This journal is c the Owner Societies 2012 Phys. Chem. Chem. Phys., 2012, 14, 11651–11656 11651 Cite this: Phys. Chem. Chem. Phys., 2012, 14, 11651–11656 Vibrational energy relaxation of the ND-stretching vibration of NH 2 D in liquid NH 3 Tim Scha¨fer, a Alexander Kandratsenka,* ab Peter Vo¨hringer, c Jo¨rg Schroeder a and Dirk Schwarzer b Received 30th April 2012, Accepted 3rd July 2012 DOI: 10.1039/c2cp41382e The vibrational energy relaxation from the first excited ND-stretching mode of NH 2 D dissolved in liquid NH 3 is studied using molecular dynamics simulations. The rate constants for inter- and intramolecular energy transfer are calculated in the framework of the quantum-classical Landau–Teller theory. At 273 K and an ammonia density of 0.642 g cm À3 the calculated ND-stretch lifetime of t = 9.1 ps is in good agreement with the experimental value of 8.6 ps. The main relaxation channel accounting for 52% of the energy transfer involves an intramolecular transition to the first excited state of the umbrella mode. The energy difference between both states is taken up by the near-resonant bending vibrations of the solvent. Less important for the ND-stretch lifetime are both the direct transition to the ground state and intramolecular relaxation via the NH 2 D bending modes contributing 23% each. Our calculations imply that the experimentally observed weak density dependence of t is caused by detuning the resonance between the ND-stretch–umbrella energy gap and the solvent accepting modes which counteracts the expected linear increase of the relaxation rate with density. 1 Introduction Studies of vibrational energy relaxation (VER) in the liquid phase can help to gain a better understanding of dynamic interactions of solutes with surrounding bath molecules. 1–3 Since these interactions play a central role in chemical reactions in the condensed phase, it is of great significance to analyze such processes in detail and quantify the contributions of different relaxation pathways. Numerous studies of this kind have focused on aqueous systems, 4–20 recently, they have been expanded to condensed ammonia. 21 Like water, liquid ammonia is a well studied ionizing solvent. Due to its potential to dissolve alkali metals and to form solvated electrons, such solutions are used in organic chemistry as reducing agents (e.g., Birch reaction). 22–24 Although liquid ammonia is often cited in chemistry textbooks as an example for associated liquids forming extended hydrogen bond networks, 25 neutron scattering experiments 26,27 and computer simulations based on Car–Parinello ab initio molecular dynamics 28 (MD) and mixed quantum/classical molecular dynamics 29 show that, in contrast to water, hydrogen bonding is of negligible importance. Consequently, the characteristics of the dynamics of VER in ammonia over a wide thermodynamical range differ from the well studied behavior of water. This was recently demonstrated by pump–probe absorption spectroscopy experiments, where the ND-stretching fundamental of NH 2 D in NH 3 was excited by a femtosecond pulse and the lifetime of the stretching mode was observed over a wide range of temperature and pressure (230–450 K, 10–1500 bar). 21 At 273 K and a solvent density of 0.642 g cm À3 the lifetime of the excited ND-stretch vibration of NH 2 D was measured to be 8.6 ps which is much larger than typical vibrational lifetimes of liquid water at comparable conditions (about 1 ps). 11,12 These experimental studies were supported by simple theoretical calculations of the ND-stretch lifetime 21 based on the Landau–Teller (LT) approach. Here a breathing-sphere model described the solute molecule, and solvent molecules were modeled as point masses. This model describes the temperature dependence but it cannot distinguish between different relaxation pathways, since it does not take into account rotations and intramolecular vibrations of the solvent. It is therefore desirable to use the more sophisticated approach already applied successfully to the water system such as advanced LT theory 30–34 and non-equilibrium molecular dynamics simulations. 35–40 In this work we use the Landau–Teller approach following the lines developed by Rey and Hynes 31 and Lawrence and Skinner 33 in order to study in detail the vibrational energy relaxation pathways from the excited N–D stretching mode of a Institut fu ¨r Physikalische Chemie, Universita ¨t Go ¨ttingen, Tammannstrasse 6, 37077 Go ¨ttingen, Germany. E-mail: akandra@gwdg.de; Fax: +49 551 201 1501; Tel: +49 551 201 2004 b Max-Planck-Institut fu ¨r Biophysikalische Chemie, Am Faßberg 11, 37077 Go ¨ttingen, Germany c Abteilung fu ¨r Molekulare Physikalische Chemie, Institut fu ¨r Physikalische und Theoretische Chemie, Rheinische Friedrich-Wilhelms-Universita ¨t, Wegelerstraße 12, 53115 Bonn, Germany PCCP Dynamic Article Links www.rsc.org/pccp PAPER Downloaded by Max Planck Institut fuer on 21 August 2012 Published on 23 July 2012 on http://pubs.rsc.org | doi:10.1039/C2CP41382E View Online / Journal Homepage / Table of Contents for this issue