Major Dissociation Channels Research Articles

Photodissociation Dynamics of Phenol

The photodissociation of phenol at 193 and 248 nm was studied using multimass ion-imaging techniques and step-scan time-resolved Fourier-transform spectroscopy. The major dissociation channels at 193 nm include cleavage of the OH bond, elimination of CO, and elimination of H(2)O. Only the former two channels are observed at 248 nm. The translational energy distribution shows that H-atom elimination occurs in both the electronically excited and ground states, but elimination of CO or H(2)O occurs in the electronic ground state. Rotationally resolved emission spectra of CO (1 <or= v <or= 4) in the spectral region of 1860-2330 cm(-1) were detected upon photolysis at 193 nm. After a correction for rotational quenching, CO (v <or= 4) shows a nascent rotational temperature of approximately 4600 K. The observed vibrational distribution of (v = 1)/(v = 2)/(v = 3)/(v = 4) = 64.3/22.2/9.1/4.4 corresponds to a vibrational temperature of 3350 +/- 20 K. An average rotational energy of 6.9 +/- 0.7 kcal mol(-1) and vibrational energy of 3.8 +/- 0.7 kcal mol(-1) are observed for the CO product. The dissociation channels, translational energy distributions of the photofragment, and vibrational and rotational energies of product CO are consistent with potential energy surfaces from quantum chemical calculations and the branching ratios from an RRKM calculation.

Photodissociation Pathways of 1,1-Dichloroacetone

We present a comprehensive investigation of the dissociation dynamics following photoexcitation of 1,1-dichloroacetone (CH(3)COCHCl(2)) at 193 nm. Two major dissociation channels are observed: cleavage of a C-Cl bond to form CH(3)C(O)CHCl + Cl and elimination of HCl. The branching between these reaction channels is roughly 9:1. The recoil kinetic energy distributions for both C-Cl fission and HCl elimination are bimodal. The former suggests that some of the radicals are formed in an excited electronic state. A portion of the CH(3)C(O)CHCl photoproducts undergo secondary dissociation to give CH(3) + C(O)CHCl. Photoelimination of Cl(2) is not a significant product channel. A primary C-C bond fission channel to give CH(3)CO + CHCl(2) may be present, but this signal may also be due to a secondary dissociation. Data from photofragment translational spectroscopy with electron impact and photoionization detection, velocity map ion imaging, and UV-visible absorption spectroscopy are presented, along with G3//B3LYP calculations of the bond dissociation energetics.

Photodissociation of methane: Exploring potential energy surfaces

The potential energy surface for the first excited singlet state (S(1)) of methane is explored using multireference singles and doubles configuration interaction calculations, employing a valence triple zeta basis set. A larger valence quadruple zeta basis is used to calculate the vertical excitation energy and dissociation energies. All stationary points found on the S(1) surface are saddle points and have imaginary frequencies for symmetry-breaking vibrations. By studying several two-dimensional cuts through the potential energy surfaces, it is argued that CH(4) in the S(1) state will distort to planar structures. Several conical intersection seams between the ground state surface S(0) and the S(1) surface have been identified at planar geometries. The conical intersections provide electronically nonadiabatic pathways towards products CH(3)((approximately)X (2)A"(2))+H, CH(2)((approximately)a (1)A(1))+H(2), or CH(2)((approximately)X (3)B(1))+H+H. The present results thereby make it plausible that the CH(3)((approximately)X (2)A"(2))+H and CH(2)((approximately)a (1)A(1))+H(2) channels are major dissociation channels, as has been observed experimentally.

Acetylene Elimination in Photodissociation of Neutral Azulene and Its Cation: An Ab Initio and RRKM Study

AbstractThe ab initio/Rice‐Ramsperger‐Kassel‐Marcus (RRKM) approach in conjunction with ab initio calculations of potential energy surfaces has been applied to investigate the photodissociation mechanism of azulene and its cation at 6.4 eV (wavelength 193 nm) upon absorption of one UV photon followed by internal conversion into the ground electronic state. Reaction pathways leading to several dissociation products have been mapped out at the Gaussian‐3 level and then the RRKM and microcanonical variational transition state theories have been applied to compute rate constants for individual reaction steps. Relative product yields (branching ratios) for dissociation products have been calculated to solve the system of master equations. It is established that the major dissociation channel of neutral azulene is H atom elimination. Pentalene and phenylacetylene as heavy fragments of acetylene elimination reaction could be observed with low yields. Azulene cation dissociates preferably through the pentalene channel. C10H7+, phenylacetylene, and benzocyclobutadiene cations can also be produced with comparable concentrations. Different kinetics of the acetylene elimination reaction have been revealed in the cases of neutral and charged azulene.

Ab initio and RRKM study of photodissociation of azulene cation

The ab initio/Rice-Ramsperger-Kassel-Marcus (RRKM) approach has been applied to investigate the photodissociation mechanism of the azulene cation at different values of the photon energy. Reaction pathways leading to various decomposition products have been mapped out at the G3(MP2,CC)//B3LYP level and then the RRKM and microcanonical variational transition state theories have been applied to compute rate constants for individual reaction steps. Relative product yields (branching ratios) for the dissociation products have been calculated using the steady-state approach. The results show that a photoexcited azulene cation can readily isomerize to a naphthalene cation. The major dissociation channels are elimination of atomic hydrogen, an H2 molecule, and acetylene. The branching ratio of the H elimination channel decreases with an increase of the photon energy. The branching ratio of the acetylene elimination as well as that of the H2 elimination rise as the photon energy increases. The main C8H6+ fragment at all photon energies considered is a pentalene cation, and its yield decreases slightly with increasing excitation energy, whereas the branching ratios of the other C8H6+ fragments, phenylacetylene and benzocyclobutadiene cations, grow.

Dissociation of Benzene Dication [C6H6]2+: Exploring the Potential Energy Surface

The singlet potential energy surface for the dissociation of benzene dication has been explored, and its three major dissociation channels have been studied: C6H6(2+) --> C3H3(+) + C3H3(+), C4H3(+) + C2H3(+), and C5H3(+) + CH3(+). The calculated energetics suggest that the products will be formed with considerable translational energy because of the Coulomb repulsion between the charged fragments. The calculated energy release in the three channels shows a qualitative agreement with the experimentally observed kinetic energy release. The formation of certain intermediates is found to be common to the three dissociation channels.

Photodissociation of Azulene at 193 nm: Ab Initio and RRKM Study

The ab initio/Rice-Ramsperger-Kassel-Marcus (RRKM) approach has been applied to investigate the photodissociation mechanism of azulene at 6.4 eV (the laser wavelength of 193 nm) upon absorption of one UV photon followed by internal conversion into the ground electronic state. Reaction pathways leading to various decomposition products have been mapped out at the G3(MP2,CC)//B3LYP level and then the RRKM and microcanonical variational transition state theories have been applied to compute rate constants for individual reaction steps. Relative product yields (branching ratios) for the dissociation products have been calculated using the steady-state approach. The results show that photoexcited azulene can readily isomerize to naphthalene and the major dissociation channel is elimination of an H-atom from naphthalene. The branching ratio of this channel decreases with an increase of the photon energy. Acetylene elimination is the second probable reaction channel and its branching ratio rises as the photon energy increases. The main C8H6 fragments at 193 nm are phenylacetylene and pentalene and the yield of the latter grows fast with the increasing excitation energy.

Photodissociation Dynamics of C6HxF6-x (x = 1−4) at 193 nm

Photodissociation of fluorine-substituted benzenes, including 1,3-difluorobenzene, 1,2,4-trifluorobenzene, 1,2,4,5-tetrafluorobenzene, and pentafluorobenzene, at 193 nm under collision-free conditions has been studied in separate experiments using multimass ion imaging techniques. HF elimination was found to be the major dissociation channel for all of these molecules. Small amounts of photofragments of C(6)H(3)F(2) and C(6)H(2)F(3) from 1,3-difluorobenzene and 1,2,4-trifluorobenzene, respectively, were also observed. They correspond to the minor dissociation channel of hydrogen elimination. Dissociation rates and fragment translational energy distributions obtained from experimental measurements suggest that HF and hydrogen elimination reactions occur in the ground electronic state. The potential energy surface obtained from ab initio calculations indicates that the four-center reaction in the ground electronic state is the major dissociation mechanism for the HF eliminations. A comparison with the RRKM calculation has been made.

Photodissociation of the Hydroxymethyl Radical from the 22A‘ ‘(3pz) State: H2CO and HCOH Products

The photodissociation of the hydroxymethyl radical in excited vibrational levels of the Rydberg 3pz state is investigated by the core-sampling time-of-flight method and infrared-ultraviolet (IR+UV) double resonance ionization spectroscopy. Translational energy distributions of dissociation products of CH2OD (3pz; υ) are measured for selected vibrational levels. CH2OD → D + CH2O (I) and CH2OD → H + CHOD (II) are identified as major dissociation channels. Secondary dissociation from internally excited H2CO and HCOD photofragments is observed when their internal energy exceeds the threshold for H/D formation. Mechanisms for secondary H/D production include direct dissociation and dissociation following HCOD ↔ HDCO isomerization. Several new overtone and combination bands of the 22A‘ ‘(3pz) ← 12A‘ ‘ transition of CH2OH are accessed by IR+UV double resonance spectroscopy. Initial vibrational excitation of ground-state CH2OH enhances the Franck−Condon factors for subsequent electronic transitions to the 3pz sta...

Dissociation of multiply ionized isocyanic acid through electron impact.

The dissociation of singly to triply ionized isocyanic acid (HNCO) has been investigated by two- and three-dimensional covariance mapping techniques through electron impact ionization at an electron energy of 200 eV. The absolute cross sections for the various dissociation channels of up to triply ionized HNCO have been measured. The HNCO dications dissociate mostly into ion pairs, while the HNCO trications dissociate mostly into ion triples, both through all the possible bond cleavages and charge allocations. Some major ion-pair dissociation channels of HNCO2+ are supposed to be sequential dissociation through initial charge separation. The metastable decay traces caused by HNCO(2+)-->H(+)+NCO+ and HNCO(+)-->HCO(+)+N have been observed on the covariance map.

Photodissociation dynamics of fluorobenzene.

Photodissociation of both fluorobenzene and d(5)-fluorobenzene at 193 nm under collision-free conditions has been studied in separate experiments using multimass ion imaging techniques. HF and DF eliminations were found to be the major dissociation channels. Small amounts of photofragments, C(6)H(4)F and C(6)D(4)F, corresponding to H and D atom eliminations were also observed. Dissociation rate and fragment translational energy distribution suggest that HF (DF) and H (D) atom elimination reactions occur in the ground electronic state. The potential energy surface obtained from ab initio calculations indicates that the four-center reaction in the ground electronic state is the major dissociation mechanism for the HF and DF eliminations. A comparison with photodissociation of benzene has been made.

Photoisomerization and Photodissociation of m-Xylene in a Molecular Beam

The photodissociation of isotope-labeled m-xylene, 1,3-C6H4CD3CD3, at 6.4 eV under collision-free conditions was studied using multimass ion imaging techniques. The masses of the photofragments were identified, and the fragment recoil velocity distributions were measured. In addition to the major dissociation channels C6H4CD3CD3 → C6H4CD3CD2 + D and C6H4CD3CD3 → C6H4CD3 + CD3, the respective photofragments C6H3DCD3, C6H2D2CD3, and C6HD3CD3 and some of their light fragment partners, CD2H and CDH2, were also observed. The results show that a small portion of the excited xylene isomerizes to a seven-membered ring (methylcycloheptatriene), followed by the isotope exchange within the seven-membered ring, and then rearomatizes to a six-membered ring prior to dissociation. The isomerization pathway competes with the direct C−C bond and C−H bond dissociation in the ground electronic state. The significance of this isomerization is that the carbon atoms and hydrogen atoms belonging to the alkyl group are involved in an exchange with those atoms in the aromatic ring.

An ab Initio/RRKM Study of Product Branching Ratios in the Photodissociation of Buta‐1,2‐ and ‐1,3‐dienes and But‐2‐yne at 193 nm

Ab initio G2M(MP2)//B3LYP/6-311G** calculations have been performed to investigate the reaction mechanism of photodissociation of buta-1,2- and -1,3-dienes and but-2-yne after their internal conversion into the vibrationally hot ground electronic state. The detailed study of the potential-energy surface was followed by microcanonical RRKM calculations of energy-dependent rate constants for individual reaction steps (at 193 nm photoexcitation and under collision-free conditions) and by solution of kinetic equations aimed at predicting the product branching ratios. For buta-1,2-diene, the major dissociation channels are found to be the single Cbond;C bond cleavage to form the methyl and propargyl radicals and loss of hydrogen atoms from various positions to produce the but-2-yn-1-yl (p1), buta-1,2-dien-4-yl (p2), and but-1-yn-3-yl (p3) isomers of C(4)H(5). The calculated branching ratio of the CH(3) + C(3)H(3)/C(4)H(5) + H products, 87.9:5.9, is in a good agreement with the recent experimental value of 96:4 (ref. 21) taking into account that a significant amount of the C(4)H(5) product undergoes secondary dissociation to C(4)H(4) + H. The isomerization of buta-1,2-diene to buta-1,3-diene or but-2-yne appears to be slower than its one-step decomposition and plays only a minor role. On the other hand, the buta-1,3-diene-->buta-1,2-diene, buta-1,3-diene-->but-2-yne, and buta-1,3-diene-->cyclobutene rearrangements are significant in the dissociation of buta-1,3-diene, which is shown to be a more complex process. The major reaction products are still CH(3) + C(3)H(3), formed after the isomerization of buta-1,3-diene to buta-1,2-diene, but the contribution of the other radical channels, C(4)H(5) + H and C(2)H(3) + C(2)H(3), as well as two molecular channels, C(2)H(2) + C(2)H(4) and C(4)H(4) + H(2), significantly increases. The overall calculated C(4)H(5) + H/CH(3) + C(3)H(3)/C(2)H(3) + C(2)H(3)/C(4)H(4) + H(2)/C(2)H(2) + C(2)H(4) branching ratio is 24.0:49.6:4.6:6.1:15.2, which agrees with the experimental value of 20:50:8:2:2022 within 5 % margins. For but-2-yne, the one-step decomposition pathways, which include mostly H atom loss to produce p1 and, to a minor extent, molecular hydrogen elimination to yield methylethynylcarbene, play an approximately even role with that of the channels that involve the isomerization of but-2-yne to buta-1,2- or -1,3-dienes. p1 + H are the most important reaction products, with a branching ratio of 56.6 %, followed by CH(3) + C(3)H(3) (23.8 %). The overall C(4)H(5) + H/CH(3) + C(3)H(3)/C(2)H(3) + C(2)H(3)/C(4)H(4) + H(2)/C(2)H(2) + C(2)H(4) branching ratio is predicted as 62.0:23.8:2.5:5.7:5.6. Contrary to buta-1,2- and -1,3-dienes, photodissociation of but-2-yne is expected to produce more hydrogen atoms than methyl radicals. The isomerization mechanisms between various isomers of the C(4)H(6) molecule including buta-1,2- and -1,3-dienes, but-2-yne, 1-methylcyclopropene, dimethylvinylidene, and cyclobutene have been also characterized in detail.

Vibrational structures of predissociating methylamines (CH3NH2 and CH3ND2) in à states: Free internal rotation of CH3 with respect to NH2

Resonantly-enhanced one-color two-photon (1+1) ionization spectra of jet-cooled methylamines (CH3NH2 and CH3ND2) reveal the vibrational structures of these molecules in predissociative à states. Rotational fine structure is clearly resolved for CH3ND2 at the origin and first wagging vibrational level in the excited state. The spectral linewidth becomes homogeneously broadened to give only vibrationally resolved spectral features for the higher vibrational energy levels of CH3ND2 (Ã). From the spectral analysis of the ÖX transition of CH3ND2, it is found that the methyl moiety rotates nearly freely about the C–N axis with respect to the amino group in the à state, indicating that the removal of an electron from the nonbonding orbital of N is responsible for the free internal rotation. Vibrational levels are only barely resolved in the ÖX excitation spectrum of CH3NH2 due to severe homogeneous line-broadening, indicating ultrashort lifetimes of ∼0.4 ps for predissociating CH3NH2 molecules in the à state. Spectral interpretation of the ÖX excitation spectrum of CH3NH2 is carried out by the comparison with that of CH3ND2, giving the confirmative vibrational assignment of methylamines in à states for the first time. The dramatic difference of CH3NH2 and CH3ND2 in their lifetimes in à states suggests that the major dissociation channel of the excited methylamine may be the N–H (or D) bond dissociation.

Dissociation of multiply ionized alkanes from methane to n-butane due to electron impact

The dissociation of up to triply ionized alkanes from methane to n-butane has been studied through electron impact ionization at an electron energy of 200 eV. The ionic fragments are recorded by a covariance mapping technique using a focusing time-of-flight mass spectrometer. The absolute cross-sections for the numerous dissociation channels of all the alkane monocations and dications, as well as propane and n-butane trications have been obtained. All the alkane dications are unstable and dissociate mostly into ion pairs, in which the production of proton pairs is found to be a major dissociation channel. The ionic products from the dissociation of ethane, propane and n-butane dications have a similar distribution, in which the first few abundant ion species are H +, CH 3 +, C 2H 3 + and C 2H 2 +. The total single ionization cross-sections agree well with the results of the binary-encounter-Bethe method. The distortion of the ion coincidence islands has been explained as due to the deviation from the space focusing condition of the mass spectrometer. Metastable decays, such as C 3H 5 2+→CH 3 ++C 2H 2 + in propane and C 4H 10 +→C 3H 7 ++CH 3 in n-butane have been observed and identified using the initial and final angles of the decay traces.

Single to triple ionization of propane due to electron impact: Cross sections for the different dissociation channels

Electron impact single to triple ionization and subsequent dissociation of propane have been studied at an electron energy of 200 eV using a covariance mapping technique employing a focusing time-of-flight mass spectrometer. The dissociation channels of up to triply ionized propane have been identified and the corresponding absolute cross sections for the different dissociation channels have been obtained. Results show that 88% of the singly ionized propane molecules are unstable and dissociate through more than 20 channels, among which 50% of the C3H8+ ions dissociate into C2H5+ or C2H4+. All the doubly ionized propane molecules are unstable and dissociate through more than 80 channels, among which 97% of the C3H82+ ions dissociate into ion pairs. The major dissociation channels for C3H82+ are C2H3++CH3+, H++H+, and C2H4++CH3+, while the major ionic products from the dissociation of C3H82+ are H+, CH3+, and C2H3+.

Determination of polychlorodibenzo-p-dioxins and polychlorodibenzofurans by gas chromatography/tandem mass spectrometry and gas chromatography/triple mass spectrometry in a quadrupole ion trap.

The determination of tetra- to octachlorodibenzo-p-dioxins and tetra- to octachlorodibenzofurans (PCCD/Fs) by high-resolution gas chromatography/tandem mass spectrometry (HRGC/MS/MS) and high-resolution gas chromatography/triple mass spectrometry (HRGC/MS(3)) in a quadrupole ion trap, equipped with an external ion source, is presented. MS/MS involves a typical four-step process, namely ionization, parent ion isolation, collision-induced dissociation (CID) and mass analysis of the daughter ions. For the MS(3) experiment, the MS/MS scan function is used with the addition of selected daughter ion isolation, their CID and the mass analysis of second-generation product ions called 'grand-daughter ions.' For both methods, the energies necessary for the CID of the 17 PCDD/Fs were determined and optimized using multiple scan functions with different CID amplitudes. The CID efficiency, defined as the signal ratio of fragment ions detected from the major dissociation channels to molecular ions isolated, was 1.15-2.40 V for parent ion dissociation (MS/MS) and 1.05-1.50 V for daughter ion dissociation (MS(3)) and for all the chloro congeners. The same sensitivity (1 pg microl(-1)) can be reached with both the MS/MS and MS(3) methods and linear responses were obtained between 1 and 100 pg microl(-1) injected.

Avoided Curve Crossing between the A1 and B1 States in CF2Br2 Photolysis at 234 and 265 nm

The photodissociation dynamics of CF2Br2 has been studied using a two-dimensional photofragment ion image technique for the two excitation wavelengths 234 and 265 nm. At 234 nm, three dissociation channels, i.e., the radical CF2Br + Br (2PJ) (J = 1/2, 3/2), the three-body CF2 + 2Br (2PJ) (J = 1/2, 3/2), and the molecular elimination channel CF2 + Br2, were observed with quantum yields of 0.84, 0.15, and trace, respectively. The difference between β (CF2Br + Br (2P3/2)) = 0.65 and β (CF2Br + Br (2P1/2)) = 0.80 suggests that the excited A1 and B1 states are strongly correlated by avoided curve crossing with a probability of 0.78. At 265 nm, the radical channel, the observed major primary dissociation channel, shows several components due to the transition close to the curve crossing point. On the basis of observations at both wavelengths, we have proposed a photodissociation dynamics model of CF2Br2 after A-band excitation.

Vacuum ultraviolet photoionization and photodissociation of ferrocene

Photoionization and photodissociation of ferrocene [Fe(C 5H 5) 2] is examined by using vacuum ultraviolet (VUV) photons from a synchrotron radiation source and a time-of-flight (TOF) photoionization mass spectrometer. VUV absorption by ferrocene results in Fe(C 5H 5) 2 ·+, FeC 5H 5 +, FeC 3H 3 +, Fe ·+ and C 10H x ·+ (x = 8 − 10). The dependency of the product distribution on photon energy indicates sequential elimination of C 5H 5· ligands as a major dissociation channel, but concerted elimination of two C 5H 5· ligands also takes place to a lesser degree. Through analysis of TOF peak shape, it is found that Fe(C 5H 5) 2 ·+ molecular ion dissociates into FeC 5H 5 + and C 5H 5· via two channels. One is nonstatistical dissociation with a fast rate, and the other is slow unimolecular decay that becomes more discernible at low photon energy. The rate of unimolecular decay, exemplified by a value of k = 2.4 ± 1.0 × 10 6 s −1 at photon energy of 14.76 eV, is well in accord with the RRKM rate.

Tandem mass spectrometric determination of polychlorodibenzo- p-dioxins and polychlorodibenzofurans in a quadrupole ion trap using multi-frequency resonant excitation

A tandem mass spectrometric analytical method is presented for the concurrent determination of tetra- to octa-chlorodibenzo- p-dioxins and tetra- to octa-chlorodibenzofurans. The method entails operation of a quadrupole ion trap mass spectrometer wherein two isolated molecular ion species are fragmented by irradiation with a band of multiple frequencies for a period of but 10 ms. Up to three co-eluting compounds have been determined individually using a software program, Toolkit, which permits successive activation of multiple scan functions customized for each compound and optimized for the resonant excitation conditions employed. Two calibration curves have been obtained for each compound over a 400-fold concentration range; one calibration curve is based on the single major fragmentation channel of COCl-loss while the other is based on multiple fragmentation channels. The CID efficiency, defined as the signal ratio of fragment ions detected from the major dissociation channels to molecular ions isolated, was 0.80–0.90 for all chlorocongeners. The calibration curves were prepared using a set of standard solutions containing labelled chlorocongeners. Optimum results were obtained when fragment ion signal intensities were normalized to those obtained from a single standard solution. In general, tandem mass spectrometric response factors display a 4-fold range with response factor decreasing with increasing chlorine substitution.