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Martin J Paterson

Publications and source records attributed to Martin J Paterson.

8 recordsLinked to original sources
PubMed2006

Linear response functions for a vibrational configuration interaction state.

Linear response functions are implemented for a vibrational configuration interaction state allowing accurate analytical calculations of pure vibrational contributions to dynamical polarizabilities. Sample calculations are presented for the pure vibrational contributions to the polarizabilities of water and formaldehyde. We discuss the convergence of the results with respect to various details of the vibrational wave function description as well as the potential and property surfaces. We also analyze the frequency dependence of the linear response function and the effect of accounting phenomenologically for the finite lifetime of the excited vibrational states. Finally, we compare the analytical response approach to a sum-over-states approach.

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PubMed2006

Two-photon absorption cross sections: an investigation of solvent effects. Theoretical studies on formaldehyde and water.

The effects of a solvent on the two-photon absorption of microsolvated formaldehyde and liquid water have been studied using hybrid coupled-cluster/molecular mechanics (CC/MM) response theory. Both water and formaldehyde were considered solvated in water, where the solvent water molecules were described within the framework of molecular mechanics. Prior to the CC/MM calculations, molecular dynamics simulations were performed on the water/formaldehyde and water/water aggregates and many configurations were generated. By carrying out CC/MM response calculations on the individual configurations, it was possible to obtain statistically averaged results for both the excitation energies and two-photon absorption cross sections. For liquid water, the comparison between one- and two-photon absorption spectra is in good agreement with the experimental data available in the literature. In particular, the lowest energy transition occurring in the one-photon absorption spectrum of water only occurs with a relatively small strength in the two-photon absorption spectrum. This result is important for the interpretation of two-photon absorption data as these results show that in the absence of selection rules that determine which transitions are forbidden, the spectral profile of the two-photon absorption spectrum can be significantly different from the spectral profile of the one-photon absorption spectrum.

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PubMed2006

A conformationally flexible, urea-based tripodal anion receptor: solid-state, solution, and theoretical studies.

Tripodal tris(urea) cationic receptors 1 and 2 containing p-tolyl or octyl substituents, respectively, have been synthesized, and their association behavior with anionic guests has been studied via a variety of methods. The receptors are based around a hexasubstituted aryl core and contain both urea and pyridinium functionalities. For 1:1 complexes, anions reside within the central cavity of the host species, held by hydrogen bonds from both NH and CH donors. The following host-anion complexes have been characterized by X-ray crystallography: 1-(Br)3, 1-(PF6)3.2(CH3)2CO, and 1-(NO3)1.5(PF6)1.5. Each structure contains the receptor in a significantly different geometry, highlighting the anion-dependent conformational flexibility of 1. Solution 1H NMR spectroscopic titrations have shown the two host species to display significant affinity for both halides and hydrogen sulfate and strongly suggest the persistence of CH...X- interactions despite the presence of "stronger" NH donor groups. Variable-temperature 1H NMR studies on the more soluble octyl derivative 2 show that there is a distinct change in conformation associated with the formation of a 1:1 host/guest complex. Computations using density functional theory (with the B3LYP functional) have been employed to aid in understanding the geometry of the 1:1 host/chloride complexes of 1 and 2. These experiments suggest that the lowest energy conformation for 1-Cl is one in which the ureidopyridinium arms are orientated upward forming a cavity that is sealed by CH...pi interactions, effectively forming a unimolecular capsule, whereas for 2 a less symmetrical "2-up, 1-down" geometry is favored.

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PubMed2006

Benchmarking two-photon absorption with CC3 quadratic response theory, and comparison with density-functional response theory.

We present a detailed study of the effects of electron correlation on two-photon absorption calculated by coupled cluster quadratic response theory. The hierarchy of coupled cluster models CCS, CC2, CCSD, and CC3 has been used to investigate the effects of electron correlation on the two-photon absorption cross sections of formaldehyde (CH2O), diacetylene (C4H2), and water (H2O). In particular, the effects of triple excitations on two-photon transition cross sections are determined for the first time. In addition, we present a detailed comparison of the coupled cluster results with those obtained from Hartree-Fock and density-functional response theories. We have investigated the local-density approximation, the pure Becke-Lee-Yang-Parr (BLYP) functional, the hybrid Becke-3-parameter-Lee-Yang-Parr (B3LYP), and the Coulomb-attenuated B3LYP (CAM-B3LYP) functionals. Our results show that the CAM-B3LYP functional, when used in conjuction with a one-particle basis-set containing diffuse functions, has much promise; however, care must still be exercised for diffuse Rydberg-type states.

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PubMed2005

Mechanism of an Exceptional Class of photostabilizers: a seam of conical intersection parallel to excited state intramolecular proton transfer (ESIPT) in o-hydroxyphenyl-(1,3,5)-triazine.

We present a detailed CASSCF study of the mechanism of excited-state intramolecular proton transfer (ESIPT) in the o-hydroxyphenyl triazine class of photostabilizers. The valence-bond analysis of the ground state and the two pipi* excited states permits a simple chemical interpretation of the mechanistic information. Our results show that the barrier to enol-keto tautomerism on the ground-state adiabatic surface is high. Following photoexcitation to the charge-transfer state, the ESIPT is predicted to take place without a barrier. Radiationless decay to the ground state is associated with an extended seam of conical intersection, with a sloped topology lying parallel to the ESIPT path, which can be accessed at any point along the reaction path. Our results show that the triazine class of photostabilizers has the photochemical and photophysical qualities associated with exceptional photostability.

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PubMed2004

The curvature of the conical intersection seam: an approximate second-order analysis.

We present a method for analyzing the curvature (second derivatives) of the conical intersection hyperline at an optimized critical point. Our method uses the projected Hessians of the degenerate states after elimination of the two branching space coordinates, and is equivalent to a frequency calculation on a single Born-Oppenheimer potential-energy surface. Based on the projected Hessians, we develop an equation for the energy as a function of a set of curvilinear coordinates where the degeneracy is preserved to second order (i.e., the conical intersection hyperline). The curvature of the potential-energy surface in these coordinates is the curvature of the conical intersection hyperline itself, and thus determines whether one has a minimum or saddle point on the hyperline. The equation used to classify optimized conical intersection points depends in a simple way on the first- and second-order degeneracy splittings calculated at these points. As an example, for fulvene, we show that the two optimized conical intersection points of C2v symmetry are saddle points on the intersection hyperline. Accordingly, there are further intersection points of lower energy, and one of C2 symmetry--presented here for the first time--is found to be the global minimum in the intersection space.

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PubMed2004

Theoretical study of benzotriazole UV photostability: ultrafast deactivation through coupled proton and electron transfer triggered by a charge-transfer state.

A theoretical CASSCF study of the reaction path for excited-state intramolecular proton transfer (ESIPT) for a model system derived from the UV absorber 2-(2'-hydroxyphenyl) benzotriazole without the fused benzo ring on the triazole has been carried out. A planar reaction path can be optimized but is shown to have no physical significance. The true reaction path involves twisted geometries. Adiabatic proton transfer is triggered by a charge-transfer from the phenol to the triazole group, and is followed by radiationless decay at the keto form. Along the nonplanar reaction path, there is a coupled proton and electron transfer in a manner similar to tryptophan. This rationalizes unexpected experimental results on the effect of electron withdrawing substituent groups on the photostability. The coupled proton and electron transfer is followed by a barrierless relaxation in the ground state to recover the enol form. An alternative photostabilization pathway from a phenyl localized state has also been documented and is similar to the channel 3 decay pathway in benzene photochemistry. Additionally, a long-lived intermediate for a twisted intramolecular charge-transfer (TICT) state has been identified as the species potentially responsible for the increase of blue fluorescence in strongly polar media.

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