Density functional theory predictions of anharmonicity and spectroscopic constants for diatomic molecules

Xe⋯OCS: relatively straightforward?

Spectroscopy meets theory in a study of Xe⋯OCS complex: accurate near-equilibrium structures, experimental interaction energies, and CCSD(T)/CBS results presented.

Local protein solvation drives direct down-conversion in phycobiliprotein PC645 via incoherent vibronic transport

The mechanisms controlling excitation energy transport (EET) in light-harvesting complexes remain controversial. Following the observation of long-lived beats in 2D electronic spectroscopy of PC645, vibronic coherence, the delocalization of excited states between pigments supported by a resonant vibration, has been proposed to enable direct excitation transport from the highest-energy to the lowest-energy pigments, bypassing a collection of intermediate states. Here, we instead show that for phycobiliprotein PC645 an incoherent vibronic transport mechanism is at play. We quantify the solvation dynamics of individual pigments using ab initio quantum mechanics/molecular mechanics (QM/MM) nuclear dynamics. Our atomistic spectral densities reproduce experimental observations ranging from absorption and fluorescence spectra to the timescales and selectivity of down-conversion observed in transient absorption measurements. We construct a general model for vibronic dimers and establish the parameter regimes of coherent and incoherent vibronic transport. We demonstrate that direct down-conversion in PC645 proceeds incoherently, enhanced by large reorganization energies and a broad collection of high-frequency vibrations. We suggest that a similar incoherent mechanism is appropriate across phycobiliproteins and represents a potential design principle for nanoscale control of EET.

Terahertz (6-15 THz) Spectroscopy and Numerical Modeling of Intermolecular Vibrations in Benzoic Acid and Its Derivatives

Terahertz spectroscopy of benzoic acid (BA) and two of its derivatives, 2-hydroxybenzoic acid (2OH-BA) and 3-hydroxybenzoic acid (3OH-BA), has been investigated in the spectral region 200 to 500 cm(-1) (6.06 to 15.15 THz). The spectra show distinct absorption features. There is agreement between some of the absorption lines observed for these compounds, despite a shift in energy, which is attributed to the differences in the molecular structures. Numerical modeling gave corresponding absorption lines, and this helped in the assignment. Temperature dependence studies revealed that most of the absorption lines are composite lines in this frequency region.

Concerted and Stepwise Reaction Mechanisms for the Addition of Ozone to Acetylene: A Computational Study

The mechanism of the reaction between acetylene and ozone to form a primary ozonide (POZ) in the gas phase has been studied theoretically. The concerted pathway, HCCH + O3 --> POZ, proceeds via a biradicaloid transition state TS0. The stepwise pathway is a three-step reaction, HCCH + O3 --> M1 --> M2 --> POZ, involving two biradical TSs and two biradical intermediates M1 and M2. The segment of the global potential energy surface (PES) for the concerted pathway is characterized as a R-PES, which is obtained from the restricted (R) density functional theory and Hartree-Fock-based methods. The RDFT and RHF solutions of TS0 and O3 are unstable toward spin-symmetry breaking. The wave function instability for TS0 and O3 results in a discontinuity between the R-PES and the region of the global PES encompassing the biradical TSs and the intermediates of the stepwise pathway, which are characterized with unrestricted (U) methods. The global PES is characterized separately as an U(R)-PES using a combination of the R and U methods. Several different values of barriers for the concerted pathway and the energy of concert (Ec) can be estimated due to complications arising from the discontinuity between the R- and the U(R)-PES and the existence of two different RDFT and UDFT O3 equilibrium geometries. RCCSD(T)//RDFT predicts a barrier of 8.2 kcal/mol. U(R)CCSD(T)/U(R)DFT predicts a barrier of 13.8 kcal/mol for the concerted and 15.3 kcal/mol for the stepwise pathway. Comparison between the R-PES barrier to the concerted pathway and the U(R)-PES barrier to the stepwise pathway suggests the former to be the only significant mechanism. Consideration of the energy difference between TS1, the TS for the first step of the stepwise mechanism, and TS0 within the global PES leads to a significantly smaller Ec. Geometry optimization with CASSCF and energy point calculations with MRMP2 are employed to characterize TS0 and TS1. MRMP2//CASSCF predicts the energy level of TS1 to be higher than that of TS0 by 2 kcal/mol. Analysis of experimental and computational data based on the low estimate of Ec shows that the possibility of the stepwise pathway being a secondary channel at elevated temperatures cannot be ruled out.

Prediction of spectroscopic constants for diatomic molecules in the ground and excited states using time-dependent density functional theory

Spectroscopic constants of the ground and next seven low-lying excited states of diatomic molecules CO, N2, P2, and ScF were computed using the density functional theory SAOP/ATZP model, in conjunction with time-dependent density functional theory (TD-DFT) and a recently developed Slater type basis set, ATZP. Spectroscopic constants, including the equilibrium distances r(e), harmonic vibrational frequency omega(e), vibrational anharmonicity omega(e)x(e), rotational constant B(e), centrifugal distortion constant D(e), the vibration-rotation interaction constant alpha(e), and the vibrational zero-point energy E(n)0 were generated in an effort to establish a reliable database for electron spectroscopy. By comparison with experimental values and a similar model with an established larger Slater-type basis set, et-QZ3P-xD, it was found that this model provides reliably accurate results at reduced computational costs, for both the ground and excited states of the molecules. The over all errors of all eight lowest lying electronic states of the molecules under study using the effective basis set are r(e)(+/-4%), omega(e)(+/-5% mostly without exceeding +/-20%), omega(e)x(e)(+/-5% mostly without exceeding 20%, much more accurate than a previous study on this constant of +/-30%), B(e)(+/-8%), D(e)(+/-10%), alpha(e)(+/-10%), and E(n)0(+/-10%). The accuracy obtained using the ATZP basis set is very competitive to the larger et-QZ3P-xD basis set in particular in the ground electronic states. The overall errors in r(e), omega(e)x(e), and alpha(e) in the ground states were given by +/-0.7, +/-10.1, and +/-8.4%, respectively, using the efficient ATZP basis set, which is competitive to the errors of +/-0.5, +/-9.2, and +/-9.1%, respectively for those constants using the larger et-QZ3P-xD basis set. The latter basis set, however, needs approximately four times of the CPU time on the National Supercomputing Facilities (Australia). Due to the efficiency of the model (TD-DFT, SAOP and ATZP), it will be readily applied to study larger molecular systems.

The Electronic Structure of Alkali Aurides. A Four-Component Dirac−Kohn−Sham Study

Spectroscopic constants, including dissociation energies, harmonic and anharmonic vibrational frequencies, and dipole moments, are calculated for the complete alkali auride series (LiAu, NaAu, KAu, RbAu, CsAu). The four-component formulation of relativistic density functional theory has been employed in this study, using the G-spinor basis sets implemented recently in the program BERTHA. The performance of four standard nonrelativistic density functionals employed is investigated by comparing the results with the best available theoretical and experimental data. The present work provides the first theoretical predictions on the molecular properties of RbAu. The intermetallic bond that occurs in the alkali auride series is highly polar and is characterized by a large charge transfer from the alkali metals to gold. The extent of this electron transfer has been investigated using several different charge analysis methods, enabling us to reach some general conclusions on their relative performance. We further report a detailed analysis of the topological properties of relativistic electron density in the bonding region, discussing the features of this approach which characterize the nature of the chemical bond. We have also computed the fully relativistic density for the alkali halides MBr and MI (M = Li, Na, K, Rb, and Cs). The comparative study shows that, on the basis of several topological properties and the variation in bond lengths, the gold atom behaves similarly to a halogen intermediate between Br and I.

Computational strategies for a four-component Dirac–Kohn–Sham program: Implementation and first applications

An implementation of the generalized gradient approximation within the four-component formulation of relativistic density-functional theory using G-spinor basis sets is presented. This approach is based on the direct evaluation of the relativistic density and its gradient from the G-spinor amplitudes and gradients without explicit reference to the total density matrix. This proves to be a particularly efficient scheme, with an intrinsic computational cost that scales linearly with the number of G-spinor basis functions. In order to validate this new implementation, incorporated in the parallel version of the program BERTHA, a detailed study of the diatomic system CsAu is also reported. The spectroscopic constants D(e),r(e),omega(e), and x(e)omega(e) and the dipole moment mu have been calculated and compared with the best available theoretical and experimental data. The sensitivity of our results to the details of the numerical schemes used to evaluate the matrix elements is analyzed in detail. Also presented is a comparative study of molecular properties in the alkali auride series which have been obtained using several standard non-relativistic density functionals.

Vibrational computations beyond the harmonic approximation: Performances of the B3LYP density functional for semirigid molecules

The performances of the B3LYP density functional in the computation of harmonic and anharmonic frequencies were tested using 14 standard basis sets of double and triple zeta quality for a set of semirigid molecules containing from 4 to 12 atoms. The quality of the results is assessed by comparison with the most reliable computations available in the literature. The study reveals that the relatively cheap 6-31+G(d,p) basis set performs a very good job for harmonic frequency calculations and that B3LYP anharmonicities are in close agreement with the reference values irrespective of the basis set used. On these grounds "hybrid force fields" are proposed to achieve the best compromise between computer time and quality of the results.

Insights into the Structure of Cyclohexane from Femtosecond Degenerate Four-Wave Mixing Spectroscopy and Ab Initio Calculations

In this paper, we report the use of femtosecond time-resolved degenerate four-wave mixing rotationally resolved spectroscopy to obtain very accurate structural information on the symmetric top cyclohexane. Apart from highlighting the versatility of this method in determining accurate structures of large and complex molecules without dipole moment, the present study also details the comparison of the experimentally determined rotational constant B(0) with that obtained from high-level ab initio calculations. The theoretical calculations, which were carried out at both the second-order Møller-Plesset (MP2) and coupled-cluster with single, double, and perturbative triple substitutions [CCSD(T)] levels of theory, also take into account vibrational averaging effects. A detailed investigation of the vibrational averaging effects reveals that the corrections emerge from only the six highly symmetric A(1g) modes, a justification of which is provided by an analysis of these modes.