Photochemical processes induced by vibrational overtone excitations: dynamics simulations for cis-HONO, trans-HONO, HNO3, and HNO3-H2O

Photochemical processes in HNO3, HNO3-H2O, and cis- and trans-HONO following overtone excitation of the OH stretching mode are studied by classical trajectory simulations. Initial conditions for the trajectories are sampled according to the initially prepared vibrational wave function. Semiempirical potential energy surfaces are used in "on-the-fly" simulations. Several tests indicate at least semiquantitative validity of the potential surfaces employed. A number of interesting new processes and intermediate species are found. The main results include the following: (1) In excitation of HNO3 to the fifth and sixth OH-stretch overtone, hopping of the H atom between the oxygen atoms is found to take place in nearly all trajectories, and can persist for many picoseconds. H-atom hopping events have a higher yield and a faster time scale than the photodissociation of HNO3 into OH and NO2. (2) A fraction of the trajectories for HNO3 show isomerization into HOONO, which in a few cases dissociates into HOO and NO. (3) For high overtone excitation of HONO, isomerization into the weakly bound species HOON is seen in all trajectories, in part of the events as an intermediate step on the way to dissociation into OH + NO. This process has not been reported previously. Well-established processes for HONO, including cis-trans isomerization and H hopping are also observed. (4) Only low overtone levels of HNO3-H2O have sufficiently long liftimes to be spectrocopically relevant. Excitation of these OH stretching overtones is found to result in the dissociation of the cluster H hopping, or dissociation of HNO3 does not take place. The results demonstrate the richness of processes induced by overtone excitation of HNO(x) species, with evidence for new phenomena. Possible relevance of the results to atmospheric processes is discussed.     

Proton transfer dynamics of the reaction H3O(+)(NH3,H2O)NH4+ studied using the crossed molecular beam technique

The proton transfer reaction of H3O+ and NH3 was studied using the crossed molecular beam technique at relative energies of 0.41, 0.81, and 1.27 eV. At all three energies, the center-of-mass flux distribution of the product ion NH4+ exhibits sharply asymmetry, and the maximum is close to the velocity and direction of the precursor ammonia beam. The reaction transforms almost all of the 1.69 eV exothermicity into internal excitation of the products at all three collision energies. At the lowest collision energy of 0.41 eV, nearly 77% of the total energy appears in NH4+ internal excitation. However, almost 100% of the incremental translational energy in the two higher-energy experiments appears in the product translational energy. Such an observation provides a classic example of the "induced repulsive energy release" mechanism that is expected to be operative on the highly skewed potential energy surfaces characteristic of the heavy+light-heavy mass combination. These results indicate that the proton transfer proceeds through a direct reaction mechanism; a Rice-Ramsperger-Kassel-Marcus theory calculation shows that the lifetime of the intermediate complex [NH3-H-H2O]+ is about 100 fs. Proton transfer occurs early on the reaction coordinate, when the incipient N-H bond is extended, and results in highly vibrationally excited NH4+ products, with excitation primarily in N-H stretching modes.     

Grotthus-type and diffusive proton transfer in 7-hydroxyquinoline.(NH(3))(n) clusters.

Proton translocation along ammonia wires is investigated in 7-hydroxyquinoline.(NH(3))(n) clusters, both experimentally by laser spectroscopy and theoretically by Hartree-Fock and density functional (DFT) calculations. These clusters serve as realistic finite-size models for proton transfer along a chain of hydrogen-bonded solvent molecules. In the enol tautomer of 7-hydroxyquinoline (7-HQ), the OH group acts as a proton injection site into the (NH(3))(n)cluster. Proton translocation along a chain of three NH(3) molecules within the cluster can take place, followed by reprotonation of 7-HQ at the quinolinic N atom, forming the 7-ketoquinoline tautomer. Exoergic proton transfer from the OH group of 7-HQ to the closest NH(3) molecule within the cluster giving a zwitterion 7-HQ-.(NH(3))(6)H+ (denoted PT-A) occurs at a threshold cluster size of n = 6 in the DFT calculations and at n = 5 or 6 experimentally. Three further locally stable zwitterion clusters denoted PT-B, PT-B', and PT-C, the keto tautomer, and several transition structures along the proton translocation path were characterized theoretically. Grotthus-type proton-hopping mechanisms occur for three of the proton transfer steps, which have low barriers and are exoergic or weakly endoergic. The step with the highest barrier involves a complex proton transfer mechanism, involving structural reorganization and large-scale diffusive motions of the cluster.     

Proton transfer reactions and dynamics of sulfonic acid group in Nafion®

Proton transfer reactions and dynamics of the hydrophilic group (-SO(3)H) in Nafion? were studied at low hydration levels using the complexes formed from CF(3)SO(3)H, H(3)O(+) and nH(2)O, 1 ≤n≤ 3, as model systems. The equilibrium structures obtained from DFT calculations suggested at least two structural diffusion pathways at the -SO(3)H group namely, the "pass-through" and "pass-by" mechanisms. The former involves the protonation and deprotonation at the -SO(3)H group, whereas the latter the proton transfer in the adjacent Zundel complex. Analyses of the asymmetric O-H stretching frequencies (ν(OH)) of the hydrogen bond (H-bond) protons showed the threshold frequencies (ν(OH*)) of proton transfer in the range of 1700 to 2200 cm(-1). Born-Oppenheimer Molecular Dynamics (BOMD) simulations at 350 K anticipated slightly lower threshold frequencies (ν(A)(OH*,MD)), with two characteristic asymmetric O-H stretching frequencies being the spectral signatures of proton transfer in the H-bond complexes. The lower frequency (ν(A)(OH,MD))) is associated with the oscillatory shuttling motion and the higher frequency (ν(B)(OH,MD))) the structural diffusion motion. Comparison of the present results with BOMD simulations on protonated water clusters indicated that the -SO(3)H group facilitates proton transfer by reducing the vibrational energy for the interconversion between the two dynamic states (Δν), resulting in a higher population of the H-bonds with the structural diffusion motion. One could therefore conclude that the -SO(3)H groups in Nafion? act as active binding sites which provide appropriate structural, energetic and dynamic conditions for effective structural diffusion processes in a proton exchange membrane fuel cell (PEMFC). The present results suggested for the first time a possibility to discuss the tendency of proton transfer in H-bond using Δν(BA)(OH,MD)) and provided theoretical bases and guidelines for the investigations of proton transfer reactions in theory and experiment.     

Dynamics of vibrational overtone excitations of H2SO4, H2SO4-H2O: hydrogen-hopping and photodissociation processes

Photochemical processes of sulfuric acid (H2SO4) and sulfuric acid monohydrate (H2SO4-H2O) following overtone excitation of the OH stretching mode are studied by classical trajectory simulations using the semiempirical PM3 potential suface in "on the fly" calculations. The main results are the following: (1) In the excitation of H2SO4 to the fifth OH-stretch overtone, hopping of the H atom between oxygen atoms is found to take place in 22% of the trajectories, only once during simulations of 400 ps. (2) All the trajectories for H2SO4 show a rapid cis-trans isomerization. (3) The photolysis of H2SO4 into SO3 + H2O takes place in 5% of the trajectories on a time scale of approximately 9 ps. (4) Only low overtone levels of H2SO4-H2O have sufficiently long lifetimes to be spectroscopically relevant. Excitation to these OH stretching overtones is found to result in the dissociation of the cluster. H hopping or dissociation of H2SO4 does not take place.     

Conformational transitions of glycine induced by vibrational excitation of the O-H stretch

Vibrational energy flow and conformational transitions following excitation of the OH stretching mode of the most stable conformer of glycine are studied by classical trajectories. "On the fly" simulations with the PM3 semiempirical electronic structure method for the potential surface are used. Initial conditions are selected to correspond to the ν=1 excitation of the OH stretch. The main findings are: (1) An an equilibrium-like ratio is established between the populations of the 3 lowest-lying conformers after about 10 picoseconds. (2) There is a high probability throughout the 150 ps of the simulations for finding the molecule in geometries far from the equilibrium structures of the lowest-energy conformers. (3) Energy from the initial excited OH (ν=1) stretch flows preferentially to 5 other vibrational modes, including the bending motion of the H atom. (4) RRK theory yields conformational transition rates that deviate substantially from the classical trajectory results. Possible implication of these results for vibrational energy flow and conformational transitions in small biological molecules are discussed.     

Intramolecular nitro-assisted proton transfer in photoirradiated 2-(2',4'-dinitrobenzyl)pyridine: polarized optical spectroscopic study and electronic structure calculations

The nitro-assisted proton transfer (NAPT), responsible for the photoactivity of ortho-nitrobenzylpyridines and a model for the nitro-based caged compounds, is studied along with the parent compound 2-(2',4'-dinitrobenzyl)pyridine (DNBP) with polarized optical spectroscopy and theoretical calculations. The transition dipole moments of a DNBP single-crystal identified oriented molecules of the long-lived enamine tautomer (NH), rather than of the aci-nitro tautomer (OH), as carriers of the photoinduced blue coloration. It is clarified that the blue second singlet transition owes to intramolecular charge transfer from the allyl-pyridinium part to the dinitrophenyl fragment of NH. The theoretical modeling of the ground-state potential energy surface showed that while NH and OH can interconvert by means of direct proton transfer, such a process between the initial form CH and either OH and NH would require significant rotation of the aromatic rings. In the ground state, OH is less stable but the kinetically preferred product over NH. Once created, regardless of whether via ground-state or excited-state routes, the aci-nitro group of OH undergoes energetically inexpensive rotation to deliver the proton to the nitrogen acceptor. The "softening" of the energy surface around OH due to its structural flexibility, that is, mediation of the proton transfer by the nitro group, is crucial to overcome the high barrier for a direct proton jump from CH to NH, even in cases of unfavorable donor-acceptor geometry. The very small structural change experienced by the surrounding of a molecule undergoing NAPT is promising for the design of photoactive systems which retain their crystallinity during a prolonged operation.     

Two competitive routes in the lactim-lactam phototautomerization of a hydroxypyridine derivative cation in water: dissociative mechanism versus water-assisted proton transfer

Ground-state tautomerism and excited-state proton-transfer processes of 2-(6'-hydroxy-2'-pyridyl)benzimidazolium in H2O and D2O have been studied by means of UV-vis absorption and fluorescence spectroscopy in both steady-state and time-resolved modes. In the ground state, this compound shows a tautomeric equilibrium between the lactim cation, protonated at the benzimidazole N3, and its lactam tautomer, obtained by proton translocation from the hydroxyl group to the pyridine nitrogen. Direct excitation of the lactam tautomer leads to its own fluorescence emission, while as a result of the increase of acidity of the OH group and basicity at the pyridine N upon excitation, the lactim species undergoes a proton translocation from the hydroxyl group to the nitrogen, favoring the lactam structure in the excited state. No fluorescence emission from the initially excited lactim species was detected due to the ultrafast rate of the excited-state proton-transfer processes. The lactim-lactam phototaumerization process takes place via two competitive excited-state proton-transfer routes: a one-step water-assisted proton translocation (probably a double proton transfer) and a two-step pathway which involves first the dissociation of the lactim cation to form an emissive intermediate zwitterionic species and then the acid-catalyzed protonation at the pyridine nitrogen to give rise to the lactam tautomer.     

Equilibration of vibrationally excited OH in atomic and diatomic bath gases

In this work, a computational model of state-to-state energy flow in gas ensembles is used to investigate collisional relaxation of excited OH, present as a minor species in various bath gases. Rovibrational quantum state populations are computed for each component species in ensembles consisting of 8000 molecules undergoing cycles of binary collisions. Results are presented as quantum state populations and as (approximate) modal temperatures for each species after each collision cycle. Equilibration of OH is slow with Ar as the partner but much faster when N(2) and/or O(2) forms the bath gas. This accelerated thermalization is shown to be the result of near-resonant vibration-vibration transfer, with vibrational de-excitation in OH matched in energy by excitation in bath molecules. Successive near-resonant events result in an energy cascade. Such processes are highly dependent on molecule pair and on initial OH vibrational state. OH rotational temperatures initially increase, but at equilibration, they are lower than those of other modes. Possible reasons for this observation in molecules such as OH are suggested. There are indications of an order of precedent in the equilibration process, with vibrations taking priority over rotations, and potential explanations for this phenomenon are discussed.     

Mechanistic study of the electrochemical oxygen reduction reaction on Pt(111) using density functional theory

Density functional theory (DFT) was used to study the electrolyte solution effects on the oxygen reduction reaction (ORR) on Pt(111). To model the acid electrolyte, an H(5)O(2)(+) cluster was used. The vibrational proton oscillation modes for adsorbed H(5)O(2)(+) computed at 1711 and 1010 cm(-1), in addition to OH stretching and H(2)O scissoring modes, agree with experimental vibrational spectra for proton formation on Pt surfaces in ultrahigh vacuum. Using the H(5)O(2)(+) model, protonation of adsorbed species was found to be facile and consistent with the activation barrier of proton transfer in solution. After protonation, OOH dissociates with an activation barrier of 0.22 eV, similar to the barrier for O(2) dissociation. Comparison of the two pathways suggests that O(2) protonation precedes dissociation in the oxygen reduction reaction. Additionally, an OH diffusion step following O protonation inhibits the reaction, which may lead to accumulation of oxygen on the electrode surface.     

Excited-state proton transfer through water bridges and structure of hydrogen-bonded complexes in 1H-pyrrolo[3,2-h]quinoline: adiabatic time-dependent density functional theory study

Proton transfer reaction is studied for 1H-pyrrolo[3,2-h]quinoline-water complexes (PQ-(H(2)O)(n), n = 0-2) in the ground and the lowest excited singlet states at the density functional theory (DFT) level. Cyclic hydrogen-bonded complexes are considered, in which water molecules form a bridge connecting the proton donor (pyrrole NH group) and acceptor (quinoline nitrogen) atoms. To understand the effect of the structure and length of water bridges on the excited-state tautomerization in PQ, the potential energy profile of the lowest excited singlet state is calculated adiabatically by the time-dependent DFT (TDDFT) method. The S(0) --> S(1) excitation of PQ is accompanied by significant intramolecular transfer of electron density from the pyrrole ring to the quinoline fragment, so that the acidity of the N-H group and the basicity of the nitrogen atom of the quinoline moiety are increased. These excited-state acid-base changes introduce a driving force for the proton transfer reaction. The adiabatic TDDFT calculations demonstrate, however, that the phototautomerization requires a large activation energy in the isolated PQ molecule due to a high energy barrier separating the normal form and the tautomer. In the 1:1 cyclic PQ-H(2)O complex, the energy barrier is dramatically reduced, so that upon excitation of this complex the tautomerization can occur rapidly in one step as concerted asynchronous movements of the two protons assisted by the water molecule. In the PQ-(H(2)O)(2) solvate two water molecules form a cyclic bridge with sterically strained and unfavorable hydrogen bonds. As a result, some extra activation energy is needed for initiating the proton dislocation along the longer hydrogen-bond network. The full tautomerization in this complex is still possible; however, the cooperative proton transfer is found to be highly asynchronous. Large relaxation and reorganization of the hydrogen-bonded water bridge in PQ-(H(2)O)(2) are required during the proton translocation from the pyrrole NH group to the quinoline nitrogen; this may block the complete tautomerization in this type of solvate.     

The effect of hydrogen bonding on the excited-state proton transfer in 2-(2'-hydroxyphenyl)benzothiazole: a TDDFT molecular dynamics study

The dynamics of the excited-state proton transfer (ESPT) in a cluster of 2-(2'-hydroxyphenyl)benzothiazole (HBT) and hydrogen-bonded water molecules was investigated by means of quantum chemical simulations. Two different enol ground-state structures of HBT interacting with the water cluster were chosen as initial structures for the excited-state dynamics: (i) an intramolecular hydrogen-bonded structure of HBT and (ii) a cluster where the intramolecular hydrogen bond in HBT is broken by intermolecular interactions with water molecules. On-the-fly dynamics simulations using time-dependent density functional theory show that after photoexcitation to the S(1) state the ESPT pathway leading to the keto form strongly depends on the initial ground state structure of the HBT-water cluster. In the intramolecular hydrogen-bonded structures direct excited-state proton transfer is observed within 18 fs, which is a factor two faster than proton transfer in HBT computed for the gas phase. Intermolecular bonded HBT complexes show a complex pattern of excited-state proton transfer involving several distinct mechanisms. In the main process the tautomerization proceeds via a triple proton transfer through the water network with an average proton transfer time of approximately 120 fs. Due to the lack of the stabilizing hydrogen bond, intermolecular hydrogen-bonded structures have a significant degree of interring twisting already in the ground state. During the excited state dynamics, the twist tends to quickly increase indicating that internal conversion to the electronic ground state should take place at the sub-picosecond scale.     

QM/MM non-adiabatic decay dynamics of formamide in polar and non-polar solvents

Non-adiabatic on-the-fly dynamics simulations of the photodynamics of formamide in water and n-hexane were performed using a QM/MM approach. It was shown that steric restrictions imposed by the solvent cage do not have an influence on the initial motion which leads to the lowest energy conical intersection seam. The initial deactivation in water is faster than in n-hexane and in the gas phase. However, most of the formamide molecules in water do not reach the ground state. The reason for the deactivation inefficiency in water is traced back to a decrease of close COHOH and NHOH(2) contacts which fall in the range of hydrogen bonds. The energy deposition into H-bond breaking events leaves molecules with less energy for surmounting the CN dissociation barrier. In both solvents, after hopping to the ground state, the solvent cage keeps the HCO and NH(2) fragments or CO and NH(3) products in close proximity. Consequently, the number of trajectories where fast recombination happens is augmented with delayed recombinations that start when the dissociation fragments hit the cage wall and return back. The hot ground state formamide is formed in an internal conversion process identical to the path leading to CN photodissociation. In the case of aqueous formamide, good agreement with experimental results is achieved by combining dynamics simulations starting from the S(1) and the S(2) excited states collecting high and low energy trajectories, respectively.     

Time-dependent quantum wave-packet description of the 1pi sigma* photochemistry of phenol

The photoinduced hydrogen elimination reaction in phenol via the conical intersections of the dissociative 1pi sigma* state with the 1pi pi* state and the electronic ground state has been investigated by time-dependent quantum wave-packet calculations. A model including three intersecting electronic potential-energy surfaces (S0, 1pi sigma*, and 1pi pi*) and two nuclear degrees of freedom (OH stretching and OH torsion) has been constructed on the basis of accurate ab initio multireference electronic-structure data. The electronic population transfer processes at the conical intersections, the branching ratio between the two dissociation channels, and their dependence on the initial vibrational levels have been investigated by photoexciting phenol from different vibrational levels of its ground electronic state. The nonadiabatic transitions between the excited states and the ground state occur on a time scale of a few tens of femtoseconds if the 1pi pi*-1pi sigma* conical intersection is directly accessible, which requires the excitation of at least one quantum of the OH stretching mode in the 1pi pi* state. It is shown that the node structure, which is imposed on the nuclear wave packet by the initial preparation as well as by the transition through the first conical intersection (1pi pi*-1pi sigma*), has a profound effect on the nonadiabatic dynamics at the second conical intersection (1pi sigma*-S0). These findings suggest that laser control of the photodissociation of phenol via IR mode-specific excitation of vibrational levels in the electronic ground state should be possible.     

Formation and characterization of protein-protein complexes in vacuo

Gas-phase reactions between multiply charged positive and negative protein ions are carried out in a quadrupole ion trap mass spectrometer. The ions react with one another by proton transfer and complex formation. Proton transfer products and complexes are formed via competitive processes in single ion/ion encounters. The relative contributions of proton transfer versus complex formation are dependent upon the charges of the ions as well as other characteristics of the ions yet to be clearly delineated. No fragmentation of covalent bonds of the protein reactants is observed. A model that considers the trajectories associated with ion/ion interactions appears to hold the most promise in accounting for the results. The formation of bound ion/ion orbits appears to play an important role in determining overall reaction kinetics as well as the distribution of ion/ion reaction products. Tandem mass spectrometry is used to compare protein complexes formed in the gas-phase with those formed initially in solution and subsequently liberated by electrospray; it is shown that both forms of complex dissociate similarly, but the complexes formed in the gas phase can retain a "memory" of their method of formation.     

Contrasts in the 77 K emission spectra, structures, and dynamics of metal-to-metal and metal-to-ligand charge-transfer excited states

The 77 K emission spectrum of trans-[(ms-Me6[14]aneN4)Cr(CNRu(NH3)5)2]5+ has components characteristic of ligand field (LF) and metal-to-metal charge transfer (MMCT) excited states (ms-Me6[14]aneN4=5,12-meso-5,7,7,12,14,14-hexamethyl-1,4,8,11-tetraazacyclotetradecane). The LF component of the emission is best resolved for irradiations at appreciably higher energies than the MMCT absorption band, while only the MMCT emission is observed for irradiations on the low-energy side of the MMCT absorption band. The LF emission component from this complex has vibronic structure that is very similar to that of the trans-[(ms-Me6[14]aneN4)Cr(CN)2]+ parent, but it is red-shifted by 560 cm-1 and the bandwidths are much larger. The red shift and the larger bandwidths of the ruthenated complex are attributed to configurational mixing between the LF and MMCT excited states, and the inferred mixing parameters are shown to be consistent with the known electron-transfer properties of the Ru(NH3)5 moieties. The MMCT excited-state lifetime is about 1 micros at 77 K and am(m)ine perdeuteration of this complex leads to an isotope effect of kNH/kND approximately 15-20. However, the contribution of the N-H stretching vibration to the emission sideband is too weak for a single vibrational mode model to be consistent with the observed lifetimes or the isotope effect. These features are very similar to those reported previously (J. Phys. Chem. A 2004, 108, 5041) for the MMCT emission of trans-[([14]aneN4)Cr{CNRu(NH3)5}2]5+ ([14]aneN4=1,4,8,11-tetraazacyclotetradecane), with the exception that the higher energy LF emission was not well resolved in the earlier work. The energies of the charge transfer absorption and emission maxima of both of these Cr(CN)Ru complexes are very similar to those of [Ru(NH3)4bpy]2+, but the latter has a 50-fold shorter 77 K excited-state lifetime, a 10-fold smaller NH/ND isotope effect, and a very different structure of its vibronic sidebands. Thus, the vibronic sidebands imply that the dominant excited-state distortions are in the metal-ligand vibrational modes for the Cr(CN)Ru complexes and in the bipyridine vibrational modes for the [Ru(NH3)4bpy]2+ complex. While an "equivalent" single vibrational mode model based on the frequencies and amplitudes of the dominant distortion modes is not consistent the observed lifetimes, such models do appear to be a good basis for qualitatively distinguishing different classes of excited-state dynamic behavior. A multimode, multichannel model may be necessary to adequately describe the excited-state dynamics of these simple electron-transfer systems.     

Localization and moving of a proton inside hydrogen-bonded complexes in aprotic solvents

The results of the study of a number of molecular and ionic H-bonded complexes in freon solutions by ¹H NMR at 100–150 K are reported. It is shown that under these conditions the signals of OH(NH) protons belonging to various complexes, self-associates and free molecules are observed separately. The spin-spin coupling of the signals is frequently discernible. The fine structure makes it possible to distinguish between complexes with fast proton migration between two wells on the potential surface and those with the proton localized in one well (in particular, the central one). Several complexes with slow (in the NMR scale) proton migration have also been found.The results of the study of the non-catalytic proton exchange kinetics between various molecules containing OH and NH groups in dilute solutions in aprotic solvents are considered. The exchange between the RCOOH and ROH molecules goes on via the intermediate formation of a cyclic ionic pair with two equivalent H-bonds even in non-polar solvents such as cyclohexane. For exchange between two RCOOH or ROH molecules a synchronous transfer of two protons in a cyclic molecular complex is likely.     

Proton/Hydrogen‐Transfer Coordinate of 2,5‐Dihydroxybenzoic Acid Investigated in a Supersonic Beam: Combined IR/UV Spectroscopy in the S0, S1, and D0 States

As a model system for intramolecular proton/hydrogen‐transfer coordinates, the structure of 2,5‐dihydroxybenzoic acid is investigated for the ground, first electronically excited and also the ionic state. Combined IR/UV spectroscopy in molecular‐beam experiments is applied and the experimental results are interpreted by the application of DFT and CASPT2 methods. No proton or hydrogen transfer is observed, but evidence is given for a hydrogen dislocation of the intramolecular hydrogen bond in the S₁ state and to lesser extent in the D₀ state. To obtain direct information on the proton/hydrogen‐transfer coordinate, IR spectra are recorded both in the region of the OH and especially the CO stretching vibrations by also applying two new variants of combined IR/UV spectroscopy for the S₁ and D₀ states. The CO groups are directly involved in the hydrogen bond and, in contrast to the hydrogen‐bonded OH groups, the CO stretching frequencies can be observed in all electronic states.     

Bond-selective photodissociation of partially deuterated ammonia molecules: photodissociations of vibrationally excited NHD(2) in the 5nu(NH) state and NH(2)D in the 5nu(ND) state

Ultraviolet photodissociation of NHD(2) excited to the fourth overtone state of the NH stretching mode (5nu(NH)) and NH(2)D excited to that of the ND stretching mode (5nu(ND)) has been investigated by using a crossed laser and molecular beams method. Branching ratio between the NH and ND bond dissociations has been determined by utilizing a (2+1) resonance enhanced multiphoton ionization scheme of H and D atoms. For the photolysis of NHD(2) in the 5nu(NH) state, the NH dissociation cross section is 5.1+/-1.4 times as large as the ND dissociation cross section per bond. On the other hand, for the photolysis of NH(2)D in the 5nu(ND) state, the ratio of the NH dissociation cross section per bond to the ND dissociation cross section decreases to 0.68+/-0.16. In comparison with the branching ratios for the photolysis of vibrationally unexcited NH(2)D and NHD(2), the present results indicate that the excitation of the NH stretching mode enhances the NH dissociation with ca. two times larger NH/ND branching ratio, whereas the excitation of the ND stretching mode results in the preferential ND dissociation with ca. 3-4 times larger ND/NH branching ratio than that for the vibrational ground states. The mechanism of the bond-selective enhancement has been discussed in terms of the energetics and dynamics of wave packet.