On the electronically nonadiabatic decomposition dynamics of furazan and triazole energetic molecules.

The combined results of ab initio electronic-structure calculations, nonadiabatic molecular dynamics simulations using ab initio multiple spawning, and previous spectroscopic investigations of jet-cooled molecules provide strong evidence of a (π,σ*)-mediated decomposition mechanism for the furazan and triazole energetic molecules. The importance of dissociative excited states formed by electron promotion from a π molecular orbital to a σ* molecular orbital is explored for the furazan and triazole energetic molecules. Dissociative (π,σ*) states of furazan and triazole energetic molecules can be populated by nonadiabatic surface jump from the (π,π*) or the (n,π*) state. Finally, conical intersections between (π,σ*) potential energy surfaces (PESs) and the ground PES influence the eventual fragmentation dynamics of the furazan and triazole energetic molecules. Due to structural similarity of the triazole molecule with the pyrrole molecule, a comparison of nonadiabatic dynamics of these two molecules is also presented. The N-N bond dissociation is found to be a barrierless pathway for the triazole molecule, whereas the N-H bond dissociation exhibits a barrierless pathway for the pyrrole molecule. The present work, thus, provides insights into the excited-state chemistry of furazan and triazole energetic functional groups. The same insight can also be relevant for other energetic molecules.

Studies of Arabinose- and Mannose-Related Anionic Species and Comparison to Ribose and Fructose.

Gas phase, isolated monosaccharides arabinose- and mannose-related anionic species generated through the matrix-assisted laser desorption ionization (MALDI) method are investigated via negative ion photoelectron spectroscopy (PES) and density functional theory (DFT) calculations. The vertical detachment energies (VDEs) of the observed anionic species are experimentally determined: the corresponding structures are assigned based on good agreement between experimental and theoretical VDEs. Arabinose- parent anion is found to exist as open chain structures in the gas phase, while mannose- parent anionic species are not observed. Both monosaccharides undergo dissociation through loss of H and loss of H2O. (saccharide-H)- anions evidence coexisting positional and conformational isomers. (saccharide-H2O)- species have only two positional isomers, each with conformational differences. The present results for arabinose and mannose are further compared to those previously reported for ribose and fructose. This comparison is based on the anions observed and identified through the same PES/DFT techniques for the four saccharides (arabinose, mannose, ribose, and fructose). The issue of natural selection of ribose as the sugar backbone constituent of RNA is thereby explored from the point of view of anionic electronic structure and stability of the four species. Saccharide phosphates are also discussed in the present work with regard to addressing the unique natural selection of ribose for the backbone support of RNA and DNA.

Fe-V sulfur clusters studied through photoelectron spectroscopy and density functional theory.

Iron-vanadium sulfur cluster anions are studied by photoelectron spectroscopy (PES) at 3.492 eV (355 nm) and 4.661 eV (266 nm) photon energies, and by density functional theory (DFT) calculations. The structural properties, relative energies of different structural isomers, and the calculated first vertical detachment energies (VDEs) of different structural isomers for cluster anions FeVS1-3- and FemVnSm+n- (m + n = 3, 4; m > 0, n > 0) are investigated at a BPW91/TZVP theory level. The experimental first VDEs for these Fe-V sulfur clusters are reported. The most probable ground state structures and spin multiplicities for these clusters are tentatively assigned by comparing their theoretical and experiment first VDE values. For FeVS1-3- clusters, their first VDEs are generally observed to increase with the number of sulfur atoms from 1.45 eV to 2.86 eV. The NBO/HOMOs of the ground state of FeVS1-3- clusters are localized in a p orbital on a S atom; the partial charge distribution on the NBO/HOMO localized site of each cluster anion is responsible for the trend of their first VDEs. A less negative localized charge distribution is correlated with a higher first VDE. Structure and steric effect differences for FemVnSm+n- (m + n = 3, m > 0, n > 0) clusters are suggested to be responsible for their different first VDEs and properties. Two types of structural isomers are identified for FemVnSm+n- (m + n = 4, m > 0, n > 0) clusters: a tower structure isomer and a cubic structure isomer. The first VDEs for tower like isomers are generally higher than those for cubic like isomers of FemVnSm+n- (m + n = 4, m > 0, n > 0) clusters. Their first VDEs are can be understood through: (1) NBO/HOMO distributions, (2) structures (steric effects), and (3) partial charge numbers on the NBO/HOMO's localized sites. EBEs for excited state transitions for all Fe-V sulfur clusters are calculated employing OVGF and TDDFT approaches at the TZVP level. The OVGF approach for these Fe/V/S cluster anions is better for the higher transition energies than the TDDTF approach. The experimental and theoretical results for these Fe/V/S cluster anions are compared with their related pure iron sulfur cluster anions. Properties of the NBO/HOMO are essential for understanding and estimating the different first VDEs for Fe/V/S, and comparing them to those of the pure Fe/S cluster anions.

Isomeric structures of isolated ammonium nitrate and its hydrogenated species identified through PES experiments and DFT calculations.

Anion photoelectron spectroscopic (PES) experiments in conjunction with density functional theory (DFT) calculations shed light on the electronic and geometric structures of gas phase, isolated ammonium nitrate related anionic species, as well as their hydrogenated species with up to five added hydrogens. These species are directly generated by laser ablation and cooled in a supersonic expansion. Their vertical detachment energies (VDE: Eneutral - Eanion, both at the anionic geometry) are experimentally determined and the corresponding anionic structures are characterized and assigned through calculations. Based on the experimentally evaluated calculation algorithm, the corresponding neutral structures are also determined. The parent anionic species exists as (NH2OH·HONO)- in the gas phase with the extra electron valence bound. Crystal structure anion NH4NO3- is not present in our experiments, as within this structure the extra electron is dipole bound (electron affinity ∼ 0 eV). The isomerization must therefore occur for ammonium nitrate upon capturing an extra electron or during the laser ablation process itself. The ammonium nitrate anion is apparently a very reactive species. The calculated global minimum for the isolated parent neutral species has an HNO3·NH3 structure, different from the crystal structure in the bulk phase. The hydrogenated cluster anions can evolve from the parent (NH2OH·HONO)- species and exhibit moieties, which bind together as a single unit through interactions between noncovalently bonded species and are stable on the experimental timescale. The hydrogenation process forms stable moieties in the cluster anions, including water (H2O), nitroxyl (HNO), ammonia (NH3), or (HNOH). The calculated global minimum structures for hydrogenated cluster neutrals (NH4NO3 + nH, n = 1,…,5) contain ammonia and water, along with stable moieties (HONO, NO, and HNO). These stable moieties, along with intermediate species NO2H2 and ONH2, offer new insights into the behavior of ammonium nitrate energetic materials.

Photoelectron spectroscopy and density functional theory studies of (FeS)mH- (m = 2-4) cluster anions: effects of the single hydrogen.

Single hydrogen containing iron hydrosulfide cluster anions (FeS)mH- (m = 2-4) are studied by photoelectron spectroscopy (PES) at 3.492 eV (355 nm) and 4.661 eV (266 nm) photon energies, and by Density Functional Theory (DFT) calculations. The structural properties, relative energies of different spin states and isomers, and the first calculated vertical detachment energies (VDEs) of different spin states for these (FeS)mH- (m = 2-4) cluster anions are investigated at various reasonable theory levels. Two types of structural isomers are found for these (FeS)mH- (m = 2-4) clusters: (1) the single hydrogen atom bonds to a sulfur site (SH-type); and (2) the single hydrogen atom bonds to an iron site (FeH-type). Experimental and theoretical results suggest such available different SH- and FeH-type structural isomers should be considered when evaluating the properties and behavior of these single hydrogen containing iron sulfide clusters in real chemical and biological systems. Compared to their related, respective pure iron sulfur (FeS)m- clusters, the first VDE trend of the diverse type (FeS)mH0,1- (m = 1-4) clusters can be understood through (1) the different electron distribution properties of their highest singly occupied molecular orbital employing natural bond orbital analysis (NBO/HSOMO), and (2) the partial charge distribution on the NBO/HSOMO localized sites of each cluster anion. Generally, the properties of the NBO/HSOMOs play the principal role with regard to the physical and chemical properties of all the anions. The change of cluster VDE from low to high is associated with the change in nature of their NBO/HSOMO from a dipole bound and valence electron mixed character, to a valence p orbital on S, to a valence d orbital on Fe, and to a valence p orbital on Fe or an Fe-Fe delocalized valence bonding orbital. For clusters having the same properties for NBO/HSOMOs, the partial charge distributions at the NBO/HSOMO localized sites additionally affect their VDEs: a more negative or less positive localized charge distribution is correlated with a lower first VDE. The single hydrogen in these (FeS)mH- (m = 2-4) cluster anions is suggested to affect their first VDEs through the different structure types (SH- or FeH-), the nature of the NBO/HSOMOs at the local site, and the value of partial charge number at the local site of the NBO/HSOMO.

Photoelectron spectroscopy and density functional theory studies of (fructose + (H2O)n)- (n = 1-5) anionic clusters.

Photoelectron spectroscopy (PES) and density functional theory (DFT) based calculations are executed to characterize gas phase, isolated (fructose + (H2O)n)- (n = 1-5) anionic species produced using a matrix assisted laser desorption ionization (MALDI) method. Gas phase, isolated (fructose + (H2O)n)- (n = 1-5) cluster anions mainly exist as open chain structures with conformational and positional isomers in the present experiments. Some cyclic structures of (fructose + (H2O)n)- (n = 3, 4) are apparently present in the experiments and their VDEs can contribute to the lower energy shoulders of PES features observed for (fructose + (H2O)n)- (n = 3, 4). Cyclic (fructose + (H2O)n)- (n = 1-5) clusters have the added electron as dipole bound, whereas open chain structures have the added electron in a valence orbital. Water molecules in open chain anions predominantly interact with the (1)C side (including (1)OH, (2)O, and (3)OH) of fructose-: they finally form a quasi-cubic structure with OH groups and carbonyl O in the most stable structures for (fructose + (H2O)4)- and (fructose + (H2O)5)- cluster anions. Water molecules solvating cyclic anions form water-water hydrogen bond networks that preferentially interact with OH groups at the (1), (2), and (3) positions of fructose pyranose anions, and the (3), (4), and (6) positions of fructose furanose anions. Structures of neutral (fructose + (H2O)n) (n = 1-5) have pyranose structures as the lower energy isomers rather than open chain structures: this observation is consistent with the fructose solution tautomeric equilibrium with neutral fructose pyranose being the preponderant species. Water molecules also tend to form water-water hydrogen bond networks, interacting with OH groups at (1), (2), and (3) positions for neutral pyranose conformations.

IR + VUV double resonance spectroscopy and extended density functional theory studies of ketone solvation by alcohol: 2-butanone·(methanol)n, n = 1-4 clusters.

Infrared plus vacuum ultraviolet (IR + VUV) photoionization vibrational spectroscopy of 2-butanone/methanol clusters [MEK·(MeOH)n, n = 1-4] is performed to explore structures associated with hydrogen bonding of MeOH molecules to the carbonyl functional group of the ketone. IR spectra and X3LYP/6-31++G(d,p) calculations show that multiple isomers of MEK·(MeOH)n are generated in the molecular beam as a result of several hydrogen bonding sites available to the clusters throughout the size range investigated. Isomer interconversion involving solvating MeOH rearrangement should probably occur for n = 1 and 2. The mode energy for a hydrogen bonded OH stretching transition gradually redshifts as the cluster size increases. Calculations suggest that the n = 3 cluster isomers adopt structures in which the MEK molecule is inserted into the cyclic MeOH hydrogen bond network. In larger structures, the cyclic network may be preserved.

Anionic ribose related species explored through PES experiments, DFT calculations, and through comparison with anionic fructose species.

Ribose related species, (ribose-H)- and (ribose-H2O)-, are investigated through anion photoelectron spectroscopy (PES) combined with density functional theory (DFT) calculations. Their vertical detachment energies (VDEs) are experimentally determined and their anionic structures with positional and conformational isomers are definitively assigned. The ribose- parent anion is not detected in the present experiments. (ribose-H)- and (ribose-H2O)- anions can be accessed as the characteristic fragmentation ions of the parent species. Generation of (ribose-H)- through the matrix assisted laser desorption ionization (MALDI) process is sample desorption substrate dependent, while generation of (ribose-H2O)- is independent of a wide range of desorption substrates. Both conformational and positional isomers of (ribose-H)- are identified in the gas phase. Two types of positional isomers of (ribose-H2O)- (both from open chain structures) are assigned to contribute to two different components of the observed PES feature. The dehydration process can be thermodynamically accessed through both the parent anion and the neutral. A comparison between the PES and DFT results of ribose and fructose leads to the general conclusions: (1) ribose- open chain structures are more likely to lose hydrogen from C atoms than fructose- open chain structures, especially from (2)C and (4)C positions; (2) ribofuranose- has a relatively higher propensity for loss of H than does fructofuranose- in our MALDI/ablation process; and (3) the ribofuranose- anion has lower decomposition probability through loss of water compared to the fructofuranose- anion.

Photoelectron Spectroscopy and Density Functional Theory Studies of Iron Sulfur (FeS)m- (m = 2-8) Cluster Anions: Coexisting Multiple Spin States.

Iron sulfur cluster anions (FeS)m- (m = 2-8) are studied by photoelectron spectroscopy (PES) at 3.492 eV (355 nm) and 4.661 eV (266 nm) photon energies, and by density functional theory (DFT) calculations. The most probable structures and ground state spin multiplicities for (FeS)m- (m = 2-8) clusters are tentatively assigned through a comparison of their theoretical and experiment first vertical detachment energy (VDE) values. Many spin states lie within 0.5 eV of the ground spin state for the larger (FeS)m- (m ≥ 4) clusters. Theoretical VDEs of these low lying spin states are in good agreement with the experimental VDE values. Therefore, multiple spin states of each of these iron sulfur cluster anions probably coexist under the current experimental conditions. Such available multiple spin states must be considered when evaluating the properties and behavior of these iron sulfur clusters in real chemical and biological systems. The experimental first VDEs of (FeS)m- (m = 1-8) clusters are observed to change with the cluster size (number m). The first VDE trends noted can be related to the different properties of the highest singly occupied molecular orbitals (NBO, HSOMOs) of each cluster anion. The changing nature of the NBO/HSOMO of these (FeS)m- (m = 1-8) clusters from a p orbital on S, to a d orbital on Fe, and to an Fe-Fe bonding orbital is probably responsible for the observed increasing trend for their first VDEs with respect to m.

Anionic fructose-related conformational and positional isomers assigned through PES experiments and DFT calculations.

Gas phase, isolated fructose anionic species, fructose-, (fructose-H)-, (fructose-OH)-, and (fructose-H2O)-, are investigated employing anionic photoelectron spectroscopy (PES) combined with density functional theory (DFT) calculations. The PES vertical detachment energies (VDEs) for these anions are determined and, based on these experimental values, their calculated anionic structures are assigned. Generation of these four species through the matrix assisted laser desorption ionization (MALDI) process is sample desorption substrate dependent. The parent anion fructose- exists as a single, dominant open chain structure in the gas phase, with substrate dependent specific conformational isomers. (Fructose-H)- and (fructose-OH)- are mainly produced from the laser ablation process rather than from fragmentation reaction pathways associated with the parent anion species. Both conformational and positional isomers are identified in the gas phase for these latter anions. (Fructose-H2O)- has two types of positional isomers, both of which contribute to two different components of the observed PES feature. The fixed positions for losing an OH group and an H atom, in addition to thermodynamic calculations, provide reaction pathways for generating a dehydration product (open chain structures) from the parent anion (open chain and furanose structures), further demonstrating the active nature of fructose upon capturing an extra electron.

Initial mechanisms for the unimolecular decomposition of electronically excited bisfuroxan based energetic materials.

Unimolecular decomposition of energetic molecules, 3,3'-diamino-4,4'-bisfuroxan (labeled as A) and 4,4'-diamino-3,3'-bisfuroxan (labeled as B), has been explored via 226/236 nm single photon laser excitation/decomposition. These two energetic molecules, subsequent to UV excitation, create NO as an initial decomposition product at the nanosecond excitation energies (5.0-5.5 eV) with warm vibrational temperature (1170 ± 50 K for A, 1400 ± 50 K for B) and cold rotational temperature (<55 K). Initial decomposition mechanisms for these two electronically excited, isolated molecules are explored at the complete active space self-consistent field (CASSCF(12,12)/6-31G(d)) level with and without MP2 correction. Potential energy surface calculations illustrate that conical intersections play an essential role in the calculated decomposition mechanisms. Based on experimental observations and theoretical calculations, NO product is released through opening of the furoxan ring: ring opening can occur either on the S1 excited or S0 ground electronic state. The reaction path with the lowest energetic barrier is that for which the furoxan ring opens on the S1 state via the breaking of the N1-O1 bond. Subsequently, the molecule moves to the ground S0 state through related ring-opening conical intersections, and an NO product is formed on the ground state surface with little rotational excitation at the last NO dissociation step. For the ground state ring opening decomposition mechanism, the N-O bond and C-N bond break together in order to generate dissociated NO. With the MP2 correction for the CASSCF(12,12) surface, the potential energies of molecules with dissociated NO product are in the range from 2.04 to 3.14 eV, close to the theoretical result for the density functional theory (B3LYP) and MP2 methods. The CASMP2(12,12) corrected approach is essential in order to obtain a reasonable potential energy surface that corresponds to the observed decomposition behavior of these molecules. Apparently, highly excited states are essential for an accurate representation of the kinetics and dynamics of excited state decomposition of both of these bisfuroxan energetic molecules. The experimental vibrational temperatures of NO products of A and B are about 800-1000 K lower than previously studied energetic molecules with NO as a decomposition product.

Photoelectron spectroscopy and density functional theory studies of N-rich energetic materials.

The geometric and electronic structures of molecular anionic energetic materials (EMs) DAAF (3,3'-diamino-4,4'-azoxyfurazan), FOX-7 (1,1-diamino-2,2-dinitroethene), 5,5'-BT (5,5'-bistetrazole), and 1,5'-BT (1,5'-bistetrazole) are explored employing anionic photoelectron spectroscopy and density functional theory calculations. The electron binding energies of the observed anionic, energetic material related species are determined and their corresponding anionic structures are assigned. Decomposition reactions for negatively charged EMs can proceed with different energy barriers, and thus mechanisms, from those for their related neutral EMs. Reactivity based on the anionic initial fragments of these EM species further reinforces their respective highly reactive and explosive nature. Fragment ions of the form EM--H-X (X = N2, N2+NH, …) are additionally observed. Detection of such species suggests that EM--H could serve as promising new candidates for EMs, assuming that such species are synthetically available, perhaps as energetic salts. Vertical detachment energies for transitions to the ground and first triplet electronic excited states of neutral matrix dye anion DCM- are additionally determined.

Properties of iron sulfide, hydrosulfide, and mixed sulfide/hydrosulfide cluster anions through photoelectron spectroscopy and density functional theory calculations.

A new magnetic-bottle time-of-flight photoelectron spectroscopy (PES) apparatus is constructed in our laboratory. The PES spectra of iron sulfide, hydrosulfide, and mixed sulfide/hydrosulfide [FeSm(SH)n-; m, n = 0-3, 0 < (m + n) ≤ 3] cluster anions, obtained at 2.331 eV (532 nm) and 3.492 eV (355 nm) photon energies, are reported. The electronic structure and bonding properties of these clusters are additionally investigated at different levels of density functional theory. The most probable structures and ground state spin multiplicity for these cluster anions are tentatively assigned by comparing their theoretical first vertical detachment energies (VDEs) with their respective experiment values. The behavior of S and (SH) as ligands in these iron sulfide, hydrosulfide, and mixed sulfide/hydrosulfide cluster anions is investigated and compared. The experimental first VDEs for Fe(SH)1-3- cluster anions are lower than those found for their respective FeS1-3- cluster anions. The experimental first VDEs for FeS1-3- clusters are observed to increase for the first two S atoms bound to Fe-; however, due to the formation of an S-S bond for the FeS3- cluster, its first VDE is found to be ∼0.41 eV lower than the first VDE for the FeS2- cluster. The first VDEs of Fe(SH)1-3- cluster anions are observed to increase with the increasing numbers of SH groups. The calculated partial charges of the Fe atom for ground state FeS1-3- and Fe(SH)1-3- clusters are apparently related to and correlated with their determined first VDEs. The higher first VDE is correlated with a higher, more positive partial charge for the Fe atom of these cluster anions. Iron sulfide/hydrosulfide mixed cluster anions are also explored in this work: the first VDE for FeS(SH)- is lower than that for FeS2-, but higher than that for Fe(SH)2-; the first VDEs for FeS2(SH)- and FeS(SH)2- are close to that for FeS3-, but higher than that for Fe(SH)3-. The first VDEs of general iron sulfide, hydrosulfide, and mixed sulfide/hydrosulfide clusters [FeSm(SH)n-; m, n = 0-3, 0 < (m + n) ≤ 3] are dependent on three properties of these anions: 1. the partial charge on the Fe atom, 2. disulfide bond formation (S-S) in the cluster, and 3. the number of hydrosulfide ligands in the cluster. The higher the partial charge on the Fe atom of these clusters, the larger the first VDE; however, cluster S-S bonding and more (SH) ligands in the cluster lower the cluster anion first VDE.

Initial mechanisms for the unimolecular decomposition of electronically excited nitrogen-rich energetic materials with tetrazole rings: 1-DTE, 5-DTE, BTA, and BTH.

Unimolecular decomposition of nitrogen-rich energetic molecules 1,2-bis(1H-tetrazol-1-yl)ethane (1-DTE), 1,2-bis(1H-tetrazol-5-yl)ethane (5-DET), N,N-bis(1H-tetrazol-5-yl)amine (BTA), and 5,5'-bis(tetrazolyl)hydrazine (BTH) has been explored via 283 nm two photon laser excitation. The maximum absorption wavelength in the UV-vis spectra of all four materials is around 186-222 nm. The N2 molecule, with a cold rotational temperature (<30 K), is observed as an initial decomposition product from the four molecules, subsequent to UV excitation. Initial decomposition mechanisms for these four electronically excited isolated molecules are explored at the complete active space self-consistent field (CASSCF) level. Potential energy surface calculations at the CASSCF(12,8)/6-31G(d) level illustrate that conical intersections play an essential role in the decomposition mechanism. The tetrazole ring opens on the S1 excited state and through conical intersections (S1/S0)CI, N2 product is formed on the ground state potential energy surface without rotational excitation. The tetrazole rings of all four energetic molecules open at the N1-N2 ring bond with the lowest energy barrier: the C-N bond opening has higher energy barrier than that for any of the N-N ring bonds. Therefore, the tetrazole rings open at their N-N bonds to release N2. The vibrational temperatures of N2 product from all four energetic materials are hot based on theoretical calculations. The different groups (CH2-CH2, NH-NH, and NH) joining the tetrazole rings can cause apparent differences in explosive behavior of 1-DTE, 5-DTE, BTA, and BTH. Conical intersections, non-Born-Oppenheimer interactions, and dynamics are the key features for excited electronic state chemistry of organic molecules, in general, and energetic molecules, in particular.

Dynamics and fragmentation of van der Waals and hydrogen bonded cluster cations: (NH3)n and (NH3BH3)n ionized at 10.51 eV.

A 118 nm laser is employed as a high energy, single photon (10.51 eV/photon) source for study of the dynamics and fragmentation of the ammonia borane (NH3BH3) cation and its cluster ions through time of flight mass spectrometry. The behavior of ammonia ion and its cluster ions is also investigated under identical conditions in order to explicate the ammonia borane results. Charge distributions, molecular orbitals, and spin densities for (NH3BH3)n and its cations are explored at both the second-order perturbation theory (MP2) and complete active space self-consistent field (CASSCF) theory levels. Initial dissociation mechanisms and potential energy surfaces for ionized NH3BH3, NH3, and their clusters are calculated at the MP2/6-311++G(d,p) level. Protonated clusters (NH3)xH(+) dominate ammonia cluster mass spectra: our calculations show that formation of (NH3)n-1H(+) and NH2 from the nascent (NH3)n(+) has the lowest energy barrier for the system. The only common features for the (NH3)n(+) and (NH3BH3)n(+) mass spectra under these conditions are found to be NHy(+) (y = 0,…,4) at m/z = 14-18. Molecular ions with both (11)B and (10)B isotopes are observed, and therefore, product ions observed for the (NH3BH3)n cluster system derive from (NH3BH3)n clusters themselves, not from the NH3 moiety of NH3BH3 alone. NH3BH2(+) is the most abundant ionization product in the (NH3BH3)n(+) cluster spectra: calculations support that for NH3BH3(+), an H atom is lost from the BH3 moiety with an energy barrier of 0.67 eV. For (NH3BH3)2(+) and (NH3BH3)3(+) clusters, a B(δ+)⋯H(δ-)⋯(δ-)H⋯(δ+)B bond can form in the respective cluster ions, generating a lower energy, more stable ion structure. The first step in the (NH3BH3)n(+) (n = 2, 3) dissociation is the breaking of the B(δ+)⋯H(δ-)⋯(δ-)H⋯(δ+)B moiety, leading to the subsequent release of H2 from the latter cluster ion. The overall reaction mechanisms calculated are best represented and understood employing a CASSCF natural bond orbital description of the valence electron distribution for the various clusters and monomers. Comparison of the present results with those found for solid NH3BH3 suggests that NH3BH3 can be a good hydrogen storage material.

Ethylene C-H Bond Activation by Neutral Mn2O5 Clusters under Visible Light Irradiation.

A photo excitation fast flow reactor coupled with a single-photon ionization (118 nm, 10.5 eV) time-of-flight mass spectrometry (TOFMS) instrument is used to investigate reactions of neutral MnmOn clusters with C2H4 under visible (532 nm) light irradiation. Association products Mn2O5(C2H4) and Mn3O6,7(C2H4) are observed without irradiation. Under light irradiation, the Mn2O5(C2H4) TOFMS feature decreases, and a new species, Mn2O5H2, is observed. This light-activated reaction suggests that the visible radiation can induce the chemistry, Mn2O5 + C2H4 + hv(532 nm) → Mn2O5*(C2H4) → Mn2O5H2 + C2H2. High barriers (0.67 and 0.59 eV) are obtained on the ground-state potential energy surface (PES); the reaction is barrierless and thermodynamically favorable on the first excited-state PES, as performed by time-dependent density functional theory calculations. The calculational and experimental results suggest that Mn2O5-like structures on manganese oxide surfaces are the appropriate active catalytic sites for visible light photocatalysis of ethylene dehydrogenation.

Three-dimensional nanoscale molecular imaging by extreme ultraviolet laser ablation mass spectrometry.

Analytical probes capable of mapping molecular composition at the nanoscale are of critical importance to materials research, biology and medicine. Mass spectral imaging makes it possible to visualize the spatial organization of multiple molecular components at a sample's surface. However, it is challenging for mass spectral imaging to map molecular composition in three dimensions (3D) with submicron resolution. Here we describe a mass spectral imaging method that exploits the high 3D localization of absorbed extreme ultraviolet laser light and its fundamentally distinct interaction with matter to determine molecular composition from a volume as small as 50 zl in a single laser shot. Molecular imaging with a lateral resolution of 75 nm and a depth resolution of 20 nm is demonstrated. These results open opportunities to visualize chemical composition and chemical changes in 3D at the nanoscale.

Initial mechanisms for the decomposition of electronically excited energetic materials: 1,5'-BT, 5,5'-BT, and AzTT.

Decomposition of nitrogen-rich energetic materials 1,5'-BT, 5,5'-BT, and AzTT (1,5'-Bistetrazole, 5,5'-Bistetrazole, and 5-(5-azido-(1 or 4)H-1,2,4-triazol-3-yl)tetrazole, respectively), following electronic state excitation, is investigated both experimentally and theoretically. The N2 molecule is observed as an initial decomposition product from the three materials, subsequent to UV excitation, with a cold rotational temperature (<30 K). Initial decomposition mechanisms for these three electronically excited materials are explored at the complete active space self-consistent field (CASSCF) level. Potential energy surface calculations at the CASSCF(12,8)/6-31G(d) level illustrate that conical intersections play an essential role in the decomposition mechanism. Electronically excited S1 molecules can non-adiabatically relax to their ground electronic states through (S1/S0)CI conical intersections. 1,5'-BT and 5,5'-BT materials have several (S1/S0)CI conical intersections between S1 and S0 states, related to different tetrazole ring opening positions, all of which lead to N2 product formation. The N2 product for AzTT is formed primarily by N-N bond rupture of the -N3 group. The observed rotational energy distributions for the N2 products are consistent with the final structures of the respective transition states for each molecule on its S0 potential energy surface. The theoretically derived vibrational temperature of the N2 product is high, which is similar to that found for energetic salts and molecules studied previously.

Initial mechanisms for the decomposition of electronically excited energetic salts: TKX-50 and MAD-X1.

Decomposition of energetic salts TKX-50 and MAD-X1 (dihydroxylammonium 5,5'-bistetrazole-1,1'-diolate and dihydroxylammonium 3,3'-dinitro-5,5'-bis-1,2,4-triazole-1,1'-diol, respectively), following electronic state excitation, is investigated both experimentally and theoretically. The NO and N2 molecules are observed as initial decomposition products from the two materials subsequent to UV excitation. Observed NO products are rotationally cold (<25 K) and vibrationally hot (>1500 K). The vibrational temperature of the NO product from TKX-50 is (2600 ± 250) K, (1100 ± 250) K hotter than that produced from MAD-X1. Observed N2 products of these two species are both rotationally cold (<30 K). Initial decomposition mechanisms for these two electronically excited salts are explored at the complete active space self-consistent field (CASSCF) level. Potential energy surface calculations at the CASSCF(8,8)/6-31G(d) level illustrate that conical intersections play an essential role in the decomposition mechanisms. Electronically excited S1 molecules can nonadiabatically relax to the lower electronic state through (S1/S0)CI conical intersections. Both TKX-50 and MAD-X1 have two (S1/S0)CI conical intersections between S1 and S0 states, related to and leading to two different reaction paths, forming N2 and NO products. N2 products are released by the opening of the tetrazole or triazole rings of TKX-50 and MAD-X1. NO products are released from the amine N-oxide moiety of TKX-50, and for MAD-X1, they are produced through nitro-nitrite isomerizations. The observed rotational energy distributions for NO and N2 products are consistent with the final structures of the respective transition states for each molecule on its S0 potential energy surface.

Experimental and theoretical studies of H2O oxidation by neutral Ti2O4,5 clusters under visible light irradiation.

A new photo excitation fast flow reactor system is constructed and used to investigate reactions of neutral TimOn clusters with H2O under visible (532 nm) light irradiation. Single photon ionization at 118 nm (10.5 eV) is used to detect neutral cluster distributions through time of flight mass spectrometry. TimOn clusters are generated through laser ablation of a titanium target in the presence of 4% O2/He carrier gas. Association products Ti2O4(H2O) and Ti2O5(H2O) are observed for reactions of H2O and TimOn clusters without irradiation. Under 532 nm visible light irradiation of the fast flow reactor, only the Ti2O5(H2O) feature disappears. This light activated reaction suggests that visible radiation can induce chemistry for Ti2O5(H2O), but not for Ti2O4(H2O). Density functional theory (DFT) and time-dependent (TD) DFT calculations are performed to explore the ground and first excited state potential energy surfaces (PES) for the reaction Ti2O5 + H2O → Ti2O4 + H2O2. A high barrier (1.33 eV) and a thermodynamically unfavorable (1.14 eV) pathway are obtained on the ground state PES for the Ti2O5 + H2O reaction; the reaction is also thermodynamically unfavorable (1.54 eV) on the first singlet excited state PES. The reaction is proposed to occur on the ground state PES through a conical intersection ((S1/S0)CI), and to generate products Ti2O4 and H2O2 on the ground state PES. This mechanism is substantiated by a multi-reference ab initio calculation at the complete active space self-consistent field (CASSCF) level. The S0-S1 vertical excitation energy of Ti2O4 (3.66 eV) is much higher than the 532 nm photon energy (2.33 eV), suggesting this visible light driven reaction is unfavorable for the Ti2O4 cluster. The TDDFT calculated optical absorption spectra of Ti2O4 and Ti2O5 further indicate that Ti2O5 like structures on a titanium oxide surface are the active catalytic sites for visible light photo-catalytic oxidation of water.