Role of Conformational Isomerism in Solvent-Mediated Charge Transfer in Chiral (S) 1,2,3,4-Tetrahydro-3-isoquinoline Methanol (THIQM): Condensed-Phase to Jet-Cooled Spectroscopic Studies

Abstract

Intramolecular charge-transfer reaction in chiral (S) 1,2,3,4-tetrahydro-3-isoquinoline methanol (THIQM) has been investigated in the condensed phase and in jet-cooled conditions by means of laser-induced fluorescence, dispersed emission, resonance-enhanced two-photon ionization, and IR-UV double resonance experiments, as well as quantum chemical calculations. In the condensed phase, THIQM only shows local emission in nonpolar and protic solvents and dual emission in aprotic polar solvents, where the solvent-polarity dependent Stokes shifted emission is ascribed to a state involving charge transfer from the nitrogen lone pair to the benzene π-cloud. Ab initio calculations reveal two low-energy conformers, which are observed in jet-cooled conditions. In the most stable conformer, THIQM(I), the CH(2)OH substituent acts as a hydrogen bond donor to the nitrogen lone pair in the equatorial position, while the second most stable conformer, THIQM(II), corresponds to the opposite NH···O hydrogen bond, with the nitrogen lone pair in the axial position. The two low-energy jet-cooled conformers of THIQM evidenced from the laser-induced fluorescence and dispersed emission spectra only show structured local emission. Complexes with usual solvents reproduce the condensed phase properties. The jet-cooled complex with aprotic polar solvent acetonitrile shows both local emission and charge transfer emission as observed in solution. The jet-cooled hydrate mainly shows local emission due to the unavailability of the nitrogen lone pair through intermolecular hydrogen bonding.

Chirality effects in gas-phase spectroscopy and photophysics of molecular and ionic complexes: contribution of low and room temperature studies

“…No CT emission is observed in jet-cooled isolated THQIM because the CT state is higher in energy than the LE state when not stabilised by the solvent. Complexation with acetonitrile results in a red-shifted emission, like observed in solution, but no stereoselectivity has been observed [171]. The hydrates show more interesting properties.…”

“…Because they account for medium-range correlation, the hybrid M05-2X functional has been shown to satisfactorily describe NCIs in bio-molecular complexes, provided that they don't involve too many long-range interactions like dispersion [169,170]. They have led to good results for complexes involving mainly strong hydrogen bonds [171] but fail at describing long-range components of the interaction energy [169]. Dispersion-corrected functionals are frequently used as they combine acceptable calculation time with accurate description of dispersion [172][173][174].…”

“…The link between intramolecular charge transfer and molecular reorganisation has been well established in model systems where the donor and the acceptor are connected by an aromatic ring [266][267][268]. THIQIM has been shown to exhibit dual fluorescence in polar non-protic solvents, namely, resonant locally excited state (LE) and red-shifted charge-transfer (CT) emissions, despite the fact that the donor and the acceptor are not conjugated [171]. In related molecules, the process has been supposed to depend on the overlap between the donor nitrogen lone pair and the acceptor π aromatic cloud which is much better when the lone pair is in axial position [269].…”

Mass Spectrometry Study and Infrared Spectroscopy of the Complex Between Camphor and the Two Enantiomers of Protonated Alanine: The Role of Higher‐Energy Conformers in the Enantioselectivity of the Dissociation Rate Constants

“…Previous results have shown that the proton is located on the α‐amino nitrogen of alanine . However, it has been shown that gas‐phase complexes can contain subunits that are not the most stable form of the bare entity …”

Diastereo-specific conformational properties of neutral, protonated and radical cation forms of (1R,2S)-cis- and (1R,2R)-trans-amino-indanol by gas phase spectroscopy

“…[18][19][20] For example, the two diastereomers of 1,2,3,4-tetrahydro-3-isoquinoline-methanol involve an OHÁ Á ÁN or an NHÁ Á ÁO interaction depending on the relative chirality of the adjacent asymmetric carbon and nitrogen atoms. 20,21 Life chemistry involves species in different pH and redox states, which motivates the study of protonated or cationic species in parallel to that of the neutral. However, few comparative studies have been reported on stereochemical effects in different charge states to assess the sensitivity of stereochemical factors (enhancement or reduction) upon ionisation or protonation.…”