Published: June 30, 2011 r2011 American Chemical Society 8920 dx.doi.org/10.1021/jp2051596 | J. Phys. Chem. A 2011, 115, 8920–8927 ARTICLE pubs.acs.org/JPCA Gas-Phase Raman Spectra and the Potential Energy Function for the Internal Rotation of 1,3-Butadiene and Its Isotopologues Praveenkumar Boopalachandran, † Norman Craig, ‡ Peter Groner, § and Jaan Laane* ,† † Department of Chemistry, Texas A&M University, College Station, Texas 77843-3255, United States ‡ Department of Chemistry & Biochemistry, Oberlin College, Oberlin, Ohio 44074, United States § Department of Chemistry, University of Missouri, Kansas City, Missouri 64110-2499, United States I. INTRODUCTION The internal rotation about the central carbonÀcarbon bond of 1,3-butadiene can produce trans, cis, or gauche conformations depending on the angle of rotation, as shown in Figure 1. The trans conformer has long been known to be the predominant one, 1À3 but whether the higher-energy conformer has a cis or gauche configuration remained a question for many years. Aston and co-workers 4 found evidence that a second conformer was present 2.3 kcal/mol higher in energy from a calorimetric study, but they could not determine its structure. Lipnick and Garbisch 5 carried out NMR studies at various temperatures and determined the energy difference to be 2.1 kcal/mol. They favored the gauche structure for the higher-energy form, but their data were not sufficient to rule out the planar cis form. Cole and co- workers 6 reported the far-infrared spectrum of 1,3-butadiene and the 1,1,4,4-d 4 and -d 6 isotopologues and observed a series of bands for the ν 13 internal rotation (torsion) for each molecule. They observed the 0 f 1 transitions at 162.5, 149.2, and 141.7 cm À1 for the d 0 , d 4 , and d 6 molecules, respectively, and calculated a potential energy barrier of 1900 ( 800 cm À1 using a quadratic/quartic potential function. However, they provided no data for a second conformer. In 1974, Carreira 7 reported the gas- phase Raman spectrum of 1,3-butadiene and observed seven sub- bands, which were assigned to double-quantum jumps of the ν 13 vibration of the trans conformer. He also observed three other features, which he assigned to a cis structure. The data were then used to calculate a periodic potential energy function that had a barrier of 2504 cm À1 (7.15 kcal/mol) at 90° rotation, where 0° corresponded to the planar trans structure. The energy for the cis form at 180° was calculated to be 873 cm À1 (2.49 kcal/mol), in reasonable agreement with the earlier studies. 4,5 Infrared, Raman, and ultraviolet spectroscopy studies of matrix-isolated 1,3- butadiene 8À10 also supported the idea that the cis structure was the minor conformer. A 1983 Raman study by Panchenko and co-workers 11 reported gas-phase Raman spectra for the 2ν 13 regions of 1,3-butadiene and its cis,cis-1,4-d 2 and -d 6 isotopolo- gues. They again assumed the minor conformer to have the cis structure. In 1991, Engeln, Consalvo, and Reuss 12 (referred to as ECR) reported new gas-phase Raman data for 1,3-butadiene and observed new features, which were assigned to the gauche conformer. Notably, they observed a band at 214.9 cm À1 , which was attributed to one of the transitions arising from a lower quantum state of the gauche conformer. ECR also calculated a periodic potential function based on cos(nϕ) terms 13 using n = 1À6. Although exact values were not reported, the reported V n values correspond to a barrier between trans and gauche forms of 2075 cm À1 , with the gauche form at 138° lying 989 cm À1 higher than the trans structure. The barrier between the two equivalent gauche forms (corresponding to the cis structure) was 408 cm À1 . A number of theoretical calculations have been carried out to determine the energy differences between the 1,3-butadiene Received: June 1, 2011 Revised: June 29, 2011 ABSTRACT: The gas-phase Raman spectra of 1,3-butadiene and its 2,3-d 2 , 1,1,4,4-d 4 , and -d 6 isotopologues have been recorded with high sensitivity in the region below 350 cm À1 in order to investigate the internal rotation (torsional) vibration. Based on more accurate structural information, the internal rotor constants F n were calculated as a function of rotation angle (ϕ). The data for all the isotopologues were then fit using a one-dimensional potential energy function of the form V = 1 / 2 ∑V n (1 À cos ϕ). Initial V n values were based on those generated from theoretical calculations. The agreement between observed and calculated frequencies is very good, although bands not taken into account were present in the spectra. The energy difference between the trans and gauche forms was determined to be about 1030 cm À1 (2.94 kcal/mol), and the barrier between the two equivalent gauche forms was determined to be about 180 cm À1 (0.51 kcal/mol), which agrees well with high-level ab initio calculations. An alternative set of assignments also fits the data quite well for all of the isotopologues. For this model, the energy difference between the trans and gauche forms is about 1080 cm À1 (3.09 kcal/mol), and the barrier between gauche forms is about 405 cm À1 (1.16 kcal/mol).