Decongestion of methylene spectra in biological and non-biological systems using picosecond 2DIR spectroscopy measuring electron-vibration–vibration coupling Paul M. Donaldson, Rui Guo, Frederic Fournier, Elizabeth M. Gardner, Ian R. Gould, David R. Klug * Department of Chemistry, Imperial College London, Exhibition Road, London SW7 2AZ, UK Received 14 October 2007; accepted 22 February 2008 Available online 4 March 2008 Abstract Methylene is found in the repeat units of many polymers including proteins. In some cases it appears to be a useful reporter of var- iation in local environment whilst in other contexts average behaviour seems to dominate. In this paper we apply a particular 2DIR tech- nique to a range of systems containing methylene groups, showing that mode frequencies, linewidths and splittings can be easily extracted even when the infrared absorption bands are too congested to allow reliable analysis. 2DIR spectra of polyethylene and several liquid alkanes are compared and it is shown for the case of L-arginine that the methylene scissor modes are split and that this can be resolved by tracking the 2DIR spectrum as a function of time. Calculations from first principles reveal that for most of the methylene modes studied, electrical anharmonicity is the dominant contributor to the 2DIR cross-peak intensity, with the mechanical anharmonicity making only a small contribution. Ó 2008 Elsevier B.V. All rights reserved. Keywords: Multidimensional infrared spectroscopy; Picosecond infrared spectroscopy; Time resolved infrared spectroscopy; Vibrational spectroscopy; Analytical spectroscopy; Methylene spectroscopy; Protein spectroscopy; Polymer science 1. Introduction Spectral congestion in infrared absorption and Raman scattering measurements can be a major impediment in the extraction of chemical information from condensed phase systems. Established methods of spectral simplifica- tion typically rely on various kinds of post hoc analyses and careful variation of measurement conditions [1]. Two dimensional infrared (2DIR) techniques are an emerging alternative to these methods [2–6]. The spectral simplifica- tions afforded by 2DIR spectroscopy occur through the use of sequences of short optical pulses to select only coupled vibrational modes from the manifold of resonant modes. Pulsed infrared techniques such as 2DIR pump probe [7] and 2DIR photon echo [8] have been used fruitfully to explore structural and dynamical issues in a diverse range of systems containing amide I and II modes, carbonyls and hydroxyl groups [9–18]. An interesting variation of 2DIR is the use of an addi- tional visible beam to retrieve the coupling information through a Raman scattering process [19–22]. Amongst the early papers on this theme were frequency domain mea- surements on CS 2 and acetonitrile by Wright and co-work- ers, achieved by measuring the infrared–infrared difference frequency component of the Raman scattered signal [23– 25]. These infrared–infrared-visible four wave mixing pro- cesses were originally termed ‘Doubly Vibrationally Enhanced Four Wave Mixing’ (DOVE-FWM) [26]. How- ever, as infrared double resonances are now a common 0301-0104/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.chemphys.2008.02.050 * Corresponding author. Tel.: +44 0 207 5945806; fax: +44 0 207 594 5880. E-mail addresses: paul.donaldson@ic.ac.uk (P.M. Donaldson), d.klu- g@imperial.ac.uk (D.R. Klug). www.elsevier.com/locate/chemphys Available online at www.sciencedirect.com Chemical Physics 350 (2008) 201–211