Single Hydrogen Transfer Research Articles

Unusual effects in single molecule tautomerization: hemiporphycene.

Parent hemiporphycene, a recently obtained constitutional isomer of porphyrin, exists in room temperature solutions and polymer matrices in the form of two trans tautomers interconverting via double hydrogen transfer. Using confocal fluorescence microscopy, it was possible to monitor tautomerization in single hemiporphycene molecules embedded in a PMMA film by monitoring the spectral and temporal evolution of their fluorescence spectra. The emission spectra of the two tautomeric forms are similar to those obtained from ensemble studies. However, the analysis of temporal spectral evolution reveals effects not detected in the bulk. For some single molecules, a large decrease of tautomerization rate was observed. This is interpreted as an indication of multidimensional character of the tautomerization coordinate and coupling of the reaction with the polymer relaxation processes. In addition, fluorescence lifetimes obtained for single molecules are significantly shorter than those measured for the bulk. It is proposed that the shortening is caused by environment-induced distortion of the molecule, which enhances the S0← S1 internal conversion rate by lowering the barrier to excited state single hydrogen transfer. This effect seems to reflect the specific morphology of thin (30 nm) polymer samples, because it is not observed in ensemble studies carried out using thick (tens of micrometers or more) PMMA films.

Role of Double Hydrogen Atom Transfer Reactions in Atmospheric Chemistry.

Hydrogen atom transfer (HAT) reactions are ubiquitous and play a crucial role in chemistries occurring in the atmosphere, biology, and industry. In the atmosphere, the most common and traditional HAT reaction is that associated with the OH radical abstracting a hydrogen atom from the plethora of organic molecules in the troposphere via R-H + OH → R + H2O. This reaction motif involves a single hydrogen transfer. More recently, in the literature, there is an emerging framework for a new class of HAT reactions that involves double hydrogen transfers. These reactions are broadly classified into four categories: (i) addition, (ii) elimination, (iii) substitution, and (iv) rearrangement. Hydration and dehydration are classic examples of addition and elimination reactions, respectively whereas tautomerization or isomerization belongs to a class of rearrangement reactions. Atmospheric acids and water typically mediate these reactions. Organic and inorganic acids are present in appreciable levels in the atmosphere and are capable of facilitating two-point hydrogen bonding interactions with oxygenates possessing an hydroxyl and/or carbonyl-type functionality. As a result, acids influence the reactivity of oxygenates and, thus, the energetics and kinetics of their HAT-based chemistries. The steric and electronic effects of acids play an important role in determining the efficacy of acid catalysis. Acids that reduce the steric strain of 1:1 substrate···acid complex are generally better catalysts. Among a family of monocarboxylic acids, the electronic effects become important; barrier to the catalyzed reaction correlates strongly with the pKa of the acid. Under acid catalysis, the hydration of carbonyl compounds leads to the barrierless formation of diols, which can serve as seed particles for atmospheric aerosol growth. The hydration of sulfur trioxide, which is the principle mechanism for atmospheric sulfuric acid formation, also becomes barrierless under acid catalysis. Rate calculations suggest that such acid catalysis play a key role in the formation of sulfuric acid in the Earth's stratosphere, Venusian atmosphere, and on heterogeneous surfaces. Over the past few years, theoretical calculations have shown that these acid-mediated double hydrogen atom transfers are important in the chemistry of Earth's atmosphere as well as that of other planets. This Account reviews and puts into perspective some of these atmospheric HAT reactions and their environmental significance.

Nonadiabatic dynamics of a truncated indigo model

Indigo (1) is stable when exposed to ultraviolet light. We employ electronic structure calculations and nonadiabatic trajectory surface-hopping dynamics simulations to study the photoinduced processes and the photoprotection mechanism of an indigo model, bispyrroleindigo (2). Consistent with recent static ab initio calculations on 1 and 2 (Phys. Chem. Chem. Phys., 2011, 13, 1618), we find an efficient deactivation process that proceeds as follows. After vertical photoexcitation, the S(1)(ππ*) state undergoes an essentially barrierless intramolecular single proton transfer and relaxes to the minimum of an S(1) tautomer, which is structurally and energetically close to a nearby conical intersection that acts as a funnel to the S(0) state; after this internal conversion, a reverse single hydrogen transfer leads back to the equilibrium structure of the most stable S(0) tautomer. This deactivation process is completely dominant in our semiempirical OM2/MRCI nonadiabatic dynamics simulations. The other two mechanisms considered previously, namely excited-state intramolecular double proton transfer and trans-cis double bond isomerization, are not seen in any of the 325 trajectories of the present surface-hopping simulations. On the basis of the computed time-dependent populations of the S(1) state, we estimate an S(1) lifetime of about 700 fs for 2 in the gas phase.

Unusual, Solvent Viscosity-Controlled Tautomerism and Photophysics: Meso-Alkylated Porphycenes

Stationary and time-resolved studies of 9,10,19,20-tetramethylporphycene and 9,10,19,20-tetra-n-propylporphycene in condensed phases reveal the coexistence of trans and cis tautomeric forms. Two cis configurations, cis-1 and cis-2, play a crucial role in understanding the excited-state deactivation and tautomer conversion dynamics. The trans-trans tautomerization, involving intramolecular transfer of two hydrogen atoms, is extremely rapid (k ≥ 10(13) s(-1)), both in the ground and lowest electronically excited states. The cis-1-trans conversion rate, even though the process is thermodynamically more favorable, is much slower and solvent-dependent. This is explained by the coupling of alkyl group rotation with the hydrogen motion. Excited-state deactivation is controlled by solvent viscosity: the S(1) depopulation rate decreases by more than 2 orders of magnitude when the chromophore is transferred from a low-viscosity solution to a polymer film. Such behavior confirms a model for excited state deactivation in porphycene, which postulates that a conical intersection exists along the single hydrogen transfer path leading from the trans to a high energy cis-2 tautomeric form. For this process, the tautomerization coordinate includes not only hydrogen translocation but also large-amplitude twisting of the two protonated pyrrole moieties attached to the opposite sides of the ethylene bridge.

A theoretical study of salicylate oxidation for ADME prediction

The salicylic acid oxidation has been explained by a mechanism involving single electron transfer oxidation and single hydrogen transfer using quantum chemistry calculations at the B3LYP theory level, together with the 6-311G** basis set. These methods were employed to obtain energy (E), ionization potential (IP), bond dissociation energies (BDE), and spin density distribution of the salicylic acid. The results show no discrepant behaviors between electron and hydrogen transfer in the regioselective hydroxylation of salicylic acid by cytochrome P-450. The unpaired electron remains localized on the O7 phenolic oxygen (0.26 and 0.38), C1 carbon atoms at the carbonyl group (0.12 and 0.28), C2 carbon atom at the hydroxyl group (0.15 and 0.00), C3 carbon atom at the hydroxyl group (0.22 and 0.30), and C5 carbon atom (0.40 and 0.41) for cation free radical and semiquinone form, respectively. Calculations of spin densities showed that chemistry reactivity is more favored in the positions C5 > C3 > C1 to form salicylate derivatives. These results supported the salicylic acid as scavenger derivatives in the lipid peroxidation. Furthermore, we suggest a conventional proton and secondary electron abstraction, and semiquinone form by [1,5] hydrogen shift between phenol and carbonyl groups.

Investigation of Reverse-Hydrogen Spillover on Zeolite-Supported Palladium Tetramer by ONIOM Method

The present study explores favorable chemical state of zeolite-supported Pd4 clusters using the ONIOM (our-own-N-layered integrated molecular orbital + molecular mechanics) approach, as implemented in the Gaussian03 code. Lowest-energy structures of Pd4 in the gas phase as well as on a zeolite support are found to be in the triplet state. The calculations reveal that reverse hydrogen spillover from bridging OH groups of zeolite support onto Pd4 results in a hydrogenated Pd4H/Zeo((m − 1)H), m = 1−3 species, which are energetically preferable over the bare zeolite-supported form of Pd4/Zeo(mH). The process of single hydrogen transfer from the zeolite support to the Pd4 cluster is found to be exothermic and more favorable than two- and three-proton transfer processes.

Hydrogen Transfer between Sulfuric Acid and Hydroxyl Radical in the Gas Phase: Competition among Hydrogen Atom Transfer, Proton-Coupled Electron-Transfer, and Double Proton Transfer

In an attempt to assess the potential role of the hydroxyl radical in the atmospheric degradation of sulfuric acid, the hydrogen transfer between H2SO4 and HO* in the gas phase has been investigated by means of DFT and quantum-mechanical electronic-structure calculations, as well as classical transition state theory computations. The first step of the H2SO4 + HO* reaction is the barrierless formation of a prereactive hydrogen-bonded complex (Cr1) lying 8.1 kcal mol(-1) below the sum of the (298 K) enthalpies of the reactants. After forming Cr1, a single hydrogen transfer from H2SO4 to HO* and a degenerate double hydrogen-exchange between H2SO4 and HO* may occur. The single hydrogen transfer, yielding HSO4* and H2O, can take place through three different transition structures, the two lowest energy ones (TS1 and TS2) corresponding to a proton-coupled electron-transfer mechanism, whereas the higher energy one (TS3) is associated with a hydrogen atom transfer mechanism. The double hydrogen-exchange, affording products identical to reactants, takes place through a transition structure (TS4) involving a double proton-transfer mechanism and is predicted to be the dominant pathway. A rate constant of 1.50 x 10(-14) cm(3) molecule(-1) s(-1) at 298 K is obtained for the overall reaction H2SO4 + HO*. The single hydrogen transfer through TS1, TS2, and TS3 contributes to the overall rate constant at 298 K with a 43.4%. It is concluded that the single hydrogen transfer from H2SO4 to HO* yielding HSO4* and H2O might well be a significant sink for gaseous sulfuric acid in the atmosphere.

The role of ion–neutral complexes in the reactions of onium ions and related species

AbstractAn account is given of the development of the proposal that ion–neutral complexes are involved in the unimolecular reactions of onium ions (R1R2CZ+R3; Z = O, S, NR4; R1, R2, R3, R4 = H, CnH2n + 1), with particular emphasis on the informative C4H9O+ oxonium ion system (Z = O; R1, R2 = H; R3 = C3H7). Current ideas on the role of ion‐neutral complexes in cation rearrangements, hydrogen transfer processes and more complex isomerizations are illustrated by considering the behaviour of isomeric CH3CH2CH2X+ and (CH3)2CHX+ species [X = CH2O, CH3CHO, H2O, CH3OH, NH3, NH2CH3, NH(CH3)2, CH2NH, CH2NCH3, CO, CH3˙, Br˙ and I˙]. Attention is focused on the importance of four energetic factors (the stabilization energy of the ion–neutral complex, the energy released by rearrangement of the cationic component, the enthalpy change for proton transfer between the partners of the ion neutral complex and the ergicity of recombination of the components) which influence the reactivity of the complexes. The nature and extent of the chemistry involving ion‐neutral complexes depend on the relative magnitudes of these parameters. Thus, when the magnitude of the stabilization energy exceeds the energy released by cation rearrangement, the ergicity of proton transfer is small, and recombination of the components in a new way is energetically favourable, extensive complex‐mediated isomerizations tend to occur. Loss of H2O from metastable CH2O+C3H7 ions is an example of such a reaction. Conversely, if the stabilization energy is small compared with the magnitude of the energy released by eation rearrangement, the opportunities for complex‐mediated processes to become manifest are decreased, especially if proton transfer is endoergic. Thus, CH3CH2CH2CO+ expels CO, with an increased kinetic energy release, after rate‐limiting isomerization of CH3CH2CH2+CO to (CH3)2CH+CO has taken place. When proton transfer between the components of the complex is strongly exoergic, fragmentation corresponding to single hydrogen transfer occurs readily. The proton‐transfer step is often preceded by cation rearrangement for CH3CH2CH2X+ species. In such circumstances, the involvement of ion–neutral complexes can be detected by the observation of unusual site selectivity in the hydrogen‐transfer step. Thus, C3H6 loss from CH2N+(R1)CH2CH2CH3 (R1 = H, CH3, C3H7) immonium ions is found by 2H‐labelling experiments to proceed via preferential α‐and γ‐hydrogen transfer; this finding is explained if the incipient +CH2CH2CH3 ion isomerizes to CH3CH+CH3 prior to proton abstraction. In contrast, the isomeric CH2N+(R1)CH(CH3)2 species undergo specific β‐hydrogen transfer because the developing CH3CH+CH3 cation is stable with respect to rearrangements involving a 1,2‐H shift.

Multimode approach to hydrogen tunneling

A procedure is developed to improve the quantitative evaluation of hydrogen transfer rates in polyatomic molecules and solids. The aim is to introduce a dynamical model that includes explicitly all vibrations participating in the transfer. This aim favors adoption of the golden-rule approach, since it treats all vibrational modes equally. To simplify the resulting multidimensional transfer integrals, two basic assumptions are introduced: (i) adiabatic separability of the hydrogenic modes directly involved in the tunneling from the other modes, and (ii) negligible anharmonicity for the latter. The number of effectively participating modes can then be reduced drastically by transformation to an appropriate local representation which allows analytical integration over most of these other modes. Those that remain involve vibrations of the atoms between which the hydrogen is transferred. Their frequency, reduced mass and displacement are expressed in terms of the harmonic force field of the system before and after transfer and can be unambiguously evaluated if these force fields are available. These modes replace the empirical effective modes used previously. The theory is applied successfully to single hydrogen transfer in dimethyl-glyoxime and double hydrogen transfer in porphine.

A mass spectral study of cyclophosphamide with special reference to differences in isomeric structures

Mass spectrometric fragmentation of cyclophosphamide (CP) has been studied in detail using linked field scanning, isotope labelling, low energy ionization and accurate mass measurements. The molecular ion is of low abundance and it favours loss of HCI and ·CH2CI to form the base peak at m/z 211. All the other ions are formed by sequential fragmentation processes. The direct bond rupture between phosphorus and the N-chloroethyl group produces two ions m/z 120 and m/z 92. The ion m/z 56 is a triplet. Many of the ions in the spectrum are produced by several competing pathways. Several of the rearrangement mechanisms involve a single hydrogen transfer. Quantitative differences between two commercial CP preparations were observed in the linked field scanning spectra. Differences in the abundances of the metastable ions strongly suggest that there is an intense isomeric competition in the decomposition pathways. The experimental evidence supports the conclusion that the two CP preparations differ in their isomeric composition and these differences are related to their stereoisomers. The rates of the individual fragmentation pathways are dependent upon the isomeric structure of the parent molecule and might also be reflected by different rates of metabolism of the isomers of the drug in vivo.

Intramolecular single and double hydrogen migrations in the mass spectra of substituted tetrahydro‐1,3,2‐oxazaphosphorin‐2‐oxides

AbstractThe mass spectral fragmentation of substituted tetrahydro‐1,3,2‐oxazaphosphorin‐2‐oxides occurs by the cleavage of ring bonds. The ions due to simple cleavage, single and double hydrogen migration, seem to be triggered by CO bond cleavage of the oxazaphosphorin ring. The variations in the relative abundance of ions arising due to similar fragmentation modes have been found to depend on the nature of the substituent and the stability of the particular fragment. The single hydrogen transfer process is supported by metastable ion and shift techniques.

Electron impact mass spectrometry of some 2,4,6-substituted s-triazines. Effect of α-and β-substitution

Comparative study of the effect of α-and β-substituents on the fragmentation pattern of two series of 2,4-bis ( N-alkylamino)-6-chloro- s-triazines ( N-alkyl being α-phenylethyl or β-phenylethyl) show that some decomposition pathways differ substantially from those of di- N-alkylamino derivatives (ref.1) and that the position of a substituent greatly influences mass spectra. In α-series C 2N bond cleavage with the loss of s-triazinyl radical to form the base peak appears to be unique. The main characterist ics of β-series are the cleavage of NC α bond with the loss of s-triazinylamino radical and α-cleavage to an amino group and the loss of p-substituted benzyl radical. The rearrangement with single hydrogen transfer and production of an ionized alken product is significant. McLafferty ions abundances can be neglected while in α-series they are rather strong.

Hydrogen rearrangements in ionized alkanoic acids. An ion lifetime study

Single and double hydrogen rearrangements involving β-bond cleavage have been studied in ionized butanoic, pentanoic and hexanoic acids specifically deuterium labelled at the hydroxy group and all carbon atoms of the side chains. The reactions have been studied in the range 6×10 −11-10 −5 s after ionization using the field ionization kinetic technique. At short ion lifetimes ( r<10 −10s) n-butanoic acid shows exclusively a single hydrogen rearrangement which specifically involves the γ-hydrogen and which is followed by β-bond cleavage. At longer ion lifetimes ( t>10 −8s) this rearrangement is preceded by hydrogen exchange between the hydroxylic hydrogen and the β- and γ-hydrogens as found previously. In ionized n-pentanoic and n-hexanoic acids, double hydrogen transfer with β-bond cleavage competes effectively with single hydrogen transfer at long ion lifetimes. At short ion lifetimes (< 10 −10s) single hydrogen rearrangement in n-pentanoic and n-hexanoic again involves predominantly the γ-hydrogen atom, the double hydrogen transfer involving those atoms from the γ- and δ-positions. In the acids extensive hydrogen exchange which is observed even on the picosecond time scale leads to participation of the β- and ϵ- (for n-hexanoic acid) positions in the hydrogen transfer reactions, although γ- and δ-hydrogens are predominantly involved even at the longest time (∼ 10 −5 s).

Mass spectra of aryl substituted N‐t‐ butylbenzamides: Substituent effects on unimolecular ion decomposition

AbstractThe major reactions of aryl substituted N‐t‐butylbenzamides upon electron‐impact involve direct cleavage of a methyl radical, the loss of a butene molecule with the transfer of one hydrogen, or the loss of a butenyl radical with the transfer of two hydrogens. The last of these processes parallels the mass spectral behavior of aliphatic amides. Substituent effects indicate that electron‐withdrawing groups on the aromatic ring enhance the two hydrogen transfer process, while electron‐donating groups enhance the single hydrogen transfer process. Ion abundances, ionization potentials and appearance potentials are discussed with respect to correlation with σ+ values.