1H-17O nuclear quadrupole double resonance in phenylphosphinic acid and phenylphosphonic acid. 17O quadrupole coupling in P = O and P-O-H bonds

The (17)O nuclear quadrupole resonance (NQR) frequencies have been measured in phenylphosphinic acid and phenylphosphonic acid using nuclear quadrupole double resonance. The quadrupole coupling constants have been determined with an uncertainty of +/- 10 kHz and the asymmetry parameter eta with an uncertainty of +/- 0.01. The results are compared to the published results of the theoretical calculation and the high-field solid-state NMR measurements. The position of hydrogen in the O-H...O hydrogen bond in phenylphosphinic acid has also been determined. On the basis of the present and the previously published data we show that the principal values of the electric-field-gradient tensor in P = O and P-O-H bonds correlate. A correlation between the nuclear quadrupole parameters and the length of the P-O bond is also observed.     

17O quadrupole coupling and chemical shielding tensors in an H-bonded carboxyl group: alpha-oxalic acid

We have used single crystal (17)O NMR and density functional theory to investigate intermolecular interactions in a strongly H-bonded system. The chemical shielding (CS) and quadrupole coupling (QC) tensors are determined in oxalic acid dihydrate by single crystal methods. With cross polarization from abundant protons, high quality spectra are obtained in 1-2 min from 10 micromol samples. In the crystal lattice, oxalic acid is H-bonded directly to the hydrate with each carboxyl group accepting two H-bonds at C=O and donating one H-bond from COH. The effects of these intermolecular interactions on the experimentally determined QC and CS tensors are modeled by density functional theory with a procedure that accurately calculates, without scaling, the known QC tensors in both gas-phase water and ice. The ice calculation uses a cluster containing 42 waters (in excess of two complete hydration shells). The same procedure applied to a similarly constructed cluster of hydrated oxalic acid gives QC and CS tensors that are within 3-6% of the observed values while isolated molecule tensors are significantly different. Comparison of the isolated molecule tensors with those from either experiment or the cluster calculation shows the magnitude and directionality of intermolecular interactions on the tensors. The isotropic shift of the COH oxygen is deshielded by 29 ppm, and C=O is shielded by 62 ppm while the spans of the CS tensors are increased by 78 ppm and decreased by 73 ppm, respectively. Magnitudes of the quadrupole coupling constants, which are proportional to the electric field gradients at the (17)O sites, decrease by 2.2 and 1.2 MHz at COH and C=O, respectively.     

Isoimides of benzene-1,2-dicarboxylic acid and napthalene-1,8-dicarboxylic acid. Suitable conditions for preparation

N-Arylisophthalimides are formed in an aprotic solvent at 20°C from equivalent quantities of phthaloyl chloride and an arylamine in the presence of not less than 1 mole of a dehydrochlorinating reagent. N-Nitrophenylisonaphthalimides were obtained by the reaction of an equimolar mixture of naphthaloyl chloride and nitroanilines in boiling toluene.Physicochemical Institute for the Protection of Man and the Environment, Ministry of Education and Academy of Sciences of Ukraine, Odessa 270100. Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 9, pp. 1251–1253, September, 1997.     

Vibrational coupling in carboxylic acid dimers

The vibrational level splitting in the ground electronic state of carboxylic acid dimers mediated by the doubly hydrogen-bonded networks are investigated using pure and mixed dimers of benzoic acid with formic acid as molecular prototypes. Within the 0-2000-cm(-1) range, the frequencies for the fundamental and combination vibrations of the two dimers are experimentally measured by using dispersed fluorescence spectroscopy in a supersonic jet expansion. Density-functional-theory calculations predict that most of the dimer vibrations are essentially in-phase and out-of-phase combinations of the monomer modes, and many of such combinations show significantly large splitting in vibrational frequencies. The infrared spectrum of the jet-cooled benzoic acid dimer, reported recently by Bakker et al. [J. Chem. Phys. 119, 11180 (2003)], has been used along with the dispersed fluorescence spectra to analyze the coupled g-u vibrational levels. Assignments of the dispersed fluorescence spectra of the mixed dimer are suggested by comparing the vibronic features with those in the homodimer spectrum and the predictions of density-functional-theory calculation. The fluorescence spectra measured by excitations of the low-lying single vibronic levels of the mixed dimer reveal that the hydrogen-bond vibrations are extensively mixed with the ring modes in the S1 surface.     

Centrifugal distortion and oxygen-17 quadrupole coupling in ethylene oxide

The microwave spectrum of ethylene oxide has been reinvestigated to determine its quartic centrifugal distortion constants and the quadrupole coupling constants of the ¹⁷O species. The measurements of the quadrupole structure were done in natural isotopic abundance (0.037%) using a computer coupled spectrograph.     

Experimental charge density study of estrogens: 17beta-estradiol.urea

To relate the molecular electrostatic potential to the biological activities of estrogens, a comparative charge density study of different derivatives has been initiated. The second completed charge density analysis of this series for 17beta-estradiol*urea is presented here. This is a large organic system with 52 atoms in a noncentrosymmetric space group, therefore special tools such as an optimal coordinate system and slow, initially constrained refinement have been used to accomplish this study. Our results for the urea molecule reasonably agree with previous experimental and theoretical results. In the 17beta-estradiol molecule, the oxygen atoms appear to be close to sp3 in shape, exhibiting two consistent, distinct lone pairs despite different chemical environments. No significant interaction of the hydroxyl group oxygen with the orbitals of the aromatic ring is observed. Analysis of the electrostatic potential revealed that the negative potential in the lone pair region of the two oxygen atoms is quite different. The topological analysis of the electron density has been performed, and the atomic charges have been estimated.     

Experimental and computational characterization of the 17O quadrupole coupling and magnetic shielding tensors for p-nitrobenzaldehyde and formaldehyde

We have used solid-state 17O NMR experiments to determine the 17O quadrupole coupling (QC) tensor and chemical shift (CS) tensor for the carbonyl oxygen in p-nitro-[1-(17)O]benzaldehyde. Analyses of solid-state 17O NMR spectra obtained at 11.75 and 21.15 T under both magic-angle spinning (MAS) and stationary conditions yield the magnitude and relative orientation of these two tensors: CQ = 10.7 +/- 0.2 MHz, etaQ = 0.45 +/- 0.10, delta11 = 1050 +/- 10, delta22 = 620 +/- 10, delta33 = -35 +/- 10, alpha = 90 +/- 10, beta = 90 +/- 2, gamma = 90 +/- 10 degrees. The principal component of the 17O CS tensor with the most shielding, delta33, is perpendicular to the H-C=O plane, and the tensor component with the least shielding, delta11, lies along the C=O bond. For the 17O QC tensor, the largest (chi(zz)) and smallest (chi(xx)) components are both in the H-C=O plane being perpendicular and parallel to the C=O bond, respectively. This study represents the first time that these two fundamental 17O NMR tensors have been simultaneously determined for the carbonyl oxygen of an aldehyde functional group by solid-state 17O NMR. The reported experimental solid-state 17O NMR results provide the first set of reliable data to allow evaluation of the effect of electron correlation on individual CS tensor components. We found that the electron correlation effect exhibits significant influence on 17O chemical shielding in directions within the H-C=O plane. We have also carefully re-examined the existing experimental data on the 17O spin-rotation tensor for formaldehyde and proposed a new set of best "experimental" 17O chemical shielding tensor components: sigma11 = -1139 +/- 80, sigma22 = -533 +/- 80, sigma33 = 431 +/- 5, and sigma(iso) = -414 +/- 60 ppm. Using this new set of data, we have evaluated the accuracy of quantum chemical calculations of the 17O CS tensors for formaldehyde at the Hartree-Fock (HF), density-functional theory (DFT), M?ller-Plesset second-order perturbation (MP2), and coupled-cluster singles and doubles (CCSD) levels of theory. The conclusion is that, while results from HF and DFT tend to underestimate the electron correlation effect, the MP2 method overestimates its contribution. The CCSD results are in good agreement with the experimental data.     

Unusual regioselectivity in the opening of epoxides by carboxylic acid enediolates

Addition of carboxylic acid dianions appears to be a potential alternative to the use of aluminium enolates for nucleophilic ring opening of epoxides. These conditions require the use of a sub-stoichiometric amount of amine (10% mol) for dianion generation and the previous activation of the epoxide with LiCl. Yields are good, with high regioselectivity, but the use of styrene oxide led, unexpectedly, to a mixture resulting from the attack on both the primary and secondary carbon atoms. Generally, a low diastereoselectivity is seen on attack at the primary center, however only one diastereoisomer was obtained from attack to the secondary carbon of the styrene oxide.     

Chemical bonding in view of electron charge density and kinetic energy density descriptors

Stalke's dilemma, stating that different chemical interpretations are obtained when one and the same density is interpreted either by means of natural bond orbital (NBO) and subsequent natural resonance theory (NRT) application or by the quantum theory of atoms in molecules (QTAIM), is reinvestigated. It is shown that within the framework of QTAIM, the question as to whether for a given molecule two atoms are bonded or not is only meaningful in the context of a well‐defined reference geometry. The localized‐orbital‐locator (LOL) is applied to map out patterns in covalent bonding interaction, and produces results that are consistent for a variety of reference geometries. Furthermore, LOL interpretations are in accord with NBO/NRT, and assist in an interpretation in terms of covalent bonding. © 2008 Wiley Periodicals, Inc.J Comput Chem, 2009.     

Two mechanisms of proton disorder in carboxylic acid dimers as studied by 17O quadrupole double resonance reply to the comment by A. Gough,M.M.I. Haq and J.A.S. Smith

The assumptions and approximations used in a previous NQDR study are considered. Consistent examination of the method employed in the previous study shows that this procedure is not suitable for the observation of the effect related to the 180° flip mechanism.     

Nitrogen-14 quadrupole coupling constants in N2O3 by isotopic labelling

Several microwave transitions of O¹⁵N…NO₂ and ON…¹⁵NO₂ have been measured under high resolution using a pulsed-nozzle Fourier transform spectrometer. The ¹⁴N quadrupole coupling constants in the inertial axis system have been determined for both nitrogen atoms. The quadrupole coupling constants for ON …¹⁵NO₂ are eQq_aa = −0.5203(20) and eQq_bb = −4.1981(19) MHz, and for O¹⁵N…NO₂ are eQq_aa= −1.7999(13) and eQq_bb = 0.0808(17) MHz. The ¹⁴N quadrpole coupling constants determined by Kukolich for the main species ON…NO₂ should be reversed with respect to the NO and NO₂ groups. Combining the present data with the main species constants allows the complete quadrupole coupling tensor to be estimated for the NO₂ group; the tensor in N₂O₃ lies within 2° of the N…N bond direction and within 0.4° of the bisector of the ONO angle. Spin-rotation effects are significant for nitrogen in the NO group; C_aa is determined to be 8 ± 2 kHz for ¹⁴N in ON…¹⁵NO₂ and −15 ± 3 kHz for ¹⁵N obtained directly from splittings in the spectrum of O¹⁵N… ¹⁵NO₂.     

Electron correlation and charge density study of N2 and O2 by high energy electron scattering

The differential elastic scattering cross sections of N₂ and O₂ for 29 keV electrons have been measured. The experiment was performed using a Möllenstedt type energy analyzer to isolate the elastically scattered electrons. The difference between the measured results and calculations from molecular Hartree-Fock wave functions reveals the electron correlation in the molecules. Using the previously measured total scattering data, the inelastic scattering cross sections are derived. Several potential energies of the target are evaluated from the cross sections. Results at small angles are analyzed in terms of molecular moments and diamagnetic susceptibilities. The scattering behavior at small angles of the N₂ measurement agrees well with several ab initio calculations.     

17O nuclear quadrupole coupling constants of water bound to a metal ion: a gadolinium(III) case study

Rotational correlation times of metal ion aqua complexes can be determined from 17O NMR relaxation rates if the quadrupole coupling constant of the bound water oxygen-17 nucleus is known. The rotational correlation time is an important parameter for the efficiency of Gd3+ complexes as magnetic resonance imaging contrast agents. Using a combination of density functional theory with classical and Car-Parrinello molecular dynamics simulations we performed a computational study of the 17O quadrupole coupling constants in model aqua ions and the [Gd(DOTA)(H2O)]- complex used in clinical diagnostics. For the inner sphere water molecule in the [Gd(DOTA)(H2O)]- complex the determined quadrupole coupling parameter chi square root of (1 + eta2/3) of 8.7 MHz is very similar to that of the liquid water (9.0 MHz). Very close values were also predicted for the the homoleptic aqua ions of Gd3+ and Ca2+. We conclude that the 17O quadrupole coupling parameters of water molecules coordinated to closed shell and lanthanide metal ions are similar to water molecules in the liquid state.     

The mass spectra of carboxylic acids—V: Carboxyl-carboxyl interaction in the fragmentation of some cycloalkene-1,2-dicarboxylic acids

The fragmentation mechanisms of the six isomeric cyclohexene-1,2-dicarboxylic acids are discussed. Only the 1-cyclohexene acid, by virtue of the major sequential losses of H₂O and CO₂ from the molecular ion, is readily distinguishable from its isomers, all of whose mass spectra are closely similar. In contrast to cis and trans cyclohexane-1,2-dicarboxylic acids, whose mass spectra were markedly different, the cis and trans cyclohexene-1,2-dicarboxylic acids fragment in a similar fashion. The mass spectra of 1-cyclopentene-1,2-dicarboxylic acid and 1-cyclobutene-1,2-dicarboxylic acid also exhibit a strong carboxyl-carboxyl interaction; the fragmentation behaviour of the 1-cyclopenteneacid is, however, more complex than that of the 1-cyclohexene and 1-cyclobutene acids.     

83 Kr Nuclear quadrupole coupling in KrHF: evidence for charge transfer

The ⁸³Kr nuclear quadrupole coupling constant in KrHF has been measured to be x = 10.227(71) MHz. Using the known ⁸³Kr nuclear quadrupole moment, the field gradient at the Kr nucleus is evaluated and interpreted in terms of charge transfer in the weak Kr-HF van der Waals bond. Rotational assignments are reported for ⁸²Kr, ⁸³Kr, ⁸⁴Kr, and ⁸⁶Kr isotopic KrHF.     

Unsaturated carboxylic acid dienolates. Michael addition to enones through tandem 1,2-addition - oxy-cope rearrangement.

Michael addition of the dienolate derived from 2-butenoic acid to 1,3-diphenylpro-penone occurs through a 1,2-addition followed by a ³,3-sigmatropic rearrangement. α,γ -re-gioselectivity found for Michael addition to other styryl ketones depends on the steric parameter of the substituents at the carbonyl group, in agreement with the same tandem addition rearrangement mechanism.     

Vibronic coupling in charge transfer complexes. II

Vibronic coupling in excited charge transfer states of crystalline donor—acceptor complexes is reconsidered in relationship to recent electromodulation experiments for anthracene—PMDA. It is demonstrated that vibronic coupling, both linear and quadratic, may substantially affect the electric field dependence of the eigenstates, which may explain, at least semiquantitatively, the observed behaviour. Some consequences for experimental determination of dipole moments by means of electromodulation spectroscopy are pointed out.     

Tuning of the alpha-terthiophene radical cation coupling reaction using mixed micelles with varying charge density.

The photophysics and photochemistry of alpha-terthiophene (alphaT), compartmentalized in mixed nonionic/anionic micelles, have been investigated with focus on the influence of the micellar surface charge density on the formation of the radical coupling product alpha-hexathiophene (alphaH). By varying the ratio of nonionic-to-anionic surfactants, and assuming ideal mixing, the charge density of the mixed micelles was varied. From Poisson-Boltzmann calculations, performed using the cell model, the electrostatic potential and the counterion activity were estimated as a function of the distance from the micellar surface. Upon excitation, the triplet state of alphaT is formed, from which the alphaT radical cation can be formed by absorption of a second photon. The radical cation can form alphaH if it encounters another alphaT radical cation. Under the experimental conditions used, this implies that the alphaH formation only occurs if the compartmentalized radical cation is able to migrate from its host micelle to another micelle, either via the surrounding bulk or by fusion of two micelles followed by mixing of their contents before micellar fission. The formation yield of the radical cation depends on the charge density of the mixed micelle; a lower charge density, that is, an increased amount of nonionic surfactant, lowers the yield. The yield of the coupling product alphaH, however, does not follow the same trend. A maximum yield of alphaH is found at intermediate nonionic surfactant molar ratios. This behavior is understood in terms of the Poisson-Boltzmann simulation results and by comparing charge-density changes as a function of molar fraction with the changes in counterion activity. The alphaH yield is a result of the balance between an increased possibility of radical cation bulk migration and a lowered electrostatic stabilization of the radical.