Extended basis abinitio calculations on conformers of propanal

Abstract

V. P. GUITA. Can. J. Chem. 63, 984 (1985). Extended basis ab initio calculations on four conformations of propanal at the 4-31G and 6-3 1G** levels followed by many-body perturbative interaction calculations MP2 and MP3 have been conducted. Optimized geometries, heights of rotational barriers, dipole moments, ionisation potentials, and diagonal harmonic force constants have been reported. The s-cis conformer (dihedral angle CCCO = 0") is found to be more stable than the gauche conformer (dihedral angle CCCO = 129.84") by 5.07 kJ mol-'. Torisonal potential barriers s-cislgauche, gauchels-cis, and gauchelgauche have values 8.81, 3.74, and 1.84 kJ mol-', respectively. It appears that meaningful values of rotational barriers can be obtained only after a careful optimization of the geometries of the conformations involved.V. P. GUITA. Can. J. Chem. 63, 984 (1985). OpCrant aux niveaux 4-3 IG et 6-3 lG**, on a effectuC des calculs ab initio sur quatre conformations du propanal; on a ensuite effectut des calculs MP2 et MP3 impliquant une interaction perturbative de plusieurs corps. On rapporte les gComCtries optimistes, les hauteurs des barrikres h la rotation, les moments dipblaires, les potentiels d'ionisation et les constantes de force harmonique diagonale. On a trouvC que le conformkre s-cis (angle diedre CCCO = 0") est plus stable que le conformkre gauche (angle dikdre CCCO = 129,84") par 5,07 kJ/mol. Les valeurs des barrikres au potentiel de torsion sont respectivement 8,81, 3,74 et 1,84 kJ/mol pour s-cislgauche, gauchels-cis et gauchelgauche. 11 semble que l'on ne peut obtenir de valeurs significatives pour les barrikres a la rotation qu'aprks avoir effectut une optimisation adCquate des gComCtries des conformations impliqubes.[Traduit par le journal] Introduction Several studies using different experimental techniques such as microwave (1-3), vibrational (4-8) and nmr (9) spectroscopy, and the theoretical techniques such as molecular dynamics (10) and molecular orbital calculations (1 1 -14) have been reported in the literature for the determination of the conformation of propanal in gaseous and liquid phases. Some statistical mechanical calculations (15, 16) of the chemical and thermodynamic properties of propanal in the gaseous state have also been reported. All these studies indicate that propanal exists as a mixture of s-cis and gauche conformers with the methyl group eclipsing the carbonyl bond in the s-cis (I) conformation and skewed out of plane to a dihedral angle of ca. 130" in the gauche (111) conformer. In a separate communication (17), we have reported photorotamerisation studies on this molecule in low temperature argon and nitrogen matrices and have found that at 10 K the concentrations of the two conformers are 95% s-cis and 5% gauche which changes to 47% and 53% respectively, on irradiation with a broad band ir radiation. A considerable uncertainty exists in the literature about the conformations of the lowest energy conformer and also about the enthalpy difference between the two stable conforme...

Near Threshold Photochemistry of Propanal. Barrier Height, Transition State Structure, and Product State Distributions for the HCO Channel

“…The addition of a methylene group introduces the complexity that several rotational conformers are now possible. Spectroscopic and theoretical work have shown that two stable conformers exist in the ground electronic state. ,− In the cis conformation, the methyl group is eclipsed with the oxygen atom (OCCC dihedral angle = 0°) whereas the skew conformation has the OCCC dihedral angle at approximately 128°. The cis conformer is found to be more stable by 5 kJ mol -1 with a barrier of 9 kJ mol -1 separating the cis and skew minima .…”

Nascent state distribution of HCO photoproduct arising from 309 nm photolysis of propionaldehyde

“…A quantitative measurement of the width of several nonoverlapped peaks in the spectrum was made by scanning very slowly over the peak several times and averaging the scans to get a final peak profile. Measurements were performed for qRo (7), qRo (10), qRo (12), qRo(13), qRo (19), qRo (20), qPo(3), and qPo(5) transitions. The splitting of transitions corresponding to Ka ^0 prevented us from making meaningful sub-Doppler measurements as there were no cases for which both the spin-rotation and asymmetry splitting were fully resolved.…”