Fluorine Substitution Effects on the Photodissociation Dynamics of Iodobenzene at 304 nm

Photodegradation of Flutamide and Halogen Derivatives of Nitrobenzotrifluoride: The NO Release Channel

The photodegradation of the nonsteroidal antiandrogen drug flutamide has been long linked to the photoisomerization involving the nitro group. In this work, the dynamics of NO photoelimination upon photolysis at 266 nm of flutamide, nitrobenzotrifluoride, and its halogen derivatives were investigated. Similar to nitrobenzene and its derivatives, a bimodal translational energy distribution was observed for the NO photofragment indicating the presence of two distinct elimination channels resulting in slow and fast components. The trends in the slow/fast branching ratio show that halogen substitution at the para position increases the triplet state yield due to the internal heavy-atom effect leading to enhancement of the fast component. Furthermore, the topology of the triplet state potential energy surface showed that the minimum energy path favors the oxaziridine ring-type intermediate over the NO2 roaming mechanism in all five molecules investigated. The steric interaction between the NO2 group and the CF3 group, which are placed in the ortho position, lowers the barrier for the formation of the oxaziridine transition state compared to that of nitrobenzene.

Combined experimental and theoretical study on the ultraviolet photodissociation dynamics of 1-bromo-2,6-difluorobenzene in 267 nm-234 nm

Thanks to their specific molecular symmetry, aromatic molecules and their derivatives represent ideal model systems in understanding photo-induced chemistry of small molecules. Herein, ultraviolet photodissociation dynamics of the 1-bromo-2,6-difluorobenzene molecule has been visualized via imaging the recoiling velocity distributions of photofragments. The measured recoiling angular distributions of the Br(²P_3/2) product vary significantly with the increasing photon energy, arguing against the simple bond-fission mechanism within the C_2v symmetry. Ab initio calculations reveal that in addition to the C-Br bond cleavage, two additional internal molecular coordinates that break the molecular symmetry are likely involved. The Br out-of-plane bending opens a direct dissociation pathway on the S₁-¹A″ (S₁-¹ππ^*) state, while the asymmetric C-F stretching significantly changes the orientation of the transition dipole moment. The present study sheds new light on the effect of symmetry breaking in the photodissociation dynamics of symmetric aryl halides, highlighting the multi-dimensional feature of excited state potential energy surfaces.

A combined theoretical and experimental study of the valence and Rydberg states of iodopentafluorobenzene

A new ultraviolet (UV) and vacuum ultraviolet (VUV) spectrum for iodopentafluorobenzene (C₆F₅I) using synchrotron radiation is reported. The measurements have been combined with those from a recent high-resolution photoelectron spectroscopic study. A major theoretical study, which includes both Franck-Condon (FC) and Herzberg-Teller (HT) analyses, leads to conclusions, which are compatible with both experimental studies. Our observation that the VUV multiplet at 7.926 eV in the VUV spectrum is a Rydberg state rather than a valence state leads to a fundamental reassignment of the VUV Rydberg spectrum over previous studies and removes an anomaly where some previously assigned Rydberg states were to optically forbidden states. Adiabatic excitation energies (AEEs) were determined from equations-of-motion coupled cluster with singles and doubles excitation; these were combined with time dependent density functional theoretical methods. Frequencies from these two methods are very similar, and this enabled the evaluation of both FC and HT contributions in the lower valence states. Multi-reference multi-root configuration interaction gave a satisfactory account of the principal UV+VUV spectral profile of C₆F₅I, with vertical band positions and intensities. The UV spectral onset consists of two very weak transitions assigned to 1¹B₁ (πσ*) and 1¹B₂ (σσ*) symmetries. The lowest unoccupied molecular orbital of a σ*(a₁) symmetry has a significant C-I* antibonding character. This results in considerable lengthening of the C-I bond for both these excited states. The vibrational intensity of the lowest 1¹B₁ state is dominated by HT contributions; the 1¹B₂ state contains both HT and FC contributions; the third band, which contains three states, two ππ*(1¹A₁, 2¹B₂) and one πσ*(2¹B₁), is dominated by FC contributions in the ¹A₁ state. In this ¹A₁ state, and the spectrally dominant bands near 6.7 (¹A₁) and 7.3 eV (¹A₁ + ¹B₂), the C-I bond length is in the normal range, and FC components dominate.

Structural and computational understanding of weak interactions in “bridge-flipped” isomeric tetrafluoro-bis-benzylideneanilines

Structural and computational insights into inter-molecular interactions in isomeric bridge-flipped tetrafluoro-bis-benzylideneanilines.

Ultrafast Photodissociation Dynamics of Highly Excited Iodobenzene on the C Band

The photodissociation dynamics of highly excited iodobenzene from the C band absorption has been studied by femtosecond time-resolved ion yields techniques. Detailed photodissociation routes are discussed with the aid of high-level, spin-orbit resolved ab initio calculations of 1D potential energy curves. Upon 200 nm excitation within the C band, iodobenzene molecules on 7B₂ and 7B₁ states decay to 7A₁ and 8B₂ states through internal conversion in 75 fs, with electronic energy converted into high vibrational energy of 7A₁ and 8B₂ states. Subsequently, 7A₁ and 8B₂ states decay through internal vibrational energy redistribution in 540 fs, accompanied by the excited C-I mode and the resulting cleavage of the C-I bond. The overall time for the reaction starting from the phenyl-type modes and ending in final C-I fragmentation for I(²P_3/2) production is 1.2 ps.

Spin-orbit ab initio investigation of photolysis of o-, m-, and p-iodotoluene

The multistate second order multiconfigurational perturbation theory in conjunction with spin-orbit interaction through complete active space state interaction (MS-CASPT2/CASSI-SO) was employed to calculate the potential energy curves for the ground and low-lying excited states of o-, m-, and p-iodotoluene along the assumed photolysis reaction coordinates. The mechanism and channels leading to products I((2)P(3/2)) and I( *)((2)P(3/2)) for o-, m-, and p-iodotoluene photolysis at 266 and 304 nm were elucidated with the computed potential energy curves and the surface crossing points. The effects of methyl substituent and heavy atom on the photodissociation mechanism were discussed by the comparison to related alkyl and aryl halides.

An ab initio investigation of the mechanisms of photodissociation in bromobenzene and iodobenzene

In concert with the latest experiment of velocity imaging technique [X. P. Zhang et al., ChemPhysChem 9, 1130 (2008)], quantum chemical calculations with relativistic effect were performed on the photodissociation of bromobenzene (BrPh) and iodobenzene (IPh) at 266 nm. The method of multistate second order multiconfigurational perturbation theory in conjunction with spin-orbit interaction through complete active space state interaction was employed to calculate the potential energy curves for the ground and low-lying excited states of BrPh and IPh along their photodissociation reaction coordinates. The dissociation mechanisms with products of X((2)P(3/2)) and X(*)((2)P(1/2)) (X = Br,I) states were clarified.

Photodissociation of dibromobenzenes at 266 nm by the velocity imaging technique

A velocity imaging technique combined with (2+1) resonance-enhanced multiphoton ionization (REMPI) is used to detect the primary Br((2)P(3/2)) fragment in the photodissociation of o-, m-, and p-dibromobenzene at 266 nm. The obtained translational energy distributions suggest that the Br fragments are produced via two dissociation channels. For o- and m-dibromobenzene, the slow channel that yields an anisotropy parameter close to zero is proposed to stem from excitation of the lowest excited singlet (pi,pi*) state followed by predissociation along a repulsive triplet (n,sigma*) state localized on the C-Br bond. The fast channel that gives rise to an anisotropy parameter of 0.53-0.73 is attributed to a bound triplet state with smaller dissociation barrier. For p-dibromobenzene, the dissociation rates are reversed, because the barrier for the bound triplet state becomes higher than the singlet-triplet crossing energy. The fractions of translational energy release are determined to be 6-8 and 29-40 % for the slow and fast channels, respectively; the quantum yields are 0.2 and 0.8, and are insensitive to the position of the substituent. The Br fragmentation from bromobenzene and bromofluorobenzenes at the same photolyzing wavelength is also compared to understand the effect of the number of halogen atoms on the phenyl ring.

Resonance Raman study of the A-band short-time photodissociation dynamics of 2-iodothiophene

Resonance Raman spectra were obtained for 2-iodothiophene in cyclohexane solution with excitation wavelengths in resonance with the A-band absorption spectrum. These resonance Raman spectra indicate that the Franck-Condon region photodissociation dynamics have multidimensional character with motion mainly along the nominal symmetric C=C stretch of the thienyl ring and accompanied by a moderate amount of motion along the nominal symmetric CSC stretch, the nominal antisymmetric CSC stretch, and the nominal C-I stretch vibrational modes. A preliminary resonance Raman intensity analysis was done for the A-band resonance Raman spectra of 2-iodothiophene. These results were compared to previous results for related iodobenzene and iodoalkane molecules that also contain a C-I chromophore and the similarities and differences in the short-time photodissociation dynamics were discussed.

Photochemistry of Bromofluorobenzenes

The photochemistry of low lying excited states of six different fluorinated bromobenzenes has been investigated by means of femtosecond laser spectroscopy and high level ab initio CASSCF/CASPT2 quantum chemical calculations. The objective of the work was to investigate how and to what extent light substituents, position on the benzene ring and number, would influence the dissociation mechanism of bromobenzene. In general, the actual position of a fluorine atom affects the dissociation rate to a less extent than the number of fluorine atoms. A clear connection between a lowering of a repulsive pisigma relative to a bound pipi state and the number of fluorine substituents exists, and the previously suggested model of coupling between dissociation rate and relative location of bound and repulsive state still holds for these molecules. A more elaborate examination of the electronic structure of the excited states in bromobenzenes than previously reported is presented.