Ultrafast coherent nuclear motions of hydrogen bonded carboxylic acid dimers Karsten Heyne, Nils Huse, Erik T.J. Nibbering, Thomas Elsaesser * Max-Born-Institut f€ ur Nichtlineare Optik und Kurzzeitspektroskopie, D-12489 Berlin, Germany Received 29 October 2002; in final form 20 December 2002 Abstract Real-time vibrational dynamics of the cyclic dimer of deuterated acetic acid in solution, a model system for hy- drogen-bonded units of biomolecules, are elucidated by ultrafast infrared spectroscopy. Upon excitation of the O–D stretching mode, we demonstrate coherent nuclear motions along several intermolecular low-frequency modes of the dimer. The anharmonic coupling of the fast O–D stretching and slow low-frequency motions is isolated in the nonlinear vibrational response whereas other couplings are suppressed. The large-amplitude motions persist for several pico- seconds, potentially allowing for optical control of (bio)chemical dynamics. Ó 2003 Elsevier Science B.V. All rights reserved. Hydrogen bonds determine the structure of important biomolecules, among them the double helix DNA structure [1] and numerous proteins [2]. In such macromolecular structures, multiple hy- drogen bonds exist between neighboring molecular units in order to stabilize specific local geometries. For instance, carboxylic groups, i.e., a COOH moiety in which the O–H group can serve as the hydrogen donor in a first hydrogen bond and the C@O group as the hydrogen acceptor in a second hydrogen bond, represent a characteristic struc- tural motif of glutamic and aspartic amino acids. Biochemical function of such structures, e.g., the making and breaking of hydrogen bonds as well as hydrogen transfer [3], involves nuclear motion of hydrogen bonded groups. The time scale of such motions is set by the underlying vi- brational modes and extends from 10 fs for the hydrogen stretching mode up to hundreds of femtoseconds for low-frequency motions. Vibra- tional dynamics of hydrogen bonds are highly complex as different modes interact with each other and with the liquid or protein environment. (i) Low-frequency modes can couple anharmoni- cally to the fast hydrogen or deuterium stretching motion and modulate the length of a hydrogen bond [4,5], a basic mechanism of structural dy- namics. (ii) Fermi resonances of the v ¼ 1 level of the hydrogen stretching oscillator with other vi- brational levels can result in an effective popula- tion relaxation of the stretching oscillator [6,7]. (iii) In systems with multiple hydrogen bonds, a cou- pling exists between resonant hydrogen stretching oscillators, resulting in a splitting of the transition Chemical Physics Letters 369 (2003) 591–596 www.elsevier.com/locate/cplett * Corresponding author. Fax: +49-30-6392-1409. E-mail address: elsasser@mbi-berlin.de (T. Elsaesser). 0009-2614/03/$ - see front matter Ó 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0009-2614(03)00023-X