Respective Potential Energy Surfaces Research Articles

Reactive chemical dynamics through conical intersections

Reaction dynamics of prototypical, D + H2 and Cl (2P) + H2, chemical reactions occurring through the conical intersections of the respective coupled multi-sheeted potential energy surfaces is examined here. In addition to the electronic coupling, nonadiabatic effects due to relativistic spin-orbit coupling are also considered for the latter reaction. A time-dependent wave packet propagation approach is undertaken and the quantum dynamical observables viz., energy resolved reaction probabilities, integral reaction cross-sections and thermal rate constants are reported.

Theoretical study on the potential energy surface of NC3P isomers

Density functional theory (DFT) calculations have been used to study the isomerization process in the NC3P system. At the DFT/B3LYP/6-311G(d) level, 28 triplet and 28 singlet minima were obtained on their respective potential energy surfaces. The linear triplet 3NCCCP is the lowest-energy structure among the isomers. On the triplet PES, only linear isomers 3NCCCP, 3CNCCP, 3CCCNP, and 3CCNCP possess great kinetic and thermodynamic stabilities to exist under low-temperature conditions (such as in the dense interstellar clouds). At the same time, one chain-like and four three-membered-ring isomers on the singlet PES have been located with high kinetic and thermodynamic stabilities. Further CCSD(T)/6-311G(2df)//QCISD/6-311G(d), CCSD(T)/cc-pVTZ//DFT/B3LYP/cc-pVTZ, and CASPT2(14,12)/cc-pVQZ//CASSCF(14,12)/cc-p VQZ calculations are performed on the structures, frequencies, and energies of the relevant species. The bonding natures were analyzed and the results were compared with the analogous NC3N and NC2P molecules so as to aid their future experimental or astrophysical detection.

Small Group IIa–VIa Clusters and Related Systems: A Theoretical Study of Physical Properties, Reactivity, and Electronic Spectra

AbstractIn the monomeric form, the title systems assume unexpected optical properties, and as oligomers they can serve as candidates for molecular devices. In bulk they are attractive in the area of material science. A broad set of quantum chemical methods ranging from density functional theory by the Hartree–Fock method and the Møller–Plesset perturbation theory to the coupled‐cluster method, in connection with nonrelativistic Dunning's basis sets as well as with relativistic SDD basis sets were used. Electronic spectra were analyzed by means of time‐dependent DFT, with the symmetry‐adapted cluster configuration interaction and with the internally contracted multireference configuration interaction. The Douglas–Kroll–Hess quasirelativistic Hamiltonian served as a basis for the estimation of the role of relativistic effects. All the diatomics representing 25 elements of a matrix formed by the combination of group IIa (Be, ..., Ba) with group VIa (O, ..., Po) atoms were characterized by calculated bond length, valence vibration, dipole moment, and the Hartree–Fock frontier orbital energies. Calculated characteristics were confronted with experimental data. The geometry of the oligomers (from dimers through hexamers) was investigated systematically, and the nature of the located stationary points on the respective potential energy surfaces was established. Special attention was paid to the four lightest elements of the group IIa–VIa matrix, i.e. BeO, BeS, MgO, and MgS. As for electronic spectra, the systems of the first column (BeO, ..., BaO), the first row (BeO, ..., BePo), and the systems of the main diagonal (BeO, ..., BaPo) were calculated in the form of monomers and dimers. Analogous calculations were performed for a few group Ia–VIIa, IIIa–Va, IVa–IVa, and IIb–VIa systems. The group IIa–VIa and IIb–VIa diatomics exhibit electronic transitions in the visible region of the spectrum, and the longest wavelength bands are located in the near‐infrared region. MgO represents an extreme with the first band appearing at about 3500 cm–1, which strictly speaking makes the use of the Born–Oppenheimer approximation questionable. Whenever experimental transitions are available, the agreement between the calculated and observed band positions is good. Passing from monomers to oligomers is always associated with a significant hypsochromic shift in the first transition.(© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2007)

Theoretical study on the potential energy surface of NC3P isomers

Density functional theory (DFT) calculations have been used to study the isomerization process in the NC3P system. At the DFT/B3LYP/6-311G(d) level, 28 triplet and 28 singlet minima were obtained on their respective potential energy surfaces. The linear triplet 3NCCCP is the lowest-energy structure among the isomers. On the triplet PES, only linear isomers 3NCCCP, 3CNCCP, 3CCCNP, and 3CCNCP possess great kinetic and thermodynamic stabilities to exist under low-temperature conditions (such as in the dense interstellar clouds). At the same time, one chain-like and four three-membered ring isomers on the singlet PES have been located with high kinetic and thermodynamic stabilities. Further CCSD(T)/6-311G(2df)//QCISD/6-311G(d), CCSD(T)/cc-pVTZ//DFT/B3LYP/cc-pVTZ, and CASPT2(14,12)/cc-pVQZ//CASSCF(14,12)/ cc-pVQZ calculations are performed on the structures, frequencies, and energies of the relevant species. The bonding natures were analyzed and the results were compared with the analogous NC3N and NC2P molecules so as to aid their future experimental or astrophysical detection.

Dynamics of Insertion Reactions of H2Molecules with Excited Atoms

Recent progress in the study of insertion reactions of hydrogen molecules with excited atoms is reviewed in this article. In particular, the dynamics of the reaction of O(1D), N(2D), C(1D), and S(1D) with H2 and its isotopomers, which have received a great deal of attention over the past decade, are examined in detail. All of these systems have in common the existence of several potential energy surfaces (PES) correlating with the reagents' states, and consequently, they can give rise to reaction following different adiabatic and nonadiabatic pathways. The main contribution, however, arises from their ground singlet PESs which feature the existence of deep wells with small or null barriers for insertion. Accordingly, these reactions proceed mainly via formation of relatively long-lived collision complexes and display an overall nearly statistical behavior. In spite of their similarities, the various reactions have peculiar characteristics caused by important differences of their respective PESs. The contribution of excited PES to the global reactivity, which has also become an important issue and a challenge both for theory and experiment, is also examined. The different theoretical approaches are discussed in the text, along with the experimental results obtained by a variety of techniques. The recent exact quantum treatments of these reactive systems together with the development of a rigorous statistical model have contributed to a very accurate description which in many cases matches very well the detailed measurements. The quasi-classical trajectory (QCT) method has also provided a fairly accurate description of the reaction dynamics for these systems. In particular, the analysis in terms of collision times has yielded interesting clues about the reaction mechanisms.

An Experimental and Theoretical Investigation of Gas‐Phase Reactions of Ca2+with Glycine

The gas-phase reactions between Ca(2+) and glycine ([Ca(gly)](2+)) have been investigated through the use of mass spectrometry techniques and B3-LYP/cc-pWCVTZ density functional theory computations. The major peaks observed in the electrospray MS/MS spectrum of [Ca(gly)](2+) correspond to the formation of the [Ca,C,O(2),H](+), NH(2)CH(2) (+), CaOH(+), and NH(2)CH(2)CO(+) fragment ions, which are produced in Coulomb explosion processes. The computed potential energy surface (PES) shows that not only are these species the most stable product ions from a thermodynamic point of view, but they may be produced with barriers lower than for competing processes. Carbon monoxide is a secondary product, derived from the unimolecular decomposition of some of the primary ions formed in the Coulomb explosions. In contrast to what is found for the reactions of Ca(2+) with urea ([Ca(urea)](2+)), minimal unimolecular losses of neutral fragments are observed for the gas-phase fragmentation processes of [Ca(gly)](2+), which is readily explained in terms of the topological differences between their respective PESs.

Structure and stability of xenon insertion compounds of hypohalous acids, HXeOX [X=F, Cl, and Br]: An ab initio investigation

The structure and stability of xenon-inserted hypohalous acids HXeOX (X=F, Cl, and Br) have been investigated theoretically using ab initio molecular orbital calculations. All these molecules are found to consist of a nearly linear HXeO moiety and a bend XeOX fragment. Geometrical parameters of HXeOX are comparable with that of experimentally observed HXeOH species. The dissociation energies corresponding to the lowest-energy fragmentation products, HOX+Xe have been computed to be -398.1, -385.5, and -386.7 kJmol for HXeOF, HXeOCl, and HXeOBr, respectively, at the MP2 level of theory. The respective barrier heights corresponding to the bent transition states (H-Xe-O bending mode) have been calculated to be 138.1, 138.4, and 138.2 kJmol with respect to HXeOX minimum. These species are found to be metastable in their respective potential-energy surface, and the dissociation energies corresponding to the H+Xe+OX products are found to be 56.8, 66.0, and 80.8 kJmol for HXeOF, HXeOCl, and HXeOBr, respectively. The energies corresponding to the H+Xe+O+X dissociation channel have been computed to be 272.0, 309.3, and 299.7 kJmol for HXeOF, HXeOCl, and HXeOBr, respectively, at the same level of theory. Energetics as well as geometrical considerations suggests that it may be possible to prepare these species experimentally similar to that of HXeOH species at low-temperature laser photolysis experiments.

A theoretical study of hydration of 4-thiouracil in the electronic singlet excited state

A comprehensive theoretical investigation was performed to study the interaction of water molecules with 4-thiouracil (4TU) in the ground and electronic lowest singlet nπ* excited state. The studied system included the isolated 4TU and the hydrated complexes of 4TU with one, two and three water molecules. The interacting water molecules were placed in between different hydrogen bond donating and accepting sites of 4TU. The ground state geometries were optimized at the HF level using the 6-311++G(d,p) basis set. The electronic excited state geometries of 4TU and different hydrated complexes were optimized at the CI-singles (CIS) level using the 6-311++G(d,p) basis set. The harmonic vibrational frequency calculations were performed to ascertained stationary points at the respective potential energy surfaces; all geometries were found minima. The lowest singlet nπ* transition of 4TU was assigned to the excitation of the lone-pair electron of the thiocarbonyl group to π*-antibonding orbitals of the molecule. It was found that the nπ* state provides a repulsive potential for water binding in the excited state. Therefore, the structures of the hydrated complexes, where water molecule was bonded at the thiocarbonyl group, were remarkably modified in the electronic singlet nπ* excited state as compared to the corresponding structure in the ground state. Further, in going from the ground state to the excited state significant changes in the features of molecular electrostatic potentials are also revealed.

How strong is the interaction between a noble gas atom and a noble metal atom in the insertion compounds MNgF (M=Cu and Ag, and Ng=Ar, Kr, and Xe)?

Ab initio molecular orbital calculations have been carried out to investigate the structure and the stability of noble gas insertion compounds of the type MNgF (M=Cu and Ag, and Ng=Ar, Kr, and Xe) through second order Moller-Plesset perturbation method. All the species are found to have a linear structure with a noble gas-noble metal bond, the distance of which is closer to the respective covalent bond length in comparison with the relevant van der Waals limit. The dissociation energies corresponding to the lowest energy fragmentation products, MF+Ng, have been found to be in the range of -231 to -398 kJ/mol. The respective barrier heights pertinent to the bent transition states (M-Ng-F bending mode) are quite high for the CuXeF and AgXeF species, although for the Ar and Kr containing species the same are rather low. Nevertheless the M-Ng bond length in MNgF compounds reported here is the smallest M-Ng bond ever predicted through any experimental or theoretical investigation, indicating strongest M-Ng interaction. All these species (except AgArF) are found to be metastable in their respective potential energy surface, and the dissociation energies corresponding to the M+Ng+F fragments have been calculated to be 30.1-155.3 kJ/mol. Indeed, in the present work we have demonstrated that the noble metal-noble gas interaction strength in MNgF species (with M=Cu and Ag, and Ng=Kr and Xe) is much stronger than that in NgMF systems. Bader's [Atoms in molecules-A Quantum Theory (Oxford University Press, Oxford, 1990)] topological theory of atoms in molecules (AIM) has been employed to explore the nature of interactions involved in these systems. Geometric as well as energetic considerations along with AIM results suggest a partial covalent nature of M-Ng bonds in these systems. The present results strengthen our earlier work and further support the proposition on the possibility of experimental identification of this new class of insertion compounds of noble gas atoms containing noble gas-noble metal bond.

NMR parameters of the tetrahedrane [Fe2(CO)6(μ-SNH)], studied by experiment and DFT

The intramolecular dynamic behavior of the tetrahedrane-type cluster [Fe2(CO)6(μ-SNH)] 1 was studied by 13C NMR spectroscopy. The 57Fe chemical shift of 1 and the coupling constants 1J(57Fe,13C) were measured. These NMR parameters, and also 1J(57Fe,15N), were found to be in good agreement with data calculated by using density functional theory (DFT) methods (B3LYP), based on the geometry calculated at the 6-311+G(d,p) level of theory. The isolobal replacement of the Fe(CO)3 with BH fragments leads to the tetrahedranes [Fe(CO)3(BH)(μ-SNH)] 2 and [(HB)2(μ-SNH)] 3. Both were identified by calculations as minima on the respective potential energy surface (PES). However, the tetrahedrane-type structure of 3 is much higher in energy when compared with the planar cyclic isomers 3a and 3b.

Rare-gas insertion compounds of perfluorobenzene: Aromaticity of some unstable species

Calculations on the novel argon insertion compounds C6F6Ar(n), n = 1-6, where the argon atoms are inserted into the C-F bonds in perfluorobenzene, suggest that all possible species, with one to six inserted argon atoms, occupy minima on their respective potential energy surfaces. Ring-current plots using the ipsocentric model indicate that there is no disruption of the aromatic pi system upon argon insertion, and consequently all insertion compounds are aromatic according to the magnetic criterion. The barrier height for decomposition of the single-insertion compound, C6F6Ar, into C6F6+Ar is 19.5 kcal/mol at HF6-311G** and 29.5 kcal/mol at B3LYP6-311G**, suggesting that, although clearly thermodynamically unstable, argon-perfluorobenzene insertion compounds may be stable kinetically. Preliminary calculations indicate that other rare gas-perfluorobenzene insertion compounds may also be metastable. Both C6F6Ne and C6F6He are predicted to occupy minima on their respective potential energy surfaces.

Effect of Hydration on the Lowest Singlet ππ* Excited-State Geometry of Guanine: A Theoretical Study

An ab-initio computational study was performed to investigate the effect of explicit hydration on the ground and lowest singlet PiPi* excited-state geometry and on the selected stretching vibrational frequencies corresponding to the different NH sites of the guanine acting as hydrogen-bond donors. The studied systems consisted of guanine interacting with one, three, five, six, and seven water molecules. Ground-state geometries were optimized at the HF level, while excited-state geometries were optimized at the CIS level. The 6-311G(d,p) basis set was used in all calculations. The nature of potential energy surfaces was ascertained via the harmonic vibrational frequency analysis; all structures were found minima at the respective potential energy surfaces. The changes in the geometry and the stretching vibrational frequencies of hydrogen-bond-donating sites of the guanine in the ground and excited state consequent to the hydration are discussed. It was found that the first solvation shell of the guanine can accommodate up to six water molecules. The addition of the another water molecule distorts the hydrogen-bonding network by displacing other neighboring water molecules away from the guanine plane.

Insertion of noble-gas atom (Kr and Xe) into noble-metal molecules (AuF and AuOH): Are they stable?

The structure and the stability of a new class of insertion compounds of noble-gas atoms of the type AuNgX (Ng=Kr, Xe and X=F, OH) have been investigated theoretically through ab initio molecular-orbital calculations. All the species are found to have a linear structure with a noble-gas-noble-metal bond, the distance of which is comparable to covalent bond length except the AuKrOH system, for which it lies in between the covalent and van der Waals limits. The dissociation energies corresponding to the lowest-energy fragmentation products, AuX+Ng have been computed to be -166.2, -276.0, -194.4, and -257.6 kJ/mol for AuXeF, AuKrF, AuXeOH, and AuKrOH, respectively, at the MP2 level of theory. The respective barrier heights corresponding to the bent transition states (Au-Ng-X bending mode) have been calculated to be 119.1, 74.9, 160.7, and 141.6 kJ/mol. However, three of these species are found to be metastable in their respective potential-energy surface, and the dissociation energies corresponding to the Au+Ng+X fragments have been calculated to be 112.9, 3.0, and 18.7 kJ/mol for AuXeF, AuKrF, and AuXeOH, respectively, at the same level of theory. An analysis of the nature of interactions involved in the Au-Ng-X systems has been performed using Bader's topological theory of atoms-in-molecules (AIM). Geometric as well as energetic considerations along with AIM results suggest a partial covalent nature of Au-Ng bonds in these systems. This work might have important implications in the preparation of a new class of insertion compounds of noble-gas atoms containing noble-gas-noble-metal bond.

Addition Reactions of 1D and 3P Atomic Oxygen with Acetylene. Potential Energy Surfaces and Stability of the Primary Products. Is Oxirene Only a Triplet Molecule? A Theoretical Study

The potential energy surfaces (PES) of the primary addition reactions of atomic oxygen O(1D) and O(3P) on acetylene were done using Density Functional Theory with the standard B3LYP hybrid functional. The crucial areas of these PES were then refined using the UMP2 and CCSD(T) schemes and, in few cases, CASPT2, MCSCF, and MR-AQCC ones. The stability of these primary products was studied. The reactivity of C2H2 and O, especially in low-temperature matrix conditions, is discussed on these bases. The singlet surface shows that the insertion of the oxygen into the C−H bond, resulting in ethynol HC⋮COH, is the main reaction. An addition onto the C⋮C bond, leading to oxirene and/or formylcarbene HC−CHO is nevertheless to be considered. A particular attention was given to the oxirene-formylcarbene region and MR-AQCC calculations predict an extremely flat area in agreement with previous CCSD(T) results. Secondary transposition of formylcarbene into ketene H2CCO could be avoided at very low temperature. From the triplet state of the oxygen atom, the triplet formylcarbene is expected as the major product. Nevertheless, a cyclic nonplanar species, the triplet state of oxirene, is stable on the triplet surface and could be trapped at low temperature, provided an intersystem triplet singlet crossing does not occur, which would lead to singlet oxirene, if stable. If not, this process would lead to singlet formyl carbene and finally to its triplet ground state. Indeed, singlet and triplet surfaces are very close to one another in this part of the PES. Hydrogen transposition giving triplet ketene can be ruled out in matrix conditions. The formation of cyclic three-member carbenes −C−O−CH2−, as well singlet as triplet, is unlikely though both species are rather stable on their respective PES.

Stabilization of Tricoordinate Pyramidal Boron: Theoretical Studies on CBSiH5, BSi2H5, CBGeH5, and CBSnH5.

Discovery of new pyramids: Theoretical studies have characterized CBSiH5, BSi2H5, CBGeH5, and CBSnH5, all of which have a tricoordinate pyramidal boron atom as minima on their respective potential energy surfaces. The extent of pyramidalization, as well as the energy for inversion through the planar structure, is dependent on the substituents of the boron atom.

Stabilization of tricoordinate pyramidal boron: theoretical studies on CBSiH5, BSi2H5, CBGeH5, and CBSnH5.

Discovery of new pyramids: Theoretical studies have characterized CBSiH5, BSi2H5, CBGeH5, and CBSnH5, all of which have a tricoordinate pyramidal boron atom as minima on their respective potential energy surfaces (see picture). The extent of pyramidalization, as well as the energy for inversion through the planar structure, is dependent on the substituents of the boron atom.

Interaction of Water Molecules with Cytosine Tautomers: An Excited-State Quantum Chemical Investigation

A detailed theoretical study of molecular geometry and electronic spectra was performed for four cytosine tautomers (amino−oxo (N1H and N3H), amino−hydroxy, and imino−oxo) and their hydrated forms with three water molecules. Geometries were optimized both in the ground and lowest singlet ππ* and nπ* excited states without any symmetry restriction. Ground-state geometries were optimized at the Hartree−Fock level of theory, whereas excited states were generated employing the configuration interaction technique involving singly excited configurations (CIS method). This was followed by excited-state geometry optimization. The nature of the corresponding potential energy surfaces was ascertained with the help of harmonic vibrational frequency analysis. All geometries were found to be minima at the respective potential energy surfaces. For the N1H tautomer, the ground-state geometry was also optimized at the B3LYP level, and the optimized geometry was used for the excitation energy calculation from the time-dependent density functional theory (TDDFT) method. The 6-311G(d,p), 6-311++G(d,p), and aug-cc-pVTZ basis sets were used in the study. However, in some singlet ππ* excited-state geometry optimizations, the 4-31G basis set was used for all atoms except the amino nitrogen, for which the 6-311+G(d) basis set was used. Although, ground-state geometries of all tautomers were found to be planar except for the amino group, corresponding excited-state geometries were found to be appreciably nonplanar. The mode of interaction of water molecules with cytosine tautomers was found to be different in the singlet nπ* excited state as compared to the ground and singlet ππ* excited states. Furthermore, in the gas phase and under hydration, cytosine is suggested to phototautomerize to the N3H tautomeric form.

Proton transfer reaction of 4-methyl-2,6-dicarbomethoxyphenol in nonpolar and weakly polar solvents

Proton transfer processes in the ground and excited state of 4-methyl-2,6-dicarbomethoxyphenol (CMOH) have been investigated by means of steady-state and nanosecond transient spectroscopy in some nonpolar and weakly polar solvents at room temperature and 77 K. The results obtained for CMOH were compared with that of 4-methyl-2,6-diformylphenol (MFOH) reported previously. Unlike MFOH the fluorescence spectra of CMOH show dual emission. This was explained as due to the presence of both the enol tautomer and intramolecularly hydrogen-bonded closed conformer as the fluorescing species. At 77 K the emission spectra show phosphorescence only in the presence of a base like triethylamine. The fluorescence decay rates of CMOH are relatively slower than that of MFOH and the nonradiative rates are always found higher than the radiative rates. A theoretical calculation at the Austin model 1 level of approximation revealed that the excited-state intramolecular proton transfer barrier in the ground singlet and excited triplet states is rather large compared to the excited singlet state in the respective potential energy surfaces.

Intermolecular Complexes of HXeOH with Water: Stabilization and Destabilization Effects

Theoretical and matrix-isolation studies of intermolecular complexes of HXeOH with water molecules are presented. The structures and possible decomposition routes of the HXeOH-(H(2)O)(n)(n = 0, 1, 2, 3) complexes are analyzed theoretically. It is concluded that the decay of these metastable species may proceed through the bent transition states (TSs), leading to the global minima on the respective potential energy surfaces, Xe + (H(2)O)(n+1). The respective barrier heights are 39.6, 26.6, 11.2, and 0.4 kcal/mol for n = 0, 1, 2, and 3. HXeOH in larger water clusters is computationally unstable with respect to the bending coordinate, representing the destabilization effect. Another decomposition channel of HXeOH-(H(2)O)(n), via a linear TS, leads to a direct break of the H-Xe bond of HXeOH. In this case, the attached water molecules stabilize HXeOH by strengthening the H-Xe bond. Due to the stabilization, a large blue shift of the H-Xe stretching mode upon complexation of HXeOH with water molecules is featured in calculations. On the basis of this computational result, the IR absorption bands at 1681 and 1742 cm(-1) observed after UV photolysis and annealing of multimeric H(2)O/Xe matrixes are assigned to the HXeOH-H(2)O and HXeOH-(H(2)O)(2) complexes. These bands are blue-shifted by 103 and 164 cm(-1) from the known monomeric HXeOH absorption.

Reactivity of Phosphate Monoester Monoanions in Aqueous Solution. 2. A Theoretical Study of the Elusive Zwitterion Intermediates RO+(H)PO32-

In a previous theoretical study [J. Am. Chem. Soc., in press], using a combination of DFT and continuum solvation (PCM) methods, the anionic zwitterion CH 3 O + (H)PO 3 2- (2) has been identified as a key intermediate in themechanism for the dissociative hydrolysis of the methyl phosphate anion CH 3 OPO 3 H - (1). To confirm this finding, DFT/B3LYP calculations in which a few solvent molecules are explicitly considered, are reported. Hydrogen-bonded complexes 2.(H 2 O) n (n = 2-4) have been fully optimized and characterized on their respective potential energy surfaces. We have shown that only two specifically solvating water molecules are sufficient to reproduce the PCM results previously obtained provided they occupy a critical bridging position between the methanol and metaphosphate fragments. Further, DFT-PCM geometry optimizations of RO + (H)PO 3 2- (R = phenyl and 2,4-dinitrophenyl) agree with predictions, based on the mechanistic picture proposed by Kirby and Varvoglis [J. Am. Chem. Soc. 1967, 89, 415] 35 years ago, according to which anionic zwitterions are assumed to exist as intermediates for the methyl and phenyl esters but not for the 2,4-dinitrophenyl ester. Moreover, biologically important counterions such as Mg 2 + can also play a crucial role in stabilizing the zwitterionic structure RO + (H)PO 3 2- in the gas phase.