Crystal Of Malonic Acid Research Articles

Lack of the 'long-distance' dynamical co-operative interactions due to low symmetry of hydrogen-bonded malonic acid aggregates in molecular crystals.

The paper explains the relationship between the energy of hydrogen bonds and the distance between associated carboxyl groups of malonic acid (MA) molecules by means of infrared spectroscopic studies of crystals of its four isotopic varieties [CH2(COOH)2, h4-MA; CH2(COOD)2, d2c-MA; CD2(COOH)2, d2m-MA; CD2(COOD)2, d4-MA]. The effects associated with impact on the isotopic dilution and changes in the temperature of spectrum registration on the fine structures of the νO-H and νO-D bands were analyzed. MA molecular crystals are characterized by a tendency to spontaneous H/D isotopic exchange both within centrosymmetric hydrogen bond cycles and methylene groups. The mono- and polycrystalline spectra obtained in the infrared range of isotopically neat and isotopically diluted by deuterons do not indicate the occurrence of anomalous temperature evolution in the course of lowering their registration temperature to 77 K. Theoretical calculations did not give clear confirmation of the nature of the phenomena analyzed.

Demonstration of Sufficient Control for Two Rounds of Quantum Error Correction in a Solid State Ensemble Quantum Information Processor

We report the implementation of a 3-qubit quantum error-correction code on a quantum information processor realized by the magnetic resonance of carbon nuclei in a single crystal of malonic acid. The code corrects for phase errors induced on the qubits due to imperfect decoupling of the magnetic environment represented by nearby spins, as well as unwanted evolution under the internal Hamiltonian. We also experimentally demonstrate sufficiently high-fidelity control to implement two rounds of quantum error correction. This is a demonstration of state-of-the-art control in solid state nuclear magnetic resonance, a leading test bed for the implementation of quantum algorithms.

Polymorphism in Containerless Crystallization

The effect of the ambient relative humidity (RH) on the crystallization of malonic and glutaric acid in aqueous microdroplets was investigated using an electrodynamic balance (EDB). Aqueous droplets containing a solute were injected into a chamber maintained at selected RH values, after which the RH was cycled to characterize crystallization and deliquescence behavior. Glutaric acid crystals that were formed at >40% RH deliquesced at 85 ± 2% RH, consistent with the metastable α polymorph. Glutaric acid crystals initially formed at ≤30% RH, however, deliquesced at 90 ± 2% RH, corresponding to the stable β polymorph. These polymorph identifications were confirmed using in situ Raman spectroscopy. In the case of malonic acid, the mass change measured with the EDB at deliquescence revealed the formation of a hydrate, C3H4O4·1.5H2O, when the crystals were initially formed at ≥20% RH. The EDB response for malonic acid crystals initially formed at ∼5% RH, however, corresponded to an anhydrous form. These observa...

Dynamical decoupling of electron spins in phosphorus-doped silicon

Quantum coherence is an important enabling feature underpinning quantum computation. However, because of couplings with its noisy surrounding environment, qubits suffer from the decoherence effects. The dynamical decoupling (DD) technique uses pulse-induced qubit flips to effectively mitigate couplings between qubits and environment. Optimal DD eliminates dephasing up to a given order with the minimum number of pulses. In this paper, we first introduce our recent work on prolonging electron spin coherence in γ-irradiated malonic acid crystals and analyze different decoherence mechanisms in this solid system. Then we focus on electron spin relaxation properties in another system, phosphorous-doped silicon (Si:P) crystals. These properties have been investigated by pulse electron paramagnetic resonance (EPR). We also investigate the performance of the dynamical decoupling technique on this system. Using 8-pulse periodic DD, the coherence time can be extended to 296 μs compared with 112 μs with one-pulse control.

Caught at the finishing line

Quantum systems habitually leak information, limiting their usefulness for practical applications. By optimally reversing the leak, this information loss has been reduced to a trickle in the solid state. If electron spins in solid materials are to be exploited in quantum computers, there is a need to minimize the quantum 'noise' that inevitably accompanies the interaction of the spins with their environment. Experiments in single crystals of malonic acid undergoing pulsed electron paramagnetic resonance now show how this can be achieved. The use of external pulses to induce the electron spins to 'flip' an optimal number of times causes their noisy interaction with the environment to be averaged towards zero, and the timescale over which spin coherence is maintained is markedly increased — from 0.04 to 30 microseconds in the system studied.

Preserving electron spin coherence in solids by optimal dynamical decoupling

To exploit the quantum coherence of electron spins in solids in future technologies such as quantum computing, it is first vital to overcome the problem of spin decoherence due to their coupling to the noisy environment. Dynamical decoupling, which uses stroboscopic spin flips to give an average coupling to the environment that is effectively zero, is a particularly promising strategy for combating decoherence because it can be naturally integrated with other desired functionalities, such as quantum gates. Errors are inevitably introduced in each spin flip, so it is desirable to minimize the number of control pulses used to realize dynamical decoupling having a given level of precision. Such optimal dynamical decoupling sequences have recently been explored. The experimental realization of optimal dynamical decoupling in solid-state systems, however, remains elusive. Here we use pulsed electron paramagnetic resonance to demonstrate experimentally optimal dynamical decoupling for preserving electron spin coherence in irradiated malonic acid crystals at temperatures from 50 K to room temperature. Using a seven-pulse optimal dynamical decoupling sequence, we prolonged the spin coherence time to about 30 mus; it would otherwise be about 0.04 mus without control or 6.2 mus under one-pulse control. By comparing experiments with microscopic theories, we have identified the relevant electron spin decoherence mechanisms in the solid. Optimal dynamical decoupling may be applied to other solid-state systems, such as diamonds with nitrogen-vacancy centres, and so lay the foundation for quantum coherence control of spins in solids at room temperature.

Electronic g-factor measurement from ENDOR-induced EPR patterns: malonic acid and guanine hydrochloride dihydrate

Measurement of electronic g-factors ( g ) from radicals in irradiated organic crystals is generally difficult because the overall EPR pattern is usually the composite of several components, e.g., from multiple radicals and from multiple magnetic sites. However, when an ENDOR line is fully resolved, the method of ENDOR-induced EPR (EI-EPR, or EIE) in principle permits identification of the EPR pattern from the individual component yielding the line. To examine this method as an approach useful for measuring g , we used it to measure those of known radicals in two different crystal systems. First, to verify correspondence of the EIE and EPR sufficient for using EIE patterns to extract g , we used both EIE and EPR to measure g of CH(COOH) 2 from irradiated crystals of malonic acid. Then, to illustrate the procedure applied to a system giving a more complex EPR pattern, we used EIE to measure g of the O6-protonated anion radical of guanine in irradiated guanine · HCl · 2H 2O crystals. EPR results from the malonic acid radical are g max=2.00374(2), g mid=2.00331(2), and g min=2.00234(3); EIE results from the same radical are g max=2.00375(2), g mid=2.00334(2), and g min=2.00238(2), where numbers in parentheses indicate statistical uncertainties in the respective least significant digits. In addition, eigenvectors from the two sets of measurements agree to approximately 1°. Results from the guanine radical are g max=2.00490(2), g mid=2.00318(4), and g min=2.00218(4). (The uncertainties should reliably indicate relative accuracy, while absolute accuracy is within ±0.0002 as indicated by simultaneous measurement of Cr 3+ in MgO.)

Weakly Coupled Proton Interactions in the Malonic Acid Radical: Single Crystal ENDOR Analysis and EPR Simulation at Microwave Saturation

The α-proton hyperfine coupling observed by electron paramagnetic resonance (EPR) spectroscopy on the radical •CH(COOH)2 in irradiated crystals of malonic acid, CH2(COOH)2, has served as a standard against which hundreds of observations of similar couplings have been held and scaled. The major doublet of the malonic acid radical is accompanied by less intense “forbidden” (f) α-proton transitions and “spin-flip” (s) transitions due to weakly interacting protons. Both s and f transition lines exhibit microwave power saturation behaviors different from that of the major doublet. At high microwave power, the prominence of these s and f lines may be misinterpreted as originating from different radical species. Computer simulations could help distinguish between the different cases, but no computer simulation programs taking into account the microwave power saturation case are commonly available. On the basis of classical line-shape theory, an algorithm describing the microwave power dependence of an EPR line shape has been developed and implemented in an existing simulation program. To test this new program, malonic acid was selected because of the simplicity of its EPR spectra. However, sufficiently detailed information about the hyperfine coupling parameters for a satisfactory simulation of the room-temperature data (including s and f lines) was not available in the literature. Therefore, a detailed room-temperature EPR/ENDOR study on a single crystal of malonic acid was performed. In addition to the major α-proton coupling, seven weaker proton interactions have been characterized and partly identified. Simulations under nonsaturating conditions reproduce very well all features of the experimental EPR spectra. Simulations under saturating conditions similarly reproduce the power-dependent EPR spectra and yield information about the relaxation behavior of the radical system, which is amenable to verification using other spin-resonance methods.

EPR and ENDOR Studies of Deuteron Hyperfine and Quadrupole Coupling in •CD(COOD)2: Experimental and Theoretical Estimates of Electric Field Gradients from an α-Carbon

In single crystals of malonic acid grown from heavy water, the methylene protons have been partially exchanged with deuterons. Upon X irradiation at room temperature, the •CD(COOD)2 radical is formed in an amount comparable to the •CH(COOD)2 radical species. In the present work, EPR and ENDOR analyses of the α-deuteron hyperfine coupling (hfc) and nuclear quadrupolar coupling (nqc) tensors at room temperature have been performed. The hyperfine coupling tensor is, when scaled with the differences in the nuclear g-factor, almost identical to the α-proton coupling of the •CH(COOH)2 radical at room temperature. The quadrupolar coupling tensor was found to be virtually coaxial with the hyperfine coupling tensor. The quadrupolar coupling constant is 149.8 ± 1 kHz, and the asymmetry factor η = 0.092 ± 0.020. It is known that, at room temperature, the malonic acid radical exhibits thermal motion between two potential energy minima separated by about ±12°. Assuming that the observed hfc and nqc tensors are the result of thermal averaging between these two conformations of the radical, a simple two-site jump model was used to estimate the rigid-limit tensors. The most significant result obtained was for the nqc tensor, for which the calculations resulted in a quadrupolar coupling constant of 160 kHz and an asymmetry factor η = 0.026. These values are fairly close to the nqc parameters for the methylene deuterons in malonic acid at low temperature. The quadrupolar coupling tensor has been theoretically modeled using Slater orbitals and formal electronic populations, as well as electron populations obtained from RHF/CI INDO-type calculations. The simple model to compute the electric field gradient at the α-deuteron caused by the charge distribution at the sp2-hybridized α-carbon was found to be as successful as more advanced methods. Furthermore, density functional theoretical (DFT) calculations for both the malonic acid radical and the native malonic acid molecule have been performed. Field gradients calculated by the DFT method significantly overestimate the quadrupolar tensors for both the α-deuteron of the radical and the methylene deuterons of the malonic acid molecule. Calculations using electron populations from the RHF/CI INDO calculations show that contributions to the quadrupolar coupling tensor from electrons and nuclei beyond the nearest-neighbor atom of the deuteron are significant.

2D correlation spectroscopy in homonuclear dipolar-coupled solids

A novel 2D proton solid-state NMR experiment is described in which the spins evolve freely during the evolution period, while they are decoupled from each other by a multiple-pulse sequence in the detection period. The resulting 2D spectrum allows the correlation of chemical shifts with dipolar interactions. It is useful for assigning proton chemical shift tensors and for recording simplified wide-line spectra whose meaning is discussed in terms of a “gedanken” experiment. The method is applied to a single crystal of malonic acid (MA). The long-standing problem of assigning the previously measured proton chemical shift tensors in MA to the proton sites is solved. For the first time line splittings are resolved which result from dipolar interactions of the carboxylic protons in MA. They are discussed in terms of the geometry of the hydrogen bonds formed by these protons.

Deuteron NMR on single crystals. EFGs, signs of quadrupole coupling constants, and assignment of σ tensors in malonic acid

The high-field NMR of two dipolar-coupled deuterons experiencing different EFGs is investigated theoretically and shown to be solvable analytically. The determination of the signs of quadrupole coupling constants from the spectral appearance is discussed. Room-temperature FT deuteron spectra of deuterated crystals of malonic acid were recorded at 54.7 MHz. The dipolar fine structure resulting from the coupling of the deuterons of the CD 2 group could be resolved. From the rotation patterns of the line splittings, the EFGs at the sites of the four inequivalent deuterons are determined and compared with the previous work of Derbyshire et al.. The results confirm the conclusion, drawn in a previous paper on the proton NMR of malonic acid, that the methylene deuteron internuclear vector and the normal of the CCC backbone of the molecule subtend an angle of about 6°. The signs of the quadrupole coupling constants of the methylene deuterons are shown to be positive. The correlation of chemical shifts with quadrupole splittings allowed to assign directly the chemical shift tensors measured previously by multiple-pulse proton NMR to the methylene hydrogen sites.

Wide-line and high-resolution proton magnetic resonance in single crystals of malonic acid

Single crystals of malonic acid were investigated by Fourier transform “wide-line” and multiple-pulse “high-resolution” techniques. Crystal spheres were used to avoid warping of the applied field by bulk susceptibility shape effects. The analysis of the wide-line spectra revealed asymmetric hydrogen bonds in malonic acid and a twist of about 6° of the methylene proton internuclear vector with respect to the normal of the CCC backbone of the molecule. All four chemically shifted lines possible by crystal symmetry could be resolved in the high-resolution spectra. The analysis of these spectra yielded the nuclear magnetic shielding tensors of all the protons in malonic acid. Particular emphasis is laid on the methylene proton shielding tensors which had not been determined previously. They turned out not to be fully congruent within experimental accuracy. Their anisotropies, 4.85 ± 0.3 and 3.80 ± 0.3 ppm, respectively, and their asymmetry parameters η 0.4 and 0.8, respectively, are compared with the corresponding parameters of methylene proton shielding tensors in a variety of compounds of which powder spectra have been recorded. The shielding anisotropy of the methylene protons in malonic acid appears to be unusually small. The problem of assigning shielding tensors to proton sites is solved in a purely experimental manner by analyzing the widths of the resonance lines in multiple-pulse spectra.

Proton magnetic shielding in malonic acid by multiple pulse techniques

The proton NMR in single crystals of malonic acid has been studied by multiple pulse line narrowing techniques. The nuclear magnetic shielding tensors σ ( i) of all protons in malonic acid could be determined from the spectra. There are two magnetically distinct carboxyl protons. The principal components of their shielding tensors are found to be σ (1) ZZ = −0.8 ppm, σ (1) YY = −19.2 ppm, σ (1) XX = −21.8 ppm, and σ (2) ZZ = −1.0 ppm, σ (2) ZZ = −21.3 ppm relative to adamantane. The error limits are estimated to be ± 1 ppm. The most shielded directions lie along the hydrogen bond directions to within 8 degrees. The least shielded directions are essentially perpendicular to the plane of the carboxyl groups. Within experimental accuracy the shielding of the aliphatic protons is axially symmetric about the CH bond axes. The anisotropy Δσ = σ ⌈ − σ ⊥ is (4 ± 1) ppm. The gross features of the anisotropy of the carboxyl protons are shown to be governed by the diamagnetic effect.

Temperature dependence of methyl group tunnelling rotation frequency

The temperature dependence is measured from 4 to 40 K of the frequency of tunnelling rotation of methyl groups in free radicals in gamma -irradiated methyl malonic acid crystals. Above 25 K the frequency, while remaining well defined, declines sharply with increasing temperature.

Electron spin-lattice and cross relaxation in irradiated malonic acid

Electron spin-lattice and cross relaxation processes in X-ray and electron irradiated single crystals of malonic acid have been investigated over the temperature range 1.1° to 300°K as a function of radiation dosage and as a function of thermal aging. The recovery of the longitudinal magnetization following pulsed saturation is due to cross relaxation among the radicals R2⋯X2, R⋯X2, and R⋯X2CO (where R = COOH or COOD and X = H or D) and to the characteristic spin-lattice relaxation rates of these radicals. The spin-lattice relaxation times in these radicals appear to be determined by phonon modulation of the spin-orbit coupling rather than of the hyperfine interaction.

Free radicals in malonic acid: a study of radical conformations at 77K and 295K by the ESR-technique.

Single crystals of malonic acid (CH2(COOH)2), have been irradiated with 4.0MeV electrons at liquid nitrogen temperature. The induced radicals have been identified by electron spin resonance technique. ESR lines due to different conformations of the ĊH2(COOH) radical are observed. ESR lines due to the ĊOOH radical are also seen. It is suggested that the fragmentation into these radicals takes place preferentially on one side of the molecule and subsequent to the ionization event. Upon warming of the crystal to room temperature, the ĊOOH radical disappears and the ĊH(COOH)2 radical is formed. The latter radical seems to exist in two slightly different modifications.

Methyl group tunnelling rotation frequency in γ-irradiated methyl malonic acid crystals

At low temperatures in molecular crystals, hindered methyl group torsional oscillator-nuclear spin states exhibit an energy splitting which is simply related to the rate of tunnelling rotation of the group through +or-120 degrees about the threefold axis. In free radicals in radiation damaged crystals where there exists a large hyperfine interaction between the methyl group protons and the unpaired electron spin, the tunnelling splitting may be measured from the electron nuclear double resonance (ENDOR) transition frequencies. A study at 4.2K of the free radical CH3C(COOH)2 formed by heat treatment of irradiated methyl malonic acid crystals yields a value of the splitting equivalent in frequency units to 160+or-30 MHz. It is found that those ENDOR transitions whose frequencies depend most upon the tunnelling splitting are anomalously broad. This implies that the splitting has a wide though strongly peaked distribution. Some methyl groups are found to be in states in which the tunnelling splitting is inverted, providing experimental support for theoretical models which have invoked low lying states of this kind to account for the averaging out of the tunnelling splitting at those higher temperatures where classical models for methyl group rotation apply.

Modulation effects in the spin-echo signals of CH(COOH)2 radicals

Results are given on the angular dependence of these effects for radicals stabilized in a γ-irradiated single crystal of malonic acid. The modulation is ascribed to anisotropic hyperfine interaction with the α proton of CH(COOH)2. The results allow electron spin echo to be used in certain cases to measure small splittings in the ESR spectra of free radicals.

Pulsed Electron-Electron Double Resonance in anS=12,I=12System

Pulsed electron-electron double resonance techniques have been applied to the \ifmmode \dot{C}\else \.{C}\fi{}H fragment in an irradiated single crystal of malonic acid. Damped Torrey oscillations about the pumping field and also about the observing field have been observed, and the spin-lattice and cross-relaxation times have been measured directly. Display of the transient signal intensity at a constant delay as a function of the pump frequency (i.e., relaxation spectroscopy) yielded very narrow EPR hyperfine lines.

Electron spin resonance of γ-irradiated malonic acid

The electron spin resonance spectrum of a γ-irradiated single crystal of malonic acid indicates the presence of at least two chemically distinct radicals. The most stable radical is ĊH(CO2H)2, but the present paper primarily concerns the structure and orientation of the radical ĊH2(CO2H). The anisotropy of the spectra of this radical with respect to the direction of the main magnetic field H shows that the free radical carbon atom has gone over to planar sp 2 hybridization, the HĊH angle being 116°±5°. Since the two ends of the undamaged malonic acid molecule are distinguishable in the crystal, two sites were expected for the radical ĊH2(CO2H). It appears, however, that approximately 80 per cent of the radicals occupy one site, and only about 20 per cent the other. Comparison of the orientation of the main ĊH2(CO2H) radical with that of the undamaged molecule indicates that in the radical the plane of the ĊH2 group is approximately perpendicular to the plane of the (CO2H) group. The spectra of the less predominant ĊH2(CO2H) radicals were too weak and too overlapped for detailed analysis.