plane of the aromatic ring remains approximately parallel to the c-axis.     

Dynamics of p-nitroaniline molecules in micoporous aluminophosphate AlPO4-5 studied by solid-state NMR

Dynamics of deuterated p-nitroaniline (pNA-d) molecules in the micropores of AlPO4-5 has been investigated by means of solid-state NMR. The adsorbed amounts of pNA-d were 5.0 and 10.1 mass % of the total mass. We have measured 13C magic-angle-spinning (MAS) and 2H NMR spectra of the guest molecules and 31P and 27Al MAS NMR spectra of the host framework. The pNA-d molecules distribute rather inhomogeneously in the channel, and do not coordinate to Al strongly like H2O. The intermolecular hydrogen bonds are formed between a part of the guest molecules only when the loading level is high. The 2H NMR spectra are successfully analyzed, elucidating the orientation and the motion of the guest molecules. The molecular axis of pNA-d is inclined to the channel axis, and the molecular plane is perpendicular to the inner wall. The guest molecule jumps among 12 sites or 12 orientations. This motion is faster in the sample of 5.0 mass % than in the sample of 10.1 mass %, suggesting that the guest-guest interaction hinders the motion. The mean residence times of the molecules are estimated from the analysis of the 2H NMR spectra, which are affected by the size of the nanospace as well as the property of the adsorbed site.     

Thermal history effects and methyl tunneling dynamics in a supramolecular complex of calixarene and para-xylene

The low-temperature structure and dynamics of guest molecules of p-xylene incorporated in the isopropyl-calix[4] arene(2:1) p-xylene complex have been investigated by solid state nuclear magnetic resonance (NMR). Using one-dimensional 1H-decoupled 13C cross-polarization magic-angle-spinning (MAS) NMR and two-dimensional 1H-13C correlation spectroscopy, a full assignment of the 13C and 1H chemical shifts has been made. Using 1H NMR relaxometry, the effects of thermal history on the structure of the system have been investigated. Rapidly cooled samples have 1H spin-lattice relaxation times T1, which at low temperature (T<60 K) are typically two orders of magnitude faster than those observed in annealed samples which have been cooled slowly over many hours. In both forms, the low-temperature relaxation is driven by the dynamics of the weakly hindered methyl rotors of the p-xylene guest. The substantial difference in T1 is attributed in the rapidly cooled sample to disorder in the structure of the complex leading to a wide distribution of correlation times and methyl barrier heights. A comparison of the linewidths and splittings in the high resolution 13C MAS spectra of the two forms provides structural insight into the nature of the disorder. Using 1H field-cycling NMR relaxometry, the methyl dynamics of the p-xylene guest in the annealed sample have been fully characterized. The B-field dependence of the 1H T1 maps out the spectral density from which the correlation times are directly measured. The methyl barrier heights are determined from an analysis of the temperature dependence.     

The dynamical properties of the aromatic hydrogen bond in NH4(C6H5)4B from quasielastic neutron scattering

NH(4)(C(6)H(5))(4)B represents a prototypical system for understanding aromatic H bonds. In NH(4)(C(6)H(5))(4)B an ammonium cation is trapped in an aromatic cage of four phenyl rings and each phenyl ring serves as a hydrogen bond acceptor for the ammonium ion as donor. Here the dynamical properties of the aromatic hydrogen bond in NH(4)(C(6)H(5))(4)B were studied by quasielastic incoherent neutron scattering in a broad temperature range (20< or =T< or =350 K). We show that in the temperature range from 67 to 350 K the ammonium ions perform rotational jumps around C(3) axes. The correlation time for this motion is the lifetime of the "transient" H bonds. It varies from 1.5 ps at T=350 K to 150 ps at T=67 K. The activation energy was found to be 3.14 kJ mol, which means only 1.05 kJ mol per single H bond for reorientations around the C(3) symmetry axis of the ammonium group. This result shows that the ammonium ions have to overcome an exceptionally low barrier to rotate and thereby break their H bonds. In addition, at temperatures above 200 K local diffusive reorientational motions of the phenyl rings, probably caused by interaction with ammonium-group reorientations, were found within the experimental observation time window. At room temperature a reorientation angle of 8.4 degrees +/-2 degrees and a correlation time of 22+/-8 ps were determined for the latter. The aromatic H bonds are extremely short lived due to the low potential barriers allowing for molecular motions with a reorientational character of the donors. The alternating rupture and formation of H bonds causes very strong damping of the librational motion of the acceptors, making the transient H bond appear rather flexible.     

Multifunctionality and crystal dynamics of a highly stable, porous metal-organic framework [Zn4O(NTB)2

A porous metal-organic framework [Zn(4)O(NTB)(2)].3DEF.EtOH (1), in which (3,6)-connected nets are doubly interpenetrated to generate curved three-dimensional channels, has been prepared. Framework 1 exhibits high permanent porosity (Langmuir surface area, 1121 m(2)/g; pore volume, 0.51 cm(3)/cm(3)), high thermal stability (up to 430 degrees C), high hydrogen adsorption capacity (1.9 wt % at 77 K and 1 atm), selective organic guest binding ability (K(f)()( )(): MeOH > pyridine > benzene > dodecane), and guest-dependent blue luminescence (lambda(max) depending on guest identity). Most interestingly, the framework sustains single crystallinity even at 400 degrees C and 10(-)(5) Torr, and the framework components undergo reversible dynamics, mainly rotational motion, in response to removal and rebinding of the guest molecules.     

A 13C NMR study of the molecular dynamics and phase transition of confined benzene inside titanate nanotubes

This work investigated the nanoconfinement effect on the molecular dynamics and phase transition of confined benzene inside titanate nanotubes with a uniform inner diameter of approximately 5.3 nm. For 13C-enriched organics, the 13C nuclear spin-spin relaxation was demonstrated as a sensitive tool to differentiate molecular translational motion and reorientation and, thus, was shown to be advantageous over the commonly employed 1H and 2H NMR for studying complex phase diagram, specifically, for separating the phase behavior of translational motion and the phase behavior of molecular reorientation. In such an approach, the melting of translational motion of confined benzene was explicitly observed to take place in a broad temperature range below the bulk melting temperature. The abrupt change of the 13C nuclear spin-spin relaxation time of the confined liquid benzene at about 260 K suggested that nanoconfinement induced two topologically distinct liquid phases.     

A switchable molecular rotator: neutron spectroscopy study on a polymeric spin-crossover compound

A quasielastic neutron scattering and solid-state (2)H NMR spectroscopy study of the polymeric spin-crossover compound {Fe(pyrazine)[Pt(CN)(4)]} shows that the switching of the rotation of a molecular fragment--the pyrazine ligand--occurs in association with the change of spin state. The rotation switching was examined on a wide time scale (10(-13)-10(-3) s) by both techniques, which clearly demonstrated the combination between molecular rotation and spin-crossover transition under external stimuli (temperature and chemical). The pyrazine rings are seen to perform a 4-fold jump motion about the coordinating nitrogen axis in the high-spin state. In the low-spin state, however, the motion is suppressed, while when the system incorporates benzene guest molecules, the movements of the system are even more restricted.     

Mobility of solid tert-butyl alcohol studied by deuterium NMR

The molecular mobility of solid deuterated tert-butyl alcohol (TBA) has been studied over a broad temperature range (103–283 K) by means of solid-state 2H NMR spectroscopy, including both line shape and anisotropy of spin–lattice relaxation analyses. It has been found that, while the hydroxyl group of the TBA molecule is immobile on the 2H NMR time scale (τC > 10(–5) s), its butyl group is highly mobile. The mobility is represented by the rotation of the methyl [CD3] groups about their 3-fold axes (C3 rotational axis) and the rotation of the entire butyl [(CD3)3-C] fragment about its 3-fold axis (C3′ rotational axis). Numerical simulations of spectra line shapes reveal that the methyl groups and the butyl fragment exhibit three-site jump rotations about their symmetry axes C3 and C3′ in the temperature range of 103–133 K, with the activation energies and preexponential factors E1 = 21 ± 2 kJ/mol, k(01) = (2.6 ± 0.5) × 10(12) s(–1) and E2 = 16 ± 2 kJ/mol, k(02) = (1 ± 0.2) × 10(12) s(–1), respectively. Analysis of the anisotropy of spin–lattice relaxation has demonstrated that the reorientation mechanism of the butyl fragment changes to a free diffusion rotational mechanism above 173 K, while the rotational mechanism of the methyl groups remains the same. The values of the activation barriers for both rotations at T > 173 K have the values, which are similar to those at 103–133 K. This indicates that the interaction potential defining these motions remains unchanged. The obtained data demonstrate that the detailed analysis of both line shape and anisotropy of spin–lattice relaxation represents a powerful tool to follow the evolution of the molecular reorientation mechanisms in organic solids.     

Compensation of steric demand by cation-pi interactions,cobaltocenium cations as guests in tetraurea calix[4]arene dimers

The affinities of ferrocene (2) and the cobaltocenium cation (3+), which have roughly the same size and differ in their charge, towards the inner cavity of the dimeric capsule formed by tetraurea calix[4]arene (1) were studied in C2D4Cl2 solutions. While 3+, which occupies more than 75% of the internal volume of the dimer, is readily encapsulated this is not the case for 2. This is probably due to cation-pi interactions, which operate only between 3+ and the aromatic rings of the calix[4]arene dimer. We found that the affinity of the cobaltocenium cation is higher than that of the tropylium cation (4+) and is only 2-3 times less than that of the tetraethylammoniun cation (5+). From the variable temperature 1H NMR spectra of this capsule, the free energy of activation at 298 K (deltaGdouble dagger(298K)) for the reorientation of the hydrogen bonded belt between the two parts of the dimer could be determined by total line shape analysis for the aromatic protons of the calixarene. The value of 14.3 +/- 0.2 kcal mol(-1) for the dimeric capsules of 3+ PF6- is very similar to the free activation energy found for dimeric capsules of 1 with 4+ PF6- and 5+ PF6- in C2D4Cl2. It becomes significantly lower, if PF6- is replaced by BF4-. We also found that ten times more DMSO is needed to disrupt the capsule 1 x 3+ x 1 than the corresponding 1 x 1 dimer containing benzene as guest. This demonstrates again the importance of the cation-pi interactions for the stability of such hydrogen-bonded dimeric capsules.     

Molecular dynamics in paramagnetic materials as studied by magic-angle spinning 2H NMR spectra

A magic-angle spinning (MAS) 2H NMR experiment was applied to study the molecular motion in paramagnetic compounds. The temperature dependences of 2H MAS NMR spectra were measured for paramagnetic [M(H2O)6][SiF6] (M=Ni2+, Mn2+, Co2+) and diamagnetic [Zn(H2O)6][SiF6]. The paramagnetic compounds exhibited an asymmetric line shape in 2H MAS NMR spectra because of the electron-nuclear dipolar coupling. The drastic changes in the shape of spinning sideband patterns and in the line width of spinning sidebands due to the 180 degrees flip of water molecules and the reorientation of [M(H2O)6]2+ about its C3 axis were observed. In the paramagnetic compounds, paramagnetic spin-spin relaxation and anisotropic g-factor result in additional linebroadening of each of the spinning sidebands. The spectral simulation of MAS 2H NMR, including the effects of paramagnetic shift and anisotropic spin-spin relaxation due to electron-nuclear dipolar coupling and anisotropic g-factor, was performed for several molecular motions. Information about molecular motions in the dynamic range of 10(2) s(-1)相似文献   

Water-soluble molecular capsules: self-assembly and binding properties

The self-assembly and characterization of water-soluble calix[4]arene-based molecular capsules (12) is reported. The assemblies are the result of ionic interactions between negatively charged calix[4]arenes 1 a and 1 b, functionalized at the upper rim with amino acid moieties, and a positively charged tetraamidiniumcalix[4]arene 2. The formation of the molecular capsules is studied by (1)H NMR spectroscopy, ESI mass spectrometry (ESI-MS), and isothermal titration calorimetry (ITC). A molecular docking protocol was used to identify potential guest molecules for the self-assembled capsule 1 a2. Experimental guest encapsulation studies indicate that capsule 1 a2 is an effective host for both charged (N-methylquinuclidinium cation) and neutral molecules (6-amino-2-methylquinoline) in water.     

Molecular motion in solid ammonia trimethylalane

Solid state ²H NMR has been used to study molecular motion in deuterated ammonia trimethylalane (CH₃)₃AlND₃. From analysis of the ²H NMR lineshape between 123 and 298 K, reorientation of the -ND₃ group about the molecular Al-N axis is shown to occur at a rate higher than 10⁸ s^-1, and simulation of partially relaxed ²H NMR lineshapes shows that the reorientation can be described as a 3-site 120° jump motion. From the temperature dependence of the ²H spin-lattice relaxation time, the activation energy for this motion is estimated to be 9.3±0.3 kJ mol^-1. There is no evidence from either ²H or ²⁷Al NMR data for any site-exchange between the sites occupied by the -ND₃ and -CH₃ groups. The anisotropy of the dynamics of (CH₃)₃AlND₃ indicates that the orientation of the Al-N bond is highly constrained, presumably by a strong interaction between the electric dipoles of neighboring molecules.     

Hexameric Capsules Studied by Magic Angle Spinning Solid‐State NMR Spectroscopy: Identifying Solvent Molecules in Pyrogallol[4]arene Capsules

Powders of pyrogallol[4]arene hexamers were produced by evaporation from organic solvents and were studied, for the first time, by magic angle spinning solid‐state NMR (MAS ssNMR). Evaporation selectively removed non‐encapsulated solvent molecules leaving stable hexameric capsules encapsulating solvent molecules. After exposure of the powder to solvent vapors, ¹H/¹³C heteronuclear correlation MAS ssNMR experiments were used to assign the signals of the external and encapsulated solvent molecules. The formed capsules were stable for months and the process of solvent encapsulation was reversible. According to the ssNMR experiments, the encapsulated solvent molecules occupy different sites and those sites differ in their mobility. The presented approach paves the way for studying guest exchange, guest affinity, and gas storage in hexamers of this type in the solid state.     

Out-of-Plane Motion of the Guest Benzene Molecule Trapped in a [Cd(dmen)2(CN)]2[Cd(CN)4] Host as Studied by 2H-NMR

An out-of-plane rotational motion of the guest benzene molecule trapped in a [Cd(dmen)₂(CN)]₂[Cd(CN)₄] host, which is a cadmium cyano complex (dmen = N,N'-dimethyl-1,2-diaminoethane), was detected by means of ²H-NMR powder patterns measured over a temperature range of 123–423 K. A simple inequivalent 8-site reorientation model which has two independent populations was used in the line shape simulation, and the potential trapping the benzene molecule was analyzed to be transformable depending on temperature. Structural flexibility of the host, which is considered to be a cause of the variable potential, was confirmed from the detection of a puckering-like motion of the dmen chelate ring that makes a part of the inner wall of the cavity trapping the guest benzene molecule.     

Metal-assembled cobalt(II) resorc[4]arene-based cage molecules that reversibly capture organic molecules from water and act as NMR shift reagents

The complex Co4 1(2)8- is a tetranuclear cobalt(II) cage compound that assembles in aqueous solutions above pH 4 and is capable of encapsulating a variety of organic guest molecules, for example, benzene, hexane, chlorobutane, butanol, and ethyl acetate. Ligand 1 is a resorc[4]arene-based molecule with iminodiacetate moieties appended to its upper rim. 1H NMR studies of Co4 1(2)8-.guest complexes demonstrate inclusion of nonpolar hydrocarbons, substituted phenyls, alcohols, halogen-containing hydrocarbons, and polar organic molecules. The complex Co4 1(2)8- acts as an NMR shift reagent and causes substantial upfield isotropic hydrogen shifts (-30 to -40 ppm) in the guest molecule and separation of the guest hydrogen chemical shifts by typically 12 ppm. The complex Co4 1(2)8- will encapsulate molecules with fewer than eight atoms in a linear chain, mono- and disubstituted benzenes, and polar molecules with greater than two carbon atoms. The solid-state structure of Ba4[Co4 1(2).C6H5C2H5] shows a disordered guest molecule encapsulated within the cavity of Co4 1(2)8-. The cavity dimensions, bond lengths, and bond angles of Ba4[Co4 1(2).C6H5C2H5] are very similar to those determined in Ba4[Co4 1(2).6H2O].     

Water‐Soluble Molecular Capsules: Self‐Assembly and Binding Properties

The self‐assembly and characterization of water‐soluble calix[4]arene‐based molecular capsules ( 1?2 ) is reported. The assemblies are the result of ionic interactions between negatively charged calix[4]arenes 1 a and 1 b , functionalized at the upper rim with amino acid moieties, and a positively charged tetraamidiniumcalix[4]arene 2 . The formation of the molecular capsules is studied by ¹H NMR spectroscopy, ESI mass spectrometry (ESI‐MS), and isothermal titration calorimetry (ITC). A molecular docking protocol was used to identify potential guest molecules for the self‐assembled capsule 1 a?2 . Experimental guest encapsulation studies indicate that capsule 1 a?2 is an effective host for both charged (N‐methylquinuclidinium cation) and neutral molecules (6‐amino‐2‐methylquinoline) in water.     

Kinetic investigations of the process of encapsulation of small hydrocarbons into a cavitand-porphyrin

Exchange of guest molecules into capsule shaped host molecules is the most fundamental process in host-guest chemistry. Several examples of quantitative measurements of guest exchange rates have been reported. However, there have been no reports on the activation energies of these processes. A molecule known as cavitand-porphyrin (H2CP) has been reported to have a flexible host structure capable of facilitating moderate guest exchange rates suitable for kinetic measurements of the guest exchange process with 1H NMR. In this article, various kinetic and thermodynamic parameters related to the process of encapsulation of small hydrocarbons into H2CP in CDCl3 solution were determined by 2D exchange spectroscopy (EXSY): association and dissociation rate constants (k(ass) = 320 M-1 s-1, k(diss) = 1.4 s-1 for methane at 25 degrees C), the corresponding activation energies (E(a,ass) = 27 kJ.mol-1, E(a,diss) = 58 kJ.mol-1), and thermodynamic parameters for each process (DeltaG++(ass) = 59 kJ.mol-1, DeltaG++(diss) = 72 kJ.mol-1, DeltaH++(ass) = 25 kJ.mol-1, DeltaH++(diss) = 55 kJ.mol-1, DeltaS++(ass) = -113 J.K-1.mol-1, and DeltaH++(diss) = 58 J.K-1.mol-1 for methane). The thermodynamic parameters (DeltaG degrees = -13 kJ.mol-1, DeltaH degrees = -31 kJ.mol-1, DeltaS degrees = -60 J.K-1.mol-1 for methane) for this encapsulation equilibrium determined by EXSY were comparable to those for methane determined by 1D 1H NMR titration (DeltaG degrees = -11 kJ.mol-1, DeltaH degrees = -33 kJ.mol-1, DeltaS degrees = -75 J.K-1.mol-1 for methane). In addition, the structure of the methane encapsulation process was revealed by ab initio MO calculations. The activation energies for methane association/dissociation were estimated from MP2 calculations (E(a,ass) = 58.3 kJ.mol-1, E(a,diss) = 89.1 kJ.mol-1, and DeltaH degrees = -30.8 kJ.mol-1). These values are in accord with the experimentally determined values. The observed guest exchange rates and energies are compared with the corresponding values of various reported capsule-shaped hosts.     

Molecular dynamics and ordering of pyridine in its cyclophosphazene inclusion compound as evaluated by solid state 2H NMR spectroscopy

Variable temperature 2H NMR experiments (line shape analysis, relaxation studies) were carried out on the pyridine-d5-tris-(1,2-dioxyphenyl)-cyclotriphosphazene inclusion compound in the temperature range between 110-300 K. It is found that the pyridine guests are highly mobile throughout the whole temperature range covered here. The observation of three superimposed 2H NMR signals can be understood in terms of a particular (motionally averaged) orientation of the pyridine molecules, which is a consequence of the molecular symmetry of the pyridine guests and the imposed channel restrictions. The experimental data are consistent with a combined rotation on cone-small angle fluctuation model, which assumes a fast molecular reorientation between two superimposed cones with an opening angle for the inner cone between 59-73 degrees (angle of fluctuation between 1-3 degrees ). On the basis of this model assumption it is possible to reproduce both the experimental 2H NMR line shapes and the spin-lattice relaxation data in a quantitative way. The analysis of the partially relaxed spectra (inversion recovery experiments) yields the correlation times for this overall motional process. They follow an Arrhenius behavior from which an activation energy of 8.7 +/- 0.4 kJ mol(-1) is derived. The results are discussed in the framework of the published data for related systems.