ARTICLE pubs.acs.org/JPCA
H/D Isotopic Recognition in Hydrogen-Bonded Systems: Strong Dynamical Coupling Effects in the Polarized IR Spectra of 3-Methylthioacetanilide and 4-Methylthioacetanilide Crystals Henryk T. Flakus,* Wioleta Smiszek-Lindert, and Barbara Hachuza Institute of Chemistry, University of Silesia, 9 Szkolna Street, Pl-40 006 Katowice, Poland ABSTRACT: 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.
1. INTRODUCTION For over six decades the IR spectroscopy has been considered to be the most powerful tool in the hydrogen bond research. Spectral studies concern the middle frequency range of IR where the νXH bands appear, ascribed to the proton stretching vibrations in the XH 3 3 3 Y hydrogen bonds.15 Complex fine structure patterns of the νXH bands are highly susceptible to diverse interaction mechanisms exerted on them, that is, the intra- and interhydrogen bond ones.25 According to contemporary quantitative theories of IR spectra of hydrogen bonded systems these bands are treated as an abundant source of information about the complex dynamics of hydrogen bonds. In terms of these theories, strong anharmonic couplings involving motions of different forms and energies are considered to be responsible for generation of the main spectral properties of hydrogen bond systems. In the understanding of the hydrogen bond system spectral properties the so-called “strong-coupling”68 and the “relaxation” theory9,10 played a particular role. These theoretical models allowed to succeed in the quantitative interpretation of the IR spectra of the hydrogen bond in simple molecular aggregates like dimers1013 as well as in hydrogen-bonded molecular crystals.1417 In earlier spectral studies of hydrogen bond dimers, suggestions were made about the generation of the system spectra that some mechanisms, nonrevealed as yet, might also codecide. It seemed that IR spectroscopy in polarized light, applied for investigation of crystalline spectra, should provide essential data in this matter. Polarized IR spectra of molecular crystals measured for spatially oriented lattices of hydrogen bonds seem to be the source of the most complete data system concerning the inter- as well as the intrahydrogen bond interactions in the systems. However, the solid-state introduces its own new effects r 2011 American Chemical Society
considerably complicating the crystal spectra interpretation, related to the interhydrogen bond couplings in the excited vibrational state. The overcoming of these interpretation problems may allow us to extend our knowledge, for instance, about the coupling mechanisms in hydrogen bond systems involving motions of diverse forms. H/D isotopic effects in the IR spectroscopy of the hydrogen bond have verified the validity of theoretical models introduced subsequently over the last decades.610 However, it seems that in the former spectral studies of H/D isotopic effects for hydrogen bonded systems the problem of the influence of the isotopic dilution on the IR spectra was treated as incidental. This was the result of studies carried out in the past and which unfortunately influenced later studies in this area.1820 The recent studies of polarized IR spectra of the hydrogen bond in diverse crystalline systems have exhibited a rich diversity of their spectral properties and allowed to reveal a number of new, nonconventional effects in the spectra. These new effects were found when model calculations, mainly performed in terms of the so-called “strong-coupling” theory,6,7,17 were applied to the quantitative interpretation of the spectra. The effects mentioned above include the so-called H/D isotopic “self-organization” effects, identified in the polarized IR spectra of isotopically diluted hydrogen-bonded molecular crystals.21,22 They result from a nonrandom distribution of protons and deuterons in the hydrogen bridges in lattices of isotopically diluted molecular crystals. It looks as if “attraction” forces appear, involving identical hydrogen isotope atoms in coupled hydrogen bond systems. Therefore, the Received: February 21, 2011 Revised: May 25, 2011 Published: May 25, 2011 7511
dx.doi.org/10.1021/jp2016903 | J. Phys. Chem. A 2011, 115, 7511–7520
The Journal of Physical Chemistry A invariability of the νXH proton stretching vibration bands, independent of the increasing concentrations of deuterons in a crystalline sample can be observed in the spectra of isotopically diluted crystalline samples. The latter effect is the result of the newly revealed interhydrogen bond mechanisms of co-operative interactions in molecular crystals, that is, the dynamical cooperation interaction mechanisms.22 Vibronic coupling in hydrogen bonded molecular systems involving the proton stretching vibrations and electronic motions was considered as the most probable source of these mechanisms.22 Our recent detailed studies of the dynamical co-operative interaction mechanisms performed for diverse solid-state systems have proved that the H/D isotopic “self-organization” processes do not occur in one precisely defined way. This observation mainly concerns a particular group of hydrogenbonded molecular crystals in whose lattices hydrogen bonds form infinitely long chains. It appeared that in relation to the electronic structure of the associating molecules, the H/D isotopic “self-organization” processes, provided they take place, might occur in two different ways. In the first way they may involve the adjacent hydrogen bonds in each individual chain. In the second case, pairs of closely spaced hydrogen bonds participate in the mechanisms, whereas each moiety in an individual pair belongs to another chain. The first group of crystals includes crystals of pyrazole23 and 4-thiopyridone.24 These molecules contain large delocalized “π”electronic systems. Surprisingly, also formic acid crystals belong to this group.25 That the discussed processes occur in the first way is evidenced by the fact that the linear dichroic effects are retained. This in turn depends on the differentiation of the dichroic properties of the two opposite branches of the νXH “residual” bands in the polarized spectra of isotopically diluted crystals. Also, the intensity distribution pattern of these bands, constituting the attribute of linear hydrogen bond dimers, supports this conclusion. The second group includes crystals of N-methylthioacetamide,26 N-methylacetamide,27 and acetic acid.28 For these latter systems, the distribution of protons and deuterons in the hydrogen bridge chains appeared to be fully random. This fact was deduced from the disappearance of the Davydow-splitting effects in the νXH “residual” bands in the spectra of the isotopically diluted crystals, attributed to the in-chain exciton couplings.2628 Also, twobranch fine structure patterns of these bands, characteristic for cyclic hydrogen bond dimers, were observed in these spectra. In this case, no essential differences in the dichroic properties between the opposite νXH “residual” band branches can be seen.2628 It is also noteworthy to add that in some rare crystalline systems (decyl alcohol18,19 and 2-oxazolidone29) the H/D “selforganization” effects were not found in the IR spectra. IR spectra of the NH 3 3 3 S hydrogen bond systems were found to be particularly strongly influenced by the electronic properties of sulfur atoms.30 Due to the fact that lattices composed of infinite hydrogen bond chains characterize the crystal lattices of various thioamides, these systems seem to be of interest for the investigation of the H/D isotopic “self-organization” process mechanisms in crystals. One might expect the diverse substituent atomic groups linked with the thioamide fragments of the molecular structures are able to regulate the electronic properties of NH 3 3 3 S hydrogen bonds by withdrawing the electronic charge from these areas. In this way they could affect the magnitude of the dynamical co- operative interaction energies because these mechanisms are basically of
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a vibronic nature.22 In a similar way, the vibrational exciton interaction mechanism could also be influenced. Crystals of diverse thioamides seemed to be very promising objects for the investigation of the problem. The IR spectra of solid-state thioamides exhibit well-developed νNH band fine structure patterns. This is a common property of many molecular systems linked by NH 3 3 3 S hydrogen bonds and, in the case of thioamide crystal spectra, it allows for their fully quantitative interpretation and for a deeper insight into the very nature of the H/D isotopic “self-organization” processes.22 However, it might also be anticipated that, in the case of small “π”-electronic systems like the ones existing in hydrogen-bonded CdO or CdS atomic groups in various amide or thioamide crystals, in some substituent atomic group cases, the H/D isotopic “self-organization” processes might proceed differently. Such a hypothetical effect, provided it exists, would manifest spectacularly the possible role of substituent atomic groups, differing by their electronic structures, in modifying the dynamical co-operative interaction mechanisms in the hydrogen bond systems. For N-methylthioacetamide26 and acetanilide22 crystals, these mechanisms strongly involved hydrogen bonds in each unit cell, which belonged to different chains of the associated molecules. In this article, we present the polarized IR spectra of 3- and 4-methylthioacetanilide crystals, which in an initial theoretical approach suggested that the course of the process might occur in a different way, because the impact of the substituent aromatic rings on these mechanisms is expected. 1.1. Crystal Structures of 3- and 4-Methylthioacetanilide. Crystals of 3-methylthioacetanilide (in the abbreviated notation 3MTA) belong to the monoclinic system.31 The space-symmetry group is P21/c C52h. There are four molecules in a unit cell (Z = 4). The lattice constants at 100 K: a = 16.023(3) Å, b = 6.988(1) Å, c = 8.067(1) Å, β = 103.35(3)°. The 3MTA molecules are linked together by NH 3 3 3 S hydrogen bonds, forming infinitely long chains that become elongated along the “c” axis. The hydrogen bond geometry parameters: rNH = 0.808(15) Å, RH 3 3 3 S = 2.520(16) Å, RN 3 3 3 S = 3.3233(14) Å,
