Conformational stability and vibrational assignment of propanal

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Abstract

Variable temperature (−52 to −100°C) studies of the infrared spectra (3500–400 cm−1) of propanal, CH3CH2CHO, dissolved in liquid xenon have been recorded and bands due to both the cis and gauche conformers have been identified. From these data, the enthalpy difference has been determined to be 370±21 cm−1 (4.43±0.25 kJ mol−1) with the cis conformer (methyl group eclipsing the oxygen atom) the more stable form. Utilizing the new infrared data from the xenon solution, along with some additional Raman data, and ab initio predictions from MP2/6-31G* calculations, several reassignments of the fundamentals have been made for the cis conformer. Additionally, several bands which have been identified as arising from the gauche conformer have also been assigned. Infrared (3500–80 cm−1) and Raman (3500–30 cm−1) spectra of gaseous, liquid and/or solid CD3CD2CHO are also reported and the fundamentals assigned for the cis conformer. The spectroscopic and theoretical results are compared to the corresponding quantities for some similar molecules.

Introduction

Recently we carried out a far infrared spectral investigation of propanal 1, 2, 3. The torsional fundamentals along with several `hot bands' were observed for the cis conformer of the normal species and three isotopomers, i.e. the d1, d2 and d5 molecules. The potential function governing the conformational interchange was determined from these data, and an assumed enthalpy difference of 320 cm−1 obtained from ab initio calculations along with the microwave splitting of the ground torsional vibrational state [4]of the gauche conformer. In order to obtain an experimental value for the enthalpy difference for comparison to the calculated value, we have carried out a variable temperature study of the infrared spectra of propanal dissolved in liquid xenon. Since there were two vibrational assignments 5, 6which differ in the assignments of six of the fundamentals, we have also investigated the infrared and Raman spectra of CD3CD2CHO. To support the vibrational assignment, ab initio calculations have been carried out to obtain the force constants and to predict the infrared and Raman spectral intensities, and the frequencies of the fundamentals. The results of this experimental and theoretical study are reported herein.

Section snippets

Experimental

The sample of CH3CH2CHO was purchased from Aldrich (Milwaukee, WI) and the sample of CD3CD2CHO was obtained from Merck (St. Louis, MO). Purification was carried out by means of a low-temperature, low-pressure fractionation column. After purification, the samples were stored at room temperature until their use for the spectroscopic investigations.

The Raman spectra of propanal-d0 and propanal-d5 were recorded with a Cary model 82 spectrophotometer equipped with a Spectra-Physics model 171 argon

Conformational stability

Variable temperature studies of the infrared spectrum of propanal dissolved in liquid xenon were conducted to determine the enthalpy difference between the two stable conformers. An important advantage of this variable temperature study is that the conformer peaks are better resolved and the areas under them are more easily measured than areas of corresponding bands observed in the infrared spectrum of the gas. Infrared spectra from 3500 to 400 cm−1 were obtained at six different temperatures

Ab initio calculations

The LCAO-MO-SCF restricted Hartree–Fock calculations were performed with the Gaussian-92 program [9]using Gaussian type basis functions. The energy minima with respect to the nuclear coordinates were obtained by the simultaneous relaxation of all of the geometric parameters using the gradient method of Pulay [10]. Calculated structural parameters were determined from RHF/6-31G*, MP2/6-31G* and MP2/6-311+G** calculations and they are listed in Table 4 for the cis and gauche conformations. At the

Vibrational assignment

As indicated earlier, there is a discrepancy in the assignment of six of the fundamentals between the two previous vibrational assignments 5, 6. Frankiss and Kynaston [5]assigned ν17, the CH2 antisymmetric stretch at 2909 cm−1 whereas Sbrana and Schettino [6]assigned this fundamental at 2941 cm−1. Our spectral data clearly indicate that the latter assignment is correct. The assignment for the other carbon–hydrogen stretches for the ethyl group are essentially the same from the two vibrational

Discussion

Three of the previous spectroscopic studies of propanal have focused on the determination of the energy separations of the cis and gauche conformers. In the original microwave study by Butcher and Wilson [13], an enthalpy difference of 315±35 cm−1 (3.77±0.42 kJ mol−1) was determined from the relative intensities of the microwave lines. In an infrared study [6], an enthalpy difference of 360±45 cm−1 (4.31±0.54 kJ mol−1) was obtained from the temperature dependence of conformer bands at 660 and

Acknowledgements

JRD would like to acknowledge partial support of these studies by the University of Missouri-Kansas City Faculty Research Grant program.

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1

Permanent address: Analytical R&D Department, ICD Division, Bayer Corporation, Bushy Park Plant, Charleston, SC 29411, USA.

2

Taken in part from the dissertation which will be submitted to the Department of Chemistry in partial fulfillment of the Ph.D. degree.

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