José L. Andrés

A systematic and feasible method for computing nuclear contributions to electrical properties of polyatomic molecules

An analytic method to evaluate nuclear contributions to electrical properties of polyatomic molecules is presented. Such contributions control changes induced by an electric field on equilibrium geometry (nuclear relaxation contribution) and vibrational motion (vibrational contribution) of a molecular system. Expressions to compute the nuclear contributions have been derived from a power series expansion of the potential energy. These contributions to the electrical properties are given in terms of energy derivatives with respect to normal coordinates, electric field intensity or both. Only one calculation of such derivatives at the field-free equilibrium geometry is required. To show the useful efficiency of the analytical evaluation of electrical properties (the so-called AEEP method), results for calculations on water and pyridine at the SCF/TZ2P and the MP2/TZ2P levels of theory are reported. The results obtained are compared with previous theoretical calculations and with experimental values.

Finite field treatment of vibrational polarizabilities and hyperpolarizabilities: On the role of the Eckart conditions, their implementation, and their use in characterizing key vibrations

In the finite field (FF) treatment of vibrational polarizabilities and hyperpolarizabilities, the field-free Eckart conditions must be enforced in order to prevent molecular reorientation during geometry optimization. These conditions are implemented for the first time. Our procedure facilities identification of field-induced internal coordinates that make the major contribution to the vibrational properties. Using only two of these coordinates, quantitative accuracy for nuclear relaxation polarizabilities and hyperpolarizabilities is achieved in π-conjugated systems. From these two coordinates a single most efficient natural conjugation coordinate (NCC) can be extracted. The limitations of this one coordinate approach are discussed. It is shown that the Eckart conditions can lead to an isotope effect that is comparable to the isotope effect on zero-point vibrational averaging, but with a different mass-dependence.

Molecular electric properties and nuclear and vibrational relaxation. An ab initio study of HF, CH4 and C2H4

The effects of nuclear relaxation and change in vibrational molecular energy are accounted for in the theoretical determination of the molecular properties of HF, CH4, and C2H4. A method based on a finite-difference technique is employed. An analysis of the influence of these contributions versus the purely electronic contribution is made, together with an appraisal of the effect of vibrational excitation.

Nuclear relaxation contribution to static and dynamic (infinite frequency approximation) nonlinear optical properties by means of electrical property expansions: Application to HF, CH4, CF4, and SF6

Electrical property derivative expressions are presented for the nuclear relaxation contribution to static and dynamic (infinite frequency approximation) nonlinear optical properties. For CF4 and SF6, as opposed to HF and CH4, a term that is quadratic in the vibrational anharmonicity (and not previously evaluated for any molecule) makes an important contribution to the static second vibrational hyperpolarizability of CF4 and SF6. A comparison between calculated and experimental values for the difference between the (anisotropic) Kerr effect and electric field induced second-harmonic generation shows that, at the Hartree–Fock level, the nuclear relaxation/infinite frequency approximation gives the correct trend (in the series CH4, CF4, SF6) but is of the order of 50% too small.

Anharmonicity contributions to the vibrational second hyperpolarizability of conjugated oligomers

Restricted Hartree–Fock 6-31G calculations of electrical and mechanical anharmonicity contributions to the longitudinal vibrational second hyperpolarizability have been carried out for eight homologous series of conjugated oligomers—polyacetylene, polyyne, polydiacetylene, polybutatriene, polycumulene, polysilane, polymethineimine, and polypyrrole. To draw conclusions about the limiting infinite polymer behavior, chains containing up to 12 heavy atoms along the conjugated backbone were considered. In general, the vibrational hyperpolarizabilities are substantial in comparison with their static electronic counterparts for the dc-Kerr and degenerate four-wave mixing processes (as well as for static fields) but not for electric field-induced second harmonic generation or third harmonic generation. Anharmonicity terms due to nuclear relaxation are important for the dc-Kerr effect (and for the static hyperpolarizability) in the σ-conjugated polymer, polysilane, as well as the nonplanar π systems polymethineimi...

Iridium Compounds with κ-P,P,Si (biPSi) Pincer Ligands: Favoring Reactive Structures in Unsaturated Complexes

The structure, coordination properties, insertion processes, and dynamic behavior in solution of the five-coordinate complexes [IrXH(biPSi)] (biPSi = kappa-P,P,Si-Si(Me){(CH(2))(3)PPh(2)}(2); X = Cl (1), Br (2), or I (3)) have been investigated. The compounds are formed as mixtures of two isomers, anti and syn, in slow equilibrium in solution. The equilibrium position depends on the halogen and the solvent. Both isomers display distorted square-based pyramidal structures in which the vacant position sits trans to silicon. The equatorial plane of the syn isomer is closer to the T structure due to distortions of steric origin. The small structural differences between the isomers trigger remarkable differences in reactivity. The syn isomers form six-coordinate adducts with chlorinated solvents, CO, P(OMe)(3), or NCMe, always after ligand coordination trans to silicon. The anti isomers do not form detectable adducts with chlorinated solvents and coordinate CO or P(OMe)(3) either trans to silicon (kinetic) or trans to hydride (thermodynamic). NCMe coordinates the anti isomers exclusively at the position trans to hydride. Qualitative and quantitative details (equilibrium constants, enthalpies, entropies, etc.) on these coordination processes are given and discussed. As a result of the different coordination properties, insertion reagents such as acetylene, diphenylacetylene, or the alkylidene resulting from the decomposition of ethyl diazoacetate selectively insert into the Ir-H bond of 1-syn, not into that of 1-anti. These reactions give five-coordinate syn alkenyl or alkyl compounds in which the vacancy also sits trans to silicon. Acetylene is polymerized in the coordination sphere of 1. The nonreactive isomer 1-anti also evolves into the syn insertion products via anti<-->syn isomerizations, the rates of which are notably dependent on the nature of the insertion reactants. H(2) renders anti<-->syn isomerization rates of the same order as the NMR time scale. The reactions are second order (k(obs) = k(anti<-->syn)[H(2)]) and do not involve H(2)/IrH hydrogen atom scrambling. A possible isomerization mechanism, supported by MP2 calculations and compatible with the various experimental observations, is described. It involves Ir(V) intermediates and a key sigma Ir-(eta(2)-SiH) agostic transition state. A similar transition state could also explain the anti<-->syn isomerizations in the absence of oxidative addition reactants, although at the expense of high kinetic barriers strongly dependent on the presence of potential ligands and their nature.

Influence of an external uniform electric field on harmonic vibrational frequencies. Analytic energy second derivatives for closed‐shell restricted Hartree–Fock wave functions with an applied uniform electric field

Expressions are given for analytic determination of energy second derivatives of closed‐shell restricted Hartree–Fock (RHF) wave functions under the effect of an external uniform electric field. In this way, harmonic vibrational frequencies can be computed. Application of the theoretical method to the methane molecule is presented, and comparison with the induced changes in vibrational bands of its experimental infrared spectrum is carried out.

Potential energy surface of cyclooctatetraene

We present a theoretical study of the cyclooctatetraene (COT) molecule. Seven COT structures are located on the singlet ground state potential energy surface. Four of them, which present D2d (tub), Cs (bicyclo[4.2.0]octa-2,4,7-triene or BOT), C2h (chair) and D4 (crown) symmetries are stable species, and the other three are transition state structures showing Cs, D4h, and D8h symmetry. We discuss the symmetry of wave functions for these stationary points. Geometries, energies, and harmonic vibrational frequencies of these structures, and energy gaps between singlet–triplet states and low-lying singlets are presented. For the planar D4h and D8h structures, Jahn–Teller and tunneling effects have also been discussed. Ring inversion, bond shifting and valence isomerization reactive channels from the tub COT conformer are discussed from the point of view of the corresponding transition state structures. Where possible, in order to lend support to this theoretical information comparisons with recent transition s...

Systematic study of the static electrical properties of the CO molecule: Influence of the basis set size and correlation energy

The influence of the basis set size and the correlation energy in the static electrical properties of the CO molecule is assessed. In particular, we have studied both the nuclear relaxation and the vibrational contributions to the static molecular electrical properties, the vibrational Stark effect ~VSE! and the vibrational intensity effect ~VIE!. From a mathematical point of view, when a static and uniform electric field is applied to a molecule, the energy of this system can be expressed in terms of a double power series with respect to the bond length and to the field strength. From the power series expansion of the potential energy, field-dependent expressions for the equilibrium geometry, for the potential energy and for the force constant are obtained. The nuclear relaxation and vibrational contributions to the molecular electrical properties are analyzed in terms of the derivatives of the electronic molecular properties. In general, the results presented show that accurate inclusion of the correlation energy and large basis sets are needed to calculate the molecular electrical properties and their derivatives with respect to either nuclear displacements or/and field strength. With respect to experimental data, the calculated power series coefficients are overestimated by the SCF, CISD, and QCISD methods. On the contrary, perturbation methods ~MP2 and MP4! tend to underestimate them. In average and using the 6-3111G(3 df ) basis set and for the CO molecule, the nuclear relaxation and the vibrational contributions to the molecular electrical properties amount to 11.7%, 3.3%, and 69.7% of the purely electronic m, a, and b values, respectively.

Theoretical study of infrared spectra perturbed by uniform electric fields: Ab initio calculations on H2O, NH3, H2CO, and C2H4

The changes in harmonic vibrational frequencies and line intensities with respect to different strengths of an applied electric field are computed using analytically calculated energy derivatives. The ground states of several small molecules are studied at the ab initio restricted Hartree–Fock self‐consistent‐field (RHF SCF) level with a double ζ plus polarization basis set. The dependence of vibrational frequencies and line intensities upon the applied field strength is discussed.