Structure Of The Photochemical Reaction Path Populated Via Promotion Of Cf2i2 Into Its First Excited State

The photochemical reaction path following the promotion of CF2I2 into its lowest-lying excited electronic singlet state has been modeled using ab initio multiconfigurational quantum chemical calculations. It is found that a conical intersection drives the electronically excited CF2I2* species either to the CF2I + I radical pair or back to the starting CF2I2 structure. The structures of the computed relaxation pathways explain the photoproduct selectivity previously observed in the gas phase. Furthermore, the results provide the basis for explaining the condensed-phase photochemistry of CF2I2

Matrix Isolation and Computational Study of Isodifluorodibromomethane (F2 Cbr-Br): A Route to Br2 Formation in Cf2 Br2 Photolysis

The photolysis products of dibromodifluoromethane (CF2 Br 2) were characterized by matrix isolation infrared and UV/Visible spectroscopy, supported by ab initio calculations. Photolysis at wavelengths of 240 and 266 nm of CF2 Br2:Ar samples (∼1:5000) held at ∼5 K yielded iso- CF2 Br2 (F2 CBrBr), a weakly bound isomer of CF2 Br2, which is characterized here for the first time. The observed infrared and UV/Visible absorptions of iso- CF2 Br2 are in excellent agreement with computational predictions at the B3LYP/aug-cc-pVTZ level. Single point energy calculations at the CCSD(T)/aug-cc-pVDZ level on the B3LYP optimized geometries suggest that the isoform is a minimum on the CF2 Br2 potential energy surface, lying some 55 kcal/mol above the CF2 Br2 ground state. The energies of various stationary points on the CF2 Br 2 potential energy surface were characterized computationally; taken with our experimental results, these show that iso- CF2 Br 2 is an intermediate in the Br+ CF2 Br→ CF 2 + Br2 reaction. The photochemistry of the isoform was also investigated; excitation into the intense 359 nm absorption band resulted in isomerization to CF2 Br2. Our results are discussed in view of the rich literature on the gas-phase photochemistry of CF2 Br2, particularly with respect to the existence of a roaming atom pathway leading to molecular products. © 2010 American Institute of Physics

Probing Vibrationally Mediated Ultrafast Excited-state Reaction Dynamics With Multireference (caspt2) Trajectories

Excited-state trajectories computed at the complete active space second-order perturbation theory (CASPT2) reveal how vibrational excitation controls the molecular approach to the intersection space that drives the photodissociation of a prototypical halogenated methyl radical, namely CF2I. Translating the Franck-Condon structure along the ground-state CASPT2 vibrational modes in this system followed by propagating the displaced structures in the first excited doublet state simulates specific vibrational excitations and vibrationally mediated dynamics, respectively. Three distinct situations are encountered: the trajectories (i) converge to an energetically flat segment of the intersection space, (ii) locate a segment of the intersection space, and (iii) access a region where the intersection space degeneracy is lifted to form a ridge of avoided crossings. The computational protocol documented herein can be used as a tool to design control strategies based on selective excitation of vibrational modes, including adaptive feedback schemes using coherent light sources

Global Sampling of the Photochemical Reaction Paths of Bromoform by Ultrafast Deep-UV Through Near-IR Transient Absorption and ab initio Multiconfigurational Calculations

Ultrafast deep-ultraviolet through near infrared (210-950 nm) transient absorption spectroscopy complemented by ab initio multiconfigurational calculations offers a global description of the photochemical reaction pathways of bromoform following 255-nm excitation in methylcyclohexane and acetonitrile solutions. Photoexcitation of CHBr3 leads to the ground-state iso-CHBr3 product in a large quantum yield (∼35), formed through two different mechanisms: concerted excited-state isomerization and cage-induced isomerization through the recombination of the nascent radical pair. These two processes take place on different time scales of tens of femtoseconds and several picoseconds, respectively. The novel ultrafast direct isomerization pathway proposed herein is consistent with the occurrence of a conical intersection between the first excited singlet state of CHBr3 and the ground electronic state of iso-CHBr3. Complete active space self-consistent field calculations characterize this singularity in the vicinity of a second order saddle point on the ground state which connects the two isomer forms. For cage-induced isomerization, both the formation of the nascent radical pair and its subsequent collapse into ground-state iso-CHBr3 are directly monitored through the deep-ultraviolet absorption signatures of the radical species. In both mechanisms, the optically active (i.e., those with largest Franck-Condon factors) C-Br-Br bending and Br-Br stretching modes of ground-state iso-CHBr3 have the largest projection on the reaction coordinate, enabling us to trace the structural changes accompanying vibrational relaxation of the non-equilibrated isomers through transient absorption dynamics. The iso-CHBr3 photoproduct is stable in methylcyclohexane, but undergoes either facile thermal isomerization to the parent CHBr3 structure through a cyclic transition state stabilized by the polar acetonitrile medium (∼300-ps lifetime), and hydrolysis in the presence of water. © 2013 American Institute of Physics

Characterization of Iso -Cf2 I2 in Frequency and Ultrafast Time Domains

The photolysis of diiododifluoromethane (CF2 I2) in condensed phases was studied by a combination of matrix isolation and ultrafast time-resolved spectroscopy, in concert with ab initio calculations. Photolysis at wavelengths of 355 or 266 nm of CF2 I2:Ar samples (1:5000) held at ∼8 K yielded iso -CF2 I2 (F2 C-I-I), a metastable isomer of CF2 I2, characterized here for the first time. The infrared (IR) spectra of this isomer were recorded in matrix experiments, and the derived positions of the C-F stretching modes are in very good agreement with the predictions of high level ab initio calculations, which show that the iso -form is a minimum on the CF2 I2 ground state potential energy surface. The formation of this isomer following 350 nm excitation of CF2 I 2 in room temperature CCl4 solutions was monitored through its intense C-F stretching mode by means of ultrafast time-resolved IR absorption. Together, matrix isolation and ultrafast IR absorption experiments suggest that the formation of iso -CF2 I2 occurs via recombination of CF2 I radical and I atom. Ultrafast IR experiments detect a delayed rise of iso -CF2 I-I absorption, placing an upper limit of 400 fs for the C-I bond dissociation and primary geminate recombination processes. The product absorption spectrum recorded 1 ns after 350 nm excitation of CF2 I2 in solution is virtually identical to the visible absorption spectrum of i so -CF2 I2 trapped in matrix isolation experiments [with subtracted I2 (X) absorption]. The formation of this isomer in solution at room temperature has direct dynamic implications for the ultrafast production of molecular iodine from electronically excited CF 2 I2. © 2010 American Institute of Physics

Radiative Recombination Of Spatially Extended Excitons In (znse/cds)/cds Heterostructured Nanorods

We report on organometallic synthesis of luminescent (ZnSe/CdS)/CdS semiconductor heterostructured nanorods (hetero-NRs) that produce an efficient spatial separation of carriers along the main axis of the structure (type II carrier localization). Nanorods were fabricated using a seeded-type approach by nucleating the growth of 20-100 nm CdS extensions at [000 +/- 1] facets of wurtzite ZnSe/CdS core/shell nanocrystals. The difference in growth rates of CdS in each of the two directions ensures that the position of ZnSe/CdS seeds in the final structure is offset from the center of hetero-NRs, resulting in a spatially asymmetric distribution of carrier wave functions along the heterostructure. Present work demonstrates a number of unique properties of (ZnSe/CdS)/CdS hetero-NRs, including enhanced magnitude of quantum confined Stark effect and subnanosecond switching of absorption energies that can find practical applications in electroabsorption switches and ultrasensitive charge detectors

Ultrafast Carrier Dynamics In Type II ZNSE/CDS/ZNSE Nanobarbells

We employ femtosecond transient absorption spectroscopy to get an insight into ultrafast processes occurring at the interface of type II ZnSe/CdS heterostructured nimocrystals fabricated via colloidal routes and comprising a barbell-like arrangement of ZnSe tips and CdS nanorods. Our study shows that resonant excitation of ZnSe tips results in an unprecedently fast transfer of excited Electrons into CdS domains of nanobarbells (\u3c0.35 ps), whereas selective pumping of CdS components loads to a relatively slow injection of photoinduced holes into ZnSe tips (tau(h) = 95 ps). A qualitative thermodynamic description of observed electron processes within the classical limit of Marcus theory was used to identify a specific charge transfer regime associated with the ultrafast electron injection into CdS. Potential photocatalytic applications of the observed fast separation of carriers along the main axis of ZnSe/CdS barbells are discussed