14N Nuclear Quadrupole Research Articles

Halogen-Bond Interactions Enhanced by Charge-Assisted Hydrogen Bonds: An Ab Initio Study

Abstract The NH4+···NCX···NCY (X = F, Cl, and Br; Y = H, F, and OH) complexes are theoretically investigated to find ways to enhance the halogen-bond interaction. Cooperative effects are observed when a charge-assisted hydrogen bond (CAHB) and halogen bond coexist in the same complex. These effects are studied in terms of equilibrium geometry, interaction energy, charge-transfer properties, and 14N nuclear quadrupole coupling constant of the complexes at the MP2/aug-cc-pVTZ level. In all triads studied, a favorable cooperativity is found with values that range between −1.44 and −4.04 kcal mol−1. However, the increased percentage of the interaction energy depends on the interaction strength. To investigate the role of different energy terms in cooperativity between hydrogen and halogen bonds, the scheme of decomposition of the interaction energy is applied. The electrostatic interaction is found to be a dominant factor in enhancing both types of interactions.

Rotational spectrum of tryptophan.

The rotational spectrum of the natural amino acid tryptophan has been observed for the first time using a combination of laser ablation, molecular beams, and Fourier transform microwave spectroscopy. Independent analysis of the rotational spectra of individual conformers has conducted to a definitive identification of two different conformers of tryptophan, with one of the observed conformers never reported before. The analysis of the (14)N nuclear quadrupole coupling constants is of particular significance since it allows discrimination between structures, thus providing structural information on the orientation of the amino group. Both observed conformers are stabilized by an O-H···N hydrogen bond in the side chain and a N-H···π interaction forming a chain that reinforce the strength of hydrogen bonds through cooperative effects.

Theoretical 14N and 17O nuclear quadrupole resonance parameters for tirapazamine and related metabolites

Density functional theory (DFT) calculations were performed to realize the effects of the N–O group on the reactivity and electronic properties of 3-amino-1,2,4-benzotriazines. The electric field gradient, EFG, tensors of 14N and 17O nuclei and natural bond orbital (NBO) analysis in the tirapazamine (TPZ) and its four derivatives were calculated at the B3LYP/6-311+G(d,p)//B3LYP/6-31G(d) method in the gas phase. The NBO analysis reveal that the bond strength, proton affinity and position of N–O group in the heterocyclic ring have major influence on the reactivity of considered molecules. Accordingly, we suggest that the TPZ and 4-oxide (d) structures due to having a weaker N–O bond with larger negative charge on the oxygen atom at the 4-position are more active than the other ones. Calculated 14N and 17O EFG tensors were used to evaluate nuclear quadrupole coupling tensors, χ, and asymmetry parameters, η Q . Results showed that oxidation of a nitrogen atom at any position have significant influence on its 14N nuclear quadrupole resonance (NQR) parameters. Also, the occupancy of nitrogen lone pair plays an important role in determination of the q zz and χ values of mentioned nuclei. It is found that the η Q and χ are appropriate parameters to study the contribution of lone pair electrons of nitrogen atom in the formation of chemical bond or conjugation with the aromatic system. Finally, a linear correlation is observed between the χ(14N) and χ(17O) values in the N–O bond which may be associated with the reactivity of these compounds.

Interaction between Freons and Amines: The C–H···N Weak Hydrogen Bond in Quinuclidine–Trifluoromethane

The 1:1 complex between quinuclidine and trifluoromethane has been investigated using pulsed jet Fourier transform microwave spectroscopy. The two constituting molecules are held together through a weak C-H···N hydrogen bond, with a H···N length of 2.070(1) Å. The C3 symmetric axes of the two moieties are collinear, making the overall molecular system a symmetric top. The HCF3 subunit is freely rotating with respect to the amine moiety. Transitions for the ground (m = 0) and for the first excited (|m| = 1) torsional states, with K up to 2, have been measured, all of them showing the (14)N nuclear quadrupole hyperfine structure. The dissociation energy of the complex has been estimated to be 10.2 kJ mol(-1).

Nuclear Quadrupole Resonance Investigation of Hydrogen Bonding in Some Cocrystals of 2,3,5,6-Tetramethylpyrazine and Carboxylic Acids

Cocrystals of 2,3,5,6-tetramethylpyrazine and several carboxylic acids have been prepared, and the complete (14)N nuclear quadrupole resonance spectra have been measured. The (14)N nuclear quadrupole resonance spectra have been used to check whether the cocrystals are indeed formed and to investigate the hydrogen bonding scheme of 2,3,5,6-tetramethylpyrazine molecules. Since a 2,3,5,6-tetramethylpyrazine molecule has two hydrogen bond acceptors, it may form either 1:1 or 1:2 cocrystals with carbocylic acids. (14)N nuclear quadrupole resonance is used to distinguish between these two possibilities. Rather large (14)N quadrupole coupling constants in the investigated cocrystals show that in these systems proton transfer O-H···N → O(-)···H-N(+) does not occur. The quadrupole coupling tensor in 2,3,5,6-tetramethylpyrazine cocrystals has been analyzed in terms of the deformation of the electron lone pair orbital and population of the π-electron orbital. The analysis shows that the two effects are correlated.

Hydrogen bonding and proton transfer in cocrystals of 4,4'-bipyridyl and organic acids studied using nuclear quadrupole resonance.

Cocrystals of 4,4'-bipyridyl and several carboxylic acids were grown from the methanol solution of the cocrystal formers. Complete (14)N NQR spectra of these cocrystals have been measured using (1)H-(14)N nuclear quadrupole double resonance. The principal values of the quadrupole coupling tensor are calculated from the (14)N NQR frequencies. A large variation in the (14)N quadrupole coupling constant between 1.3 MHz and 4.7 MHz is observed. A very low (14)N quadrupole coupling constant, characteristic for proton transfer O-H···N → O(-)···H-N(+), is observed in 4,4'-bipyridyl-oxalic acid (1 : 1). In 4,4'-bipyridyl-5-chlorosalycilic acid (1 : 1) the (14)N NQR data show the presence of a short, strong N···H···O hydrogen bond. A correlation of the principal values of the (14)N quadrupole coupling tensor is observed. The correlation is analyzed in the model, where the deformation of the lone pair electron orbital and the change of the population of the π-electron orbital produce the variation of the (14)N quadrupole coupling tensor in the hydrogen bonded 4,4'-bipyridyl. The temperature variation of the (14)N quadrupole coupling tensor in 4,4'-bipyridyl-5-chlorosalycilic acid (1 : 1) is analyzed. Proton displacement within the N···H···O hydrogen bond and the change of the population of the π-electron orbital at the two nitrogen positions in a 4,4'-bipyridyl molecule in the temperature interval between 157 K and 323 K are determined.

THE CM-, MM-, AND SUB-MM-WAVE SPECTRUM OF ALLYL ISOCYANIDE AND RADIOASTRONOMICAL OBSERVATIONS IN ORION KL AND THE SgrB2 LINE SURVEYS

Organic isocyanides have an interesting astrochemistry and some of these molecules have been detected in the interstellar medium (ISM). However, rotational spectral data for this class of compounds are still scarce. We provide laboratory spectra of the four-carbon allyl isocyanide covering the full microwave region, thus allowing a potential astrophysical identification in the ISM. We assigned the rotational spectrum of the two cis (synperiplanar) and gauche (anticlinal) conformations of allyl isocyanide in the centimeter-wave region (4-18 GHz), resolved its 14N nuclear quadrupole coupling (NQC) hyperfine structure, and extended the measurements into the millimeter and submillimeter-wave (150-900 GHz) ranges for the title compound. Rotational constants for all the monosubstituted 13C and 15N isotopologues are additionally provided. Laboratory observations are supplemented with initial radioastronomical observations. Following analysis of an extensive dataset (>11000 rotational transitions), accurate ground-state molecular parameters are reported for the cis and gauche conformations of the molecule, including rotational constants, NQC parameters, and centrifugal distortion terms up to octic contributions. Molecular parameters have also been obtained for the two first excited states of the cis conformation, with a dataset of more than 3300 lines. The isotopic data allowed determining substitution and effective structures for the title compound. We did not detect allyl isocyanide either in the IRAM 30 m line survey of Orion KL or in the PRIMOS survey toward SgrB2. Nevertheless, we provided an upper limit to its column density in Orion KL.

Investigation of the electronic structure and the structural stability of selected penicillins by density functional calculations of 14N nuclear quadrupole resonance parameters

A computational study was carried out by density functional theory (DFT) to investigate the relative stability and reactivity in three selected penicillins: penicillin-G, penicillin-V and carbenicillin. The geometry of the investigated molecules was optimized at the B3LYP/6-31G(d) level of theory. Then, the nuclear quadrupole resonance (NQR) parameters of 14N and 2H nuclei and natural bond orbital (NBO) analysis in these molecules were calculated on the geometrically optimized models at the B3LYP level using 6-311++G(d,p) basis sets in the gas phase. The NBO analysis shows that the occupancy of the LP(N) decreases with increasing p character of the lone pair of nitrogen. A comparison between the results obtained for these penicillins and related 6-APA structures of them indicates that the presence of a bulky side group in the acyl side chain can lead to more stability of the β-lactam ring. On the other hand, NBO analysis was applied to rationalize the 14N NQR parameters in the charge distribution around nitrogen atoms. Inspection of the present results illustrates that the largest component of EFG tensor (q zz ), the nuclear quadrupole coupling constant, C Q, and the NQR frequency values of nitrogens decrease with decreasing occupancy values of LP(N). We suggest that the reason for this trend can be found in increasing contribution of delocalized electrons of nitrogen in the intramolecular interactions and hence stability of these structures increases in the order: PG < PV < CA. Finally, a good relationship is found between most of the calculated 2H NQR parameters and the related intramolecular hydrogen bonds.

WURST–QCPMG sequence and “spin-lock” in 14N nuclear quadrupole resonance

14N nuclear quadrupole resonance (NQR) is a promising method for the analysis of pharmaceuticals or for the detection of nitrogen based illicit compounds, but so far, the technique is still not widely used, mostly due to the very low sensitivity. This problem is already acute in the preliminary NQR stage, when a compound is being examined for the first time and the NQR frequencies are being searched for, by scanning a wide frequency range step-by-step. In the present work, we experimentally show how to increase the efficiency of this initial stage by using a combination of a wideband excitation achieved with frequency swept pulses (WURST) and a “spin-lock” state obtained with a quadrupolar-CPMG (QCPMG) sequence. In the first part we show that WURST pulses provide a much larger excitation bandwidth compared to common rectangular pulses. This increased bandwidth allows to increase the frequency step and reduces the total number of steps in a scanning stage. In the second part we show that the “spin-lock” decay time T2eff obtained with the WURST–QCPMG combination is practically identical with the T2eff obtained with the most common “spin-lock” sequence, the SLSE, despite a very different nature and length of excitation pulses. This allows for a substantial S/N increase through echo averaging in every individual step and really allows to exploit all the advantages of the wider excitation in the NQR frequency scanning stage. Our experimental results were obtained on a sample of trinitrotoluene, but identical behavior is expected for all compounds where a “spin-lock” state can be created.

Hydrogen Bonds in Cocrystals and Salts of 2-Amino-4,6-dimethylpyrimidine and Carboxylic Acids Studied by Nuclear Quadrupole Resonance

(14)N and (17)O nuclear quadrupole resonance frequencies have been measured in 1:1 cocrystals and salts of 2-amino-4,6-dimethylpyrimidine and several carboxylic acids. A systematic decrease of the (17)O quadrupole coupling constant on increasing strength of the hydrogen bond is observed in cocrystals bound by O-H···N hydrogen bonds. The O-H distances deduced from the line widths of the (17)O NQR lines show that the hydrogen atom is in a hydrogen bond formed by a carboxylic groups for about 0.01 nm displaced from the oxygen atom toward the center of the hydrogen bond. In the O-H···N hydrogen bond formed by the hydroxyl group, which is only slightly longer than the hydrogen bonds formed by the carboxyl group, the hydrogen atom is much less displaced. A linear relation between the (14)N quadrupole coupling constant and the sum of the inverse third powers of the H···A (A = O or N) distances is deduced for the amino group. A linear correlation of the principal values of the (14)N quadrupole coupling tensor in -NH2, as observed in the solid phase and in the gas phase, is analyzed in a simple model assuming a displacement of the electron charge in the N-H σ bond and simultaneous deformation of the nitrogen lone pair electron orbital. At the ring nitrogen position, hydrogen bonding and proton transfer produce a large decrease of the (14)N quadrupole coupling constant. A linear correlation of the principal values of the (14)N quadrupole coupling tensor is observed in cocrystals and salts of 2-amino-4,6-dimethylpyrimidine. This correlation differs from the correlation observed in substituted pyrimidine, where the hydrogen atoms are replaced by other atoms or functional groups. The difference is analyzed in a model, which assumes that the hydrogen bonding and substituents affect the nitrogen lone pair and π electron orbitals. The analysis shows that the two effects are nearly independent.

The Influence of Pressure in Paracetamol Tablet Compaction on 14N Nuclear Quadrupole Resonance Signal

14N nuclear quadrupole resonance (14N NQR) of several commercially available paracetamol tablets was measured. The spectra of two polymorphs are presented. The linewidths of the correspondent 14N NQR lines in all the measured samples containing the room-temperature stable monoclinic polymorph were noticeably different. We proved experimentally that the linewidth differences are the consequence of different compacting pressure in the production of tablets.

Tautomerism and Possible Polymorphism in Solid Hydroxypyridines and Pyridones Studied by 14N NQR

(14)N nuclear quadrupole resonance frequencies have been measured in solid 2-pyridone, 3-hydroxypyridine, and 4-pyridone by (1)H-(14)N nuclear quadrupole double resonance. Two slightly nonequivalent nitrogen positions are observed in solid 3-hydroxypyridine, whereas only one nitrogen position has been observed in 2-pyridone and 4-pyridone within the experimental resolution. Rather low (14)N quadrupole coupling constants in pyridones are the consequence of the delocalization of the nitrogen lone pair electrons in the aromatic rings. Two different compounds have been obtained by crystallization of 4-pyridone from ethanol in a normal and in a dry atmosphere. The compound obtained in the dry atmosphere is identical to the commercial sample. The compound obtained in the normal atmosphere cannot be converted to the commercial polymorph by melting. It is thus not a polymorph of anhydrous 4-pyridone. The temperature coefficient of the (14)N quadrupole coupling constant is negative in 3-hydroxypyridine and positive in 2- and 4-pyridone. Therefore, in 3-hydroxypyridine, molecular librations dominate the temperature variation of the quadrupole coupling constant, whereas in 2- and 4-pyridone, the changes in the hydrogen bonding interactions with temperature seem to give the dominant effect.

Microwave spectrum, structure, tautomeric, and conformational composition of 4-vinylimidazole

The microwave spectra of the two conformers each, of the 1H and 3H tautomers of 4-vinylimidazole, have been measured in the 48-72 GHz spectral region. The 4-vinylimidazole was generated in situ by the facile decarboxylation of urocanic acid at its vaporization temperature of 220 °C. The recognition of this reaction casts doubt on the reliability of a previous published spectroscopic study apparently mistakenly thought to be of uncontaminated vaporized urocanic acid, a natural product of great interest in skin cancer etiology. Quantum chemical theoretical predictions of the structures of each of ten possible conformers∕tautomers of urocanic acid and four of 4-vinylimidazole were performed at the ab initio MP2∕cc-pVTZ level, with vibrational predictions at the B3LYP∕cc-pVTZ and M062X∕cc-pVTZ levels. The predicted values of rotational constants for all the urocanic acid species were found to be quite inconsistent with those of the four observed spectra. For the 4-vinylimidazole isomers, the calculated relative energies suggested that all four species would have substantial equilibrium mole fractions at 220 °C. The isomers were identified by matching the observed and calculated rotational constants. The resulting assignment was found to be consistent with the predicted and observed (14)N nuclear quadrupole hyperfine multiplet patterns for a suitable rotational transition, and with the observed versus empirically calculated inertial defects. With one exception, the predicted structures were found to be planar. Resembling the case of 1-vinylimidazole, where one conformer is nonplanar, one isomer of 4-vinylimidazole was found to be quasiplanar. This seems to belong to a class of spontaneous symmetry-breaking observed in the molecular structure of some otherwise planar vinyl aromatic compounds.

Rotational spectrum and structure of the pyridine–CO2 van der Waals complex

The rotational spectrum of the pyridine–CO2 van der Waals complex has been recorded in a cold supersonic jet using pulsed-nozzle Fourier transform microwave spectroscopy. Spectroscopic constants are reported for the parent species and for all four singly-substituted 13C derivatives. The data indicate a planar structure in which the pyridine nitrogen approaches the CO2 carbon with the C2 axis of the pyridine perpendicular to the CO2. No evidence of internal rotation is observed. The zero point vibrationally averaged N⋯C van der Waals distance is 2.7977(64)Å and the O⋯ortho-hydrogen distance is 3.090(6)Å. The 14N nuclear quadrupole coupling constants are consistent with a bending vibrational amplitude of the pyridine moiety of about 9° away from the C2v geometry. The observed structure is in excellent agreement with that previously calculated to correspond to the lowest of several nearly isoenergetic isomers, suggesting that it is, indeed, the global minimum-energy structure. Somewhat surprisingly, the C⋯N van der Waals bond distance is about 0.2Å shorter than those observed in the related systems HCN⋯CO2 and H3N⋯CO2.

Microwave Spectra and Gas Phase Structural Parameters for N-Hydroxypyridine-2(1H)-thione

The microwave spectrum for N-hydroxypyridine-2(1H)-thione (pyrithione) was measured in the frequency range 6-18 GHz, providing accurate rotational constants and nitrogen quadrupole coupling strengths for three isotopologues, C(5)H(4)(32)S(14)NOH, C(5)H(4)(32)S(14)NOD, and C(5)H(4)(34)S(14)NOH. Pyrithione was found to be in a higher concentration in the gas phase than the other tautomer, 2-mercaptopyridine-N-oxide (MPO). Microwave spectroscopy is best suited to determine which structure predominates in the gas phase. The measured rotational constants were used to accurately determine the coordinates of the substituted atoms and provided sufficient data to determine some of the important structural parameters for pyrithione, the only tautomer observed in the present work. The spectra were obtained using a pulsed-beam Fourier transform microwave spectrometer, with sufficient resolution to allow accurate measurements of the (14)N nuclear quadrupole hyperfine interactions. Ab initio calculations provided structural parameters and quadrupole coupling strengths that are in very good agreement with measured values. The experimental rotational constants for the parent compound are A = 3212.10(1), B = 1609.328(7), and C = 1072.208(6) MHz, yielding the inertial defect Δ(0) = -0.023 amu·Å(2) for the C(5)H(4)(32)S(14)NOH isotopologue. The observed near zero inertial defect clearly indicates a planar structure. The least-squares fit structural analysis yielded the experimental bond lengths R(O-H) = 0.93(2) Å, R(C-S) = 1.66(2) Å, and angle (N-O-H) = 105(4)° for the ground state structure.

Zeeman shift – A tool for assignment of 14N NQR lines of nonequivalent 14N atoms in powder samples

The use of Zeeman perturbed 14N nuclear quadrupole resonance (NQR) to determine the ν + and ν − 14N lines in polycrystalline samples with several nonequivalent nitrogen atoms was investigated. The 14N NQR line shift due to a weak external Zeeman magnetic field was calculated, assuming isotropic distribution of EFG tensor directions. We calculated the broad line distribution of the ν + and ν − line shifts and experimentally confirmed the calculated Zeeman field dependence of singularities (NQR peaks) in cyclotrimethylenetrinitramine (RDX) and aminotetrazole monohydrate (ATMH). The calculated and measured frequency shifts agreed well. The proposed measurement method enabled determination of which 14N NQR lines in ATMH belong to ν + and which to ν − transitions.

The Singular Gas-Phase Structure of 1-Aminocyclopropanecarboxylic Acid (Ac3c)

The natural nonproteinogenic α-amino acid 1-aminocyclopropanecarboxylic acid (Ac(3)c) has been vaporized by laser ablation and studied in the gas phase by molecular-beam Fourier transform microwave spectroscopy. Comparison of the experimental rotational and (14)N nuclear quadrupole coupling constants with the values predicted ab initio for these parameters has allowed the unambiguous identification of three Ac(3)c conformers differing in the hydrogen bonding pattern. Two of them resemble those characterized before for the coded aliphatic α-amino acids. Remarkably, a third conformer predicted to be energetically accessible for all of these amino acids but never observed (the so-called "missing conformer") has been found for Ac(3)c, close in energy to the global minimum. This is the first time that such a conformer, stabilized by an N-H···O(H) hydrogen bond, is detected in the rotational spectrum of a gaseous α-amino acid with a nonpolar side chain. The conjugative interaction established between the cyclopropane ring and the adjacent carbonyl group seems to be responsible for the unique conformational properties exhibited by Ac(3)c.

Partial Proton Transfer in a Molecular Complex: Assessments From Both the Donor and Acceptor Points of View

Microwave spectra have been observed for the gas phase complexes (CH(3))(3)(14)N-H(14)NO(3) and (CH(3))(3)(15)N-H(14)NO(3) and rotational and nuclear quadrupole coupling constants are reported. The structure and binding energy have also been calculated at the MP2 level of theory using the 6-311++G(d,p) and 6-311++G(2df,2pd) basis sets both with and without corrections for basis set superposition error. The HNO(3) forms a near-linear hydrogen bond to the amine nitrogen with a rather short hydrogen bond distance of about 1.5-1.6 Å (depending on the basis set and method of computation). The C(3) axis of the trimethylamine lies in the plane of the nitric acid. For both the H(14)NO(3) and the (CH(3))(3)(14)N moieties of the parent species, the component of the nuclear quadrupole coupling tensor perpendicular to the molecular symmetry plane, χ(cc), is sensitive to the electronic structure at the corresponding nitrogen but independent of relative orientation within the plane. Its value, therefore, provides a convenient experimental measure of the degree of proton transfer within the complex. For the HNO(3), χ(cc) lies 62% of the way between those of free HNO(3) and aqueous NO(3)(-), indicating a substantial degree of proton transfer. A similar comparison of the quadrupole coupling constant of (CH(3))(3)N in the (CH(3))(3)N-HNO(3) complex with those of free (CH(3))(3)N and (CH(3))(3)NH(+) indicates only about 31% proton transfer, about half that determined from the HNO(3) coupling constant. Though surprising at first, this disparity is to be expected if the quadrupole coupling constants vary nonlinearly with the position of the proton relative to the donor and acceptor atoms. Calculations of the (14)N nuclear quadrupole coupling constants as a function of proton position using density functional theory are reported and confirm that this is the case. We suggest that when proton transfer is assessed according to changes in individual monomer molecular properties, the overall process may be best described in terms of a dual picture involving proton release by the acid and proton acquisition by the base.

A 14N nuclear quadrupole resonance study of phase transitions and molecular dynamics in hydrogen bonded organic antiferroelectrics 55DMBP–H2ca and 1,5-NPD–H2ca

The temperature dependence of the (14)N NQR frequencies has been measured in antiferroelectric and paraelectric 55DMBP-H(2)ca and 1,5-NPD-H(2)ca. In both compounds we observe two non-equivalent nitrogen positions (N(+)-H···O(-) and N···H-O) in the antiferroelectric phase. The two nitrogen positions become equivalent (N···H···O) in the paraelectric phase. The critical exponent of the local antiferroelectric order parameter has been determined from the NQR data. The principal values of the quadrupole coupling tensor correlate in both compounds. The correlation diagrams clearly show how a proton migrates from the antiferroelectric position towards the paraelectric position in the bifurcated hydrogen bond on increasing the temperature. A slow motion has been observed in 55DMBP-H(2)ca by the (1)H and (14)N spin-lattice relaxation. An analysis of the spin-lattice relaxation data suggests a slow exchange between two non-planar conformations of the bipyridine molecule.

The Application of the Two Frequency Composite Pulses for NQR Detection of Nitrogen-Based Compounds

Multifrequency 14N nuclear quadrupole resonance (NQR) signals induced due to irradiation by two-frequency composite pulses have been studied. The experiments have been performed on RDX explosive at room temperature. It has been shown that the NQR signal at the observed transition can be essentially enhanced by use of the two-frequency composite pulses. Moreover, it is possible to excite and maintain the precession signal at the third, nonirradiated frequency. We have also shown that the two-frequency composite pulse can be applied as the preparatory pulse for increasing the NQR signal in multipulse sequences.