Andreas F. Voegele

Towards the Experimental Decomposition Rate of Carbonic Acid (H2CO3) in Aqueous Solution

Dry carbonic acid has recently been shown to be kinetically stable even at room temperature. Addition of water molecules reduces this stability significantly, and the decomposition (H2CO3 + nH2O --> (n+1)H2O + CO2) is extremely accelerated for n = 1, 2, 3. By including two water molecules, a reaction rate that is a factor of 3000 below the experimental one (10 s(-1)) at room temperature was found. In order to further remove the gap between experiment and theory, we increased the number of water molecules involved to 3 and took into consideration different mechanisms for thorough elucidation of the reaction. A mechanism whereby the reaction proceedes via a six-membered transition state turns out to be the most efficient one over the whole examined temperature range. The determined reaction rates approach experimental values in aqueous solution reasonably well; most especially, a significant increase in the rates in comparison to the decomposition reaction with fewer water molecules is found. Further agreement with experiment is found in the kinetic isotope effects (KIE) for the deuterated species. For water-free carbonic acid, the KIE (i.e., kH2CO3/kD2CO3) for the decomposition reaction is predicted to be 220 at 300 K, whereas it amounts to 2.2-3.0 for the investigated mechanisms including three water molecules. This result is therefore reasonably close to the experimental value of 2 (at 300 K). These KIEs are in much better accordance with the experiment than the KIE for decomposition with fewer water entities.

The optimal tunneling path for the proton transfer in malonaldehyde

The proton tunneling reaction in malonaldehyde at low temperatures is investigated. The principal aim of this study is to find the optimal tunneling path at 0 K in the framework of the semiclassical theory with a global optimization method. An amount of 11366 ab inito points was determined in the reaction swath (i.e., the conformational space enclosed by the minima and the transition state) of malonaldehyde. With a simulated annealing approach, the path with the smallest integral of the imaginary action through the swath from minimum to minimum was determined. Surprisingly the optimal tunneling path was found to be quite far off the large curvature tunneling path [i.e., the straight connection of the two minima large-current tunneling (LCT path)]. At the beginning, it is following the minimum energy path (MEP) (i.e. the path with the lowest energy connecting the two minima and passing through the transition state), and then it is describing a curved path through the reaction swath. This curve was determin...

The ground-state tunneling splitting of various carboxylic acid dimers

Carboxylic acid dimers in gas phase reveal ground-state tunneling splittings due to a double proton transfer between the two subunits. In this study we apply a recently developed accurate semiclassical method to determine the ground-state tunneling splittings of eight different carboxylic acid derivative dimers (formic acid, benzoic acid, carbamic acid, fluoro formic acid, carbonic acid, glyoxylic acid, acrylic acid, and N,N-dimethyl carbamic acid) and their fully deuterated analogs. The calculated splittings range from 5.3e-4 to 0.13 cm(-1) (for the deuterated species from 2.8e-7 to 3.3e-4 cm(-1)), thus indicating a strong substituent dependence of the splitting, which varies by more than two orders of magnitude. One reason for differences in the splittings could be addressed to different barriers heights, which vary from 6.3 to 8.8 kcal/mol, due to different mesomeric stabilization of the various transition states. The calculated splittings were compared to available experimental data and good agreement was found. A correlation could be found between the tunneling splitting and the energy barrier of the double proton transfer, as the splitting increases with increased strength of the hydrogen bonds. From this correlation an empirical formula was derived, which allows the prediction of the ground-state tunneling splitting of carboxylic acid dimers at a very low cost and the tunneling splittings for parahalogen substituted benzoic acid dimers is predicted.

An accurate semiclassical method to predict ground-state tunneling splittings

A new method for calculating the ground-state tunneling splitting is presented. It is based on the semiclassical theory including recently derived corrections and it is the first method, which explicitly takes into account the whole conformational space between the minima and the transition state. The density-functional theory is used to determine the qualitative shape of the potential energy surface (PES) and high level ab initio calculations provide information about the stationary points. With a dual level scheme, the low-level energy surface is mapped onto the high-level points to get a good quantitative description of the high-level PES. Therefore, the new method requires no adjustment of additional parameters like scaling of the energy barrier as is necessary in other methods. Once the high-level PES is calculated, the most probable tunneling paths are determined with a global optimization procedure. Along this representative tunneling path, the tunneling splitting is calculated with additional cons...

About the Stability of Sulfurous Acid (H2SO3) and Its Dimer

The characterization and isolation of sulfurous acid (H2SO3) have never been accomplished and thus still remain one of the greatest open challenges of inorganic chemistry. It is known that H2SO3 is thermodynamically unstable. In this study, however, we show that a Ci-symmetric dimer of sulfurous acid (H2SO3)2 is 3.5 kcal mol-1 more stable than its dissociation products SO2 and H2O at 77 K. Additionally, we have investigated the kinetic stability of the sulfurous acid monomer with respect to dissociation into SO2 and H2O and the kinetic isotope effect (KIE) on this reaction by transition-state theory. At 77 K, the half-life of H2SO3 is 15 x 10(9) years, but for the deuterated molecule (D2SO3) it increases to 7.9 x 10(26) years. At room temperature, the half-life of sulfurous acid is only 24 hours; however, a KIE of 3.2 x 10(4) increases it to a remarkable 90 years. Water is an efficient catalyst for the dissociation reaction since it reduces the reaction barrier tremendously. With the aid of two water molecules, one can observe a change in the reaction mechanism for sulfurous acid decomposition with increasing temperature. The most likely mechanism below 170 K is via an eight-membered transition-state ring; yet, above 170 K, a mechanism with a six-membered transition state ring becomes the predominant one. For deuterated sulfurous acid, this change in reaction mechanism can be observed at 120 K. Consequently, between 120 and 170 K, different predominant reaction mechanisms occur for the decomposition of normal and deuterated sulfurous acid when assisted by two water molecules. However, the much longer half-life of deuterated sulfurous acid and the stability of the sulfurous acid dimer at 77 K are encouraging for future synthesis and characterization under laboratory conditions.

The ground state tunneling splitting of the 2-pyridone·2-hydroxypyridine dimer

Abstract The symmetric tautomerization of the 2-pyridone · 2-hydroxypyridine complex has been investigated by quantum chemical methods. This reaction is accomplished by an intermolecular double proton transfer between the monomer units. The barrier of the proton transfer is found to be 9.45–9.85 kcal/mol depending on the level of theory. The ground state tunneling splitting that can be expected for proton exchange in a symmetric double well potential is computed by a recently developed accurate method and was shown to be 1×108 s−1. This finding is in contradiction to the analysis of recent experimental results of Borst et al. [Chem. Phys. 283 (2002) 341], as they propose that their measured splitting of 5×108 s−1 exclusively arises from the splitting in the first excited state. Our results suggest that the measured splitting may be interpreted as either the difference between the tunneling splittings of the ground state and the first excited state or the ground state tunneling splitting itself.

Toward elimination of discrepancies between theory and experiment: The gas-phase reaction of N2O5 with H2O

New reaction mechanisms are presented and the corresponding reaction rate constants are calculated for the homogeneous gas-phase reaction N2O5 + nH2O ↔ 2HNO3 + (n − 1)H2O with n = 1,2,3 using ab initio methods and canonical variational transition state theory including tunneling corrections. The reaction barriers for the new mechanisms are 21.1 kcal mol−1 for n = 1, 18.9 kcal mol−1 for n = 2 and for the two mechanisms with three water molecules 14.2 and 19.2 kcal mol−1. Using the new reaction mechanism the rate constant for N2O5 hydrolysis with n = 1 is k1 = 5.2 × 10−25 cm3 molecule−1 s−1 at 298 K, which is in much better agreement with the experimental value being only two orders of magnitude smaller, compared to the old mechanism which is ten orders of magnitude smaller than the experimental value. Also the rate constant for the third order process—second order with respect to [H2O]—is in better agreement with experiment compared with the old mechanism (seven compared to approximately twelve orders of magnitude). For possible future confirmation of the new reaction mechanisms we determined kinetic isotope effects for the reactions and obtained KIEs of 1.55 and 1.09 for n = 1 and n = 2 water molecules, respectively, compared to 1.11 and 1.44 for the old mechanisms.

Reactions of HOBr + HCl + nH2O and HOBr + HBr + nH2O

Abstract The reactions of HOBr with HCl and HBr in the presence of n =0, 1, 2 and 3 water molecules are investigated by hybrid density functional theory methods in combination with canonical, variational transition state theory including tunneling corrections. Compared to the reactions of HOCl with HCl and HBr [J. Phys. Chem. A 106 (2002) 7850], we found that the barriers of the title reactions are significantly lower yielding much higher rate constants. Support of only two water molecules makes the reaction of HOBr with HBr barrierless. Under stratospheric conditions the reactions of HOBr with HBr are the most reactive ones.

Extended method for adiabatic mode reordering

The task of vibrational mode reordering is very important for reaction valley studies and for the determination of small curvature tunneling effects. An extended algorithm for adiabatic mode reordering is presented. It is based on the method introduced by Konkoli et al. [J Comput Chem 1997, 18, 1282], which is shown to suffer from numerical problems in the region of frequency‐crossings and avoided crossings. One improvement is the use of cubic splines for the interpolation of the projected matrix of force constants, which allows larger step sizes between the discrete points along the reaction path, where vibrational analysis is performed. The main improvement is the use of perturbation theory to resolve crossings and avoided crossings. Within this theoretical framework it becomes clear why the method of the maximal overlap between the normal modes cannot work properly, as eigenvectors associated with nearby eigenvalues tend to become “wobbly”. Thus a perturbative procedure is designed that is used for all cases where two harmonic frequencies approach each other and the overlap of the associated normal modes is of no practical use. Advantages of the new procedure are the use of larger step sizes along the minimum energy path and the much more reliable resolution of mode‐crossings and avoided crossings independent of the systems symmetry. In addition to that it is shown that one should be very cautious in all computational situations when working with eigenvectors associated with nearby eigenvalues.