Letter pubs.acs.org/JPCL
Monitoring Intramolecular Proton Transfer with Two-Dimensional Infrared Spectroscopy: A Computational Prediction Z. L. Terranova and S. A. Corcelli* Department of Chemistry and Biochemistry, University of Notre Dame, Notre Dame, Indiana 46556, United States S Supporting Information *
ABSTRACT: Proton transfer processes are ubiquitous and play a vital role in a broad range of chemical and biochemical phenomena. The ability of two-dimensional infrared (2D IR) spectroscopy with a carbon−deuterium (C−D) reporter to monitor the kinetics of proton transfer in the model compound malonaldehyde was demonstrated computationally. One of the two carbonyl/enol carbon atoms in malonaldehyde was labeled with a C−D bond. The C−D stretch vibrational frequency provides ∼150 cm−1 of sensitivity to the two tautomers of malonaldehyde. Mixed quantum mechanics/ molecular mechanics simulations employing the self-consistent-charge density functional tight binding (SCC-DFTB) method were used to compute 2D IR line shapes for the C− D stretch of labeled malonaldehyde in aqueous solution. The 2D IR spectra reveal cross peaks from the chemical exchange of the proton. The kinetics for the growth of the crosspeaks (and the decay of the diagonal peaks) precisely match the proton transfer rate observed in the SCC-DFTB simulations. SECTION: Kinetics and Dynamics
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The use of C−D to monitor the kinetics of PT processes using chemical exchange 2D IR spectroscopy was demonstrated and analyzed computationally for the model compound malonaldehyde (3-hydroxy-2-propenal, Figure 1). Malonalde-
roton transfer (PT) plays a critical role in many chemical, photochemical, catalytic, and biomolecular processes.1,2 It remains an important challenge to elucidate the factors that affect the kinetic rates and mechanisms of PT in realistic contexts. Novel spectroscopic approaches for monitoring PT processes in real-time are therefore essential. Because of its subpicosecond time resolution and superb sensitivity of infrared reporters to their local environments, chemical exchange twodimensional infrared (2D IR) spectroscopy3,4 has the ability to offer tremendous insight regarding the dynamics of thermally activated PT. Chemical exchange 2D IR spectroscopy requires a suitable IR-active mode whose vibrational frequency is sensitive to the PT reaction coordinate. A potential strategy is to directly monitor the bonds in which the transferring proton participates. While such an approach is necessarily completely nonperturbative, a likely drawback is the complexity of the resulting spectra because of tunneling and nonadiabatic effects.5 An alternative approach is to introduce a spectroscopic reporter whose vibrational frequency is sensitive to the PT process, but whose presence is minimally perturbative to the motion (e.g., mechanism and kinetics) of the transferring proton of interest. Selective deuteration of carbon atoms in the vicinity of the PT event can potentially offer a means to monitor the reaction with minimal perturbation. Site-specific C−D bonds generally absorb between 2100 and 2300 cm−1, a region of the IR spectrum that is mostly absent of other vibrational transitions in biomolecular contexts, and are known to be excellent reporters of their chemical environment (in terms of solvatochromic shift and vibrational Stark tuning rates).6,7 Moreover, since carbon is rarely the donor or acceptor in most PT processes of interest, C−D vibrations will be adiabatically decoupled from the PT reaction. © 2012 American Chemical Society
Figure 1. PT reaction coordinate in C−D labeled malonaldehyde. The transferable proton is depicted with a white sphere, and the deuterium atom is labeled with a purple sphere.
hyde contains an intramolecular hydrogen bond across which a proton can transfer between symmetric donor and acceptor oxygen atoms (OA−H···OB), a process that has been studied extensively by both theory8−11 and experiment.12−15 The wealth of previous investigations further motivates the use of malonaldehyde as a model system for the development of new spectroscopic approaches. In this study, one of the carbonyl/ enol carbon atoms, a direct neighbor to the PT event, is deuterated. This particular C−D vibrational reporter is likely to Received: June 1, 2012 Accepted: June 21, 2012 Published: June 21, 2012 1842
dx.doi.org/10.1021/jz300714t | J. Phys. Chem. Lett. 2012, 3, 1842−1846
The Journal of Physical Chemistry Letters
Letter
into account). In this study, the 2D IR spectra of C−D labeled malonaldehyde in aqueous solution will be calculated to demonstrate how the technique is applicable to the study of thermally induced PT reactions. Because the intramolecular PT reaction is symmetric in malonaldehyde, the resulting 2D IR spectra will be symmetric, and the diagonal peaks should decay at the same rate as the off-diagonal peaks grow (neglecting vibrational population relaxation). In order to calculate the 2D IR spectrum of C−D labeled malonaldehyde, it is first necessary to map the instantaneous C−D vibrational frequency to the PT reaction coordinate:
be exquisitely sensitive to the PT event because of (1) its spatial proximity, (2) the known sensitivity of C−D vibrational frequencies to the movement of charge via the vibrational Stark effect,16 and, most importantly, (3) the coupling of the C−D bond to the dramatic change in the electronic structure of the malonaldehyde molecule upon intramolecular PT. The third effect is likely to be the most dominant, by far, for malonaldehyde. The chemical environment of the carbonatom to which the deuterium probe is attached changes markedly from HOA−CDC to OACD−C when the proton transfers. In essence, this change in the electronic structure of the malonaldehyde molecule results in a substantial alteration of the force constant of the C−D bond. In the gas phase, density functional theory (DFT) at the B3LYP/6311+G(d, p) level of theory/basis set predicts a 179 cm−1 shift in the harmonic vibrational frequency of C−D vibrational probe. This large shift contrasts significantly with the
