Effects of dynamical couplings in IR spectra of the hydrogen bond in N-phenylacrylamide crystals

This article presents the investigation results of the polarized IR spectra of the hydrogen bond in N-phenylacrylamide crystals measured in the frequency range of the proton and deuteron, ν(N-H) and ν(N-D), stretching vibration bands. The basic spectral properties of the crystals were interpreted quantitatively in terms of the "strong-coupling" theory. The proposed model of the centrosymmetric dimer of hydrogen bonds facilitated the explanation of the well-developed, two-branch structure of the ν(N-H) and ν(N-D) bands as well as the isotopic dilution effects in the spectra. The vibronic mechanism of the generation of the long-wave branch of the ν(N-H) band ascribed to the excitation of the totally symmetric proton vibration was elucidated. The complex fine structure pattern of ν(N-H) and ν(N-D) bands in N-phenylacrylamide spectra in comparison with the spectra of other secondary amide crystals (e.g., N-methylacetamide and acetanilide) can be accounted for in terms of the vibronic model for the forbidden transition breaking in the dimers. On the basis of the linear dichroic and temperature effects in the polarized IR spectra of N-phenylacrylamide crystals, the H/D isotopic "self-organization" effects were revealed.     

H/D isotopic recognition in hydrogen bonded systems: H/D isotopic self-organization effects in the IR spectra of the hydrogen bond in 2-methylimidazole crystals

Polarized IR spectra of H12(3)45 2-methylimidazole and of its H1D2(3)45, D1H2(3)45 and D12(3)45 deuterium derivative crystals are reported and interpreted within the limits of the "strong-coupling" theory. The spectra interpretation facilitated the recognition of the H/D isotopic "self-organization" phenomenon, which depends on a non-random distribution of protons and deuterons in the lattices of isotopically diluted crystal samples. The H/D isotopic "self-organization" mechanism engaged all four hydrogen bonds from each unit cell. These effects basically resulted from the dynamical co-operative interactions involving adjacent hydrogen bonds in each hydrogen bond chain. A weaker exciton coupling involved the closely spaced hydrogen bonds; each belonging to a different chain of associated 2-methylimidazole molecules. The high intensity of the narrow band at ca. 1880cm(-1) was interpreted as the result of coupling between the γ(N-H?N) proton bending "out of plane" vibration overtone and the ν(N-H) proton stretching vibration.     

Polarization IR spectra of hydrogen bonded pyrazole crystals: self-organization effects in proton and deuteron mixture systems. Long-range H/D isotopic effects

This paper deals with experimental studies of the polarization IR spectra of solid-state pyrazole H1345, as well as of its H1D345, D1H345 and D1345 deuterium derivatives. Spectra were measured for the vN-H and vN-D band frequency ranges at temperatures of 298 and 77 K. The spectra were found to strongly change their intensity distribution and their polarization properties with the decrease of temperature. These effects were ascribed to some temperature-induced conformational changes in the hydrogen bond lattices. The studies reported allowed the finding of new kind of isotopic effects H/D in the open-chain hydrogen bond systems, i.e. the self-organization effects. It was found that the spectrally active aggregates of hydrogen bonds remain unchanged despite the growing isotope H/D exchange rate. This statement was supported by analysis of the residual polarized vN-H and vN-D band properties, measured for the isotopically diluted crystalline samples. Analysis of the band shapes of the four hydrogen isotope derivative crystals proved the existence of another kind of H/D isotopic effect, i.e. the long-range isotopic effect. It depends on an influence of the pyrazole ring hydrogen atoms onto the vN-H and vN-D band widths and onto the band fine structures.     

Polarized IR spectra of the hydrogen bond in acetic acid crystals

The paper presents the results of our investigations of the polarized IR spectra of the hydrogen bond in crystals of acetic acid, CH₃COOH, as well as in crystals of three deuterium isotopomers of the compound: CH₃COOD, CD₃COOH and CD₃COOD. The spectra were measured at 283 K and at 77 K by a transmission method using polarized light. Theoretical analysis of the results concerned the linear dichroic effects, together with 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 ν_O–H and the ν_O–D bands, respectively. Basic spectral properties of the crystals can be interpreted satisfactorily in terms of one of the quantitative theories of the IR spectra of the hydrogen bond, i.e. the “strong-coupling” theory or the “relaxation” theory when a hydrogen bond dimer model is used. From the spectra obtained it resulted that the strongest exciton coupling involved the closely spaced hydrogen bonds, belonging to different chains of associated acetic acid molecules. These results contradict the former explanation of the spectra within a model, which assumed a strong vibrational exciton coupling between four hydrogen bonds in a unit cell. On analyzing the spectra of isotopically diluted crystalline samples of acetic acid it has been proved that a non-random distribution of the protons and deuterons takes place in the hydrogen bond lattices. This non-conventional isotopic effect is a result of dynamical co-operative interactions involving hydrogen bonds in the system. Simultaneously it has been also found that in an individual hydrogen bonded chain in the crystals, distribution of the hydrogen isotope atoms H and D was fully random. The H/D isotopic “self-organization” mechanism most probably involves a pair of hydrogen bonds from each unit cell where each hydrogen bond belongs to a different chain.     

H/D isotopic recognition in hydrogen-bonded systems: strong dynamical coupling effects in the polarized IR spectra of 3-methylthioacetanilide and 4-methylthioacetanilide crystals

This paper presents the investigation results of the polarized IR spectra of the hydrogen bond in crystals of 3- and 4-methylthioacetanilide. The spectra were measured at 293 and 77 K by a transmission method, with the use of polarized light. The main spectral properties of the crystals can be interpreted satisfactorily in terms of the "strong-coupling" theory, on the basis of the hydrogen bond centrosymmetric 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 3- and 4-methylthioacetanilide molecules. A weaker exciton coupling involved the adjacent hydrogen bonds in each individual chain. It was proven that a nonrandom distribution of the protons and deuterons took place in the lattices of isotopically diluted crystalline samples of 3- and 4-methylthioacetanilide. In each case, the H/D isotopic "self-organization" mechanism involved all four hydrogen bonds from each unit cell.     

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.     

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.     

Polarized IR spectra of the hydrogen bond in two different oxindole polymorphs with cyclic dimers in their lattices

This article focuses on the problem of remarkably strong changes in the fine structure patterns of the ν(N-H) and ν(N-D) bands attributed to the hydrogen and deuterium bonds accompanying the phase transition, which occurs between two polymorphic forms of oxindole. The lattices of these two different crystals contain hydrogen-bonded cyclic dimers differ in their geometry parameters. The source of these differences in the polymorph spectral properties results from the geometry relations concerning the dimers constituting the lattice structural units. In the case of the "alpha" phase, the hydrogen bond lengths of the dimers differ by 0.18 ?. This leads to the "off-resonance exciton coupling" weakly involving the dimer hydrogen bonds. For the "beta" phase, with practically symmetric dimers in the lattice, the spectra become typical for centrosymmetric hydrogen bond systems due to the full resonance of the proton or deuteron vibrations.     

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.     

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.     

Electron-induced phase transition in hydrogen-bonded solid-state 2-pyridone

This paper presents the results of experimental studies of hydrogen-bonded 2-pyridone crystal IR spectra. Spectral studies have demonstrated the existence of two anhydrous solid-state phases of each compound, namely the α and the β phases. Hydrogen bonds in the high-temperature α phase of these crystals have been estimated to be 40% stronger than the hydrogen bonds in the β phase, which are stable at room temperature. The mechanism of the phase transition in the solid-state 2-pyridone is proposed on the basis of the IR spectral data. This was possible by taking into account small changes in the geometry of heterocyclic molecular skeletons, which accompany the electron density redistribution in the hydrogen bonds occurring during the transition. The phase transition is connected with a partial change in the hydrogen bond nature from the N(+)-H···O(-) in the α phase, to the N-H···O hydrogen bonds in the β phase crystals.     

The progression of strong and weak hydrogen bonds in a series of ethylenediammonium dithiocyanate derivatives--a new bonding protocol for macromolecules?

The experimental charge densities for a series of sym-N-methyl-substituted ethylenediammonium dithiocyanate salts have been investigated based on low-temperature and high-resolution X-ray diffraction data. This series of organic dications provides both strong and weak hydrogen bonding networks that vary depending on the N-H : SCN(-) (donor/acceptor) ratios. The number of N-HN hydrogen bonds connected to each cation increases (linear to bifurcated) as the number of N-H donor groups increases. The bifurcated thiocyanate anions also form a less energetic N-HS hydrogen bond. The presence of more than one hydrogen bond acceptor on each thiocyanate anion results in a competition between the sulfur and nitrogen atoms in forming both strong and weak hydrogen bonds. The formation of a significant number of weak hydrogen bonds is shown to play a crucial role in stabilizing these organic ionic crystals. The progression of these organic dications (smaller to larger N-H : SCN(-) ratios) results in the weaker hydrogen bonds playing a smaller role in stabilizing the crystalline structures. In addition, the electron density along the saddle point has been shown to vary significantly from weak hydrogen bonds to van der Waals interactions. This has led to a better understanding of the progression of hydrogen bonding in the crystalline states of sym-N-methyl substituted ethylenediammonium dithiocyanate salts and provides insight into the relationship between strong and weak hydrogen bonds in organic ionic crystals.     

Excited-state N-H···S hydrogen bond between indole and dimethyl sulfide: time-dependent density functional theory study

The intermolecular hydrogen bond N-H···S between indole and dimethyl sulfide is theoretically investigated. The formation of N-H···S hydrogen bonds between indole and dimethyl sulfide in ground and excited states is confirmed by the analysis of geometric structure, Mulliken charge, and infrared spectra. The result shows that the S(1) state of hydrogen bonded indole-Me(2)S is mainly a charger transfer state, while the S(2) state is a local excited state and also the state corresponding to the experiment. More importantly, it is demonstrated that the intermolecular hydrogen bond N-H···S of indole-Me(2)S is strengthened in the S(1) and S(2) states compared to that in ground state. Moreover, the strengthening of intermolecular N-H···S hydrogen bond in excited state induces the fluorescence emission peak of indole shifts to the red. These findings may provide insights for further study of N-H···S hydrogen bonds existing in many biomolecular systems.     

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.     

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.     

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

This paper deals with the polarized IR spectra of the hydrogen bond in pimelic acid crystals and their quantitative interpretation. The spectra were measured for the vO-H and vO-D band frequency regions, at temperatures of 298 and 77 K, for the 'alpha' crystalline form of pimelic acid. Two kinds of transmission spectra were obtained, for the beam perpendicular to the two different crystalline faces: 'ac' and 'ab'. The spectra of the hydrogen and the deuterium bonded systems were quantitatively reproduced, with sufficient accuracy, on the basis of the 'strong-coupling' model, assuming the (COOH)2 cycles to be the structural units responsible for the basic spectral properties. It was found that the spectra could be reproduced only, when assuming spectral activity in the IR of the totally symmetric proton stretching vibrations in centrosymmetric cyclic dimers of hydrogen bonds. The polarization effects in the crystal spectra were interpreted as directly connected with the hydrogen bond orientation in the crystal. However, in the pimelic acid crystalline spectra there were not observed another polarization effects characteristic for another carboxylic acid crystals, depending on differentiation of the long- and of the short-wave branch properties of each of the vO-H and vO-D bands. The temperature variation only affect the intensity ratio between the lower- and the higher-frequency branches of the vO-H and vO-D bands. These spectral effects were ascribed to less strained hydrogen bond structures in the pimelic acid lattices of the alpha' crystalline form and a relatively low concentration of defects in the lattices.     

Hydrogen bonds not only provide a structural scaffold to assemble donor and acceptor moieties of zinc porphyrin-quinone dyads but also control the photoinduced electron transfer to afford the long-lived charge-separated states

A series of zinc porphyrin-quinone linked dyads [ZnP-CONH-Q, ZnP-NHCO-Q, and ZnP-n-Q (n = 3, 6, 10)] were designed and synthesized to investigate the effects of hydrogen bonds which can not only provide a structural scaffold to assemble donor and acceptor moieties but also control the photoinduced electron-transfer process. In the case of ZnP-CONH-Q and ZnP-NHCO-Q, the hydrogen bond between the N-H proton and the carbonyl oxygen of Q results in the change in the reduction potential of Q. The strong hydrogen bond between the N-H proton and the carbonyl oxygen of Q*- in ZnP-CONH-Q*-,ZnP-NHCO-Q*-, and ZnP-n-Q*- (n = 3, 6, 10) generated by the chemical reduction has been confirmed by the ESR spectra, which exhibit hyperfine coupling constants in agreement those predicted by the density functional calculations. In the case of ZnP-n-Q (n = 3, 6, 10), on the other hand, the hydrogen bond between two amide groups provides a structural scaffold to assemble the donor (ZnP) and the acceptor (Q) moiety together with the hydrogen bond between the N-H proton and the carbonyl oxygen of Q, leading to attainment of the charge-separated state with a long lifetime up to a microsecond.     

Crystal structure and vibrational spectra of piperazinium bis(4-hydroxybenzenesulphonate) molecular-ionic crystal

The piperazinium bis(4-hydroxybenzenesulphonate) crystallizes from water solution at room temperature in P2(1)/c space group of monoclinic system. The crystals are built up of doubly protonated piperazinium cations and ionized 4-hydroxybenzenesulphonate anions that interact through weak hydrogen bonds of O-H...O and N-H...O type. Mutual orientation of anions is determined by non-conventional hydrogen bonds of C-Hcdots, three dots, centeredpi type. Room temperature powder FT IR and FT Raman measurements were carried out. The vibrational spectra are in full agreement with the structure obtained from X-ray crystallography. The big single crystals of the title salt can be grown.     

The quasi-symmetric OHN and ODN bridges in the adducts of 3,5-dimethylpyridine with 3,5-dinitrobenzoic acid

The crystal structure of the adduct of 3,5-dimethylpyridine and 3,5-dinitrobenzoic acid (DMP-DNB) has been determined at room temperature and 80 K for both undeuterated and deuterated compounds. The monoclinic crystals are isomorphous, space group P2₁/c and Z = 4. Very strong OHN hydrogen bonds are almost linear with fully disordered (1:1) bridge hydrogen atoms between oxygen and nitrogen atoms. This is well reflected in the difference in electron density maps the contours of which depend both on cooling and deuteration. The intramolecular hydrogen bond lengths are 2.550(2) Å for the (OHN) and 2.563(2) Å for (ODN) at room temperature and 2.529(2) Å for (OHN) and 2.531(2) Å for (ODN) at 80 K. Therefore, there is a small but meaningful isotope effect upon the O…N hydrogen bridge length at room temperature and no Ubbelohde isotope effect is observed at 80 K. The infra-red spectra show very broad stretching protonic bands in the 200–1600 cm⁻¹ range. The isotopic ratio v(H)/v(D) at room temperature is about 1.1.