F. Fillaux

Proton dynamics in the hydrogen bond. The inelastic neutron scattering spectrum of potassium hydrogen carbonate at 5 K

Abstract We report the inelastic neutron scattering (INS) spectrum from KHCO 3 powder (5 K). The spectrum has been analysed by the frequency and intensity normal coordinate refinement program CLIMAX. The final force field indicates that the proton bending modes in the OH…O bond are independent of other deformation modes, but that the stretching modes interact strongly. This is interpreted in terms of a significant ionic character for the hydrogen bond. Based upon simple calculations for its position and intensity an INS mode at ≈ 220 cm −1 is tentatively assigned to a tunneling transition for the proton.

Proton transfer dynamics in the hydrogen bond. Inelastic neutron scattering, infrared and Raman spectra of Na3H(SO4)2, K3H(SO4)2 and Rb3H(SO4)2

Abstract Na 3 H(SO 4 ) 2 , K 3 H(SO 4 ) 2 and Rb 3 H(SO 4 ) 2 crystals are composed of (SO 4 HSO 4 ) −3 dimers linked by rather strong hydrogen bonds ( R O…O =2.43 A for Na 3 H(SO 4 ) 2 , R O…O =2.48 A for Rb 3 H(SO 4 ) 2 and R O…O =2.49 A for K 3 H(SO 4 ) 2 ). Crystallographic data of the salts at room temperature indicate either asymmetric (Na 3 H(SO 4 ) 2 ) or symmetric (K 3 H(SO 4 ) 2 and Rb 3 H(SO 4 ) 2 ) hydrogen bonds. Inelastic neutron scattering (INS), infrared and Raman spectra of crystal powders at 20 K are reported for these three compounds. The OH bending modes, which give large INS intensities, appear only weakly in the infrared. The two bending modes are degenerate in Na 3 H(SO 4 ) 2 which has the shortest hydrogen bond but are well separated in K 3 H(SO 4 ) 2 and Rb 3 H(SO 4 ) 2 . The OH stretching band profiles in INS are also quite different from those in the infrared. Strong INS bands at 57 and 44 cm −1 for K 3 H(SO 4 ) 2 and Rb 3 H(SO 4 ) 2 , respectively, are assigned to 0→1 transitions in quasi-symmetric double-minimum potentials for the OH stretching coordinates. For K 3 H(SO 4 ) 2 the frequency is unaffected by temperature between 2 and 100 K. Potential functions are calculated and the dynamics of the proton transfer are discussed. Infrared spectra are thus dominated by OH stretching transitions in asymmetric double-minimum potentials with low barriers, with relative intensities indicating a large electrical anharmonicity.

Inelastic neutron-scattering study of the proton dynamics in N-methylacetamide at 20 K

Abstract Inelastic neutron-scattering (INS) spectra of four isotopic derivatives of N-methylacetamide (CH 3 CONHCH 3 , CD 3 CONHCH 3 , CH 3 CONHCD 3 and CD 3 CONHCD 3 ) at 20 K are presented from 30 to 4000 cm −1 . The band frequencies are compared with those observed in the infrared and Raman at low temperature. The quantitative simulation of the INS intensities, in the harmonic force field approximation, shows that the proton dynamics for the (N)H proton are totally different from those proposed previously. The valence-bond approach is not consistent with observation and the proton dynamics are independent of the molecular frame. A phenomenological approach is proposed in terms of localized modes. The calculated intensities reveal that the (N)H stretching mode is at ∼ 1575 cm −1 . This is a dramatic change compared to all former assignments at ∼ 3250 cm −1 based on the infrared and Raman data. These unforeseen proton dynamics are associated with the weakening of the NH bond due to the ionic character of the hydrogen bond (N δ− …H + …O δ′− ) and proton transfer. The infrared and Raman spectra are reconsidered and a new assignment scheme is proposed for the amide bands in terms of dynamicalproton exchange between the amidic (…OCNH…) and imidolic (…HOCN…) forms in infinite chains of hydrogen-bonded molecules.

Vibrational spectra and dynamics of conformation and hydrogen bonding of n-methylacetamide. I. Conformational dynamics of the CH3CONHCH3 molecule and NH out of plane band splitting

Abstract The infrared spectra of CH3CONHCH3 and CH3CONDCH3 have been investigated as low temperature crystals, pure liquids and solutions in various solvents in the 400-800 cm−1 range. A new assignment of the bands associated with the NH group is given. Multiplets of γ NH and γ ND fundamentals have generally been observed and have been interpreted in terms of a double minimum potential function of the γ NH mode with a tunnelling between two minima. The potential functions of γ NH and τ CN modes are similar and can be combined to give a potential surface with four minima corresponding to four molecular conformations. The influence of the hydrogen bonding on the γ NH splitting and barrier height is discussed.

Inelastic neutron-scattering study of the proton dynamics in coals

Abstract The inelastic neutron-scattering (INS) spectra from 30 to 4000 cm −1 of four coal samples of different rank (subbituminous A, low-volatile bituminous, semianthracite and anthracite) at 20 K are reported. For each coal, rather broad bands superimposed on a continuum with almost constant intensity over the whole frequency range are observed. This continuum is assigned to recoiling free protons located between the graphite-like planes. Band frequencies are compared with those observed in the infrared for the same samples in KBr matrices at room temperature, and consistent assignments in terms of chemical-grouping vibrations are proposed. The INS spectra of the coals are also compared with that of perylene (C 20 H 12 ). The spectrum of anthracite is dominated by CH and skeletal modes of polyaromatic entities. Coals of lower rank show bands due to alkyl and vinyl CH 2 and CH 3 groups. A comparison of INS intensities in the CH stretching region for coals and perylene gives an estimate of the concentration of bound protons which ranges from ∼40% to ∼80%. A simple model is proposed to estimate the mean size of the graphite-like domains in the coals.

An inelastic neutron scattering study of the proton dynamics in γ-MnO2

Abstract The inelastic neutron scattering (INS) spectra between ≈0 and 4000 cm −1 of electrodeposited manganese dioxide (γ-MnO 1.96 ) at 4.5 K are reported. The spectrum of the commercial product is compared to those obtained from various samples heated at 150, 250, 350 and 450°C during 16 h in presence of air. Three different types of bands are distinguished in the spectra. (i) A broad continuum rising at ≈ 30 cm −1 and extending over the whole frequency range is interpreted in terms of an ideal gas of protons moving almost freely in the crystal. (ii) Bands between 500 and 1100 cm −1 are assigned to protons trapped in crystal defects. (iii) Sharp and relatively intense bands between 30 and 90 cm −1 are assigned to covalently bonded protons strongly coupled to the libration modes of the crystal. They are likely to correspond to oxyhydroxide MnO(OH). The concentration in MnO(OH) increases substantially after ambiant atmosphere rehydratation of the sample previously treated at 450°C. The dynamics of these different protons and their stabilities with respect to thermal treatments are discussed. It appears that most of the protons are not covalently bonded to the oxygen atoms in the crystal but are rather mobile. Moreover, INS spectra provide direct evidence that some of the protons are localized by crystal defects which may thus play a key role in the electroactivity of the material.

Inelastic neutron scattering study of proton dynamics in carbon blacks

Abstract Inelastic neutron scattering spectra from 16 to 4000 cm −1 of various carbon blacks (rubber blacks, graphitized black, post-oxidized gas black) and activated carbon at 30 K are presented. They reveal new aspects of proton dynamics in these samples. The nearly proton-free graphitized sample is similar to pure graphite. All spectra reveal a continuum of intensity which is attributed to the recoil of free protons inserted in the basic structural units. On the top of the continuum, more or less well-defined bands are attributed to protons either chemically bound at the border of the basic structural units or, in the case of the post-oxidized gas black, trapped in topological defects. The activated carbon is dominated by bands due to OH groups. The integrated intensities are not simply related to proton concentrations given by analytical techniques.

Inelastic neutron-scattering study of the proton dynamics in carbons and coals

Abstract The INS spectra from 30 to 4000 cm −1 of four coal samples at 20 K with various ranks (subbituminous A, low volatile bituminous, semi-anthracite, and anthracite) are reported. For each sample, rather broad bands superimposed on a continuum with constant intensity over the whole frequency range are observed. This continuum is assigned to recoiling free protons located between the graphite-like planes. The INS spectra of the coals are also compared to that of perylene (C 20 H 12 ). The spectrum of the anthracite is dominated by the C:H and skeletal modes of polyaromatic entities. Samples of lower rank show bands due to alkylic and vinylic CH 2 and CH 3 groups. The integrated intensities of the INS spectra are not proportional to the numbers of protons given by the elemental analysis. This is due to different proportions of free and bound protons in the samples. A comparison of the INS intensities in the CH stretching region for coals and perylene gives an estimation of the concentration of bound protons which ranges from ∼40% to ∼80%.

Vibrational force field of solid imidazole from inelastic neutron scattering

Inelastic neutron scattering (INS) measurements are presented for solid imidazole at 20 K. Both powder and oriented platelets have been measured. Assignments of bands are consistent with earlier infrared and Raman studies except for a band at 961 cm−1 which is definitely assigned as A″. A fit with the proton‐weighted vibrational density of states is presented and a full harmonic force field has been established. The mean deviation between measured eigenvalues and those predicted from the force field is ±3 cm−1. The potential energy distribution is compared to quantum chemistry calculations and is found to be in best agreement with an unscaled calculation [J. Sadlej, A. Jaworski, and K. Miaskiewicz, J. Mol. Struct. 274, 247 (1992)]. The INS spectra from a scaled ab initio force field [K. Fan, Y. Xie, and J. Boggs, Theochem, 29, 339 (1986)] are calculated both before and after refinement against only infrared frequencies and compared to experiment. It is demonstrated that comparison with the INS spectrum is...

The anisotropy of the proton momentum distribution in KHCO3: A deep inelastic neutron scattering study

We report the anisotropy of the proton momentum distribution in potassium hydrogen carbonate, KHCO3. Using the deep inelastic neutron scattering technique, the mean proton momenta along, and across, the O–H–O bond have been determined. While the momentum across the bond corresponds to a proton in a harmonic potential, that along the bond shows this potential is consistent with a previously proposed double well shape.