Characterization of cyclic and linear C3H- and C3H via anion photoelectron spectroscopy

Anion photoelectron spectroscopy of C3H- and C3D- is performed using both field-free time-of-flight and slow electron velocity-map imaging. We observe and assign transitions originating from linear/bent (l-C3H) and cyclic (c-C3H) anionic isomers to the corresponding neutral ground states and low-lying excited states. Transitions within the cyclic and linear manifolds are distinguished by their photoelectron angular distributions and their intensity dependence on the neutral precursor. Using calculated values for the energetics of the neutral isomers [Ochsenfeld et al., J. Chem. Phys. 106, 4141 (1997)], which predict c-C3H to lie 74 meV lower than l-C3H, the experimental results establish c-C3H- as the anionic ground state and place it 229 meV below l-C3H-. Electron affinities of 1.999+/-0.003 and 1.997+/-0.005 eV are determined for C3H and C3D from the X 2B2<--X 1A1 photodetachment transition of c-C3H. Term energies for several low-lying states of c-C3H and l-C3H are also determined. Franck-Condon simulations are used to make vibrational assignments for the bands involving c-C3H. Simulations of the l-C3H bands were more complicated owing to large amplitude bending motion and, in the case of the neutral A 2Pi state, strong Renner-Teller coupling.     

Effects of alkyl substitution on the energetics of enolate anions and radicals.

The photoelectron spectra of the structural isomers of the three- and four-carbon enolate anions, n-C3H5O(-), i-C3H5O(-), n-C4H7O(-), s-C4H7O(-), and i-C4H7O(-) have been measured at 355 nm. Both the X(2A' ') ground and A(2A') first excited states of the corresponding radicals were accessed from the X(1A') ground state of the enolate anions. The separation energies of the ground and first excited states (T0) were determined: T0[(E)-n-C3H5O] = 1.19 +/- 0.02 eV, T0[(Z)-n-C3H5O] = 0.99 +/- 0.02 eV, T0[i-C3H5O] = 1.01 +/- 0.02 eV, T0[n-C4H7O] = 1.19 +/- 0.02 eV, T0[(2,3)-s-C4H7O] = 1.25 +/- 0.02 eV, T0[(1,2)-s-C4H7O] = 0.98 +/- 0.02 eV, and T0[i-C4H7O] = 1.36 +/- 0.02 eV. The effects of alkyl substitution on the vibronic structure and energetics previously observed in the vinoxy radical are discussed. The X(1A')-X(2A' ') relative stability is strongly influenced by substitution whereas the X(1A')-A(2A') relative stability remains nearly constant for all of the observed structural isomers. Alkyl substitution at the carbonyl carbon affects vibronic structure more profoundly than the energetics, while the converse is observed upon alkyl substitution at the alpha carbon.     

Positive and negative photoion spectroscopy study of monochlorothiophenes

Photolysis dynamics of monochlorothiophenes (2- and 3-chlorothiophenes) is investigated using positive and negative photoion mass spectrometry combined with the synchrotron vacuum ultraviolet radiation. A dozen of the daughter cations are observed in the time-of-flight mass spectra, and their appearance energies are determined by the photoion efficiency spectroscopy measurements. At the energetic threshold, the concerted process rather than a stepwise reaction for C(4)H(3)SCl(+) → C(2)HSCl(+) + C(2)H(2) and the ring-open isomers of the dehydrogenated thiophene cations (C(4)H(3)S(+) and C(4)H(2)S(+)) formed in C(4)H(3)SCl(+) → C(4)H(3)S(+) + Cl and C(4)H(2)S(+) + HCl are proposed on the basis of the B3LYP/6-311+G(3df,3pd) calculations. The chlorine anion (Cl(-)) is observed as the product of the photoion-pair dissociations in the energy range of 10.70-22.00 eV. A set of valence-to-Rydberg state transitions 12a' → np (n = 6, 7, 8, 9, 10, etc.) and several series of vibrational excitations are tentatively assigned in the Cl(-) spectrum of 2-chlorothiophene in the lower energy range of 10.90-12.00 eV.     

Heats of formation of Co(CO)(2)NOPR(3), R = CH(3) and C(2)H(5), and its ionic fragments

A joint threshold photoelectron photoion coincidence spectrometry (TPEPICO) and collision-induced dissociation (CID) study on the thermochemistry of Co(CO)(2)NOPR(3), R = CH(3) (Me) and C(2)H(5) (Et), complexes is presented. Adiabatic ionization energies of 7.36 +/- 0.04 and 7.24 +/- 0.04 eV, respectively, were extracted from scans of the total ion and threshold electron signals. In the TPEPICO study, the following 0 K onsets were determined for the various fragment ions: CoCONOPMe(3)(+), 8.30 +/- 0.05 eV; CoNOPMe(3)(+), 9.11 +/- 0.05 eV; CoPMe(3)(+) 10.80 +/- 0.05 eV; CoCONOPEt(3)(+), 8.14 +/- 0.05 eV; CoNOPEt(3)(+), 8.92 +/- 0.05 eV; and CoPEt(3)(+), 10.66 +/- 0.05 eV. These onsets were combined with the Co(+)-PR(3) (R = CH(3) and C(2)H(5)) bond dissociation energies of 2.88 +/- 0.11 and 3.51 +/- 0.17 eV, obtained from the TCID experiments, to derive the heats of formation of the neutral and ionic species. Thus, the Co(CO)(2)NOPR(3) (R = CH(3) and C(2)H(5)) 0 K heats of formation were found to be -350 +/- 13 and -376 +/- 18 kJ x mol(-)(1), respectively. These heats of formation were combined with the published heat of formation of Co(CO)(3)NO to determine the substitution enthalpies of the carbonyl to phosphine substitution reactions. Room-temperature values of the heats of formation are also given using the calculated harmonic vibrational frequencies. Analysis of the TCID experimental results provides indirectly the adiabatic ionization energies of the free phosphine ligands, P(CH(3))(3) and P(C(2)H(5))(3), of 7.83 +/- 0.03 and 7.50 +/- 0.03 eV, respectively.     

Photoelectron imaging of large anionic methanol clusters: (MeOH)n - (n approximately 70-460)

Electron solvation in methanol anion clusters, (MeOH)(n) (-) (n approximately 70-460), is studied by photoelectron imaging. Two isomers are observed: methanol I, with vertical binding energies (VBE) ranging from 2-2.5 eV, and methanol II, with much lower VBE's between 0.2 and 0.5 eV. The VBE's of the two isomers depend linearly on n(-1/3) with nearly identical slopes. We propose that the excess electron is internally solvated in methanol I clusters, whereas in methanol II it resides in a dipole-bound surface-state. Evidence of an excited state accessible at 1.55 eV is observed for methanol I.     

Thermochemical studies of N-methylpyrazole and N-methylimidazole

The 351.1 nm photoelectron spectra of the N-methyl-5-pyrazolide anion and the N-methyl-5-imidazolide anion are reported. The photoelectron spectra of both isomers display extended vibrational progressions in the X2A' ground states of the corresponding radicals that are well reproduced by Franck-Condon simulations, based on the results of B3LYP/6-311++G(d,p) calculations. The electron affinities of the N-methyl-5-pyrazolyl radical and the N-methyl-5-imidazolyl radical are 2.054 +/- 0.006 eV and 1.987 +/- 0.008 eV, respectively. Broad vibronic features of the A(2)A' ' states are also observed in the spectra. The gas-phase acidities of N-methylpyrazole and N-methylimidazole are determined from measurements of proton-transfer rate constants using a flowing afterglow-selected ion flow tube instrument. The acidity of N-methylpyrazole is measured to be Delta(acid)G(298) = 376.9 +/- 0.7 kcal mol(-1) and Delta(acid)H(298) = 384.0 +/- 0.7 kcal mol(-1), whereas the acidity of N-methylimidazole is determined to be Delta(acid)G(298) = 380.2 +/- 1.0 kcal mol(-1) and Delta(acid)H(298)= 388.1 +/- 1.0 kcal mol(-1). The gas-phase acidities are combined with the electron affinities in a negative ion thermochemical cycle to determine the C5-H bond dissociation energies, D(0)(C5-H, N-methylpyrazole) = 116.4 +/- 0.7 kcal mol(-1) and D(0)(C5-H, N-methylimidazole) = 119.0 +/- 1.0 kcal mol(-1). The bond strengths reported here are consistent with previously reported bond strengths of pyrazole and imidazole; however, the error bars are significantly reduced.     

Negative ions of nitroethane and its clusters

Valence and dipole-bound negative ions of the nitroethane (NE) molecule and its clusters are studied using photoelectron spectroscopy (PES), Rydberg electron transfer (RET) techniques, and ab initio methods. Valence adiabatic electron affinities (EA(a)s) of NE, C(2)H(5)NO(2), and its clusters, (C(2)H(5)NO(2))(n), n=2-5, are estimated using vibrationally unresolved PES to be 0.3+/-0.2 eV (n=1), 0.9+/-0.2 eV (n=2), 1.5+/-0.2 eV (n=3), 1.9+/-0.2 eV (n=4), and 2.1+/-0.2 eV (n=5). These energies were then used to determine stepwise anion-neutral solvation energies and compared with previous literature values. Vertical detachment energies for (C(2)H(5)NO(2))(n)(-) were also measured to be 0.92+/-0.10 eV (n=1), 1.63+/-0.10 eV (n=2), 2.04+/-0.10 eV (n=3), and 2.3+/-0.1 eV (n=4). RET experiments show that Rydberg electrons can be attached to NE both as dipole-bound and valence bound anion states. The results are similar to those found for nitromethane (NM), where it was argued that the diffuse dipole state act as a "doorway state" to the more tightly bound valence anion. Using previous models for relating the maximum in the RET dependence of the Rydberg effective principle number n(max)(*), the dipole-bound electron affinity is predicted to be approximately 25 meV. However, a close examination of the RET cross section data for NE and a re-examination of such data for NM finds a much broader dependence on n(*) than is seen for RET in conventional dipole bound states and, more importantly, a pronounced [l] dependence is found in n(max)(*) (n(max)(*) increases with [l]). Ab initio calculations agree well with the experimental results apart from the vertical electron affinity value associated with the dipole bound state which is predicted to be 8 meV. Moreover, the calculations help to visualize the dramatic difference in the distributions of the excess electron for dipole-bound and valence states, and suggest that NE clusters form only anions where the excess electron localizes on a single monomer.     

Addition of water to Al5O4- determined by anion photoelectron spectroscopy and quantum chemical calculations

The anion photoelectron spectra of Al5O4- and Al5O5H2- are presented and interpreted within the context of quantum chemical calculations on these species. Experimentally, the electron affinities of these two molecules are determined to be 3.50(5) eV and 3.10(10) eV for the bare and hydrated cluster, respectively. The spectra show at least three electronic transitions crowded into a 1 eV energy window. Calculations on Al5O4- predict a highly symmetric near-planar structure with a singlet ground state. The neutral structure calculated to be most structurally similar to the ground state structure of the anion is predicted to lie 0.15 eV above the ground state structure of the neutral. The lowest energy neutral isomer does not have significant Franck-Condon overlap with the ground state of the anion. Dissociative addition of water to Al5O4- is energetically favored over physisorption. The ground state structure for the Al5O4- +H(2)O product forms when water adds to the central Al atom in Al5O4- with -H migration to one of the neighboring O atoms. Again, the ground state structures for the anion and neutral are very different, and the PE spectrum represents transitions to a higher-lying neutral structure from the ground state anion structure.     

Theoretical characterization of the SiC3H- anion

Highly correlated ab initio methods are used to predict the equilibrium structures and spectroscopic parameters of the SiC(3)H(-) anion. The total energies and physical properties are reported using CASSCF/MRCI, RCCSD(T), and RCCSD(T)-F12 approaches and extended basis sets. The search of stable geometries leads to a total of 12 isomers (4 linear and 8 cyclic), for which electronic ground states have close-shell configurations. The stability of the linear form, l-SiC(3)H(-), is prominent. For the most stable linear isomer, the B(e) equilibrium rotational constant has been calculated with RCCSD(T) and a complete basis set. Core-correlation and vibrational effects have been taken into account to predict a B(0) of 2621.68 MHz for l-SiC(3)H(-) and 2460.48 MHz for l-SiC(3)D(-). The dipole moment of l-SiC(3)H(-) was found to be 2.9707 D with CASSCF/aug-cc-pV5Z and the electron affinity to be 2.7 eV with RCCSD(T)-F12A/aug-cc-pVTZ. Anharmonic spectroscopic parameters are derived from a quadratic, cubic, and quartic RCCSD(T)-F12A force field and second order perturbation theory. CASSCF/MRCI vertical excitations supply three metastable electronic states, (1)Σ(+) (3)Σ(+) and (3)Δ. Electron affinities calculated for a series of chains type SiC(n)H and SiC(n) (n=1-5) allow us to discuss the anion formation probabilities.     

Probing the structural and electronic properties of Ag(n)H(-) (n = 1-3) using photoelectron imaging and theoretical calculations

Structural and electronic properties of silver hydride cluster anions (Ag(n)H(-); n = 1-3) have been explored by combining the negative ion photoelectron imaging spectroscopy and theoretical calculations. The photoelectron spectrum of AgH(-) exhibits transitions from AgH(- 2)Σ(+) to AgH (1)Σ(+) and AgH (3)Σ(+), with the electron affinity (EA) 0.57(3) eV. For Ag(2)H(-), the only observed transition is from Ag(2)H(-) (C(∞v)) (1)Σ(+) to Ag(2)H (C(2v)) (2)A(') and the electron affinity is 2.56(5) eV. Two obvious electron bands are observed in photoelectron imaging of Ag(3)H(-), which are assigned to the transitions from Ag(3)H(-) (C(2v)-T, which means C(2v) geometry with top site hydrogen) (2)B(2) to Ag(3)H (C(2v)-T) (1)A(1) and Ag(3)H (C(2v)-T) (3)B(2). The electron affinity is determined to be 1.61(9) eV. The Ag-H stretching modes in the ground states of AgH and Ag(2)H are experimentally resolved and their frequencies are measured to be 1710(80) and 1650(100) cm(-1), respectively. Aside from the above EAs and the vibrational frequencies, the vertical detachment energies to all ground states and some excited states of Ag(n)H (n = 1-3) are also obtained. Theoretical calculations reproduce the experimental energies quite well, and the results are used to assign the geometries and electronic states for all related species.     

Electronic absorption spectra of linear and cyclic C(n)(+) n=7-9 in a neon matrix

The C(n)(+) n=7-9 cations were produced by electron-impact ionization of perchloronaphthalene, mass selected, and their electronic absorption spectra in 6 K neon matrices recorded. The linear and cyclic isomers of C7(+) and C8(+) are detected. Three systems of linear C7(+) are observed with origin bands near 770, 332, and 309 nm. The cyclic C7(+) shows two transitions near 676 and 448 nm. One system of linear C9(+) is observed commencing at 371 nm. Linear C8(+) shows five dipole-allowed electronic transitions from the X 2pi(g) ground state, and the strongest ones have the origin bands at 890.8 and 308.1 nm. Five electronic transitions of cyclic C8(+) are also discernible.     

Ab initio molecular orbital study of structures and energetics of Si(3)H(2), Si(3)H(2) (+), and Si(3)H(2) (-)

The geometric structures, isomeric stabilities, and potential energy profiles of various isomers and transition states in Si(3)H(2) neutral, cation and anion are investigated at the coupled-cluster singles, doubles (triples) level of theory. For the geometrical survey, the basis sets used are of the Dunning's correlation consistent basis sets of triple-zeta quality (cc-pVTZ) for the neutral and cation and the Dunning's correlation consistent basis sets of double-zeta quality with diffuse functions (aug-cc-pVDZ) for the anion. For the final energy calculations, the aug-cc-pVTZ: Dunning's correlation consistent basis sets of triple-zeta quality with diffuse functions and cc-pVQZ: Dunning's correlation consistent basis sets of quadruple-zeta quality basis sets are used for the neutral and the aug-cc-pVTZ ones for the cation and anion. The global minimum neutral (I-1: (1)A(1)) has the same framework as that (cyclopropenylidene) of the C(3)H(2) molecule. Other low-lying three isomers (I-2, I-3, and I-4) are also predicted to be within 20 kJ/mol. Five transition states are optimized and their energy relationships with the isomers are clarified. The geometric structure of the global minimum cation (C-1: (2)A(1)) has the same framework as that of the neutral, but that of the anion (A-1: (2)A(')) differs very much from those of the neutral and cation. The calculated vertical and adiabatic ionization potentials from the global minimum neutral (I-1) are 7.85 and 7.77 eV, respectively. The adiabatic electron affinity of the neutral I-1 and the electron detachment energy of the global minimum anion (A-1) are predicted to be 1.21 and 1.92 eV, respectively. The two-electron three-centered bond is widely observed in the present Si(3)H(2) neutral, cation, and anion. The contour plots of their localized molecular orbitals clearly show the existence of such nonclassical chemical bonds.     

Anion photoelectron imaging of deprotonated thymine and cytosine

We report the anion photoelectron spectra of deprotonated thymine and cytosine at 3.496 eV photodetachment energy using velocity-mapped imaging. The photoelectron spectra of both species exhibit bands resulting from detachment transitions between the anion ground state and the ground state of the neutral radical. Franck-Condon simulations identify the anion isomers that contribute to the observed photoelectron spectrum. For both thymine and cytosine, the photoelectron spectra are consistent with anions formed by removal of a proton from the N atom that normally attaches to the sugar in the nucleotide (N1). For deprotonated thymine, the photoelectron spectrum shows a band due to a ring breathing vibration excited during the photodetachment transition. The electron affinity for the dehydrogenated thymine radical is determined as 3.250 +/- 0.015 eV. For deprotonated cytosine, the photoelectron spectrum lacks any resolved structure and the electron affinity of the dehydrogenated cytosine radical is determined to be 3.037 +/- 0.015 eV. By combining the electron affinity with previously measured gas phase acidities of thymine and cytosine, we determine the bond dissociation energy for the N-H bond that is broken.     

On the electronic structure and chemical bonding in the tantalum trimer cluster

The electronic structure and chemical bonding in the Ta 3 (-) cluster are investigated using photoelectron spectroscopy and density functional theory calculations. Photoelectron spectra are obtained for Ta 3 (-) at four photon energies: 532, 355, 266, and 193 nm. While congested spectra are observed at high electron binding energies, several low-lying electronic transitions are well resolved and compared with the theoretical calculations. The electron affinity of Ta 3 is determined to be 1.35 +/- 0.03 eV. Extensive density functional calculations are performed at the B3LYP/Stuttgart +2f1g level to locate the ground-state and low-lying isomers for Ta 3 and Ta 3 (-). The ground-state for the Ta 3 (-) anion is shown to be a quintet ( (5)A 1') with D 3 h symmetry, whereas two nearly isoenergetic states, C 2 v ( (4)A 1) and D 3 h ( (6)A 1'), are found to compete for the ground-state for neutral Ta 3. A detailed molecular orbital analysis is performed to elucidate the chemical boding in Ta 3 (-), which is found to possess multiple d-orbital aromaticity, commensurate with its highly symmetric D 3 h structure.     

Structure and stability of (CuNO) isomers

Quantum-mechanical calculations have been performed on various isomers of the (CuNO)⁺ system. A ²Π ground state is found for the linear CuNO⁺ and CuON⁺ isomers and a ²A′ state for the bent CuNO⁺ and CuON⁺ isomers. Energy calculations indicate that the linear CuNO⁺ structure is the most stable, the bent CuNO⁺ and CuON⁺ and the linear CuON⁺ structures are at 0.86 eV, 0.99 eV and 1.04 eV above this respectively. In the CuNO⁺ → CuON⁺ interconversion between the linear isomers, three transition states are involved, whereas the bent CuNO⁺ isomer is found to be an intermediate species. The isomerization barriers, dissociation energies, equilibrium geometries and vibration frequencies are given for all isomers in their ground and first excited states.     

Photoionization of C(4) molecular beam: ab initio calculations

Large computations are performed on the C(4) (+) cation in order to characterize its stable isomers and its lowest electronic excited states using configuration interaction methods and large basis sets. Several stable isomers are found including a linear C(4) (+)(l-C(4) (+)), a rhombic C(4) (+)(r-C(4) (+)) (or cyclic), and a branched (d-C(4) (+)) structure. Our calculations show a high density of electronic states for all of these isomers favoring their interactions. By combining the present ab initio data and those on neutral C(4), the l-C(4)(X)+hnu-->l-C(4) (+)(X(+))+e(-), d-C(4)(X)+hnu-->d-C(4) (+)(X(+))+e(-), and r-C(4)(X)+hnu-->r-C(4) (+)(X(+))+e(-) vertical photoionization transition energies are computed at 10.87, 10.92, and 10.77 eV, respectively. Photoionizing a C(4) molecular beam results on an onset at 10.4-10.5 eV and then to a linear increase of the signal due to the opening of several ionization channels involving most of the C(4) and C(4) (+) isomers and electronic states.     

Electron attachment and detachment: electron affinities of isomers of trifluoromethylbenzonitrile

Rate constants for electron attachment to the three isomers of trifluoromethylbenzonitrile [(CF(3))(CN)C(6)H(4), or TFMBN] were measured over the temperature range of 303-463 K in a 133-Pa He buffer gas, using a flowing-afterglow Langmuir-probe apparatus. At 303 K, the measured attachment rate constants are 9.0 x 10(-8) (o-TFMBN), 5.5 x 10(-8) (m-TFMBN), and 8.9 x 10(-8) cm(3) s(-1) (p-TFMBN), estimated accurate to +/-25%. The attachment process formed only the parent anion in all three cases. Thermal electron detachment was observed for all three anion isomers, and rate constants for this reverse process were also measured. From the attachment and detachment results, the electron affinities of the three isomers of TFMBN were determined to be 0.70(o-TFMBN), 0.67(m-TFMBN), and 0.83 eV (p-TFMBN), all +/-0.05 eV. G3(MP2) [Gaussian-3 calculations with reduced M?ller-Plesset orders (MP2)] calculations were carried out for the neutrals and anions. Electron affinities derived from these calculations are in good agreement with the experimental values.     

Photoelectron spectroscopy and electronic structures of fullerene oxides: C60Ox- (x = 1-3)

We report a photoelectron spectroscopy (PES) study on a series of fullerene oxides, C60Ox- (x = 1-3). The PES spectra reveal one isomer for C60O-, two isomers for C60O2, and multiple isomers for C60O3-. Compared to C60, the electronic structures of C60Ox are only slightly perturbed, resulting in similar anion photoelectron spectra. The electron affinity of C60Ox was observed to increase only marginally with the number of oxygen atoms, x, from 2.683 eV for C60, to 2.745 eV for C60O, and 2.785 eV/2.820 eV for C60O2 (two isomers). We also carried out theoretical calculations, which confirmed the observed isomers and showed that all the fullerene oxides are in the form of epoxide. The PES and theoretical calculations, as well as molecular orbital analysis, indicate that addition of oxygen atoms to the C60 cage only modifies the local carbon network and leave the rest of the fullerene cage largely intact geometrically and electronically.     

Identification of C5Hx isomers in fuel-rich flames by photoionization mass spectrometry and electronic structure calculations

The isomeric composition of C(5)H(x) (x = 2-6, 8) flame species is analyzed for rich flames fueled by allene, propyne, cyclopentene, or benzene. Different isomers are identified by their known ionization energies and/or by comparison of the observed photoionization efficiencies with theoretical simulations based on calculated ionization energies and Franck-Condon factors. The experiments combine flame-sampling molecular-beam mass spectrometry with photoionization by tunable vacuum-UV synchrotron radiation. The theoretical simulations employ the rovibrational properties obtained with B3LYP/6-311++G(d,p) density functional theory and electronic energies obtained from QCISD(T) electronic structure calculations extrapolated to the complete basis set limit. For C(5)H(3), the comparison reveals the presence of both the H(2)CCCCCH (i-C(5)H(3)) and the HCCCHCCH (n-C(5)H(3)) isomer. The simulations also suggest a modest amount of cyclo-CCHCHCCH-, which is consistent with a minor signal for C(5)H(2) that is apparently due to cyclo-CCHCCCH-. For C(5)H(4), contributions from the CH(2)CCCCH(2) (1,2,3,4-pentatetraene), CH(2)CCHCCH, and CH(3)CCCCH (1,3-pentadiyne) isomers are evident, as is some contribution from CHCCH(2)CCH (1,4-pentadiyne) in the cyclopentene and benzene flames. Signal at m/z = 65 originates mainly from the cyclopentadienyl radical. For C(5)H(6), contributions from cyclopentadiene, CH(3)CCCHCH(2), CH(3)CHCHCCH, and CH(2)CHCH(2)CCH are observed. No signal is observed for C(5)H(7) species. Cyclopentene, CH(2)CHCHCHCH(3) (1,3-pentadiene), CH(3)CCCH(2)CH(3) (2-pentyne), and CH(2)CHCH(2)CHCH(2) (1,4-pentadiene) contribute to the signal at m/z = 68. Newly derived ionization energies for i- and n-C(5)H(3) (8.20 +/- 0.05 and 8.31 +/- 0.05 eV, respectively), CH(2)CCHCCH (9.22 +/- 0.05 eV), and CH(2)CHCH(2)CCH (9.95 +/- 0.05 eV) are within the error bars of the QCISD(T) calculations. The combustion chemistry of the observed C(5)H(x) intermediates and the impact on flame chemistry models are discussed.     

Microsolvation of the dicyanamide anion: [N(CN)(2)(-)](H(2)O)n (n = 0-12)

Photoelectron spectroscopy is combined with ab initio calculations to study the microsolvation of the dicyanamide anion, N(CN)(2)(-). Photoelectron spectra of [N(CN)(2)(-)](H2O)n (n = 0-12) have been measured at room temperature and also at low temperature for n = 0-4. Vibrationally resolved photoelectron spectra are obtained for N(CN)(2)(-), allowing the electron affinity of the N(CN)2 radical to be determined accurately as 4.135 +/- 0.010 eV. The electron binding energies and the spectral width of the hydrated clusters are observed to increase with the number of water molecules. The first five waters are observed to provide significant stabilization to the solute, whereas the stabilization becomes weaker for n > 5. The spectral width, which carries information about the solvent reorganization upon electron detachment in [N(CN)(2)(-)](H2O)n, levels off for n > 6. Theoretical calculations reveal several close-lying isomers for n = 1 and 2 due to the fact that the N(CN)(2)(-) anion possesses three almost equivalent hydration sites. In all the hydrated clusters, the most stable structures consist of a water cluster solvating one end of the N(CN)(2)(-) anion.