S-trans Isomer Research Articles

Molecular structure of S-ethylthioacrylate Combined vibrational spectroscopic and ab initioSCF-MO study

Ab initio 6-31G* SCF-MO calculations have been carried out on S-ethyl thioacrylate [CH2CHC(O)SCH2CH3]. Fully optimized geometries, relative stabilities, dipole moments and harmonic force fields for several conformers of this molecule have been determined and the results compared with those for similar molecules. Together with FTIR and Raman spectroscopic data, the theoretical results demonstrate that S-ethyl thioacrylate exists in two different conformations about the Cα–C bond (the s-cis and s-trans forms, with CC–CO dihedral angles equal to 0° and 180°, respectively); the s-cis conformation being more stable than the s-trans form by ca. 6 kJ mol-1 for the isolated molecule. Comparison of the experimental and theoretical vibrational spectra confirms that, as concluded from our previous study on the analogous trans-S-ethyl thiocrotonate molecule (R. Fausto, P. J. Tonge and P. R. Carey, J. Chem. Soc., FaradayTrans., 1994, 90, 3491), the presence of the s-trans isomer of an α,β-unsaturated thioester can be successfully monitored by the IR band at ca. 1170 cm-1, ascribed to the Cα–C stretching mode of this form. In addition, we were also able to identify some IR bands sensitive to the conformation of the ethyl group that may be used as spectroscopic probes to study conformational equilibria associated with this internal degree of freedom in more complex S-ethyl thioesters.

Conformational Study of SnCl4Complexes of Aldehydes, α,β-Unsaturated Aldehydes, and Esters: Observation of Both s-Cis and s-Trans Isomers

A variable-temperature NMR study shows that 2-methylpropanal (1a), ethyl 2-pentenoate (2), ethyl 4-methyl-2-pentenoate (3a), 2-pentenal (4), and 4-methyl-2-pentenal (5a) prefer the C-CH3 eclipsed c...

Molecular structures of cis- and trans-S-Ethyl thiocrotonate. A combined vibrational spectroscopic and ab initio SCF-MO study

Ab initio 6-31G* SCF-MO calculations have been carried out for cis- and trans-S-ethyl thiocrotonate [cis- and trans- CH3—CHCH—C(O)SCH2CH3]. Fully optimized geometries, relative stabilities, dipole moments and harmonic force fields for several conformers of these molecules have been determined and the results compared with those of similar molecules. Combined with FTIR spectroscopic data, the theoretical results demonstrate that trans-S-ethyl thiocrotonate exists in two different confromations about the Cα–C bond (the s-cis and s-trans forms, with CC–CO dihedral angles equal to 0° and 180°, respectively), the s-cis conformation being more stable than the s-trans form by ca. 7 kJ mol–1 for the isolated molecule situation, while the cis-s-ethyl thiocrotonate molecule adopts only the s-cis conformation about this bond. A comparison of the experimental and theoretical vibrational spectra also shows that the existence of the less stable s-trans isomer of an α,β-unsaturated thioester can be successfully monitored by the IR band at ca. 1165 cm–1, ascribed to the Cα–C stretching mode of this form. Additional conformationally sensitive bands are also identified, but these are difficult to use in the IR spectrum because of overlap with other features.

Conformational stability and force field of chloromonophosphazenes: MNDO calculations, vibrational spectra and normal coordinate analyses of Cl3PNP(X)Cl2 (X = O, S) and [Cl3PNPCl3][Y] (Y = Cl, PCl6)

The potential energy around the PN bonds for the Cl3PNP(X)Cl2 (X = O, S) molecules and [Cl3PNPCl3]+ cation have been derived from MNDO (modified neglect of diatomic overlap) calculations. The most stable conformations are two s-trans isomers in nearly eclipsed forms. The calculated structural parameters agree well with the X-ray experimental data. Barriers of 6 and 1.5 kJ/mol for the rotations of the POCl2 and PCl3 groups are predicted. In addition, the Raman spectra and the qualitative depolarization measurements for these molecules in the liquid phase have been obtained. All the data indicate that the molecules exist as a mixture of two rotamers in the molten phase. These two conformers are stabilized in the crystal packing of Cl3PNP(O)Cl2. The observed frequencies are in good agreement with the calculated values obtained by normal coordinate analysis. The MNDO calculation of the harmonic force field is in reasonable agreement with the experimental values. The force-constant values assigned to the torsional modes around the PN bonds correspond to low barriers for the internal rotations. These easy internal rotations around the PN and PN bonds can explain the flexibility of the phosphazene backbone and the elastomeric properties of the polyphosphazene polymers.

Conformational studies on (+)-anatoxin-a and derivatives.

Anatoxin-a (AnTX) is a highly potent agonist acting at the nicotinic acetylcholine receptor (nAChR) and represents a valuable tool in the study of this receptor. Molecular mechanics, semi-empirical and ab initio molecular orbital energy minimization procedures were conducted to investigate the conformation of AnTX. For each minimization procedure, the s-trans enone isomer of protonated AnTX was the energetically favoured conformer due to intramolecular electrostatic interactions. Our studies are discussed in the light of previous experimental observations and conformational studies, in addition to their importance in the development of future pharmacophore models for nAChR agonist binding.

Photochemical generation and detection of an elusive rotamer of matrix-isolated mesityl oxide. Dispersive kinetics of the thermal isomerization: twisted s-trans? s-cis forms

The photochemical behaviour of mesityl oxide (4-methylpent-3-en-2-one) was investigated in a number of rare-gas matrices. A ‘new’ conformer, most probably the twisted s-trans form, generated by irradiation with UV light (λmax 250 nm), was trapped and its UV and IR absorption spectra were recorded. The twisted s-trans isomer was unstable upon warming of the matrix and rearranged to starting material (s-cis isomer). Kinetic measurements of the twisted s-trans→ s-cis rotational isomerization were performed in an argon matrix in the temperature range 37–41 K, and these data were analysed in terms of dispersive kinetics with time-dependent rate coefficients.

Molecular structures of the rotational isomers of methyl vinyl ether from infrared and raman spectra and non-empirical calculations

The IR spectrum of the vapour phase and the Raman spectra of the solid, liquid and vapour phases of methyl vinyl ether have been recorded. All of the fundamentals of the s-cis isomer have been assigned and approximately described; for the s-trans isomer new bands have been detected in the Raman spectra and now eleven Fundamentals are known. Reassignment of the low-frequency region leads to a value for the barrier to methyl internal rotation of the s-cis isomer in close agreement with that derived from microwave data: V 3 = 1427 cm −1 (4.08 kcal mol −1) and V 6 = −40 cm −1 (−0.11 kcal mol −1). Ab initio calculations at the minimal basis SCF level including electron correlation have been performed in order to study the two possible internal rotations in the molecule. The correlation effects lower the energy difference between the s-trans and the s-cis isomers to 1.3 kcal mol −1 and enhance the barrier to interconversion, whereas they have very little effect on the barrier to methyl internal rotation; in the s-trans isomer this barrier is calculated to be as low as 0.67 kcal mol −1. A comparison is made between the first two measured and computed ionization potentials for the two isomers. The choice of the best set of structural parameters for the s-cis form is discussed. Repulsive overlap interactions between bond orbitais and conjugation may explain the stability of the s-cis form of methyl vinyl ether and other molecules.

A Proton Magnetic Resonance Study of the Amide Configurations of 2-Acetoacetamido Derivatives of 4-Methylpyridine and Pyrimidine

Abstract It has been found by 1H NMR spectroscopy that 2-acetoacetamido-4-methylpyridine (1) and 2-acetoacetamidopyrimidine (2) exist predominantly in keto forms in CDCl3 and DMSO. The amide group of 1 seems to be in a s-trans configuration in both solvents, whereas that of 2 is in a s-cis form in CDCl3, but predominantly in a s-trans isomer in DMSO at room temperature. Configurations of the enol forms are discussed.

Konformation und Schwingungsspektren des 1,1'-Biaziridyls

Infrared and Raman spectra indicate that 1, 1'-biaziridine exists only as the s-trans conformational isomer in the gaseous, liquid, and solid states. Based on i.r. band contours, Raman depolarisation data, and crystal splittings, complete assignments of i.r. and Raman bands to normal modes of vibration are given.

Dipole moments of some substituted acetophenones : Influence of ortho substitutents on the position of the acetyl group

The electric dipole moments of a number of substituted acetophenones, with at least one substituent (methyl, halo, nitro or amino) in the ortho position, have been measured in benzene solution and compared with the values calculated for free rotation as well as for s-trans and s-cis orientations of the acetyl group. 2-Methylacetophenones have a greater proportion of the s-cis isomer while 2-haloacetophenones have a greater proportion of the s-trans isomer. With increase in temperature there is an increase in the proportion of the less stable isomer. The observed dipole moment of o-aminoacetophenone indicates that the molecule exists predominantly in the s-cis configuration stabilized by intramolecular hydrogen bonding.

SOME STEREOCHEMICAL STUDIES ON α,β-UNSATURATED KETONES USING RAMAN SPECTROPHOTOMETRY

The Raman spectra of 12 α,β-unsaturated open-chain and cyclic ketones have been measured in the region of the C = O and C = C stretching vibrations, and the band positions and intensities have been compared with the corresponding bands in the infrared spectra. If due allowance is made for solvent effects, the infrared and Raman bands are observed at the same positions, but the relative intensities of the C = O and C = C bands are different; in particular, the Raman C = O stretching bands of the ketones in the s-trans configuration are from 5 to 10 times more intense than the Raman bands for the s-cis isomers. The cyclic s-trans ketones with a hydrogen atom on the α-carbon atom exhibit a solvent-sensitive doubling of the carbonyl maximum similar to that noted previously for certain α,β-unsaturated lactones, unsaturated acid anhydrides, and Δ2-cyclopentenone derivatives. These doublet carbonyl maxima are observed also in the infrared spectra. The C = C frequency is not similarly perturbed and the effect is distinct from the doubling of both the C = O and the C = C bands exhibited by some open-chain α,β-unsaturated ketones that exist in the liquid phase as equilibrium mixtures of s-cis and s-trans isomers.

Microwave Spectrum, Structure, and Dipole Moment of s-trans Acrolein

The microwave spectrum of acrolein has been studied in the 19–37 kmc region. A series of R-type transitions was identified for the ground and three excited torsional states of the s-trans isomer and these spectra were successfully fitted in the rigid rotor approximation. Using the ground state moments and additional electron diffraction data a reasonable though not unique structure has been obtained. The torsional frequency was found to be 200±50 cm—1 from relative intensity measurements, and a lower limit of about 2300 cal was estimated for the barrier hindering trans-cis conversion. The dipole moment for the ground state was found to be 3.11±0.04 d, oriented about 14° from the carbonyl group. The results obtained and the absence of a strong s-cis spectrum establish the s-trans isomer as the predominant form.

Two Forms of 1,3 Butadiene

IN a previous publication1, experimental evidence was adduced for the existence of butadiene molecules in two forms, s-cis and s-trans. This evidence came from a study of the ionization potentials of butadiene gas, both by spectroscopic and electron impact methods. The existence of the two forms has recently received strong confirmation from the thermodynamic studies of Aston, Szasz, Woolley and Brickwedde2. A conflict of opinion has, however, appeared as to which of the isomers is the lower-energy form. Rasmussen, Tunnicliff and Brattain3 concluded that the s-cis form had the lower energy ; Aston et al. followed Mulliken4 in believing that the s-trans was the lower-energy form. We wish to note here that the ionization potential data indicate that the s-trans isomer is the lower-energy form.