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Notes: The mechanism of interconversion between the neutral and zwitterionic forms of glycine in aqueous solution is studied theoretically. It is argued that indirect transfer via a water bridge is a plausible alternative to the generally assumed direct transfer mechanism. The argument is based on model calculations in which the system glycine-water is represented by a 1:6 supermolecule embedded in a dielectric continuum. Optimized geometries and vibrational frequencies are obtained at the Hartree–Fock level with a 6-31G* basis set, and at the second-order Møller–Plesset frozen-core level with the 6-31+G* basis set for the neutral and zwitterionic forms, and for their transition state. At both levels the energetics are corrected by single-point quadratic configuration interaction calculations, including single and double substitutions with frozen-core inner-shell orbitals. Both models reproduce the observed endothermicity of the transfer better than models that use only a limited number of discrete water molecules without a continuum and models solely based on the continuum approximation. In the optimized structures of this complex and of complexes with fewer water molecules, one of the water molecules always bridges the two functional groups. In the 1:6 complex, two of the other water molecules form hydrogen bonds with the amino hydrogens, two others with the carboxyl oxygens, and the sixth water molecule forms a bridge between the two water molecules attached to the amino group. The interaction of this supermolecule with the bulk solvent is treated by means of the Onsager model. The transition state calculated with the two models implies that the mechanism of interconversion is concerted transfer of two protons along the amino–water–carboxyl bridge. The dynamics calculations are performed with a multidimensional instanton model that includes solvent reorganization. For both models the calculated transfer rate constants are about an order of magnitude larger than the observed rate constants, indicating that the indirect mechanism can easily account for the observed dynamics. These results confirm the plausibility of the indirect mechanism of proton transfer via a water bridge in aqueous solutions of glycine. © 2000 American Institute of Physics.

Notes: The ground and excited electronic states conformations of various tautomers of salicylidene methylamine (SMA) are calculated by ab initio methods. It is demonstrated that the cis-keto form, more stable in the ππ* S1 state, can rapidly decay to a strongly nonplanar configuration of the nπ* state, which subsequently undergoes very fast internal conversion to one of the trans-keto tautomers. This we identify with the photochromic species of SMA. The calculated photochromic absorption spectrum reproduces very well the observed spectrum. The rates of the proton transfer processes in the ground and excited states of SMA are calculated with the help of the ab initio geometries, force fields, and the instanton method. It is shown that the excited state proton transfer occurs faster than 50 fs for the CIS barrier of 1.6 kcal/mol. The ground state backproton transfer between the cis-keto and enol tautomers is predicted to take ∼30 ps and to exhibit a large deuterium effect. © 2000 American Institute of Physics.

Notes: The theoretical ab initio studies of the singlet states of salicylideneaniline (SA) are presented. The enol, cis-keto and trans-keto tautomers were treated by the HF/6-31G* (geometries and force fields of the ground states), and the CIS (excited states), methods. For the dynamic calculations of the rates of proton transfer (PT) in S1 states, the instanton approach was applied. It was found that the SA molecule in S0 and S1 states of both tautomers needs nonplanarity to stabilize. In the ground state the corresponding angle was calculated as 44° vs the experimental value, 49°. Upon twist of the excited system, the conical intersection of (π,π*) and (n,π*) potential surfaces takes place. In enol form the absolute minimum on the S1 potential energy surface belongs to a strongly twisted (n,π*) state. In keto-form this minimum corresponds to a planar (π,π*) state, while the twisted (n,π*) has the energy (approximate)1055 cm−1 higher. The angles of distortion are equal 93° and 80°, for the enol and keto form, respectively. Both (n,π*) excited tautomers are practically unable to undergo the PT reaction. This, according to the calculations, goes via the planar unstable (π,π*) state. The calculated structures and force-fields lead to the (calculated) proton transfer rate of the order observed experimentally. The calculation of the transfer rate includes multimode nature of the transfer process. The PT cycle of the SA molecule involving the creation of photochromic transient has been proposed. © 2000 American Institute of Physics.

Notes: Multidimensional ab initio proton tunneling rate constants are reported for the tautomerization of singlet-excited 7-azaindole complexed with water, represented by discrete water molecules with and without a dielectric continuum. The results are compared with experimental observations in cold beams and in room-temperature aqueous solutions. For complexes with one and two water molecules, potential-energy surfaces are calculated at the complete active space multiconfiguration self-consistent-field [CASSCF(8,8)] level. For comparison with solution data, the structures are reoptimized inside a spherical cavity according to the Onsager model. To compare the effect of the dielectric with that of a secondary solvent shell, the structure of 1:1 and 1:2 complexes solvated by four and three additional water molecules so as to form 1:5 complexes, are optimized at the CASSCF(8,8) level with single-point Onsager corrections. Based on these potential-energy surfaces, temperature-dependent multidimensional proton transfer rate constants are calculated with a recently developed version of the instanton approach. It is found that in gas-phase 1:1 and 1:2 complexes tautomerization occurs through concerted double and triple proton transfer, respectively. The calculated low-temperature rate constants agree with the observation that in these complexes no tautomerization occurs within the fluorescence lifetime of about 8 ns. Addition of a dielectric continuum within the Onsager model cannot explain the room-temperature rate constant of about 1010 s−1 observed as the fast tautomerization component of excited 7-azaindole in protic solutions. Addition of a secondary solvent shell of four water molecules to the 1:1 complex has only a minor effect on the proton transfer rate, but addition of a secondary shell of three water molecules to the cyclic 1:2 complex yields rate constants of the observed order of magnitude. This happens because the double bridge facilitates charge separation, which stabilizes an ion-pair structure for the transition state. As a result the barrier is lowered drastically and although the proton effective mass is also increased, the effect of the lower barrier dominates, leading to much faster proton transfer. It is concluded that the fast rate component observed in room-temperature tautomerization of excited 7-azaindole in water and alcohols corresponds to proton transfer through a bridge of two hydrogen-bonded water molecules, rather than through a single-molecule water bridge as previously assumed. The predicted mechanism involves a (meta)stable intermediate state. © 2001 American Institute of Physics.

Notes: Ab initio density functional theory and approximate instanton methods are used to study proton transfer processes in the first excited electronic state of 3,6-bis(benzoxazolyl)pyrocatechol (BBPC). Geometries of di-enol, keto-enol, and di-ketone tautomers as well as transition states for single and double proton transfer processes and the corresponding force fields are obtained with the CIS/6-31G* method and verified with CISD/6-31G* single point calculations. It is shown that keto-enol tautomer is the most stable in the S1 state while the least stable is di-ketone. The single proton transfer in the 2A1 state of di-enol leads to a somewhat more stable keto-enol tautomer. This result nicely reproduces the experimental assignment stating that BBPC, a symmetric molecule with two equivalent proton transfer reaction sites, undergoes a single proton transfer in the S1 state. The excited system has to overcome the barrier of about 9 kcal/mol and proton transfer is therefore dominated by tunneling. Dynamics calculations with the instanton method yield the rate of transfer of 9.8×1010 s−1, again in a very good agreement with the experimental value of kPT=(5.1±0.4)×1010 s−1 [Chem. Phys. Lett. 169, 450 (1990)]. Theory predicts a large kinetic isotope effect on this process. It is also shown that the reverse proton transfer leading back to di-enol has the rate strongly dependent on the stabilization energy of keto-enol. It effectively competes with the radiative decay of the latter, resulting in the observed weak di-enol fluorescence of BBPC. Finally, the calculations demonstrate why BBPC is not a photochrome unlike many typical Schiff bases. © 2002 American Institute of Physics.

Notes: It is shown that the direct precursor for the formation of a photochrome in Schiff bases is a "twisted" geometry S1 state of a keto tautomer in which the O(centered ellipsis)N–H hydrogen bond is broken. In photochromic Schiff bases this state is close in energy to the ππ* state of the cis-ketone obtained from the initially optically excited enol species by an ultrafast excited-state proton transfer, in particular, in N-salicylidene-α-methylbenzylamine (SMB), it lies within 1 kcal/mol from the ππ* state of cis-ketone and ∼16 kcal/mol below the initial ππ* state of enol. The excited-state proton transfer in SMB is predicted to take ∼15 fs and to be nearly barrierless. It is also shown that the initial ππ* state of enol can decay via competing route to a twisted geometry nNπ* state of enol with the energy ∼20 kcal/mol below that state. © 2001 American Institute of Physics.