Rotamers of m-cresol cation studied by mass-analyzed threshold ionization spectroscopy

We have applied two-color resonant two-photon mass-analyzed threshold ionization technique to record the vibrational spectra of the selected rotamers of m-cresol. The adiabatic ionization energies of cis and trans m-cresol are determined to be 66,933+/-5 and 67,084+/-5 cm(-1), respectively. Frequencies of the in-plane ring vibrations 6a, 1, and 9b are measured to be 528, 720, 1167 cm(-1) for the cis and 520, 698, and 1153 cm(-1) for the trans m-cresol cation. This indicates that different orientation of the OH group with respect to the CH(3) group slightly influences these ring vibrations.     

Serine-phosphoric acid cluster ions studied by electrospray ionization and tandem mass spectrometry

More than 310 kinds of cluster ions of S(m) P(n) H(k) (k+) are observed in a single ESI mass spectrum of a mixed solution of serine and phosphoric acid. Some typical cluster ions are selected, activated by collision in a FT ICR cell, and the dissociation pathways were deduced in detail. For large singly protonated ions, the collisions cause the ejection of subunits of serine or phosphoric acid subsequently producing the ions of S(2) P(4) H(1) (1+) , which can be further dissociated by the loss of phosphoric acid molecules in turn and form the protonated serine dimer and monomer. However, for the doubly protonated ions, the dissociation pathways change from the loss of a protonated serine dimer for the ions of S(7) P(9) H(2) (2+) to the neutral loss of H(3) PO(4) for the ions of S(7) P(12) H(2) (2+) or the neutral loss of serine or H(3) PO(4) for the larger clusters, indicating the effect of cluster sizes on the process of dissociation. The structure of S(2) P(4) H(1) (1+) is suggested based on B3LYP/6-31G(d,p) calculations. The diversity and structural orderliness of the hetero-cluster ions are mainly attributed to the network of hydrogen bonds inside the cluster ions and the extraordinary amphotericity of the components.     

Photodissociation dynamics of various conformers of iodobutane isomer ions prepared selectively by vacuum ultraviolet mass-analyzed threshold ionization

Various conformers of 1-C(4)H(9)I(+*), 2-C(4)H(9)I(+*), and i-C(4)H(9)I(+*) were prepared selectively by mass-analyzed threshold ionization with coherent vacuum ultraviolet radiation. Conformer-selective photodissociation of these ions was studied in the 560-730 nm spectral region, which corresponds to excitation to the first excited electronic state. Rapid dissociation was observed as manifested by noticeable laser polarization dependence of the product signals. In particular, photodissociation of i-C(4)H(9)I(+*) was found to be conformer specific, occurring without interconversion between conformers. The product's asymptote energies estimated from the experimental data were compared with the reaction enthalpies at 0 K to get information on the structures and states of products. It was found that a simple S(N)2-type mechanism deduced from the previous study of 1-C(3)H(7)I(+*) was compatible with the present observations. Validity of the widely adopted postulate in stereochemistry that different conformations can be gateways to different reactions has been demonstrated in the gas phase.     

Effect of noncovalent interactions on the n-butylbenzene...Ar cluster studied by mass analyzed threshold ionization spectroscopy and ab initio computations

Clusters of Ar bound to isomers of the aromatic hydrocarbon n-butylbenzene (BB) have been studied using two-color REMPI (resonance enhanced multiphoton ionization) and MATI (mass analyzed threshold ionization) spectroscopy to explore noncovalent vdW interactions between these two moieties. Blue shifts of excitation energy were observed for gauche-BB...Ar clusters, and red shifts for anti-BB...Ar clusters were observed. Adiabatic ionization energies (IEs) of the conformer BB-I...Ar and BB-V...Ar were determined as 70052 and 69845 +/- 5 cm (-1), respectively. Spectral features and vibrational modes were interpreted with the aid of UMP2/cc-pVDZ ab initio calculations. Data of complexation shifts of the alkyl-benzenes and their argon clusters were collected and discussed. Using the CCSD(T) method at complete basis set (CBS) level, interaction energies for the neutral ground states of BB-I...Ar and BB-V...Ar were obtained as 650 and 558 cm (-1), respectively. Combining the CBS calculation results and the REMPI and MATI spectra allowed further the determination of the interaction energies and the energetics of BB...Ar in the excited neutral S 1 and the D 0 cationic ground states.     

Site-specific H/D exchange of p-methoxyphenol studied by resonant two-photon ionization and mass-analyzed threshold ionization spectroscopy

The origin of the S(1) <-- S(0) transition (E(1)) and the adiabatic ionization energy (IE) of cis-p-methoxyphenol-d(1)-OD are determined to be 33 660 and 62 302 cm(-1), whereas those of cis-p-methoxyphenol-d(1)-OCH(2)D are 33 669 and 62 323 cm(-1), respectively. Similarly, the E(1) and IE of trans-p-methoxyphenol-d(1)-OD are determined to be 33 563 and 62 191 cm(-1) and those of trans-p-methoxyphenol-d(1)-OCH(2)D are 33 575 and 62 216 cm(-1), respectively. Comparing these data with those of p-methoxyphenol suggests that the H/D exchange on the OH substituent gives rise to a red shift in both the E(1) and IE, whereas that on the OCH(3) group yields a blue shift. The mass-analyzed threshold ionization spectra of the selected isomers can be used as the fingerprints for molecular identification. Analysis of these cation spectra shows that the substituent-sensitive in-plane C-OH and C-OCH(3) bending (mode 9b) and breathing (mode 1) vibrations are active for all of these isomeric cations.     

Photodissociation of thioglycolic acid studied by femtosecond time-resolved transient absorption spectroscopy

Steady-state and time-resolved spectroscopies were employed to study the photodissociation of both the neutral (HS-CH(2)-COOH) and doubly deprotonated ((-)S-CH(2)-COO(-)) forms of thioglycolic acid (TGA), a common surface-passivating ligand used in the aqueous synthesis and organization of semiconducting nanostructures. Room temperature UV-Vis absorption spectroscopy indicated strong absorption by the S(1) and S(2) excited states at 250 nm and 185 nm, respectively. The spectrum also contained a weaker absorption band that extended to approximately 550 nm, which was assigned to the π(CO) (*)←n(O) transition. Femtosecond time-resolved transient absorption spectroscopy was performed on TGA using 400 nm excitation and a white-light continuum probe to provide the temporally and spectrally resolved data. Both forms of TGA underwent a photoinduced dissociation from the excited state to form an α-thiol-substituted acyl radical (α-TAR, S-CH(2)-CO(●)). For the acidic form of TGA, radical formation occurred with an apparent time constant of 60 ± 5 fs; subsequent unimolecular decay took 400 ± 60 fs. Similar kinetics were observed for the deprotonated form of TGA (70 ± 10 fs radical formation; 420 ± 40 fs decay). The production of the α-TAR was corroborated by the observation of its characteristic optical absorption. Time-resolved data indicated that the photoinduced dissociation of TGA via cleavage of the C-OH bond occurred rapidly (≤100 fs). The prevalence of TGA in aqueous semiconducting nanoparticles makes its absorption in the visible spectral region and subsequent dissociation key to understanding the behavior of nanoscale systems.     

Photodissociation yield spectroscopy of vinyl bromide cation generated by mass-analyzed threshold ionization: vibrational spectroscopy and decay dynamics in the (~)B state

A new technique [mass-analyzed threshold ionization (MATI)-photodissociation yield spectroscopy] to probe bound excited states of a cation was developed, which measures photodissociation yield of the cation generated by mass-analyzed threshold ionization. A vibrational spectrum of vinyl bromide cation in the (~)B state was obtained using this technique. Optical resolution in the low vibrational energy range of the spectrum was far better than in conventional MATI spectra. The origin of the (~)B state was found at 2.2578+/-0.0003 eV above the first ionization onset. Almost complete vibrational assignment was possible for peaks appearing in the spectrum. Analysis of time-of-flight profiles of C(2)H(3) (+) product ion obtained with different laser polarization angles suggested that photoexcited vinyl bromide cation remained in the (~)B state for several hundred picoseconds prior to internal conversion to the ground state and dissociation therein.     

Formation of negatively-charged cluster ions in thermal energy collisions with state-selected rydberg atoms

Using crossed atomic, molecular cluster, and cw laser beams in conjunction with mass spectrometric ion detection, we have obtained for the first time results for electron transfer fromstate-selected Rydberg atoms to molecular clusters. We report negative ion mass spectra for (CO₂) _k ^? (4≦k≦25) and (O₂) _k ^? (1≦k≦13) cluster ions, resulting from collisions of Ar** (nd) Rydberg atoms (12≦n≦40) with (CO₂) _m and (O₂) _m clusters at relative velocities around 830 m/s, and, for comparison, positive ion mass spectra due to Ne(3s ³ P _{2, 0}) Penning ionization. For both CO ₂ ^? and O₂-clusters, the negative and the positive ion mass spectra are very different. For (CO₂) _k ^Emphasis>/? cluster ions, the mass spectra show distinct variations with principal quantum number of the Rydberg atom, corresponding to differentn-dependences of the effective rate constant for selected cluster ions, as measured relative to the knownn-dependence for SF ₆ ^? formation in collisions with SF₆. For (O₂) _k ^? cluster ions, on the other hand, the mass spectra are almost independent ofn with ion intensities, which clearly reflect their thermochemical stabilities (O ₄ ^? as dominant species).     

Dissociation kinetics of energy-selected Cp(2)Mn(+) ions studied by threshold photoelectron-photoion coincidence spectroscopy

Threshold photoelectron-photoion coincidence spectroscopy has been used to investigate the dissociation kinetics of the manganocene ion, Cp(2)Mn(+) (Cp = eta(5)-cyclopentadienyl). The Cp loss reaction was found to be extremely slow over a large ion internal energy range. By simulating the measured asymmetric time-of-flight peak shapes and breakdown diagram, the 0 K thermochemical dissociation limit for CpMn(+) production was determined to be 9.55 +/- 0.15 eV. A CpMn(+)-Cp bond energy of 3.43 eV was obtained by combining this CpMn(+) + Cp dissociation limit with the Cp(2)Mn adiabatic ionization energy of 6.12 +/- 0.07 eV. Combining the measured onset with known heats of formation of Cp and Mn(+), the Cp-Mn(+) bond energy was determined to be 3.38 +/- 0.15 eV. These results lead to 298 K heats of formation of Cp(2)Mn(+) and CpMn(+) of 863 +/- 7 and 935 +/- 16 kJ/mol, respectively. Finally, by combining these results with a previous measurement of the CpMn(CO)(3) --> CpMn(+) + 3CO + e(-) dissociation limit, we arrive at a new value for Delta(f)H degrees (298K)(CpMn(CO)(3)) of -424 +/- 17 kJ/mol.     

Two-photon dissociation spectroscopy of state-selected HCl+ and DCl+ ions.

HCl+ and DCl+ ions were formed via the R(1) pump line of the f3delta2(v'=0)<--sigma+(v'=0) REMPI process. For these ions, the two-photon dissociation spectroscopy, resonance-enhanced via the A2sigma+(v')<--pi3/2(v'=0) transition, was investigated for various intermediate states of HCl+ (v'=4,5,6) and DCl+ (v'=6,7,8,9). From the analysis of the data, spectroscopic parameters of the X and the A states were derived (including the lambda-doubling in the X state and the spin-rotation coupling in the A state). Some of the parameters deviate considerably from literature data. The spectra provide clear evidence that the REMPI process employed for forming the ions has a very high rotational selectivity.     

Amino acid cluster formation studied by electrospray ionization mass spectrometry

The association properties of natural and non-natural amino acids were studied in detail using electrospray ionization mass spectrometry. The results show a highly diverse cluster formation behavior of amino acids. There are differences regarding the degree of clustering (average cluster size), the presence or absence of one or several 'magic' clusters of special stability and the influence of chirality on cluster stability. Cluster formation does not show a good correlation with simple physico-chemical properties (such as solubility), indicating that it is a specific process and not only a simple aggregation during evaporation/ionization. A systematic study of cluster formation of serine derivatives reveals that all functional groups play a prominent role in the binding of the magic octamer. The results support the idea of the zwitterionic character of the octamer. Electrospray ionization of the side-chain acetylated serine shows the formation of a very stable tetramer with a strong preference for homochirality. The results suggest that Ser8 is made up of two tetramer subunits, held together by hydrogen bonds of the side-chain.     

Mass analyzed threshold ionization spectroscopy of p-fluorostyrene

Adiabatic ionization energy (AIE) and two-color threshold ion vibrational spectra of p-fluorostyrene have been measured by mass analyzed threshold ionization (MATI) method via three different intermediate levels in the first excited state, vibrationless S1 origin, 42(1)41(1), and 23(1) vibronic levels. Features of the ion vibrational spectra indicates that the geometry of the molecular ion including the conformation of the vinyl chain in the ionic ground state (D0) is almost identical to that of its neutral ground state (S0), and ionization has very little effect on the vibrational potentials of the aromatic ring modes. Comparison of the AIE with the reported value of styrene shows that fluorination at the para position of the aromatic ring has little effect on energy of the electron ejected in ionization process from the styrene chromophore.     

Vibrational structures of dimethyl sulfide and ethylene sulfide cations studied by vacuum-ultraviolet mass-analyzed threshold ionization (MATI) spectroscopy

Adiabatic ionization energies of dimethyl sulfide (DMS) and ethylene sulfide (thiirane) are both accurately and precisely determined to be 8.6903 +/- 0.0009 and 9.0600 +/- 0.0009 eV, respectively, by vacuum-UV mass-analyzed threshold ionization (MATI) spectroscopy. Also reported are vibrational frequencies of DMS and thiirane monocations. Simulations using a Franck-Condon analysis based on ab initio molecular structures reproduce the experimental findings quite well. Detailed vibrational structures are discussed with the aid of ab initio calculations. Ionization-induced structural changes provide the information about the role of the sulfur nonbonding orbital in the geometrical layout of the title compounds.     

The dissociation kinetics of energy-selected CpMn(CO)(3)(+) ions studied by threshold photoelectron-photoion coincidence spectroscopy

Threshold photoelectron-photoion coincidence spectroscopy has been used to investigate the dissociation kinetics of the cyclopentadienyl manganese tricarbonyl ion, CpMn(CO)(3)(+). The ionization energy of CpMn(CO)(3) was measured from the threshold photoelectron spectrum to be 7.69 +/- 0.02 eV. The dissociation of the CpMn(CO)(3)(+) ion proceeds by the sequential loss of three CO molecules. The first and third CO loss reactions were observed to be slow (lifetimes in the microsecond range). By simulating the resulting asymmetric time-of-flight peak shapes and breakdown diagram, 0 K onsets for three product ions were determined to be 8.80 +/- 0.04, 9.43 +/- 0.04, and 10.51 +/- 0.06 eV, respectively. Combined with the adiabatic ionization energy, the three successive Mn-CO bond energies in the CpMn(CO)(3)(+) were found to be alternating with values of 1.11 +/- 0.04, 0.63 +/- 0.04, and 1.08 +/- 0.06 eV, respectively. Using a scaled theoretical Cp-Mn(+) bond energy of 3.10 +/- 0.10 eV and the combined results from theory and experiment, the 298 K gas-phase heat of formation of CpMn(CO)(3) is suggested to be -419 +/- 15 kJ/mol. Based on this value, the 298 K heats of formation of CpMn(CO)(3)(+), CpMn(CO)(2)(+), CpMnCO(+), and CpMn(+) are 325 +/- 15, 546 +/- 15, 719 +/- 15, and 938 +/- 15 kJ/mol, respectively. By scaling theoretical calculated neutral bond energies with the experimental information derived in this study, the successive Mn-CO bond energies were estimated to be 1.88, 1.10, and 1.03 eV, respectively, while the Cp-Mn bond energy was found to be 2.16 eV. Comparison between the quantum chemical calculations and experimental values shows that the loss of CO groups follows the lowest energy adiabatic path, in which electronic spin on the metal center is not conserved.     

(2+1) resonance-enhanced ionization spectroscopy of a state-selected beam of OH radicals

A state-selected beam of hydroxyl radicals is generated using a pulsed discharge source and hexapole field. The OH radicals are characterized by resonance-enhanced multiphoton ionization (REMPI) spectroscopy via the nested D 2Sigma- and 3 2Sigma- Rydberg states. Simplified spectra are observed from the selected |MJ|=3/2 component of the upper Lambda-doublet level of the lowest rotational state (J=32) in ground (v"=0) and excited (v"=1-3) vibrational levels of the OH X 2Pi3/2 state. Two-photon transitions are observed to the D 2Sigma-(v'=0-3) and 3 2Sigma-(v'=0,1) vibronic levels, extending previous studies to higher vibrational levels of the Rydberg states. Spectroscopic constants are derived for the Rydberg states and compared with prior experimental studies. Complementary first-principle theoretical studies of the properties of the D 2Sigma- and 3 2Sigma- Rydberg states [see M. P. J. van der Loo and G. C. Groenenboom, J. Chem. Phys. 123, 074310 (2005), following paper] are used to interpret the experimental findings and examine the utility of the (2+1) REMPI scheme for sensitive detection of OH radicals.     

Photodissociation of trapped ions

The ion-trapping ion cyclotron resonance spectrometer, or Fourier transform mass spectrometer, provides a powerful and convenient environment for the study of photodissociation of gas-phase ions. This capability has been explored for about 30 years in a number of laboratories including our own. A variety of developments and applications, historical and current, are organized here under five broad headings: (1) optical spectroscopy of ions; (2) kinetics of the dissociation process; (3) dynamics of the dissociation process; (4) thermochemistry of dissociation; and (5) probing the structure and energy of the ions.     

Sorption of uranyl ions on titanium oxide studied by ATR-IR spectroscopy

ATR-IR spectroscopy was used to study the sorption of uranyl ions (10(-4) M) onto titanium oxide (mixture of rutile and anatase). A circulation setup, filled with a solution in D(2)O, allowed recording of the evolution of the antisymmetric O=U=O stretching of uranyl species onto titanium oxide particles deposited on the ATR crystal. The band centered at 915 cm(-1) has been decomposed in two Gaussian peaks at 920 and 905 cm(-1). From these values, and the observation that the ratio of the areas of the two peaks vs pH was constant, we have proposed that uranyl sorption on titanium oxide in the pH range 4-7 leads to the formation of one surface complex where uranium atoms have two different chemical environments. A trimer surface complex linked by two uranium atoms to the titanium oxide surface would be consistent with this interpretation.     

Mass analyzed threshold ionization spectroscopy of 7-azaindole cation

The vibrationally resolved mass analyzed threshold ionization (MATI) spectra of jet-cooled 7-azaindole have been recorded by ionizing via four different intermediate levels. The adiabatic ionization energy of this molecule is determined to be 65 462±5 cm⁻¹, which is greater than that of indole by 2871 cm⁻¹. The vibrational spectra of 7-azaindole in the S₁ and D₀ states have been successfully assigned by comparing the measured frequencies with those of indole as well as the predicted values from the ab initio calculations. Detailed analysis on the MATI spectra shows that the structure of the cation is somewhat different from that of this species in the neutral S₁ state.