Sören Knuts

THE OH VIBRATIONAL-SPECTRUM OF LIQUID WATER FROM COMBINED ABINITIO AND MONTE-CARLO CALCULATIONS

The infrared vibrational OH stretching spectrum of isotopically isolated HDO molecules in liquid water has been calculated by ab initio methods at the MP2 level for a number of geometrical configurations taken from a Monte Carlo simulation. Each vibrating water molecule with its environment was described by a pentamer supermolecule, surrounded by a large number of point charges representing polarized water molecules. The anharmonic stretching potentials (MP2 force constants up to fifth order) for 40 uncoupled OH water vibrators were calculated. The average computed re distance found for liquid water is 0.01 A longer than the free‐water value. The frequencies were obtained by solving the one‐dimensional Schrodinger equation variationally for each OH potential curve. Using the squared dipole moment derivatives, which vary by a factor of 7 over the frequency band, the density‐of‐states histograms were converted to intensities. The resulting computed average frequency downshift is ∼260 cm−1, compared to ∼310 ...

The vibronically induced phosphorescence in benzene

Abstract The phosphorescence spectrum of benzene is particularly rich in vibronic structure and provides the best-known example of a spectrum of electronic transitions that by symmetry is both spin and orbital forbidden. Such transitions, only allowed through the coupling of nuclear and electronic motions, are notoriously difficult to analyze both on theoretical and experimental grounds. We investigate the vibronically induced phosphorescence by means of multi-configuration quadratic response theory calculations and explore vibronic intensities, polarization directions, transition moments for benzene phosphorescence and the radiative lifetime of triplet benzene. We find that the radiative decay of the 3 B 1u state takes place predominantly through vibronic coupling among the e 2g CC stretching modes ν 8 =1601 cm −1 and ν 9 =1178 cm −1 , with a close to complete out-of-plane polarization. The calculations predict relative intensities of different vibronic bands in good agreement with experiment. The oscillator strength for the 3 B 1u ← 1 A 1g absorption is predicted to 0.74×10 −10 , to be compared with the experimental value of ≈10 −10 . The computed averaged radiative lifetime falls in the interval of 22 to 96 seconds, depending on the quality of the basis set, with the best prediction being 64 seconds.

On the interpretation of the external heavy atom effect on singlet-triplet transitions

Abstract We have studied the cause for the external heavy atom (EHA) effect on radiative singlet-triplet (S-T) transition in π-electronic molecules by means of the quadratic response calculations accounting for spin-orbit coupling. The model systems employed, X+C 2 H 4 , X=F − , Cl − , Br − , HCl, Ar, take into account all essential interactions behind the EHA effect and provide an explanation for the principal physical effect, namely how the intermolecular electrostatic interaction can enhance the magnetic spin-orbit coupling responsible for the S-T transitions. The present study, including the first ab initio investigation of this kind, clearly indicates that the EHA effect should be interpreted as an increased spin-orbit coupling due to back-charge-transfer from the heavy atom. The conclusions are also verified by symmetry and molecular orbital arguments and by sum-over-state decompositions of the respons function results. A strong spin-sublevel selectivity of the EHA effect is obtained, with the in-plane sub-level, the most active one in pure hydrocarbon S-T radiative transitions, being greatly enhanced in the complex. The S-T transition moment exhibits a strong intermolecular distance ( r ) dependence due to the EHA effect. This effect is found quite significant at an equilibrium distance corresponding to halide anion van der Waals complexes in gas phase. Our model is also relevant for interpreting EHA effects in liquid or rigid alkali halide solutions of dyes in polar solvents, when accounting for an increased distance r due to solvation shells, and for intersystem crossings in supersonic jet molecular beams. For the neutral systems (HCl, Ar) there is no significant EHA effect even for those complexes where a strong enhancement is obtained for the isoelectronic Cl − perturber.

Solvatochromatic shifts studied by multi-configuration self-consistent reaction field theory. Application to azabenzenes

Abstract We explore red and blue shifts of low-lying electronic excitation bands accompanying solvation by means of a self-consistent reaction field (SCRF) theory where the chromophore is treated quantum mechanically. A previously devised multi-configuration SCRF (MCSCRF) method has been enhanced to encompass restricted active space MCSCF wave functions, D 2h point group symmetries and higher multipole expansions of the charge and the reaction fields. Applications focus on the series of azabenzenes which show large solvation blue shifts for π→π* transitions but small solvation red shifts for n→π* transitions. It is found that the magnitude of the reaction field contributions to the shifts are of about the same order of magnitude as the polarization contri- butions and that the multipole expansion of the polarizing field cannot be truncated to include only a dipole. The solvation shifts are found to be sensitive to the parameters of the model, such as cavity radius and dielectric constant, but fairly insensitive to the parametrization of the solute wave functions. The role of symmetry broken, local descriptions of the excitations is discussed.

Response theory calculations of the vibronically induced 1A1g−1B2u two-photon spectrum of benzene

Abstract We perform multiconfiguration quadratic response theory calculations of vibronically induced symmetry and parity forbidden 1 A 1g − 1 B 2u two-photon transitions of benzene. The following order of vibronic activity is found: ν 14 > ν 15 > ν 18 > ν 19 > ν 17 > ν 16 > ν 20 . The two-photon activity is crucially dependent on the description of the carbon skeletal motion in the ν 14 (b 2u ) mode, which gives a good illustration of the fact that two-photon spectroscopy can be used to derive information on vibronic motions and force fields. It is suggested that the so far unobserved ν 19 mode should appear in a high-resolution experiment. Two-photon polarization ratios are predicted and discussed.

AN AB-INITIO STUDY OF THE OH STRETCHING FREQUENCIES IN ICE-II, ICE-VIII, AND ICE-IX

Ab initio studies of the uncoupled, anharmonic OH and OD stretching frequency shifts in the three proton‐ordered ice phases known, ice II, ice VIII, and ice IX, are presented. The ice structures are simulated by (H2O)5 supermolecules surrounded by point charges representing the correct crystal potentials. The calculations include electron correlation at the MP2 (DZP) level. For the eight different OH (OD) vibrators studied, the crystal environment leads to a downshift of the anharmonic OD frequency in the range 195–265 cm−1, in good agreement with experimental values (222–281 cm−1) when corrections are made for the limited supermolecular size (∼−45 cm−1), and, for ice VIII, also for the effects of the nonhydrogen bonded network (∼+75 cm−1). Also the agreement between absolute experimental and theoretical OD frequencies is good when errors due to basis set limitation (∼−75 cm−1) are taken into account. The calculations suggest a reassignment of two of the experimental OD bands in ice II and all three exper...

Multiconfiguration response theory calculations of singlet and triplet spectra of the azabenzenes

Abstract Multiconfigurational linear response theory calculations of singlet and triplet excitations of the azabenzenes are presented and used for assignment of their electron energy loss and UV/optical spectra up to the region of the first ionization potentials at approximately 10 eV. Particular attention is given to the role of n-π∗ versus π-π∗ transitions, and to the onset of triplet versus singlet states in the low energy part of the spectra. Calculations test dependences on geometry, one-particle basis set, reference wavefunction with Hartree-Fock, complete and restricted active spaces, and the fulfilment of gauge invariance and sum rules.

The phosphorescence of benzene obtained byab initio and semi-empirical calculations

SummaryRadiative decay and phosphorescence of triplet stare benzene is doubly -orbital and spin- forbidden and is only activated through vibronic coupling among the manifold of triplet states. For this reason the determination of lifetime and transition moments for the decay of triplet benzene has posed a considerable challenge to both theory and experiment. In the present work we have addressed the triplet benzene problem at several levels of theory; by truncated perturbation theory and semiempirical, CNDO/S-CI, calculations; by complete sum-over-state calculations as implemented in recentab initio multiconfiguration quadratic response (MCQR) theory; and by direct MCQR calculations of vibronic phosphorescence. The vibronic coupling is in the two former cases treated by the Herzberg-Teller (H-T) perturbation theory, involving four main mechanisms for the phosphorescent decay of triplet benzene. The results and interpretations given by these approaches as well as their merits and limitations are presented and discussed in some detail. Our calculations indicate that the phosphorescent decay of the3B1u state takes place predominantly through vibronic coupling along thee2g mode. We obtain a phosphorescence that is almost completely out-of-plane polarized, which is in line with more recent measurements by the microwave-induced delayed phosphorescence technique, and could reproduce quite well the intensity ratios for different vibronic bands obtained in that experiment. The final triplet state lifetime is the result of a delicate sum of contributions from several vibronic degenerate and non-degenerate modes. The direct vibronic phosphorescence calculations predict a long lifetime, about one minute — 68 seconds for the best wavefunction — and seem to focus on a doubling of the assumed, albeit not established, “best experimental” value for the radiative lifetime of triplet benzene; ⋍ 30 seconds.

The two-photon spectrum of pyrimidine. Role of vibronic coupling

Abstract The one- and two-photon spectra of the lowest electronic states of pyrimidine are analyzed by means of response theory calculations. Particular attention is paid to the lower one-photon forbidden (nπ*) 1 A 2 and one-photon allowed (ππ*) 1 B 1 and 1 B 2 states. Despite being electronically allowed it is found that the vibronically induced intensity for the two-photon transition to the first 1 B 2 state is much larger than the intensity derived from the pure electronic contribution. A detailed theoretical two-photon vibronic spectrum is presented showing strong excitations of the b 2 mode and a shift of the two-photon absorption maximum as much as 1600 cm −1 from the corresponding one-photon maximum, in good agreement with experiment

Phosphorescence of aromatic molecules

Abstract Quadratic response theory for singlet and triplet operators (O. Vahtras et al., J. Chem. Phys., 97 (1992) 9178) have recently been applied to series of small molecules as well as to several aromatic compounds. A comparative analysis of results of such calculations on the phosphorescence effect in benzene, naphthalene and various azabenzenes and azanaphthalenes is presented. The information gained from such calculations concern polarization directions, oscillator strengths, radiative lifetimes and excitation energies for the triplet states. These quantities either refer to values averaged over the triplet states or to the specific triplet state spin sublevels. The vibronically induced phosphorescence problem, with specific reference to benzene phosphorescence which is forbidden both by spin and orbital symmetry and only allowed through the coupling of nuclear and electronic motions is also discussed. Results are compared with vapor phase data concerning total radiative lifetimes, and with data from phosphorescence microwave double resonance (PMDR) measurements of matrix isolated samples concerning the spin sublevel rates.