Hydrogen Atom Tunneling Research Articles

Conformational Switching of HOCO Radical: Selective Vibrational Excitation and Hydrogen-Atom Tunneling.

Conformers of carboxyl radical (HOCO) have been studied by IR spectroscopy in argon and nitrogen matrices. In an argon matrix, only the lower-energy conformer trans-HOCO is observed, whereas both cis and trans conformers are found for deuterated carboxyl radical DOCO. In a nitrogen matrix, both conformers of HOCO and DOCO isotopologues can be prepared, indicating strong stabilization of the higher-energy cis conformer by a nitrogen matrix. Selective vibrational excitation promotes the trans-to-cis and cis-to-trans conversions of DOCO in an argon matrix and HOCO and DOCO in a nitrogen matrix, which is the first conformational photoswitching of an open-shell species. In a nitrogen matrix, the cis-to-trans and trans-to-cis conversions of HOCO is also found upon broadband IR light of the spectrometer, and the ratio of the quantum yields of these processes is about 3.3. The photoswitching peculiarities are in agreement with the available theoretical energy barriers. The higher-energy cis conformer decays to the lower-energy trans conformer via hydrogen-atom tunneling through the torsional barrier, which is also a unique observation for an open-shell species. The tunneling mechanism of the cis-to-trans switching is supported by the low-temperature limit of the reaction rate and by the H/D kinetic isotope effect. Our results suggest a large difference in the H/D kinetic isotope effects in nitrogen and argon matrices (∼5 and >100, respectively). The stabilizing effect on cis-DOCO by a nitrogen matrix (by 2 orders of magnitude versus an argon matrix) is much smaller than that on cis-HOCO (estimated to be >104).

Hydrogen Tunneling above Room Temperature Evidenced by Infrared Ion Spectroscopy.

While hydrogen tunneling at elevated temperatures has, for instance, often been postulated in biochemical processes, spectroscopic proof is thus far limited to cryogenic conditions, under which thermal reactivity is negligible. We report spectroscopic evidence for H-tunneling in the gas phase at temperatures around 320-350 K observed in the isomerization reaction of a hydroxycarbene into an aldehyde. The charge-tagged carbene was generated in situ in a tandem mass spectrometer by decarboxylation of oxo[4-(trimethylammonio)phenyl]acetic acid upon collision induced dissociation. All ion structures involved are characterized by infrared ion spectroscopy and quantum chemical calculations. The charge-tagged phenylhydroxycarbene undergoes a 1,2-H-shift to the corresponding aldehyde with an half-life of about 10 s, evidenced by isomer-selective two-color (IR-IR) spectroscopy. In contrast, the deuterated (OD) carbene analogue showed much reduced 1,2-D-shift reactivity with an estimated half-life of at least 200 s under the experimental conditions, and provides clear evidence for hydrogen atom tunneling in the H-isotopologue. This is the first spectroscopic confirmation of hydrogen atom tunneling governing 1,2-H-shift reactions at noncryogenic temperatures, which is of broad significance for a range of (bio)chemical processes, including enzymatic transformations and organocatalysis.

Structure-dependent reactivity of Criegee intermediates studied with spectroscopic methods.

Criegee intermediates are very reactive carbonyl oxides that are formed in reactions of unsaturated hydrocarbons with ozone (ozonolysis). Recently, Criegee intermediates have gained significant attention since a new preparation method has been reported in 2012, which employs the reaction of iodoalkyl radical with molecular oxygen: for instance, CH2I + O2 → CH2OO + I. This new synthesis route can produce Criegee intermediates with a high number density, which allows direct detection of the Criegee intermediate via various spectroscopic tools, including vacuum UV photoionization mass spectrometry, absorption and action spectroscopy in the UV and IR regions, and microwave spectroscopy. Criegee intermediates have been thought to play important roles in atmospheric chemistry, such as in OH radical formation as well as oxidation of atmospheric gases such as SO2, NO2, volatile organic compounds, organic and inorganic acids, and even water. These reactions are relevant to acid rain and aerosol formation. Kinetics data including rate coefficients, product yields and their temperature and pressure dependences are important for understanding and modeling relevant atmospheric chemistry. In fundamental physical chemistry, Criegee intermediates have unique and interesting features, which have been partially revealed through spectroscopic, kinetic, and dynamic investigations. Although previous review articles have discussed Criegee intermediates, new data and knowledge on Criegee intermediates are still being accumulated. In this tutorial review, we have focused on structure-dependent reactivity of Criegee intermediates and various spectroscopic tools that have been utilized to probe the kinetics of Criegee intermediates.

Rate constants and isotope effects for the reaction of H-atom abstraction from RH substrates by PINO radicals

The kinetics of the reactions of hydrogen atom abstraction from the C–H bonds of substrates of different structures by phthalimide-N-oxyl radicals is studied. The rate constants of this reaction are measured and the kinetic isotope effects are determined. It is shown that in addition to the thermodynamic factor, Coulomb forces and donor–acceptor interactions affect the reaction between phthalimide-N-oxyl radicals and substrate molecules, altering the shape of the transition state. This favors the tunneling of hydrogen atoms and leads to a substantial reduction in the activation energy of the process.

Tunneling isomerization of small carboxylic acids and their complexes in solid matrixes: a computational insight.

We have studied hydrogen-atom tunneling in the cis-to-trans conformational change of some carboxylic acid monomers and formic acid (FA) complexes and dimers at the MP2(full) and CCSD(T) levels of theory within the Wentzel-Kramers-Brillouin approximation. The barrier for the minimum energy path, where the OH bond length and the COH bending angle are optimized, is found to be a good approximation for providing the highest barrier transparency. The matrix effect on the transmission coefficients of cis-FA monomer, trans-cis FA dimer (tc1), and cis-acetic acid monomer are modeled by the polarizable continuum model (PCM) at the MP2(full) level of theory in different environments. For the cis-FA monomer and trans-cis FA dimer (tc1), the calculated transmission coefficients agree with the experimental lifetimes observed in noble-gas solids. However, this method cannot reproduce the experimental results obtained for cis-acetic acid. Moreover, the long lifetime of cis-FA and cis-acetic acid in the N2 environment cannot be reproduced either, which is most probably due to specific interactions that are not included in the PCM. The calculation for cis-HCOOD shows a strong decrease of the barrier transparency compared to that for cis-HCOOH, which is consistent with the experiments. In general, good agreement is observed between the calculated barrier transparency (including PCM) and experimental tunneling rate. However, some exceptions are found, which shows that additional factors influence the tunneling rate.

Understanding the Mechanisms of Unusually Fast HH, CH, and CC Bond Reductive Eliminations from Gold(III) Complexes

Carbon-carbon bond reductive elimination from gold(III) complexes are known to be very slow and require high temperatures. Recently, Toste and co-workers have demonstrated extremely rapid CC reductive elimination from cis-[AuPPh3 (4-F-C6 H4 )2 Cl] even at low temperatures. We have performed DFT calculations to understand the mechanistic pathway for these novel reductive elimination reactions. Direct dynamics calculations inclusive of quantum mechanical tunneling showed significant contribution of heavy-atom tunneling (>25 %) at the experimental reaction temperatures. In the absence of any competing side reactions, such as phosphine exchange/dissociation, the complex cis-[Au(PPh3 )2 (4-F-C6 H4 )2 ](+) was shown to undergo ultrafast reductive elimination. Calculations also revealed very facile, concerted mechanisms for HH, CH, and CC bond reductive elimination from a range of neutral and cationic gold(III) centers, except for the coupling of sp(3) carbon atoms. Metal-carbon bond strengths in the transition states that originate from attractive orbital interactions control the feasibility of a concerted reductive elimination mechanism. Calculations for the formation of methane from complex cis-[AuPPh3 (H)CH3 ](+) predict that at -52 °C, about 82 % of the reaction occurs by hydrogen-atom tunneling. Tunneling leads to subtle effects on the reaction rates, such as large primary kinetic isotope effects (KIE) and a strong violation of the rule of the geometric mean of the primary and secondary KIEs.

Hydroxyl radical recycling in isoprene oxidation driven by hydrogen bonding and hydrogen tunneling: the upgraded LIM1 mechanism.

The Leuven isoprene mechanism, proposed earlier to aid in rationalizing the unexpectedly high hydroxyl radical (OH) concentrations in isoprene-rich, low-nitric-oxide (NO) regions ( Peeters ; et al. Phys. Chem. Chem. Phys . 2009 , 11 , 5935 ), is presented in an upgraded and extended version, LIM1. The kinetics of the crucial reactions in the proposed isoprene-peroxy radical interconversion and isomerization pathways are re-evaluated theoretically, on the basis of energy barriers computed at the much higher CCSD(T)/aug-cc-pVTZ//QCISD/6-311G(d,p) level of theory, and using multiconformer partition functions obtained at the M06-2X/6-311++G(3df,2p) level that, different from the B3LYP level used in our earlier work, accounts for the crucial London dispersion effects in the H-bonded systems involved. The steady-state fraction of the specific Z-δ-OH-peroxy radical isomers/conformers that can isomerize by a 1,6-H shift is shown to be largely governed by hydrogen-bond strengths, whereas their isomerization itself is found to occur quasi-exclusively by hydrogen atom tunneling. The isomer-specific Z-δ-OH-peroxy 1,6-H-shift rate coefficients are predicted to be of the order of 1 s(-1) at 298 K, but the experimentally accessible bulk rate coefficients, which have to be clearly distinguished from the former, are 2 orders of magnitude lower due to the very low Z-δ-OH-peroxy steady-state fractions that are only around or below 0.01 at low to moderate NO and depend on the peroxy lifetime. Two pathways subsequent to the peroxy radical 1,6-H shift are identified, the earlier predicted route yielding the photolabile hydroperoxy-methylbutenals (HPALDs), and a second, about equally important path, to dihydroperoxy-carbonyl peroxy radicals (di-HPCARP). Taking this into account, our predicted bulk peroxy isomerization rate coefficients are about a factor 1.8 higher than the available experimental results for HPALD production ( Crounse ; et al. Phys. Chem. Chem. Phys. 2011 , 13 , 13607 ), which is within the respective uncertainty margins. We also show that the experimental temperature dependence of the HPALD production rates as well as the observed kinetic isotope effect for per-deuterated isoprene support quantitatively our theoretical peroxy interconversion rates. Global modeling implementing LIM1 indicates that on average about 28% of the isoprene peroxys react via the 1,6-H-shift isomerization route, representing 100-150 Tg carbon per year. The fast photolysis of HPALDs we proposed earlier as primary OH regeneration mechanism ( Peeters and Muller . Phys. Chem. Chem. Phys . 2010 , 12 , 14227 ) found already experimental confirmation ( Wolfe ; et al. Phys. Chem. Chem. Phys. 2012 , 14 , 7276 ); based on further theoretical work in progress, reaction schemes are presented of the oxy coproduct radicals from HPALD photolysis and of the di-HPCARP radicals from the second pathway following peroxy isomerization that are both expected to initiate considerable additional OH recycling.

Reaction of atomic hydrogen with formic acid

We study the reaction of atomic hydrogen with formic acid and characterize the radical products using IR spectroscopy in a Kr matrix and quantum chemical calculations. The reaction first leads to the formation of an intermediate radical trans-H2COOH, which converts to the more stable radical trans-cis-HC(OH)2via hydrogen atom tunneling on a timescale of hours at 4.3 K. These open-shell species are observed for the first time as well as a reaction between atomic hydrogen and formic acid. The structural assignment is aided by extensive deuteration experiments and ab initio calculations at the UMP2 and UCCSD(T) levels of theory. The simplest geminal diol radical trans-cis-HC(OH)2 identified in the present work as the final product of the reaction should be very reactive, and further reaction channels are of particular interest. These reactions and species may constitute new channels for the initiation and propagation of more complex organic species in the interstellar clouds.

Direct evidence of hydrogen-atom tunneling dynamics in the excited state hydrogen transfer (ESHT) reaction of phenol–ammonia clusters

The photodynamics of phenol-ammonia clusters, PhOH(NH3)(3-5), after one UV photon absorption, has been investigated using velocity map imaging of the NH4(NH3)(2-4) cluster products. The dependence of the NH4(NH3)2 translational energy distributions on the available energy reveals three dynamical regions in close correspondence with the photodissociation of bare phenol. At low excitation energies (between 282 and 260 nm), the NH4(NH3)2 distribution mirrors the hydrogen-atom passage through the 1(1)ππ*-1(1)πσ* barrier, constituting the first evidence of hydrogen-atom tunneling dynamics in an excited state hydrogen transfer (ESHT) reaction. At excitation wavelengths below 260 nm, the product distributions are consistent with two separate barrierless dissociation processes associated, respectively, with excitation to the 1(1)ππ* and 2(1)ππ* excited electronic states. Similar conclusions can be derived from the velocity map imaging results on the larger NH4(NH3)(3,4) cluster products.

Theory and simulation of atom tunneling in chemical reactions

Quantum tunneling of atoms, the penetration of energy barriers higher than the total energy of the system, plays a role in many chemical systems. While any chemical reaction is dominated by tunneling at low enough temperature, there is evidence for hydrogen atom tunneling even in enzymatic reactions at ambient conditions. The smaller the mass of the atoms, the lower and thinner the barrier is, the stronger the tunneling effect increases the reaction rate. Different methods to calculate tunneling rates are available. They range from full solutions of the time‐dependent Schrödinger equation via the semiclassical method to ad hoc corrections of classical transition state theory. The basis of different methods, their accuracy, and applicability is discussed in the present overview, with a particular focus on instanton theory, a Feynman‐path‐based approach using the semiclassical approximation.This article is categorized under: Theoretical and Physical Chemistry > Reaction Dynamics and Kinetics

H/D kinetic isotope effect in HCOOH cis–trans conversion of formic acid in noble gas matrices

Intramolecular tunneling of hydrogen and deuterium atoms in noble gas matrices (Ar, Kr and Xe) has been studied theoretically for cis–trans isomerization of formic acid at low temperatures. The temperature dependence of the tunneling rate constant and quantum kinetic isotope effect taking into account the media reorganization upon the H/D atomic transfer is described in terms of the modified theory of radiationless transitions. The theoretical results are found to be in a reasonable agreement with the literature experimental data on the rate constants and the anomalous quantum kinetic H/D isotope effect.

Tunneling Lifetime of the ttc/VIp Conformer of Glycine in Low-Temperature Matrices

Conformer ttc/VIp of glycine and glycine-N,N,O-d(3) has been prepared in low-temperature Ar, Kr, Xe, and N(2) matrices by near-infrared (NIR) laser irradiation of the first OH stretching overtone of conformer ttt/Ip. Glycine (and glycine-N,N,O-d(3)) ttc/VIp was found to convert back to ttt/Ip in the dark by hydrogen-atom tunneling. The observed half-lives of ttc/VIp in Ar, Kr, and Xe matrices at 12 K were 4.4 ± 1 s (50.0 ± 1 h), 4.0 ± 1 s (48.0 ± 1 h), and 2.8 ± 1 s (99.3 ± 2 h), respectively. In correspondence with the observation for the cis-to-trans conversion of formic and acetic acid, the tunneling half-life of glycine ttc/VIp in a N(2) matrix is more than 3 orders of magnitude longer (6.69 × 10(3) and 1.38 × 10(4) s for two different sites) than in noble gas matrices due to complex formation with the host molecules. The present results are important to understand the lack of experimental observation of some computationally predicted conformers of glycine and other amino acids.

ChemInform Abstract: Atom Tunneling in Organic Transformations.

AbstractA short list of illustrative examples of hydrogen and deuterium atom tunneling is highlighted.

Tunneling proton transfer in biological systems. Role of temperature and pressure

The possibilities of hydrogen atom tunneling transfer in biological liquids are discussed. Basic mechanisms of temperature and pressure effects on the tunneling rate constant are considered: the reorganization of reagents and the medium due to the transfer of H atoms and changes in the value and shape of the chemical reaction potential barrier upon intermolecular and soft intramolecular vibrations. Expressions are derived for the tunneling transition rate constant and kinetic isotopic effect as functions of temperature and pressure. It is found that the temperature dependence of the isotope effect is mainly affected by the second mechanism only. The theory is compared with the literature’s experimental data on the temperature dependence of the isotope effect. It is shown that experiments are described well by the theory at sensible values of the fitting parameters.

Near-Infrared Radiation Induced Conformational Change and Hydrogen Atom Tunneling of 2-Chloropropionic Acid in Low-Temperature Ar Matrix

Former assignments of the matrix-isolation infrared (MI-IR) spectrum of 2-chloropropionic acid are revised with the help of near-infrared (NIR) laser irradiation induced change in conformer ratios. This method allows not only the unambiguous assignment of each band in the MI-IR spectrum to the two trans (Z) and the cis (E) conformers but also the assignment of the spectral bands to different matrix sites. The tunneling decay of the higher-energy cis conformer prepared from both trans conformers in different sites is also investigated. It is shown that the tunneling decay time is very sensitive to the matrix site, especially if the in situ prepared high-energy conformer has a strained geometry in the matrix cage. The analysis shows that the kinetics of some cis → trans back conversion processes cannot be fitted by a single exponential decay. The possible reasons of this observation are examined and discussed. The present and former results clearly show that, in addition to tunneling processes, the decay rates strongly depend on solid-state effects. Therefore, simple theoretical predictions of decay rates, which do not take into account the solid-state effects, can only be compared to experimental observations only if experimentally proven that these effects do not significantly affect the experimentally measured tunneling rates.

Intramolecular hydrogen-atom tunneling and photoreaction mechanism of 4-bromo-2-chloro-6-fluorophenol in low-temperature argon matrices

Infrared spectra of 4-bromo-2-chloro-6-fluorophenol in low-temperature argon matrices were measured with an FTIR spectrometer. Despite that an F-type geometrical isomer, which has an intramolecular hydrogen bond of OH⋯F, is as stable in the CCl4 solution at room temperature as a Cl-type geometrical isomer, which has that of OH⋯Cl, no infrared bands of the F-type were observed in an argon matrix at 20K. This implied that the F-type changed to the more stable Cl-type by hydrogen-atom tunneling in the argon matrix. To confirm the tunneling isomerization, similar experiments on the deuterated species were performed, resulting in that an OD stretching band of the F-type appeared as well as that of the Cl-type. This finding suggested that a deuterium is inhibited from the tunneling migration. The UV-induced photoreaction pathways of 4-bromo-2-chloro-6-fluorophenol were additionally examined, resulting in that 2-fluoro-4-bromocyclopentadienylidenemethanone was produced from the Cl-type by Wolff rearrangement after dissociation of the H atom in the OH group and the Cl atom. However no cyclohexadienone derivative, which is analogous to a photoproduct of a parent molecule, 2-chloro-6-fluorophenol, was produced upon UV irradiation. It is concluded that the bromine atom located at the 4th position disturbs the intramolecular migration of the Cl atom.

Hydrogen-Atom Tunneling in Isomerization Around the C–O Bond of 2-Chloro-6-Fluorophenol in Low-Temperature Argon Matrixes

Infrared spectra of 2-chloro-6-fluorophenol in argon matrixes at 20 K revealed the presence of a "Cl-type" isomer, which has the OH···Cl hydrogen bond, but no "F-type" isomer with OH···F bonding, in striking contrast to the existence of both isomers in the gas and liquid phases at room temperature. This finding suggests that the F-type isomer changes to the more stable Cl-type one by hydrogen-atom tunneling in the matrixes. Similar experiments on the OD···X analog species were performed to confirm the tunneling isomerization, resulting in an O-D stretching band of the F-type isomer appearing as well as that of the Cl type, like the spectra reported in the gas and liquid phases. This implies that tunneling migration of the D atom is inhibited in the argon matrix. In addition, UV-induced photoreactions of 2-chloro-6-fluorophenol were studied by a joint use of matrix-isolation IR spectroscopy assisted by density functional theory calculations. It was found that 2-fluorocyclopentadienylidenemethanone and 4-chloro-2-fluorocyclohexadienone were produced from the Cl type; the former was by the Wolff rearrangement after dissociation of the H atom in the OH group and the Cl atom, and the latter was by intramolecular migration of the H and Cl atoms. As for the deuterated F-type isomer, however, 2-chlorocyclopentadienylidenemethanone was produced by the Wolff rearrangement after dissociation of the D atom in the OD group and the F atom, besides other photoproducts of the deuterated Cl-type isomer. It is thus concluded that the tunneling isomerization around the C-O bond occurs in the OH···X species but not in the OD···X species.

Quantum Tunneling of Hydrogen Atom in Dissociation of Photoexcited Methylamine

The probability of hydrogen atom release, following photoexcitation of methylamine, CH(3)NH(2), is found to increase extensively as higher vibrational states on the first excited electronic state are accessed. This behavior is consistent with theoretical calculations, based on the probability of H atom tunneling through an energy barrier on the excited potential energy surface, implying that N-H bond breaking is dominated by quantum tunneling.

Matrix reorganization with intramolecular tunneling of H atom: Formic acid in Ar matrix

The intramolecular tunneling of hydrogen atom in formic acid HCOOH at low temperatures is considered based on literature experimental data on C-O internal rotation. The energetic and geometric parameters as well as vibrational frequencies for formic acid in cis and trans configurations surrounded by 12 Ar atoms are calculated in the frame of the MP2 approach with extended basis sets. The temperature and pressure dependence of the rate constant is analyzed taking into consideration the matrix reorganization for the Debye model of lattice motion. It has been shown that the available experimental data can be explained by the suggested matrix reorganization mechanism. Theoretical expressions for the temperature dependence of the rate constant agree well with the experimental data on the cis to trans tunneling reactions in formic acid with fitting parameters attaining reasonable values. A mechanism describing pressure dependence of the rate constant for H-atom intramolecular tunneling reactions is also proposed.

Tunneling dynamics of a hydroxyl group adsorbed on Cu(110)

Dynamics of a hydroxyl (OH) group and its dimer adsorbed on Cu(110) is investigated at 6 K with a scanning tunneling microscope (STM). For the monomer, the inclined OH axis switches back and forth between the two equivalent orientations via hydrogen-atom tunneling. The motion is enhanced by tunneling electron that excites the OH bending mode that directly correlates with the reaction coordinate. The inelastic electron-tunneling spectra exhibit either peak or dip, depending on the position of the STM tip over the molecule. The switch motion is quenched for the hydroxyl dimer since it is stabilized by hydrogen-bond formation, whereas it is induced by inelastic excitation of the OH stretch mode.