Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy
FTIR studies of intermolecular hydrogen bonding in halogenated ethanols
Introduction
Intermolecular hydrogen bonding in aliphatic alcohols has been of interest for the past half century [1], [2], [3], [4]. These compounds represent a model system in which to study how hydrogen bonding can effect the properties of strongly associated liquids. Our interest in these systems stems from a desire to understand how hydrogen bonding effects the efficiency of photoproduct recombination (ie, geminate recombination) for dissociation reactions occurring in condensed environments [5], [6]. In particular, it has been shown that the quantum yield for photofragment recombination is highly solvent dependent, and that the yields are typically larger in solvents where hydrogen bonding is operative [7], [8], [9], [10], [11], [12]. Presumably, this effect is due to increased rigidity of the solvent cage accompanying intermolecular hydrogen bonding; however, this hypothesis has yet to be tested. We attempted such a test by investigating the photodissociation of OClO in ethanol and 2,2,2-trifluoroethanol (TFE) [13]. It was hoped that the increase in hydrogen-bond enthalpy accompanying halogenation would be manifested as an increase in the geminate recombination quantum yield. However, we observed a reduction in the recombination yield in TFE relative ethanol. This observation motivated the following questions: What is the change in hydrogen-bonding enthalpy accompanying halogenation? How does the extent of intermolecular hydrogen bonding vary as a function of halogenation?
In this paper, the intermolecular hydrogen-bonding enthalpies of ethanol, TFE, and 2,2,2-trichloroethanol (TCE) are investigated using Fourier-transform infrared (FTIR) spectroscopy. It is known that alcohols are capable of forming extended, polymeric structures through hydrogen bonding [2], [3], [4]. Numerous spectroscopic techniques including x-ray diffraction [14], NMR [15], [16], [17], and Raman [18] as well as indirect methods such as dielectric constant and thermodynamic data have been employed to study hydrogen bonding in aliphatic alcohols [3], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19], [20], [21], [22], [23]. However, it has been recognized that inherent to FTIR spectroscopy is the ability to define the structures formed via hydrogen bonding [18], [21], [24], [25], [26], [27], [28], [29], [30], [31], [32], [33], [34], [35], [36], [37], [38], [39], [40], [41], [42], [43]. Specifically, dramatic evolution is observed in the OH stretching region of the IR spectrum upon dimerization and polymerization. This sensitivity was exploited by Coggeshall and Saier to study intermolecular hydrogen bonding in phenols [27]. Subsequently, IR spectroscopy was used to study hydrogen-bonding in ethanol, with dimerization enthalpy values ranging from −3 to −10 kcal/mol being reported [21], [30], [31], [34], [36], [40], [42]. Recently, Marand and co-workers performed FTIR studies in conjunction with the equilibrium constant model of Coggeshall and Saier, and determined that the enthalpy for dimer and multimer formation ethanol was −4.61 and −4.15 kcal/mol, respectively [40].
In this paper, the FTIR spectra of ethanol, TFE, and TCE as a function of concentration and temperature are obtained and used in conjunction with the Coggeshall and Saier model to ascertain the effect of halogenation on dimer and multimer formation. As mentioned above, numerous studies have been performed on ethanol; therefore, its inclusion here could be viewed as redundant. However, in an attempt to systematically determine how halogenation affects hydrogen bonding, we felt it necessary to perform a study in which all alcohols of interest are investigated under identical conditions. Information regarding the hydrogen-bonding enthalpies for halogenated ethanols is rare, and we have found a single study where intermolecular bonding enthalpies are reported [25]. Qualitatively, it has been noted that most halogenated ethanols demonstrate a single OH stretching band suggesting that the self-association is reduced in these solvents [32]. The results presented here establish that the enthalpy for dimerization increases from −4.2±0.3 kcal/mol in ethanol to −6.8±1.0 kcal/mol in TFE. A more modest evolution of is observed for multimer formation, with enthalpies decreasing from −3.7±0.5 kcal/mol in ethanol to −2.1±1.4 kcal/mol in TFE. The results presented here demonstrate that relative to ethanol, TCE and TFE prefer to exist in the monomeric form due to the formation of intramolecular hydrogen bonds involving the halogen substituents.
Section snippets
Experimental
Anhydrous USP grade ethanol (Pharmco Supply), TFE (Aldrich, 99.5%+purity), 2,2,2-TCE (Aldrich, 99%+purity), and anhydrous carbon tetrachloride (Aldrich, 99%+ purity) were purchased and used as received. Sample solutions were constructed by diluting a measured volume of an alcohol in carbon tetrachloride using standard volumetric glassware. Both a micropipet and a Drummond microdispenser were used to measure the alcohol volumes. Care was taken to calibrate both dispensers using the alcohol
Results
Fig. 1 presents FTIR absorption spectra for 320-mM solutions of ethanol, TCE, and TFE dissolved in carbon tetrachloride at 293 K. In all alcohols, three absorption bands dominate the OH-stretching region of the spectrum. The observation of three distinct transitions has been rationalized in terms of various hydrogen-bonded structures [30]. Specifically, monomeric or uncomplexed hydroxyl groups are assigned to the highest-energy transition at ∼3600 cm−1. Dimeric structures are assigned to the
Discussion
The hydrogen-bonding enthalpies corresponding to dimer and multimer formation determined in this work for ethanol are −4.2±0.3 and −3.7±0.5 kcal/mol, respectively. These values are consistent with the values determined in earlier studies, and the results of this earlier work are summarized in Table 5. Inspection of the table demonstrates that differences in hydrogen bonding enthalpies are observed between various methods and bonding models employed. Reported enthalpies range from −2.5 [18] to
Conclusions
FTIR spectra of ethanol, TFE, and TCE dissolved in carbon tetrachloride as a function of temperautre and concentration were obtained and used to determine the effect of halogen substitution on the enthalpy for hydrogen-bond formation. The enthalpy for dimerization was found to evolve from −4.2±0.3 kcal/mol in ethanol to −6.8±1.0 kcal/mol in TFE. An opposite trend was found for multimer formation with enthalpies of −3.7±0.5 in ethanol and −2.1±1.4 kcal/mol in TFE. A portion of this evolution was
Acknowledgements
Acknowledgement is made to the donors of the Petroleum Research Fund, administered by the American Chemical Society (PJR). This work was also supported by the NSF (CHE-9701717/CHE-0091320). PJR is an Alfred P. Sloan Fellow and is a Cottrell Fellow of the Research Corporation. PCB is a Mary Gates Fellow of the University of Washington.
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