Potential Energy Hypersurface Research Articles

Molecular structure and intramolecular rearrangements in tris-2,2,6,6-tetramethyl-heptane-3,5-dione complexes, M(thd) 3 (M = B, Al, Ga, In, Tl) by DFT calculations

The DFT calculations for tris-2,2,6,6-tetramethyl-heptane-3,5-dionato complexes of trivalent group 13 ions, M(thd) 3 (M = B, Al, Ga, In, Tl) were performed. The most likely geometrical structures of D 3 , D 3 h and C 2 v symmetry were examined. The D 3 molecular structure with distorted antiprismatic MO 6 coordination polyhedron corresponds to the minimum of potential energy hyper-surface (PES) for Al, Ga, In and Tl complexes. Calculated structures of MO 6 coordination polyhedron of studied complexes are in good agreement with the literature experimental results. The D 3 h and C 2 v configurations correspond to the first-order saddle points on the PES. The analysis of the different ways of intramolecular rearrangements was performed. Structure with D 3 symmetry was found to be a second-order saddle point on PES for B(thd) 3 complex. The configuration with one bidentate ligand and two monodentate bonded ligands and almost tetrahedron BO 4 is energetically most preferred for B(thd) 3.

On the Relaxation Mechanisms of 6-Azauracil

The nonadiabatic photochemistry of 6-azauracil has been studied by means of the CASPT2//CASSCF protocol and double-ζ plus polarization ANO basis sets. Minimum energy states, transition states, minimum energy paths, and surface intersections have been computed in order to obtain an accurate description of several potential energy hypersurfaces. It is concluded that, after absorption of ultraviolet radiation (248 nm), two main relaxation mechanisms may occur, via which the lowest (3)(ππ*) state can be populated. The first one takes place via a conical intersection involving the bright (1)(ππ*) and the lowest (1)(nπ*) states, ((1)ππ*/(1)nπ*)(CI), from which a low-energy singlet-triplet crossing, ((1)nπ*/(3)ππ*)(STC), connecting the (1)(nπ*) state to the lowest (3)(ππ*) triplet state is accessible. The second mechanism arises via a singlet-triplet crossing, ((1)ππ*/(3)nπ*)(STC), leading to a conical intersection in the triplet manifold, ((3)nπ*/(3)ππ*)(CI), evolving to the lowest (3)(ππ*) state. Further radiationless decay to the ground state is possible through a (gs/(3)ππ*)(STC).

Development of real-time vibrational spectroscopy of molecules in electronic excited states: toward mapping molecular potential energy hypersurfaces

We have developed an experimental system for real-time vibrational spectroscopy of molecules in a specific electronic excited state. A synchronized UV light source and a few-cycle visible laser pulse were utilized to prepare and investigate the ultrafast dynamics of molecules in the electronic excited states. A multichannel lock-in amplifier detection system operating in a tandem double lock-in detection mode was used to extract the signal correlated only to the UV and visible pump pulses. Real-time vibrational spectroscopy of chrysene in the triplet state and 1′, 3′-dihydro-1′, 3′, 3′-trimethyl-6-nitrospiro [2H-1-benzopyran-2, 2′-(2H)-indole] in a photochromic state was demonstrated.

Quantum chemical study of p-toluenesulfonic acid, p-toluenesulfonate anion and the water–p-toluenesulfonic acid complex. Comparison with experimental spectroscopic data

The 1:1 p-toluenesulfonic acid–water complex, p-toluenesulfonic acid itself and the p-toluenesulfonate anion were studied at HF and B3LYP/6-31+G(d,p) levels of theory. Full geometry optimizations of the aforementioned species reveal non-existence of ionic minima on the explored 1:1 p-toluenesulfonic acid–water complex potential-energy hypersurfaces (PEHSs), implying that two or three p-toluenesulfonate ions (+crystal field) are required to stabilize the ionic H 3O +⋯C 6H 4(CH 3)SO 3 − species found in the crystal structure of p-toluenesulfonic acid monohydrate (in fact, oxonium p-toluenesulfonate). Harmonic vibrational analyses of the p-toluenesulfonic acid–water complex as well as of the p-toluenesulfonate anion were used to confirm some of our previous reassignments of bands in the vibrational spectra of p-toluenesulfonic acid monohydrate and several metal p-toluenesulfonates. According to the quantum chemical results, the symmetric SO 3 bending mode should appear at higher frequencies than the antisymmetric one. A more consistent interpretation of the region of appearance of the SO 3 stretching modes is proposed which is in excellent agreement with the experimental spectroscopic data. The frequency of the multireference benzenoid ν 14 (B 2u) mode (the “Kekulé” type vibration) is excellently predicted at the B3LYP level of theory, while the HF methodology performs significantly poorer in this respect. The interaction energies as well as the vibrational frequency shifts of the most relevant modes are also presented for the 1:1 p-toluenesulfonic acid–water complex. The NBO analysis is employed to analyze the charge transfer interaction within the complex.

Ab initio intermolecular potential energy surface and thermophysical properties of hydrogen sulfide

A six-dimensional potential energy hypersurface (PES) for two interacting rigid hydrogen sulfide molecules was determined from high-level quantum-mechanical ab initio computations. A total of 4016 points for 405 different angular orientations of two molecules were calculated utilizing the counterpoise-corrected supermolecular approach at the CCSD(T) level of theory and extrapolating the calculated interaction energies to the complete basis set limit. An analytical site-site potential function with eleven sites per hydrogen sulfide molecule was fitted to the interaction energies. The PES has been validated by computing the second pressure virial coefficient, shear viscosity, thermal conductivity and comparing with the available experimental data. The calculated values of volume viscosity were not used to validate the potential as the low accuracy of the available data precluded such an approach. The second pressure virial coefficient was evaluated by means of the Takahashi and Imada approach, while the transport properties, in the dilute limit, were evaluated by utilizing the classical trajectory method. In general, the agreement with the primary experimental data is within the experimental error for temperatures higher than 300 K. For lower temperatures the lack of reliable data indicates that the values of the second pressure virial coefficient and of the transport properties calculated in this work are currently the most accurate estimates for the thermophysical properties of hydrogen sulfide.

The leapfrog principle for boron fullerenes: a theoretical study of structure and stability of B112

Two leapfrog isomers of a B(112) boron fullerene are constructed from small C(28) fullerenes (T(d) and D(2) symmetries) by the leapfrog transformation combined with omnicapping of the new hexagons. Their electronic structure is analyzed using the density functional theory at the B3LYP/SVP and BHLYP/SVP levels. Both isomers are characterized as minima on the potential energy hypersurface with a HOMO-LUMO gap at B3LYP/SVP of 1.7 eV and 1.6 eV (3.1 and 3.0 eV at BHLYP/SVP), respectively. The optimized structure of the helical D(2)-leapfrog is asymmetric, due to radial displacements of the capping atoms. The computed cohesive energies amount to -4.2 eV (∼0.04 eV lower than B(80)). The B(112) isomers are isoelectronic to T(d)-C(84) and D(2)-C(84), and HOMO and LUMO orbitals in both isomers closely resemble those of their C(84) homologues. Energetic stability of leapfrog boron fullerenes depends on the isolation of empty hexagon criterion, which is defined by the empty hexagon index based on the total number of empty hexagon pairs and empty hexagon-pentagon fused pairs. The switch of the cap atom to the nearest or farthest empty hexagon destabilizes the cage by 1.6 and 2.7 eV, respectively. The destabilization becomes more enhanced in non-leapfrog structures wherein more caps are displaced.

Intramolecular addition of oxyradicals to benzene rings: A DFT study

The reactivity of a series of oxyradicals related to the triplet state of β-phenylpropiophenone was investigated by density functional theory. Analysis of the potential energy hypersurfaces indicates that radical addition to the β-phenyl ring should occur with a smaller barrier than intramolecular hydrogen abstraction from the benzylic position, although the latter reaction is far more exothermic. Addition can occur in ipso- and ortho-position of the β-phenyl ring, with ortho addition being slightly more favourable. As both addition reactions are predicted to be mildly exothermic and exergonic, intermolecular trapping of the resulting cyclohexadienyl- type radicals should be feasible.

Intramolecular hydroxycarbene C-H-insertion: The curious case of (o-methoxyphenyl)hydroxycarbene.

The first C–H insertion of a hydroxycarbene species in the gas phase has been observed experimentally by means of high vacuum flash pyrolysis (HVFP) and subsequent matrix isolation: (o-Methoxyphenyl)glyoxylic acid gives non-isolable (o-methoxyphenyl)hydroxycarbene upon pyrolysis at 600 °C, which rapidly inserts into the methyl C–H bond. The insertion product, 2,3-dihydrobenzofuran-3-ol, was trapped in an excess of Ar at 11 K and characterized by infrared spectroscopy. The insertion process kinetically outruns the alternative [1,2]H-tunneling reaction to o-anisaldehyde, a type of reaction observed for other hydroxycarbenes. Traces of the dehydration product, benzo[b]furan, were also detected. The potential energy hypersurface including the insertion and hydrogen migration processes was computed at the all-electron coupled-cluster level of theory encompassing single and double substitutions and perturbatively included triple excitations [AE-CCSD(T)] in conjunction with a correlation-consistent double-ζ basis set (cc-pVDZ) by utilizing density functional theory (DFT) optimized geometries (M06-2X/cc-pVDZ) with zero-point vibrational energy (ZPVE) corrections. Exchange of the methoxy for a trifluoromethoxy group successfully prevents insertion and (o-trifluoromethoxy)benzaldehyde is produced instead; however, the carbene cannot be observed under these conditions. Thermal decomposition of (o-methoxyphenyl)glyoxylic acid in refluxing xylenes does not give the insertion product but yields o-anisaldehyde. This unanticipated outcome can be rationalized by protonation of the hydroxycarbene intermediate leading to the tautomeric formyl group. Thermochemical computations at M06-2X/cc-pVDZ in conjunction with a self-consistent solvent reaction field model support this suggested reaction pathway.

Analysis of the nitrile and methyl torsional vibrations of n‐propyl cyanide in its S0 electronic state

AbstractThis work presents a structural and vibrational theoretical study of n‐propyl cyanide as a function of the nitrile and methyl torsional modes. A potential energy hypersurface is built at the MP4(SDQ)/aug‐cc‐pVTZ//MP2/aug‐cc‐pVTZ theory level. The equilibrium structure is found in a gauche conformation. Another minimum is found for the trans form. The maximum appears in a cis conformation. For the first time, the interconversion barriers between the different forms are calculated. A two‐dimensional anharmonic vibrational Hamiltonian is built for the nitrile and methyl torsional modes. We find the vibrational energy levels to organize in two stacks associated to the gauche and trans forms. Fundamental frequencies of 113.12 and 220.54 cm−1 are predicted for the nitrile and methyl torsions in the equilibrium, gauche, conformer. In addition, we find symmetry allowed transitions between the gauche and trans energy levels stacks. The lowest transition is predicted to appear at 24.49 cm−1. © 2010 Wiley Periodicals, Inc. Int J Quantum Chem, 2011

A resonance Raman spectroscopic and CASSCF investigation of the Franck–Condon region structural dynamics and conical intersections of thiophene

Resonance Raman spectra were acquired for thiophene in cyclohexane solution with 239.5 and 266 nm excitation wavelengths that were in resonance with ∼240 nm first intense absorption band. The spectra indicate that the Franck-Condon region photodissociation dynamics have multidimensional character with motion mostly along the reaction coordinates of six totally symmetry modes and three nontotally symmetry modes. The appearance of the nontotally symmetry modes, the C-S antisymmetry stretch +C-C=C bend mode ν(21)(B(2)) at 754 cm(-1) and the H(7)C(3)-C(4)H(8) twist ν(9)(A(2)) at 906 cm(-1), suggests the existence of two different types of vibronic-couplings or curve-crossings among the excited states in the Franck-Condon region. The electronic transition energies, the excited state structures, and the conical intersection points (1)B(1)/(1)A(1) and (1)B(2)/(1)A(1) between 2 (1)A(1) and 1 (1)B(2) or 1 (1)B(1) potential energy surfaces of thiophene were determined by using complete active space self-consistent field theory computations. These computational results were correlated with the Franck-Condon region structural dynamics of thiophene. The ring opening photodissociation reaction pathway through cleavage of one of the C-S bonds and via the conical intersection point (1)B(1)/(1)A(1) was revealed to be the predominant ultrafast reaction channel for thiophene in the lowest singlet excited state potential energy hypersurface, while the internal conversion pathway via the conical intersection point (1)B(2)/(1)A(1) was found to be the minor decay channel in the lowest singlet excited state potential energy hypersurface.

ChemInform Abstract: Ab initio and Density Functional Electronic Structure Study of Molybdenum Oxide Clusters

First principles quantum chemical calculations at various levels of theory (HF, MP2, B3LYP), using the LANL2DZ basis set, provide a satisfactory description of structural, energetic and electronic properties of molybdenum(VI) oxide species of molecular dimensions, formulated as MoO3, MoO42−, MoO4H2, MoO4Na2, Mo2O52+ and Mo2O6. The energetics of all topomers corresponding to global or local minima and saddle points in the potential energy hypersurfaces were computed at the more sophisticated QCISD(T) level as well. The proton affinity of the MoO42− dianion was found to be equal to 1544, 1536 and 1527 kJ mol−1 at the HF, MP2 and B3LYP levels of theory. The formation processes of Mo2O52+(C1) dication from MoO3 and MoO22+(C2v) dication and the Mo2O6 dimer upon dimerization of the MoO3 species are predicted to be exothermic, the energies of formation being equal to 636.0 and 388.7 kJ mol−1 at the B3LYP level, respectively. Finally, the computed spectroscopic properties (harmonic vibrational frequencies and corresponding normal modes, electronic transitions and NMR chemical shifts) of the molecular molybdenum oxides are thoroughly discussed in relation with available experimental data.

C58Xe: A heterofullerene molecule containing a xenon atom

Abstract A novel heterofullerene-C 58 Xe-C 2V molecule has been reported here. It is a molecule that one xenon atom replaces a 6–6 C–C bond on the surface of C 60 . Density functional calculations and minimization techniques have been employed to characterize the structural and electronic properties of the molecule. Geometry, electronic properties, vibrational frequencies and spectral signals of nuclear magnetic resonance (NMR) of the molecule have been calculated at the B3LYP/6-31G* level of theory. The absence of imaginary vibrational frequency confirms that the molecule corresponds to a true minimum on the potential energy hypersurface. Its heat of formation is estimated, 840.7 kcal/mol.

Efficient computation of compliance matrices in redundant internal coordinates from Cartesian Hessians for nonstationary points

We present an extension to the theory of compliance matrices, which is valid for arbitrary nonstationary points on the potential energy hypersurface. It is shown that compliance matrices computed as the inverse of the covariant Hessian matrix obey the same invariance properties with respect to different internal coordinate systems as they do for stationary points. Furthermore, we demonstrate how the computation of compliance matrices in arbitrary sets of redundant internal coordinates starting from a Cartesian Hessian can be achieved efficiently, and we discuss their potential usefullness in geometry optimization processes

Heuristic computation of the rovibrational G matrix in optimized molecule-fixed axes. Gmat 2.1

Gmat 2.1 is a program able to compute the rovibrational G matrix in different molecule-fixed axes extending the capabilities of Gmat 1.0. The present version is able to select optimal molecule-fixed axes minimizing the pure rotational kinetic elements, the rovibrational kinetic elements or both simultaneously. To such an end, it uses a hybrid minimization approach. Thus, it combines a global search heuristic based in simulated annealing with a gradient-free local minimization. As the previous version, the program handles the structural results of potential energy hypersurface mappings computed in computer clusters or computational Grid environments. However, since now more general molecule-fixed axes can be defined, a procedure is implemented to ensure the same minimum of the cost function is used in all the molecular structures. In addition, an algorithm for the unambiguous definition of the molecule-fixed axes orientation is used.

Franck–Condon profiles in photodetachment-photoelectron spectra of and based on vibrational configuration interaction wavefunctions

Explicitly electron correlating coupled cluster calculations, CCSD(T)-F12a, were performed to determine three-dimensional potential energy hypersurfaces of disulphanide and disulphanyl in an automated approach. Surfaces for different electronic states were used in a Watson rovibrational Hamiltonian ansatz to obtain the correlated anharmonic vibrational wavefunctions. Subsequently the anharmonic Franck–Condon overlap integrals were evaluated. The computed Franck–Condon profiles were compared to experimental photodetachment-photoelectron spectra and confirm essentially the assignments made previously. The profiles indicate, however, additional weaker, and as of yet unresolved, additional features.

Low Frequency Vibrations of Biological Solids: Terahertz, FTIR, INS, Raman, DFT, and BOMD Molecular Dynamics of the L-Serine Crystal

Molecular dynamics simulations provide our most realistic description of biological events at the molecular level. Motions below 200 wave-numbers are of particular interest since they contribute most of the vibrational entropy and probably infuence many biological processes. Measurements of the vibrational spectrum in this region yield direct information about the potential energy hyper-surface, and these measurements can be used to refine molecular mechanics potential functions. We present here a vibrational analysis of polycrystalline L-serine using experimental vibrational spectra,calculated inelastic neutron scattering (INS), and Born-Oppenheimer molecular dynamics (BOMD) simulations.

The photophysics of alloxazine: a quantum chemical investigation in vacuum and solution

(Time-dependent) Kohn-Sham density functional theory and a combined density functional/multi-reference configuration interaction method (DFT/MRCI) were employed to explore the ground and low-lying electronically excited states of alloxazine, a flavin related molecule. Spin-orbit coupling was taken into account using an efficient, nonempirical mean-field Hamiltonian. Intersystem crossing (ISC) rate constants for S --> T transitions were computed, employing both direct and vibronic spin-orbit coupling. Solvent effects were mimicked by a conductor-like screening model and micro-hydration with up to six explicit water molecules. Multiple minima were found on the first excited singlet (S(1)) potential energy hypersurface (PEH) with electronic structures (1)(npi*) and (1)(pipi*), corresponding to the dark 1 (1)A'' (S(1)) state and the nearly degenerate, optically bright 2 (1)A' (S(2)) state in the vertical absorption spectrum, respectively. In the vacuum the minimum of the (1)(npi*) electronic structure is clearly found below that of the (1)(pipi*) electronic structure. Population transfer from (1)(pipi*) to (1)(npi*) may proceed along an almost barrierless pathway. Hence, in the vacuum, internal conversion (IC) between the 2 (1)A' and the 1 (1)A'' state is expected to be ultrafast and fluorescence should be quenched completely. The depletion of the (1)(npi*) state is anticipated to occur via competing IC and direct ISC processes. In aqueous solution this changes, due to the blue shift of the (1)(npi*) state and the red shift of the (1)(pipi*) state. However, the minimum of the (1)(npi*) state still is expected to be found on the S(1) PEH. For vibrationally relaxed alloxazines pronounced fluorescence and ISC by a vibronic spin-orbit coupling mechanism is expected. At elevated temperatures or excess energy of the excitation laser, the (1)(npi*) state is anticipated to participate in the deactivation process and to partially quench the fluorescence.

Carbon nitride:Ab initioinvestigation of carbon-rich phases

We have examined the potential energy hypersurfaces for the carbon-rich phases of carbon nitride, CN and ${\text{C}}_{3}\text{N}$, and discovered low-energy structures different from those reported previously. Trends in the preferred local bonding environments have been analyzed as a function of nitrogen content. For each composition, several structures with similar energies were found, but they have very different equilibrium volumes; the structure produced during synthesis will strongly depend on the preparation conditions. When low densities are favored, conjugated planar-ring structures with $s{p}^{2}$ hybridized carbon are most likely to be formed. These structures are similar to those suggested as potential photocatalytic materials. At high pressures, the preferred structures contain three-coordinate nitrogen and $s{p}^{3}$ hybridized carbon, including the $\ensuremath{\beta}\text{-InS}$ structure, which we predict to be the thermodynamically preferred structure for CN under positive hydrostatic pressures. This structure has a moderately high bulk modulus with a lower formation energy than $\ensuremath{\beta}{\text{-C}}_{3}{\text{N}}_{4}$ and so should be more easily synthesized.

Quantum chemistry study of symmetric methyne substitution patterns in the boron buckyball

We report the electronic structure of methyne boron buckyballs B 68(CH) 12, B 72(CH) 8 and B 76(CH) 4 obtained by substituting, respectively, 12, 8 and 4 boron cap atoms by methyne CH groups on the boron buckyball B 80. DFT calculations and minimization techniques have been employed to characterize the structural and electronic properties of endo and exo isomers of these molecules in T h and T symmetry. A vibrational frequencies analysis predicts that only endo-B 72(CH) 8 and endo-B 76(CH) 4 correspond to a true minimum on the potential energy hypersurface with a cohesive energy similar to the boron buckyball. The stable substitutions are characterized by six rhombohedral B 4 bonding motifs.

Gas-phase reaction between calcium monocation and fluoromethane: Analysis of the potential energy hypersurface and kinetics calculations

The gas-phase reaction between calcium monocation and fluoromethane: Ca(+)+CH(3)F-->CaF(+)+CH(3) was theoretically analyzed. The potential energy hypersurface was explored by using density functional theory methodology with different functionals and Pople's, Dunning's, Ahlrichs', and Stuttgart-Dresden basis sets. Kinetics calculations (energy and total angular momentum resolved microcanonical variational/conventional theory) were accomplished. The theoretically predicted range for the global kinetic rate constant values at 295 K (7.2x10(-11)-5.9x10(-10) cm(3) molecule(-1) s(-1)) agrees reasonably well with the experimental value at the same temperature [(2.6+/-0.8)x10(-10) cm(3) molecule(-1) s(-1)]. Explicit consideration of a two transition state model, where the formation of a weakly bounded prereactive complex is preceded by an outer transition state (entrance channel) and followed by an inner transition state connecting with a second intermediate that finally leads to products, is mandatory. Experimental observations on the correlation, or lack of correlation, between reaction rate constants and second ionization energies of the metal might well be rationalized in terms of this two transition state model.