The full nine-dimensional potential energy surfaces (PESs) of the 3Q0 and 1Q1 states of CH3I have been calculated with the ab initio contracted spin–orbit configuration interaction method. The results are fitted to three diabatic potential terms and their couplings as functions of all the internal degrees of freedom. The transition dipole at the Franck–Condon region has also been calculated. Surface hopping quasiclassical trajectory calculations on these potential energy surfaces have been performed to examine the photodissociation dynamics of both CH3I and CD3I in the A-continuum. The results are in general good agreement with the recent experimental findings. The reasonable I*/(I*+I) branching ratio can be obtained with these PESs when the contribution of direct transition to the 1Q1 state is considered. The rotational distribution of the CH3 and CD3 fragments and its I*/(I*+I)-channel selectivity are determined by the shape of the PESs with respect to the bending angle outside the conical intersection region. The vibrational distribution of umbrella mode is closely related to the shape of PESs for the umbrella angle; the sudden switch of reaction coordinate from 3Q0 to 1Q1 at the conical intersection is the origin of vibrational excitation in the I* channel. The larger umbrella excitation of the CD3 fragment in both I and I* channels, in comparison with the CH3 fragment, is related to the larger separation of the reaction coordinate from the Franck–Condon geometry. The symmetric stretching energy increases during the dissociation, which is related to the shape of PESs with respect to this coordinate, and the excitation of symmetric stretching mode seems to be possible.

A b initio contracted spin–orbit configuration interaction (SOCI) calculations have been carried out to obtain potential energy surfaces of 3Q0 and 1Q1 excited states of methyl iodide as functions of all the geometrical parameters except for the three C–H stretches. The results are fitted to six-dimensional diabatic potential functions and their couplings. Classical trajectory calculations have been performed using these potential functions. The rotation of the CH3 product in the I channel has been calculated to be perpendicular to the top axis and to have a peak at N=5 and extend up to N=8, whereas it is cold in the I* channel, in good agreement with recent experiments. The CH3 rotation is excited by the time trajectories arrive at the conical intersection region; this excitation is retained in the I-channel product because the 1Q1 surface has a small bending force constant outside the conical intersection, whereas it is damped in the I* channel because 3Q0 still has a large bending force constant. The calculated distribution in the ν2 umbrella vibrational mode of the CH3 product is hot and has a peak at v=2 for the I channel, and is cool for the I* channel, in good agreement with recent experiments. This channel selectivity is due to the difference in the preferred structure of the CH3 group outside the conical intersection region; while the 3Q0 surface prefers a bent CH3 until the CH3–I distance becomes very large, 1Q1 wants a planar CH3. The location of conical intersection and the ground-excited energy difference there are in good agreement with those deduced from experiment if a dynamical effect is taken into account.

The photodissociation reaction of ICN in the A continuum has been theoretically studied based on ab initio potential energy surfaces and classical trajectories. Ab initio contracted spin–orbit configuration interaction calculations have been carried out to obtain potential energy surfaces (PES’s) of 3Π1, 3Π0+ and 1Π1 excited states, where results are fit to five diabatic potential functions and their couplings as functions of all three internal degrees of freedom. The transition dipoles at the Franck–Condon region have also been calculated. All the PES’s involved in photodissociation are bent near the Franck–Condon region. Classical trajectory calculations performed on these potential surfaces have produced results that are in agreement with various experimental findings and provide a basis for their interpretation. The calculations indicate that the absorption is a mixture of parallel and perpendicular transition. A reasonable I/I* branching ratio can be obtained by considering the effect of initial bending vibrations in addition to the character of mixed transitions. The I/I* channel selectivity of the CN rotation can be compared to the shape of PES’s with respect to the bending angle. The rotational excitation of the CN fragment is determined by the shape of PES’s on which trajectories travel before and after the transition. The higher rotational component in the I channel is attributed to the energy gradient of 1Π1 with respect to the bending angle at the transition region where the C–I distance is between 5.0 and 8.0 a.u. The lower component in the I channel emerges from 3Π1. The average rotational distribution obtained with the proper weight of Boltzmann populations and transition intensities is in agreement with the experiment. This interpretation can also be applied to the rotational quantum number dependence of anisotropy parameters. Trajectory calculations on the 3Π1 surface alone, give a single Boltzmann rotational distribution. Reflecting the shape of PES’s with respect to the CN distance, the product CN vibration on 3Π0+ and 1Π becomes suppressed while that on 3Π1 becomes slightly more excited. The anisotropic parameter was also analyzed. Some comments on the femtosecond transition spectroscopy are also made.

We report three-dimensional quantum mechanical calculations on the photodissociation dynamics of CH3I and CD3I on new ab initio potential energy surfaces. The improved potentials are obtained in the contracted spin−orbit configuration interaction framework by using a larger basis set and more spin-free configurations. The dynamical model includes the C−I stretch, C−H3 umbrella bend, and I−C−H3 bend and allows the overall rotation. The wave packet is propagated in the Chebyshev order domain. The absorption spectrum, product vibrational and rotational distributions, I* quantum yield, and state-resolved angular distributions are calculated for the parent states of |JMK〉 = |000〉 and |111〉, and compared with experiments. The new potential energy surfaces yield a much better agreement with the experimental absorption spectrum, thanks to small potential gradients in the Franck−Condon region. The calculated rovibrational distributions of the methyl fragment are also in good agreement with experimental data. It is shown that the overall rotation has significant effects on the methyl rotational and vibrational distributions as well as the I* yield.

Ab initio calculations have been performed to examine the photochemical behavior of 4-(dimethylamino)benzenzonitrile (DMABN). The conical intersection between S2 and S1 (S2/S1-CIX), where the internal conversion takes place after the main transition of S0-S2 at the equilibrium geometry in S0, is characterized by a dimethylamino-twisted quinoid structure where aromaticity of the benzene ring is lost. The optimized geometry of the charge transfer (CT) state in S1 has a feature similar to that of S2/S1-CIX but is not energetically stabilized so much. Consequently, electronically excited DMABN with CT character relaxes into the most stable locally excited (LE) state in S1 through a recrossing at S2/S1-CIX in gas phase or nonpolar solvent. In polar solvent, in contrast, the equilibration between LE and CT takes place in S1 so that the CT state is more stable because of electrostatic interaction. The excited states of DMABN derivatives have been also examined. On the basis of the present computational results, a new and simple guiding principle of the emission properties is proposed, where conventional twisted intramolecular CT (TICT) and planar intramolecular CT (PICT) models are properly incorporated.

Ab initio complete active space self-consistent field (CASSCF) and second-order multireference Möller−Plesset (MRMP2) calculations have been performed to examine the photochemical behavior of diphenylacetylene (DPA) theoretically. The stable structure of DPA in S0 (S0-geometry) is optimized to be D 2 h . DPA at S0-geometry is mainly excited into the S3(B1u) state and then relaxes into the stable geometry in the B1u state (B1u-geometry) which is characterized as a quinoid structure. The B1u-geometry further relaxes into the globally stable geometry in S1 (tS1-geometry) which takes a trans-bent form. Around tS1-geometry, DPA moves into the lowest triplet state through intersystem crossing and finally relaxes into the stable geometry in T1 with D 2 h . The vibrational analyses at the important conformations mentioned above are in good agreement with the experimental findings of time-resolved transient spectroscopy.

We present a time-dependent quantum mechanical calculation of ICN photodissociation in the à continuum, using the ab initio potential surfaces of Morokuma and co-workers [S. Yabushita and K. Morokuma, Chem. Phys. Lett. 175, 518 (1990); Y. Amatatsu, S. Yabushita, and K. Morokuma, J. Chem. Phys. 100, 4894 (1994)]. Five excited state potential energy surfaces are included in this model, 3Π0+, 1Π1 (A′,A″), and 3Π1 (A′,A″), which are accessed, respectively, by parallel, perpendicular, and perpendicular transitions from the ground state. The calculated absorption spectrum, β parameters, the I/I* branching ratio, and the rotational product distribution are in good agreement with experiment. The I/I* branching ratio for photodissociation from vibrationally excited states of ICN has been calculated. The results are in good agreement with the recent measurements at different vibrational temperatures by Kash and Butler [P. W. Kash and L. J. Butler, J. Chem. Phys. 96, 8923 (1992)] at 249 nm but, interestingly, predict opposite trends at higher and lower excitation energies.

The mechanism of charge transfer (CT) state formation of excited state 4-(N,N-dimethylamino)benzonitrile (DMABN) in an aqueous solution has been studied theoretically. Ab initio configuration interaction (CI) calculations were carried out for the potential energy surfaces of ground and excited state DMABN. The potential surface of second excited S2 state was represented by a superposition of three diabatic states, one is of the ion pair type and the other two of the neutral ones, to facilitate the calculations in a polar solution. The intermolecular pair potentials between DMABN and H2O were developed with the aid of electron distributions in DMABN obtained from ab initio calculations. These potential functions were applied to determine the geometries of DMABN–H2O complex and the results were compared with the available experimental data. Monte Carlo simulation calculations were further performed for the aqueous solution of DMABN. The potentials of mean force for the torsional angle of dimethylamino group revealed that the S2 state potential profile is remarkably altered due to the solvation and the twisted intramolecular CT state becomes a stable point on the surface while this point corresponds to the top of potential barrier in the gas phase. The origin of broad emission band at a longer wavelength region observed in the experiments were discussed on the basis of present calculations. In order to elucidate the mechanism of CT state formation, the reaction free energy surfaces were constructed as the function of solvation coordinate and amino torsional angle. The results obtained here were that: (a) the shape of free energy curve of S2 state is far from a parabolic form along the solvation coordinate while the S1 state curve is nearly parabolic; and (b) the torsional coordinate is required to undergo a deformation to reach the transition state region of CT state formation reaction.