Proton Rearrangement Research Articles

LASER INDUCED HYDROGEN MOTION IN ORGANIC CRYSTALS AT LOW TEMPERATURE

We study here a laser induced reaction leading to a rearrangement of the acid protons in benzoic acid crystals at very low temperature. This hydrogen motion results from the electronic excitation of pentacene molecules substitutionally included into a benzoic acid host matrix, protonated or deuterated. The growth of a deuterated benzoic acid crystal doped with pentacene leads to a partial deuteration of pentacene molecules, resulting in several origin bands in the absorption spectrum. The dynamics of the system following non selective and selective laser irradiations of the differently deuterated pentacene molecules is analyzed in terms of hydrogen Introduction. Hydrogen motion in solids at low temperature is intensively studied with a wide variety of methods. Among them, optical spectroscopy is a very sensitive one, specially when performed on crystals giving narrow spectral lines. Carboxylic acid crystals represent a class of model systems commonly used for studies on proton transfer 11-51, In such a system where two molecules associate to form a dimer bound by two hydrogen bonds, the hydrogen motion consists in a double exchange of the acid protons along the two hydrogen bonds giving rise to a tautomer form. For an isolated dimer the two tautomer forms are identical and the potential energy may be represented by a symmetrical double well. In the crystal the two forms are no longer equivalent and the double well becomes asymmetric, modifying the protons dynamics. An optical way of monitoring the proton movement is to replace a dimer by a dye impurity: any change in the dye environment, a tautomerization in the neighboring dimer for example, is reflected in its absorption spectrum. A particularly interesting feature of such a system is that the optical excitation of the dye molecule induces the desired environmental change which is then probed using the same dye molecule. By such a method, it was possible to study the protons tunneling mechanism in mixed thioindigolbenzoic acid crystals at very low temperature 161. In the system studied here, composed of benzoic acid (BA) as the matrix and pentacene (PC) as the impurity, it has been proved that the induced modification of the neighborhood was not a double proton exchange but a more severe rearrangement 171. This conclusion was drawn from the fact that the induced changes were very long-lived (10 orders of magnitude longer than in the tautomerization case), and that the energy difference between the two states was at least 30 times larger than the one reported in the literature for a tautomerization in a BA crystal 181. A proposed mechanism was an

ChemInform Abstract: Collision‐Induced Dissociations of Deprotonated Phosphorus Esters. Specific Proton Transfer Accompanying Fragmentation

The collision-induced dissociations of deprotonated phosphorus esters are dominated by elimination of neutral species from the alkoxy groups with accompanying proton rearrangement. For example, [(MeO)2P(CH2)(O)]– eliminates CH2O and CH2 with specific proton transfer to carbon to produce, respectively, (MeO)(Me)PO– and (MeO)(Me)PO2–, whilst [(EtO)2P(CH2)(O)]– similarly eliminates MeCHO and C2H4. The loss of C2H4 from this ion occurs by the competitive losses of CH2CH2 and MeCH; both losses form the product ion Me(EtO)PO2–. Thus neutral eliminations are accompanied by specific proton transfer to carbon.

Reaction of CS2 with OH: a quantum chemical study

Ab initio calculations on the reaction between carbon disulphide, CS2, and an OH radical predict that an intermediary complex is formed with OH attached to the carbon atom, the reaction being exothermic without an energy barrier, while unimolecular decomposition of this complex into OCS and SH by proton rearrangement is found to be hindered by a very high potential energy barrier.

Collision-induced dissociations of deprotonated phosphorus esters. Specific proton transfer accompanying fragmentation

The collision-induced dissociations of deprotonated phosphorus esters are dominated by elimination of neutral species from the alkoxy groups with accompanying proton rearrangement. For example, [(MeO)2P(CH2)(O)]– eliminates CH2O and CH2 with specific proton transfer to carbon to produce, respectively, (MeO)(Me)PO– and (MeO)(Me)PO2–, whilst [(EtO)2P(CH2)(O)]– similarly eliminates MeCHO and C2H4. The loss of C2H4 from this ion occurs by the competitive losses of CH2CH2 and MeCH; both losses form the product ion Me(EtO)PO2–. Thus neutral eliminations are accompanied by specific proton transfer to carbon.

Biosynthetic formation of the S-methyl group of the angucycline antibiotic urdamycin E

Biosynthetic studies on the angucycline antibiotic urdamycin E (1), produced by Streptomyces fradiae(strain Tu 2717), resulted in methionine being the precursor of the S-methyl group which is transfered in a unique way as an intact unit from methionine thus showing a new structural element biogenetically deriving from this amino acid; the discussed mechanism for this unusual reaction is an enzymatic cleavage of methionine yielding SMe– which attacks the electrophilic 5,6-double bond of the (1)-precursor urdamycin A (2), thus forming (1) by a non-enzymatic Michael addition followed by a proton rearrangement and oxidation by molecular oxygen.

A Phenomenological Interpretation of Anomalous Quickening of Dielectric Relaxation by Doping of KOH in Ice Ih

A mechanism of the drastic reduction of the relaxation time by doping of a small amount of KOH into ice Ih is discussed using the Eyring's theory of the elementary rate process by introducing a phenomenological parameter. It is deduced that this reduction of the relaxation time is strongly attributed to the release of water molecules from the ice rule which acts as a strong limitation to proton rearrangement.

Direct optical studies of proton tunneling in hydrogen bonded mixed molecular crystals

Absorption and fluorescence spectra of dye molecules doped substitutionally into benzoic acid crystals provide very sensitive probes of the environment. Processes related to a rearrangement of the acid protons can be probed directly at very low temperatures.

An electron spin resonance investigation of ester cation radicals at low temperatures. Proton transfer and rearrangement reactions

The cation radicals of a series of esters have been produced by γ-irradiation of CFCl3 matrices containing the ester at 77 K. In previous work cations of methyl and ethyl esters were investigated. In this work we report results for larger esters. The cations of these esters are found to undergo immediate internal proton-transfer reactions involving specific sites on one of the alkyl substituents. For example, deuteration studies show that proton transfer occurs from the terminal methyl group in propyl formate to produce —OCH2CH2CH·2, whereas in propyl acetate —OCH2ĊHCH3 is produced. The proton lost from these groups is assumed to add to an oxygen on the ester functional group. In the case of propyl acetate and esters with branched side chains we find that fragmentation reactions follow the proton transfer. In the cases of t-butyl acetate and isobutyl formate the fragmentation process occurs at 77 K and results in the isobutylene cation. Neopentyl formate gives evidence for the cation radical, an RCH·2 radical and the fragmentation radical cation as the sample is annealed. These results show that ester cation radicals are highly reactive even at low temperatures where proton-transfer and fragmentation reactions are found.

ChemInform Abstract: PHOTOCHEMICAL HOLE BURNING: A MEANS TO OBSERVE HIGH RESOLUTION OPTICAL STRUCTURES IN PHYCOERYTHRIN

We have investigated the lowest electronic transitions of phycoerythrobilin using laser‐induced low temperature photochemistry. Narrow band irradiation into the absorption bands of the native chromophore leads to photochemical holes of the width of about 1–2 cm−1. We attribute the hole burning mechanism to reversible proton rearrangement processes. The high resolution of the experiments allows the resolution of well‐defined substructures in the broad absorption bands which we interpret as being due to discrete vibrational states or due to a nonresonant energy transfer mechanism within the highly ordered chromophore–protein configuration.

Photochemical hole burning: A means to observe high resolution optical structures in phycoerythrin

We have investigated the lowest electronic transitions of phycoerythrobilin using laser-induced low temperature photochemistry. Narrow band irradiation into the absorption bands of the native chromophore leads to photochemical holes of the width of about 1–2 cm−1. We attribute the hole burning mechanism to reversible proton rearrangement processes. The high resolution of the experiments allows the resolution of well-defined substructures in the broad absorption bands which we interpret as being due to discrete vibrational states or due to a nonresonant energy transfer mechanism within the highly ordered chromophore–protein configuration.

Light induced proton transfer in dihydroxyanthraquinone as studied by photochemical hole burning

The low temperature photochemistry of dihydroxyanthraquinone in various host matrices has been studied via photochemical hole burning. It has been shown that the observed photochemistry is due to the rearrangement of protons involving both intra- and intermolecular hydrogen bonds. The model is supported by spectroscopic data on the low temperature photoproduct and by optical experiments on isotopically substituted and deprotonated samples. A brief discussion of the observed homogeneous linewidth links the optical data to possible radiationless deactivation processes (T1) or to optical dephasing processes (T2) on the time scale of several picoseconds.

The Effect of Water on Proton Transfer in Pyrazole

Abstract An NMR 13C study has shown that proton rearrangement in pyrazole is decreased by adding small amounts of water to the sample. Single water molecules as coordination centres pertrub the mechanism of cooperative proton transfer occurring in pyrazole selfassociates. The decrease of the rearrangement effect due to water is similar to the action of transition metal ions studied by the authors earlier.

Acylation of vicinal dianions. Further observations concerning proton transfer and rearrangements during reaction

Acylation of vicinal dianions. Further observations concerning proton transfer and rearrangements during reaction

Kink velocities on dislocations in ice

Abstract This short paper presents an alternative model for the velocity of kinks on a dislocation in an ice lattice to that presented by Whitworth, Paren and Glen (1976). The physics of the two models is identical; but they involve quite different methods for computing the statistical sums. We calculate the same expression for the kink velocity, and reach the same conclusion: that, if the time constant for the rearrangement of protons is twice the dielectrical relaxation time, the predicted kink velocity is too slow to account satisfactorily for the experimentally observed dislocation velocities. We use our model to calculate the asymptotic solution at high stresses.

Protonic States and the Mechanism of Acid‐Catalysed Esterification

AbstractProtons can be very mobile in condensed phases; the length of time they are present in any one place is minute. The significance of proton mobility in the course of chemical reactions is considered in the case of acid‐catalysed esterification by the normal carbonyl addition mechanism. – The properties of the protons in the carboxylic acid/alcohol system can be discussed on the basis of a semiconductor term diagram. The high mobility of the catalysing protons in the semiconductor system suggests that formation of the esterification intermediates occurs in two reaction steps which proceed at different rates. In the first rapid step the transition state is reached by proton transfers via hydrogen bridges. The proton rearrangement involved proceeds so rapidly that the heavy molecular skeletons retain their mutual spatial positions (Franck‐Condon principle for proton‐catalysed reactions). In the second slower step, the intermediate is formed by rearrangement of the groups of atoms. – Decomposition of the intermediate, with formation of either the starting materials or the end products, proceeds via a mechanism similar to that for the formation.