Multireference configuration interaction calculations of the low‐lying electronic states of ClO2

Chlorine Dioxide: An Exception that Proves the Rules of Localized Chemical Bonding

We employ natural bond orbital (NBO) and natural resonance theory (NRT) tools to analyze the enigmatic properties of the C2v-symmetric isomer of chlorine dioxide radical (ClO2), whose many challenges to Pauling-type localized bonding concepts were recognized by Linus Pauling himself. Although spin-contamination is minimal in this species, ClO2 exhibits an unusually strong form of "different Lewis structures for different spins" bonding pattern, intrinsically outside the framework of "maximal pairing" concepts. We show how the novel spin-unpaired donor-acceptor interactions lead to weakened bonding in the supramolecular domain of polyradical (ClO2)n homoclusters and aqueous ClO2(H2O)n heteroclusters. Despite feeble binding energies and large inter-radical separations, the polyradical clusters are found to maintain coherent spin patterns in each cluster component, attesting to the quantal donor-acceptor nature of their interactions and the cooperative and anticooperative couplings that govern intra- and intermolecular spin distributions in such spin-clusters.

Dichlorine peroxide (ClOOCl), chloryl chloride (ClCl(O)O) and chlorine chlorite (ClOClO): very accurateab initiostructures and actinic degradation

The structural parameters of the three most stable isomers with formula Cl₂O₂, dichlorine peroxide, chloryl chloride and chlorine chlorite, were determined by high-levelab initiotheory. The photodissociation pathways were investigated.

Basis Set Effects in the Description of the Cl-O Bond in ClO and XClO/ClOX Isomers (X = H, O, and Cl) Using DFT and CCSD(T) Methods

The performance of a group of density functional methods of progressive complexity for the description of the ClO bond in a series of chlorine oxides was investigated. The simplest ClO radical species and the two isomeric structures XClO/ClOX for each X = H, Cl, and O were studied using the PW91, TPSS, B3LYP, PBE0, M06, M06-2X, BMK, and B2PLYP functionals. Geometry optimizations and reaction enthalpies and enthalpies of formation for each species were calculated using Pople basis sets and the (aug)-cc-pVnZ Dunning sets, with n = D, T, Q, 5, and 6. For the calculation of enthalpies of formation, atomization and isodesmic reactions were employed. Both the precision of the methods with respect to the increase of the basis sets, as well as their accuracy, were gauged by comparing the results with the more accurate CCSD(T) calculations, performed using the same basis sets as for the DFT methods. The results obtained employing composite chemical methods (G4, CBS-QB3, and W1BD) were also used for the comparisons, as well as the experimental results when they are available. The results obtained show that error compensation is the key for successful description of molecular properties (geometries and energies) by carefully selecting the method and basis sets. In general, expansion of the one-electron basis set to the limit of completeness does not improve results at the DFT level, but just the opposite. The enthalpies of formation calculated at the CCSD(T)/aug-cc-pV6Z for the species considered are generally in agreement with experimental determinations and the most accurate theoretical values. Different sources of error in the calculations are discussed in detail.

Theoretical Study of ClOO + NO Reaction: Mechanism and Kinetics

Theoretical investigations are performed on mechanism and kinetics of the reaction of halogen peroxy radical ClOO with NO radical. The electronic structure information for both of the singlet and triplet potential energy surfaces (PESs) is obtained at the MP2/6-311 + G(2df) level of theory, and the single-point energies are refined by the CCSD(T)/6-311 + G(2df) level. The rate constants for various product channels of the reaction in the pressure range of 1-7600 Torr are predicted. The main results are as follows: On the singlet surface, the addition-elimination mechanism is the most important. First, the N atom of the NO radical can attack the O atom of the ClOO radical to form an energy-riched intermediate IM1 ClOONOtp (21.3 kcal/mol) barrierlessly, then IM1 could isomerizes to IM2 ClOONOcp (22.1 kcal/mol) via a low energy barrier. Both IM1 and IM2 can dissociate to the primary product P₁ ClNO + ¹O₂ and the secondary product P₂ ClO + NO₂. On the triplet surface, the direct Cl-abstraction reaction is the most feasible pathway. The Cl-abstraction can take place via a van der Waals complex, ³IM1 ONClOO (4.1 kcal/mol), then it fragments readily to give P₁' ClNO + ³O₂ with a small barrier. The kinetic calculations show that at low temperatures, the singlet bimolecular product P₁ is the primary product, while at high temperatures, the triplet product P₁' becomes the primary one; only at high pressures and low temperatures, the unimolecular products IM1 and IM2 can be found with quite small yields. At experimentally measured temperature 213 K, ClNO is the primary product in the whole pressure range, which is consistent with the previous experiment. The present study may be useful for further experimental studies for the title reaction.

Dynamics of the O + ClO reaction: reactive and vibrational relaxation processes

Classical trajectories have been integrated to study the O + ClO reaction, both reactive and vibrational energy transfer processes, for the range of temperatures 100 ≤ T/K ≤ 500 using momentum Gaussian binning. The employed potential energy surface is the recently proposed single-sheeted double many-body expansion potential energy surface for the (2)A" ground-state of ClO2 based on multireference ab initio data. A capture-type regime with a room-temperature rate constant of (17.8 ± 0.5) × 10(-12) cm(3) s(-1) and temperature dependence of k(T/K)/cm(3) s(-1) = 22.4 × 10(-12) × T(-0.81) exp(-39.2/T) has been found. Although the value reported here is half of the experimental and recommended one, tentative explanations are given. Other dynamical attributes are also examined for the title reaction, with state-to-all and state-to-state vibrational relaxation and excitation rate constants reported for temperatures of relevance in stratospheric chemistry.

DENSITY FUNCTIONAL THEORY STUDIES OF SPECTROSCOPIC CONSTANTS AND ANHARMONIC FORCE FIELD OF O35ClO

The equilibrium structure, spectroscopy constants and anharmonic force field of O35ClO have been calculated at B3PW91 and B3LYP levels of theory with two basis sets 6-311++G(2df,2pd) and 6-311++G(3df,3pd) , respectively. The computed geometries, dipole moment, rotational constants, vibration–rotation interaction constants, vibrational band origins, anharmonic constants, quartic, and sextic centrifugal distortion constants are compared with the available experimental data. The cubic and quartic force constants are predicted. The calculated results show that the B3PW91 results are in excellent agreement with experiment and represent a substantial improvement over the results obtained from B3LYP.

Femtosecond pump-probe studies of actinic-wavelength dependence in aqueous chlorine dioxide photochemistry

The actinic or photolysis-wavelength dependence of aqueous chlorine dioxide (OClO) photochemistry is investigated using femtosecond pump-probe spectroscopy. Following photoexcitation at 310, 335, and 410 nm the photoinduced evolution in optical density is measured from the UV to the near IR. Analysis of the optical-density evolution illustrates that the quantum yield for atomic chlorine production (Phi(Cl)) increases with actinic energy, with Phi(Cl)=0.16+/-0.02 for 410 nm excitation and increasing to 0.25+/-0.01 and 0.54+/-0.10 for 335 and 310 nm excitations, respectively. Consistent with previous studies, the production of Cl occurs through two channels, with one channel corresponding to prompt (<5 ps) Cl formation and the other corresponding to the thermal decomposition of ClOO formed by OClO photoisomerization. The partitioning between Cl production channels is dependent on actinic energy, with prompt Cl production enhanced with an increase in actinic energy. Limited evidence is found for enhanced ClO production with an increase in actinic energy. Stimulated emission and excited-state absorption features associated with OClO populating the optically prepared (2)A(2) surface decrease with an increase in actinic energy suggesting that the excited-state decay dynamics are also actinic energy dependent. The studies presented here provide detailed information on the actinic-wavelength dependence of OClO photochemistry in aqueous solution.

A variational approach for calculating Franck-Condon factors including mode-mode anharmonic coupling

We have implemented our new procedure for computing Franck-Condon factors utilizing vibrational configuration interaction based on a vibrational self-consistent field reference. Both Duschinsky rotations and anharmonic three-mode coupling are taken into account. Simulations of the first ionization band of ClO(2) and C(4)H(4)O (furan) using up to quadruple excitations in treating anharmonicity are reported and analyzed. A developer version of the MIDASCPP code was employed to obtain the required anharmonic vibrational integrals and transition frequencies.

Ultrafast processes in OClO molecules excited by femtosecond laser pulses at 386–409nm

Ultrafast dissociation dynamics in OClO molecules is studied, induced by femtosecond laser pulses in the wavelength region from 386 to 409 nm, i.e., within the wide absorption band to the (approximately)A (2)A(2) electronic state. The decay of the initially excited state due to nonadiabatic coupling to the close lying (2)A(1) and (2)B(2) electronic states proceeds with a time constant increasing from 4.6 ps at 386 nm to 30 ps at 408.5 nm. Dissociation of the OClO molecule occurs after internal conversion within about 250 fs. In addition, a minor channel of direct excitation of the (2)A(1) electronic state has been identified, the lifetime of which increases from a few 100 fs at 386 nm to 2.2 ps at 408.5 nm. Simultaneous excitation of two neighboring vibrational bands in the (approximately)A (2)A(2) state leads to a coherent oscillation of the parent ion signal with the frequency difference of both modes.

Theoretical study of the electronic nonadiabatic transitions in the photoelectron spectroscopy of F2O

The photoelectron spectrum of F2O pertaining to ionizations to the ground (X2B1) and low-lying excited electronic states (A2B2, B2A1, and C2A2) of F2O+ is investigated theoretically. The near equilibrium potential energy surfaces of the ground electronic state (X2B1) of F2O and the mentioned ground and excited electronic states of F2O+ reported by Wang et al. ( J. Chem. Phys. 2001, 114, 10682) for the C2v configuration are extended for the Cs geometry assuming a harmonic vibration along the asymmetric stretching mode. The vibronic interactions between the A2B2 and B2A1 electronic states of F2O+ are treated within a linear coupling approach, and the strength of the vibronic coupling parameter is calculated by an ab initio method. The nuclear dynamics is simulated by both time-independent quantum mechanical and time-dependent wave packet approaches. Although the first photoelectron band exhibits resolved vibrational progression along the symmetric stretching mode, the second one is highly overlapping. The latter is attributed to the nonadiabatic interactions among the energetically close A2B2, B2A1, and C2A2 electronic states of F2O+. The theoretical findings are in good accord with the available experimental results.

A theoretical investigation of the excited states of OCLO radical, cation, and anion using the CASSCF/CASPT2 method

Using the complete active space self-consistent field method with a large atomic natural orbital basis set, 10, 13, and 9 electronic states of the OClO radical, OClO(+) cation, and OClO(-) anion were calculated, respectively. Taking the further correlation effects into account, the second-order perturbation (CASPT2) calculations were carried out for the energetic calibration. The photoelectron spectroscopy of the OClO radical and OClO(-) anion were extensively studied in the both case of the adiabatic and vertical ionization energies. The calculated results presented the relatively complete assignment of the photoelectron bands of the experiments for OClO and its anion. Furthermore, the Rydberg states of the OClO radical were investigated by using multiconfigurational CASPT2 (MS-CASPT2) theory under the basis set of large atomic natural orbital functions augmented with an adapted 1s1p1d Rydberg functions that have specially been built for this study. Sixteen Rydberg states were obtained and the results were consistent with the experimental results.

Photodissociation Dynamics of Chlorine Peroxide Adsorbed on Ice

Chlorine peroxide plays an important role in the chlorine-ozone chemistry in the antarctic stratosphere. Adsorption by ice crystals may alter its photochemistry in different ways. We have simulated the photodissociation of a ClOOCl molecule adsorbed on ice by means of a semiclassical representation of the excited state dynamics. Electronic energies and wave functions of ClOOCl are computed by an ad hoc reparametrized semiempirical method, and the interaction with ice is taken into account by a QM/MM strategy. The reaction mechanism is similar to what was previously found for the isolated molecule: sequential or almost simultaneous breaking of both Cl-O bonds leads to the 2Cl + O2 reaction products in most cases. The Cl atoms remain temporarily adsorbed on the ice surface, whereas O2 is ejected. The main effect for the overall chlorine chemistry is probably an increase of the photodissociation rates at long wavelengths, due to the change of adsorption cross sections induced by the interaction with ice.

Time resolved infrared absorption studies of geminate recombination and vibrational relaxation in OClO photochemistry

Ultrafast time-resolved infrared absorption studies of aqueous chlorine dioxide (OClO) photochemistry are reported. Following photoexcitation at 401 nm, the evolution in optical density at frequencies between 1000 to 1100 cm(-1) is monitored to investigate vibrational energy deposition and relaxation along the asymmetric-stretch coordinate following the reformation of ground-state OClO via geminate recombination of the primary photofragments. The measured kinetics are compared to two proposed models for the vibrational-relaxation dynamics along the asymmetric-stretch coordinate. This comparison demonstrates that the perturbation model derived from molecular dynamics studies is capable of qualitatively reproducing the observed kinetics, where the collisional model employed in previous UV-pump, visible probe experiments demonstrates poor agreement with experiment. The ability of the perturbation model to reproduce the optical-density evolution observed in these studies demonstrates that for aqueous OClO, frequency dependence of the solvent-solute coupling is important in defining the level-dependent vibrational relaxation rates along the asymmetric-stretch coordinate. The absence of optical-density evolution corresponding to the population of higher vibrational levels (n>8) along the asymmetric-stretch coordinate suggests that following geminate recombination, energy is initially deposited into a local Cl-O stretch, with the relaxation of vibrational energy from this coordinate providing for delayed vibrational excitation of the asymmetric- and symmetric-stretch coordinates relative to geminate recombination, as previously observed.

A different approach for calculating Franck–Condon factors including anharmonicity

An efficient new procedure for calculating Franck-Condon factors, based on the direct solution of an appropriate set of simultaneous equations, is presented. Both Duschinsky rotations and anharmonicity are included, the latter by means of second-order perturbation theory. The critical truncation of basis set is accomplished by a build-up procedure that simultaneously removes negligible vibrational states. A successful test is carried out on ClO2 for which there are experimental data and other theoretical calculations.

Fourier transform microwave spectroscopy and Fourier transform microwave–millimeter wave double resonance spectroscopy of the ClOO radical

Pure rotational spectra of the ClOO radical for the (35)Cl and (37)Cl isotopomers have been observed using Fourier transform microwave and Fourier transform microwave-millimeter wave double resonance spectroscopy. The rotational, centrifugal, spin-rotation coupling, and hyperfine coupling constants have been determined by least-squares fits of the observed transition frequencies. The molecular constants indicate that the electronic ground state is 2A". The r(0) structure is determined to be r(0)(ClO)=2.075 A, r(0)(OO)=1.227 A, and theta;(0)(ClOO)=116.4 degrees . Several highly accurate ab initio calculations have also been performed. Some of them turned out to be inaccurate because it is necessary to take into account both static and dynamic electronic correlations. Only multireference (single and double) configuration interaction calculations with large basis sets reproduce the present experimental results. The anharmonic force constants obtained by the ab initio calculations are used to determine the r(e) structure, r(e)(ClO)=2.084(1) A, r(e)(OO)=1.206(2) A, and theta;(e)(ClOO)=115.4(1) degrees . Unique features of the ClOO radical have become clear by the present experiment and the ab initio calculations.

Chlorine oxide radicals ClOx (x = 1-4) studied by matrix isolation spectroscopy

Low pressure flash thermolysis of different precursor molecules containing-ClO, -ClO3 or -OClO3 yield, when highly diluted in Ne or O2 and subsequent quenching of the products in a matrix at 5 or 15 K, ClOx (x = 1, 3, 4) radicals, respectively. If Ne or O2 gas is directed over solid ClO2 at -120 degrees C and the resulting gas mixtures are immediately deposited as a matrix, a high fraction of (OClO)2 is trapped. This enables recording of IR and UV spectra of weakly bonded (OClO)2 dimers and detailed studying of their photochemistry. For Ne or O2 matrix isolated ClO radicals the vibrational wavenumbers and electronic transitions are only slightly affected compared with the gas phase. In this study strong evidence is found for long lived ClO in the electronically excited 2 [symbol: see text] 1/2 state. A comprehensive IR study of Ne matrix isolated ClO3 (fundamentals at 1081, 905, 567, 476 cm-1) yield i) a reliable force field; ii) a OClO bond angle of alpha e = 113.8 +/- 1 degrees and iii) a ClO bond length of 148.5 +/- 2 pm in agreement with predicted data from quantum chemical calculations. The UV/Vis spectrum of ClO3 isolated in a Ne matrix (lambda max at 32,100 and 23,150 cm-1) agrees well with the photoelectron spectrum of ClO3- and theoretical predictions. The origin of the structured high energy absorption is at 22,696 cm-1 and three fundamentals (794, 498, 280 cm-1) are detected in the C2E state. By photolysis of ClO3 with visible light the complex ClO.O2 with ClO in the 2 [symbol: see text] 1/2 state is formed. In an extended spectroscopic study of the elusive ClO4 radical, isolated in a Ne or O2 matrix, three additional IR bands, a complete UV spectrum and a strong interaction with O2 are found. This leads to the conclusion that ClO4 exhibits C2v or Cs symmetry with a shallow potential minimum and forms with O2 the previously unknown peroxy radical O3ClO-O2. All these results are discussed in the context of recent developments in the chemistry and spectroscopy of the important and interesting ClOx (x = 1-4) family of radicals.