N‐Methyl Inversion and Accurate Equilibrium Structures in Alkaloids: Pseudopelletierine

Abstract

A rotational spectroscopy investigation has resolved the conformational equilibrium and structural properties of the alkaloid pseudopelletierine. Two different conformers, which originate from inversion of the N-methyl group from an axial to an equatorial position, have been unambiguously identified in the gas phase, and nine independent isotopologues have been recorded by Fourier-transform microwave spectroscopy in a jet expansion. Both conformers share a chair-chair configuration of the two bridged six-membered rings. The conformational equilibrium is displaced towards the axial form, with a relative population in the supersonic jet of N /N ≈2/1. An accurate equilibrium structure has been determined by using the semiexperimental mixed-estimation method and alternatively computed by quantum-chemical methods up to the coupled-cluster level of theory. A comparison with the N-methyl inversion equilibria in related tropanes is also presented.

Molecular Rotation Spectrum of Tetracyclic Quinolizidines: Observation of trans-Matrine and the Elusive cis-Matrine

“…The high resolution (10 -7 cm -1 ) of rotational spectra is critically sensitive to molecular structure, precisely resolving all populated species and offering the most accurate description of the intrinsic structural properties and plausible conformational equilibria in matrine. Our work follows previous rotational investigations of other quinolizidine alkaloids like sparteine 26 and lupinine, 27 and tropane 28,29 and norbornane 30 bicycles and related derivatives. 31…”

Axial–equatorial isomerism and semiexperimental equilibrium structures of fluorocyclohexane

“…As observed in fructose, deoxyribose, 12 and pseudopelletierine, 15 the MP2 method fails to deliver accurate dihedral angles, the error being sometimes as large as several degrees. This error may be easily explained by the fact it requires much less energy to modify a dihedral angle than a bond angle (it requires about 4.2 kJ mol À1 to distort a +(CCC) bond angle by 101 and only 0.8 kJ mol À1 to distort a t(CCCC) dihedral angle by 101).…”

“…49 For the molecules investigated up to now, the accuracy was found to be much less sensitive to the basis set than to the method, and the CCSD/cc-pVTZ level of theory was found to be a significant improvement over the MP2 method. 12,15 However, although the CCSD method can be easily used in the case of the cyclohexane derivatives investigated in this work, it is significantly more expensive than the MP2 method. For this reason, it would be useful to find a cheaper approximation.…”

“…This fact allows us to predict the torsional angles with a standard deviation of 0.371, which is acceptable for predicates. The resulting equation (complete list of angles in Table S15, ESI †) is given by t e À t[MP2_FC/cc-pVTZ] = À0.2916 (15)…”

“…11 It is easy to obtain reliable predicates for the CH bond lengths, the single CC bond lengths and related bond angles. However, as observed in fructose, 12 deoxyribose, 12 proline, 13 succinic anhydrid, 14 pseudopelletierine 15 and several other molecules the prediction of accurate dihedral angles is a problem, the error being sometimes as large as several degrees. Indeed, a provisional criticism of the ME method is the difficulty to obtain reliable estimates for a full set of internal coordinates and to simultaneously estimate their accuracy.…”