Structure of 1‑Alkyl-1-methylpyrrolidinium Bis(trifluoromethylsulfonyl)amide Ionic Liquids with Linear, Branched, and Cyclic Alkyl Groups Hemant K. Kashyap, † Cherry S. Santos, ‡ N. Sanjeeva Murthy, § Jeevapani J. Hettige, † Kijana Kerr, ∥ Sharon Ramati, ⊥ JinHee Gwon, ⊥ Masao Gohdo, ∥ Sharon I. Lall-Ramnarine, ⊥ James F. Wishart, ∥ Claudio J. Margulis,* ,† and Edward W. Castner, Jr.* ,‡ † Department of Chemistry, University of Iowa, Iowa City, Iowa 52242, United States ‡ Department of Chemistry and Chemical Biology, Rutgers, The State University of New Jersey, Piscataway, New Jersey 08854, United States § New Jersey Center for Biomaterials, Rutgers, The State University of New Jersey, Piscataway, New Jersey 08854, United States ∥ Chemistry Department, Brookhaven National Laboratory, Upton, New York 11973-5000, United States ⊥ Department of Chemistry, Queensborough Community College-CUNY, Bayside, New York 11364, United States * S Supporting Information ABSTRACT: X-ray scattering and molecular dynamics simulations have been carried out to investigate structural differences and similarities in the condensed phase between pyrrolidinium-based ionic liquids paired with the bis(trifluoromethylsulfonyl)amide (NTf 2 − ) anion where the cationic tail is linear, branched, or cyclic. This is important in light of the charge and polarity type alternations that have recently been shown to be present in the case of liquids with cations of moderately long linear tails. For this study, we have chosen to use the 1-alkyl-1-methylpyrrolidinium, Pyrr 1,n + with n = 5 or 7, as systems with linear tails, 1-(2- ethylhexyl)-1-methylpyrrolidinium, Pyrr 1,EtHx + , as a system with a branched tail, and 1-(cyclohexylmethyl)-1-methylpyrrolidinium, Pyrr 1,ChxMe + , as a system with a cyclic tail. We put these results into context by comparing these data with recently published results for the Pyrr 1,n + /NTf 2 − ionic liquids with n = 4, 6, 8, and 10. 1,2 General methods for interpreting the structure function S(q) in terms of q-dependent natural partitionings are described. This allows for an in-depth analysis of the scattering data based on molecular dynamics (MD) trajectories that highlight the effect of modifying the cationic tail. ■ INTRODUCTION Modern room-temperature ionic liquids (RTILs) are novel materials that are being pursued for a variety of applications including energy storage in solar cells, supercapacitor devices, lithium ion batteries, as well as catalysis and separation processes. 3−8 In the past decade, a significant number of X- ray 1,9−34 and neutron 35−37 scattering experiments as well as MD simulations 23,28,31,32,38−51 have been carried out to unravel the local and mesoscale structure of RTILs. As our group has indicated in several recent publications, 2,23,28,50 the X-ray structure function S(q) of RTILs typically has either two or three peaks in the relevant intermolecular region at q values <2 Å −1 . If a peak is present at q values <0.5 Å −1 , it is often called a prepeak or first sharp diffraction peak (FSDP). Unless cancellations between the constituent partial structure functions occur, two other peaks are located in the range 0.5 < q/Å −1 < 2. If we consider the situation for an ionic liquid (IL) that shows three peaks (prepeak + two other peaks in S(q)), the peak at larger q values is often associated with adjacency correla- tions. 23,28,50 The intermediate peak, at higher q values than the prepeak but at lower q values than the adjacency peak, commonly results from cation−anion charge alternation. 23,28 Assigning a structural feature to the prepeak has been challenging and the source of some controversy. 29,30,37,50,52,53 In a set of recent articles, 2,54 we have provided a full mathematical description of its origin for systems with larger linear alkyl tails. In this article, we report consolidated results from X-ray and MD simulation of the 1-alkyl-1-methylpyrrolidinium (Pyrr 1,n + ) with n = 5 or 7, 1-(2-ethylhexyl)-1-methylpyrrolidinium (Pyrr 1,EtHx + ), and 1-(cyclohexylmethyl)-1-methylpyrrolidinium (Pyrr 1,ChxMe + ) cations paired with the bis(trifluoromethyl- sulfonyl)amide (NTf 2 − ) anion. See Figure 1 for the chemical structures of the cations and anion studied here. We also Special Issue: Michael D. Fayer Festschrift Received: April 10, 2013 Revised: June 5, 2013 Article pubs.acs.org/JPCB © XXXX American Chemical Society A dx.doi.org/10.1021/jp403518j | J. Phys. Chem. B XXXX, XXX, XXX−XXX