Potassium Hydrogen Maleate Research Articles

Preparation of potassium hydrogen maleate

KHM is useful to have in the stockroom: this note provides instructions on its preparation.

X-ray diffraction and vibrational spectra of the guanidine:maleic acid (1:1) complex—two crystal forms

From an equimolar mixture of guanidine and maleic acid, two different crystal modifications of guanidinium hydrogen maleate, CH6N3+·C4H3O4−, were isolated and their crystal structures determined using three-dimensional diffractometer data. Crystals of both forms are orthorhombic:Pca21,a=19.012(8)b=3.758(1),c=11.119(5) A,Z=4 for form I;Fdd2,a=18.505(2),b=20.007(3),c=8.406(2) A,Z=16 for form II. The structures were solved by direct methods and refined by full-matrix least squares to conventionalR values of 0.064 for form I and 0.043 for form II, with 488 and 605 reflections respectively. The structures consist of planar guanidinium cations C(NH2)3+ and hydrogen-maleate residues C4H3O4−. There are no significant differences in distances and angles within the guanidinium and hydrogen-maleate units. However, a substantial difference appears in the length and geometry of the intramolecular hydrogen bond O⋯H⋯O. The packing of cations and anions is also completely different in the two forms. Infrared and Raman spectra of the title compound are presented, and salient differences between them and the spectrum of the symmetrically bonded potassium hydrogen maleate are discussed briefly.

Vibrational spectra of potassium hydrogen maleate crystal and solution

Polarized i.r. transmission and reflection and Raman spectra of potassium hydrogen maleate (KHM) single crystals were measured. The reflection spectra were analysed by means of the four parameter dielectric function. Less complete data were obtained for deuterated single crystals. Solutions of KHM in H 2O, D 2O and DMSO were also studied. In analysing the data attention was focused on the hydrogen bond modes. The in-plane modes are strongly mixed with skeletal modes. The analysis of the reflection spectra shows that the B 1 u modes in the region below 1100 cm −1, where the hydrogen bond stretchings are located, are strongly anharmonic.

Transport properties and thermal studies on pure and Cu2+-doped potassium hydrogen maleate single crystals

Conductivity, thermally stimulated depolarisation (tsdc) and thermal studies are carried out on pure and Cu2+-doped potassium hydrogen maleate single crystals. Bothdta anddsc reveal first-order phase transition at 503 K.dsc further shows that the crystal decomposes even before the transition is complete. A ‘knee’ is observed around 505 K in conductivity plots at this transition. Thetsdc of Cu2+-doped system gave a single peak at 241 K and the reasons are analysed. Activation energies from conductivity andtsdc suggest proton conduction. However no direct evidence in coulometric experiment could be observed. But thermoelectric power measurements reveal the charge carriers to be positive.

Inelastic neutron scattering studies of some intramolecular hydrogen bonded complexes: A new correlation of γ(OHO) vs R (OO)

Inelastic neutron scattering spectra of several complexes {potassium hydrogen maleate, nickel dimethylglyoxime, and tripotassium cis-dichloro (hydrogen disulfito) and tripotassium cis-dichloro (hydrogen disulfito) platinate (II)} with strong intramolecular hydrogen bonds have been obtained and the out-of-plane bending modes γ(OHO) of the hydrogen bond assigned. Using previously published data, primarily for intermolecularly hydrogen bonded complexes, it is shown that as the oxygen–oxgyen distance {R(OO)} decreases γ(OHO) increases. Our new data do not follow this correlation since γ(OHO) is also observed to decrease as R(OO) decreases. It is suggested that this observation is due not to a fundamental difference in the behavior of inter- and intramolecular hydrogen bonds but rather to the different range of values of R(OO) found for each type. For the intermolecular systems R(OO) is greater than ∼ 2.44 Å, while for the intramolecular complexes (including potassium monochloromaleate) R(OO) is less than this value. It appears that hydrogen bonds undergo a change in dynamic properties at R(OO) 2.44 Å and some geometric properties of ring systems are discussed which support this idea.

Electron spin resonance and optical absorption studies of Cu 2+ doped in potassium hydrogen maleate single crystals

The results of electron spin resonance studies on Cu 2+ doped in potassium hydrogen maleate single crystals at 300 K are presented. The spin Hamiltonian parameters evaluated indicate a square planar environment for Cu 2+ ion in this lattice. From the crystal structure data and the direction cosines of the principal axes of the g tensor, it is concluded that the Cu 2+ ion enters the lattice in an interstitial position. Charge compensation is achieved by the release of protons. Using the optical absorption and ESR data, bonding parameters and orbital reduction factors are also evaluated.

Hydrogen bonding in potassium hydrogen maleate and some simple derivatives studied by inelastic neutron scattering spectroscopy

Inelastic neutron scattering spectra (300–2500 cm −1) of KH(CHCO 2) 2, KD(CHCO 2) 2, KH(CDCO 2) 2 and KH(CHCO 2 · CClCO 2) have been obtained and the vibrations of the hydrogen bond, with the exception of ν 2(OHO), assigned. This is the first assignment of these vibrations in a centrosymmetric intramolecularly hydrogen bonded complex. ν as(OHO) was found to be heavily mixed and to give rise to a strong doublet in the INS spectra.

17O and 2H quadrupole double resonance in some acid carboxylates

An 17O and 2H quadrupole double resonance study has been made of a number of strong O—H—O hydrogen bonds in acid carboxylates and related compounds and an analysis of the 17O⋯1H dipolar structure in one (potassium hydrogen maleate). The compounds selected show a range of hydrogen-bond structures varying from symmetric and centred to asymmetric and non-centred; with certain assumptions, it is possible to follow the consequent changes in sign and orientation of the 17O quadrupole coupling tensor. The arguments are based on the proposal that the 17O quadrupole coupling constant is negative in symmetric hydrogen bonds, its orientation being related to that of the 17O tensor in the C—17OH group in carboxylic acids by a rotation of 25 ° with respect to the O—H direction. The behaviour of the 2H quadrupole coupling constant and asymmetry parameter in strong O—H—O hydrogen bonds is discussed in terms of the position of the hydrogen atom.

Hydrogen bonds in fluorosubstituted dicarboxylic acids: vibrational spectra and crystal structure of potassium hydrogen difluorofumarate and potassium hydrogen difluoromaleate

The crystal structures of potassium hydrogen difluoromaleate (KHDFM) and potassium hydrogen difluorofumarate (KHDFF) have been determined by single crystal X-ray methods. The hydrogen difluoromaleate ion has a closed ring structure with a short intramolecular hydrogen bond. The O· O distance is 2.415(1) Å The hydrogen difluorofumarate ions form infinite chains via short intermolecular hydrogen bonds.The O· ;O distance is 2.450(5) Å. In both compounds the hydrogen bonds are across crystallographic symmetry elements. The vibrational spectra of both compounds and their deuterated derivatives have been recorded and assigned. The IR spectrum of KHDFF is of Speakman's A 2 type. The spectra of KHDFM and potassium hydrogen maleate are very similar.

The electric field gradient tensor at the olefinic deuterons of potassium hydrogen maleate

The quadrupole coupling tensors at the four magnetically inequivalent olefinic deuterons in potassium hydrogen maleate have been determined. The quadrupole coupling constant is 176.4 ± 0.3 kHz and the asymmetry parameter 0.035 ± 0.003. The largest (absolute value) principal component is along the CD bond direction (±0.3°). The intermediate component is near to the normal to the molecular plane. Line splittings have been observed and interpreted in terms of the dipolar interaction of a particular deuteron with a proton from a neighboring molecule.

17O and 2H quadrupole double resonance in potassium hydrogen maleate and related compounds

2 H, 17O, 39K and 85,87Rb quadrupole resonance signals have been recorded in potassium and rubidium hydrogen maleates and several related compounds by double resonance methods. The 17O signals were detected in natural abundance by a novel technique of high sensitivity based on the “solid effect” and thermal mixing. The symmetric hydrogen bonds in potassium hydrogen maleate and chloromaleate are shown to have characteristic values of the 17O and 2H quadrupole coupling constants which can be used to identify this kind of bonding in other related compounds. The orientation of the quadrupole tensor within the bonding system is also discussed.

270 MHz multiple pulse study of the olefinic protons in potassium hydrogen maleate

The nuclear magnetic shielding tensors of the protons in potassium hydrogen maleate (KHM) have been determined in single crystals by means of multiple pulse line narrowing techniques at 270 MHz. The increase of resolution in solid state NMR spectra on switching the spectrometer frequency from 90 MHz to 270 MHz is demonstrated. The results for the carboxylic protons in KHM agree fully with those of a previous 90 MHz study. Unlike at 90 MHz separate lines from individual olefinic proton sites could be resolved at 270 MHz allowing a straightforward determination of the corresponding shielding tensors. The principal shielding components found are 0.9, −2.0, −3.1 ± 0.2 ppm relative to liquid water. An assignment of the four experimental shielding tensors to the four olefinic sites in the crystal is proposed on the basis of molecular and local symmetry. According to this assignment the most shielded direction of the olefinic protons is in the molecular plane of the maleate anion and perpendicular to the CH bond axis.

Proton shielding tensors in potassium hydrogen maleate

The proton NMR in single crystals of potassium hydrogen maleate has been sttudied by means of multiple-pulse line-narrowing techniques. The magnetic shielding tensors of all magnetically inequivalent protons in the unit cell could be determined independently. Two of these protons are carboxylic, forming hydrogen bonds. The orientations of the shift tensors are consistent with the position of the hydrogens at the midpoints of the 0–0 intervals. The range of anisotropy of 32 ppm, found for the shift tensor of the caboxylic hydrogen, is larger than that found for hydrogen bonds in acids and seems to be characteristics of acidic salts. The other protons in the unit cell are olefinic. Two features distinguish this type of protons from those studied so far: (1) The magnetic shielding tensor is not even approximately axially symmetric, the principal values being −2.4, −5.1, −7.3 ± 05 ppm (from adamantane); and (2) the principal directions reflect all characteristic directions of the carboncarbon double bond (while the CH direction is of no importance). The principal value in the direction perpendicular to the sp 2 system is the least shielded one.

ELDOR enhancement arising from admixture of nuclear spin states

ELDOR enhancement effects arising from the admixture of nuclear spin states have been studied with the irradiated single crystals of potassium hydrogen maleate, which give strong forbidden ESR lines. When the magnetic field was applied along the c axis, all of the ELDOR transitions involving no level in common showed enhancement while those involving a level in common showed reduction. In the different orientations of the crystal, some of the enhanced lines changed into reduced ones with decreasing forbidden ESR transition probabilities. The temperature changes of the ELDOR spectra were also studied. With decreasing temperature, all of the enhanced lines at room temperature changed into reduced lines giving maximum R factors at around − 100°C. These results were interpreted in terms of large intramolecular cross relaxation probabilities that are due to the admixture of the nuclear spin states in our system. To calculate the R factors for our system, the generalized expression has been derived for the R factor in multiobserving and multipumping ELDOR experiments. The variation of the observed R factors with pumping transitions is qualitatively understood from the R factors calculated with appropriate assumptions on the relaxation probabilities. The calculated R factors also show that increasing nuclear spin relaxation probabilities with decreasing temperature are responsible for the temperature change of the observed R factors.

ESR Study of Radical Structure and Symmetrical Hydrogen Bond in a Single Crystal of Potassium Hydrogen Maleate Irradiated at 77°K

ESR study has been carried out on the structure of radicals produced in an irradiated single crystal of potassium hydrogen maleate which is known as a typical compound having a symmetrical intramolecular hydrogen bond. It was found that in the radical formed by removal of the acidic proton the unpaired electron occupies the σ antibonding orbital between the in-plane p orbitals of the two ring oxygen atoms, when the crystal is irradiated at 77°K. The principal values of the g tensor for this radical were determined to be gxx=2.0043, gyy=2.0039, and gzz=2.0079. The hyperfine couplings for the two vinylene protons are equivalent and have the nearly isotropic value of 5±1 G. On the other hand, if the crystal is warmed up to room temperature after irradiation at 77°K, the same species of radical is formed but the unpaired electron occupies the delocalized π orbital. This radical is the same as that found in room-temperature irradiation which was previously reported. Both the radicals possess the mirror symmetry perpendicular to the C=C bond as does the mother molecule. The formations of such symmetrical radicals are closely related to the symmetrical structure and the extremely short O···O distance of the parent molecule, which are due to the symmetrical hydrogen bond. The cause of the formations of the σ and π electron radicals from the same mother molecule is also discussed.

ESR Study of the Effects of Alkali Metal Substitution on the Radical Species Produced and the Stereospecific Formation of Radicals in Irradiated Carboxylic Acids

A remarkable difference has been found in the radical species produced when the alkali metal was changed from potassium to sodium in irradiated single crystals of acid salts of maleic acids. The room temperature stable species in an irradiated single crystal of potassium hydrogen maleate was previously found to be the symmetrical delocalized π radical formed by removal of the acidic proton. In contrast to this result, the stable species found in sodium hydrogen maleate was the radical formed by the addition of hydrogen to the double bond. Furthermore, it has been found that the addition takes place selectively to one of the two vinylene carbon atoms, as is unexpected from the symmetrical structure of the hydrogen maleate ion. A similar difference was also observed in the unstable species at 77°K. The potassium crystal gave the symmetrical σ radical formed by removal of the acidic proton while the sodium crystal gave the unsymmetrical σ radical. Here again the unpaired electron is selectively located in one of the two COO groups. These results indicate that the formation of radicals in crystalline solids is strongly affected by the surrounding crystalline field, since the sodium crystal is known to have a different crystal structure with hydrated water.

The Electron Spin Resonance of an Irradiated Single Crystal of Potassium Hydrogen Maleate

Electron spin resonance absorptions of x-irradiated single crystals of potassium hydrogen maleate and potassium deuterium maleate were observed. Both compounds gave the same hyperfine structures, although the slightly sharper line widths were observed for the deuterium exchanged compound.

Electron Magnetic Resonance of X-Irradiated Potassium Hydrogen Maleate

X irradiation of a single crystal of potassium hydrogen maleate HOOCCH = CHCOOK was found to produce a free radical in which the unpaired electron interacts with only two protons. The electron magnetic resonance of this radical, shown to be OOCCH = CHCOO-, indicates that it is oriented in the host crystal in the same way as the parent hydrogen maleate ion. The principal values of the proton hyperfine tensors of the radical are; -19.0, -28.4, and -8.0 Mc. The rather small principal values and their directions indicate that the unpaired electron in the radical is in a p pi orbital delocalized over the whole radical, including the carboxyl groupa. The directions of the principal values are in agreement with the cis-structure of the parent molecule. The utilization of emr spectra in differentiating between cis- and trans-structures of radicals is briefly discussed.

Infra-red and proton magnetic resonance spectra of solid substances containing very short hydrogen bonds

According to the type of spectra obtained, the substances investigated are divided into two groups. The infra-red spectra of the first group are characterized by the presence of two OH stretching bands in the region 3000 to 1900 cm −1 separated by 300 to 500 cm −1. The PMR signals are strong and narrow (3–5 G) at room temperature and slightly broader at −180°. To this group belong some metal hydrogen phosphates, arsenates, periodates, phthalates and p-nitrobenzoates. The i.r. spectra of the second group are characterized by the absence of the OH stretching band in the region above 1800 cm −1. The PMR signals are narrow and weak or very weak at room temperature and remain essentially unchanged at low temperature. To this group belong potassium hydrogen bisphenylacetate, bisbenzoate, sodium sesquicarbonate and potassium hydrogen maleate. The latter compound shows a particularly weak signal. These experimental results are interpreted in terms of a potential function for the proton motion having two minima of potential energy separated by a barrier of different size in each of the two groups. In the first one, the barrier is sufficiently small to allow for the tunnelling of protons with subsequent splitting of vibrational levels. With regard to PMR such a function results in a short correlation time which is temperature independent at low temperature when the proton is tunnelling at the lowest vibrational level. In the second group the potential barrier is vanishingly small, the hydrogen bond approaching the symmetrical type. The OH stretching frequency becomes small and the characteristic property of the vibration is lost owing to strong vibrational interactions with other modes. In the limiting case the tunnelling motion ceases. As there is no more large proton motion the relaxation time becomes long: the signal is weak and saturation occurs even at low r.f. power. This extreme case seems to be approached by KH-maleate.

Structure of Potassium Hydrogen Maleate by Neutron Diffraction

The preliminary results of a neutron diffraction study are presented which confirm the existence in potassium hydrogen maleate of a short, strong, hydrogen bond and show the ion to be at least statistically symmetrical. The hydrogen is strongly linked to both neighboring oxygen atoms, and there is an existing mode of correlated motion of considerable amplitude in which the oxygen atoms are displaced but hydrogen is not. (J.R.D.)