Ab initioconformational analysis of 1,2,3,4-tetrahydroquinoline and the high-resolution rotational spectrum of its lowest energy conformer

Abstract

The saturated part of the 1,2,3,4-tetrahydroquinoline (THQ) molecule allows for the possibility of multiple conformers' existence. High-resolution microwave spectroscopy, supported by high-level quantum chemistry calculations, was used to determine the precise molecular structures of the conformers of THQ. Via the MP2 calculations, we were able to discriminate four stable conformations, i.e. two pairs of energetically equivalent enantiomorphic conformers. The results of the calculations also indicate that energetically non-equivalent conformers are separated by a low energy barrier (104 cm-1) that allows for conformational cooling to occur. The high resolution rotational spectrum with resolved hyperfine structure in the frequency range of 7-20 GHz was obtained using both the In-phase/quadrature-phase-Modulation Passage-Acquired-Coherence Technique (IMPACT) and the coaxially oriented beam resonator arrangement (COBRA) to perform Fourier transform microwave (FTMW) spectroscopy. The precise values of the rotational constants, 14N nuclear hyperfine coupling parameters and centrifugal distortion parameters were determined from the measured transition frequencies. Based on our experimental results, only the most stable enantiomeric pair of THQ contributes to the rotational spectrum under the conditions of our experiment as the less stable conformers seem to efficiently relax to the lower energy conformers. Thus the experimentally evaluated molecular constants unambiguously define the lowest energy conformer of 1,2,3,4-tetrahydroquinoline.

Ealamines A–H, a Series of Naphthylisoquinolines with the Rare 7,8′-Coupling Site, from the Congolese Liana Ancistrocladus ealaensis, Targeting Pancreatic Cancer Cells

“…The most populated conformer of 4b , by contrast, was not the corresponding half-chair form 4b-I (see Figure , bottom, left), but the tetrahydroisoquinoline portion of the new compound 4b was found to preferably adopt the conformation 4b-II (see Figure , bottom, center), with both the nitrogen and C-3 “down” and with Me-1 now occupying a pseudoequatorial position and, in turn, with H-1 becoming pseudoaxial. Such a rarely observed array has been referred to as a half-boat conformation in the literature . With the Me-1 group in 4b-II in an equatorial position, the spatial distance to H-1′ was largely increased (up to ca.…”

“…Such a rarely observed array has been referred to as a half-boat conformation in the literature. 39 With the Me-1 group in 4b-II in an equatorial position, the spatial distance to H-1′ was largely increased (up to ca. 4.7 Å), now, instead, permitting detection of an unexpected long-range NOE interaction between Me-1 and H-7′, which would not have been imaginable in the case of 4b-I.…”

Tautomers of homophthalic anhydride in the ground and excited electronic states: analysis through energy, hardness and vibrational signatures

“…Initially, we used density functional theory (DFT) [37]. It is a wellknown fact that its increase in popularity is more on its fast computational time, inclusion of electron correlations and above all reproduction of a large number of experimental observations [38][39][40]. We will start with the three-parameter hybrid exchange functional, B3LYP, which also incorporates correlation through Lee-Yang-Parr correlational functional [41,42].…”