Flexibility of the Saturated Five-Membered Ring in 2,5-Pyrrolidinedione (Succinimide): Electron Diffraction and Quantum-Chemical Studies with Use of Vibrational Spectroscopy Data

Abstract

The flexibility of succinimide molecule has been studied for the first time by quantum-chemical (at the MP2 level with up to the 6-311G(3df,2p) basis sets) and gas-phase electron diffraction (GED) methods using vibrational spectroscopy data from literature. The analysis of vibrational spectra, performed for the molecular model of C(2v) symmetry (predicted by high-level ab initio calculations) using the scaling procedure, has shown that the two out-of-plane ring motions, that is, ring-bending and ring-twisting, are practically pure modes of different symmetry types and can be considered separately. The one-dimensional potential curves for the ring-bending and ring-twisting vibrations calculated at the MP2(full)/6-311G(3df,2p) level could be approximated by harmonic and anharmonic functions, respectively. The diverged energy levels for the ring-twisting vibration and the constant transition frequencies for the bending motion, obtained by the solution of the direct one-dimensional problem for the nonrigid model, demonstrate this statement. In the GED analysis, the succinimide molecule with a large-amplitude ring-twisting motion was described by a dynamic model with the distribution of pseudoconformers according to the calculated potential function taking into account structural relaxation effects from the MP2(full)/6-311G(3df,2p) calculations. This model greatly improved the fit of the GED intensities (R factor decreased from 4.6% for static model to 2.8%). The equilibrium molecular parameters r(e) determined in the dynamic approximation are very close to the corresponding values from the ab initio calculations. At the same time, the parameters of the -CH(2)-CH(2)- fragment involved in the ring-twisting motion deviate considerably from those obtained for the static model (C-C bond length by 0.027 A, =C-C-H, C-C-H, and H-C-C-N angles by up to 7 degrees). The flexibility influence on the C-C bond length is several times larger than the calculated vibrational correction (r(e) - r(a)) as well as the experimental uncertainty.

Molecular structure of N-chlorosuccinimide studied by gas-phase electron diffraction and quantum-chemical methods

“…This conclusion is in agreement with results of our previous studies of succinimide [7] and NMS [6]. The planarity of the succinimide ring can be explained by influence of two carbonyl groups with C sp2 atoms which need enlarging of the adjusted ring angles and thus lead to stabilization of planar ring conformation although the C-C-C-C torsion angle should deviate from zero due repelling of two CH 2 groups.…”

“…Therefore, the C(O)-N and C-C bond lengths refined in GED analysis with differences from high-level (MP2(fc)/cc-pVTZ) calculation seem to be reliable. The two equilibrium C-C bond lengths derived from the experimental data for NCS (r e (C sp2 -C sp3 ) = 1.515(2) Å , r e (C sp3 -C sp3 ) = 1.530(2) Å ) are very close to those for NMS (r e (C sp2 -C sp3 ) = 1.517 (3), r e (C sp3 -C sp3 ) = 1.527(3) [6]) as well as for succinimide (r e (C sp2 -C sp3 ) = 1.520(4), r e (C sp3 -C sp3 ) = 1.526(5) [7]). In contrast to the present and previous [6] studies, based on the static molecular model, the dynamic model was applied in very extensive study of succinimide [7].…”

“…The two equilibrium C-C bond lengths derived from the experimental data for NCS (r e (C sp2 -C sp3 ) = 1.515(2) Å , r e (C sp3 -C sp3 ) = 1.530(2) Å ) are very close to those for NMS (r e (C sp2 -C sp3 ) = 1.517 (3), r e (C sp3 -C sp3 ) = 1.527(3) [6]) as well as for succinimide (r e (C sp2 -C sp3 ) = 1.520(4), r e (C sp3 -C sp3 ) = 1.526(5) [7]). In contrast to the present and previous [6] studies, based on the static molecular model, the dynamic model was applied in very extensive study of succinimide [7]. Very good agreement between the C-C bond lengths for succinimide and those for NMS and NCS means that the static molecular model is an appropriate approximation for the N-derivatives of succinimide with relatively low frequency of ringtwisting motion (for NCS m = 87 cm -1 ).…”

“…The C(O)-N endocyclic bond in NCS is also longer than those in its recently studied analogues NMS (r g = 1.403(3) Å [6]) and succinimide (r g = 1.389(2) Å [7]). In this case, the elongation of the C(O)-N bond in NCS seems to be a result of repelling of chlor and oxygen from each other.…”

“…In the solid states, the ring conformations of the succinimide, N-methylsuccinimide (NMS), N-chlorosuccinimide (NCS), and Nbromosuccinimide (NBS) have been found to be nonplanar with the C-C-C-C angle of 2-11° [1][2][3][4][5]. However, our recent gas-phase electron diffraction (GED) and quantumchemical studies of NMS [6] and succinimide [7] have shown that both molecules have a planar ring as well as a planar configuration of the bonds around the N atom, i.e., a planar -C(=O)-M\ fragment like that in amides (see formamide [8]) among them peptides and proteins. On the other side, several molecules with -C(=O)-MH-linkage, such as urea, carbamic acid, methyl carbamate, etc., seem to have a pyramidalized nitrogen at equilibrium [9].…”