A theoretical approach to the vibrational analysis of the nitroenamine system

Abstract

A comparative ab initio and semi-empirical study of the vibrational spectra of a series of nitroenamines with intramolecular hydrogen-bonds has been performed. Although the semi-empirical MN DO/H calculations reproduce qualitatively some of the observed spectral properties, the v( NO,) frequencies are considerably overestimated. The minimal basis set STO-3G gives the correct order for the frequencies of the characteristic vibrations of the nitroenamine system, and reproduces many of the observed structural effects. Better results still are obtained with the split-valence 3-21G basis set. The calculation of force constants, infrared and Raman intensities, and isotope frequency shifts has confirmed the previous assignment of the experimental spectra. The analysis of the normal modes reveals the complex vibrational couplings occurring in these molecules and explains the unusual vibrational spectra of the nitroenamine system. The comparison between theoretical and experimental spectra also allowed an independent assignment of the nature of the isomers present in the molecules studied.

Spectral properties and isomerism of nitroenamines. Part 4.1 β-Amino-α-nitro-α,β-unsaturated ketones

“…In compounds 7 (s-trans,Z geometry), the coupling affects mainly ν a (NO 2 ) and to a lesser extent the enamine band; the ν(C᎐ ᎐ O) vibration, that shows a negligible effect, does not seem to be coupled to δ(N᎐H). The results of the ab initio 3-21G theoretical calculations 18 for compound 5 (R 1 = H) are in accordance with these observations and also indicate that, in the strongest coupled isomer A, the enamine mode can be described as the symmetric combination of the C᎐ ᎐ O, C᎐ ᎐ C and C(1)᎐N stretching motions, with contribution of the C(1)᎐H and the N᎐H bending modes. The frequency shift (∆ν Ϫ32 cm Ϫ1 in CDCl 3 ) of the enamine band of E-6 relative to the same band of E-5b can be attributed to the lack of the δ[C(1)᎐H] mode in the composition of the band in the former compound, though the electronic effects introduced by the methyl group may also contribute.…”

Configurational isomerization of push–pull thiazolidinone derivatives controlled by intermolecular and intramolecular RAHB: 1H NMR dynamic investigation of concentration and temperature effects

“…the COPh substituent, via the C --C bond, as found for other push-pull alkenes. [20][21][22][23][24][25][26][27] The key factor controlling the Z/E ratio is the strength of inter-and intramolecular hydrogen bonds which depends on, among other factors, the polarity of the medium 28,29 (Table 2). Polar solvents (EtOH, DMSO, acetone) enhance sulfur or nitrogen participation in the ground-state polarization, favoring the resonance forms A and B (Scheme 1).…”

Ab initio studies of push–pull systems

“…Push-pull substituents reduce the double bond character yielding dipolar species thus lowering the rotational barriers. As mentioned, substituted ethylenes have attracted both experimental [5][6][7][8][9] and theoretical interest [5,6,10]. Theoretical calculations on 3-aminoacrylonitrile, 2, showed that the trans isomer was about 2 kcal/mol higher in energy than the cis isomer [6].…”

“…Gate and coworkers reported the ratio of cis/trans 2-nitroethylenamine, 3, to be 68:32 by 1 H-NMR spectroscopy in (CD 3 ) 2 SO 3 [8], and the X-ray crystal structure of (cis)-3 has been reported by Evans and Gilardi [9]. Vibrational frequencies as well as IR and Raman intensities have been calculated for 2-nitroethylenamine, 3, using low-level ab initio methods [10]. The trans isomer of 3-aminoprop-2-en-l-ylium cation, 4, was found to be 3 kcal/mole more stable than the cis isomer and the rotational barrier was determined to be 11.2 kcal/mol based on Hartree-Fock calculations employing the 3-21G basis set [11].…”