Rapid Allylic 1,6 H-Atom Transfer in an Unsaturated Criegee Intermediate.

A novel allylic 1,6 hydrogen-atom-transfer mechanism is established through infrared activation of the 2-butenal oxide Criegee intermediate, resulting in very rapid unimolecular decay to hydroxyl (OH) radical products. A new precursor, Z/E-1,3-diiodobut-1-ene, is synthesized and photolyzed in the presence of oxygen to generate a new four-carbon Criegee intermediate with extended conjugation across the vinyl and carbonyl oxide groups that facilitates rapid allylic 1,6 H-atom transfer. A low-energy reaction pathway involving isomerization of 2-butenal oxide from a lower-energy (tZZ) conformer to a higher-energy (cZZ) conformer followed by 1,6 hydrogen transfer via a seven-membered ring transition state is predicted theoretically and shown experimentally to yield OH products. The low-lying (tZZ) conformer of 2-butenal oxide is identified based on computed anharmonic frequencies and intensities of its conformers. Experimental IR action spectra recorded in the fundamental CH stretch region with OH product detection by UV laser-induced fluorescence reveal a distinctive IR transition of the low-lying (tZZ) conformer at 2996 cm-1 that results in rapid unimolecular decay to OH products. Statistical RRKM calculations involving a combination of conformational isomerization and unimolecular decay via 1,6 H-transfer yield an effective decay rate keff(E) on the order of 108 s-1 at ca. 3000 cm-1 in good accord with the experiment. Unimolecular decay proceeds with significant enhancement due to quantum mechanical tunneling. A rapid thermal decay rate of ca. 106 s-1 is predicted by master-equation modeling of 2-butenal oxide at 298 K, 1 bar. This novel unimolecular decay pathway is expected to increase the nonphotolytic production of OH radicals upon alkene ozonolysis in the troposphere.

Energy-resolved and time-dependent unimolecular dissociation of hydroperoxyalkyl radicals (˙QOOH).

Hydroperoxyalkyl radicals (˙QOOH) are transient intermediates in the atmospheric oxidation of volatile organic compounds and combustion of hydrocarbon fuels in low temperature (<1000 K) environments. The carbon-centered ˙QOOH radicals are a critical juncture in the oxidation mechanism, but have generally eluded direct experimental observation of their structure, stability, and dissociation dynamics. Recently, this laboratory demonstrated that a prototypical ˙QOOH radical [˙CH2(CH3)2COOH] can be synthesized by an alternative route, stabilized in a pulsed supersonic expansion, and characterized by its infrared (IR) spectroscopic signature and unimolecular dissociation rate to OH radical and cyclic ether products. The present study focuses on a partially deuterated ˙QOOD analog ˙CH2(CH3)2COOD, generated in the laboratory by H-atom abstraction from partially deuterated tert-butyl hydroperoxide, (CH3)3COOD. IR spectral features associated with jet-cooled and isolated ˙QOOD radicals are observed in the vicinity of the transition state (TS) barrier leading to OD radical and cyclic ether products. The overtone OD stretch (2νOD) of ˙QOOD is identified by IR action spectroscopy with UV laser-induced fluorescence detection of OD products. Direct time-domain measurement of the unimolecular dissociation rate for ˙QOOD (2νOD) extends prior rate measurements for ˙QOOH. Partial deuteration results in a small increase in the TS barrier predicted by high level electronic structure calculations due to changes in zero-point energies; the imaginary frequency is unchanged. Comparison of the unimolecular decay rates obtained experimentally with those predicted theoretically for both ˙QOOH and ˙QOOD confirm that unimolecular decay is enhanced by heavy-atom tunneling involving simultaneous O-O bond elongation and C-C-O angle contraction along the reaction pathway.

Infrared spectroscopic signature of a hydroperoxyalkyl radical (•QOOH).

Infrared (IR) action spectroscopy is utilized to characterize a prototypical carbon-centered hydroperoxyalkyl radical (•QOOH) transiently formed in the oxidation of volatile organic compounds. The •QOOH radical formed in isobutane oxidation, 2-hydroperoxy-2-methylprop-1-yl, •CH2(CH3)2COOH, is generated in the laboratory by H-atom abstraction from tert-butyl hydroperoxide (TBHP). IR spectral features of jet-cooled and stabilized •QOOH radicals are observed from 2950 to 7050 cm-1 at energies that lie below and above the transition state barrier leading to OH radical and cyclic ether products. The observed •QOOH features include overtone OH and CH stretch transitions, combination bands involving OH or CH stretch and a lower frequency mode, and fundamental OH and CH stretch transitions. Most features arise from a single vibrational transition with band contours well simulated at a rotational temperature of 10 K. In each case, the OH products resulting from unimolecular decay of vibrationally activated •QOOH are detected by UV laser-induced fluorescence. Assignments of observed •QOOH IR transitions are guided by anharmonic frequencies computed using second order vibrational perturbation theory, a 2 + 1 model that focuses on the coupling of the OH stretch with two low-frequency torsions, as well as recently predicted statistical •QOOH unimolecular decay rates that include heavy-atom tunneling. Most of the observed vibrational transitions of •QOOH are readily distinguished from those of the TBHP precursor. The distinctive IR transitions of •QOOH, including the strong fundamental OH stretch, provide a general means for detection of •QOOH under controlled laboratory and real-world conditions.

Coupling of torsion and OH-stretching in tert-butyl hydroperoxide. II. The OH-stretching fundamental and overtone spectra.

The vibrational spectra of gas phase tert-butyl hydroperoxide have been recorded in the OH-stretching fundamental and overtone regions (ΔvOH = 1-5) at room temperature using conventional Fourier transform infrared (ΔvOH = 1-3) and cavity ring-down (ΔvOH = 4-5) spectroscopy. In hydroperoxides, the OH-stretching and COOH torsion vibrations are strongly coupled. The double-well nature of the COOH torsion potential leads to tunneling splitting of the energy levels and, combined with the low frequency of the torsional vibration, results in spectra in the OH-stretching regions with multiple vibrational transitions. In each of the OH-stretching regions, both an OH-stretching and a stretch-torsion combination feature are observed, and we show direct evidence for the tunneling splitting in the OH-stretching fundamental region. We have developed two complementary vibrational models to describe the spectra of the OH-stretching regions, a reaction path model and a reduced dimensional local mode model, both of which describe the features of the vibrational spectra well. We also explore the torsional dependence of the OH-stretching transition dipole moment and show that a Franck-Condon treatment fails to capture the intensity in the region of the stretch-torsion combination features. The accuracy of the Franck-Condon treatment of these features improves with increasing ΔvOH.

Coupling of torsion and OH-stretching in tert-butyl hydroperoxide. I. The cold and warm first OH-stretching overtone spectrum.

The infrared (IR) spectrum of tert-butyl hydroperoxide (TBHP) in the region of the first OH-stretching overtone has been observed under jet-cooled and thermal (300 K, 3 Torr) conditions at ∼7017 cm-1. The jet-cooled spectrum is recorded by IR multiphoton excitation with UV laser-induced fluorescence detection of OH radical products, while direct IR absorption is utilized under thermal conditions. Prior spectroscopic studies of TBHP and other hydroperoxides have shown that the OH-stretch and XOOH (X = H or C) torsion vibrations are strongly coupled, resulting in a double well potential associated with the torsional motion about the OO bond that is different for each of the OH-stretching vibrational states. A low barrier between the wells on the torsional potential results in tunneling split energy levels, which leads to four distinct transitions associated with excitation of the coupled OH-stretch-torsion states. In order to interpret the experimental results, two theoretical models are used that include the OH-stretch-torsion coupling in TBHP. Both methods are utilized to compute the vibrational transitions associated with the coupled OH-stretch-torsion states of TBHP, revealing the underlying transitions that compose the experimentally observed features. A comparison between theory and experiment illustrates the necessity for treatments that include OH-stretch and COOH torsion in order to unravel the spectral features observed in the first OH-stretching overtone region of TBHP.

Incidence rates and characteristics of pediatric onset psychogenic nonepileptic seizures.

BACKGROUND: Pediatric onset psychogenic nonepileptic seizures (PNES) is a highly disabling disorder and potentially misdiagnosed as epilepsy. Still, knowledge regarding PNES in children and adolescents is limited and data on both incidence and characteristics are scarce. This study investigated the incidence rate (IR) and clinical characteristics of pediatric onset PNES, including possible differences when having comorbid epilepsy. METHODS: A population-based study of children and adolescents aged 5-17 years with an incident diagnosis of PNES in the Danish healthcare registries between 1996 and 2014. In total, 386 children and adolescents were included after assessment of diagnostic validity using medical record data. RESULTS: The IR increased during the study period with the maximum IR observed in 2014 (7.4 per 100,000 person-years). A history of both neurologic and psychiatric problems as well as negative life events was identified. Comorbid epilepsy was confirmed for 55 cases (14.2%) and was associated with intellectual disabilities, school support and prolonged delay in PNES diagnosis. CONCLUSIONS: PNES are increasingly diagnosed in children and adolescents, and the clinical profile of both neurologic and psychiatric health problems underscores the need for collaborative pediatric and mental healthcare. These findings provide important information for future healthcare planning in this area. IMPACT: This nationwide study is the first to report population-based incidence rates of pediatric onset PNES documenting markedly increasing incidence rates between 1996 and 2014. A history of both neurologic and psychiatric problems as well as negative life events was identified for pediatric onset PNES. Comorbid epileptic seizures were associated with intellectual disabilities, school support and prolonged delay in PNES diagnosis. The clinical profile of both neurologic and psychiatric health problems underscores the need for collaborative pediatric and mental healthcare. The increasing number of children and adolescents diagnosed with PNES is important information for future healthcare planning in this area.

Room Temperature Gas-Phase Detection and Gibbs Energies of Water Amine Bimolecular Complex Formation.

We have detected the H2O·DMA and H2O·TMA (DMA, dimethylamine; TMA, trimethylamine) bimolecular complexes at room temperature in the gas phase using Fourier transform infrared spectroscopy. For both complexes, five vibrational bands associated with the H2O molecule are observed and assigned. Within a reduced dimensional local mode framework, we set up a six-dimensional model, including the three H2O vibrational modes and three of the six intermolecular modes, all described with internal curvilinear coordinates. The single points on the potential energy surface and Eckart corrected dipole moment surface are calculated with the CCSD(T)-F12a/cc-pVDZ-F12 method. Combining the measured and calculated transition intensities, we determine the Gibbs energy of complex formation of both complexes from each of the observed bands. The multiple determinations give similar Gibbs energies, for each complex, and increase the confidence in the combined experimental and theoretical approach, and improve the accuracy of the determined Gibbs energies. The average Gibbs energies of complex formation are found to be 5.0 ± 0.2 and 3.8 ± 0.2 kJ/mol for H2O·DMA and H2O·TMA, respectively. In addition to the experimental uncertainty, there is a potential error on the calculated intensities corresponding to 0.4 kJ/mol. However, the small spread among the four determinations suggests that this error is even less. The Gibbs energies of these complexes serve as accurate benchmarks for theoretical approaches that are prevalent in hydrogen bonding and nucleation studies.

Unraveling Conformer-Specific Sources of Hydroxyl Radical Production from an Isoprene-Derived Criegee Intermediate by Deuteration.

Ozonolysis of isoprene, the most abundant volatile organic compounds emitted into the Earth's troposphere after methane, yields three distinct Criegee intermediates. Among these, methyl vinyl ketone oxide (MVK-oxide) is predicted to be the major source of atmospheric hydroxyl radicals (OH) from isoprene ozonolysis. Previously, Barber et al. [ J. Am. Chem. Soc., 2018, 140, pp 10866-10880] demonstrated that syn-MVK-oxide conformers undergo unimolecular decay via 1,4-hydrogen (H) transfer from the methyl group to the adjacent terminal oxygen atom, followed by the prompt release of OH radical products. Here, we selectively deuterate the methyl group of MVK-oxide (d3-MVK-oxide) and record its IR action spectrum in the vinyl CH stretch overtone (2νCH) region. The resultant time-dependent appearance of OD radical products, detected by laser-induced fluorescence, demonstrates that a unimolecular decay of d3-MVK-oxide proceeds by an analogous 1,4-deuterium (D) atom transfer mechanism anticipated for syn conformers. The experimental spectral and temporal results are compared with the calculated IR absorption spectrum and unimolecular decay rates predicted by the Rice-Ramsperger-Kassel-Marcus (RRKM) theory for syn-d3-MVK-oxide, as well as the prior study on syn-MVK-oxide. The d3-MVK-oxide IR action spectrum is similar to that for MVK-oxide, yet exhibits notable changes as the overtone and combination transitions involving CD stretch shift to a lower frequency. The unimolecular decay rate for d3-MVK-oxide is predicted to be a factor of 40 times slower than that for MVK-oxide in the 2νCH region. Experimentally, the temporal profile of the OD products reflects the slower unimolecular decay of d3-MVK-oxide compared to that for MVK-oxide to OH products as well as experimental factors. Both experiment and theory demonstrate that quantum mechanical tunneling plays a very important role in the 1,4-H/D-transfer processes at energies in the vicinity of the transition-state barrier. The similarities of the IR action spectra and changes in the unimolecular decay dynamics upon deuteration indicate that syn conformers make the main contribution to the IR action spectra of MVK-oxide and d3-MVK-oxide.

Experimental and theoretical studies of the doubly substituted methyl-ethyl Criegee intermediate: Infrared action spectroscopy and unimolecular decay to OH radical products.

The infrared (IR) action spectrum of the doubly substituted methyl-ethyl Criegee intermediate (MECI) is observed in the CH stretch overtone region with detection of OH products. The MECI exhibits four conformers, all of which undergo unimolecular decay via a 1,4 H-atom transfer mechanism, followed by the rapid release of OH products. Conformers with different orientations of the carbonyl oxide group with respect to the methyl and ethyl substituents (i.e., anti and syn) decay via distinct transition state barriers (16.1 kcal mol-1 and 15.4 kcal mol-1, respectively). The observed IR action spectrum is in good agreement with the predicted anharmonic IR absorption spectrum, but exhibits significant congestion, which is attributed to couplings between spectroscopic bright states and nearby dark states. Energy-dependent OH appearance rates are measured upon IR excitation of the strongest features in the IR action spectrum and are found to be on the order of 106-107 s-1. The experimental rates are in good agreement with computed Rice-Ramsperger-Kassel-Marcus rates for the unimolecular decay of MECI at these energies, which incorporate quantum mechanical tunneling and sophisticated hindered rotor treatments, as well as high-level theoretical calculations of the TS barrier heights, rovibrational properties, and torsional barriers associated with the MECI conformers. Master equation modeling is used to predict thermal rates for the unimolecular decay of anti- and syn-MECI of 473 s-1 and 660 s-1, respectively. Comparison with other previously studied Criegee intermediate systems provides insights into substituent effects on unimolecular decay under both energy-dependent and thermal conditions.

Hybridization of Nitrogen Determines Hydrogen-Bond Acceptor Strength: Gas-Phase Comparison of Redshifts and Equilibrium Constants.

Gas-phase Fourier transform infrared spectroscopy and quantum chemical calculations are combined to illustrate the effect of hybridization on the hydrogen-bond acceptor strength of nitrogen by a comparison of nine bimolecular complexes. We present gas-phase results for the complexes of methanol, ethanol, and 2,2,2-trifluoroethanol with acetonitrile (sp-hybridized N) and find that the structure of these complexes is nearly linear and dominated by the OH···N hydrogen bond with no experimental indication of an OH-π bonded structure. We compare experimental redshifts and equilibrium constants, obtained by combining experiments and theory, for these complexes to the corresponding complexes with pyridine (sp2-hybridized N) and trimethylamine (sp3-hybridized N). The comparison clearly illustrates that increasing the s-character of the nitrogen lone pair decreases the hydrogen-bond acceptor strength (sp3 > sp2 > sp). The observed trend correlates with the basicity of the acceptors and can be explained by the partial charge on the accepting nitrogen atom and the degree of localization of the nitrogen lone pair.

Dimethyl Sulfoxide Complexes Detected at Ambient Conditions.

Dimethyl sulfoxide (DMSO) is an important intermediate in the atmospheric oxidation of dimethyl sulfide (DMS). DMS, emitted from the ocean, is the main natural sulfur source, and its oxidation products are essential in the formation of sulfate aerosols. The high atmospheric concentration of water makes hydrogen bonding with DMS and its oxidation products likely. Through hydrogen bonding, water can potentially catalyze and affect the steps of the oxidation. We investigate binary hydrogen-bound complexes involving DMSO. Both water·DMSO and methanol·DMSO complexes are identified in an Ar matrix, and at room temperature, a Gibbs free energy of 0.7 kJ/mol for the formation of the methanol·DMSO complex is determined. Assuming a similar Gibbs free energy of the hydrate, it would suggest a relatively high abundance of the DMSO hydrate relative to the monomer in the atmosphere. The effect of changing the atom divalently bound to the hydrogen bond accepting oxygen, from S to C (DMSO to acetone), is found to significantly decrease the equilibrium constant of complex formation.

Kinetic Energy Density as a Predictor of Hydrogen-Bonded OH-Stretching Frequencies.

This work considers the nature of the intermolecular hydrogen bond in a series of 15 different complexes with OH donor groups and N, O, P, or S acceptor atoms. To complement the existing literature, room-temperature gas-phase vibrational spectra of the methanol-pyridine, ethanol-pyridine, and 2,2,2-trifluoroethanol-pyridine complexes were recorded. These complexes were chosen, as they exhibit hydrogen bonds of intermediate strength as compared to previous investigations that involved strong or weak hydrogen bonds. Non Covalent Interactions (NCI) theory was used to calculate various properties of the intermolecular hydrogen bonds, which were compared to the experimental OH-stretching vibrational red shifts. We find that the experimental OH-stretching red shifts correlate strongly with the kinetic energy density integrated within the reduced density gradient volume that describes a hydrogen bond [G(s0.5)]. Given that vibrational red shifts are commonly used as a metric of the strength of a hydrogen bond, this suggests that G(s0.5) could be used as a predictor of hydrogen bonding strength.

Solar Thermal Energy Storage in a Photochromic Macrocycle.

The conversion and efficient storage of solar energy is recognized to hold significant potential with regard to future energy solutions. Molecular solar thermal batteries based on photochromic systems exemplify one possible technology able to harness and apply this potential. Herein is described the synthesis of a macrocycle based on a dimer of the dihydroazulene/vinylheptafulvene (DHA/VHF) photo/thermal couple. By taking advantage of conformational strain, this DHA-DHA macrocycle presents an improved ability to absorb and store incident light energy in chemical bonds (VHF-VHF). A stepwise energy release over two sequential ring-closing reactions (VHF→DHA) combines the advantages of an initially fast discharge, hypothetically addressing immediate energy consumption needs, followed by a slow process for consistent, long-term use. This exemplifies another step forward in the molecular engineering and design of functional organic materials towards solar thermal energy storage and release.

Accurate thermodynamic properties of gas phase hydrogen bonded complexes.

We have measured the infrared spectra of ethanol·dimethylamine and methanol·dimethylamine complexes in the 299-374 K temperature range, and have determined the enthalpy of complex formation (ΔH) to be -31.1 ± 2 and -29.5 ± 2 kJ mol(-1), respectively. The corresponding values of the Gibbs free energy (ΔG) are determined from the experimental integrated absorbance and a calculated oscillator strength of the OH-stretching vibrational transition to be 4.1 ± 0.3 and 3.9 ± 0.3 kJ mol(-1) at 302 and 300 K, respectively. The entropy, ΔS is determined from the values of ΔH and ΔG to be -117 ± 7 and -111 ± 10 J (mol K)(-1) for the ethanol·dimethylamine and methanol·dimethylamine complexes, respectively. The determined ΔH, ΔG and ΔS values of the two complexes are similar, as expected by the similarity to their donor molecules ethanol and methanol. Values of ΔH, ΔG and ΔS in chemical reactions are often obtained from quantum chemical calculations. However, these calculated values have limited accuracy and large variations are found using different methods. The accuracy of the present ΔH, ΔG and ΔS values is such that the benchmarking of theoretical methods is possible.

Towards solar energy storage in the photochromic dihydroazulene-vinylheptafulvene system.

One key challenge in the field of exploitation of solar energy is to store the energy and make it available on demand. One possibility is to use photochromic molecules that undergo light-induced isomerization to metastable isomers. Here we present efforts to develop solar thermal energy storage systems based on the dihydroazulene (DHA)/vinylheptafulvene (VHF) photo/thermoswitch. New DHA derivatives with one electron-withdrawing cyano group at position 1 and one or two phenyl substituents in the five-membered ring were prepared by using different synthetic routes. In particular, a diastereoselective reductive removal of one cyano group from DHAs incorporating two cyano groups at position 1 turned out to be most effective. Quantum chemical calculations reveal that the structural modifications provide two benefits relative to DHAs with two cyano groups at position 1: 1) The DHA-VHF energy difference is increased (i.e., higher energy capacity of metastable VHF isomer); 2) the Gibbs free energy of activation is increased for the energy-releasing VHF to DHA back-reaction. In fact, experimentally, these new derivatives were so reluctant to undergo the back-reaction at room temperature that they practically behaved as DHA to VHF one-way switches. Although lifetimes of years are at first attractive, which offers the ultimate control of energy release, for a real device it must of course be possible to trigger the back-reaction, which calls for further iterations in the future.

Controlling two-step multimode switching of dihydroazulene photoswitches.

We present the synthesis and switching studies of systems with two photochromic dihydroazulene (DHA) units connected by a phenylene bridge at either para or meta positions, which correspond to a linear or cross-conjugated pathway between the photochromes. According to UV/Vis absorption and NMR spectroscopic measurements, the meta-phenylene-bridged DHA-DHA exhibited sequential light-induced ring openings of the two DHA units to their corresponding vinylheptafulvenes (VHFs). Initially, the VHF-DHA species was generated, and, ultimately, after continued irradiation, the VHF-VHF species. Studies in different solvents and quantum chemical calculations indicate that the excitation of DHA-VHF is no longer a local DHA excitation but a charge-transfer transition that involves the neighboring VHF unit. For the linearly conjugated para-phenylene-bridged dimer, electronic communication between the two units is so efficient that the photoactivity is reduced for both the DHA-DHA and DHA-VHF species, and DHA-DHA, DHA-VHF, and VHF-VHF were all present during irradiation. In all, by changing the bridging unit, we can control the degree of stepwise photoswitching.

Gas Phase Detection of the NH-P Hydrogen Bond and Importance of Secondary Interactions.

We have observed the NH···P hydrogen bond in a gas phase complex. The bond is identified in the dimethylamine-trimethylphosphine complex by a red shift of the fundamental NH-stretching frequency observed using Fourier transform infrared spectroscopy (FT-IR). On the basis of the measured NH-stretching frequency red shifts, we find that P is a hydrogen bond acceptor atom similar in strength to S. Both are stronger acceptors than O and significantly weaker acceptors than N. The hydrogen bond angle, ∠NHP, is found to be very sensitive to the functional employed in density functional theory (DFT) optimizations of the complex and is a possible parameter to assess the quality of DFT functionals. Natural bonding orbital (NBO) energies and results from the topological methods atoms in molecules (AIM) and noncovalent interactions (NCI) indicate that the sensitivity is caused by the weakness of the hydrogen bond compared to secondary interactions. We find that B3LYP favors the hydrogen bond and M06-2X favors the secondary interactions leading to under- and overestimation, respectively, of the hydrogen bond angle relative to a DF-LCCSD(T)-F12a calculated angle. The remaining functionals tested, B3LYP-D3, B3LYP-D3BJ, CAM-B3LYP, and ωB97X-D, as well as MP2, show comparable contributions from the hydrogen bond and the secondary interactions and are close to DF-LCCSD(T)-F12a results.

Computational methodology study of the optical and thermochemical properties of a molecular photoswitch.

We assess how the utilization of different DFT functionals for obtaining the equilibrium geometries and vibrational frequencies affect the description of the thermochemistry and subsequent calculation of the optical properties of a dihydroazulene-vinylheptafulvene photoswitch. The assessment covers nine popular DFT functionals (BLYP, B3LYP, CAM-B3LYP, M06-L, M06, M06-2X, PBE, PBE0, and ωB97X-D) in conjugation with five different Pople style basis sets (6-31+G(d), 6-31++G(d,p), 6-311+G(d), 6-311++G(d,p), and 6-311++G(3df,3pd)). It is identified that only CAM-B3LYP, M06-2X, and PBE0 are able to quantitatively describe the correct trends in the thermochemical properties. The subsequent calculation of the optical properties using the CAM-B3LYP functional shows that there is little difference in whether the CAM-B3LYP, M06-2X, or PBE0 functionals have been used to calculate the equilibrium geometries. Utilizing the identified functionals, we investigate how the number of electron withdrawing cyano substituents influence the thermochemistry and optical properties of the molecular photoswitch.

The effect of fluorine substitution in alcohol-amine complexes.

The effect of fluorine substitution on the hydrogen bond strength in alcohol-amine molecular complexes was investigated, with a combination of vapour phase infrared spectroscopy and theoretical calculations. The complexes were combined from methanol (MeOH), ethanol (EtOH) and trifluoroethanol (TFE) as the hydrogen bond donor, and either dimethylamine (DMA) or trimethylamine (TMA) as the acceptor. The fundamental OH-stretching vibration involved in hydrogen bonding was measured for all complexes, as well as the weak second NH-stretching overtone for the DMA complexes. Equilibrium constants for complex formation were determined by combining a calculated intensity and the measured integrated absorbance. The observation of two transitions in the alcohol-DMA complexes provides an opportunity for two independent determinations of the equilibrium constants. Molecular interactions between the monomers were elucidated by Natural Bond Orbital, Atoms in Molecules and Non-covalent Interactions analysis. We find that the alcohol-amine complexes with TFE as the hydrogen bond donor form stronger hydrogen bonds and that secondary interactions between the monomers increase from MeOH to EtOH to TFE. TFE is a stronger acid than EtOH and MeOH making the OH bond weaker, and the OH-stretching frequency is redshifted in TFE relative to EtOH. This redshift is small in the monomers but significantly enhanced upon complexation.