Investigation of hydrogen bonds in proton transfer complexes derived from the reaction of 2- and 4-(N,N-dimethylamino)pyridines with chloranilic acid.

New complexes of 2-(N,N-dimethylamino)pyridine with chloranilic acid (2-DMAP + CLA) and 4-(N,N-dimethylamino)pyridine with chloranilic acid (4-DMAP + CLA) were synthesized and characterized by single crystal X-ray diffraction, infrared spectroscopy, thermal analysis methods and 1H, 13C and 15N NMR spectroscopy. The NMR spectroscopies were carried out in both, DMSO solution and in the solid state (CPMAS NMR). The 2-DMAP + CLA and 4-DMAP + CLA complexes crystallize in centrosymmetric P-1 and P21/c space group, respectively. In both complexes, the phenomenon of proton transfer is observed, which results in the formation of strong N+-H···O- hydrogen bonds. Thermal decompositions of 2-DMAP + CLA and 4-DMAP + CLA complexes were studied by thermogravimetric analysis. Temperature dependent IR spectra revealed that methyl groups of 4-DMAP + CLA perform fast stochastic reorientational motion at room temperature which is slowed on cooling while in 2-DMAP + CLA reonrientational motion of CH3 groups is much slower due to steric effects.

Low temperature investigations of dynamic properties in l-leucine - chloranilic acid complex.

Inelastic neutron scattering (INS) and infra-red (IR) spectroscopy methods were used for determination of dynamic structure of l-leucine - chloranilic acid complex. A theoretical dynamic pattern calculated by the density functional theory (DFT) method for periodic boundary conditions accompanied the experimental ones. Normal modes in the vibrational spectra were defined and described. The characteristic presence of the Hadzi's trio enriched by numerous submaxima is observed in the wavenumber range 3200-800 cm-1. Bands assigned to CH3 torsion vibrations in the leucine cation were observed at 231 cm-1 and 258 cm-1 in the INS spectrum. Temperature-dependent far-infrared spectra in the temperature range 9 K-290 K were obtained. Vibrational bands were analyzed as a function of temperature. Activation energies for reorientational motion of CH3 and CH2 groups were determined by means of the band shape analysis performed for torsional and twisting vibrations of these groups. The estimated energy is equal to Ea = 2.7 ±â€¯0.2 kJ/mol and Ea = 2.17 ±â€¯0.12 kJ/mol for CH3 and CH2 groups, respectively. A phase transition at about 130 K in the l-leucine - chloranilic acid complex was observed.

4,4'-, 5,5'-, and 6,6'-dimethyl-2,2'-bipyridyls: the structures, phase transitions, vibrations, and methyl group tunneling of their complexes with chloranilic acid.

The crystal and molecular structures of 4,4(')- and 6,6(')-dimethyl-2,2(')-bipyridyl complexes with 2,5-dichloro-3,6-dihydroxy-p-benzoquinone (chloranilic acid, CLA) have been determined and compared with those of the complex with the 5,5(')-derivative, which is known to possess interesting antiferroelectric properties. In the crystalline state, all three compounds form hydrogen bonded chains with N(+)-H···O(-) and O-H···N bridges on both sides of the bipyridyl constituent. The comparison of three derivatives indicates that the N(+)-H···O(-) hydrogen bonds are shortest for the 5,5(')-dimethyl complex. The 4,4(')- and 6,6(')-derivatives do not show any ferroelectric feature. The 6,6(')-one is, however, characterized by a continuous phase transition, revealed in the differential scanning calorimetry, dilatometric, and dielectric characteristics. The tunneling splitting measured by neutron backscattering in the energy range ±30 µeV for the neat dimethyl bipyridyls and their complexes with CLA indicates that the different splittings are primarily due to the crystal packing effect and that charge transfer between interacting compounds plays only a minor role.

The structure of diaminodurene and the dynamics of the methyl groups.

Diaminodurene crystallizes in the orthorhombic space group Pbca, with eight molecules in the unit cell. Four inequivalent methyl groups with different environments exist in a molecule. The amino groups are also different, which is well reflected in infrared spectra. Two tunneling modes are resolved at 23.7 and 7.0 microeV at 4.5 K. Their intensities are consistent with the presence of two further unresolved tunneling modes. Quasielastic spectra are composed of three Lorentzians of equal intensities. The two low activation energies and tunnel modes are modeled into consistent rotational potentials. The third activation energy and a librational band are used to guess the strength of the two stronger rotational potentials. The internal modes related to the torsional/librational vibrations mix with ring torsions in the range of 70-220 cm(-1). This way the tunnel modes couple to ring torsions whose energy determines the broadening of both tunnel bands. The calculations for free molecules yield mode frequencies a little bit lower than the experimental inelastic neutron scattering (INS) values. Application of theoretical methods elaborated for the crystalline state leads to a satisfactory consistency. It is also valid for bending modes of NH(2) groups, which in the solid state show much higher frequencies than in the gas phase, as expected.

Inelastic neutron scattering study of methyl groups rotation in some methylxanthines.

The three isomeric dimethylxanthines and trimethylxanthine are studied by neutron spectroscopy up to energy transfers of 100 meV at energy resolutions ranging from 0.7 microeV to some meV. The loss of elastic intensity with increasing temperature can be modeled by quasielastic methyl rotation. The number of inequivalent methyl groups is in agreement with those of the room temperature crystal structures. Activation energies are obtained. In the case of theophylline, a doublet tunneling band is observed at 15.1 and 17.5 microeV. In theobromine, a single tunneling band at 0.3 microeV is found. Orientational disorder in caffeine leads to a 2.7 microeV broad distribution of tunneling bands around the elastic line. At the same time, broad low energy phonon spectra characterize an orientational glassy state with weak methyl rotational potentials. Librational energies of the dimethylxanthines are clearly seen in the phonon densities of states. Rotational potentials can be derived which explain consistently all observables. While their symmetry in general is threefold, theophylline shows a close to sixfold potential reflecting a mirror symmetry.

X-ray diffraction and inelastic neutron scattering study of 1:1 tetramethylpyrazine chloranilic acid complex: temperature, isotope, and pressure effects.

The x-ray diffraction studies of the title complex were carried out at room temperature and 14 K for H/D (in hydrogen bridge) isotopomers. At 82 K a phase transition takes place leading to a doubling of unit cells and alternation of the hydrogen bond lengths linking tetramethylpyrazine (TMP) and chloranilic acid molecules. A marked H/D isotope effect on these lengths was found at room temperature. The elongation is much smaller at 14 K. The infrared isotopic ratio for O-H(D)...N bands equals to 1.33. The four tunnel splittings of methyl librational ground states of the protonated complex required by the structure are determined at a temperature T=4.2 K up to pressures P=4.7 kbars by high resolution neutron spectroscopy. The tunnel mode at 20.6 microeV at ambient pressure shifts smoothly to 12.2 microeV at P=3.4 kbars. This is attributed to an increase of the strength of the rotational potential proportional to r(-5.6). The three other tunnel peaks show no or weak shifts only. The increasing interaction with diminishing intermolecular distances is assumed to be compensated by a charge transfer between the constituents of deltae/e approximately 0.02 kbar(-1). The phase transition observed between 3.4 and 4.7 kbars leads to increased symmetry with only two more intense tunneling bands. In the isotopomer with deuterated hydrogen bonds and P=1 bar all tunnel intensities become equal in consistency with the low temperature crystal structure. The effect of charge transfer is confirmed by a weakening of rotational potentials for those methyl groups whose tunnel splittings were independent of pressure. Density functional theory calculations for the model TMP.(HF)2 complex and fully ionized molecule TMP+ point out that the intramolecular rotational potential of methyl groups is weaker in the charged species. They do not allow for the unequivocal conclusions about the role of the intermolecular charge transfer effect on the torsional frequencies.

Low frequency internal modes of 1,2,4,5-tetramethylbenzene, tetramethylpyrazine and tetramethyl-1,4-benzoquinone INS, Raman, infrared and theoretical DFT studies.

The results of inelastic neutron scattering (INS), Raman and infrared (IR) studies on 1,2,4,5-tetramethylbenzene (durene), tetramethylpyrazine (TMP) and tetramethyl-1,4-benzoquinone (TMBQ) in the solid state are reported. The observed frequencies are analyzed on the basis of DFT calculations. The low frequency region, below 400 cm(-1), related to the torsional and bending out-of-plane vibrations of the CH(3) groups, is of particular interest. The detailed analysis is possible due to the simulation of the INS spectra by using the auntie-CLIMAX program. It is shown that the observed low frequency INS bands are dramatically shifted, compared to the calculated ones, towards higher frequencies. Although one cannot exclude deficiencies of theoretical methods as applied to low frequency modes, it seems more probable the interpretation based on an existence of non-conventional CH(...)pi, CH(...)N, CH(...)O hydrogen bonds formed by the methyl groups in crystalline phases.

l-Cysteine: Neutron spectroscopy, Raman, IR and ab initio study.

Inelastic incoherent neutron scattering (IINS) spectra were obtained at 10K for normal and deuterated l-cysteine. Raman and infrared spectra were also recorded. Geometry of l-cysteine molecule was optimized for the zwitterion form using ab initio HF/6-31G* level. The theoretical frequencies of normal and d(4)-l-cysteine were compared with INS, Raman and IR spectra. Normal coordinate analysis and band assignments based on ab initio calculations and experimental data are presented.

Inelastic neutron scattering study of tetramethylpyrazine in the complex with chloranilic acid.

The tunnel splitting of the methyl librational ground states in the hydrogen bonded tetramethylpyrazine-chloranilic acid (TMP-CLA) complex are determined for temperatures T≤28 K by high resolution neutron spectroscopy. Three tunnel modes are resolved at T = 2.4 K. Their relative intensities show that the crystal structure must be different from the proposed space group. Tunnelling and methyl librational modes from the measured density of states are combined into rotational potentials. There are discrepancies of activation energies calculated for these potentials and those obtained from quasielastic scattering of neutrons at T≥50 K due to structural differences in the two respective temperature regimes. Rotational potentials in TMP-CLA are significantly weaker as in pure TMP.

Inelastic neutron scattering studies on dichloro-1,4-benzoquinones.

Inelastic neutron scattering (INS) spectra of 2,6-dichloro- and 2,5-dichloro-1,4-benzoquinone were compared with Raman and infra-red (IR) spectra and analysed in detail below 1800 cm(-1). The analysis was based on calculations tending towards simulation of spectra by using GAUSSIAN (HF, DFT/B3LYP and BLYP/6-31G(d,p)), and auntieCLIMAX programs. The correlations between calculated and experimental (either INS or Raman and IR) frequencies enabled to analyse the problem of scaling factors (SFs). The advantages of INS technique was shown in studies of low frequency vibrations with participation of H-atoms. The macroscopic lattice effect at low temperatures on INS spectra is discussed.

DFT studies of the structure and vibrational spectra of 8-hydroxyquinoline N-oxide.

The geometry, frequency and intensity of the vibrational bands of 8-hydroxyquinoline N-oxide (8-HQNO) and its deuterated derivative (8-DQNO) were obtained by the density functional theory (DFT) with the BLYP and B3LYP functionals and 6-31G(d,p) basis set. The optimized bond lengths and bond angles are in good agreement with the X-ray data. The IR and INS spectra of 8-HQNO and 8-DQNO computed at the DFT level reproduce the vibrational wavenumbers and intensities with an accuracy, which allows reliable vibrational assignments.

Inelastic neutron scattering (INS) studies on 2,5-dihydroxy-1,4-benzoquinone (DHBQ).

Inelastic neutron scattering (INS) spectra of solid 2,5-dihydroxy-1,4-benzoquinone were measured and compared with IR and Raman data. The INS spectrum is very well reproduced in the region below 1000 cm(-1) by DFT calculations on the B3LYP/6-311++G** level using Gaussian and Climax programs. To get a better agreement one should take into account additional interactions of OH groups in the solid state leading to an increase of the gamma(OH) frequency and to a decrease of frequencies for modes in which the delta(OH) participates. The studies of the deuterated compound in IR enabled to correct the assignment of gamma(OH) vibrations. Highly asymmetric nu(OH) band observed in IR spectrum with sharp maximum at about 3300 cm(-1) is discussed in terms of a stochastic approach to the analysis of hydrogen bonded systems.

Neutron spectroscopic study of hydrogen bonding dynamics in L-serine.

Inelastic incoherent neutron scattering (IINS) spectra were obtained at 10 K for normal and deuterated L-serine. The geometry of L-serine molecule was optimized for the zwitterion form using ab initio HF, MP2 and DFT (B3LYP) levels with 6-31G* and 6-311 + +G4** basis sets. The theoretical frequencies of normal and d4-L-serine were compared with IINS spectra. Normal coordinate analysis and band assignments based on ab initio calculations and experimental data were presented. IINS frequencies due to the out-of-plane gamma(N-H...O) hydrogen bond motions were observed and identified.

Inelastic neutron scattering studies on low frequency vibrations of pentachlorophenol.

Inelastic neutron scattering (INS) and DFT theoretical studies on pentachlorophenol (PCP) and d-PCP were performed. IR and Raman spectra were also measured for comparison. A special attention was focused on low frequency modes in INS spectra, which provide information about modes into which the co-ordinates of the hydrogen and chlorine atoms are involved. The intensity of respective INS bands is discussed based on the cross-sections of nuclei and calculated relative amplitudes of vibrations. The appearance of overtones and summation frequencies in INS spectra was evidenced.

Neutron scattering, infra red, Raman spectroscopy and ab initio study of L-threonine.

Inelastic incoherent neutron scattering spectra (IINS) were obtained for normal and deuterated L-threonine. Raman and infrared spectra were also recorded. Geometries were optimized for the zwitterion form using ab initio Hartree-Fock (HF) levels with 6-31G*, 6-311G*, 6-311G** and 6-311 + + G** basis sets. Force fields and normal modes were calculated and used as basis for an assignment of the spectral features.

Vibrational spectra of the adduct of 1,8-bis(dimethylamino)naphthalene with dichloromaleic acid (DMAN x DCM).

The infra-red (IR), Raman (R) and inelastic incoherent neutron scattering (IINS) spectra, particularly in low frequency region, of the title ionic adduct were studied. It is shown that all low frequency vibrations (below 200 cm(-1)) of (CH3)2N groups of protonated 1,8-bis(dimethylamino)naphthalene (DMAN)--clearly observed in IINS spectra--are sensitive to the environment, i.e. to the type of counterion forming short contacts with C-H bonds of methyl groups. The internal frequencies were also calculated by ab initio method. The results are consistent with numerous observations of the counteranion effect on the geometry of the protonated DMAN. The conclusions are compared with structural and NMR studies reported recently for the 1,8-bis(dimethylamino)naphthalene with dichloromaleic acid (DMAN x DCM) adduct. The single crystal R polarized spectra taken over the frequency range 20-3200 cm(-1) were analyzed in detail. We have shown that a substantial difference in the IR spectrum of the dichloromaleic acid (DCM) anion in the DMAN adduct and in the potassium salt results from different geometries of OHO hydrogen bonds. In the case of potassium salt the chains of longer intermolecular hydrogen bonds are formed described by means of a double minimum potential.