Hassan M. Badawi

Analysis of the infrared and Raman spectra of phenylacetic acid and mandelic (2-hydroxy-2-phenylacetic) acid

The structural stability of phenylacetic acid and mandelic acid was investigated by the DFT-B3LYP and the ab initio MP2 calculations with the 6-311G** basis set. The two molecules were predicted at the DFT and MP2 levels of calculation to have the non-planar (Np) forms as their lowest energy structures. The observed spectral intensities of the acids were consistent with the Np conformation being the predominant form at room temperature. The vibrational wavenumbers were computed at the B3LYP level of theory and tentative vibrational assignments were provided on the basis of combined theoretical and experimental infrared and Raman data of the molecules. The sharpness of the methylenic O-H stretching mode in the IR spectrum of mandelic acid suggests the absence of intermolecular dimerization in the acid which is supported by the observation of no splitting of its CO stretching mode.

A comparative study of the infrared and Raman spectra of aniline and o-, m-, p-phenylenediamine isomers

The structural stabilities of o-, m- and p-phenylenediamine (PDA) isomers were investigated by DFT-B3LYP and ab initio MP2 calculations with the 6-311G(**) basis set. From the calculations the three isomers were predicted to exist predominantly in an anti (transoid) structure. In the o-isomer, the syn (cisoid) form is calculated to turn to the anti (transoid) form with the two HNCC torsional angles of about 44 and 10° and the NH2 inversion barrier of 3-4 kcal/mol. The CCNH torsional angles in the m-PDA and p-PDA isomers were calculated to be about 25-26° as compared to 20° in aniline. A comparison of the Raman spectra of the three PDA-s with those of aniline shows the high sensitivity of the ring breathing mode to the nature of substituents in the aniline ring. The vibrational wavenumbers were computed at the DFT-B3LYP for aniline and the o-, m- and p-PDA isomers for the purpose of comparison. Complete vibrational assignments were made on the basis of normal coordinate analyses and potential energy distributions for aniline and the o-, m- and p-PDA molecules.

C-C and C-N rotational barriers in vinyl ketene and vinyl isocyanate

Abstract The conformational behavior and structural stability of vinyl ketene and vinyl isocyanate were investigated by utilizing ab initio calculations with 6-311++G∗∗ basis set at the Density Functional (B3LYP) level. Both molecules were predicted to have the s-cis⇔s-trans conformational equilibrium with the s-trans form being the predominant conformer for the two molecules. Full optimization was performed at the ground and transition states in the two molecules. The calculated rotational barriers in the systems were compared with each other and it was found that the barrier of the internal rotation around the C–C single bond in vinyl ketene was much higher than that of the corresponding C–N bond in the vinyl isocyanate. The vibrational frequencies were computed at the DFT-B3LYP level and the calculated infrared and Raman spectra of the cis–trans mixtures of the two molecules were plotted. Complete vibrational assignments were made on the basis of normal coordinate calculations for both stable conformers of the molecules.

Raman spectra, conformational stability, structural parameters, vibrational assignment, and ab initio calculations for epifluorohydrin

The Raman spectra (3200–100 cm−1) of epifluorohydrin, OCH2CH(CH2F), in variable solvents, as well as that of the gas have been recorded and several of the bands due to the two less stable conformers have been identified. The variable solvent studies were inconclusive on the relative conformer stabilities. The conformational energy differences and optimized geometries for all three conformers have been obtained from ab initio calculations with the 3–21G, 4–31G and 6–31G* basis sets. A normal coordinate analysis has also been performed for each conformer with a force field determined from the 3–21G basis set. Assignment of the vibrational fundamentals observed in the Raman spectra of the fluid phases is proposed based on the normal coordinate calculations. In the liquid phase, one of the conformers with a large dipole moment predominates and it appears to be the gauche-I form which is the only one found in the solid. Utilizing the three rotational constants previously reported for each conformer, along with restricted relative distances for several of the structural parameters among the conformers from ab initio calculations, r0 structural parameters for the heavy atoms have been determined.

Structural stability, C-N internal rotations and vibrational spectral analysis of non-planar phenylurea and phenylthiourea

The structural stability and C-N internal rotations of phenylurea and phenylthiourea were investigated by DFT-B3LYP and ab initio MP2 and MP4//MP2 calculations with 6-311G** and/or 6-311+G** basis sets. The complex multirotor internal rotations in phenylurea and phenylthiourea were investigated at the B3LYP/6-311+G** level of theory from which several clear minima were predicted in the calculated potential energy scans of both molecules. For phenylurea two minima that correspond to non-planar- (CNCC dihedral angle of about 45 degrees ) cis (CNCO dihedral angle is near 0 degrees ) and trans (CNCO dihedral angle is near 180 degrees ) structures were predicted to have real frequency. For phenylthiourea only the non-planar-trans structure was predicted to be the low energy minimum for the molecule. The vibrational frequencies of the lowest energy non-planar-trans conformer of each of the two molecules were computed at the B3LYP level and tentative vibrational assignments were provided on the basis of normal coordinate analysis and experimental infrared and Raman data.

An investigation of structural stability and analysis of vibrational spectra of formyl ketene based on ab initio calculations

Abstract The conformational behavior and structural stability of formyl ketene were investigated by utilizing ab initio calculations with 6-311++G∗∗ basis set at restricted Hartree–Fock (RHF) and density functional (DFT-B3LYP) levels. The molecule was predicted to have the s -cis⇔ s -trans conformational equilibrium. Full optimization was performed at the transition state and the rotational barrier was calculated. The π–π interaction between the carbonyl and ketene groups was found to stabilize the planar s-cis and s-trans conformers only with a relatively high rotational barrier. The vibrational frequencies were computed at the RHF and DFT-B3LYP levels and the zero-point corrections were included into the calculated rotational barrier. Complete vibrational assignments were made on the basis of normal coordinate calculations for both stable conformers of the molecule.

Molecular structure and vibrational assignments of 2,4-dichlorophenoxyacetic acid herbicide.

The structural stability of 2,4-dichlorophenoxyacetic acid was investigated by the DFT-B3LYP and the ab initio MP2 calculations with the 6-311G** basis set. From the calculations at both levels of theory the Cgcpp structure was predicted to be the lowest energy minimum for the acid. The DFT and the MP2 levels disagreed about the nature of the second stable structure of 2,4-dichlorophenoxyacetic acid. At the DFT-B3LYP level of calculation the planar Tttp (transoid O=C-O-H) and the non-planar Tgcpp (cisoid O=C-O-H) forms were predicted to be 0.7 and 1.5 kcal/mol, respectively higher in energy than the Cgcpp conformation. At the MP2 level the two high energy Tttp and Tgcpp forms were predicted to be 2.7 and 1.4 kcal/mol, respectively higher in energy than the ground state Cgcpp structure. The Tgcpp form was adopted as the second possible structure of 2,4-dichlorophenoxyacetic acid on the basis of the fact that the Møller-Plesset calculations account better than the DFT ones for the non-bonding Ocdots, three dots, centeredH interactions. The vibrational frequencies of the lowest energy Cgcpp conformer were computed at the B3LYP level of theory and tentative vibrational assignments were provided on the basis of normal coordinate analysis and experimental infrared and Raman data.

DFT and MP2 vibrational spectra and assignments for gauche N-methyleneformamide CH2N–CHO

Abstract The conformational behavior of N-methyleneformamide CH2N–CHO was investigated by DFT-B3LYP and MP2 calculations with 6-311+G∗∗ basis set. The molecule was predicted by both levels to exist predominantly in the non-planar gauche structure as a result of pronounce repulsive interaction between the carbonyl oxygen and the lone-pair on the nitrogen that overcame the conjugation between the CN and CO moieties. The vibrational frequencies were computed at the DFT-B3LYP and the MP2 levels and the calculated infrared and Raman spectra of the molecule in the gauche conformation were plotted. On comparison there was no significant change in the calculated vibrational line intensities of the infrared spectra by both levels. In the Raman spectra of the molecule the line intensity of some skeletal modes of the heavy atoms were noticed to significantly change as going from DFT to MP2 calculations. Complete vibrational assignments were made on the basis of normal coordinate calculations for the molecule.

Vibrational infrared and raman spectra and density functional calculation of C–S rotational barrier in vinyl sulfonyl chloride and fluoride

Abstract The structure and conformational stability of vinyl sulfonyl chloride and fluoride were investigated using ab initio calculations at DFT-B3LYP/6-311++G∗∗ level. From the calculations the molecules were predicted to exist predominantly in the non-planar gauche conformations with the vinyl CC group being nearly eclipsing one of the sulfonyl SO groups as a result of significant conjugation between the two moieties. The asymmetric potential function for the internal rotation was determined for the molecule. The vibrational frequencies were computed. Normal coordinate calculations were carried out and potential energy distributions were calculated for the two molecules in the gauche conformation.

Conformational equilibria and structure of cyclopropylcarboxaldehyde and 1-methyl and 1-halo derivatives

Abstract The conformational equilibria of cyclopropylcarboxaldehyde and its 1-methyl, 1-fluoro and 1-chloro derivatives were investigated using ab initio calculations with 4–31G and 6–31G basis sets. The results indicate that cyclopropylcarboxaldehyde exhibits a cis←trans conformational equilibrium, with the cis form being the more stable conformer at room temperature. The conformational stability of the molecule was found to be dependent on the nature of the α-group substituent of the three-membered ring. For 1-halocyclopropylcarboxaldehyde the equilibrium is found to shift more towards the cis conformation, in which the dipolar repulsion between the C0 group and the halogen destabilizes the molecule in the trans conformation. In the case of 1-methylcyclopropylcarboxaldehyde, the trans conformation is predicted to be more stable than the cis conformer at ambient temperature. For the methyl group the energetically preferred conformation is predicted to have the hydrogen atom eclipsing the cyclopropane ring in a staggered configuration.