OH Stretch Research Articles

Observing vibronic coupling in a strongly hydrogen bonded system with coherent multidimensional vibrational-electronic spectroscopy.

The coupled structural and electronic parameters of intramolecular hydrogen bonding play an important role in ultrafast chemical reactions, such as proton transfer processes. We perform one- and two-dimensional vibrational-electronic (1D and 2D VE) spectroscopy experiments to understand the couplings between vibrational and electronic coordinates in 10-Hydroxybenzo[h]quinoline, an ultrafast proton transfer system. The experiments reveal that the OH stretch (νOH) is strongly coupled to the electronic excitation, and Fourier analysis of the 1D data shows coherent oscillations from the low frequency backbone vibrational modes coupled to the νOH mode, resulting in an electronically detected vibronic signal. In-plane low-frequency vibrations at 242 and 386 cm-1 change the hydrogen bond distance and modulate the observed electronic signal in the polarization-selective 1D VE experiment through orientation-dependent coupling with the νOH mode. Resolution of the excitation frequency axis with 2D VE experiments reveals that excitation frequency, detection frequency, and experimental delay affect the frequency and strength of the vibronic transitions observed. Our results demonstrate evidence of direct coupling of the high frequency νOH mode with the S1 ← S0 electronic transition in 10-Hydroxybenzo[h]quinoline (HBQ), and orientation-dependent couplings of the low-frequency 242 and 386 cm-1 modes to the νOH mode and the electronic transition. This demonstration of multidimensional VE spectroscopy on HBQ reveals the potential of using 1D and 2D VE spectroscopy to develop a quantitative understanding of the role of vibronic coupling in hydrogen bonding and ultrafast proton transfer for complex systems.

Revisiting the Thickness of the Air-Water Interface from Two Extremes of Interface Hydrogen Bond Dynamics.

The air-water interface plays a crucial role in many aspects of science because of its unique properties, such as a two-dimensional hydrogen bond (HB) network and completely different HB dynamics compared to bulk water. However, accurately determining the boundary of interfacial and bulk water, that is, the thickness of the air-water interface, still challenges experimentalists. Various simulation-based methods have been developed to estimate the thickness, converging on a range of approximately 3-10 (Å). In this study, we introduce a novel approach, grounded in density functional theory-based molecular dynamics and deep potential molecular dynamics simulations, to measure the air-water interface thickness, offering a different perspective based on prior research. To capture realistic HB dynamics in the air-water interface, two extreme scenarios of the interface HB dynamics are obtained: one underestimates the interface HB dynamics, while the other overestimates it. Surprisingly, our results suggest that the interface HB dynamics in both scenarios converges as the thickness of the air-water interface increases to 4 (Å). This convergence point, indicative of the realistic interface thickness, is also validated by our calculation of anisotropic decay of OH stretch and the free OH dynamics at the air-water interface.

Infrared Fingerprint and Unimolecular Decay Dynamics of the Hydroperoxyalkyl Intermediate (•QOOH) in Cyclopentane Oxidation.

A transient carbon-centered hydroperoxyalkyl intermediate (•QOOH) in the oxidation of cyclopentane is identified by IR action spectroscopy with time-resolved unimolecular decay to hydroxyl (OH) radical products that are detected by UV laser-induced fluorescence. Two nearly degenerate •QOOH isomers, β- and γ-QOOH, are generated by H atom abstraction of the cyclopentyl hydroperoxide precursor. Fundamental and first overtone OH stretch transitions and combination bands of •QOOH are observed and compared with anharmonic frequencies computed by second-order vibrational perturbation theory. An OH stretch transition is also observed for a conformer arising from torsion about a low-energy CCOO barrier. Definitive identification of the β-QOOH isomer relies on its significantly lower transition state (TS) barrier to OH products, which results in rapid unimolecular decay and near unity branching to OH products. A benchmarking approach is utilized to compute high-accuracy stationary point energies, most importantly TS barriers, for cyclopentane oxidation (C5H9O2), building on higher level reference calculations for ethane oxidation (C2H5O2). The experimental OH product appearance rates are compared with computed statistical microcanonical rates using RRKM theory, including heavy-atom tunneling, thereby validating the computed TS barrier. The results are extended to thermal unimolecular decay rate constants at temperatures and pressures relevant to cyclopentane combustion via master-equation modeling. The various torsional and ring puckering states of the wells and transition states are explicitly considered in these calculations.

Solution and Surface Solvation of Nitrate Anions with Iron(III) and Aluminum(III) in Aqueous Environments: A Raman and Vibrational Sum Frequency Generation Study.

Hydrated trivalent metal nitrate salts, Fe(NO3)3·9H2O and Al(NO3)3·9H2O, in both solid and aqueous phases are investigated. Raman and surface-selective vibrational sum frequency generation (SFG) spectroscopy, are used to shed light on ion-ion interactions and hydration in several spectral regions spanning low frequency (440-550 cm-1) to higher frequency modes of nitrate and water (720, 1050, 1250-1450, and 2800-3750 cm-1). These frequencies span the metal-water mode, nitrate in-plane deformation, nitrate symmetric and asymmetric modes, and the OH stretch of condensed phase water molecules. Comparison to NaNO3, and in some cases KNO3, is also shown, providing insight. Splitting and frequency shifts are observed and discussed for both the solid state and solution phase. The Lewis acidity of Fe3+ and Al3+ ions plays a significant role in the observed spectra, in particular for the nitrate asymmetric band splitting and frequency shift. The spectral response from water solvation for iron and aluminum nitrates is nonlinear as compared to linear for sodium nitrate, suggesting significantly different solvation environments that are limited by water hydration capacity at higher concentrations. Moreover, a non-hydrogen bonded OH, dangling OH, from hydrating water molecules is observed spectroscopically for Al and Fe nitrate solutions. Furthermore, aluminum nitrate perturbs the surface water structure more than iron nitrate despite aluminum being a weaker Lewis acid. The surface water structure is thus found to be unique for the Al(NO3)3 solutions as compared to both Fe(NO3)3 and NaNO3, such that surface solvation is more pronounced. This observation exemplifies the nature of the Fe(III) and Al(III) ions and their substantial influence on the surface water structure.

Rearrangement of Proline Complexes with Zn2+: An Infrared Multiple Photon Dissociation and Theoretical Investigation.

Complexes of proline (Pro) cationized with Zn2+ and Cd2+ were examined by infrared multiple photon dissociation (IRMPD) action spectroscopy using light generated from a free electron laser. Complexes of intact Pro with CdCl+, CdCl+(Pro), a complex of (Zn+Pro-H)+ where a proton has been lost, as well as Zn+(Pro-H)(Pro) were formed by electrospray ionization. In order to identify the structures formed experimentally, the IRMPD spectra were compared to those calculated from optimized structures at the B3LYP/6-311+G(d,p) level for zinc complexes and B3LYP/def2-TZVP level with an effective core potential on cadmium for the CdCl+(Pro) system. For the latter complex, the main binding motif observed has a zwitterionic proline ligand structure, [CO2-]cc, where the metal binds to the two carboxylate oxygens. In contrast, for Zn+(Pro-H)(Pro), both ligands interact with zinc via a [N,CO-][N,CO] binding motif, where binding is observed at the carbonyl oxygens and nitrogens for both ligands, consistent with previous work. In both cases, contributions from different puckers of the proline ring may contribute. For (Zn+Pro-H)+, we identify that the structure is actually ZnH+(Pro-2H), in which the proline has been dehydrogenated and one of the hydrogens has migrated to form a covalent bond with Zn, which verifies a previous report relying on a single OH stretch band.

In Situ Raman Spectroscopy of Plasma Electrochemical and Plasma Catalytic Reactors

Plasma electrochemistry (PEC) has been under investigation for an expanding number of potential applications such as agriculture, water treatment, decarbonized chemical processing, and nanomaterials synthesis [1–4]. PEC in water leads to the formation of gas-phase species that undergo a cascade of processes in the liquid to produce aqueous species such as OH, H2O2, ONOOH, NO2 -, and NO3 - that are sought after in these applications. Plasma-assisted catalysis (PAC) is another growing research field for some of the same applications and others that occur in the gas phase, such as industrial catalyst decoking, VOC abatement, gas reforming, and CO2 conversion [5]. For such problems, popular catalysts include zeolites, TiO2, and CeO2 [6]. Similar to PEC, in PAC the operating conditions for catalysis become more relaxed (e.g. lower temperature to activate the catalyst). There can be plasma-catalyst synergistic effects that improve the process yield well beyond that using plasma or catalyst alone.In both PEC and PAC, the plasma produces energetic photons, electrons, ions, and excited species, and enhances the electric field near plasma-liquid and plasma-solid interfaces. These strongly non-equilibrium conditions enable the plasma to perform the required processes for these applications in many cases in a single step and without assistance, compared to conventional methods that can require multiple steps, potentially toxic chemical agents (e.g. acids, reducing agents), and/or external heating.A major challenge facing the development of PEC- and PAC-related applications is the direct and detailed experimental investigation of the plasma-liquid and plasma-catalyst interfacial regions, respectively. The majority of existing liquid- and solid-phase diagnostics must be performed ex situ, removed from the plasma reactor and after treatment. To extend the scope of diagnostics, we employ an in situ approach using multiple diagnostic techniques to study a wide range of physical and chemical properties at the plasma-water interface. The centrepiece of this platform is in situ spontaneous Raman microspectroscopy, which is advantageous because of its non-intrusiveness, selectivity, versatility, and straightforward calibration. Shaping the laser beam into a light sheet enables probing of the interfacial region with micron-scale spatial resolution. For PEC in pure water, in situ Raman spectra showed that the concentrations of aqueous H2O2 and NO3 - both exceed those of the bulk liquid at a depth of a few tens of microns from the plasma-liquid interface [7].We will focus on transient changes to Raman spectra that appear in the presence of plasma, then disappear when the plasma is switched off. Such changes are observable in situ, not ex situ, and indicate the energy exchange from the plasma to water or the catalyst surface. The general approach will be to establish relationships between plasma properties and changes to the Raman spectra. Two primary cases will be studied. First, we will study PEC in air plasma-water systems at atmospheric pressure, using both batch reactor and electrospray configurations. We will focus on the spectral profile of the –OH stretch band of water and of probe molecules such as NO3 -. Analysis of the –OH stretch band reveals that the plasma weakens the hydrogen bonding network of water. To help pinpoint the cause, we will track the broadening of the N-O symmetric stretch mode (v 1) of NO3 - at less than 20 µm depth from the plasma-liquid interface. Second, we will investigate a PAC reactor consisting of a low- to medium-pressure CO2 plasma in contact with CeO2 as a catalyst. In this case, the catalyst temperature will be tracked using the Stokes-to-anti-Stokes ratio of the Raman intensities of the first-order optical phonon of CeO2, as well as the spectral shift of this spectral feature. Acknowledgments Financial support: Campus France Franco-Thai PhD fellowship (French Embassy in Thailand, E4C Engie sponsorship), IP Paris Energy4Climate “E4C” Interdisciplinary Center, Campus France PHC Stefanik project (49885SJ), CNRS-IEA “GRAFMET”, EUR PLASMAScience, Fédération Plas@Par, ANR grant ANR-15-CE06-0007-01 References [1] T. Orriere, D. Kurniawan, Y.-C. Chang, D. Z. Pai, and W.-H. Chiang, Nanotechnology 31, 485001 (2020).[2] J.-S. Yang, D. Z. Pai, and W.-H. Chiang, Carbon 153, 315 (2019).[3] S. Kooshki, P. Pareek, R. Mentheour, M. Janda, and Z. Machala, Environmental Technology & Innovation 32, 103287 (2023).[4] D. S. Mallapragada et al., Joule 7, 23 (2023).[5] E. Baratte, C. A. Garcia-Soto, T. Silva, V. Guerra, V. I. Parvulescu, and O. Guaitella, Plasma Chemistry and Plasma Processing s11090-023-10421-z (2023).[6] C. A. Garcia-Soto, E. Baratte, T. Silva, V. Guerra, V. I. Parvulescu, and O. Guaitella, Plasma Chemistry and Plasma Processing s11090-023-10419-7 (2023).[7] D. Z. Pai, Journal of Physics D: Applied Physics 54, 355201 (2021).

Study of the perturbed hydrogen-bonded water structure of the hydration shell of N-Methyl-Acetamide by Raman multivariate curve resolution spectroscopy

In this work, Raman scattering measurement combined with multivariate curve resolution (Raman-MCR) technique has been used to obtain the solute correlated spectra (SC) of N-methyl-acetamide (NMA) in an aqueous medium. The SC spectra is composed of NMA intramolecular vibrations and NMA induced-perturbations of water OH stretch vibrations. The SC spectra obtained for different mole-fractions of NMA reveal that as the mole-fraction of NMA increases the perturbed water interacting with the amide group is retained whereas it gets depleted from the hydrophobic hydration shell of the methyl group of NMA. The high wavenumber region of the OH spectrum shows that the fraction of the weakly hydrogen-bonded population is higher in the bulk water compared to perturbed water. The Raman spectra of perturbed OH spectral features due to hydration of the hydrophobic methyl group of NMA have been extracted from SC spectra obtained at low mole-fraction of NMA by the MCR technique. Its spectral feature resembles the SC-OH spectra of hydrophobic hydration of the methyl group of methanol.

Infrared signature of the hydroperoxyalkyl intermediate (·QOOH) in cyclohexane oxidation: An isomer-resolved spectroscopic study.

Infrared (IR) action spectroscopy is utilized to characterize carbon-centered hydroperoxy-cyclohexyl radicals (·QOOH) transiently formed in cyclohexane oxidation. The oxidation pathway leads to three nearly degenerate ·QOOH isomers, β-, γ-, and δ-QOOH, which are generated in the laboratory by H-atom abstraction from the corresponding ring sites of the cyclohexyl hydroperoxide (CHHP) precursor. The IR spectral features of jet-cooled and stabilized ·QOOH radicals are observed from 3590 to 7010cm-1 (∼10-20kcal mol-1) at energies in the vicinity of the transition state (TS) barrier leading to OH radicals that are detected by ultraviolet laser-induced fluorescence. The experimental approach affords selective detection of β-QOOH, arising from its significantly lower TS barrier to OH products compared to γ and δ isomers, which results in rapid unimolecular decay and near unity branching to OH products. The observed IR spectrum of β-QOOH includes fundamental and overtone OH stretch transitions, overtone CH stretch transitions, and combination bands involving OH or CH stretch with lower frequency modes. The assignment of β-QOOH spectral features is guided by anharmonic frequencies and intensities computed using second-order vibrational perturbation theory. The overtone OH stretch (2νOH) of β-QOOH is shifted only a few wavenumbers from that observed for the CHHP precursor, yet they are readily distinguished by their prompt vs slow dissociation rates to OH products.

High-resolution spectroscopic study of asymmetric [formula omitted] stretch [formula omitted] Torsion [formula omitted] combination band and other higher-lying vibrational fundamental, overtone, combination, and hot bands of astrophysically significant methanol [formula omitted] molecule

In this paper, the spectrum of high energy vibrational bands has been recorded in the range 1900-6000cm-1with the highest possible resolution. New assignments have been obtained for a large number of transitions belonging to several fundamental and combination bands. Varios bands of importance in the range 1950 − 3420 cm-1 are shown in Fig. 1. Notable are those belonging to a new combination band comprising the asymmetric CH3- Stretch band and OH internal rotation (or torsion) band lying in a very crowded region of the three coupled fundamental CH3- stretching bands. A large number of ΔK=±1 transitions have been observed from the ground and the first excited torsional states of the ground vibrational state. In addition, new transitions have been obtained for the torsionally excited OH- stretch band. The majority of the transitions have been confirmed using closed combination loops. In addition, detailed assignment work has been performed with improved accuracy on the overtone CO stretch band 2ν8, CO- Stretch:CH3-rock (ν8+ν7) combination band, and the overtone asymmetric CH3-bending band 2ν4. Due to an accidental state mixing in the first excited torsional state of the ground vibrational state, many forbidden transitions have been detected in the torsional band of the OH–stretch state (ν1). Lastly, a new and elegant method has been employed to obtain the pure torsional energies of many vibrational states. The most exciting finding from these studies has been the inversion of A/E energy structure. Some explanations from the literature have been reviewed, and more work is needed to understand the coupling between torsion and vibration in methanol. The work should be helpful in the astronomical search for methanol in interstellar space.

Evaluation of anthelmintic and antioxidant efficacy of green-synthesized copper nanoparticles derived from Erioglossum rubiginosum leaf and seed aqueous extracts

The green synthesis of copper nanoparticles refers to the preparation of nanoparticles using eco-friendly and sustainable processes, often involving plant extracts, microorganisms, or other natural sources. As a sustainable approach with many potential applications, plant extracts could be used for the green synthesis of copper nanoparticles. The green synthesis of copper nanoparticles from plants, such as Erioglossum rubiginosum, involves using leaf and seed extracts to reduce copper ions to form nanoparticles. Primarily, copper nanoparticles are confirmed by visual observation of the aluminum vessel’s inner wall exhibiting a glossy, reddish-brown precipitate. Further, the UV–Vis spectrophotometer is employed to detect the presence of CuNPs. The peak is at 631 nm, which is unique to CuNPs. FTIR data show the presence of a capping agent which stabilizes copper nanoparticles. The peaks at 3,265 cm−1 for the hydroxy group (H-bonded OH stretch) indicate alcohols and phenols; 1610 cm−1 for aromatic ring stretch; and 1,075 cm−1 for aliphatic fluoro compounds (C–F stretch). DPPH reducing power assay is used to verify the antioxidant activity. Both plant extracts and the synthesized CuNPs showed considerable antioxidant activity. The seed and leaf extract of Erioglossum rubiginosum used for the green synthesis of copper nanoparticles served as a capping agent due to the presence of antioxidant phytoconstituents. Then, we used Paramphistomum cervi to evaluate the anthelmintic activity of aqueous extracts of leaves and seeds of Erioglossum rubiginosum. After that, we assessed the anthelmintic activity of the green synthesized copper nanoparticles (CuNPs). We observed that the extracts showed a significant and dose-dependent decrease in the paralysis time and death time of parasites whereas better activity was found from the synthesized CuNPs. At last, we claim that comprehensive investigation on plant extracts and CuNPs could be carried out for further study and might pave the way for development of novel therapeutic agents that show efficacy against diverse parasitic infections.

Molecular docking analysis of mosquito ribosomal protein in selenium nanoparticle bio-synthesis: Implications for pest and pathogen mitigation

BackgroundInsects, rich in diverse proteins, are crucial in nanoparticle biofabrication. This study explores mosquito proteins' potential to reduce selenium ions and produce Selenium nanoparticles (SeNPs). MethodsMosquito larval protein composition was determined via liquid chromatography-mass spectrometry (LC-MS). Molecular docking validated cytoskeletal proteins' efficacy, notably 40S ribosomal protein S7 (binding energy: -6.23) and β-tubulin (binding energy: -5.94), in facilitating selenium ion conversion to SeNPs. SeNPs, mediated by these proteins, were characterized using Ultra Violet-visible spectroscopy, revealing a peak at 295 nm. FT-IR analysis identified biomolecules responsible for capping SeNPs and converting selenious acid to SeNPs. Significant findingsLC-MS detected β-tubulin (1 peptide, 76 amino acids, 8.2 kDa) and 40S ribosomal protein S7 (2 peptides, 192 amino acids, 22.1 kDa). The FT-IR analysis showed peaks at 3420.89 (OH stretch), 2928.35 (aromatic CH stretch), 2344.02 (CC alkynes stretches), etc. X-Ray Diffraction confirmed SeNPs' crystalline structure. Scanning electron microscopy and transmission electron microscopy revealed smooth-surfaced SeNPs in spherical and rod-shaped forms. SDS PAGE analysis post-SeNPs synthesis confirmed 40S ribosomal protein S7 and β-tubulin presence. SeNPs exhibited antibacterial, anti-biofilm, antioxidant, photocatalytic, and insecticidal activities. Median lethal concentrations were 17.674 μg/mL for Culex quinquefasciatus, 31.117 μg/mL for Aedes aegypti, and 81.95 μg/mLfor R. dominica at 48 h. SeNPs suppressed S. aureus biofilm by 33.06 ± 8.26 % and E. coli by 13.8 ± 3.63 %. Vero cells showed no harm at 250 µg/mL SeNPs. At 100 μg/mL, SeNPs exhibited 52.94 % free radical scavenging, compared to 74 % for ascorbic acid. Protein-mediated nanoparticle preparation is favoured for biomedical applications due to ease of synthesis, eco-friendliness, conjugation capability, simple preparation, and utilization of pests.

Fully Quantum Simulation of Polaritonic Vibrational Spectra of Large Cavity-Molecule System.

The formation of molecular vibrational polaritons, arising from the interplay between molecular vibrations and infrared cavity modes, is a quantum phenomenon necessitating accurate quantum dynamical simulations. Here, we introduce the cavity vibrational self-consistent field/virtual state configuration interaction method, enabling quantum simulation of the vibrational spectra of many-molecule systems within the optical cavity. Focusing on the representative (H2O)21 system, we showcase this parameter-free quantum approach's ability to capture both linear and nonlinear vibrational spectral features. Our findings highlight the growing prominence of molecular couplings among OH stretches and bending excited bands with increased light-matter interaction, revealing distinctive nonlinear spectral features induced by vibrational strong coupling.

Site-Specific Photochemistry along a Protonated Peptide Scaffold.

We present a detailed study of the time-dependent photophysics and photochemistry of a known conformation of the two protonated pentapeptides Leu-enkephalin (Tyrosine-Glycine-Glycine-Phenylalanine-Leucine, YGGFL) and its chromophore-swapped analogue FGGYL, carried out under cryo-cooled conditions in the gas phase. Using ultraviolet-infrared (UV-IR) double resonance, we record excited state IR spectra as a function of time delay between UV and IR pulses. We identify unique Tyr OH stretch transitions due to the S1 state and the vibrationally excited triplet state(s) formed by intersystem crossing, Tn(v). Photofragment mass spectra are recorded out of the S1 origin and following UV-IR double resonance. Several competing site-specific fragmentation pathways are discovered involving peptide backbone cleavage, Tyr side chain loss, and N-terminal NH3 loss mediated by electron transfer. In YGGFL, IR excitation in the S1 state promotes electron transfer (ET) from the aromatic ring to the N-terminal R-NH3+ group leading to loss of neutral NH3. This product channel is missing in FGGYL due to the larger distance for ET from Y(4) to NH3+. Selective loss of the Tyr side chain occurs out of an excited state process following UV excitation and is further enhanced by IR excitation in S1 and Tn(v) states of both YGGFL and FGGYL. Finally, IR excitation in the S1 or Tn(v) states fragments the peptide backbone exclusively at amide(4), producing the b4 cation. We postulate that this selective fragmentation results from intersystem crossing to produce vibrationally excited triplets with enough energy to launch the proton along a proton conduit present in the known starting structure.

No Headache for PIPs: A PIP Potential for Aspirin Runs Much Faster and with Similar Precision Than Other Machine-Learned Potentials.

Assessments of machine-learning (ML) potentials are an important aspect of the rapid development of this field. We recently reported an assessment of the linear-regression permutationally invariant polynomial (PIP) method for ethanol, using the widely used (revised) rMD17 data set. We demonstrated that the PIP approach outperformed numerous other methods, e.g., ANI, PhysNet, sGDML, and p-KRR, with respect to precision and notably with respect to speed [Houston et al., J. Chem. Phys. 2022, 156, 044120]. Here, we extend this assessment to the 21-atom aspirin molecule, using the rMD17 data set, with a focus on the speed of evaluation. Both energies and forces are used for training, and the precision of several PIPs is examined for both. Normal mode frequencies, the methyl torsional potential, and 1d vibrational energies for an OH stretch are presented. We show that the PIP approach achieves the level of precision obtained from other ML methods, e.g., atom-centered neural network methods, linear regression ACE, and kernel methods, as reported by Kovács et al. in J. Chem. Theory Comput. 2021, 17, 7696-7711. More significantly, we show that the PIP PESs run much faster than all other ML methods, whose timings were evaluated in that paper. We also show that the PIP PES extrapolates well enough to describe several internal motions of aspirin, including an OH stretch.

Unified picture of vibrational relaxation of OH stretch at the air/water interface

The elucidation of the energy dissipation process is crucial for understanding various phenomena occurring in nature. Yet, the vibrational relaxation and its timescale at the water interface, where the hydrogen-bonding network is truncated, are not well understood and are still under debate. In the present study, we focus on the OH stretch of interfacial water at the air/water interface and investigate its vibrational relaxation by femtosecond time-resolved, heterodyne-detected vibrational sum-frequency generation (TR-HD-VSFG) spectroscopy. The temporal change of the vibrationally excited hydrogen-bonded (HB) OH stretch band (ν=1→2 transition) is measured, enabling us to determine reliable vibrational relaxation (T1) time. The T1 times obtained with direct excitations of HB OH stretch are 0.2-0.4 ps, which are similar to the T1 time in bulk water and do not noticeably change with the excitation frequency. It suggests that vibrational relaxation of the interfacial HB OH proceeds predominantly with the intramolecular relaxation mechanism as in the case of bulk water. The delayed rise and following decay of the excited-state HB OH band are observed with excitation of free OH stretch, indicating conversion from excited free OH to excited HB OH (~0.9 ps) followed by relaxation to low-frequency vibrations (~0.3 ps). This study provides a complete set of the T1 time of the interfacial OH stretch and presents a unified picture of its vibrational relaxation at the air/water interface.

Effect of Temperature on the OH-Stretching Bands of the Methanol Dimer.

We present a conceptually simple model for understanding the significant spectral changes that occur with the temperature in the infrared spectra of hydrogen-bound complexes. We have measured room-temperature spectra of the methanol dimer and two deuterated isotopologues in the OH(D)-stretching region. We correctly predict spectral changes observed in the gas phase for the bound OH stretch in the methanol dimer from jet-cooled to room temperature and corroborate this with experimental and theoretical results for deuterated isotopologues. The origin of the observed spectral features is explained based on a reduced-dimensional vibrational model, which includes the two high-frequency OH stretches, the two methyl torsions, and the six intermolecular low-frequency vibrations. Key to the success of the model is a new coordinate definition to describe the intrinsic large-amplitude curvilinear motion of low-frequency vibrations. Despite the deceivingly simple appearance of the room temperature bound OH-stretching fundamental band, it consists of ∼107 vibrational transitions.

Vibrational spectroscopy and dissociation dynamics of cyclohexyl hydroperoxide.

Organic hydroperoxides (ROOH) are ubiquitous in the atmospheric oxidation of volatile organic compounds (VOCs) as well as in low-temperature oxidation of hydrocarbon fuels. The present work focuses on a prototypical cyclic hydroperoxide, cyclohexyl hydroperoxide (CHHP). The overtone OH stretch (2νOH) spectrum of jet-cooled CHHP is recorded by IR multiphoton excitation with UV laser-induced fluorescence detection of the resulting OH products. A distinctive IR feature is observed at 7012.5 cm-1. Two conformers of CHHP are predicted to have similar stabilities (within 0.2 kcal mol-1) and overtone OH stretch transitions (2νOH), yet are separated by a significant interconversion barrier. The IR power dependence indicates that absorption of three or more IR photons is required for dissociation of CHHP to cyclohexoxy (RO) and OH radical products. Accompanying high-level single- and multi-reference electronic structure calculations quantitatively support the experimental results. Calculations are extended to a range of organic hydroperoxides to examine trends in bond dissociation energies associated with RO + OH formation and compared with prior theoretical results.

IR spectrum of SiH3OH2+SiH4: cationic OH⋯HSi dihydrogen bond versus charge-inverted SiH⋯Si hydrogen bond.

The low electronegativity of Si gives rise to a variety of nonconventional intermolecular interactions in clusters of silanes and their derivatives, which have not been well characterized yet. Herein, we characterize the structures of various isomers of bare and Ar-tagged SiH3OH2+SiH4 dimers composed of protonated silanol and silane by infrared photodissociation (IRPD) of mass-selected ions and dispersion-corrected density functional calculations (B3LYP-D3/aug-cc-pVTZ). The analysis of the IRPD spectra recorded in the OH stretch range reveals the competition between two types of nonconventional hydrogen bonds (H-bonds). The first one represents a OH⋯HSi ionic dihydrogen bond (DHB), in which SiH4 interacts with the H2O moiety of SiH3OH2+. The second one represents a charge-inverted SiH⋯Si ionic H-bond (CIHB), in which the SiH4 ligand interacts with the SiH3 moiety of SiH3OH2+. The latter may also be considered as a weak three-centre two-electron (3c-2e) bond. Although both types of H-bonds are computed to have comparable interaction strengths for SiH3OH2+SiH4 (D0 ≈ 35-40 kJ mol-1), DHB isomers dominate the population in the supersonic plasma expansion, while the abundance of CIHB isomers is roughly one order of magnitude lower, probably as a result of entropic factors.

Theoretical Study of the Two-Dimensional Vibrational Sum Frequency Generation Spectroscopy of the Air-Water Interface at Varying Temperature and Its Connections to the Interfacial Structure and Dynamics.

We performed a theoretical study of the temperature variation of two-dimensional vibrational sum frequency generation (2D-VSFG) spectra of the OH stretch modes at air-water interfaces in the mid-IR region. The calculations are performed at four different temperatures from 250 to 325 K by using a combination of techniques involving response function formalism of nonlinear spectroscopy, electronic structure calculations, and molecular dynamics simulations. Also, the calculations are performed for isotopically dilute solutions so that the intra- and intermolecular coupling between the vibrational modes of interest can be ignored. We have established the connections of temperature variation of various frequency- and time-dependent features of the calculated spectra to the changes in the underlying structure and dynamics of the interfaces. The results reveal that interfacial water is dynamically more heterogeneous than bulk water, with three dominant dynamical processes exhibiting their corresponding time-dependent features in the 2D-VSFG spectrum. These are the spectral diffusion of hydrogen-bonded OH groups at the interface, conversion of an initially hydrogen-bonded OH group to a dangling OH which is a stable state for surface water, unlike the bulk water, and the third one, which involves the conversion of an initially free or dangling OH group to its hydrogen-bonded state at the interface. The temporal appearance of the cross peaks corresponding to interconversion of the hydrogen-bonded state to the dangling state or vice versa of an interfacial OH group is found to take place at a slower rate than the dynamics of spectral diffusion of hydrogen-bonded molecules at the interface, which, in turn, is slower than the corresponding spectral diffusion of bulk water molecules. The temperature variation of these dynamic processes can be linked to the decay of appropriate hydrogen-bond and non-hydrogen-bond time correlation functions of interfacial water molecules for the different air-water systems studied in this work.

A temperature study of methanol interstellar ice analogues

Methanol ice has been studied in the field of astrochemistry for many years. As a simple organic compound, CH3OH is extremely interesting in the study of ice formation and evolution. There are three forms of methanol ice – amorphous, α-crystalline, and β-crystalline. Although there are studies focused on the comparison of crystalline and amorphous phases in terms of chemical properties, there is a lack of systematic studies on phase transition. Factors such as pressure, temperature, and rate of ice formation have an influence on the process of phase transition. In this study, the influence of temperature on the vibrational properties of methanol ice is considered. For the experimental section, a Portable Astrochemistry Chamber (PAC) with the ability to control deposition pressure and temperature was used. The temperature of phase transition was experimentally determined at 103 K and for analysis of the ice structure, two vibrational modes were chosen – OH stretch and CH3 in-plane rocking. Moreover, depositions at higher temperatures were performed with the result of different vibrational properties depending on ice formation conditions. This study presents the temperature-dependant properties of methanol ice in the mid-infrared spectrum and considers the possibility of thermal history determination.