The effect of low-temperature dynamics of the dimethylammonium group in [(CH3)2NH2]3Sb2Cl9 on proton spin-lattice relaxation and narrowing of the proton NMR line

This paper reports the temperature dependence of the relaxation time T1 (55.2 and 90 MHz) and the second moment of the NMR line for protons in a polycrystalline sample of [NH2(CH3)2]3Sb2Cl9 (DMACA). The fundamental aspects of molecular dynamics from quantum tunneling at low temperatures to thermally activated reorientation at elevated temperatures have been studied. The experimentally observed spin-lattice relaxation rate is a consequence of dipolar interactions between the spin pairs inside the methyl group (1/T(1AE) contribution) as well as the spins belonging to neighboring methyl groups and pairs, methyl spin-outer methyl spin (1/T(1EE) contribution). These contributions are considered separately. Two methyl groups in the dimethylammonium (DMA) cations are dynamically inequivalent. The values of the tunnel splitting of separate methyl groups are obtained from the T1 (55.2 MHz) experiment. The tunneling dynamics taking place below the characteristic temperatures 74 and 42 K for separate methyl groups are discussed in terms of the Schr?dinger equation. These temperatures point to the one at which thermal energy C(p)T and potential barrier take the same value. It is established that the second moment of the proton NMR line below 74 K up to liquid helium temperature is much lower than the rigid lattice value, which is due to a tunneling stochastic process of the methyl groups.     

1H NMR study of internal motions and quantum rotational tunneling in (CH3)4NGeCl3

(CH3)4NGeCl3 is prepared, characterized and studied using 1H NMR spin lattice relaxation time and second moment to understand the internal motions and quantum rotational tunneling. Proton second moment is measured at 7 MHz as function of temperature in the range 300-77 K and spin lattice relaxation time (T1) is measured at two Larmor frequencies, as a function of temperature in the range 270-17 K employing a homemade wide-line/pulsed NMR spectrometers. T1 data are analyzed in two temperature regions using relevant theoretical models. The relaxation in the higher temperatures (270-115 K) is attributed to the hindered reorientations of symmetric groups (CH3 and (CH3)4N). Broad asymmetric T1 minima observed below 115 K down to 17 K are attributed to quantum rotational tunneling of the inequivalent methyl groups.     

Application of high resolution NMR spectroscopy on the solid state to the degradation of polyethylene by high dosages of γ-radiation

High resolution (HR) NMR spectroscopy of solid polymer by the cross-polarization (CP) and magic angle spinning (MAS) method was applied to the study of the degradation of polyethylene (PE) by γ-irradiation. No change in the HR NMR spectra was observed in high density polyethylene irradiated up to 300 M rad. Above 300 M rad, deviation of chemical shifts of the main peak corresponding to methylene carbons occurred to higher field. Fractions of the methylene in the gauche form were quantitatively estimated from the observed deviations of the main peaks. Several new peaks, including methyl and carbonyl peaks, were detected after irradiation. The relative concentrations of the methyl and carbonyl groups produced by irradiation were estimated from the observed intensities of the corresponding peaks. Chemical reactions resulting in the formation of the methyl and carbonyl groups are discussed on the basis of the observed changes in the relative concentrations of these groups.     

X-Ray photoelectron spectra of inequivalent atoms in inorganic compounds

X-Ray photoelectron spectra of compounds containing inequivalent atoms were studied. The spectra of some compounds did not have separate peaks corresponding to inequivalent atoms, but their spectra were summations of the spectra of the compounds containing single kinds of atoms. In determining the chemical states of the atoms with XPS, the spectra of the compounds must be compared with the spectra of compounds having the same oxidation and coordination numbers, and the photoelectron spectral intensities are summations of the intensities of the primary and satellite peaks.     

Isotope effects associated with tunneling and double proton transfer in the hydrogen bonds of benzoic acid

The isotope effects associated with double proton transfer in the hydrogen bonds of benzoic acid (BA) dimers have been measured using field-cycling (1)H NMR relaxometry and quasielastic neutron scattering. By studying mixed isotope (hydrogen and deuterium) samples, the dynamics of three isotopologues, BA-HH, BA-HD, and BA-DD, have been investigated. Low temperature measurements provide accurate measurements of the incoherent tunneling rate, k(0). This parameter scales accurately with the mass number, m, according to the formula k(0)=(E/m)e(-Fm) providing conclusive evidence that the proton transfer process is a strongly correlated motion of two hydrons. Furthermore, we conclude that the tunneling pathway is the same for the three isotopologue species. Measurements at higher temperatures illuminate the through barrier processes that are mediated via intermediate or excited vibrational states. In parallel with the investigation of proton transfer dynamics, the theoretical and experimental aspects of studying spin-lattice relaxation in single crystals of mixed isotope samples are investigated in depth. Heteronuclear dipolar interactions between (1)H and (2)H isotopes contribute significantly to the overall proton spin-lattice relaxation and it is shown that these must be modeled correctly to obtain accurate values for the proton transfer rates. Since the sample used in the NMR measurements was a single crystal, full account of the orientation dependence of the spin-lattice relaxation with respect to the applied B field was incorporated into the data analysis.     

Inelastic neutron scattering study of methyl groups rotation in some methylxanthines

The three isomeric dimethylxanthines and trimethylxanthine are studied by neutron spectroscopy up to energy transfers of 100 meV at energy resolutions ranging from 0.7 microeV to some meV. The loss of elastic intensity with increasing temperature can be modeled by quasielastic methyl rotation. The number of inequivalent methyl groups is in agreement with those of the room temperature crystal structures. Activation energies are obtained. In the case of theophylline, a doublet tunneling band is observed at 15.1 and 17.5 microeV. In theobromine, a single tunneling band at 0.3 microeV is found. Orientational disorder in caffeine leads to a 2.7 microeV broad distribution of tunneling bands around the elastic line. At the same time, broad low energy phonon spectra characterize an orientational glassy state with weak methyl rotational potentials. Librational energies of the dimethylxanthines are clearly seen in the phonon densities of states. Rotational potentials can be derived which explain consistently all observables. While their symmetry in general is threefold, theophylline shows a close to sixfold potential reflecting a mirror symmetry.     

Design and evaluation of a crystalline hybrid of molecular conductors and molecular rotors

Combining recent concepts from the fields of molecular conductivity and molecular machinery we set out to design a crystalline molecular conductor that also possesses a molecular rotor. We report on the structures, electronic and physical properties, and dynamics of two solids with a common 1,4-bis(carboxyethynyl)bicyclo[2.2.2]octane (BABCO) functional rotor. One, [nBu(4)N(+)](2)[BABCO][BABCO(-)](2), is a colorless insulator where the dicarboxylic acid cocrystallizes with two of its monoanionic conjugated bases. The other is self-assembled by electrocrystallization in the form of black, shiny needles, with highly conducting molecular slabs of (EDT-TTF-CONH(2))(2)(+) (EDT-TTF = ethylenedithiotetrathiafulvalene) and anionic [BABCO(-)] rotors. Using variable-temperature (5-300 K) proton spin-lattice relaxation, (1)H T(1)(-1), we were able to assign two types of Brownian rotators in [nBu(4)N(+)](2)[BABCO][BABCO(-)](2). We showed that neutral BABCO groups have a rotational frequency of 120 GHz at 300 K with a rotational barrier of 2.03 kcal mol(-1). Rotors on the BABCO(-) sites experience stochastic 32 GHz jumps at the same temperature over a rotational barrier of 2.72 kcal mol(-1). In contrast, the BABCO(-) rotors within the highly conducting crystals of (EDT-TTF-CONH(2))(2)(+)[BABCO(-)] are essentially "braked" at room temperature. Notably, these crystals possess a conductivity of 5 S cm(-1) at 1 bar, which increases rapidly with pressure up to 50 S cm(-1) at 11.5 kbar. Two regimes with different activation energies E(a) for the resistivity (180 K above 50 and 400 K below) are observed at ambient pressure; a metallic state is stabilized at ca. 8 kbar, and an insulating ground state remains below 50 K at all pressures. We discuss two likely channels by which the motion of the rotors might become slowed down in the highly conducting solid. One is defined as a low-velocity viscous regime inherent to a noncovalent, physical coupling induced by the cooperativity between five C(sp3)-H···O hydrogen bonds engaging any rotor and five BABCO units in its environment. The rotational barrier calculated with the effect of this set of hydrogen bonds amounts to 7.3 kcal mol(-1). Another is quantum dissipation, a phenomenon addressing the difference of dynamics of the rotors in the two solids with different electrical properties, by which the large number of degrees of freedom of the low dimensional electron gas may serve as a bath for the dissipation of the energy of the rotor motion, the two systems being coupled by the Coulomb interaction between the charges of the rotors (local moments and induced dipoles) and the charges of the carriers.     

Proton dynamics in the hydrogen bond. Inelastic neutron scattering by single crystals of CsH2PO4 at 20 K

Inelastic neutron scattering spectra (INS) of the powder and of oriented single crystals of cesium dihydrogen phosphate (CsH₂PO₄, or CDP) at 20 K have been investigated. For single crystals the incident neutron beam was perpendicular to either the [100] or [001] crystal planes in order to distinguish between the short and long hydrogen bonds. The proposed assignments are based on previous infrared and Raman data and on the INS band intensities and polarisation. The optical and INS OH stretching band profiles are compared. Their shapes are described in terms of mechanical and electrical coupling of the two stretching modes of a O---H…O hydrogen bond. For the longer bond a Fermi resonance of the OH stretching mode with an overtone of the bending mode is observed. Finally, a broad central mode observed in the INS at very low frequency has been tentatively assigned to the relaxation of the proton transfer along the hydrogen bond.     

Ultra-fast rotors for molecular machines and functional materials via halogen bonding: crystals of 1,4-bis(iodoethynyl)bicyclo[2.2.2]octane with distinct gigahertz rotation at two sites

As a point of entry to investigate the potential of halogen-bonding interactions in the construction of functional materials and crystalline molecular machines, samples of 1,4-bis(iodoethynyl)bicyclo[2.2.2]octane (BIBCO) were synthesized and crystallized. Knowing that halogen-bonding interactions are common between electron-rich acetylenic carbons and electron-deficient iodines, it was expected that the BIBCO rotors would be an ideal platform to investigate the formation of a crystalline array of molecular rotors. Variable temperature single crystal X-ray crystallography established the presence of a halogen-bonded network, characterized by lamellarly ordered layers of crystallographically unique BIBCO rotors, which undergo a reversible monoclinic-to-triclinic phase transition at 110 K. In order to elucidate the rotational frequencies and the activation parameters of the BIBCO molecular rotors, variable-temperature (1)H wide-line and (13)C cross-polarization/magic-angle spinning solid-state NMR experiments were performed at temperatures between 27 and 290 K. Analysis of the (1)H spin-lattice relaxation and second moment as a function of temperature revealed two dynamic processes simultaneously present over the entire temperature range studied, with temperature-dependent rotational rates of k(rot) = 5.21 × 10(10) s(-1)·exp(-1.48 kcal·mol(-1)/RT) and k(rot) = 8.00 × 10(10) s(-1)·exp(-2.75 kcal·mol(-1)/RT). Impressively, these correspond to room temperature rotational rates of 4.3 and 0.8 GHz, respectively. Notably, the high-temperature plastic crystalline phase I of bicyclo[2.2.2]octane has a reported activation energy of 1.84 kcal·mol(-1) for rotation about the 1,4 axis, which is 24% larger than E(a) = 1.48 kcal·mol(-1) for the same rotational motion of the fastest BIBCO rotor; yet, the BIBCO rotor has three fewer degrees of translational freedom and two fewer degrees of rotational freedom! Even more so, these rates represent some of the fastest engineered molecular machines, to date. The results of this study highlight the potential of halogen bonding as a valuable construction tool for the design and the synthesis of amphidynamic artificial molecular machines and suggest the potential of modulating properties that depend on the dielectric behavior of crystalline media.     

Rotational dynamics of methyl groups in m-xylene

Methyl group dynamics of m-xylene was investigated by using incoherent inelastic and quasi-elastic neutron scattering. Inelastic measurements were carried out at the high flux backscattering spectrometer HFBS at the National Institute of Standards, quasi-elastic measurements at the time-of-flight spectrometer NEAT at the Hahn-Meitner-Institute. Rotational potentials are derived which describe the tunnel splittings, first librational, and activation energies of the two inequivalent CH(3) groups. Indications for coupling of the methyl rotation to low-energy phonons have been found. The finite width of one tunneling transition at He temperature is described by direct methyl-methyl coupling. The combined results of the experiments and the calculations allow a unique assignment of rotor excitations to crystallographic sites.     

Hydrogen adsorption in nanoporous nickel(II) phosphates

Hydrogen sorption in the nanoporous nickel phosphates VSB-1 and VSB-5 has been studied with a combination of BET, temperature programmed desorption (TPD), and inelastic neutron scattering (INS) measurements. H(2) BET isotherms for VSB-1 are similar to those seen in nanoporous zeolites, while VSB-5 adsorbs substantially more hydrogen due to a steep initial uptake at low partial pressures. TPD data show that hydrogen interacts strongly with VSB-5, with desorption peaks at 109 and 149 K in a nitrogen flow, whereas the absence of similar peaks for VSB-1 suggests a weaker interaction. INS spectra of the rotational tunnel transition of the adsorbed H(2) also reveal a strong interaction with the VSB-5 host. These data strongly suggest the existence of coordinatively unsaturated Ni(2+) sites accessible to H(2) molecules in the pores of VSB-5.     

Molecular rotor of Cs2([18]crown-6)3 in the solid state coupled with the magnetism of [Ni(dmit)2

Nanoscale molecular rotors that can be driven in the solid state have been realized in Cs2([18]crown-6)3[Ni(dmit)2]2 crystals. To provide interactions between the molecular motion of the rotor and the electronic system, [Ni(dmit)2]- ions, which bear one S=1/2 spin on each molecule, were introduced into the crystal. Rotation of the [18]crown-6 molecules within a Cs2([18]crown-6)3 supramolecule above 220 K was confirmed using X-ray diffraction, NMR, and specific heat measurements. Strong correlations were observed between the magnetic behavior of the [Ni(dmit)2]- ions and molecular rotation. Furthermore, braking of the molecular rotation within the crystal was achieved by the application of hydrostatic pressure.     

N’-苄基酰腙分子的氮-氮键旋转位阻及分子构象

采用动态核磁共振波谱(DNMR)和密度泛函理论(DFT)对N'-苄基酰腙化合物进行构象研究. 实验和理论计算表明, 1H NMR图谱中三组不同质子的双峰裂分是由N—N键旋转位阻造成的, 而这三个双峰裂分的化学位移差异随温度升高而减小. 通过模拟化学位移差异与温度的关系, 得到了交换速率常数, 采用Eyring方程计算出N—N键旋转位阻. 提出顺式和反式共存的模型来分析酰胺质子信号分裂的原因, 并利用DFT计算得出优化的异构体构象及其最低能量. 端甲基质子和次甲基质子信号裂分也来源于N—N键旋转受阻. N'-苄基酰腙通过缩合反应转变成1,3,4-二唑化合物, 消除了甲基空间取向的差异, 其信号变为单峰.     

Vibrational overtone spectroscopy of phenol and its deuterated isotopomers

We have measured the OH- and OD-stretching fundamental and overtone spectra of phenol and its deuterated isotopomers under jet-cooled conditions using nonresonant ionization detection spectroscopy and vapor-phase infrared (IR) and near-infrared (NIR) spectra at room temperature using conventional and photoacoustic spectroscopy. The OH- and OD-stretching bands in the jet-cooled spectra are about 1-10 cm(-1) wide and generally show a few Lorentzian shaped peaks. The bands in the room-temperature spectra have widths of 20-30 cm(-1) and display clear rotational profiles. The band profiles in the jet-cooled spectra arise mostly from nonstatistical intramolecular vibrational redistribution (IVR) with specific coupling to "doorway" states, which are likely to involve CH- and CD-stretching vibrations. The transition dipole moment that determines the rotational structure is found to rotate significantly from the fundamental to the third overtone and is not directed along the OH(D) bond. We use these calculated transition dipole moments to simulate the rotational structure. We determine the rotational temperature in the jet-cooled spectra to be about 0.5 K. Anharmonic oscillator local mode calculations of frequencies and intensities of the OH- and OD-stretching transitions are compared with our measured results. The calculated intensities are in good agreement with the absolute intensities obtained from conventional spectroscopy and with the relative intensities obtained from the room-temperature laser spectroscopy.     

Pulsed field ionization-ZEKE spectroscopy of cresoles and their aqueous complexes: internal rotation of methyl group and intermolecular vibrations

Pulsed field ionization-ZEKE photoelectron spectroscopy and (1 + 1) R2PI spectroscopy have been applied to the cis- and trans-m-cresol.H2O clusters. The internal rotational structure in the S1 state has been re-assigned, and the potential curve has been determined for the cluster. The PFI-ZEKE spectra of the cis- and trans-isomers show low-frequency bands up to 1000 cm-1 above the adiabatic ionization potential IP0. The low-frequency bands are assigned to the internal rotation of the methyl group, the intermolecular stretching and their combination bands in the m-cresol.H2O cluster cation. Level energies and relative transition intensities are reproduced well by a one-dimensional rotor model with a three-fold axis potential. Potential curves for the internal rotation have been determined for both cis- and trans-isomers of m-cresol.H2O cations. The effect of the cluster formation upon the internal methyl rotation, and the interaction between the methyl rotation and the intermolecular vibration are discussed.     

Control of rotor function in light-driven molecular motors

A study is presented on the control of rotary motion of an appending rotor unit in a light-driven molecular motor. Two new light driven molecular motors were synthesized that contain aryl groups connected to the stereogenic centers. The aryl groups behave as bidirectional free rotors in three of the four isomers of the 360° rotation cycle, but rotation of the rotors is hindered in the fourth isomer. Kinetic studies of both motor and rotor functions of the two new compounds are given, using (1)H NMR, 2D-EXSY NMR, and UV-vis spectroscopy. In addition, we present the development of a new method for introducing a range of aryl substituents at the α-carbon of precursors for molecular motors. The present study shows how the molecular system can be photochemically switched between a state of free rotor rotation and a state of hindered rotation and reveals the dynamics of coupled rotary systems.     

X-ray diffraction and inelastic neutron scattering study of 2,6-dimethylpyrazine (DMP) chloranilic acid (CLA) complex

The DMP · CLA and DMP · CLA-d₂ crystals below 100 K are monoclinic, space group P2₁/c with four molecules in the unit cells. Infinite chains of hydrogen bonded counterparts are formed with O?N distances equal to 2.767(2) and 2.639(2) for non-deuterated DMP · CLA. Deuteration leads to the Ubbelohlde effect particularly well manifested in the second short O–H?N bridge (elongation by ca. 0.02 Å). In the IR spectra a Had?i’s trio of the broad absorption is observed characteristic of strong hydrogen bonds. In the INS spectra the vibrational density of states and methyl rotational tunnel splittings were determined. The temperature dependence of tunneling bands enabled to make mode assignments and to determine the methyl rotational potentials. Comparison of the results to the pure electron donor DMP was made and the difference found can be almost completely assigned to the steric changes of the environment. A weak isotope effect with deuteration of the OH?N bridges of the DMP · CLA complex is assigned to a charge transfer of δe/e = 0.006.     

Determination of natural abundance 15N-1H and 13C-1H dipolar couplings of molecules in a strongly orienting media using two-dimensional inverse experiments

NMR spectra of molecules oriented in liquid crystals provide homo- and heteronuclear dipolar couplings and thereby the geometry of the molecules. Several inequivalent dilute spins such as 13C and 15N coupled to protons form different coupled spin systems in their natural abundance and appear as satellites in the proton spectra. Identification of transitions belonging to each spin system is essential to determine heteronuclear dipolar couplings, which is a formidable task. In the present study, using 15N-1H and 13C-1H HSQC, and HMQC experiments we have selectively detected spectra of each rare spin coupled to protons. The 15N-1H and 13C-1H dipolar couplings have been determined in the natural abundance of 13C and 15N for the molecules pyrazine, pyrimidine and pyridazine oriented in a thermotropic liquid crystal.     

Methyl group tunneling rotation in the lowest nπ* state of toluquinone. An optically detected ENDOR,LAC and CR study

In this paper we report and discuss the effects of methyl group tunneling rotation on the methyl proton ENDOR, LAC and CR spectra in the lowest triplet state of toluquinone at 1.8 K. From a detailed analysis of the ENDOR spectra in the lowest rotational state (A) we obtain for the methyl protons the following isotropic hyperfine interaction constants: A₁ = A₂ = 34.4 MHz and A₃ = ?53.7 MHz. The tunneling frequency (3J) is calculated to be 2.9 GHz. The most likely equilibrium configuration of the torsional oscillator is found to be the one whereby the molecular plane is a mirror plane and the out-of-plane methyl protons point towards the closest oxygen atom. We also show that cross-relaxation between |A> and |E> states of translational equivalent toluquinone molecules is responsible for the observed sidebands in the level-anti-crossing spectra of toluquinone.