Hydrogen bonds in liquid water are broken only fleetingly

doi:10.1073/pnas.0505125102

Hydrogen bonds in liquid water are broken only fleetingly

^*Department of Chemistry and George R. Harrison Spectroscopy Laboratory, Massachusetts Institute of Technology, Cambridge, MA 02139; and ^**Department of Chemistry, University of California, and Physical Biosciences and Material Sciences Divisions, Lawrence Berkeley National Laboratory, Berkeley, CA 94720

Communicated by Richard J. Saykally, University of California, Berkeley, CA, June 17, 2005 (received for review May 13, 2005)

Abstract

Although it is widely accepted that the local structure of liquid water has tetrahedral arrangements of molecules ordered by hydrogen bonds, the mechanism by which water molecules switch hydrogen-bonded partners remains unclear. In this mechanism, the role of nonhydrogen-bonded configurations (NHBs) between adjacent molecules is of particular importance. A molecule may switch hydrogen-bonding partners either (i) through thermally activated breaking of a hydrogen bond that creates a dangling hydrogen bond before finding a new partner or (ii) by infrequent but rapid switching events in which the NHB is a transition state. Here, we report a combination of femtosecond 2D IR spectroscopy and molecular dynamics simulations to investigate the stability of NHB species in an isotopically dilute mixture of HOD in D₂O. Measured 2D IR spectra reveal that hydrogen-bonded configurations and NHBs undergo qualitatively different relaxation dynamics, with NHBs returning to hydrogen-bonded frequencies on the time scale of water's fastest intermolecular motions. Simulations of an atomistic model for the OH vibrational spectroscopy of water yield qualitatively similar 2D IR spectra to those measured experimentally. Analysis of NHBs in simulations by quenching demonstrates that the vast majority of NHBs are in fact part of a hydrogen-bonded well of attraction and that virtually all molecules return to a hydrogen-bonding partner within 200 fs. The results from experiment and simulation demonstrate that NHBs are intrinsically unstable and that dangling hydrogen bonds are an insignificant species in liquid water.

Footnotes

↵ ¶ To whom correspondence should be addressed. E-mail: tokmakof{at}mit.edu.
↵ † J.D.E. and J.J.L. contributed equally to this work.
↵ ‡ Present address: Department of Chemistry, Columbia University, New York, NY 10027.
↵ § Present address: Department of Applied and Engineering Physics, Cornell University, Ithaca, NY 14853.
Author contributions: J.D.E., J.J.L., C.J.F., A.T., and P.L.G. designed research; J.D.E., J.J.L., C.J.F., and S.T.R. performed research; J.D.E., J.J.L., C.J.F., S.T.R., A.T., and P.L.G. analyzed data; and J.D.E., J.J.L., C.J.F., S.T.R., A.T., and P.L.G. wrote the paper.
Abbreviations: HB, hydrogen-bonded configuration; NHB, nonhydrogen-bonded configuration; MD, molecular dynamics.
↵ †† The duration of the pulses (45 fs full width at half maximum) determines the instrumental time resolution, which is fast enough to resolve all spectral relaxations in water. Even so, the time resolution for the 2D IR experiment is not dictated by pulse length alone but by the time scales of spectral fluctuations. A finite time period Δτ is required to select a frequency resolution of width Δω ≈ 2π/Δτ. The microscopic dynamics blur the transition frequencies during Δτ so that the observation loses dynamic information over this interval. The shortest measurable time interval in a 2D IR experiment of the OH stretch of HOD in D₂O, determined by the inverse of the antidiagonal linewidth, is Δτ₁ + τ₂ +Δτ₃ ≳ 150 fs. This time is faster than all intermolecular motions in water except for librations.
↵ ‡‡ Quenching liquid configurations to nearby potential energy minima is a useful strategy for exploring basins of attraction provided the NHB state is not stabilized by entropy. An appreciable entropic contribution seems a priori unlikely, given the few constraints imposed by a single hydrogen bond. This expectation is confirmed by the rapid decay of NHB populations shown in Fig. 4. Details of the quenching procedure can be found in Supporting Text.
↵ §§ Simulation strategies with various different water models (9–12) and methods for computing vibrational frequencies yield qualitatively similar results for C(t). Intramolecular electronic polarizability, absent in any fixed-charge model of water, increases hydrogen bond strength and nearly doubles the value of the high-frequency dielectric constant in water. Indeed, including molecular polarizability can improve quantitative agreement between a calculated C(t) and that measured by experiment, but these effects have little bearing on our conclusions. In fact, polarizable models predict a higher degree of stabilization for HB over NHB species, further supporting our observation that presumed NHBs return back to HBs on the time scales of the fastest intermolecular motions.