Electron-Induced Phase Transition in Hydrogen-Bonded Solid-State 2

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Electron-Induced Phase Transition in Hydrogen-Bonded Solid-State 2-Pyridone Henryk T. Flakus,*,† Aleksandra Tyl,† and Andrzej Maslankiewicz‡ † ‡

Institute of Chemistry, University of Silesia, 9 Szkolna Street, Pl-40 006 Katowice, Poland Department of Organic Chemistry, The Medical University of Silesia, 4 Jagiellonska Street, Pl-41 200 Sosnowiec, Poland ABSTRACT: 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 R and the β phases. Hydrogen bonds in the high-temperature R 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 Nþ-H 3 3 3 O- in the R phase, to the N-H 3 3 3 O hydrogen bonds in the β phase crystals.

1. INTRODUCTION IR spectroscopy has always been considered to be a powerful tool in hydrogen bond research. Spectral properties of hydrogen bonds were the subject of particularly intensive research interest during the 1970s. For hydrogen bonds of a general structural formula X-H 3 3 3 Y, the higher frequency spectral range, attributed to the proton stretching vibrations νX-H, was thoroughly investigated,1-5 since the most characteristic spectral effects ascribed to the hydrogen bond appear in this spectral region. Problems connected with the theoretical interpretation of the band intensity distribution, with the H/D isotopic effects and with very complex temperature effects in the spectra, became a challenge for the theory and led to a rapid development of numerous theoretical models.2-9 The researchers became particularly interested in spectral properties not only of single, isolated hydrogen bonds but also of far more complex aggregates such as hydrogen bond dimers, including those formed in the gas phase.6,7,10-13 Nevertheless, even for simple systems such as cyclic, centrosymmetric hydrogen bond dimers, spectral properties are still not fully understood, especially in reference to the H/D isotopic effects on the intensity distribution in the IR spectra. Spectra of hydrogenbonded molecular crystals are a potentially rich source of information. Molecular crystals provide a diversity of spatially ordered hydrogen bond systems, which cannot be generated in the gaseous or in the liquid phase. The solid state, however, generates some new and specific theoretical problems, which need to be solved to make the interpretation of crystalline spectra possible. These involve mutual interactions between hydrogen bonds in crystal lattices. r 2011 American Chemical Society

The IR spectra of the hydrogen bond are strongly influenced by temperature, by the condensation state of matter and, in the case of the solid state, by the spatial arrangement of the hydrogen bonds in the lattice.3,5 The latter effect seems to be of particular interest since it could constitute the basic reason for the diversity of spectral properties in spatially ordered hydrogen bond systems. To date, there still exists an incomplete understanding of the influence of the hydrogen bond structure on the spectral properties of molecular crystals and of the specific mechanisms governing interactions in hydrogen bond aggregates. In most recent studies it has been found that molecular crystals containing infinite chains of hydrogen bonds form very promising model systems for such investigations. For elaboration of a general theory on such phenomena the understanding of the influence of phase transitions in the solid state on the spectral properties of hydrogen-bonded molecular systems is of particular importance. Unfortunately, reports on such studies are extremely rare in literature and the observed spectral effects accompanying the phase transitions in crystals are usually small (e.g., oxalic acid 14-16 and 2-butynoic acid crystals 17-19 ). In this paper, results of experimental and theoretical studies are presented for 2-pyridone crystals.20-22 For many years the problem of tautomerism of 2-pyridone dimers has been intensively investigated using advanced experimental and theoretical methods.23-26 This molecular system is also particularly interesting since by chance we managed to identify a phase Received: September 13, 2010 Revised: December 17, 2010 Published: January 19, 2011 1027

dx.doi.org/10.1021/jp108717v | J. Phys. Chem. A 2011, 115, 1027–1039

The Journal of Physical Chemistry A transition in the solid state. In this case the phase transition is responsible for some spectacular but quite puzzling spectral changes in the frequency ranges of the proton and deuteron stretching vibrations in the hydrogen and deuterium bonds of the crystal.

2. CRYSTAL STRUCTURE OF 2-PYRIDONE When we started our studies, only one commercial polymorph form of 2-pyridone was known. Crystals of the commercial 2-pyridone are orthorhombic, space group P212121, a = 13.585(2) Å, b = 5.806(3) Å, c = 5.603(2) Å, Z = 4. The associated molecules form twisting open molecular chains linked via the N-H 3 3 3 O hydrogen bonds20-22 that elongate along the “c” axis. The only nonconventional C-H 3 3 3 O interactions (H 3 3 3 O 2.57 Å) are presumably weak. 3. EXPERIMENTAL SECTION 2-Pyridone used for our studies was a commercial substance (Sigma-Aldrich). It was used without further purification. Crystals of 2-pyridone suitable for spectral studies were obtained by crystallization from melt between two closely spaced CaF2 windows. In this way sufficiently thin crystals could be obtained, characterized by their maximum absorbance at the νO-H band frequency range close to 0.5. From the crystal mosaic, monocrystalline fragments were selected and then spatially oriented with the help of a polarization microscope. The investigated crystals obtained in this way appeared to be unstable. Measurements of IR spectra of crystals grown from the melt, performed at different times after crystallization, incidentally revealed a phase transition in the solid state. This irreversible process occurs at room temperature, up to ca. 3 days after crystal forming the high-temperature phase (which we term the R phase). As a result of this transformation, a new crystalline phase (the β phase) is obtained, which is stable at room temperature. Its properties appeared to be identical with those of commercial 2-pyridone. It was also found that the phase transition, from the R toward the β phase, could be easily induced by mechanical perturbation exerted on the R phase sample. The single crystals of the R phase formed rectangular plates. For the purpose of the experiment they were exposed by using on a tin diaphragm with a 1.5 mm hole diameter. The solid-state spectra were measured by a transmission method at room temperature and at the temperature of liquid nitrogen, with the help of the FT-IR Nicolet Magna 560 spectrometer using a nonpolarized beam and in the latter case with the use of polarized IR radiation. In this case two component spectra were recorded for two mutually perpendicular polarization directions of the “E” vector. Each component polarized spectrum corresponded to an individual optical axis (“E” parallel to one individual axis of the indicatrix). The spectra were measured at 2 cm-1 resolution. Measurements of spectra were completed in a similar way for the crystals of deuterium derivative of 2-pyridone, which was synthesized by evaporation of the solution in D2O under reduced pressure. 4. RESULTS The spectra of 2-pyridone dissolved in CCl4, measured in the frequency ranges of the νN-H proton stretching vibration band, are presented in Figure 1. The corresponding spectrum of commercial polycrystalline sample of the 2-pyridone, measured

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in KBr pellets at room temperature, is also shown. The figure also presents the νN-H band shapes, measured at room temperature, for samples of the R and the β phase crystals. They illustrate the changes in the hydrogen bond spectra of 2-pyridone accompanying the phase transition. Spectra of deuterium-bonded 2-pyridone recorded in similar experimental conditions (i.e., for CCl4 solution and by utilizing the KBr pellet technique) and measured in the frequency ranges of the νN-D deuteron stretching vibration band are shown in Figure 2. These spectra are compared with the νN-D band shapes, measured at room temperature, for samples of the deuterated R and the β phase crystals. They serve to illustrate the changes in the hydrogen bond spectra of 2-pyridone, accompanying the phase transition. A comparison of the band shapes from the CCl4 spectra of 2-pyridone with the corresponding spectra measured for KBr pellets reveals some substantial differences. The νN-H band positions differ considerably, since the polycrystalline sample spectrum is shifted toward the higher frequencies by ca. 250 cm-1 for the νN-H band whereas for the νN-D band the frequency difference of the compared bands is markedly lower and is equal to ca. 10 cm-1. In the case of the νN-H bands these frequency changes can be intuitively ascribed to different structural units from which cyclic dimers in the liquid phase and infinite, open chains in the solid state can originate. It is symptomatic that similar changes in the band location accompany the phase transition from the R toward the β phase. Moreover, there is a close correspondence between the compared spectra: the CCl4 solution spectrum resembles the R phase spectrum and the commercial sample spectrum measured in KBr pellet is very much alike the β phase spectrum recorded immediately after the phase transition. Surprisingly, a similar conclusion is not valid for the νN-D bands. In the case of the solid state the R and the β phases of their νN-D bands exhibit a qualitatively similar effect of the band frequency change: the R phase band appears at ca. 85 cm-1 lower frequency than the location of the β phase band (see Figure 2b). However, an analogous νN-D band position shift effect is practically absent when we compare the νN-D band positions in the spectra measured for the CCl4 solution and in the KBr pellet for deuterated samples of the commercial 2-pyridone. The two bands are almost identical with the β phase νN-D band. The discussed band frequency shift effect is, therefore, only an attribute of the two crystalline phases of deuterium- bonded 2-pyridone obtained from the melted substance. Polarized IR spectra of the R phase crystals measured in the frequency range of the νN-H band are given in Figure 3. In this case each component spectrum corresponds with the orientation of the electric field vector “E” parallel to an individual axis of the crystal indicatrix. It is noteworthy that the contours of the two component polarized bands in the spectrum are mutually proportional, although the integral component band intensities considerably differ, one from the other. Analogous spectra of isotopically diluted R phase crystals of 2-pyridone measured in the frequency range of the νN-D band are shown in Figure 4.

5. NATURE OF THE PHASE TRANSITION IN THE SCOPE OF THE IR SPECTROSCOPY The nature of the phase transition can be identified in an indirect way by the analysis of the νN-H and the νN-D band 1028

dx.doi.org/10.1021/jp108717v |J. Phys. Chem. A 2011, 115, 1027–1039

The Journal of Physical Chemistry A

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Figure 1. (a) IR spectra of 2-pyridone in the CCl4 solution and the spectra of polycrystalline samples of commercial 2-pyridone dispersed in KBr pellets, measured at room temperature. The νN-H band. (b) IR spectra of the two solid-state phases of 2-pyridone, measured at the room temperature. The νN-H band.

shapes as well as from other spectral properties of the two crystalline phases, including band frequencies. The νN-H bands for the R phase crystals are fairly similar in shape to the corresponding bands registered for the CCl4 solution of 2-pyridone. From literature data it results that this compound forms cyclic dimers in nonpolar solvents.27 Analogous observations are also valid for the CCl4 solution and the solid-state spectra of 2-thiopyridone.28,29 It is noteworthy that the νN-H band shape in the IR spectra of 5-nitro-2-pyridone crystals is also similar to the compared spectra of R 2-pyridone. In the latter case the molecules are also held together via N-H 3 3 3 O hydrogen bonds, forming cyclic dimers.27 The room-temperature spectra of 2-pyridone in CCl4 solution, crystals of the R 2-pyridone, 2-thiopyridone and 5-nitro-2-pyridone, measured in the νN-H band frequency range, are compared in Figure 5. This comparison supports the hypothesis that in both cases of 2-pyridone, i.e., in the crystal and in the CCl4 solution, identical structural units are the source of spectral properties.

It is also noteworthy that spectra of 2-pyridone and 2-thiopyridone in CCl4 solution are strikingly similar in terms of their νN-H band shapes. This behavior is by no means coincidental. Polarized IR spectra of thin monocrystalline layers of this polymorph species can provide an essential information on the way in which the molecules associate via hydrogen bonds. To gain this knowledge, the polarized spectra were measured in the frequency ranges of the νN-H and νN-D bands for two mutually perpendicular directions of the “E” vector of the incident light. In these circumstances any complexity of the band structure should be reliably evidenced. The key property of the spectra from Figures 3 and 4 is the proportionality relation between the component band contours in each polarized spectrum. This proves the invariability of the vibrational transition moment vector direction within the frequency range of each band in the crystalline spectra. This is a common spectral property of crystals whose lattices are formed by centrosymmetric cyclic hydrogen-bonded dimers.30-35 In the case of chain structures of hydrogen-bonded associates in crystal 1029



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Figure 2. (a) IR spectra of deuterium-bonded 2-pyridone in the CCl4 solution and the spectra of polycrystalline samples of commercial deuteriumbonded 2-pyridone dispersed in KBr pellets measured at room temperature. The νN-D band. (b) IR spectra of the two solid-state phases of deuteriumbonded 2-pyridone, measured at room temperature. The νN-D band.

lattices a characteristic linear dichroic effect in the spectra is expected. This effect helps to differentiate the spectral properties of the two opposite branches of each band, νN-H and νN-D. The dichroic properties of the R phase crystal spectra strongly support the choice of the hypothetical crystal structure with centrosymmetric cyclic dimers in the sites of the lattice. The same X-ray structure was postulated in the case of 2-thiopyridone crystal.36-38 We therefore felt justified in assuming that only cyclic dimers of 2-pyridone exist in the crystal lattice of the R form, which is analogous to the established structure of 2-thiopyridone. Crystal spectra of the two compounds reveal some striking similarities, which serve as an indirect indication of the R phase crystal structure.

6. STRUCTURE OF THE R PHASE CRYSTALS IN THE SCOPE OF THE MOST RECENT X-RAY STUDIES Only recently, during preparing the manuscript of the main part of this article, the X-ray structural data of a new monoclinic

crystalline phase of 2-pyridone was published.39 These crystals belong to the P21/n space group. Lattice constants: a = 6.2027(13) Å, b = 16.327(4) Å, c = 9.1046(18) Å, β = 92.242(7)°, and Z = 8. In the crystal, hydrogen-bonded centrosymmetric dimers constitute the structural units of the lattice. These new crystalline phase corresponds to the popular monoclinic phase of 2-thiopyridone.36-38 At this point the relation between the R phase and the monoclinic phase of 2-pyridone had to be estimated. Following the published method of growing the monoclinic phase crystals given in ref 39 these crystals were successfully prepared. In the next step IR spectra of the monoclinic crystals were measured. It was found that their spectra were identical with the corresponding spectra of the R phase. The monoclinic crystals also appeared to be unstable, changing to the familiar β phase crystals. From these estimations it results that the R phase crystals are identical with crystals of the monoclinic phase described in ref 39. It appeared that the IR spectral study of the R phase crystals, before performing a successful crystal structure determination by 1030



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Figure 3. Polarized spectra of the R phase single crystal of 2-pyridone measured in the νN-H band frequency range: (a) room temperature; (b) 77 K. The component spectra (I) and (II), drawn in a common scale, were obtained for two perpendicular orientations of the electric field vector E: Spectrum (I) - E parallel to the longer edge of a rectangular crystalline plate. The crystalline face was nonidentified.

X-ray, gave a reliable identification of the structure of the molecular associates constituting the structural units of this polymorph form of 2-pyridone. This illustrates the value of the IR spectra analysis in the determination of molecular complex structures.

7. INITIAL APPROACHES The main problem to be solved is why such substantial changes in the hydrogen bond IR spectra of 2-pyridone crystal accompany the evolution of cyclic dimers of hydrogen bonds for the R phase toward infinite open chains for the β phase. The center of gravity of the νN-H band in the spectrum of the β phase is shifted by about 250 cm-1 toward higher frequencies with respect to the corresponding band location in the spectrum of the R phase. This fact may be interpreted in terms of the diminution of the hydrogen bond energies by ca. 40%, due to phase transition.2-5 It is extremely surprising, however, that the 2-pyridone crystals of the R phase, characterized by stronger hydrogen bonds are unstable and transform into the β phase

crystals with appreciably weaker hydrogen bonds. This is observed if these interactions mainly determine the crystal lattice energy. Qualitatively similar, although much weaker, dependence concerns the corresponding spectral properties of two crystalline phases of the oxalic acid. Also in this case the phase containing cyclic hydrogen bond dimers is characterized by the νO-H band, which is more strongly shifted toward the lower frequencies than the spectral properties of the second phase with hydrogen bond chains in the lattice.14-16 A possible approach to the problem of the nature of phase transition could be based on the assumption of proton transfer in each N-H 3 3 3 O hydrogen bond. It would occur from the nitrogen atom toward the oxygen atom and lead to an O-H 3 3 3 N hydrogen bond. The two crystalline phases of 2-pyridone would then differ in the proton positions in the N 3 3 3 O bridges. The X-ray structure determination of the stable (β) form of 2-pyridone by Penfold, in the early 1950s identified the tautomeric form of the compound not as 2-hydroxypyridine, but as 2-pyridone, with protons bonded to nitrogen atoms.20 The structure was redetermined recently using modern methods.21,22 1031



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Figure 4. νN-D bands in the polarized spectra of the R phase single crystal of deuterium-bonded 2-pyridone measured at two different temperatures. Other experimental conditions and the presentation of the spectra are identical to those given in Figure 3.

The proton positions were also confirmed in our studies. Hence it is absolutely certain that the hydrogen positions in the hydrogen bonds have been fully proved. We do not believe that the nature of the phase transition can be ascribed to the change in the contribution of energy by the socalled nonconventional hydrogen bonds to the crystal lattice.40 This is due to the fact such energetic effects should be negligibly small in comparison to the hydrogen bond energy changes deduced from the νN-H band shift. Therefore, another cause of the phase transition has to be sought. However, this requires, further systematic investigations of these spectral phenomena, involving the use of various experimental methods.

8. TENTATIVE EXPLANATION OF THE PHASE TRANSITION MECHANISM We will show that the phase transition spectral effects most probably result from the changes in the electronic structure of 2pyridone hydrogen-bonded cyclic dimers occurring in the R phase. We present a list of arguments in support of the hypothesis

that assumes that in crystals of the R phase the molecules are linked by Nþ-H 3 3 3 O- hydrogen bonds to form centrosymmetric cyclic dimers. These are most probably the structural units responsible for the basic crystal properties. There are certain facts that suggest that the hydrogen bond energy change accompanying the phase transition is determined by a more complex rearrangement in the associated molecular structures than just a simple jump of protons in the hydrogen bonds. This process would involve mainly the electronic structure of 2-pyridone molecules together with the pyridine rings. Changes in the electronic structure might also be coupled with some slight conformational changes within the pyridine rings, corresponding to a transition from the ketone form of the compound toward the zwitterion form (Scheme 1). The results of our spectroscopic studies have proved that in the case of cyclic dimers in the R phase of 2-pyridone a single hydrogen bond is considerably stronger than a single hydrogen bond in a chain of molecules in the β phase. This fact suggests that the Nþ-H 3 3 3 O- bonds exist only in the R phase and that these bonds are obviously stronger than the N-H 3 3 3 O 1032



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Figure 5. Comparison of the νN-H band shapes from the spectra of 2-pyridone, 2-thiopyridone and 5-nitro-2-pyridone: (a) 2- pyridone in CCl4 solution; (b) the R phase of 2-pyridone; (c) 2-thiopyridone in CCl4 solution; (d) polycrystalline 5-nitro-2-pyridone sample in KBr pellet.

Scheme 1. Electronic Structures of 2-Pyridone Molecules in the Two Phases

hydrogen bonds in the β phase, since ionic hydrogen bonds are generally known to be stronger than the neutral ones.2,3,5,40 This corresponds with the structural data characterizing the geometry of hydrogen bonds in the two different polymorphs: In the R phase the hydrogen bonds are shorter when compared with the hydrogen bond lengths in the β phase.20-22,39 For the tautomeric ketone form the electronic structure of the pyridine ring is perturbed compared with the one of

2-hydroxypyridine. This is associated with the routine assumption of sp3 hybridization of the nitrogen atoms in 2-pyridone rings. This also means a loss of aromatic properties on forming the ketone structure. A reduction of aromaticity should be accompanied by a slight bending in the pyridine ring and should not be energetically neutral. The N-H bond direction should also be nonparallel to the ring plane. Therefore, some tension effects in the molecules can be expected in the crystal exerted by intermolecular interaction forces. This, in turn, should cause some energetic destabilization of the lattice, despite the fact that the hydrogen bonds are more stable. 8.1. Electronic Structure Identification of 2-Pyridone Molecules in the Two Phases. There are some other arguments supporting the statement that the β phase crystals of 2-pyridone, which appeared thermodynamically more stable, contain N-H 3 3 3 O hydrogen-bonded chains formed by the nonaromatic molecules. On the other hand, the R phase crystals most probably contain the zwitterion form of 2-pyridone molecules, which form centrosymmetric dimers constituting the lattice structural units. 1033


The Journal of Physical Chemistry A Scheme 2. Electronic Structures of Pyridone and Thiopyridone Derivatives

The transformation of the molecules, leading from the zwitterion-type in the R phase, toward neutral ones in the β phase, should result in a nonplanar geometry of the pyridine rings and in a slight differentiation of C-C bond lengths, which in these conditions is energetically privileged structure. Such a nonideally planar ring structure in 2-pyridone in the β phase, with differentiated C-C bond lengths confirmed by X-ray studies,20,21 corresponds to a nonaromatic aromatic character of the pyridine rings. In the case of the related structure of 2-thiopyridone crystal, an almost ideally plain ring structure of pyridine rings, with less differentiated C-C bond lengths than in 2-pyridone β phase crystal has been observed. In its lattice there are also centrosymmetric cyclic dimers constituting the lattice structural units.29,36-38 This means a better-pronounced contribution of the aromatic character of the pyridine rings to the molecular electronic structure as well as to the zwitterion electronic structure of 2-thiopyridone molecules in the crystal. At this point it is worth recalling that the compared spectra of 2-thiopyridone and 2-pyridone measured in CCl4 solution and in the R phase are fairly similar. Unfortunately, the hypothetical crystalline phase of 2-thiopyridone with infinite chains of hydrogen-bonded molecules constituting the counterpart of the β phase of 2-pyridone still remains unknown. On comparison of the molecular geometry of 2-pyridone in the two different solid-state phases it results that in the monoclinic R phase chemical bond lengths in pyridine rings are only slightly less differentiated than in the molecules in the β phase. However, this effect remains in the experimental error limits of X-ray diffraction methods of the chemical bond length determination.22,39 Similarly uncertain structural data relation concerns the pyridine ring bending in the crystalline lattices of the two compared phases. The comparison of the molecular structural data is also complicated by the fact that in a unit cell of the monoclinic crystal there are two independent molecules, which differ, one from the other, by their geometry.39 Therefore, from the X-ray data no decisive conclusion about the value of the proposed mechanism can be derived. Our hypothesis concerning the mechanism of the discussed phase transition finds strong support in X-ray studies on crystal structures of relative molecular systems. The zwitterion structures appeared, in fact, to be the widespread forms of diverse pyridone and thiopyridone derivatives in crystals. For instance, it was found that 1-alkyl-4(1H)-pyridine and quinolinethiones41,42 as well as 1-methyl-4(1H)-quinoline43 revealed two interesting molecular features (Scheme 2), namely: (i) planarity of pyridine rings and (ii) planarity around the endocyclic nitrogen atom

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(indicating the sp2 hybridization of the N atom). From these structural data it was concluded that the pyridinium-type resonance form is the major contributor to the real molecular structure in the solid state of the pyridine derivatives mentioned above. All the above examples indicate that, in contrast to molecular geometry rules resulting from the chemical bond hybridization theory, planar ring structures are common in the case of stable crystalline phases. However, in some cases, crystals with plain geometry of pyridine rings, provided they are formed, may remain thermodynamically unstable due to the influence of the force field of relatively low symmetry generated by the surrounding molecules (e.g., the R phase crystal of 2-pyridone). At this point it should be remembered that the total energetic effect of hydrogen bonding also includes changes in the geometry of component molecules of a hydrogen-bonded complex. Such an approach is applied when quantum chemical calculations of molecular complex bonding energies are performed.3-5 From the IR spectroscopic data only a change in the N-H bond force constant can be estimated, which provides information about the hydrogen bond energies of the two different phases.2-5 Other contributions to the crystal lattice energy result from electronic structure and conformational changes in associating molecules. Generation of even slightly deformed pyridine ring structures in associated 2-pyridone molecules could critically influence the total energy balance of lattice formation for β phase crystals, despite their weaker hydrogen bonds. It seems that such a change in the ring geometry might be stimulated by forces of a nonspherical symmetry acting on each 2-pyridone molecule in the R phase crystal. As a consequence, the crystal lattice is rebuilt by cyclic hydrogen-bonded dimers forming the lattice structural units in the direction of the chain structure of the associates. A relatively low energy barrier separates the two energetic states of 2-pyridone, corresponding to the two phases, which is fairly easy to overcome at room temperature. The proposed mechanism of the phase transition may be supported by the fact that at the same time we have observed a similar phase transition in spectra of anhydrous 4-pyridone crystals.44 In this case, the phase transition also leads to weaker hydrogen bonds for the stable room-temperature phase. This in turn results in an increase of the proton vibration frequency in IR spectra. With regard to the mutual locations of functional groups in 4-pyridone molecules, any idea of cyclic dimers must be excluded and only a chain arrangement of hydrogen bonds is possible for each crystalline anhydrous phase.45 8.2. Arguments Resulting from the Analysis of the νN-H and νN-D Band Shapes. Some arguments supporting the choice of the electronic structure of 2-pyridone molecules in each individual crystalline phase can be derived from the quantitative analysis of the νN-H and νN-D band contour shapes, performed within the terms of the so-called “strong- coupling” theory. This model succeeded many times in the quantitative interpretation of IR spectra of hydrogen-bonded molecular systems.6,7,10-12,29-35,46-48 For this purpose a subtle, nonconventional spectral effect in the unique intensity distribution in the νN-H and νN-D bands should be analyzed. It has only recently been identified and applied for the quantitative interpretation of the spectra of crystals with the O-H 3 3 3 S and N-H 3 3 3 S bonded dimers.29,30,46,47 We will base our interpretation of the R phase spectra on the observed similarity of this spectrum to the spectrum of 2-pyridone measured in the CCl4 solution as well as on their similarity 1034


The Journal of Physical Chemistry A to the spectra of 2-thiopyridone in the solid-state phase and in the CCl4 solution.29 We will also take into account the interpretation results of the polarized IR spectra of 2-thiopyridone29 and 3-hydroxy-4-methyl-2(3H)-thiazolothione crystals.47 The basic conclusions of the cited works may be applied for the interpretation of the R phase spectra. From the comparison of the spectra presented above it results that cyclic centrosymmetric 2-pyridone dimers are most likely to form the structural units of the R phase crystal lattice. The X-ray structure of the crystal probably resembles the crystal structure of 2-thiopyridone.36-38 Basing on the results of the investigation of the IR spectra of the hydrogen bond in diverse molecular crystals it may be concluded that the characteristic shapes of the νN-H band are unique and typical for a small group of centrosymmetric hydrogen bond dimeric systems, including the 2-thiopyridone29 and 3-hydroxy-4-methyl-2(3H)-thiazolothione crystals.47 Spectra of this type of hydrogen bond dimer are generated by two component transitions, which form two branches of the νN-H band. In this case the lower-frequency branch of the band corresponds with the non totally symmetric proton stretching vibrations in the centrosymmetric dimers, which are anharmonically coupled with the low-energy N 3 3 3 S hydrogen bridge stretching vibrations.29,47 These vibrations are active in IR by the symmetry rules. In turn, the higher-frequency spectral branch corresponds to the totally symmetric proton vibrations. These vibrations, although theoretically nonactive in IR, are activated by a vibronic coupling mechanism, which is a kind of reverse of the familiar Herzberg-Teller mechanism from the electronic spectroscopy of aromatic hydrocarbons.49 In this case proton-stretching vibrations of different symmetries are mixed one with another via coupling with electronic motions in the dimers.50 In the case of the R 2-pyridone crystal the spectra generation mechanism is probably very similar. 8.3. Model Calculations of the νN-H and νN-D Band Contours. To elucidate the mechanisms of the unusual νN-H band shape generation, we utilized a simple quantitative theoretical model elaborated in the past for simulation of the spectra of the hypothetical hydrogen bond dimer IR. First, the model calculations aiming at reconstituting the “residual” νN-H and νN-D band shapes were performed in terms of the “strong-coupling” theory, for a centrosymmetric cyclic hydrogen-bonded dimer model. In the past such an approach allowed for a successful quantitative reconstitution of the νX-H band shapes of many spectra of hydrogen-bonded dimer systems.6,7,46 The model calculations aiming at interpreting the analyzed spectra quantitatively were able to deliver results, which effectively illuminated the problems of the intra- and the interdimer interactions in the crystal. A typical νX-H band in the spectrum of a centrosymmetric cyclic dimeric hydrogen bond system is usually composed of two spectral branches.7,46,50 The lower-frequency branch of the νX-H band in cyclic dimer spectra usually has a regular fine structure pattern, with a regular Franck-Condon type vibrational progression. However, the narrower band contour characterizes the higher-frequency branch of the dimeric band, with a tendency to blur the fine structure of the band. The higher-frequency component branch of the band used to usually correspond with the symmetry allowed transition to the excited state of the nontotally symmetric X-H stretching vibration. On the other hand, the lower-frequency sub-band was usually connected with the symmetry forbidden totally symmetric X-H vibrations in the dimer and was usually of a lower intensity. The forbidden

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transition in IR is activated via a vibronic mechanism.50 The relative intensity of the spectral branch of the activated transition depends on the effectiveness of the promotion mechanism, which remains in close relation with the electronic properties of the associating molecules.30,50 In the limits of the “strong coupling” theory applied to a centrosymmetric, hydrogen-bonded cyclic dimer model, the νX-H band shape basically depends on three coupling parameters: bH, C0, and C1.6,7,46 The bH parameter describes the change in equilibrium geometry for the low-frequency stretching vibration of the hydrogen bridge, accompanied by the excitation of the high-frequency X-H stretching vibration to its first excited state. This parameter determines the νX-H bandwidth in the IR spectra of dimers and also governs its fine structure pattern. The C0 and C1 parameters are responsible for the energy of the resonance interactions in the system of hydrogen bonds. They denote the subsequent expansion coefficients in the series, which are derived by developing the resonance interaction integral C with respect to the normal coordinates of the low frequency νX 3 3 3 Y hydrogen bond stretching vibrations in a dimer. The C0 parameter determines the splitting of the component bands of the dimeric spectrum corresponding to the excitation of the proton vibration motions of different symmetry. The C1 parameter is connected with the bandwidths of an individual νX-H band component.46 However, on the basis of the generally applied version of the “strong-coupling” theory proposed for centrosymmetric cyclic dimers of hydrogen bonds, it appeared impossible to quantitatively reconstitute the fine structure pattern of X-H and X-D stretching bands in the crystalline spectra of 2-pyridone. The main reason for this was the fact that the spectra look like a mirror reflection of a typical νX-H band contour from the spectra of the previously examined ca. 40 systems of centrosymmetric dimers of hydrogen bonds (e.g., in carboxylic acids crystals).30-35,46 Theoretical reproduction of the νN-H and νN-D band contours was only possible in one particular case, when a reverse sign of the exciton coupling energy in hydrogen bond dimeric systems was assumed. As a result of this assumption, a change of the spectral branch sequence of the component bands in the hydrogen bond dimer IR spectra was possible. We performed appropriate model calculations, aiming at reconstituting the proton and deuteron stretching vibration bands, νN-H and νN-D, respectively. This was done in the limits of the “strong coupling” model by assuming the “reversal” mechanism of the exciton interactions47 in 2-pyridone dimers. 8.3.1. νN-H Band. Figure 6 shows the results of model calculations of the νN-H band contour for the IR spectra of crystalline 2-pyridone samples, isotopically diluted by deuterium (ca. 20% H and 80% D). On simulating the νN-H band contours, the so-called dimeric “minus” sub-band, corresponding to the “in-phase” N-H stretching vibrations has reproduced the higher-frequency branches of the band. This spectral branch corresponds to the “forbidden” vibrational transition. The lower-frequency branch of the νN-H band has been reproduced by the so-called “plus” dimeric sub-band related to the “out-of-phase” proton vibrations. This fragment of the band corresponds to the dipole allowed transition of the Au symmetry excited state of the N-H stretching vibrations. The results of the calculations suggest that the two dimeric component sub-bands, “minus” and “plus”, contribute to the νN-H band generation mechanism with their appropriate statistical weights given by the F- and Fþ parameter values, 1035



Figure 6. Theoretical reconstitution of the most intense νN-H band component from the low-temperature spectra of the R phase 2-pyridone crystal. (I) The “plus” dimeric band reconstituting the symmetryallowed transition subband, (II) the “minus” dimeric band reproducing the forbidden transition subband and, (III) the superposition of the “plus” and “minus” bands taken with their appropriate statistical weight parameters Fþ and F-. The coupling parameter values: bH = 1.65, C0 = 1.8, C1 = 0.2, Fþ = 1.0, F- = 0.5, ΩN 3 3 3 O = 50 cm-1. The νN-H band center is located at 2800 cm-1. The experimental spectrum is shown in inset.

respectively. The spectrum was calculated for the following coupling parameter values: bH = 1.65, C0 = 1.8, C1 = 0.2, Fþ = 1.0, F- = 0.5, ΩN 3 3 3 O = 50 cm-1, where the ΩN 3 3 3 O parameter denotes the N 3 3 3 O hydrogen bond stretching frequency. As a consequence of the assumed model, a characteristic fine structure pattern of the νN-H band was satisfactorily simulated. In its higher-frequency part, a regular progression with the lowfrequency νN 3 3 3 O vibrational quantum is formed. In the lowerfrequency branch, the band structure has a more dense and compact fine structure pattern, with a tendency to become blurred. Our calculation results have quantitatively reproduced some general properties of the analyzed bands. In the case of the simulation of the νN-H bands it was necessary to take into account the dimeric “minus” sub-band, corresponding to the “inphase” νN-H vibrations, with a relatively high statistical weight F-. This approach makes it possible reproduce the higherfrequency branches of the bands. The allowed transition occurring to the Au-state of the nontotally symmetric proton vibration coordinates participates in the band generation mechanism with the Fþ statistical weight factor. This transition forms the longerwave part of the “residual” νN-H band and was reproduced by the so-called “plus” sub-band. The two sub-bands, “plus” and “minus”, appeared in the theoretical spectrum in an order reversed from the one expected from the results of model calculations of the νX-H band shapes for other hydrogen bond dimeric systems in molecular crystals.19,30-35,46 This means that the vibrational exciton energy

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Figure 7. Theoretical reconstitution of the most intense νN-D band component from the low-temperature spectra of the “R” phase 2-pyridone crystal: (I) The “plus” dimeric band, (II) the “minus” dimeric band reproducing the forbidden transition.The coupling parameter values: bD = 1.0, C0 = 0.9, C1 = -0.3, Fþ = 1.0, F- = 0.8, ΩN 3 3 3 O = 50 cm-1. The νN-D band center is located at 2170 cm-1. The experimental spectrum is shown in inset.

for the hydrogen bonds in the 2-pyridone dimer is of a different sign than estimated for the vast majority of the IR spectra of hydrogen-bonded dimeric systems in the crystals. In the case of the 2-thiopyridone crystal spectra this effect was at least partially ascribed to the relatively long N-H 3 3 3 S bonds in the dimers, which in terms of the dipole-dipole approximation of the exciton interaction energy led to the change of the sign of the exciton energy parameter.47 The model calculations have proved that the assumed centrosymmetric dimeric hydrogen bond model, with the “reversal” exciton coupling mechanism, satisfactorily explains the basic spectral properties of the R phase 2-pyridone crystals. The relatively weak higher-frequency branch of the νN-H band ascribed to the symmetry-forbidden transition appears in the IR spectra via the vibronic mechanism of selection rule breaking in the spectra of the hydrogen-bonded centrosymmetric dimers.50 For the oriented hydrogen-bonded dimeric systems, the two subtransitions contributing to the νX-H band generation should be characterized by the same polarization of their transition moment vectors.50 8.3.2. νN-D Band. Figure 7 presents a quantitative simulation of the νN-D band contour for the spectra of solid-state samples of 2-pyridone, isotopically diluted by hydrogen (ca. 80% D and 20% H). The coupling parameter values used for calculation were bD ¼ 1:0, C0 ¼ 0:9; C1 ¼ - 0:3; F þ ¼ 1:0, F ¼ 0:8; ΩN 3 3 3 O ¼ 50cm - 1 In the case of the νN-D band the component theoretical bands, “plus” and “minus”, reproducing via the model calculations of the band contour, appear in a sequence reverse to the sequence of the component bands used for the quantitative 1036



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Table 1. Electronic Structures of 2-Pyridone Molecules in Different Phases

reconstitution of the νN-H band contour shape. It represents a regular composition of the spectral branches similar to the νX-H and νX-D bands in IR spectra of diverse cyclic hydrogen bond dimeric systems.30-35,46 A smaller contribution of electronic effects to the exciton interaction involving the vibrationally excited deuterium bonds in 2-pyridone dimers due to a reduced amplitude and a smaller anharmonicity of the deuteron vibrations versus the proton vibration properties seem to be a likely cause of this effect. This in turn leads to the change in the way of the interdimer exciton interactions from the “head-to-tail” in the hydrogen-bonded dimers toward the “side-to-side” type in the deuterium-bonded ones. 8.4. Mechanism of the Spectra Generation for the r Phase Crystals. The proposed sequence of the νN-H band components in the analyzed spectra of the R 2-pyridone and 2-thiopyridone crystals differs from the one widely accepted for the description of the corresponding band structures in the spectra of carboxylic acid crystals and crystals of many other molecular systems.30-35,46 In this respect, the abnormal properties of the spectra of 2-thiopyridone29 and 3-hydroxy-4-methyl-2(3H)thiazolothione47 crystals were ascribed to the extremely long N-H 3 3 3 S and O-H 3 3 3 S hydrogen bonds in their lattices (ca. 3.38 Å).36-38,47 The “dipole-dipole” interaction model applied for the estimation of the energy of the inter-hydrogen bond vibrational exciton interactions predicted that in these circumstances a reverse in the component band sequence, contributing to the νN-H and νO-H band contour generation mechanism, may occur.29,47 In the case of the interpretation of the spectra of R phase 2-pyridone crystal, this interpretation cannot be relied on since the N-H 3 3 3 O bonds are much shorter than the sulfur atoms containing hydrogen bonds. Therefore, another source of the unique spectral properties of the R phase 2-pyridone, 2-thiopyridone29 and 3-hydroxy-4-methyl-2(3H)-thiazolothione47 dimers in the crystals should be found. Most probably, the purely vibrational

approach to solving the problem of the exciton interactions in the hydrogen bond dimers should be abandoned. However, the contribution of electronic interactions to the vibrational exciton interactions between the hydrogen bonds in the dimers, expressed by the electronic coordinates, should be taken account. This means that the exciton couplings in the dimers are not exclusively of the “side-toside” type, i.e., “through-space” but may also be of the “head-to-tail” type occurring around the rings formed by the hydrogen bonds. This way of occurring of the exciton interactions in the cyclic hydrogen bond dimers changes the sign of the exciton interaction energy between the dimer vibrational excited states. Hence, the reversion of the sequence of the component bands in the νN-H band contours may appear. From these considerations some consequences result for the electronic structure of 2-pyridone dimers in the R phase crystals. This is most probably the structure of a larger contribution of the zwitterion electronic structure. Such a continuous structure of the π-electron cloud in the pyridine rings, in cyclic Nþ-H 3 3 3 O- hydrogen bond dimers allows for an effective (“head-to-tail”) coupling of the hydrogen bonds in the cyclic dimers.

9. ELECTRONIC STRUCTURE OF 2-PYRIDONE MOLECULES IN DIFFERENT PHASES In Table 1 our estimations concerning the electronic structure of 2-pyridone molecules in the different-phase samples, i.e., in the CCl4 solution and in the R and β crystalline phases, are collected. It is shown that in the case of hydrogen-bonded 2-pyridone, in CCl4 solution and in the crystalline R phase, the molecules of the compound exist in the zwitterion, partly aromatic form. In the case of the deuterium-bonded isotopomer of 2-pyridone, the zwitterion form exists exclusively in the R phase, while the ketone form exists in CCl4 solution in cyclic dimers of deuteriumbonded 2-pyridone and in the β phase. The different spectral properties of the deuterium-bonded 2-pyridone cyclic dimers existing in CCl4 solution may result 1037


The Journal of Physical Chemistry A from a diminution of the aromatic character of pyridone rings in these associates. This fact probably results from the smaller amplitude of the deuteron stretching vibrations in the dimer deuterium bonds. In these circumstances the deuteron vibrations are unable to average effectively enough the π electron cloud in the ring via a vibronic coupling mechanism and the chemical bond system become more pronounced, which is typical for the ketone form of the molecules. Therefore, the R phase of the deuterium-bonded 2-pyridone crystals were found to be less stable than the hydrogen-bonded R phase crystals.

10. ANALYSIS OF THE OTHER BAND PROPERTIES Some arguments helpful in the interpretation of the IR spectra illustrating the phase transition in solid-state 2-pyridone might be derived from the analysis of the other band parameters in the spectra of the two crystalline phases, e.g., the CdO bond stretching vibration bands. Unfortunately, these bands are of complex structures and their interpretation is as yet highly uncertain.51 The same problems occur in the case of the other bands present in the two compared spectra. 11. CONCLUSION The two crystalline phases of 2-pyridone contain molecules existing in different electronic structures. The source of the phase transition in the solid state is interpreted as being connected with an electronic charge rearrangement occurring not only in hydrogen bonds but also in a more distant molecular skeleton. This latter effect is probably accompanied by a slight conformational transition in pyridine rings. These changes provide the basic energetic effect for the R-β phase transition in 2-pyridone crystals. It seems to be a paradox that this rearrangement leads toward the phase characterized by weaker hydrogen bonds. These thermodynamic effects basically do not depend upon the molecular association pattern, i.e., cyclic dimers or infinite chains, as in the case of related systems. Namely, such a duality simply does not exist for crystals of 4-pyridone, which exhibit similar spectral effects of the phase transition.44 ’ AUTHOR INFORMATION Corresponding Author

*E-mail: fl[email protected].

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Electron-Induced Phase Transition in Hydrogen-Bonded Solid-State 2

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