Vibrational features of oxyamines: a comparative study of N,N-diethylhydroxylyamine and N,N-diethylacetyloxyamine.

The ability of oxyamines to undergo homolytic cleavage of the O-C bond, leading to the formation of stable radicals, is widely used in polymerization processes and to prevent oxidative stress in materials. We present a mid and near-infrared spectroscopy study on two model compounds, the commercial N,N-diethylhydroxyloxyamine (C4H11NO) and the non-commercial N,N-diethylacetyloxyamine (C6H13NO2) in the liquid phase. The analysis of the spectra is based on a complete exploration of the conformational space, coupled to harmonic and anharmonic calculations performed using the generalized second-order vibrational perturbation theory (GVPT2) formalism at the B3LYP-D3(BJ)/Def2-TZVP level of calculation and potential energy distribution analysis. In the most stable species out of 25, the three amine chains present an all-anti arrangement, with the carbonyl oxygen atom pointing towards the nitrogen lone pair. The simulated spectra are in overall good agreement with the experimental ones, and suitable for the assignment of the main observed bands. Furthermore, similarities and divergences between the two molecules are discussed.

A Rotational Study of 2-tert-Butylphenol and Its 1 : 1 Argon Complex.

The chirped-pulse Fourier Transform microwave spectrum of 2-tert-butylphenol, an industrial intermediate for the production of antioxidants, has been investigated in the 2-8 GHz frequency range. The spectral analysis has allowed obtaining precise structural information on the most stable conformer and its complex with argon. The conformation of the monomer reveals that the hydroxyl group is coplanar with the ring but points in the opposite direction to the tert-butyl group, reducing steric interactions. In the tert-butyl group one methyl group is coplanar and the other two are symmetrically staggered respect to the ring. The complex shows the rare gas sitting above the aromatic ring. Interestingly, neither the monomer nor the complex exhibit large-amplitude hydroxyl torsion motions, previously observed in 2,6-disubstituted phenols such as 2,6-di-tert-butylphenol or propofol. The experimental results are supported by computational calculations, validating the molecular structure. Additionally, symmetry-adapted perturbation theory has allowed determining the van der Waals intermolecular interaction energy of the complex.

The Structure of 2,6-Di-tert-butylphenol-Argon by Rotational Spectroscopy.

The molecular structure of a van der Waals-bonded complex involving 2,6-di-tert-butylphenol and a single argon atom has been determined through rotational spectroscopy. The experimentally derived structural parameters were compared to the outcomes of quantum chemical calculations that can accurately account for dispersive interactions in the cluster. The findings revealed a π-bound configuration for the complex, with the argon atom engaging the aromatic ring. The microwave spectrum reveals both fine and hyperfine tunneling components. The main spectral doubling is evident as two distinct clusters of lines, with an approximate separation of 179 MHz, attributed to the torsional motion associated with the hydroxyl group. Additionally, each component of this doublet further splits into three components, each with separations measuring less than 1 MHz. Investigation into intramolecular dynamics using a one-dimensional flexible model suggests that the main tunneling phenomenon originates from equivalent positions of the hydroxyl group. A double-minimum potential function with a barrier of 1000 (100) cm-1 effectively describes this extensive amplitude motion. However, the three-fold fine structure, potentially linked to internal motions within the tert-butyl group, requires additional scrutiny for a comprehensive understanding.

Rotational Spectroscopy Probes Lone Pair···π-Hole Interactions in Hexafluorobenzene-Tertiary Alkylamines Complexes.

We employed microwave spectroscopy to investigate the 1:1 complexes of hexafluorobenzene with trimethylamine and quinuclidine, respectively. These complexes exhibit a C3v symmetry and are stabilized by nitrogen lone pair···π-hole interactions along the C3 axes. The N···π-center distances were determined to be 3.110(1) and 3.040(2) Å, respectively, which are shorter than that of hexafluorobenzene-ammonia at 3.2685(3) Å. Additionally, the strength of the intermolecular interaction increases with cluster size. While it was initially expected that the electron-donating effect of alkyl groups was responsible for changing the N···π interaction, the symmetry-adapted perturbation theory analysis revealed that, from hexafluorobenzene-ammonia to both hexafluorobenzene-alkylamines, electrostatic interaction actually decreases while dispersion interaction increases and becomes dominant. Interestingly, dispersion interaction decreases while electrostatic interaction increases from C6F6-N(CH3)3 to C6F6-NC7H13. The splitting pattern of the spectra indicates hexafluorobenzene rotates freely relative to its partners along the axis of the N···π-hole interactions.

A Competition between Relative Stability and Binding Energy in Caffeine Phenyl-Glucose Aggregates: Implications in Biological Mechanisms.

Hydrogen bonds and stacking interactions are pivotal in biological mechanisms, although their proper characterisation within a molecular complex remains a difficult task. We used quantum mechanical calculations to characterise the complex between caffeine and phenyl-ß-D-glucopyranoside, in which several functional groups of the sugar derivative compete with each other to attract caffeine. Calculations at different levels of theory (M06-2X/6-311++G(d,p) and B3LYP-ED=GD3BJ/def2TZVP) agree to predict several structures similar in stability (relative energy) but with different affinity (binding energy). These computational results were experimentally verified by laser infrared spectroscopy, through which the caffeine·phenyl-ß-D-glucopyranoside complex was identified in an isolated environment, produced under supersonic expansion conditions. The experimental observations correlate with the computational results. Caffeine shows intermolecular interaction preferences that combine both hydrogen bonding and stacking interactions. This dual behaviour had already been observed with phenol, and now with phenyl-ß-D-glucopyranoside, it is confirmed and maximised. In fact, the size of the complex's counterparts affects the maximisation of the intermolecular bond strength because of the conformational adaptability given by the stacking interaction. Comparison with the binding of caffeine within the orthosteric site of the A2A adenosine receptor shows that the more strongly bound caffeine·phenyl-ß-D-glucopyranoside conformer mimics the interactions occurring within the receptor.

Analysis of isomeric mixtures by molecular rotational resonance spectroscopy.

Recent developments in molecular rotational resonance (MRR) spectroscopy that have enabled its use as an analytical technique for the precise determination of molecular structure are reviewed. In particular, its use in the differentiation of isomeric compounds-including regioisomers, stereoisomers and isotopic variants-is discussed. When a mixture of isomers, such as resulting from a chemical reaction, is analyzed, it is highly desired to be able to unambiguously identify the structures of each of the components present, as well as quantify them, without requiring complex sample preparation or reference standards. MRR offers unique capabilities for addressing this analytical challenge, owing to two factors: its high sensitivity to a molecule's structure and its high spectral resolution, allowing mixtures to be resolved without separation of components. This review introduces core theoretical principles, an introduction to MRR instrumentation and the methods by which spectra can be interpreted with the aid of computational chemistry to correlate the observed patterns to molecular structures. Recent articles are discussed in which this technique was applied to help chemists complete challenging isomer analyses. Developments in the use of MRR for chiral analysis and in the measurement of isotopically labeled compounds are also highlighted.

How Water Interacts with the NOH Group: The Rotational Spectrum of the 1:1 N,N-diethylhydroxylamine·Water Complex.

The rotational spectrum of the 1:1 N,N-diethylhydroxylamine-water complex has been investigated using pulsed jet Fourier transform microwave spectroscopy in the 6.5-18.5 GHz frequency region. The most stable conformer has been detected as well as the 13C monosubstituted isotopologues in natural abundance and the 18O enriched water species, allowing to determine the nitrogen nuclear quadrupole coupling constants and the molecular structure in the vibrational ground state. The molecule has a Cs symmetry and the water lies in the bc symmetry plane forming two hydrogen bonds with the NOH frame with length: dHOH·NOH = 1.974 Å and dH2O·HON = 2.096 Å. From symmetry-adapted perturbation theory calculations coupled to atoms in molecule approach, the corresponding interaction energy values are estimated to be 24 and 13 kJ·mol-1, respectively. The great strength of the intermolecular interaction involving the nitrogen atom is in agreement with the high reactivity of hydroxylamine compounds at the nitrogen site.

Accuracy of quantum chemistry structures of chiral tag complexes and the assignment of absolute configuration.

The absolute configuration of a molecule can be established by analysis of molecular rotational spectra of the analyte complexed with a small chiral molecule of known configuration. This approach of converting the analyte enantiomers, with identical rotational spectra, into diastereomers that can be distinguished spectroscopically is analogous to chiral derivatization in nuclear magnetic resonance (NMR) spectroscopy. For the rotational chiral tag method, the derivatization uses noncovalent interactions to install the new chiral center and avoids complications due to possible racemization of the analyte when covalent chemistry is used. The practical success of this method rests on the ability to attribute assigned rotational spectra to specific geometries of the diastereomeric homochiral and heterochiral tag complexes formed in the pulsed jet expansion that is used to introduce samples into the microwave spectrometer. The assignment of a molecular structure to an experimental rotational spectrum uses quantum chemistry equilibrium geometries to provide theoretical estimates of the spectrum parameters that characterize the rotational spectrum. This work reports the results of a high-sensitivity rotational spectroscopy study of the complexes formed between (3)-butyn-2-ol and verbenone. The rotational spectra of four homochiral and four heterochiral complexes are assigned. In addition, the 14 distinct, singly-substituted 13C isotopomer spectra of five of these species are assigned in natural abundance. Analysis of these spectra provides direct structural characterization of the complexes through determination of the carbon atom position coordinates. This data set is used to benchmark quantum chemistry calculations of candidate equilibrium geometries of the chiral tag complexes. The quantum chemistry calculations are limited to methods commonly used in the field of rotational spectroscopy. It is shown that the accuracy of the structures from quantum chemistry provides a high-confidence assignment of cluster geometries to the observed spectra. As a result, a high-confidence determination of the analyte (verbenone) absolute configuration is achieved.

Competition between In-Plane vs Above-Plane Configurations of Water with Aromatic Molecules: Non-Covalent Interactions in 1,4-Naphthoquinone-(H2O)1-3 Complexes.

Non-covalent interactions between aromatic molecules and water are fundamental in many chemical and biological processes, and their accurate description is essential to understand molecular relative configurations. Here we present the rotational spectroscopy study of the water complexes of the polycyclic aromatic hydrocarbon 1,4-naphthoquinone (1,4-NQ). In 1,4-NQ-(H2O)1,2, water molecules bind through O-H···O and C-H···O hydrogen bonds and are located on the plane of 1,4-NQ. For 1,4-NQ-(H2O)3, in-plane and above-plane water configurations are observed exhibiting O-H···O, C-H···O, and lone pair···π-hole interactions. The observation of different water arrangements for 1,4-NQ-(H2O)3 allows benchmarking theoretical methods and shows that they have great difficulty in predicting energy orderings due to the strong competition of C-H···O binding with π and π-hole interactions. This study provides important insight into water interactions with aromatic systems and the challenges in their modeling.

Characterizing the Interactions of Dimethyl Sulfoxide with Water: A Rotational Spectroscopy Study.

The interaction of dimethyl sulfoxide with water has been investigated by Fourier-transform microwave spectroscopy of the 1:1 complex and its isotopologues, complemented with quantum chemical calculations. The rotational spectra of 34S and 13C isotopologues in natural abundance and the H218O and deuterated water enriched isotopologues have been measured, allowing a partial structure determination and establishing the position of water in the complex. In the most stable conformation water was found to be the donor of a primary OH···OS bond to the oxygen atom of dimethyl sulfoxide and acceptor of two weak CH···OH bonds with the methyl hydrogen atoms of dimethyl sulfoxide. From the structural determination confirmed by quantum chemical calculations, the water molecule lies in the symmetry plane of dimethyl sulfoxide, and the complex has an overall Cs symmetry. The experimental findings are supported by atoms in molecules and symmetry-adapted perturbation theories, which allowed for determining the hydrogen bond and intermolecular interaction energies, respectively.

Spectroscopic and quantum mechanical study of a scavenger molecule: N,N-diethylhydroxylamine.

We report a combination of quantum mechanical calculations and a range of spectroscopic measurements in the gas phase of N,N-diethylhydroxylamine, an important scavenger compound. Three conformers were observed by pulsed jet Fourier transform microwave spectroscopy in the 6.5-18.5 GHz frequency range. They are characterized by the hydroxyl hydrogen atom being in trans orientation with respect to the bisector of the CNC angle while the side alkyl chains can be both trans (global minimum, Cs symmetry, A = 7608.1078(4), B = 2020.2988(2) and C = 1760.5423(2) MHz) or one trans and the other gauche (second energy minimum, A = 5302.896(1), B = 2395.9822(4) and C = 1804.8567(3) MHz) or gauche' (third energy minimum, A = 5960.8025(6), B = 2273.6627(4) and C = 1975.8074(4) MHz). For the global minimum, the 13Cα,13Cß and 15N isotopologues were observed in natural abundance, allowing for an accurate partial structure determination. Moreover, several lines were detected by free jet absorption millimeter wave spectroscopy in the 59.6-74.4 GHz spectral range. The electron binding energies of the highest occupied molecular orbital and the next-to-highest occupied molecular orbital, determined by photoelectron spectroscopy, are 8.95 and 10.76 eV, respectively. Supporting calculations evidence that, (i) upon ionization of the HOMO, the molecular structure changes from an amine to an N-oxoammonium arrangement and (ii) the 0-0 of the HOMO-1 photoionization is 10.46 eV. The K-shell binding energies, determined by X-ray photoelectron spectroscopy, are 290.42 eV (Cß), 291.45 eV (Cα), 405.98 eV (N) and 538.75 eV (O). The Fourier transform near infrared spectrum is reported and a tentative assignment is proposed. The equilibrium wavenumber (ω̃ = 3811 cm-1) and the anharmonicity constant (ω̃χ = -87.5 cm-1) of the hydroxyl stretching mode were estimated using a quadratic model.

Skeletal Torsion Tunneling and Methyl Internal Rotation: The Coupled Large Amplitude Motions in Phenyl Acetate.

The rotational spectrum of phenyl acetate, CH3COOC6H5, is measured using a free jet absorption millimeter-wave spectrometer in the range from 60 to 78 GHz and two pulsed jet Fourier transform microwave spectrometers covering a total frequency range from 2 to 26.5 GHz. The features of two large amplitude motions, the methyl group internal rotation and the skeletal torsion of the CH3COO group with respect to the phenyl ring C6H5 (tilted at about 70°), characterize the spectrum. The vibrational ground state is split into four widely spaced sublevels, labeled as A0, E0, A1, and E1, each of them with its set of rotational transitions and with additional interstate transitions. A global fit of the line frequencies of the four sublevels leads to the determination of 51 spectroscopic parameters, including the ΔEA0/A1 and ΔEE0/E1 vibrational splittings of ~36.4 and ~33.5 GHz, respectively. The V3 barrier to methyl internal rotation (~136 cm-1) and the skeletal torsion B2 barrier to the orthogonality of the two planes (~68 cm-1) are deduced.

Hydrogen Bonding in the Dimer and Monohydrate of 2-Adamantanol: A Test Case for Dispersion-Corrected Density Functional Methods.

Weakly-bound intermolecular clusters constitute reductionist physical models for non-covalent interactions. Here we report the observation of the monomer, the dimer and the monohydrate of 2-adamantanol, a secondary alcohol with a bulky ten-carbon aliphatic skeleton. The molecular species were generated in a supersonic jet expansion and characterized using broadband chirped-pulse microwave spectroscopy in the 2-8 GHz frequency region. Two different gauche-gauche O-H···O hydrogen-bonded isomers were observed for the dimer of 2-adamantanol, while a single isomer was observed for the monomer and the monohydrate. The experimental rotational parameters were compared with molecular orbital calculations using density functional theory (B3LYP-D3(BJ), B2PLYP-D3(BJ), CAM-B3LYP-D3(BJ), ωB97XD), additionally providing energetic and electron density characterization. The shallow potential energy surface makes the dimer an interesting case study to benchmark dispersion-corrected computational methods and conformational search procedures.

Portable light detectors for bioluminescence biosensing applications: A comprehensive review from the analytical chemist's perspective.

Bioluminescence, that is the emission of light in living organisms, has been extensively explored and applied for diverse bioanalytical applications, spanning from molecular imaging to biosensing. The unprecedented technological evolution of portable light detectors opened new possibilities to implement bioluminescence detection into miniaturized devices. We are witnessing a number of applications, including DNA sequencing, reporter gene assays, DNA amplification for point-of care and point-of need analyses relying on BL. Several photon detectors are currently available for measuring low light emission, such as photomultiplier tubes (PMT), charge-coupled devices (CCD), complementary metal oxide semiconductors (CMOS), single photon avalanche diodes (SPADs), silicon photomultipliers (SiPMs) and smartphone-integrated CMOS. Each technology has pros and cons and several issues, such as temperature dependence of the instrumental specific noise, the power supply, imaging capability and ease of integration, should be considered in the selection of the most appropriate detector for the selected BL application. These issues will be critically discussed from the perspective of the analytical chemist together with relevant examples from the literature with the goal of helping the reader in the selection and use of the most suitable detector for the selected application and to introduce non familiar readers into this exciting field.

Characterizing hydrogen and tetrel bonds in clusters of CO2 with carboxylic acids.

The interaction between carbon dioxide and planar carboxylic acids has been investigated through the analysis of the microwave spectrum of the acrylic acid·CO2 complex and quantum chemical modeling of the R-COOH·(CO2)1,16 clusters, where R = H, CH2CH. As regards the 1 : 1 compounds, two species, involving the s-cis and s-trans conformers of acrylic acid were observed. For both of them, a similar bidentate interaction arises between the carbonyl group of CO2 and the carboxylic group of the organic acid, leading to the formation of a planar six-membered ring. The binding energy is estimated to be De ≃ 21 kJ mol-1, 1/3 being the energy contributions of the tetrel to hydrogen bonds, respectively. In the 1 : 16 clusters, the ring arrangement is broken, allowing for the interaction of the acid with several CO2 molecules. The CO2 molecules completely surround formic acid, whereas, in the case of acrylic acid, they tend to avoid the allyl chain.

σ-Hole activation and structural changes upon perfluorination of aryl halides: direct evidence from gas phase rotational spectroscopy.

Enhancement of the σ-hole on the halogen atom of aryl halides due to perfluorination of the ring is demonstrated by use of the Extended Townes-Dailey (ETD) model coupled to a Natural Atomic Orbital Bond analysis on two perfluorinated aryl halides (C6F5Cl and C6F5Br) and their hydrogenated counterparts. The ETD analysis, which quantifies the halogen p-orbitals populations, relies on the nuclear quadrupole coupling constants which in this work are accurately determined experimentally from the rotational spectra. The rotational spectra investigated by Fourier-transform microwave spectroscopy performed in supersonic expansion are reported for the parent species of C6F5Cl and C6F5Br and their 13C, 37Cl or 81Br substituted isotopologues observed in natural abundance. The experimentally determined rotational constants combined with theoretical data at the MP2/aug-cc-pVTZ level provide precise structural information from which an elongation of the ring along its symmetry axis due to perfluorination is proved.

A rotational study of the 1:1 adduct of ethanol and 1,4-dioxane.

The pure rotational spectra of the 1:1 ethanol - 1,4-dioxane complex and its OD mono-deuterated species have been measured using pulsed-jet Fourier transform microwave spectroscopy. Conformational predictions for the plausible isomers of ethanol - 1,4-dioxane have been carried out considering the spatial orientation of gauche/trans ethanol with respect to the chair/boat and twisted conformations of 1,4-dioxane. Using Helium for the supersonic expansion, the microwave spectrum has been observed for the most stable structure. In the observed isomer, the two subunits are linked together by an OH⋯O hydrogen bond with gauche ethanol acting as proton donor to dioxane in the chair conformation. The non-covalent interactions have been characterized using different computational approaches. A small inverse Ubbelohde effect was observed after H â†’ D isotopic substitution in the OH⋯O hydrogen bond.

The Role of Non-Covalent Interactions on Cluster Formation: Pentamer, Hexamers and Heptamer of Difluoromethane.

The role of non-covalent interactions (NCIs) has broadened with the inclusion of new types of interactions and a plethora of weak donor/acceptor partners. This work illustrates the potential of chirped-pulse Fourier transform microwave technique, which has revolutionized the field of rotational spectroscopy. In particular, it has been exploited to reveal the role of NCIs' in the molecular self-aggregation of difluoromethane where a pentamer, two hexamers and a heptamer were detected. The development of a new automated assignment program and a sophisticated computational screening protocol was essential for identifying the homoclusters in conditions of spectral congestion. The major role of dispersion forces leads to less directional interactions and more distorted structures than those found in polar clusters, although a detailed analysis demonstrates that the dominant interaction energy is the pairwise interaction. The tetramer cluster is identified as a structural unit in larger clusters, representing the maximum expression of bond between dimers.

Characterizing the lone pairπ-hole interaction in complexes of ammonia with perfluorinated arenes.

When hydrogen is completely replaced by fluorine, arenes become prone to forming a lone pairπ-hole non-covalent bond with ligands presenting electron rich regions. Such a species is ammonia, which confirms this behavior engaging its lone pair as the electron donor counterpart in the 1 : 1 adducts with hexafluorobenzene and pentafluoropyridine. In this work, the geometrical parameters of the interaction have been unambiguously identified through the detection, by means of Fourier transform microwave spectroscopy, of the rotational spectra of both normal species and their 15NH3 isotopologues. An accurate analysis of the experimental data, including internal dynamics effects, endorsed by quantum chemical calculations, both with topological analysis and energy decomposition method, extended to the hydrogenated arenes and their water complexes, proved the ability of ammonia to create a stronger and more flexible lone pairπ-hole interaction than water. Interestingly, the higher binding energies of the ammonia lone pairπ-hole interactions correspond to larger intermolecular distances.

Rotational studies of adducts between carboxylic acids and tertiary alcohols: Formic acid - tert-butyl alcohol.

The rotational spectra of the parent and eight isotopologues of the 1:1 complex formic acid - tert-butyl alcohol (FA-TBA) have been measured by pulsed jet Fourier transform microwave spectroscopy. The spectra have been observed in the supersonic expansion of a mixture of FA and TBA in Helium, differently with respect to the mixtures of FA with primary and secondary alcohols, which undergo the esterification reaction upon supersonic expansion. In the complex, the two subunits are linked to each other by two different O-H···O hydrogen bonds (HB) in which FA and TBA are alternate their roles of bond acceptor and donor. Upon H â†’ D substitution of the corresponding O-H···O HB, a small Ubbelohde effect is observed.