Search results for "Calculation"
showing 10 items of 594 documents
A Two-State Computational Investigation of Methane C-H and Ethane C-C Oxidative Addition to [CpM(PH3)]n+ (M=Co, Rh, Ir;n=0, 1)
2006
Reductive elimination of methane from methyl hydride half-sandwich phosphane complexes of the Group 9 metals has been investigated by DFT calculations on the model system [CpM(PH(3))(CH(3))(H)] (M = Co, Rh, Ir). For each metal, the unsaturated product has a triplet ground state; thus, spin crossover occurs during the reaction. All relevant stationary points on the two potential energy surfaces (PES) and the minimum energy crossing point (MECP) were optimized. Spin crossover occurs very near the sigma-CH(4) complex local minimum for the Co system, whereas the heavier Rh and Ir systems remain in the singlet state until the CH(4) molecule is almost completely expelled from the metal coordinati…
Density functional theory description of random Cu-Au alloys
2019
Density functional alloy theory is used to accurately describe the three core effects controlling the thermodynamics of random Cu-Au alloys. These three core effects are exchange correlation (XC), local lattice relaxations (LLRs), and short-range order (SRO). Within the real-space grid-based projector augmented-wave (GPAW) method based on density functional theory (DFT), we adopt the quasinonuniform XC approximation (QNA), and take into account the LLR and the SRO effects. Our approach allows us to study the importance of all three core effects in a unified way within one DFT code. The results demonstrate the importance of the LLR term and show that going from the classical gradient level a…
Exact Coulomb cutoff technique for supercell calculations in two dimensions
2009
We present a reciprocal space technique for the calculation of the Coulomb integral in two dimensions in systems with reduced periodicity, i.e., finite systems, or systems that are periodic only in one dimension. The technique consists in cutting off the long-range part of the interaction by modifying the expression for the Coulomb operator in reciprocal space. The physical result amounts in an effective screening of the spurious interactions originated by the presence of ghost periodic replicas of the system. This work extends a previous report [C. A. Rozzi et al., Phys. Rev. B 73, 205119 (2006)], where three-dimensional systems were considered. We show that the use of the cutoffs dramatic…
Theoretical absorption spectrum of the Ar–CO van der Waals complex
2003
The three-dimensional intermolecular electric dipole moment surface of Ar–CO is calculated at the coupled cluster singles and doubles level of theory with the aug-cc-pVTZ basis set extended with a 3s3p2d1f1g set of midbond functions. Using the rovibrational energies and wave functions of our recent study [J. Chem. Phys. 117, 6562 (2002)], temperature-dependent spectral intensities are evaluated and compared to available experimental data. Based on the theoretical spectrum, alternative assignments of the experimentally observed lines in the fundamental band of CO around 2160 and 2166 cm−1 are suggested. Thomas.Bondo@uv.es
Coupled cluster calculations of the vertical excitation energies of tetracyanoethylene
2003
The vertical spectrum of tetracyanoethylene was studied using coupled cluster theory. It was found that the lowest singlet-singlet transition, which corresponds to the excitation from the highest occupied molecular orbital (HOMO) to the lowest unoccupied molecular orbital (LUMO) excitation, occurs at 5.16 eV in the gas phase and is lowered approximately 0.1 eV due to solvent effects in acetonitrile. A parallel study on the ethene spectrum showed the quality of the basis sets and methods used, by placing the V state 7.92 eV above the ground state and giving an energy for the 0-0 transition of 5.42 eV to be compared with the experimental value of 5.50 eV.
Karplus-Type Dependence of Vicinal119Sn-13C and119Sn-1H Spin-Spin Couplings in Organotin(IV) Derivatives: A DFT Study
2009
The empirical Karplus-type dependence of (3)J((119)Sn,(13)C) and (3)J((119)Sn,(1)H) couplings in organotin(IV) derivatives has been computationally validated by DFT methods both at the nonrelativistic and scalar ZORA relativistic level. A preliminary calibration of the computational protocols, by comparing experimental and calculated couplings for a Set Of Suitable rigid molecules, revealed their high predictive power: in particular, relativistic results for (3)J((119)Sn,(13)C) have a mean absolute error of just above 2 Hz, over a range of values up to about 70 Hz. The latter protocol has then been used to study in detail the influence of substituents and multiple paths connecting the coupl…
One-bond 1 J (15 N,H) coupling constants at sp2 -hybridized nitrogen of Schiff bases, enaminones and similar compounds: A theoretical study
2020
1 J(15 N,H) coupling constants for enaminones and NH-forms of intramolecularly hydrogen-bonded Schiff bases as model compounds for sp2 -hybridized nitrogen atoms are evaluated using density functional theory (DFT) to find the optimal functionals and basis sets. Ammonia is used as a test molecule and its one-bond coupling constant is compared with experiment. A methylamine Schiff base of a truncated molecule of gossypol is used for checking the performance of selected B3LYP, O3LYP, PBE, BHandH, and APFD density functionals and standard, modified, and dedicated basis sets for coupling constants. Both in vacuum and in chloroform, modeled by the simple continuum model of solvent, the modified b…
A DFT study of the vicinal 3J(119Sn,13C) and 3J(119Sn,1H) coupling constants in trimethyl- and chlorodimethyl-stannyl propanoates.
2013
Abstract We have tested the performance of DFT protocols, both at the Scalar relativistic ZORA and non-relativistic level of theory, for the calculation of the 3 J( 119 Sn, 13 C) and 3 J( 119 Sn, 1 H) vicinal couplings for a series of flexible organotin(IV) derivatives with formula XMe2SnCHRCHR′COOMe (X = Me, Cl; R, R′ = Me, Ph). A satisfactory agreement between experimental and calculated vicinal couplings has been obtained by taking into account the conformational behaviour of the compounds investigated. The protocols used, mainly the relativistic one, were found to give a correct picture of the populations and a sufficiently high overall performance in calculating the vicinal couplings, …
Copper-Assisted Hemiacetal Synthesis: A Cu II Chain Obtained by a One-Step in situ Reaction of Picolinaldehyde
2014
International audience; The 1D polymer complex [Cu2(L)2(SCN)2]n (1 ) has been synthesised in a one‐step in situ reaction of picolinaldehyde with sodium thiocyanate. The complex 1 was characterised by FTIR spectroscopy, UV/Vis spectrophotometry and elemental analysis. The crystal structure of complex 1 shows that chains of dimer complexes are formed with tetra‐ and pentacoordinate copper centres alternately linked by one thiocyanato and two alkoxido bridges. Variable‐temperature magnetic measurements showed a strong antiferromagnetic interaction between the copper centres within the dimer mediated by the two alkoxido bridges with a J value of –374 cm–1, which is in agreement with the DFT‐cal…
Ab initio determination of the ionization potentials of DNA and RNA nucleobases
2006
Quantum chemical high level ab initio coupled-cluster and multiconfigurational perturbation methods have been used to compute vertical and adiabatic ionization potentials of the five canonical DNA and RNA nucleobases: uracil, thymine, cytosine, adenine, and guanine. Several states of their cations have been also calculated. The present results represent a systematic compendium of these magnitudes, establishing theoretical reference values at a level not reported before, calibrating computational strategies, and guiding the assignment of the features in the experimental photoelectron spectra. Daniel.Roca@uv.es Mercedes.Rubio@uv.es Manuela.Merchan@uv.es Luis.Serrano@uv.es