MAGNETIC RESONANCE IN CHEMISTRY Magn. Reson. Chem. 2004; 42: 71–75 Published online in Wiley InterScience (www.interscience.wiley.com). DOI: 10.1002/mrc.1318 Evidence for spin diffusion in a H,H-NOESY study of imidazolium tetrafluoroborate ionic liquids † Norman E. Heimer, 1 Rico E. Del Sesto 1 and W. Robert Carper 1,2* 1 Department of Chemistry, USAF Academy, Colorado 80840-6230, USA 2 Department of Chemistry, Wichita State University, Wichita, Kansas 67260-0051, USA Received 7 July 2003; Revised 22 September 2003; Accepted 29 September 2003 The ionic liquids 1-ethyl-3-methylimidazolium tetrafluoroborate ([EMIM][BF4]) and 1-methyl-3- propylimidazolium tetrafluoroborate ([PMIM][BF4]) were studied by H,H-NOESY NMR using a cross- relaxation matrix analysis. Cross-peak intensities are seen to increase with increasing mixing time. Experimental and theoretical hydrogen–hydrogen distances are in agreement at short mixing times (50 ms). Mixing times longer than 50 ms result in an increasing contribution of spin diffusion that pro- duces unrealistically short hydrogen–hydrogen distances. Gas-phase ab initio molecular structures are obtained using Hartree–Fock (HF) and density functional theory (B3LYP) methods at the 6311 + G(2d,p) basis set level. The hydrogen–hydrogen distances obtained from the theoretical structures are in reason- able agreement with those calculated from the cross-relaxation matrices. Published in 2004 by John Wiley & Sons, Ltd. KEYWORDS: NMR; H,H-NOESY; cross-relaxation; matrix analysis; ionic liquids; ab initio; density functional theory INTRODUCTION Over the past several years, room-temperature ionic liquids have generated considerable excitement as potential ‘green chemistry’ designer solvents. Ionic liquids are environmen- tally benign as they have little or no vapor pressure. 1–3 Consequently, ionic liquids can be recycled, thus making syn- thetic processes less expensive and potentially more efficient and environmentally friendly. Ionic liquids may provide real practical advantages over earlier systems because of their relative insensitivity to air and water; 1–3 however, only limited information is available concerning the fundamental physico-chemical behavior of ionic liquids. 4–30 The study of viscous solutions (i.e. ionic liquids, pro- tein solutions, organometallic complexes, etc.) by NMR methods can provide useful information concerning rota- tional motion, transport properties, molecular structure and molecular interactions. Consequently, the dynamics and conformational properties of ionic liquids are of great importance for their host characteristics when performing as a recyclable solvent system. Previous studies of ionic liquids and molten salts have included NMR relaxation methods, diffusion studies, theoretical (ab initio) meth- ods, viscosity and conductivity studies. 4–30 In this paper, we examine the results of a H,H-NOESY study of the ionic liquids 1-ethyl-3-methylimidazolium tetrafluoroborate L Correspondence to: W. Robert Carper, Department of Chemistry, Wichita State University, Wichita, Kansas 67260-0051, USA. E-mail: bob.carper@wichita.edu † This article is a US Government work and is in the public domain in the USA ([EMIM][BF4]) and 1-methyl-3-propylimidazolium tetraflu- oroborate ([PMIM][BF4]). The calculated gas-phase struc- tures of [EMIM][BF4] and [PMIM][BF4] are shown in Figs 1 and 2, respectively. The effect of mixing time on the H,H-NOESY spectra was used to evaluate the degree of spin diffusion in [EMIM][BF4] and [PMIM][BF4]. Proton distances determined at short mixing times compare favorably with those obtained using Hartree–Fock (HF) and density functional theory (DFT) methods at the 6311 C G(2d,p) level of theory. EXPERIMENTAL The syntheses of [EMIM][BF4] and [PMIM][BF4] are described elsewhere. 13,31 The NMR spectra were obtained with a 4.70 T Varian Unity-Plus spectrometer operating at 30 ° C using a D 2 O capillary for a lock. Phase-sensitive NOESY spectra of [EMIM][BF4] and [PMIM][BF4] were recorded under the following conditions: SW D 1860 Hz, SW1 D 3720 Hz, 1024 experiments (310 increments), relax- ation delay D 3.0 s, acquisition time D 0.275 s, mixing times D 0.05, 0.10, 0.15, 0.2, 0.4, 0.6 and 0.8 s, 90 ° pulse width D 9.7 μs and measuring time 4–6 h. ([EMIM][BF4]) SW D 1860 Hz, SW1 D 3720 Hz, 1024 experiments (310 incre- ments), relaxation delay D 3.0 s, acquisition time D 0.275 s, mixing times D 0.05, 0.10, 0.15, 0.2, 0.25, 0.4, 0.6 and 0.8 s, 90 ° pulse width D 9.7 μs and measuring time 5 – 6 h. The υ H chemical shift values for [EMIM][BF4] are shown in Fig. 3 and are 1.53 ppm for the terminal methyl (C6), 4.00 ppm for the imidazolium ring methyl (C5), 4.30 ppm for the methylene protons (C4), 7.65 and 7.58 ppm for the C2 and C3 imidazolium ring protons and 8.73 ppm for the Published in 2004 by John Wiley & Sons, Ltd.