The ICN-INC system: experiment and quantum chemical calculations

Ultrafast polarized pump–probe spectroscopy has been used to study the ICN Ã band photodissociation dynamics in several molecular liquids (water, ethanol, methanol, and chloroform). By exciting at 224 and 255 nm and probing at 385 nm near the gas phase CN B←X transition, the population and vector correlations in the resulting products have been followed with ∼150 fs time resolution. The exhaustive gas-phase reaction dynamics charted for this system indicate that a substantial torque is imparted to the CN fragment in the ground state I dissociation channel while the CN is rotationally cold in the I* channel. The fate of the highly rotationally excited fragment in a liquid environment is explored in the present study. The transient pump–probe anisotropy in ethanol and methanol shows two time scales for decay, ∼400 fs and 3–7 ps depending on excitation energy; the two time scales are assigned to the two product channels. The subpicosecond time scale is characteristic of rotational diffusion of thermalized CN while the longer several picosecond time scale suggests the rotationally hot CN rotate nearly freely for several picoseconds in the plane defined by the initial dissociative event. The rotational behavior of a diatomic fragment is considered between the free rotor and small-angle rotational diffusion limits. The CN photoproduct population dynamics are quite varied in the different solvents. In alcohols and chloroform, CN undergoes abstraction reactions with the solvent while in water there is no abstraction. Caging and diffusive geminate recombination dynamics apparently vary markedly in the different solvents; these effects are compared to molecular dynamics results for this photodissociation system. A large isotope effect is observed between H2O and D2O solvents in the product recombination dynamics.

Section: Introductionmentioning

confidence: 99%

Photodissociation of ICN in polar solvents: Evidence for long lived rotational excitation in room temperature liquids

Moskun

Bradforth

2003

“…It has been used to calculate the properties of molecules such as ethylene, 4 butadiene and bicyclobutane, 5 hexatriene, 6 benzene, 7 and others. Double and triple excitation ͓imple-mented in various configuration interaction schemes such as QICSD and QICSD͑T͔͒ have been used to improve the accuracy of the computations, 8,9 but they become very computationally expensive when applied to large molecules, and at the moment do not seem to be feasible for conveniently treating molecules as large as styrene.…”

Section: Introductionmentioning

confidence: 99%

Ab initio study of styrene and β-methyl styrene in the ground and in the two lowest excited singlet states

Zilberg

Haas

1995

The structure and vibrational frequencies of styrene and trans-β-methyl styrene in the lowest three singlet states (S0, S1, and S2) have been calculated using ab initio quantum chemical methods. The frequencies are compared with experimental data obtained in the bulk and in a supersonic jet. The calculation shows that in the ground state the molecules have a broad shallow potential as a function of the torsional angle, are essentially planar, but may be slightly bent. In the S1 and S2 states, the molecules are planar; In S1, the main structural change is in the aromatic ring, that is somewhat expanded. In S2, the C=C vinyl double bond elongates, while the C1—Cα single bond becomes shorter, bringing these two bonds to almost equal length. Correlation diagrams connecting ground state vibrational modes with ones belonging to electronically excited states are given; they show that for many out-of-plane modes the vibrational frequencies decrease upon electronic excitation. This is accounted for in terms of the changes in the π electron distribution taking place upon optical excitation that result in decreasing the force constants characterizing these vibrations. The frequencies of most in-plane modes change very little, but mixing between S0 modes is indicated in some cases, and a few vibrations, among them a Kekulé-type mode, undergo considerable change. The relation to the spectroscopy of the corresponding transitions in benzene is briefly discussed.

“…20 the ab initio diagonal force constants of the ground state potential have been scaled by 0.93 in order to fit the experimental frequencies. 38 Using this scaled potential our converged energy levels give transition frequencies which are lower than the experimental values. 38 To improve the transition frequencies we have not applied the 0.93 scale factor to the two bond-length force constants.…”

Section: The Ground State Wave Functionmentioning

confidence: 77%

A time-dependent calculation of the alignment and orientation of the CN fragment of the photodissociation of ICN

Carrington

1996