For the widely used anticancer, antiinflammatory, and immunosuppressant drug 6-thioguanine (6-TG), its unique photochemistry of absorbing UVA, generating singlet oxygen (1O2), and inducing oxidative DNA damage can cause severe phototoxicity side effects in biomedical applications. Here we raise a new strategy of loading 6-TG onto gold nanoparticles (AuNPs) and demonstrate from systematic ultraviolet irradiation experiments that the 6-TG photochemistry can be successfully modulated by binding with AuNPs and the phototoxicity under UVA irradiation are effectively removed, not only for single base 6-TG but also for 6-TG embedded in DNA. In conjunction with excited state ab initio calculations, the molecular mechanisms are further unveiled, accounting for the protection effects of AuNPs in prohibiting the 6-TG photoactivation and 1O2 oxidation. These findings point to tremendous possibilities of using AuNPs as inert carrier to modify excited state photochemistry and prevent unwanted phototoxicity effects of the adsorbed drug molecules.

The rich photo-oxidation pathways and products of terrylenediimide (TDI) with singlet oxygen (1O2) have been examined by powerful computational approaches. Potential energy profiles and product fluorescence properties are characterized. A variety of new products are unraveled and predicted to emit fluorescence at both visible and near-infrared ranges, which could open the possibility for interesting applications of using TDI as a fluorescence probe for the single-molecule detection of 1O2 and designing multicolor photoconvertible fluorophores based on 1O2 oxidation.

Learning nature’s approach to modulate photophysical properties of NIR porphyrinoids by fine-tuning β-substituents including the number and position, in a manner similar to naturally occurring chlorophylls, has the potential to circumvent the disadvantages of traditional “extended π-conjugation” strategy such as stability, molecular size, solubility, and undesirable π–π stacking. Here we show that such subtle structural changes in Pt(II) or Pd(II) cis/trans-porphodilactones (termed by cis/trans-Pt/Pd) influence photophysical properties of the lowest triplet excited states including phosphorescence, Stokes shifts, and even photosensitization ability in triplet–triplet annihilation reactions with rubrene. Prominently, the overall upconversion capability (η, η = ε·ΦUC) of Pd or Pt trans-complex is 104 times higher than that of cis-analogue. Nanosecond time-resolved infrared (TR-IR) spectroscopy experiments showed larger frequency shift of ν(C═O) bands (ca. 10 cm–1) of cis-complexes than those of trans-complexes in the triplet excited states. These spectral features, combining with TD-DFT calculations, suggest the strong electronic coupling between the lactone moieties and the main porphyrin chromophores and thus the importance of precisely positioning β-substituents by mimicking chlorophylls, as an alternative to “extended π-conjugation”, in designing NIR active porphyrinoids.

Although numerous studies have been devoted to the charge transfer through double-stranded DNA (dsDNA), one of the major problems that hinder their potential applications in molecular electronics is the fast deprotonation of guanine cation (G+•) to form a neutral radical that can cause the termination of hole transfer. It is thus of critical importance to explore other DNA structures, among which G-quadruplexes are an emerging topic. By nanosecond laser flash photolysis, we report here the direct observation and findings of the unusual deprotonation behavior (loss of amino proton N2–H instead of imino proton N1–H) and slower (1–2 orders of magnitude) deprotonation rate of G+• within G-quadruplexes, compared to the case in the free base dG or dsDNA. Four G-quadruplexes AG3(T2AG3)3, (G4T4G4)2, (TG4T)4, and G2T2G2TGTG2T2G2 (TBA) are measured systematically to examine the relationship of deprotonation with the hydrogen-bonding surroundings. Combined with in depth kinetic isotope experiments and pKa analysis, mechanistic insights have been further achieved, showing that it should be the non-hydrogen-bonded free proton to be released during deprotonation in G-quadruplexes, which is the N2–H exposed to solvent for G bases in G-quartets or the free N1–H for G base in the loop. The slower N2–H deprotonation rate can thus ensure less interruption of the hole transfer. The unique deprotonation features observed here for G-quadruplexes open possibilities for their interesting applications as molecular electronic devices, while the elucidated mechanisms can provide illuminations for the rational design of G-quadruplex structures toward such applications and enrich the fundamental understandings of DNA radical chemistry.

Intensified research interests are posed with the thionucleobase 4-thiouracil (4-TU), due to its important biological function as site-specific photoprobe to detect RNA structures and nucleic acid–nucleic acid contacts. By means of time-resolved IR spectroscopy and density functional theory (DFT) studies, we have examined the unique photophysical and photochemical properties of 4-TU. It is shown that 4-TU absorbs UVA light and results in the triplet formation with a high quantum yield (0.9). Under N2-saturated anaerobic conditions, the reactive triplet undergoes mainly cross-linking, leading to the (5–4)/(6–4) pyrimidine–pyrimidone product. In the presence of O2 under aerobic conditions, the triplet 4-TU acts as an energy donor to produce singlet oxygen 1O2 by triplet–triplet energy transfer. The highly reactive oxygen species 1O2 then reacts readily with 4-TU, leading to the products of uracil (U) with a yield of 0.2 and uracil-6-sulfonate (USO3) that is fluorescent at ∼390 nm. The product formation pathways and product distribution are well rationalized by the joint B3LYP/6-311+G(d,p) calculations. From dynamics and mechanistic point of views, these results enable a further understanding for 4-TU acting as reactive precursors for photochemical reactions relevant to 1O2, which has profound implications for photo cross-linking, DNA photodamage, as well as photodynamic therapy studies.

We report a new transient spectral method utilizing triplet excited state as sensitive reporters to monitor and differentiate the multiplex G-quadruplex/ligand interactions in a single assay, which is a difficult task and usually requires a combination of several techniques. From a systematic study on the interactions of porphyrin (TMPyP4) with each telomeric G-quadruplex: AG3(T2AG3)3, G2T2G2TGTG2T2G2, (G4T4G4)2, and (TG4T)4, it is convincingly shown that the ligand triplet decay lifetimes are sensitive to the local bound microenvironment within G-quadruplexes, from which the coexisting binding modes of end-stacking, intercalation, and sandwich are distinguished and their respective contribution are determined. The complete scenario of mixed interaction modes is thus revealed, shedding light on the past controversial issues. Additional control experiments demonstrate the sensitivity of this triplet reporter method, which can even capture the binding behavior change as the G-quadruplex structures are adjusted by Na+ or K+.Keywords: binding; G-quadruplex; microenvironment; porphyrin; triplet reporter;

•The complex nonadiabatic reaction pathways are newly revealed.•The most probable reaction mechanism is described, which can rationalize the experimentally observed product distributions logically.•New product channels are predicted, which involves the formation of cyclopentenol, one kind of enols believed to be the key reaction intermediate in the flame of hydrocarbons.The reaction mechanism of the ground state oxygen atom O(3P) with cyclopentene is investigated theoretically. The triplet and singlet potential energy surfaces are calculated at the CCSD(T)//MP2/6-311G(d,p) level and the minimum energy crossing points (MECPs) between the two surfaces are located by means of the Newton–Lagrange method, from which the complex nonadiabatic reaction pathways are revealed. Based on the theoretical results, the most probable reaction mechanism of O(3P) with c-C5H8 is described, which agrees with the experimental results nicely, including the condensed phase experiment. At the same time, the newly revealed reaction mechanism clarifies the previous controversial product distribution, and predicts the possible existence of the new enol product, cyclopentenol.

The potential energy profiles toward formation of cyclobutane pyrimidine dimers CPD and the physical quenching after UV excitation were explored for the dinucleotide thymine dinucleoside monophosphate (TpT) using density functional theory (ωB97XD) and the time-dependent density functional theory (TD-ωB97XD). The ωB97XD functional that includes empirical dispersion correction is shown to be an appropriate method to obtain rational results for the current large reaction system of TpT. Photophysical quenching is shown to be predominant over the photochemical CPD formation. Following the initial excitation to the 1ππ* state, the underlying dark 1nπ* state bifurcates the excited population to the prevailing IC to S0 and the small ISC to the long-lived triplet state T1 via T4 (3ππ*) state that has negligible energy gap with 1nπ* state. Even for the reactive T1 state, two physical quenching pathways resulting in the conversion back to ground-state reactant via the T1/S0 crossing points are newly located, which are in strong competition with CPD formation. These results provide rationale for the recently observed nanosecond triplet decay rates in the single-stranded (dT)18 and inefficiency of deleterious CPD formation, which allow for a deeper understanding of DNA photostability.

As an end metabolism product of the widely used thiopurine drugs, 6-thioguanine (6-TG) absorbs UVA and produces 1O2 by photosensitization. This unusual photochemical property triggers a variety of DNA damage, among which the oxidation of 6-TG itself by 1O2 to the promutagenic product guanine-6-sulfonate (GSO3) represents one of the major forms. It has been suspected that there exists an initial intermediate, GSO, prior to its further oxidation to GSO2 and GSO3, but GSO has never been observed. Using density functional theory, we have explored the energetics and intermediates of 6-TG and 1O2. A new mechanism via GSOOH → GSO2 → GSO4 → GSO3 has been discovered to be the most feasible energetically, whereas the anticipated GSO mechanism is found to encounter an inaccessibly high barrier and thus is prevented. The mechanism through the GSOOH and GSO4 intermediates can be validated further by joint experimental measurements, where the fast rate constant of 4.9 × 109 M–1 s–1 and the reaction stoichiometry of 0.58 supports this low-barrier new mechanism. In addition to the dominant pathway of GSOOH → GSO2 → GSO4 → GSO3, a side pathway with higher barrier, GSOOH → G, has also been located, providing a rationalization for the observed product distributions of GSO2 and GSO3 as major products and G as minor product. From mechanistic and kinetics points of view, the present findings provide new chemical insights to understand the high phototoxicity of 6-TG in DNA and point to methods of using 6-TG as a sensitive fluorescence probe for the quantitative detection of 1O2, which holds particular promise for detecting 1O2 in DNA-related biological surroundings.

Remarkable optical properties are posed with gold nanoparticles (AuNPs) due to the excitation of localized surface plasmon resonances, which makes AuNPs affect strongly both the ground state and the excited state of adjacent organic molecules. Compared with the ground state, the effect of AuNPs on excited state of organic molecules is not always fully understood. Here, we performed transient UV–vis absorption experiments to monitor the triplet excited state formation of three cationic dyes and one anionic dye in the presence of two types of gold nanoparticles: the citrate-stabilized AuNPs and ATP-protected AuNPs. It is found that the three cationic dyes can cause efficient aggregation of citrate-stabilized AuNPs, leading to AuNPs aggregates with varied size, whereas the ATP-protected AuNPs can be sustained in the monodispersed state. By comparing the circumstances of aggregated AuNPs and monodispersed AuNPs, we demonstrate that the enhancement effect on triplet excited state formation results from the aggregation of gold nanoparticles and depends on the aggregation size. These findings reveal the aggregation induced plasmon field interaction of AuNPs with excited state population dynamics and may enable new applications of aggregated metal nanoparticles, where aggregates can serve as stronger plasmonic nanoantennas.

The o-hydroxycinnamic derivatives represent efficient caged compounds that can realize quantification of delivery upon uncaging, but there has been lack of time-resolved and mechanistic studies. We used time-resolved infrared (TRIR) spectroscopy to investigate the photochemical uncaging dynamics of the prototype o-hydroxycinnamic compound, (E)-3-(2-hydroxyphenyl)-acrylic acid ethyl ester (HAAEE), leading to coumarin and ethanol upon uncaging. Taking advantage of the specific vibrational marker bands and the IR discerning capability, we have identified and distinguished two key intermediate species, the cis-isomers of HAAEE and the tetrahedral intermediate, in the transient infrared spectra, thus providing clear spectral evidence to support the intramolecular nucleophilic addition mechanism following the trans–cis photoisomerization. Moreover, the product yields of coumarin upon uncaging were observed to be greatly affected by the solvent polarity, suppressed in CH2Cl2 but enhanced in D2O/CH3CN with the increasing volume ratio of D2O. The highly solvent-dependent behavior indicates E1 elimination of the tetrahedral intermediate to give rise to the final uncaging product coumarin. The photorelease rate of coumarin was directly characterized from TRIR (3.6 × 106 s–1), revealing the promising application of such o-hydroxycinnamic compound in producing fast alcohol jumps. The TRIR results provide the first time-resolved detection and thus offer direct dynamical information about this photochemical uncaging reaction.

By means of transient UV–visible absorption spectra/fluorescence spectra, combined with electronic structure calculations, the present work focuses on characterizing the photophysical and electronic properties of five PCBM-like C60 derivatives (F1, F2, F3, F4, and F5) and understanding how these properties are expected to affect the photovoltaic performance of polymer solar cells (PSCs) with those molecules as acceptors. Spectral data reveal that the fluorescence quantum yields (ΦF) are enhanced and the triplet quantum yields (ΦT) are lowered for the five PCBM-like C60 derivatives as compared to those of the pristine C60, suggesting that functionalization of a C═C double bond perturbs the fullerene’s π-system and breaks the Ih symmetry of pristine C60, which results in modifications of photophysical properties of the fullerene derivatives. PBEPBE/6-311G(d,p)//PBEPBE/6-31G(d) level of electronic structure calculations yields the HOMO–LUMO gaps and LUMO energies, showing that the electron-withdrawing effect induced by the side chain functional groups perturbs LUMO energies, from which different open circuit voltages Voc are resulted. The predicted Voc from our calculation agrees with previous experiment results. Basically, we found that functionalization of a C═C double bond sustains the fullerene structure and its electron affinitive properties. Adducted side chains contribute to adjust the HOMO–LUMO gap and LUMO levels of the acceptors to improve open circuit voltage. The results could provide fundamental insights for understanding how structural modifications influence the photovoltaic performance, which paves a way for guiding the synthesis of new fullerene derivatives with improved performance in polymer solar cells.

We have explored the potential energy profiles of TpT dinucleotides toward formation of a DNA photolesion product, spore photoproduct (SP), along the S0, S1, and T1 states, by means of density functional theory and time-dependent density functional theory. Together with the spin density analysis, the consecutive mechanism for the SP formation can be established. The detailed reaction pathways have been revealed. All the adiabatic reaction pathways proceeding though S1, T1, or S0 alone are shown to be energetically infeasible, while the nonadiabatic pathway involving both the T1 and S0 states corresponds to the lowest-energy path and is the most favorable in energy. The nonadiabatic pathway is rate-limited by the step of the hydrogen atom transfer proceeding in the T1 state with a barrier of 14.2 kcal mol–1 (11.9 kcal mol–1 in bulk solution), whereas the subsequent C5–CH2 bond formation toward the final SP formation occurs readily in S0 after intersystem crossing from T1 to S0 via the singlet–triplet interaction. The results provide a rationale for the experimentally observed kinetic isotope effect after deuterium substitution at the 3′-T methyl group of TpT.

The 193 nm photodissociation dynamics of CH2CHCOCl in the gas phase has been examined with the technique of time-resolved Fourier transform infrared emission (TR-FTIR) spectroscopy. Vibrationally excited photofragments of CO (ν ⩽ 5), HCl (ν ⩽ 6), and C2H2 were observed and two photodissociation channels, the C-Cl fission channel and the HCl elimination channel have been identified. The vibrational and rotational state distributions of the photofragments CO and HCl have been acquired by analyzing their fully rotationally resolved ν→ν−1 rovibrational progressions in the emission spectra, from which it has been firmly established that the mechanism involves production of HCl via the four-center molecular elimination of CH2CHCOCl after its internal conversion from the S1 state to the S0 state. In addition to the dominant C-Cl bond fission along the excited S1 state, the S1→S0 internal conversion has also been found to play an important role in the gas phase photolysis of CH2CHCOCl as manifested by the considerable yield of HCl.

The products and mechanisms of the atmospherically and environmentally important reaction, C2Cl3 + NO, are investigated comprehensively by step-scan time-resolved Fourier transform infrared emission spectroscopy and the CCSD(T)/6-311+G(d)//B3LYP/6-311G(d) level of electronic structure calculations. Vibrationally excited products of Cl2CO, ClNCO, CCl3NCO and NCO have been observed in the IR emission spectra. Cyclic intermediates are found to play important roles leading to the rich variety of the chemical transformations of the reaction. Mainly two competitive reaction pathways are revealed: the four-membered ring intermediate pathway leading to the products Cl2CO + ClCN which is essentially barrierless and the bicyclic ring intermediate pathway leading to the product channels of ClNCO + CCl2,CCl3NCO and CCl3 + NCO which is rate-limited by a barrier of 42.9 kJ mol−1 higher than the reactants. By photolyzing the precursor at 248 and 193 nm, respectively, C2Cl3 radicals with different internal energy are produced to observe the product branching ratios as a function of reactant energy. The Cl2CO channelvia the four-membered ring intermediate pathway is shown to be overwhelmingly dominant at low energy (temperature) but become less important at high energy while the ClNCO and CCl3NCO channels via the bicyclic ring intermediate pathway are greatly enhanced and compete effectively. The experimental observation of the products and their branching ratios varying with reactant energy is well consistent with the calculated potential energy profiles.

Taking the 266 nm excited pyrimidine (uracil or thymine) with cyclopentene as model reaction systems, we have examined the photoproduct formation dynamics from the [2 + 2] photocycloaddition reactions of triplet pyrimidines in solution and provided mechanistic insights into this important DNA photodamage reaction. By combining two compliment methods of nanosecond time-resolved transient IR and UV–vis laser flash-photolysis spectroscopy, the photoproduct formation dynamics as well as the triplet quenching kinetics are measured. Characteristic IR absorption bands due to photoproduct formation have been observed and product quantum yields are determined to be ∼0.91% for uracil and ∼0.41% for thymine. Compared to the measured large quenching rate constants of triplet uracil (1.5 × 109 M–1s–1) or thymine (0.6 × 109 M–1s–1) by cyclopentene, the inefficiency in formation of photoproducts indicates competitive physical quenching processes may exist on the route leading to photoproducts, resulting in very small product yields eventually. Such an energy wasting process is found to be resulted from T1/S0 surface crossings by the hybrid density functional calculations, which compliments the experiments and reveals the reaction mechanism.

In intense femtosecond laser fields, molecules could be tunnel ionized from multiple valence orbitals, which produces molecular ions in different electronic states. In this article, we have used a reaction microscope to study double ionization of nitrogen by intense femtosecond laser pulses. It is found that some doubly charged molecular ions N22+ are metastable while others dissociate through charge symmetric dissociation (N22+ → N+ + N+) or charge asymmetric dissociation (N22+ → N2+ + N). The kinetic energy releases and angular distributions of atomic ions are measured for the dissociation channels. With the aid of the CASSCF and MRCI calculations, the electronic states are identified and the contributions of the valence orbitals are discussed for these dissociated molecular dications.

Investigations on the dissociation kinetics of hydrated protonium ions, (H2O)2H+ and their deuterated species (D2O)2D+, are reported based on the harmonic and anharmonic oscillator model using the transition state theory and ab initio calculations. We find that the dissociation of (H2O)2H+ and (D2O)2D+ exhibits a distinct threshold behavior due to the existence of activation energies. Moreover, the deviation between the harmonic and anharmonic dissociation rate constants becomes larger in the high energy or temperature range, with the rate constants becoming unreasonably large under the harmonic oscillator model. The isotope effect is found to become more distinct but only in the case of the anharmonic oscillator model. These results show that the anharmonic Rice−Ramsperger−Kassel−Marcus (RRKM) theory can provide a reasonably good description for the dissociation of (H2O)2H+ and (D2O)2D+. Furthermore, a theoretical model to demonstrate the principle of vibrational predissociation spectroscopy (VPS) is established from the viewpoint of RRKM theory and applied in determining the experimental conditions and understanding the role of the dissociation rate constant k(E) played in the VPS experiment, using (H2O)2H+ and (D2O)2D+ as examples.

We present the structure-dependent nonlinear optical (NLO) properties of fully conjugated tri(perylene bisimides) (triPBIs) toward the understanding of the role of conformational flexibility and π-electron conjugation in molecular NLO properties of model graphene-nanoribbon (GNR)-like molecules. In the present paper, we report the NLO absorption properties of the triPBIs in toluene excited at 532 nm with nanosecond laser pulses, where the observed transient excited state is determined to be a triplet and presented in the nonlinear process similar to the NLO properties that occur in C60. As a result, the all-optical switching in both visible and near-infrared regions upon excitation at 532 nm was demonstrated, suggesting that the chemically synthesized model GNRs act well as smart all-optical switching devices without the need of external control. Furthermore, Raman spectral measurement was further used to characterize the conjugated structure properties of model compounds of functionalized graphene nanoribbons (F-GNRs), while the dispersion and splitting of the G-band and D-band in both frequency and intensity can help to distinguish the π-conjugation and conformational flexibility of the two different triPBI isomers, showing the opportunity to tailor their optoelectronic properties by precisely controlling the edge orientation, edge width, and chemical termination of the edges in the synthesized F-GNRs.

Time-resolved Fourier transform infrared absorption spectroscopy measurements and B3LYP/cc-pVDZ calculations have been conducted to characterize the reaction dynamics of a remarkable photoinduced 1,3-Cl sigmatropic rearrangement reaction upon 193 or 266 nm excitation of the model systems acryloyl chloride (CH2CHCOCl) and crotonyl chloride (CH3CHCHCOCl) in solution. The reaction is elucidated to follow nonadiabatic pathways via two rapid ISC processes, S1 → T1 and T1 → S0, and the S1/T1 and T1/S0 surface intersections are found to play significant roles leading to the nonadiabatic pathways. The S1 → T1 → S0 reaction pathway involving the key participation of the T1 state is the most favorable, corresponding to the lowest energy path. It is also suggested that the photoinduced 1,3-Cl migration reaction of RCHCHCOCl (R = H, CH3) proceeds through a stepwise mechanism involving radical dissociation−recombination, which is quite different from the generally assumed one-step concerted process for pericyclic reactions.

For the reaction of O(3P) with propyne, the product channels and mechanisms are investigated both theoretically and experimentally. Theoretically, the CCSD(T)//B3LYP/6-311G(d,p) level of calculations are performed for both the triplet and singlet potential energy surfaces and the minimum energy crossing point between the two surfaces are located with the Newton−Lagrange method. The theoretical calculations show that the reaction occurs dominantly via the O-addition rather than the H-abstraction mechanism. The reaction starts with the O-addition to either of the triple bond carbon atoms forming triplet ketocarbene 3CH3CCHO or 3CH3COCH which can undergo decomposition, H-atom migration or intersystem crossing from which a variety of channels are open, including the adiabatic channels of CH3CCO + H (CH2CCHO + H), CH3 + HCCO, CH2CH + HCO, CH2CO + CH2, CH3CH + CO, and the nonadiabatic channels of C2H4 + CO, C2H2 + H2 + CO, H2 + H2CCCO. Experimentally, the CO channel is investigated with TR-FTIR emission spectroscopy. A complete detection of the CO product at each vibrationally excited level up to v = 5 is fulfilled, from which the vibrational energy disposal of CO is determined and found to consist with the statistical partition of the singlet C2H4 + CO channel, but not with the triplet CH3CH + CO channel. In combination with the present calculation results, it is concluded that CO arises mainly from the singlet methylketene (1CH3CHCO) dissociation following the intersystem crossing of the triplet ketocarbene adduct (3CH3CCHO). Fast intersystem crossing via the minimum energy crossing point of the triplet and singlet surfaces is shown to play significant roles resulting into nonadiabatic pathways for this reaction. Moreover, other interesting questions are explored as to the site selectivity of O(3P) atom being added to which carbon atom of the triple bond and different types of internal H-atom migrations including 1,2-H shift, 3,2-H shift, and 3,1-H shift involved in the reaction.

The atmospherically and environmentally important reaction of chlorinated vinyl radical with nitrogen dioxide (C2Cl3 + NO2) is investigated by step-scan time-resolved Fourier transform infrared emission spectroscopy and electronic structure calculations. Vibrationally excited products of CO, NO, Cl2CO, and NO2 are observed in the IR emission spectra. Geometries of the major intermediates and transition states along the potential energy surface are optimized at the B3LYP/6-311G(d) level, and their energies are refined at the CCSD(T)/6-311+G(d) level. The reaction mechanisms are characterized to be barrierless addition−elimination via nitro (C2Cl3−NO2) and nitrite (C2Cl3−ONO) adducts. Four energetically accessible reaction routes are revealed, i.e., the decomposition of the nitrite adduct forming C2Cl3O + NO and its sequential dissociation to CO + NO + CCl3, the elimination of ClNO from the nitrite adduct leading to ClNO + Cl2CCO, the Cl-atom shift of the nitrite adduct followed by the decomposition to CCl3CO + NO, and the O-atom shift of the nitro adduct followed by C−C bond cleavage forming ClCNO + Cl2CO. In competition with these reactive fluxes, the back-decomposition of nitro or nitrite adducts leads to the prompt formation of vibrationally excited NO2 and the long-lived reaction adducts facilitate the vibrational energy transfer. Moreover, the product channels and mechanisms of the C2Cl3 + NO2 reaction are compared with the C2H3 + NO2 reaction to explore the effect of chlorine substitution. It is found that the two reactions mainly differ in the initial addition preferentially by the N-attack forming nitro adducts (only N-attack is plausible for the C2H3 + NO2 reaction) or the O-attack forming nitrite adducts (O-attack is slightly more favorable and N-attack is also plausible for the C2Cl3 + NO2 reaction). The addition selectivity can be fundamentally correlated to the variation of the charge density of the end carbon atom of the double bond induced by chlorine substitution due to the electron-withdrawing effect of chlorine groups.

The products and mechanisms of the atmospherically and environmentally important reaction, C2Cl3 + NO, are investigated comprehensively by step-scan time-resolved Fourier transform infrared emission spectroscopy and the CCSD(T)/6-311+G(d)//B3LYP/6-311G(d) level of electronic structure calculations. Vibrationally excited products of Cl2CO, ClNCO, CCl3NCO and NCO have been observed in the IR emission spectra. Cyclic intermediates are found to play important roles leading to the rich variety of the chemical transformations of the reaction. Mainly two competitive reaction pathways are revealed: the four-membered ring intermediate pathway leading to the products Cl2CO + ClCN which is essentially barrierless and the bicyclic ring intermediate pathway leading to the product channels of ClNCO + CCl2,CCl3NCO and CCl3 + NCO which is rate-limited by a barrier of 42.9 kJ mol−1 higher than the reactants. By photolyzing the precursor at 248 and 193 nm, respectively, C2Cl3 radicals with different internal energy are produced to observe the product branching ratios as a function of reactant energy. The Cl2CO channelvia the four-membered ring intermediate pathway is shown to be overwhelmingly dominant at low energy (temperature) but become less important at high energy while the ClNCO and CCl3NCO channels via the bicyclic ring intermediate pathway are greatly enhanced and compete effectively. The experimental observation of the products and their branching ratios varying with reactant energy is well consistent with the calculated potential energy profiles.