The temperature dependence of 14N pure quadrupole resonance in 4- and 3-cyanopyridines

In this paper it is shown that the temperature dependence of the ¹⁴N pure quadrupole resonance for both cyanide and pyridine nitrogen atoms in cyanopryidines may be used to relate the orientation of the electric field gradient tensor at the cyanide site to that at the pyridine. This provides a means to discuss the population of the two π orbitals on the cyanide nitrogen atom.     

14N Pulsed nuclear quadrupole resonance. 2. Effect of a single radio-frequency pulse in the general case

A novel theory, based on density operator calculations, is provided for assessing the nuclear quadrupole resonance behaviour of a spin 1 (¹⁴N) subjected to a single radio-frequency pulse. It is for a powder sample in zero magnetic field for an electric field gradient tensor without symmetry. A complete set of equations is obtained for the quantities of interest. It is derived from the general Liouville–von Neumann equation and from a proper basis on which the density operator is expanded. Theoretical results, in terms of signal evolution as a function of the pulse length (nutation experiments), show that the same nutation curve is expected for the three different transitions which exist when the electric field gradient tensor is without symmetry. This latter nutation curve is, however, different from that which prevails in the case of an axially symmetric tensor, this apparent discrepancy being easily resolved on theoretical grounds. Experimental data (for NaNO₂, electric field gradient tensor without symmetry) are checked against values of the radio-frequency field amplitude provided by NMR measurements performed with the same equipment. Good agreement between theory and experiment is obtained.     

Secondary signals in two-frequency nuclear quadrupole resonance on (14)N nuclei with I=1

Our experimental and theoretical studies show that using two-frequency excitation of (14)N nuclei it is possible to observe secondary NQR signals at one of the three possible transitions due to irradiation of another adjacent transition. As a result of the pulse sequence applied to the adjacent transition the spin-echo signals on the detected transition are observed after essential time interval from the initial single pulse on this frequency. Experiments have been performed on the (14)N nuclei in the sodium nitrite (NaNO(2)) and the military explosive hexahydro-1,3,5-trinitro-s-triazine C(3)H(6)N(6)O(6) (RDX).     

14N pulsed nuclear quadrupole resonance. 3. Effect of a pulse train. Optimal conditions for data averaging

The calculations developed in this paper aim at determining the optimal conditions of a NQR experiment when a transition is monitored by means of a pulse train with pulses of identical duration and signal acquisition after each pulse; coherences are assumed to vanish by effective transverse relaxation prior to every new pulse. These calculations demonstrate that, as in NMR, a steady state is effectively reached for any value of the recycle time. However, by contrast with NMR, it is shown that, for optimal data averaging under steady state conditions, the recycle time T can be kept as low as possible (the only limitation is the acquisition time). Nutation curves (signal amplitude versus pulse length) calculated in the steady state case are shown to depend strongly on the ratio T/T ₁ (T ₁: longitudinal relaxation time). The signal growth as a function of T/T ₁under averaging of the first transients has been evaluated as well as the number of pulses necessary for reaching a steady state.     

Line shape analysis in nuclear quadrupole resonance spectroscopy (I=3/2)

Following the treatment of Vega (1973), the theoretical expressions for the second moments in nuclear quadrupole resonance (nqr) due to dipole-dipole interactions have been derived for the system containing the nuclei with spinI=3/2. Cases with the orientation of the static magnetic field and the interactions with the other magnetic nuclei are also dealt with.     

Decrease of free amino acids in high-pressure treated cheese

Abstract

High-pressure treatment of cheese has been studied with a view to preserving fresh cheese [1], and also of developing a method of accelerating its ripening [2-7]. While these objectives can be reached, other changes also occur in cheese when high pressure is applied. One of those changes is the decrease of free amino acids (FAA) in cheese treated in pressures over 200-300MPa. In this paper we will try to summarize the results obtained by different research groups, as well as our own. These results have been published separately, but not previously tied together and explained.     

14N Pulsed nuclear quadrupole resonance. 4. Two-pulse sequences for the determination of T1 and T2 relaxation times

A general theory, based on density matrix calculations, has been developed for the special case of a two-pulse sequence applied to spin 1 (¹⁴N) nuclear quadrupole resonance (NQR) of a powder sample. It is shown that the homolog of the NMR inversion-recovery experiment leads easily to the spin-lattice relaxation time T ₁ (associated with the diagonal elements of the density matrix) provided that an appropriate phase cycling is used. Conversely, in spite of two-step phase cycling schemes adapted to spin-spin relaxation measurements, the homolog of the NMR Hahn spin-echo sequence may pose some problems if the results are displayed in the magnitude mode. First, at short decay times, the echo may be corrupted by unwanted signals. Secondly, in that case, the amplitude of the resulting signal can evolve unexpectedly and differently as a function of the phase of the second pulse. Thirdly, at long decay times, the echo maximum occurs earlier than expected. All these problems in principle disappear with a complete four-step phase cycling scheme and the echo decay curve yields reliably the spin-spin relaxation time T ₂ (associated with off-diagonal elements). This theory allowed the exploitation of many test experiments performed at different frequencies on hexamethylenetetramine (HMT) and sodium nitrite.     

14N and35Cl nuclear quadrupole double resonance study of the structural phase transitions in N-octylammoniumchloride

The structural phase transitions between the non-intercalated phases of the layer structure compound N-octylammoniumchloride have been studied via proton-¹⁴N and proton-³⁵Cl nuclear quadrupole double resonance. The results have been related to an order-disorder model of the octylammonium chains. The temperature dependence of the order parameters of the –NH₃ head groups has been determined.     

A DFT studies of structural and quadrupole coupling constants properties in C-doped BeO nanotubes

In the framework of density functional theory (DFT), we calculated the electronic structures and the quadrupole coupling constants (CQ) in the pristine and carbon doped (C-doped) beryllium oxide nanotubes (BeONTs) for the first time. The pristine and C-doped forms of representative (10, 0) zigzag and (5, 5) armchair models of BeONTs were considered in this study. The structures are allowed to relax by performing all atomic optimization. Formation energies indicate that C-doping of Be atom (CBe form) could be more favorable than C-doping of O atom (CO form) in both zigzag and armchair BeONTs. Gap energies and dipole moments detected the effects of dopant in the (5, 5) armchair models; however, those parameters did not detect any significant changes in the C-doped (10, 0) zigzag BeONT models. The calculated nuclear quadrupole coupling constant for the Be and O nuclei reveal that the pristine models can be divided into layers of nuclei with an equivalent electrostatic environment such that those nuclei at the ends of tubes end up in a strong electrostatic environment when compared to the other nuclei along the length of tubes. Comparison with the available data on the pristine BeONTs reveals the influence of C-doping on the CQ parameters of Be and O atoms in the C-doped structures. For most lattice sites, the degree of influence on the CQ parameters of the zigzag model is larger than that of the armchair model. The calculations were performed based on the B3LYP DFT method and 6-31G^∗ standard basis sets using the Gaussian 09 program package.     

Mass spectrometric study of the decomposition pathways of canonical amino acids and α‐lactones in the gas phase

The dissociation pathways of a gas‐phase amino acid with a canonical (non‐zwitterionic) α‐amino acid moiety are studied by using mass spectrometry. Investigation of the canonical amino acid moiety is possible because the ionized amino acid, a sulfonated phenylalanine, has a charge center that is separated from the amino acid, and dissociation occurs by charge‐remote fragmentation. The amino acid is found to dissociate only by loss of NH₃ upon collision‐induced dissociation to form a substituted α‐lactone. The dissociation is consistent with what has been observed previously upon pyrolysis of other α‐substituted carboxylic acids. Decarboxylation, which has also been reported previously for amino acid pyrolysis, is not observed, likely because the product would be a high‐energy, ammonium ylide. The resulting α‐lactone is found to undergo dissociation by decarbonylation to give an aldehyde, and by loss of CO₂. Decarboxylation is calculated to occur through a transition state involving hydride shift coupled with lactone ring‐opening. The transition state is found to be stabilized by the negative charge, and therefore, decarboxylation is more favorable for anions. The results show that remote ionic groups can be used as mostly inert charge carriers to enable mass spectrometry to be used to investigate the gas‐phase physical and chemical properties of different types of functional groups, including amino acids. Copyright © 2015 John Wiley & Sons, Ltd.     

Outdoor use of mobile Raman spectrometers to study the solutions and ices of amino acids

Selected amino acids (glycine, alanine, and threonine) were analysed in the form of water solutions, mixtures of these amino acids in solution, as well as their respective ices using two hand‐held instruments under outdoor and winter mountain conditions. The hand‐held instruments featured 785 and 532 nm excitations; the latter being one of the first using this wavelength on the market. The changes to the Raman spectra of the solutions and ices are discussed in relationship to the degree of ambiguity of the detection of the amino acids within the mixtures. The hand‐held instruments were able to detect two amino acids out of the three in solution. The results showed that the hand‐held instruments provide sufficient quality spectra when analysing ices prepared from the solutions; however, an unambiguous detection of these amino acids within mixtures still remains a challenge. Copyright © 2014 John Wiley & Sons, Ltd.     

14N and35Cl nuclear quadrupole double resonance study of the phase transitions in n-decylammoniumchloride and n-hexylammoniumchloride

The structural phase transitions between the intercalated and the non-intercalated phases of the layer structure compound n-decylammoniumchloride and n-hexylammoniumchloride have been studied via proton-¹⁴N and proton-³⁵Cl nuclear quadrupole double resonance. The results have been related to an order-disorder model of the alkylammonium chains. The temperature dependence of the order parameters of the –NH₃ head groups has been determined.     

Hyperfine interaction study of electric field gradient in hafnium phosphide using181Ta probe

The hyperfine interaction of¹⁸¹Ta in HfP has been investigated by time differential perturbed angular correlation method which yielded interaction frequencyv _Q=630.20(15) MHz. The observed electric field gradient is calculated to be 1.66(25) × 10²⁰ V/m².     

Effect of a weak static magnetic field on nitrogen-14 quadrupole resonance in the case of an axially symmetric electric field gradient tensor

The application of a weak static B₀ magnetic field (less than 1 mT) may produce a well-defined splitting of the ¹⁴N Quadrupole Resonance line when the electric field gradient tensor at the nitrogen nucleus level is of axial symmetry. It is theoretically shown and experimentally confirmed that the actual splitting (when it exists) as well as the line-shape and the signal intensity depends on three factors: (i) the amplitude of B₀, (ii) the amplitude and pulse duration of the radio-frequency field, B₁, used for detecting the NQR signal, and (iii) the relative orientation of B₀ and B₁. For instance, when B₀ is parallel to B₁ and regardless of the B₀ value, the signal intensity is three times larger than when B₀ is perpendicular to B₁. This point is of some importance in practice since NQR measurements are almost always performed in the earth field. Moreover, in the course of this study, it has been recognized that important pieces of information regarding line-shape are contained in data points at the beginning of the free induction decay (fid) which, in practice, are eliminated for avoiding spurious signals due to probe ringing. It has been found that these data points can generally be retrieved by linear prediction (LP) procedures. As a further LP benefit, the signal intensity loss (by about a factor of three) is regained.     

Nuclear quadrupole resonance studies of209Bi in Bi4(GeO4)3 and Bi4(SiO4)3

Nuclear quadrupole resonance (NQR) of²⁰⁹Bi has been studied in Bi₄ (GeO₄)₃ and Bi₄ (SiO₄)₃ using a wide band coherence-controlled superregenerative oscillator-detector. All the four allowed (ΔM _I=±1) transitions are observed. In both cases the electric field gradient (EFG) tensor is axially symmetric (η=0.0). The quadrupole coupling constante ² qQ is measured to be 490.8±1 MHz and 470.4±1 MHz respectively. It is pointed out that the purely ionic model is inadequate to understand these results. With the available experimental accuracy and the strength of the applied electric field (∼ 6 KV/cm), no field-induced effects on the NQR spectrum could be observed in the case of Bi₄ (SiO₄)₃.     

Kinetics and mechanism of the reaction of an arenediazonium ion with hydrophilic aminocarboxylic acids in aqueous buffered solution

The reaction of 4‐nitrobenzenediazonium ion, 4NBD, with the aminocarboxylic acids (AA) glycine and serine was studied under acidic conditions by using Linear Sweep Voltammetry (LSV), which allows simultaneous monitoring of 4NBD loss and product formation. Voltammograms of the reaction mixture are complex, showing up to five reduction peaks. The reduction peaks at E_p = ?0.5 and ?1.0 V, not detected in the absence of AA, are associated to products formed in the course of the reaction. The variation of their peak current, i_p, with time shows a complex behavior; that of i_p (E_p = ?1.0 V) follows a biphasic profile with i_p increasing with time up to a maximum after which a decrease is detected, suggestive of formation and subsequent decomposition of a transient intermediate, meanwhile i_p (E_p = ?0.5 V) increases with time after an induction period. The peaks at E_p = ?0.1 and ?0.8 V are associated to the reduction of the diazonium group of 4NBD and, in the presence of AA ([AA] >>> [4NBD]), their peak currents decrease exponentially with time following clean first‐order kinetics for more than 3t_1/2. The variation of k_obs with [AA] at a given pH is linear with an intercept equal to zero and that of log(k_obs) with pH at constant [AA] is also linear. Kinetic evidence is consistent with a reaction mechanism involving an irreversible, rate‐limiting bimolecular step which leads to the formation of an unstable triazene, which further decomposes yielding 4‐nitroaniline among other reaction products. Copyright © 2007 John Wiley & Sons, Ltd.     

Nuclear hyperfine coupling interactions in the rotational spectra of the linear and bent isomers of HF-N2O

The rotational spectra of six isotopomers of the linear and bent isomers of HF-N₂O have been collected in the 7-18 GHz region with a Fourier transform microwave spectrometer. The nuclear hyperfine structure in the spectra produced by HF spin-spin coupling interaction and nuclear quadrupole coupling interactions due to the D nucleus of DF and the nuclei of N₂O have been resolved and analyzed. In the linear isomer, H in HF is bonded to the terminal N in N₂O. The NF bond lengths are 2.9808(2) Å for the HF-containing isotopomers and 2.9732(2) Å for the DF-containing isotopomers. The zero point angles are 23.1° for HF and 31-34° for N₂O. The hyperfine constants suggest that the HF bond is lengthened by 0.0105 Å upon complexation and that the electric field gradients of the two nitrogen nuclei in N₂O are perturbed differently in the complex. In the bent isomer, the hydrogen bond is formed between HF and O in N₂O. The intermolecular distances are 3.4942(2) Å for the HF-containing isotopomers and 3.4436(2) Å for the DF-containing isotopomers, with HF and N₂O forming angles of 34° and 46°, respectively, with the intermolecular axis. The nuclear quadrupole coupling constants of the two nitrogen nuclei do not indicate electric field gradient perturbation in this isomer.     

Non-covalently bonded diastereomeric adducts of amino acids and (S)-1-phenylethanol in low-energy dissociative collisions

We have studied the collision induced dissociation reactions of proton-bound diastereomeric adducts of S-1-phenylethanol and enantiomers of three different amino acids (tryptophan, phenylalanine, methionine). In all cases, the loss of S-1-phenylethanol from the adduct ion is the only observed process, and the relative abundance is found to be independent of the chirality of the amino acid. This is in contrast to earlier experiments on the dissociation of protonated tryptophan–2-butanol adducts, where chirality affected the results. Results obtained from quantum chemical computations support and provide a rationale for the experimental observations and highlight temperature as a possible factor of importance for the chiral effect in these types of systems.     

Solvation thermodynamics of xylitol in water and in aqueous amino acids at 298.15 K

The scaled particle theory has been applied to calculate the free energy, ΔG_solv, enthalpy, ΔH_solv, and entropy, ΔS_solv of solvation for xylitol in water and in aqueous amino acids (glycine, alanine and valine) at 298.15 K. The solvation energy, enthalpy and entropy of xylitol are expressed in terms of their various ingredients. The results show that the interaction terms contribute favorably to the process of solvation. The results suggest that the cavity formation for accommodation of xylitol molecules in aqueous amino acids is an enthalpy‐dominated process. Furthermore, the investigated parameters indicate that xylitol–amino acid interactions follow the sequence: glycine alanine valine water. The findings of the present work may help to throw light on the role that xylitol can play to stabilize macromolecules like proteins in aqueous solutions. Copyright © 2012 John Wiley & Sons, Ltd.