Heterogeneous Oxysulfide@Fluoride Core/Shell Nanocrystals for Upconversion-Based Nanothermometry.

Lanthanide (Ln3+)-doped upconversion nanoparticles (UCNPs) often suffer from weak luminescence, especially when their sizes are ultrasmall (less than 10 nm). Enhancing the upconversion luminescence (UCL) efficiency of ultrasmall UCNPs has remained a challenge that must be undertaken if any practical applications are to be envisaged. Herein, we present a Ln3+-doped oxysulfide@fluoride core/shell heterostructure which shows efficient UCL properties under 980 nm excitation and good stability in solution. Through epitaxial heterogeneous growth, a ∼4 nm optically inert ß-NaYF4 shell was coated onto ∼5 nm ultrasmall Gd2O2S:20%Yb,1%Tm. These Gd2O2S:20%Yb,1%Tm@NaYF4 core/shell UCNPs exhibit a more than 800-fold increase in UCL intensity compared to the unprotected core, a 180-fold increase in luminescence decay time of the 3H4 → 3H6 Tm3+ transition from 5 to 900 µs, and an upconversion quantum yield (UCQY) of 0.76% at an excitation power density of 155 W/cm2. Likewise, Gd2O2S:20%Yb,2%Er@NaYF4 core/shell UCNPs show a nearly 5000-fold increase of their UCL intensity compared to the Gd2O2S:20%Yb,2%Er core and a maximum UCQY of 0.61%. In the Yb/Er core-shell UCNP system, the observed variation of luminescence intensity ratio seems to originate from a change in lattice strain as the temperature is elevated. For nanothermometry applications, the thermal sensitivities based on thermally coupled levels are estimated for both Yb/Tm and Yb/Er doped Gd2O2S@NaYF4 core/shell UCNPs.

Cation-π Interactions between a Noble Metal and a Polyfunctional Aromatic Ligand: Ag+ (benzylamine).

The structure of an isolated Ag+ (benzylamine) complex is investigated by infrared multiple photon dissociation (IRMPD) spectroscopy complemented with quantum chemical calculations of candidate geometries and their vibrational spectra, aiming to ascertain the role of competing cation-N and cation-π interactions potentially offered by the polyfunctional ligand. The IRMPD spectrum has been recorded in the 800-1800 cm-1 fingerprint range using the IR free electron laser beamline coupled with an FT-ICR mass spectrometer at the Centre Laser Infrarouge d'Orsay (CLIO). The resulting IRMPD pattern points toward a chelate coordination (N-Ag+ -π) involving both the amino nitrogen atom and the aromatic π-system of the phenyl ring. The gas-phase reactivity of Ag+ (benzylamine) with a neutral molecular ligand (L) possessing either an amino/aza functionality or an aryl group confirms N- and π-binding affinity and suggests an augmented silver coordination in the product adduct ion Ag + ( benzylamine ) ( L ) .

Photoactive Organic/Inorganic Hybrid Materials with Nanosegregated Donor-Acceptor Arrays.

The synthesis of the first mesogenic donor-acceptor polyoxometalate (POM)-based hybrid is herein described. The structural and electronic properties of the hybrid compound were evaluated through combination of small- and wide-angle X-ray scattering, optical microscopy, electrochemistry and photoluminescence. In the solid state, the compound behaves as a birefringent solid, displaying a lamellar organization in which double-layers of POMs and bis(thiophene)thienothiophene organic donors alternate regularly. Noticeably, the sub-unit organizations in the composite are similar to that observed for the individual POM and organic donor precursors. Photophysical studies show that in the hybrid, the fluorescence of the organic donor unit is considerably quenched both in solution and in the solid state, which is attributed to occurrence of intramolecular charge-separated state.

Importance of the Mixing and High-Temperature Heating Steps in the Controlled Thermal Coprecipitation Synthesis of Sub-5-nm Na(Gd-Yb)F4:Tm.

In order to achieve a significant size reduction to get ultrasmall upconverting nanoparticles (UCNPs) following a thermal coprecipitation pathway, we identified two critical points: the UCNP precursor mixing and high-temperature heating steps. Significant differences could be observed according to the way the inorganic sodium and fluoride sources were mixed to the rare-earth oleate before the high-temperature heating step. More interestingly, accurate monitoring of the high-temperature heating step using microwave (MW) dielectric heating yielded major improvement toward ultrasmall UCNPs. Thus, hexagonal, Tm-doped sub-5-nm UCNPs with an unusual Na(Yb-Gd)F4 matrix with 53% Yb were produced, displaying satisfactory luminescence. Noticeably, MW heating was achieved in a weakly MW-absorbing oleic acid (OA)/octadecene mixture, and the influence of the OA content composition on the MW heating efficiency is discussed in this report.

Probing chirality recognition of protonated glutamic acid dimers by gas-phase vibrational spectroscopy and first-principles simulations.

The homochirality of the amino acid metabolism still puzzles biochemists. Vibrational spectroscopy of mass-selected gas-phase amino acids and their clusters can precisely reveal their conformation and might ultimately help to decode the interactions responsible for chirality recognition. Infrared photodissociation (IRPD) and conformer-selective IR-IR hole burning spectra of protonated glutamic acid dimers (LL-/LD-Glu2H+) recorded in the fingerprint and XH stretch ranges (1100-1900 and 2600-3600 cm-1) provide direct insight into their stereospecific interactions. Glu2H+ dimers are generated by electrospray ionization and stored in a cryogenic quadrupole ion trap held at 10 K. The assignment of the IRPD spectra is supported by vibrational analysis using many-body dispersion-corrected hybrid density-functional theory. Sampling of the conformational space is accomplished by basin hopping and replica-exchange molecular dynamics simulations. The most stable LD-Glu2H+ dimer (LD1) is predicted to be more stable than the most stable LL-Glu2H+ dimer (LL1) by ΔE0 = 4.0 kJ mol-1, which relies on stronger secondary interactions in LD1 as demonstrated by the noncovalent interaction method. IR-IR hole burning spectroscopy reveals the coexistence of at least four LD-Glu2H+ and three LL-Glu2H+ conformers. Their IR-dip spectra are assigned to the most stable conformers at room and cryogenic temperature, revealing incomplete thermalization of the ions by kinetic trapping in the cold trap. We observe different population ratios of LL and LD conformers of Glu2H+, as revealed by specific νNH2 and νCO intensities (fingerprints of chirality recognition).

Effect of puckering motion and hydrogen bond formation on the vibrational circular dichroism spectrum of a flexible molecule: the case of (S)-1-indanol.

The influence of flexibility and hydrogen bond formation on the IR absorption and vibrational circular dichroism (VCD) spectrum of a floppy protic molecule, namely, (S)-1-indanol, is studied in both non-polar CCl4 and polar DMSO solvents. The experimental IR absorption and VCD spectra obtained by Fourier transform spectroscopy are interpreted using both static density functional theory (DFT) calculations and first principles molecular dynamics (FPMD) within DFT, using the nuclear velocity perturbation theory (NVPT). Simulation of the spectra based on static optimised geometries is not sufficient in CCl4 and going beyond static calculations is mandatory for satisfactorily reproducing the VCD spectra. The FPMD results obtained in DMSO indicate that (S)-1-indanol is hydrogen-bonded to one DMSO molecule. As a result, static "cluster-in-the-bulk" DFT calculations in which the solute-solvent interaction is modeled as the most stable (S)-1-indanol:DMSO complexes in a DMSO continuum yield satisfactory agreement with the experiment. Correspondence between experimental and simulated spectra is slightly improved when the VCD spectrum is calculated as the summed contributions of snapshots extracted from FPMD trajectories, due to better sampling of the potential-energy surface. Finally, NVPT calculations further improve the description of experimental spectra by taking into account higher-energy structures, which are not necessary local minima.

Stereochemistry-dependent structure of hydrogen-bonded protonated dimers: the case of 1-amino-2-indanol.

To understand the role of chirality in shaping biological supramolecular systems it is instructive to visualize the subtle effects of stereochemistry on the structure of model aggregates at the molecular level. Here, we apply conformer-specific IR-UV double-resonance laser spectroscopy in a cold ion trap to derive a detailed description of the protonated homodimers of (1R,2S)-cis- and (1R,2R)-trans-1-amino-2-indanol (c-AI2H+, t-AI2H+). Although the protonated monomers (c-AIH+, t-AIH+) only differ by the chirality of one carbon atom, their conformations are clearly distinct. c-AIH+ has an intramolecular NH+O hydrogen bond (H-bond), while t-AIH+ lacks such an interaction. This has crucial consequences on the geometry and stability of the corresponding c-AI2H+ and t-AI2H+ dimers. While there is a competition between intra- and intermolecular H-bonds in c-AI2H+, the formation of t-AI2H+ does not require deformation of the monomers. This difference results in higher binding energies of t-AI2H+ compared to c-AI2H+. To optimize the H-bond network, the two dimers do not necessarily involve the corresponding most stable monomers. c-AI2H+ and t-AI2H+ differ in their UV photodissociation mass spectra and in their electronic spectra, which suggests different geometries also in the excited state.

Cation-Size-Dependent Conformational Locking of Glutamic Acid by Alkali Ions: Infrared Photodissociation Spectroscopy of Cryogenic Ions.

Consolidated knowledge of conformation and stability of amino acids and their clusters is required to understand their biochemical recognition. Often, alkali ions interact with amino acids and proteins. Herein, infrared photodissociation (IRPD) spectra of cryogenic metalated glutamic acid ions (GluM+, M = Li-Cs) are systematically analyzed in the isomer-specific fingerprint and XH stretch ranges (1100-1900, 2600-3600 cm-1) to provide a direct measure for cation-size-dependent conformational locking. GluM+ ions are generated by electrospray ionization and cooled down to 15 K in a cryogenic quadrupole ion trap. The assignment of the IRPD spectra is supported by density functional theory calculations at the dispersion-corrected B3LYP-D3/aug-cc-pVTZ level. In the global minimum of GluM+, the flexibility of Glu is strongly reduced by the formation of rigid ionic CO···M+···OC metal bridges, corresponding to charge solvation. The M+ binding energy decreases monotonically with increasing cation size from D0 = 314 to 119 kJ/mol for Li-Cs. Whereas for Li and Na only the global minimum of GluM+ is observed, for K-Cs at least three isomers exist at cryogenic temperature. The IRPD spectra of cold GluM+ ions are compared to IR multiple-photon dissociation spectra measured at room temperature. Furthermore, we elucidate the differences of the impact of protonation and metalation on the structure and conformational locking of Glu.

Microsolvation of the pyrrole cation (Py+) with nonpolar and polar ligands: infrared spectra of Py+-Ln with L = Ar, N2, and H2O (n ≤ 3).

The solvation of aromatic (bio-)molecular building blocks has a strong impact on the intermolecular interactions and function of supramolecular assemblies, proteins, and DNA. Herein we characterize the initial microsolvation process of the heterocyclic aromatic pyrrole cation (Py+) in its 2A2 ground electronic state with nonpolar, quadrupolar, and dipolar ligands (L = Ar, N2, and H2O) by infrared photodissociation (IRPD) spectroscopy of cold mass-selected Py+-Ln (n ≤ 3) clusters in a molecular beam and dispersion-corrected density functional theory calculations at the B3LYP-D3/aug-cc-pVTZ level. Size- and isomer-specific shifts in the NH stretch frequency (ΔνNH) unravel the competition between various ligand binding sites, the strength of the respective intermolecular bonds, and the cluster growth. In Py+-Ar, linear H-bonding of Ar to the acidic NH group (NHAr) is competitive with π-stacking to the aromatic ring, and both Py+-Ar(H) and Py+-Ar(π) are observed. For L = N2 and H2O, the linear NHL H-bond is much more stable than any other binding site and the only observed binding motif. For the Py+-Ar2 and Py+-(N2)2 trimers, the H/π isomer with one H-bonded and one π-bonded ligand strongly competes with a 2H isomer with two bifurcated nonlinear NHL bonds. The latter are equivalent for Ar but nonequivalent for N2. Py+-H2O exhibits a strong and linear NHO H-bond with charge-dipole configuration and C2v symmetry. IRPD spectra of cold Py+-H2O-L clusters with L = Ar and N2 reveal that Ar prefers π-stacking to the Py+ ring, while N2 forms an OHN2 H-bond to the H2O ligand. The ΔνNH frequency shifts in Py+-Ln are correlated with the strength of the NHL H-bond and the proton affinity (PA) of L, and a monotonic correlation between ΔνNH of the Py+-L(H) dimers and PA is established. Comparison with neutral Py-L dimers reveals the strong impact of the positive charge on the acidity of the NH group, the strength of the NHL H-bond, and the preferred ligand binding motif.

Conformation of protonated glutamic acid at room and cryogenic temperatures.

Recognition properties of biologically relevant molecules depend on their conformation. Herein, the conformation of protonated glutamic acid (H+Glu) isolated in quadruple ion traps is characterized by vibrational spectroscopy at room and cryogenic temperatures and dispersion-corrected density functional theory calculations at the B3LYP-D3/aug-cc-pVTZ level. The infrared multiple photon dissociation (IRMPD) spectrum recorded in the fingerprint range at room temperature using an IR free electron laser is attributed to the two most stable and nearly isoenergetic conformations (1-cc and 2-cc) with roughly equal population (ΔG298 = 0.0 kJ mol-1). Both have bridging C[double bond, length as m-dash]O(HNH)+O[double bond, length as m-dash]C ionic H-bonds of rather different strengths but cannot be distinguished by their similar IRMPD spectra. In contrast, the higher-resolution single-photon IRPD spectrum of H2-tagged H+Glu recorded in the conformation-sensitive X-H stretch range in a trap held at 10 K distinguishes both conformers. At low temperature, 1-cc is roughly twice more abundant than 2-cc, in line with its slightly lower calculated energy (ΔE0 = 0.5 kJ mol-1). This example illustrates the importance of cryogenic cooling, single-photon absorption conditions, and the consideration of the X-H stretch range for the identification of biomolecular conformations involving hydrogen bonds.

Infrared spectrum of the cold ortho-fluorinated protonated neurotransmitter 2-phenylethylamine: competition between NH+π and NH+F interactions.

Halogenation of pharmaceutical molecules is a common tool to modify their physiological properties. The geometric, vibrational, and electronic properties of the ortho-fluorinated protonated neurotransmitter 2-phenylethylamine (oF-H+PEA) are characterized by infrared photodissociation (IRPD) spectroscopy in the NH stretch range using the messenger technique and dispersion-corrected density functional theory calculations at the B3LYP-D3/aug-cc-pVTZ level to elucidate the drastic effect of site-specific ortho-fluorination. The IRPD spectra of cold oF-H+PEA-Rg dimers (Rg = Ne, Ar) are assigned to the most stable gauche conformer (Gf1) of oF-H+PEA, which benefits from both NH+π and NH+F interactions. A minor contribution (∼5%) of the slightly less stable Gf2 gauche conformer (E0 = +1.1 kJ mol-1) is also identified. Comparison of oF-H+PEA with unsubstituted H+PEA reveals a much stronger NH+π interaction in H+PEA resulting in a large red shift of the bonded NH stretch frequency. This behavior is confirmed by natural bond orbital (NBO) analysis and noncovalent interaction (NCI) calculations. The Rg ligand prefers a binding site at which it can maximize the interaction with the aromatic π electron system and the ammonium group. Although the intermolecular interactions with the Rg atoms can compete with the noncovalent intramolecular bonds, they induce only minor spectral shifts in the NH stretch range.

Effects of Aromatic Fluorine Substitution on Protonated Neurotransmitters: The Case of 2-Phenylethylamine.

Fluorination of pharmaceutical compounds is a common tool to modulate their physiochemical properties. We determine the effects of site-specific aromatic fluorine substitution on the geometric, energetic, vibrational, and electronic properties of the protonated neurotransmitter 2-phenylethylamine (xF-H(+) PEA, x=ortho, meta, para) by infrared multiphoton photodissociation (IRMPD) in the fingerprint range (600-1750 cm(-1) ) and quantum chemical calculations at the B3LYP-D3/aug-cc-pVTZ level. The IRMPD spectra of all ions are assigned to their folded gauche conformers stabilized by intramolecular NH(+) ⋅⋅⋅π hydrogen bonds (H-bonds) between the protonated amino group and the aromatic ring. H→F substitution reduces the symmetry and allows for additional NH(+) ⋅⋅⋅F interactions in oF-H(+) PEA, leading to three distinct gauche conformers. In comparison to oF-H(+) PEA, the fluorination effects on the energy landscape (energy ordering and isomerization barriers) in pF-H(+) PEA and mF-H(+) PEA with one and two gauche conformers are less pronounced. The strengths of the intramolecular NH(+) ⋅⋅⋅F and NH(+) ⋅⋅⋅π bonds are analyzed by the noncovalent interaction (NCI) method.

Competing Insertion and External Binding Motifs in Hydrated Neurotransmitters: Infrared Spectra of Protonated Phenylethylamine Monohydrate.

Hydration has a drastic impact on the structure and function of flexible biomolecules, such as aromatic ethylamino neurotransmitters. The structure of monohydrated protonated phenylethylamine (H(+) PEA-H2 O) is investigated by infrared photodissociation (IRPD) spectroscopy of cold cluster ions by using rare-gas (Rg=Ne and Ar) tagging and dispersion-corrected density functional theory calculations at the B3LYP-D3/aug-cc-pVTZ level. Monohydration of this prototypical neurotransmitter gives an insight into the first step of the formation of its solvation shell, especially regarding the competition between intra- and intermolecular interactions. The spectra of Rg-tagged H(+) PEA-H2 O reveal the presence of a stable insertion structure in which the water molecule is located between the positively charged ammonium group and the phenyl ring of H(+) PEA, acting both as a hydrogen bond acceptor (NH(+) ⋅⋅⋅O) and donor (OH⋅⋅⋅π). Two other nearly equivalent isomers, in which water is externally H bonded to one of the free NH groups, are also identified. The balance between insertion and external hydration strongly depends on temperature.

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.

Chirality effects on the intramolecular interactions strongly depend on the charge and protonation states. Here, the influence of chirality on the structure of the neutral, protonated, and radical cation forms of (1R,2S)-cis- and (1R,2R)-trans-1-amino-2-indanol diastereomers, prototypical molecules with two chiral centers, is investigated in a molecular beam by laser spectroscopy coupled with quantum chemical calculations. The neutral systems are structurally characterised by double resonance IR-UV spectroscopy, while IR-induced dissociation spectroscopy is employed for the charged molecules. The sterical constraints due to the cyclic nature of the molecule emphasise the chirality effects, which manifest themselves by the formation of an intramolecular hydrogen bond in neutral or protonated (1R,2S)-cis-amino-indanol. In contrast, this interaction is not possible in (1R,2R)-trans-amino-indanol. In the protonated species, chirality also influences the spectroscopic probes in the NH/OH stretch range by fine-tuning subtle effects such as the hyperconjugation between the σ(OH) orbital and σ* orbitals localised on the alicyclic ring. The radical cation undergoes opening of the alicyclic ring, which results in an ionisation-induced loss of the chirality effects.

IR spectrum of the protonated neurotransmitter 2-phenylethylamine: dispersion and anharmonicity of the NH3(+)-π interaction.

The structure and dynamics of the highly flexible side chain of (protonated) phenylethylamino neurotransmitters are essential for their function. The geometric, vibrational, and energetic properties of the protonated neutrotransmitter 2-phenylethylamine (H(+)PEA) are characterized in the N-H stretch range by infrared photodissociation (IRPD) spectroscopy of cold ions using rare gas tagging (Rg = Ne and Ar) and anharmonic calculations at the B3LYP-D3/(aug-)cc-pVTZ level including dispersion corrections. A single folded gauche conformer (G) protonated at the basic amino group and stabilized by an intramolecular NH(+)-π interaction is observed. The dispersion-corrected density functional theory calculations reveal the important effects of dispersion on the cation-π interaction and the large vibrational anharmonicity of the NH3(+) group involved in the NH(+)-π hydrogen bond. They allow for assigning overtone and combination bands and explain anomalous intensities observed in previous IR multiple-photon dissociation spectra. Comparison with neutral PEA reveals the large effects of protonation on the geometric and electronic structure.

Cation-π interactions in protonated phenylalkylamines.

Phenylalkylamines of the general formula C6H5(CH2)nNH2 (n = 1-4) have been delivered to the gas phase as protonated species using electrospray ionization. The ions thus formed have been assayed by IRMPD spectroscopy in two different spectroscopic domains, namely, the 600-1800 and the 3000-3500 cm(-1) regions using either an IR free electron laser or a tabletop OPO/OPA laser source. The interpretation of the experimental spectra is aided by density functional theory calculations of candidate species and vibrational frequency analyses. Protonated benzylamine presents a relatively straightforward instance of a single stable conformer, providing a trial case for the adopted approach. Turning to the higher homologues, C6H5(CH2)nNH3(+) (n = 2-4), more conformations become accessible. For each C6H5(CH2)nNH3(+) ion (n = 2-4), the most stable geometry is characterized by cation-π interactions between the positively charged ammonium group and the aromatic π-electronic system, permitted by the folding of the polymethylene chain. The IRMPD spectra of the sampled ions confirm the presence of the folded structures by comparison with the calculated IR spectra of the various possible conformers. An inspection of the NH stretching region is helpful in this regard.

Weak hydrogen bonding motifs of ethylamino neurotransmitter radical cations in a hydrophobic environment: infrared spectra of tryptamine(+)-(N2)n clusters (n ≤ 6).

Size-selected clusters of the tryptamine cation with N2 ligands, TRA(+)-(N2)n with n = 1-6, are investigated by infrared photodissociation (IRPD) spectroscopy in the hydride stretch range and quantum chemical calculations at the ωB97X-D/cc-pVTZ level to characterize the microsolvation of this prototypical aromatic ethylamino neurotransmitter radical cation in a nonpolar solvent. Two types of structural isomers exhibiting different interaction motifs are identified for the TRA(+)-N2 dimer, namely the TRA(+)-N2(H) global minimum, in which N2 forms a linear hydrogen bond (H-bond) to the indolic NH group, and the less stable TRA(+)-N2(π) local minima, in which N2 binds to the aromatic π electron system of the indolic pyrrole ring. The IRPD spectrum of TRA(+)-(N2)2 is consistent with contributions from two structural H-bound isomers with similar calculated stabilization energies. The first isomer, denoted as TRA(+)-(N2)2(2H), exhibits an asymmetric bifurcated planar H-bonding motif, in which both N2 ligands are attached to the indolic NH group in the aromatic plane via H-bonding and charge-quadrupole interactions. The second isomer, denoted as TRA(+)-(N2)2(H/π), has a single and nearly linear H-bond of the first N2 ligand to the indolic NH group, whereas the second ligand is π-bonded to the pyrrole ring. The natural bond orbital analysis of TRA(+)-(N2)2 reveals that the total stability of these types of clusters is not only controlled by the local H-bond strengths between the indolic NH group and the N2 ligands but also by a subtle balance between various contributing intermolecular interactions, including local H-bonds, charge-quadrupole and induction interactions, dispersion, and exchange repulsion. The systematic spectral shifts as a function of cluster size suggest that the larger TRA(+)-(N2)n clusters with n = 3-6 are composed of the strongly bound TRA(+)-(N2)2(2H) core ion to which further N2 ligands are weakly attached to either the π electron system or the indolic NH proton by stacking and charge-quadrupole forces.

Structural Characterization of the UV-Induced Fragmentation Products in an Ion Trap by Infrared Multiple Photon Dissociation Spectroscopy.

Protonated cinchona alkaloids and their dimers undergo photochemical reaction in the gas phase, leading to UV-specific photofragments, not observed by collision-induced dissociation. Simultaneous coupling of UV and IR lasers with a Paul ion trap has been achieved for obtaining the vibrational spectrum of the fragments arising from the photodissociation. The structure of the photoproduced radical has been fully characterized by comparing the experimental spectrum to that simulated by DFT calculations.

Chirality-dependent balance between hydrogen bonding and London dispersion in isolated (±)-1-indanol clusters.

The aggregation behavior of racemic and enantiopure 1-indanol has been studied by FTIR spectroscopy, resonant ion dip IR spectroscopy, and spontaneous Raman scattering in supersonic jets. This triple experimental approach, augmented by homology to related molecular fragments and dispersion-corrected DFT predictions, allows disentangling the complex spectroscopic signature in the OH stretch range. Evidence for chirality-sensitive aggregation via isolated OH···π bonds in competition with cooperative ···OH···OH···π patterns is collected. An accurate description of London dispersion forces provides the key to its explanation.

Conformational analysis of quinine and its pseudo enantiomer quinidine: a combined jet-cooled spectroscopy and vibrational circular dichroism study.

Laser-desorbed quinine and quinidine have been studied in the gas phase by combining supersonic expansion with laser spectroscopy, namely, laser-induced fluorescence (LIF), resonance-enhanced multiphoton ionization (REMPI), and IR-UV double resonance experiments. Density funtional theory (DFT) calculations have been done in conjunction with the experimental work. The first electronic transition of quinine and quinidine is of π-π* nature, and the studied molecules weakly fluoresce in the gas phase, in contrast to what was observed in solution (Qin, W. W.; et al. J. Phys. Chem. C2009, 113, 11790). The two pseudo enantiomers quinine and quinidine show limited differences in the gas phase; their main conformation is of open type as it is in solution. However, vibrational circular dichroism (VCD) experiments in solution show that additional conformers exist in condensed phase for quinidine, which are not observed for quinine. This difference in behavior between the two pseudo enantiomers is discussed.