A. Pawlukojć

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.

The IINS spectroscopy of amino acids: l- and dl-valine

Abstract Results of the inelastic incoherent neutron scattering (IINS) study on normal and N-deuterated l and dl -valine at pulsed reactor IBR-2 are reported. l -Valine spectra show additional bands as compared with dl -valine due to the difference in the respective molecular conformation. Both in l - and dl -valine, lines corresponding to out-of-plane γ(N-H ··· O) hydrogen bond vibrations are observed at 279, 505, 513 cm−1 and 276, 559, 592 cm−1, respectively.

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.

The infrared, Raman and inelastic neutron scattering studies on 5-nitro-N-salicylideneethylamine

Abstract The title compound in the solid state exists in the proton transfer − δ O⋯H–N + δ state. This is due, most probably, to the fact that dimeric species are formed that promotes more polar charge distribution. The studies performed in various non-polar and medium-polar solvents show that in such cases no proton transfer takes place. Only in very polar solvents like acetonitrile we are dealing with intramolecular ion-pairs. The ab initio calculations confirm such finding. The detailed vibrational analysis was performed based on ab initio calculations, Infrared (IR), Raman (R) and inelastic neutron scattering (INS) studies, over the whole frequency range. Of particular interest were the frequencies sensitive to the proton transfer with participation of the bridge atoms.

The IINS spectroscopy of amino acids: l-isoleucine

Abstract Results of inelastic incoherent neutron scattering (IINS) supplemented by infrared (IR) and Raman scattering studies on normal and N -deuterated l -isoleucine in the energy transfer up to 800 cm −1 are reported. The IINS spectra of l -isoleucine show additional bands as compared to the spectra obtained by optical spectroscopic methods. In both, normal and d 3 - l -isoleucine, extra bands correspond to out-of-plane γ (N–H⋯O) (518, 505, 250 cm −1 ) and γ (N–DO⋯O) (362 cm −1 ) hydrogen bond vibrations. Methyl and carboxyl groups vibrations are observed at 219, 228, 250, 281 cm −1 and 175, 543, 686, 749 cm −1 respectively.

The dynamical pattern of the 2-aminopyrazine-3-carboxylic acid molecule by inelastic and incoherent neutron scattering, Raman spectroscopy and ab initio calculations

Abstract Inelastic and incoherent neutron scattering and Raman spectra were measured for 3-amino-2-carboxylic acid. Geometries were optimised and harmonic frequencies calculated at both semiempirical (AM1, MNDO, PM3) and ab initio Hartree–Fock (HF) levels using the 3-21G, 6-31G, 6-31G**, 6-311G and 6-311** basis sets. The observed frequencies were assigned using the HF/6-311G results.

Inelastic and quasielastic neutron scattering and IR and R spectroscopic studies of 1,2,4,5-tetracyanobenzene(TCNB)-1,2,4,5-tetramethylbenzene (durene) complex¥

The infrared and Raman spectra for durene, tetracyanobenzene and their 1 : 1 complex were studied. Simultaneously the neutron scattering experiments (INS and QENS) were performed. A detailed analysis of the modes assigned to the methyl groups vibrations were analyzed based on the simulated frequencies and intensities by using the GAUSSIAN-03 and auntie-CLIMAX programs. A good agreement between calculated frequencies and INS experimental ones was found. Moreover the calculations generate, with a quite high accuracy, the observed lattice phonons below 70 cm−1. The QENS studies have shown that the activation energy for the 120° CH3 jumps equal to 49 ± 13 meV for durene and 35 ± 12 meV for the complex confirming that the complexation leads to a decrease of the potential barrier for the methyl group rotations. ¥ Dedicated to Professor Jerzy A. Janik on the occasion of his 80th birthday.

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.

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.

Elastic, quasielastic, and inelastic neutron-scattering studies on the charge-transfer hexamethylbenzene-tetracyanoquinodimethane complex.

The 1:1 hexamethylbenzene (HMB)-tetracyanoquinodimethane (TCNQ) complex shows a first-order phase transition at 230/218 K (heating/cooling) with no change of the space group. The neutron-diffraction studies reveal that this transition is related to a freezing of the rotation of methyl groups. The results for 100 K enabled precise determination of configuration of HMB.TCNQ complexes. The planes of HMB and TCNQ molecules from small angle (6 degrees) so that the dicyanomethylene group approaches the HMB molecule to a distance of 3.34 angstroms. The conformation of methyl groups was exactly determined. The quasielastic neutron-scattering spectra can be interpreted in terms of 120 degrees jumps with different activation barrier in low- and high-temperature phases, equal to 3.7 and 1.8 kJ/mol, respectively. These values are lower than that for neat HMB (6 kJ/mol). The conclusion can be drawn that the methyl groups can reorient more freely in the complex. This conclusion is in agreement with the results of inelastic neutron-scattering studies of low-frequency modes assigned to torsional vibrations of methyl groups. These frequencies are lower than those for neat HMB. The analyzed increase of frequencies of these modes as compared with free molecules can be interpreted as due to formation of unconventional C-H...Y hydrogen bonds which are more pronounced in crystals of neat HMB than in those of HMB.TCNQ. The low-frequency librational modes can be treated as a sensitive measure of unconventional hydrogen bonds formed by the CH3 groups.