Effects of dynamical couplings in hydrogen bond systems in the polarized IR spectra of 3-hydroxybenzaldehyde and 4-hydroxybenzaldehyde crystals

This paper presents the investigation results of the polarized IR spectra of 3-hydroxybenzaldehyde and 4-hydroxybenzaldehyde crystals measured at 293 and 77 K. Analysis of the results concerned the linear dichroic, H/D isotopic and temperature effects observed in the spectra of the hydrogen and deuterium bond at the frequency ranges of the ν_O–H and the ν_O–D bands, respectively. The main spectral properties of the crystals were interpreted in terms of the “strong-coupling” theory on the basis of the hydrogen bond dimer model. The spectra revealed that the strongest vibrational exciton coupling involved the closely-spaced hydrogen bonds, each belonging to a different chain of associated molecules. The reason for two different crystalline systems, are characterized by almost identical ν_O–H and ν_O–D band shapes, is explained. It was proved that a random distribution of the protons and deuterons took place in the lattices of the isotopically diluted crystals.     

Effects of strong inter-hydrogen bond dynamical couplings in the polarized IR spectra of adipic acid crystals

This paper presents the results of the re-investigation of polarized IR spectra of adipic acid and of its d₂, d₈ and d₁₀ deuterium derivative crystals. The spectra were measured at 77 K by a transmission method using polarized light for two different crystalline faces. Theoretical analysis concerned linear dichroic effects and H/D isotopic effects observed in the spectra of the hydrogen and deuterium bonds in adipic acid crystals at the frequency ranges of the ν_O–H and the ν_O–D bands. The two-branch fine structure pattern of the ν_O–H and ν_O–D bands and the basic linear dichroic effects characterizing them were ascribed to the vibronic mechanism of vibrational dipole selection rule breaking for IR transitions in centrosymmetric hydrogen bond dimers. It was proved that for isotopically diluted crystalline samples of adipic acid, a non-random distribution of protons and deuterons occurs in the dimers (H/D isotopic “self-organization” effect). This effect results from the dynamical co-operative interactions involving the dimeric hydrogen bonds.     

Inter-hydrogen bond dynamical coupling effects in polarized IR spectra of N-methylacetamide single crystals

This article presents the investigation results of polarized IR spectra of the hydrogen bond in N-methylacetamide (NMA) crystals measured in the frequency range of the proton and deuteron stretching vibration bands, ν_N–H and ν_N–D. A similar study was also performed for crystals of the deuterium isotopomers of the compound, D₇-NMA (CD₃CONDCD₃) and D₆-NMA (CD₃CONHCD₃). On the basis of the analysis of the linear dichroic and temperature effects, the two-branch structure of the ν_N–H bands in the spectra was ascribed to centrosymmetric hydrogen bond pairs in the lattice. Each hydrogen bond in such a dimeric system belonged to another chain of the associated molecules. The exciton interactions involving the dimer hydrogen bonds were considered to be responsible for the band shape generation. For the deuterium-bonded crystals the exciton interactions were found to be weaker since the ν_N–D bands were less split. Within an individual hydrogen or deuterium bond chain the in-chain exciton couplings involving hydrogen bonds were estimated as considerably weaker than the inter-chain ones. The exciton dilution retains the two-branch fine structure pattern of the “residual” ν_N–H and ν_N–D bands. This means that the inter-chain couplings involving hydrogen bonds do not change, when the in-chain couplings vanish. These results are the evidence of the influence of non-conventional co-operative interactions occurring in the hydrogen bond systems on the spectra. These co-operative interactions are responsible for the non-random distribution of the hydrogen isotope atoms in the hydrogen bridge lattices, namely for the grouping of identical hydrogen isotope atoms in the dimers. The proposed interpretation of the IR spectra of the hydrogen bond in N-methylacetamide (NMA) crystals casts light on the spectra generation mechanisms and gives a new meaning to the traditional nomenclature applied for describing the ν_N–H band structure pattern in IR spectra of amides.     

Influence of molecular electronic properties on the IR spectra of dimeric hydrogen bond systems: polarized spectra of 2-hydroxybenzothiazole and 2-mercaptobenzothiazole crystals

We have studied the polarized IR spectra of the hydrogen-bonded molecular crystals of 2-hydroxybenzothiazole (HBT) and 2-mercaptobenzothiazole (MBT). The crystal structure of 2-hydroxybenzothiazole was determined by X-ray diffraction. The polarized spectra of the crystals were measured, in the frequency ranges of the ν_N-H and ν_N-D bands, at room temperature, and at 77 K. In both systems an extremely strong H/D isotopic effect in the spectra was observed, involving reduction of the well-developed ν_N-H band fine structure to a single prominent ν_N-D line only. The two ν_N-H bands were also shown to exhibit almost identical properties, band shapes, temperature and dichroic properties included. The spectra were quantitatively reconstituted, along with the strong isotopic effect, when calculated using the ‘strong-coupling’ theory, assuming the centrosymmetric dimers of HBT or MBT to be the structural units responsible for the crystalline spectral properties. The similarity of the spectra of the two crystalline systems was considered to be a result of longer-distance couplings between the proton vibrations in the dimers, via the aromatic ring electrons. When investigating the ‘residual’ ν_N-H band shapes for crystals isotopically diluted by deuterium, we observed some ‘self-organization’ effects in the spectra, indicating the energetically favored presence of two identical hydrogen isotopes in each hydrogen bond dimer.     

H-bonds in acetic acid crystals: Influence of temperature and of isotopic dilution on OH(O-D) stretching BANDS

We describe the evolution of the ν_s bands of CH₃COOH, CD₃COOH, CH₃COOD and CD₃COOD crystals when varying temperature from 10 K to 270 K, and also under isotopic dilutions. We discuss the results obtained and we show how we can estimate from these experiments the values of the anharmonic constant coupling ν_s with ν_σ (parameter b) and also how we can evaluate the energy of resonance interactions of ν_s with other modes. The origin of the shift of the centre of gravity of the ν_s, bands with temperature is discussed and we give arguments against an anharmonic one-dimesional potential for the ν_σ mode being at the origin of such a shift in acetic acid crystals. We suggest that this shift might rather be due to a modulation of ν_s or ν_σ by lower frequency modes of the H-band. In the conclusion we summarize our results on acetic acid crystals and stress the interest of performing such quantitative studies.     

Fermi resonance in crystals of acetic and d-acetic acid

Band profiles of the IR spectra of CH₃COOD and CD₃COOH crystals are calculated, assuming two or three Fermi resonances and adiabatic interaction between high- and low-frequency vibrations. The parameters of the model are evaluated from experimental spectra. The agreement between calculated and experimental spectra shows that both mechanisms are important in determining the overall fine structure of the IR spectra and confirms Novak's qualitative interpretation.     

Study of polarized IR spectra of the hydrogen bond system in crystals of styrylacetic acid

We have investigated the polarized IR spectra of the hydrogen bond system in crystals of trans-styrylacetic acid C(6)H(5)CHCHCH(2)COOH, and also in crystals of the following three deuterium isotopomers of the compound: C(6)H(5)CHCHCH(2)COOD, C(6)H(5)CHCHCD(2)COOH and C(6)H(5)CHCHCD(2)COOD. The spectra were measured at room temperature and at 77K by a transmission method. The spectral studies were preceded by determination of the X-ray crystal structure. Theoretical analysis of the results concerned linear dichroic effects, the H/D isotopic and temperature effects, observed in the solid-state IR spectra of the hydrogen and of the deuterium bond, at the frequency ranges of the nu(OH) and the nu(OD) bands, respectively. Basic spectral properties of the crystals can be interpreted satisfactorily in terms of the "strong-coupling" theory, when based on a hydrogen bond dimer model. This model sufficiently explained not only a two-branch structure of the nu(OH) and the nu(OD) bands, and temperature-induced evolution of the crystalline spectra, but also the linear dichroic effects observed in the band frequency ranges. A vibronic mechanism was analyzed, responsible for promotion of the symmetry-forbidden transition in the IR for the totally symmetric proton stretching vibrations in centrosymmetric hydrogen bond dimers. It was found to be of minor importance, when compared with analogous spectral properties of arylcarboxylic acid, or of cinnamic acid crystals. These effects were ascribed to a substantial weakening of electronic couplings between the hydrogen bonds of the associated carboxyl groups and the styryl radicals, associated with the separation of these groups in styrylacetic acid molecules by methylene groups in the molecules.     

The source of similarity of the IR spectra of acetic acid in the liquid and solid-state phases

This paper explains the problem of the fair similarity of the ν_O-H band contour shapes in the spectra of liquid and crystal samples of hydrogen-bonded acetic acid, in contrast to the corresponding spectral properties of formic acid. It has been proven that regardless of the relative arrangement of hydrogen bonds in each phase, the hydrogen-bonded dimeric systems decide about the profile of the bands. Moreover, the model dimers of different symmetry, i.e., the C_i or C_2v one of the “side-to-side” type arrangement of moieties, yield nearly identical spectra. Qualitatively similar spectral properties in the ν_O-D band frequency range were found for deuterium-bonded formic and acetic acid isotopologues.     

Infrared spectra of the hydrogen bond in benzoic acid crystals: temperature and polarization effects.

This paper deals with experimental studies and with quantitative interpretation of the polarized IR crystalline spectra of benzoic acid and of its deuterium isotopomers: C6H5COOD, C6D5COOH and C6D5COOD. The spectra were measured in the vO-H and in the vO-D band frequency ranges at temperatures of 298 and 77 K. The intensity distribution in the bands was quantitatively reproduced on the basis of the 'strong-coupling' model, when assuming that the isolated (COOH)2 and (COOD)2 cycles were the source of the spectral properties of the crystals. Such approach appeared to be sufficient for explaining most of the isotopic and the temperature effects in the spectra. Another band shaping mechanism, i.e. a vibronic mechanism, promoting the symmetry forbidden transition in the IR for the totally symmetric proton stretching vibrations in the centrosymmetric dimers of hydrogen bonds was also considered. A new kind of the 'long-range' isotopic effects H/D in the spectra was indicated, depending on the influence of the aromatic ring hydrogen atoms onto the vO-H band fine structure patterns. The role of the aromatic rings was also discussed, in order to explain mechanisms of extremely effective promotion of the forbidden transition, as well as of the Fermi resonance impact on the crystalline spectra.     

Strong vibrational exciton coupling effects in polarized IR spectra of the hydrogen bond in 2-thiopyridone crystals

This paper presents the results of investigation on polarized IR spectra of the hydrogen bond in 2-thiopyridone crystals. The spectra were measured in the frequency range of the NH and ND bond stretching vibrations, for two different crystalline forms, having developed ab or bc crystal faces. The spectra exhibited extremely strong vibrational exciton coupling effects characterized by a large Davydov-splitting (correlation field splitting), whose existence was confirmed by a strong difference between the polarized spectra of the two forms of 2-thiopyridone crystals. It was shown that extremely strong exciton interactions involving the translationally non-equivalent hydrogen bonds in the unit cell are responsible for these effects. Isotopic dilution in the crystals caused the disappearance of the spectral effects, ascribed to the inter-dimer exciton couplings, and the simultaneous retaining of the dimeric character of the “residual” ν_NH and ν_ND bands. This spectral behavior of the isotopically diluted crystals was interpreted as the result of the dynamical co-operative interactions involving the hydrogen bonds in the lattice. These interactions lead to a non-random distribution of the protons and deuterons in the cyclic hydrogen bond dimeric systems and in consequence to the so-called H/D isotopic “self-organization” effects in the crystal spectra. The fine structure of the “residual” ν_NH and ν_ND bands is also influenced by such non-conventional spectral effects as the selection rule breaking for IR transitions, as well as the “reversal” exciton coupling effect for centrosymmetric hydrogen bonded dimers. This statement is supported by model calculations of the analyzed band shapes. They are performed in terms of the “strong-coupling” theory which assumes a strong anharmonic coupling involving different frequency hydrogen bond normal vibrations in the dimers, namely the high-frequency NH stretching and the low-frequency ν_N⋯S hydrogen bond stretching vibrations.     

Energy randomization in the reactions of F atoms at the carboxyl sites of CH3COOH and CF3COOH

The HF and DF vibrational distributions for the hydrogen abstraction reactions between F atoms and CH₃COOD, CD₃COOH, CF₃COOD and CF₃COO have been measured by arrested relaxation infrared chemiluminescence. Phase space calculations have been carried out which accurately reproduce the dis observed for the reactions at the carboxyl site in each case. The calculations suggest that the energy is not completely randomised during the acetic a whereas the trifluoroacetic acid reactions show ergodic behaviour.     

Polarized Infrared Spectra of the H(D) Bond in 2‐Thiophenic Acid Crystals: A Spectroscopic and Computational Study

Polarized IR spectra of the hydrogen bond in 2‐thiophenic acid crystals, isotopically neat and of mixed H/D isotopic content, are measured at 298 and 77 K in the “residual” νO? H and νO? D band frequency ranges. This crystalline system provides spectra in these band frequency ranges that differ considerably in intensity distribution from the spectra of other H‐bonded centrosymmetric dimeric species. This change in the spectral properties of the crystals is probably due to the influence of the sulfur atoms from the thiophene aromatic rings, which are directly linked to the (COOH)2 or (COOD)2 cycles. The magnitude of this effect correlates with the net electronic charge distribution at the 2‐ and 3‐positions of substituted thiophene rings, which in a different way influences the electron charge density in the hydrogen bonds of the two thiophenic acid isomers. The experimental results for spectral structures are compared to predictions obtained with theoretical calculations involving the combined effects of anharmonicities, Davydov coupling, Fermi resonances, and direct and indirect relaxations within the framework of the linear response theory. Numerical results show that mixing of all these effects allows satisfactory reproduction of the main features of the experimental IR line shapes of crystalline H‐ and D‐bonded 2‐thiophenic acid at room and liquid‐nitrogen temperatures.     

Polarization IR spectra of the hydrogen bond in phenylacetic acid crystals: H/D isotopic effects temperature and polarization effects

This paper deals with experimental studies and with quantitative interpretation of the polarized IR crystalline spectra of phenylacetic acid and of its deuterium isotopomers d2 and d7. The spectra were measured in the v(O-H) and in the v(O-D) band frequency ranges at temperatures of 298 and 77 K. The intensity distribution in the bands was quantitatively reproduced on the basis of the strong-coupling model, when assuming that the isolated (COOH)2 and (COOD)2 cycles were the source of the spectral properties of the crystals. Such approach appeared to be sufficient for explaining most of the isotopic and the temperature effects in the spectra. A vibronic mechanism, promoting the symmetry forbidden transition in the IR for the totally symmetric proton stretching vibrations in centrosymmetric hydrogen bond dimers, was found to be of a considerably minor importance, when compared with analogous properties of arylcarboxylic acid crystals. The spectra of phenylacetic acid crystals, unlike the spectra of arylacetic acid crystals do not exhibit the so-called H/D long-range isotopic effects, depending on an influence of the aromatic ring hydrogen atoms on the v(O-H) band fine structure patterns. Also no Fermi resonance impact on the v(O-H) band shape was identified in the phenylacetic acid crystal spectra. These effects were ascribed to weakening of electronic couplings between the hydrogen bonds and the phenyl rings, due to the separation of these groups in phenylacetic acid molecules by methylene groups.     

Freezing Point of Mixtures of C6H6 with C6D6 or 13C6H6 and of CH3COOH with CH3COOD or CD3COOD

The freezing points of mixtures of benzene, C₆H₆, with one of its isotopes, C₆D₆ and ¹³C₆H₆, and those of acetic acid CH₃COOH with its isotopes, CH₃COOD and CD₃COOD, were measured as functions of the molal concentrations of C₆D₆ and ¹³C₆H₆, CH₃COOD and CD₃COOD, respectively. They changed linearly or non-linearly with increasing molal concentration of C₆D₆ and ¹³C₆H₆, CH₃COOD, and CD₃COOD, respectively. These findings confirm Kiyosawas previous conclusion drawn from experiments on the freezing points of mixtures of H₂¹⁶O with H₂¹⁸O or H₂¹⁷O. This hypothesis states that even a difference in the number of neutrons in the hydrogen or oxygen atoms of water molecules makes water molecules behave as different entities with respect to the colligative properties of solutions. This concept can be extended to mixtures of ordinary benzene with either of its isotopes, C₆D₆ or ¹³C₆H₆, and those of ordinary acetic acid CH₃COOH with either of its isotopes, CH₃COOD or CD₃COOD.     

Uncommon oxidative transformations of acetic and propionic acids

The oxidative decarbonylation of acetic and propionic acids with the formation of the corresponding alcohol and alkyl carboxylate is observed in the Rh^III/Cu^I,II/Cl⁻ catalytic system in the presence of O₂ and CO. The decarbonylation of propionic acid in a deuterated solvent results in the substitution of hydrogen atoms by deuterium in the alkyl part of the products to form CH₂DCOOD (CHD₂COOH) and CHD₂COOD (CD₃COOH). The subsequent decarbonylation of deuterated acetic acids affords the corresponding deuteromethanols detected as esters with propionic and deuteroacetic acids. The substitution of the hydrogen atom by deuterium in the alkyl part of molecules of the products of oxidative decarbonylation of propionic acid, when the reaction is carried out in a deuterated solvent, indicates that propionic acid behaves as saturated hydrocarbon and blocks the oxidation of poorly soluble methane. Unlike propionic acid, acetic acid enters only the oxidative decarbonylation reaction and does not block methane oxidation.     

Molecular structure and spectroscopic properties of N-methylmorpholine betaine complex with 4-hydroxybenzoic acid

Crystal structure of the 1:1 complex of N-methylmorpholine betaine (MMB) with 4-hydroxybenzoic acid (4-HBA) has been determined by X-ray diffraction. Crystals are orthorhombic, space group Pna2₁ with a=7.933(2), b=15.336(3), and Z=4, R=0.033. The acid molecule forms two O-H?O hydrogen bonds with two betaine molecules. The COOH group of the acid forms shorter hydrogen bond with betaine (2.587(2) Å), than the hydroxyl group (2.677(2) Å). The carbonyl oxygen atom of the acid also interacts with the methylene hydrogen atom of the betaine through C-H?O hydrogen bond (3.256(2) Å). Thus formed infinite chains parallel to the z axis are connected by other C-H?O hydrogen bonds into layers perpendicular to the x axis. The morpholine ring has a chair conformation with the methyl group in the equatorial position and CH₂COO⁻ group in the axial one. The powder FTIR and Raman spectra and semiempirical calculations of the isolated molecule confirm the structure of the complex investigated. The ¹H and ¹³C spectra indicate that in DMSO-d₆ solution, protons are not transferred from the acid to the betaine molecules.     

Shape of the proton potential in an intramolecular hydrogen-bonded system. Part II

The crystals of 5,5′-dibromo-3-diethylaminomethyl-2,2′-biphenol N-oxide were studied by X-ray and FT-IR spectroscopy. Within this molecule two short OHO intramolecular hydrogen bonds are formed. The NO?H⁺?O⁻ bond between the OH and the N-oxide groups is very strong, of 2.419(7) Å between the oxygen atoms. The proton potential of this hydrogen bond is flat, broad and has probably no barrier—consequently it could not be located from X-ray diffraction data. The other hydrogen bond formed between two hydroxyl groups appears asymmetrical from FT-IR spectra, and shows also relatively limited proton polarizability. The molecular conformation is non-planar, due to strong overcrowding effect between the oxygen atoms involved in the hydrogen bonds.     

Tautomeric behaviour and isotopic multiplets in the 13C NMR spectra of partially deuterated 5‐arylazo‐pyrimidine (1H,3H,5H)‐2,4,6‐triones and 5‐arylazo‐2‐thioxo‐pyrimidine (1H,3H,5H)‐4,6‐diones—evidence for elucidation of tautomeric forms

NMR spectra of the synthesized azo dyes, 5‐arylazo‐pyrimidine (1H,3H,5H)‐2,4,6‐triones (5a–g), 1,3‐dimethyl‐5‐arylazo‐pyrimidine (1H,3H,5H)‐2,4,6‐triones (6a–g), and 5‐arylazo‐2‐thioxo‐pyrimidine (1H,3H,5H)‐4,6‐diones (7a–g) were studied in (CD₃)₂SO (three drops of CD₃OD were added into solutions of the dyes in two different concentrations). All dyes showed intramolecular hydrogen bonding. Dyes 5a–7a showed bifurcated intramolecular hydrogen bonds. Tautomeric behaviours of some of N‐methylated azo dyes (6a‐g) were studied in two different concentrations. The solvent–substrate proton exchange of dyes 5a–d, 6a and 7a–e was examined in presence of three drops of CD₃OD. The dyes which were soluble in (CD₃)₂SO containing CD₃OD showed isotopic splitting (β‐isotope effect) in the ¹³C NMR spectra. Copyright © 2009 John Wiley & Sons, Ltd.