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The Hydrogen-Bonded 2-Pyridone Dimer Model System. 2. Femtosecond Mid-Infrared Pump-Probe Study Ming Yang, Łukasz Szyc, Jens Dreyer, Erik T. J. Nibbering,* and Thomas Elsaesser Max Born Institut fu¨r Nichtlineare Optik und Kurzzeitspektroskopie, Max Born Strasse 2A, D-12489 Berlin, Germany ReceiVed: August 26, 2010; ReVised Manuscript ReceiVed: October 7, 2010
2-Pyridone (PD) tautomerises to 2-hydroxypyridine (HP) in liquid solution, the equilibrium of which is solvent dependent. Dimerization of PD and HP leads to the cyclic dimers (PD)2, (HP)2, and (PD-HP). A combined NMR and FT-IR study [Szyc, Ł.; et al. J. Phys. Chem. A 2010, 114, 7749-7760] has shown that solutions of 2-pyridone in CD2Cl2 constitute mainly PD-CD2Cl2 solute-solvent complexes and cyclic dimers (PD)2. Because of a lack of specific marker modes, a contribution of the cyclic dimer (HP)2 to the NH/OH stretching absorption between 2400 and 3300 cm-1 could not be fully ruled out. Here, we present the first ultrafast infrared (IR) pump-probe experiments on the NH/OH stretching region of a solution of 2-pyridone in CD2Cl2. The temporally and spectrally resolved data reveal different rate-like relaxation processes with time constants between 150 fs and 20 ps as well as coherent low-frequency oscillations due to hydrogen bond modes. An analysis shows that the transient behavior is dominated by a single hydrogen bonded species. We compare the low-frequency wavepacket motions, observed with 99 and 150 cm-1 frequencies, with literature values as well as our quantum chemical calculations and conclude that this single molecular species is cyclic (PD)2. 1. Introduction 2-Pyridone (PD), widely studied as a nucleotide base pairing model for the pyrimidines uracil or thymine,1-11 is subject to lactam-lactim tautomerism, whereby 2-hydroxypyridine (HP) is formed (Scheme 1). Both tautomers may be expected to be present in solution because of relatively small energy differences, with a solvent-dependent tautomerism equilibrium constant. In addition to that PD and HP tautomers can dimerize to form three possible cyclic combinations, the two cyclic homodimers (PD)2 and (HP)2 and the cyclic mixed dimer (PD-HP; Scheme 1). Gas phase experiments can single out particular tautomers or complexes, thereby providing detailed spectroscopic information on PD or HP monomers,10,12 and on (PD)2 or (PD-HP) dimers,3,4,8,9 including the dimer low-frequency intermolecular modes.5-7,13 Solution-phase experiments have mainly dealt with the spectroscopic assignment of the measured signals to the monomer tautomers and their dimers, and the determination of the chemical equilibria between them, using NMR,14-17 UV,15,16,18-22 and IR spectroscopy.14,15,23-28 Such methods have been complemented by theoretical methods.17,26,29-36 In a previous paper,37 we have combined FT-IR and NMR methods to determine the role of the different tautomers in this complexation chemistry in the weakly polar solvent dichloromethane-d2. We were able to clearly identify the existence of PD-CD2Cl2 monomer-solvent complexes, and of cyclic (PD)2 dimers in solution. The NMR results show that the (PD)2 dimer is the dominant species at low temperatures which exchanges with PD in monomeric form or complexed with the solvent. At room temperature the presence of the proton exchange between PD units is inferred from the NMR spectra. Whereas the linear FT-IR spectra show congested vibrational transitions in the CdO stretching region as well as in the NH/OH stretching region, a 2D-IR correlation analysis of concentration-dependent and of * Corresponding author. E-mail:
[email protected].
temperature-dependent data enable us to clearly assign most of the spectral features to either PD-CD2Cl2 solute-solvent complexes, or to (PD)2 cyclic dimers. A few spectral features hint at the presence of an additional species that we assigned to PD-PD chain-like dimers (Scheme 1). The absence of free OH stretching signals indicate that HP monomers, and HP-HP or PD-HP chain-like dimers are of minor importance. With such a combined NMR and FT-IR spectroscopic approach, the chemical speciation of 2-pyridone/2-hydroxypyridine in dichloromethane solution has been determined to a large extent. An open question to be addressed is the potential role of cyclic dimer (HP)2. (HP)2 has not been detected with NMR, suggesting that the average lifetime of (HP)2 is definitely much shorter than milliseconds. It may, however, exist as a short-lived transient species, making possible routes for proton transfer between different PD units. A detection of this species in stationary FT-IR spectroscopy is hampered by the lack of distinct marker modes of (HP)2. Here, time-resolved, in particular ultrafast nonlinear infrared spectroscopy, should provide specific insight. Ultrafast infrared spectroscopy has provided a wealth of dynamical information on hydrogen-bonded systems on these ultrashort time scales.38 In such studies hydrogen bond exchange dynamics have been investigated for weakly hydrogen-bonded systems between solute-solvent and solute-solute complexes, including acetamide-dimethylsufoxide39 and phenol dimer in CCl4.40 Hydrogen bond exchange on ultrafast time scales is mainly absent in medium strong hydrogen bonded systems such as acetic acid dimer and 7-azaindole dimer. They are subject to stronger anharmonic couplings, existing between the hydrogen stretching oscillators with low-frequency hydrogen bond deformation modes, and with combination and overtone states of fingerprint vibrational modes. These anharmonic couplings have been probed in ultrafast IR pump-probe and photon echo experiments and determined in a quantitative way by theoretical modeling of the results.41-54
10.1021/jp108096y 2010 American Chemical Society Published on Web 11/01/2010
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J. Phys. Chem. A, Vol. 114, No. 46, 2010
Yang et al.
SCHEME 1
In this paper, we present femtosecond IR pump-probe measurements on the NH/OH stretching band of 2-pyridone/2hydroxpyridine in dichloromethane-d2 solution. We analyze the obtained results in terms of contributions by vibrational population kinetics and by coherent low-frequency wavepacket motions. From these results we can ascribe the vibrational manifold in the 2400-3000 cm-1 to NH stretching transitions of cyclic (PD)2 and can firmly exclude contributions by OH stretching resonances of cyclic (HP)2 in the same frequency range. 2. Experimental Section 2-Pyridone was purchased from Fluka and used as received. Dichloromethane-d2 (CD2Cl2) was obtained from Deutero GmbH (99.6% isotopic purity, water content
