Conformational preferences for N, N-dimethyl-2-haloacetamides (halo=F, Cl, Br and I) through theoretical and experimental studies: An unexpected orbital interaction (original) (raw)
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Journal of Molecular Structure, 2010
The analysis of the IR carbonyl band of the 2-substituted N-methoxy-N-methylacetamides Y-CH 2 C(O)-N(OMe)Me (Y = F 1, OMe 2, OPh 3, Cl 4), supported by B3LYP/6-311++G(3df, 3pd) calculations along with the NBO analysis for 1-4, indicated the existence of cis-gauche conformers i.e. (c) and (g) for 1 and 3, (c 1 , c 2 ) and (g 1 , g 2 ) for 2, and (c) and (g 1 , g 2 ) for 4. In the gas phase, the g conformer population prevails over the c one, for 1 and 3, the (c 1 + c 2 ) population prevails over the (g 1 + g 2 ) one for 2, and the (g 1 + g 2 ) conformer population is more abundant than (c) one for 4. In n-hexane solution, the cis conformer is more abundant for 1-3. The occurrence of Fermi resonance in the m CO region, in n-hexane, precludes the estimative of relative populations of the (c, g 1 , g 2 ) conformers for 4. The SCI-PCM calculations agree with the solvent effect on the m CO band component relative intensities for 1-3. NBO analysis showed that the n N ? p à co orbital interaction is the main factor which stabilizes the gauche (g, g 1 , g 2 ) conformers for 1-4 into a larger extent relative to the cis (c, c 1 , c 2 ) ones. The n Y ? p à co , r CAY ? p à co , p co ? r à CAY and p à co ? r à CAY orbital interactions still contribute, but into a minor extent for the stabilization of the gauche conformers relative to the cis ones. The existence of some pyramidalization at the nitrogen atom of the Weinreb amides 1-4 is responsible for the occurrence of Y dÀ (4)ÁÁÁO dÀ (9) and Y dÀ (4)ÁÁÁN dÀ (7) short contacts in the gauche (g, g 1 , g 2 ) conformers, which originates strong repulsive Coulombic interactions, acting in opposition to the large orbital stabilization of the gauche conformer with respect to the cis one. Therefore, a delicate balance of the Coulombic and orbital interactions seems to be responsible for the observed stabilization of the gauche (g, g 1 , g 2 ) and cis (c, c 1 , c 2 ) conformers, both in the gas phase and in the solution for 1-4. However, the cis conformer predominance, in non polar solvents, for the 2-substituted N-methoxy-N-methyl acetamides 1-3, bearing in a first raw (fluorine and oxygen) atoms, is in the opposite direction to the gauche conformer preference for the corresponding 2-substituted N,N-dialkyl-acetamides.
Journal of the Chemical Society, Perkin Transactions 2, 2000
The solvent and temperature dependence of the 1 H and 13 C NMR spectra of N,N-dimethylfluoroacetamide (DMFA) and N,N-dimethyl-α-fluoropropionamide (DMFP) are reported and the 5 J CF , 1 J CF and 4 J CF couplings analysed by solvation theory. Density function theory (DFT) at the B3LYP/6-311ϩG(d,p) level with ZPE (zero point energy) corrections was used to obtain the conformer geometries. In DMFA, the DFT method gave only two minima for the cis (F-C-C᎐ ᎐ O, 0Њ) and gauche (F-C-C᎐ ᎐ O, 140.6Њ) rotamers. The trans rotamer was not a minimum in the energy surface. Assuming only the cis and gauche forms, the observed couplings when analysed by solvation theory gave the energy difference (E cis Ϫ E g) of 2.5 kcal mol Ϫ1 in the vapour phase, (cf. the ab initio value of 2.3 kcal mol Ϫ1) decreasing to 0.87 kcal mol Ϫ1 in CCl 4 and to Ϫ1.29 kcal mol Ϫ1 in DMSO. In DMFP the ab initio calculations gave three minima; the cis (F-C-C᎐ ᎐ O, 30.4Њ), gauche-1 (F-C-C᎐ ᎐ O, 144.7Њ) and gauche-2 (F-C-C᎐ ᎐ O, Ϫ124.1Њ) rotamers with (E cis Ϫ E g2) equal to 2.5 kcal mol Ϫ1 and (E g1 Ϫ E g2) equal to 0.3 kcal mol Ϫ1. The observed couplings were analysed by solvation theory assuming one "average" gauche conformer to give (E cis Ϫ E g(AV)) equal to 2.1 kcal mol Ϫ1 in the vapour phase, decreasing to 0.83 kcal mol Ϫ1 in CCl 4 and to Ϫ1.11 kcal mol Ϫ1 in DMSO.
Chemical Physics Letters, 2004
The rotational isomerism of a-fluoroacetaldehyde (FAL), a-chloroacetaldehyde (CAL) and a-bromoacetaldehyde (BAL) has been studied through ab initio and density functional theory. Potential energy surfaces have been obtained and the equilibrium was found to be between the cis and trans forms for FAL and between the cis and gauche forms for CAL and BAL. The trans or gauche conformer was found to be more stable in the three cases. Natural bond orbital analysis was conducted and led to the conclusion that steric repulsion prevail over electron delocalization in determining the conformational preference.
The Journal of …, 2007
Conformational preferences and electronic interactions of trans-2-fluorocyclopentanol (1), trans-2-chlorocyclopentanol (2), and trans-2-bromocyclopentanol (3) were analyzed using experimental and theoretical 3 J HH coupling constants, theoretical calculations, and natural bond orbital (NBO) analysis. The conformational equilibria of compounds 1-3 can be represented by their diaxial and diequatorial conformers as supported by theoretical calculations. From 3 J HH coupling constant values, it can be found that the diequatorial conformer is present in the equilibrium as 55% for compound 1 and as 60% for compounds 2 and 3. This behavior is in agreement with orbital interaction analyses obtained from NBO.
Egyptian Journal of Chemistry, 2019
I N THIS investigation, the stability of the keto-enol forms and conformers of 2-fluoroacetaldehyde was investigated by LC-wPBE, B3LYP and M06-2X functionals and 6-311++G (d,p) basis set. The total energy, the energies of frontier orbitals, HOMO-LUMO gaps and total dipole moment of these molecules were estimated. These calculations show that more stability of the keto form in compared to enol form. Calculations at the LC-wPBE/6-311++G (d,p) level of theory reveal the more stability of I-conformer in compared to other conformers.Natural bond orbital (NBO) analysis was applied for illustrating the negative hyperconjugative effect on the conformers. In the basis of NBO analysis, the LP (O2)→σ*(C-H) and LP (O2)→σ*(C-C) interactions were responsible of the negative hyperconjugation in the examined compounds. The interaction energy, off-diagonal elements, and total steric energy values of these interactions were reported.
Halogenated C,N-diarylacetamides: molecular conformations and supramolecular assembly
Acta crystallographica. Section C, Structural chemistry, 2014
The structures of four halogenated N,2-diarylacetamides are reported and compared with a range of analogues. N-(4-Chloro-3-methylphenyl)-2-phenylacetamide, C(15)H(14)ClNO, (I), and N-(4-bromo-3-methylphenyl)-2-phenylacetamide, C(15)H(14)BrNO, (II), are isostructural in the space group P-1. The molecules of (I) and (II) are linked into chains of rings by a combination of N-H...O and C-H...π(arene) hydrogen bonds. The molecules of N-(4-chloro-3-methylphenyl)-2-(2,4-dichlorophenyl)acetamide, C(15)H(12)Cl(3)NO, (III), and N-(4-bromo-3-methylphenyl)-2-(2-chlorophenyl)acetamide, C(15)H(13)BrClNO, (IV), are linked into simple C(4) chains by N-H...O hydrogen bonds, but significant C-H...π(arene) interactions are absent. The N-aryl groups in compounds (III) and (IV) adopt a different orientation, by ca 180°, from that of the corresponding groups in compounds (I) and (II), but otherwise the conformations of (I)-(IV) are very similar. Comparisons are drawn between compounds (I) and (IV) and a ...
Journal of Molecular Structure, 1999
Ab initio molecular orbital calculations at HF/4-31G level and infrared spectroscopic data for the frequencies are applied to analyse the grouping in a series model aromatic secondary amides: formanilide; acetanilide; o-methylacetanilide; 2,6-dimethylformanilide, 2,6-dimethylacetanilide; N-benzylacetamide and N-benzylformamide. The theoretical and experimental data obtained show that the conformational state of the molecules studied is determined by the fine balance of several intramolecular factors: resonance effect between the amide group and the aromatic ring, steric interaction between various substituents around the -NH-CO-grouping in the aromatic ring, conjugation between the carbonyl bond and the nitrogen lone pair as well as direct field influences inside the amide group. ᭧
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy, 2023
The structural and vibrational properties of naproxen, an inhibitor of cyclooxygenase (COX) enzyme, were investigated by molecular modeling and experimental IR and Raman spectroscopic techniques. Possible conformers of the molecule were searched via a molecular dynamics simulation carried out with MM2 force field. The total energies, equilibrium geometries, force fields, IR and Raman spectral data of the found stable conformers were determined by means of geometry optimization and harmonic frequency calculations carried out using the B3LYP method and Pople-style basis sets of different size. The stability order obtained for the lowest-energy conformers was confirmed by high-accuracy thermochemistry calculations performed with G3MP2B3 composite method. Some electronic structure parameters of naproxen and the anharmonicity characters of its vibrational modes were determined by means of natural population analysis (NPA) and anharmonic frequency calculations at B3LYP/6-31++G(d,p) and B3LYP/6-311++G(d,p) levels of theory. A part of these calculations carried out for free naproxen molecule were repeated also for its energetically most favored dimer forms. Two different scaling procedures ((1) "SQM-FF methodology" and (2) "Dual scale factors") were independently applied to the obtained harmonic vibrational spectral data to fit them to the corresponding experimental data. In the light of the obtained calculation results, which confirm the remarkable effects of conformation and intermolecular hydrogen bonding on the structural and vibrational spectral data, in particular, on those associated with the functional groups in the propanoic acid chain, a reliable assignment of the fundamental bands observed in the experimental IR and Raman spectra of the molecule was achieved.
Canadian Journal of Chemistry, 1973
Classical energy calculations are used in combination with Abraham's electrostatic theory of solvation energy to estimate rotamer energy differences for haloethane derivatives. The calculations are tested by comparing experimental and calculated ΔE values for several di- and tetrahaloethanes. There is good agreement for chloro and bromo derivatives but poorer agreement for fluoro and iodo derivatives. ΔE values in solution are also estimated for l4 complex chloro- and bromoethanes which we have previously investigated by n.m.r. spectroscopy. The calculations generally parallel the experimental results as reflected by vicinal coupling constants. They are particularly useful for trends in conformational preference for closely related compounds and are used in conjunction with vicinal coupling constants to identify diastereomers produced by halogenation of alkenes. Steric interactions, dipole–dipole interactions and solvation energy are all important in determining conformational p...
The Journal of Physical Chemistry A, 2004
The structure of N-methyldichloroacetamide (MeNHCOCHCl 2 ) has been elucidated in the gaseous and solid states experimentally by gas electron diffraction and X-ray crystallography, and computationally with ab initio and plane-wave DFT methods. Although the main structural parameters generally agree well, the orientation of the CHCl 2 group relative to the carbonyl oxygen was found to be very different in the solid and gas phases. X-ray crystallography and solid-state plane-wave DFT methods indicate that the bond torsion angle φ(HCCO) is 180.0°, while ab initio and gas electron diffraction methods return φ(HCCO) as -13.1°a nd -31.8(22)°, respectively. Further investigation of this phenomenon was carried out by using various computational methods. The possibility of intermolecular H‚‚‚O and Cl‚‚‚O bonds, which would stabilize the solid-state structure, was investigated by both solid-state plane-wave DFT and single-point ab initio methods. Ab initio SCRF calculations were also employed to evaluate solvent effects on the structure, using the Onsager reaction field model.