S1 excitation and zero kinetic energy spectra of partly deuterated 1:1 phenol–water complexes

Vibronic and Cationic Features of 2-Fluorobenzonitrile and 3-Fluorobenzonitrile Studied by REMPI and MATI Spectroscopy and Franck-Condon Simulations

Fluorinated organic compounds have superior physicochemical properties than general organic compounds due to the strong C-F single bond; they are widely used in medicine, biology, pesticides, and materials science. In order to gain a deeper understanding of the physicochemical properties of fluorinated organic compounds, fluorinated aromatic compounds have been investigated by various spectroscopic techniques. 2-fluorobenzonitrile and 3-fluorobenzonitrile are important fine chemical intermediates and their excited state S₁ and cationic ground state D₀ vibrational features remain unknown. In this paper, we used two-color resonance two photon ionization (2-color REMPI) and mass analyzed threshold ionization (MATI) spectroscopy to study S₁ and D₀ state vibrational features of 2-fluorobenzonitrile and 3-fluorobenzonitrile. The precise excitation energy (band origin) and adiabatic ionization energy were determined to be 36,028 ± 2 cm^-1 and 78,650 ± 5 cm^-1 for 2-fluorobenzonitrile and 35,989 ± 2 cm^-1 and 78,873 ± 5 cm^-1 for 3-fluorobenzonitrile, respectively. The density functional theory (DFT) at the levels of RB3LYP/aug-cc-pvtz, TD-B3LYP/aug-cc-pvtz, and UB3LYP/aug-cc-pvtz were used to calculate the stable structures and vibrational frequencies for the ground state S₀, excited state S₁, and cationic ground state D₀, respectively. Franck-Condon spectral simulations for transitions of S₁ ← S₀ and D₀ ← S₁ were performed based on the above DFT calculations. The theoretical and experimental results were in good agreement. The observed vibrational features in S₁ and D₀ states were assigned according to the simulated spectra and the comparison with structurally similar molecules. Several experimental findings and molecular features were discussed in detail.

Detailed analysis of the vibronic structure of phenetole in its first excited state and ionic ground state

The vibronic structure of the first electronically excited state S₁ and ionic ground state D₀ of phenetole has been investigated by means of resonance enhanced multi photon ionization (REMPI) and mass analyzed threshold ionization (MATI) spectroscopy. The vibronic levels were assigned with the aid of quantum chemical calculations at the (TD)DFT level of theory and a multidimensional Franck-Condon approach. The S₁ excitation energy of phenetole has been determined to be 36370 ± 4 cm^-1 (4.5093 ± 0.0005 eV). The adiabatic ionization energy was determined to be 65665 ± 7 cm^-1 (8.1415 ± 0.0008 eV). The vibronic structure has been analyzed whereby the in-plane bending vibration ν_bend shows high activity in the first excited state but is more pronounced in the ionic ground state. Moreover, a strong Duschinsky rotation effect can be observed for several D₀←S₁ transitions that causes violations of the Δv = 0 propensity rule.

Real-time observation of the photoionization-induced water rearrangement dynamics in the 5-hydroxyindole-water cluster by time-resolved IR spectroscopy

Solvation plays an essential role in controlling the mechanism and dynamics of chemical reactions in solution. The present study reveals that changes in the local solute-solvent interaction have a great impact on the timescale of solvent rearrangement dynamics. Time-resolved IR spectroscopy has been applied to a hydration rearrangement reaction in the monohydrated 5-hydroxyindole-water cluster induced by photoionization of the solute molecule. The water molecule changes the stable hydration site from the indolic NH site to the substituent OH site, both of which provide a strongly attractive potential for hydration. The rearrangement time constant amounts to 8 ± 2 ns, and is further slowed down by a factor of more than five at lower excess energy. These rearrangement times are slower by about three orders of magnitude than those reported for related systems where the water molecule is repelled from a repulsive part of the interaction potential toward an attractive well. The excess energy dependence of the time constant is well reproduced by RRKM theory. Differences in the reaction mechanism are discussed on the basis of energy relaxation dynamics.

Exploring microsolvation of the anesthetic propofol

Propofol (2,6-diisopropylphenol) is a broadly used general anesthetic. By combining spectroscopic techniques such as 1- and 2-color REMPI, UV/UV hole burning, infrared ion-dip spectroscopy (IRIDS) obtained under cooled and isolated conditions with high-level ab initio calculations, detailed information on the molecular structure of propofol and on its interactions with water can be obtained. Four isomers are found for the bare propofol, while only three are detected for the monohydrated species and two for propofol·(H(2)O)(2). The isopropyl groups do not completely block the OH solvation site, but reduce considerably the strength of the hydrogen bonds between propofol and water. Such results may explain the high mobility of propofol in the GABA(A) active site, where it cannot form a strong hydrogen bond with the tyrosine residue.

Ion spectroscopy: where did it come from; where is it now; and where is it going?

The ASMS conference on ion spectroscopy brought together at Asilomar on October 16-20, 2009 a large group of mass spectrometrists working in the area of ion spectroscopy. In this introduction to the field, we provide a brief history, its current state, and where it is going. Ion spectroscopy of intermediate size molecules began with photoelectron spectroscopy in the 1960s, while electronic spectroscopy of ions using the photodissociation "action spectroscopic" mode became active in the next decade. These approaches remained for many years the main source of information about ionization energies, electronic states, and electronic transitions of ions. In recent years, high-resolution laser techniques coupled with pulsed field ionization and sample cooling in molecular beams have provided high precision ionization energies and vibrational frequencies of small to intermediate sized molecules, including a number of radicals. More recently, optical parametric oscillator (OPO) IR lasers and free electron lasers have been developed and employed to record the IR spectra of molecular ions in either molecular beams or ion traps. These results, in combination with theoretical ab initio molecular orbital (MO) methods, are providing unprecedented structural and energetic information about gas-phase ions.

IR Spectroscopy of Microsolvated Aromatic Cluster Ions: Ionization-Induced Switch in Aromatic Molecule–Solvent Recognition

IR spectroscopy, mass spectrometry, and quantum chemical calculations are employed to characterize the intermolecular interaction of a variety of aromatic cations (A+) with several types of solvents. For this purpose, isolated ionic complexes of the type A+–L n , in which A+ is microsolvated by a controlled number (n) of ligands (L), are prepared in a supersonic plasma expansion, and their spectra are obtained by IR photodissociation (IRPD) spectroscopy in a tandem mass spectrometer. Two prototypes of aromatic ion–solvent recognition are considered: (i) microsolvation of acidic aromatic cations in a nonpolar hydrophobic solvent and (ii) microsolvation of bare aromatic hydrocarbon cations in a polar hydrophilic solvent. The analysis of the IRPD spectra of A+–L dimers provides detailed information about the intermolecular interaction between the aromatic ion and the neutral solvent, such as ion–ligand binding energies, the competition between different intermolecular binding motifs (H-bonds, π-bonds, charge–dipole bonds), and its dependence on chemical properties of both the A+ cation and the solvent type L. IRPD spectra of larger A+–L n clusters yield detailed insight into the cluster growth process, including the formation of structural isomers, the competition between ion–solvent and solvent–solvent interactions, and the degree of (non)cooperativity of the intermolecular interactions as a function of solvent type and degree of solvation. The systematic A+–L n cluster studies are shown to reveal valuable new information about fundamental chemical properties of the bare A+ cation, such as proton affinity, acidity, and reactivity. Because of the additional attraction arising from the excess charge, the interaction in the A+–L n cation clusters differs largely from that in the corresponding neutral A–L n clusters with respect to both the interaction strength and the most stable structure, implying in most cases an ionization-induced switch in the preferred aromatic molecule–solvent recognition motif. This process causes severe limitations for the spectroscopic characterization of ion–ligand complexes using popular photoionization techniques, due to the restrictions imposed by the Franck–Condon principle. The present study circumvents these limitations by employing an electron impact cluster ion source for A+–L n generation, which generates predominantly the most stable isomer of a given cluster ion independent of its geometry.

Vibrational overtone spectroscopy of phenol and its deuterated isotopomers

We have measured the OH- and OD-stretching fundamental and overtone spectra of phenol and its deuterated isotopomers under jet-cooled conditions using nonresonant ionization detection spectroscopy and vapor-phase infrared (IR) and near-infrared (NIR) spectra at room temperature using conventional and photoacoustic spectroscopy. The OH- and OD-stretching bands in the jet-cooled spectra are about 1-10 cm(-1) wide and generally show a few Lorentzian shaped peaks. The bands in the room-temperature spectra have widths of 20-30 cm(-1) and display clear rotational profiles. The band profiles in the jet-cooled spectra arise mostly from nonstatistical intramolecular vibrational redistribution (IVR) with specific coupling to "doorway" states, which are likely to involve CH- and CD-stretching vibrations. The transition dipole moment that determines the rotational structure is found to rotate significantly from the fundamental to the third overtone and is not directed along the OH(D) bond. We use these calculated transition dipole moments to simulate the rotational structure. We determine the rotational temperature in the jet-cooled spectra to be about 0.5 K. Anharmonic oscillator local mode calculations of frequencies and intensities of the OH- and OD-stretching transitions are compared with our measured results. The calculated intensities are in good agreement with the absolute intensities obtained from conventional spectroscopy and with the relative intensities obtained from the room-temperature laser spectroscopy.

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.

The absorption spectra of anisole-h8, anisole-d3 and anisole-d8. The assignment of fundamental vibrations in the S0and the S1states

The REMPI spectra of anisole-h8, anisole-d3 and anisole-d8 have been measured. The assignment of the fundamental vibrations of anisole in the S(1) state is supported by quantum chemical model calculations, the isotopic shifts, the comparison with the frequencies of corresponding vibrations in other monosubstituted benzenes, especially phenol, and the overtones, combinations and progressions observed in the spectrum of anisole-h8. The frequencies of the 42 fundamental vibrations of anisole in the S(1) state are evaluated and compared with the frequencies of the corresponding vibrations in the electronic ground state. Some assignments given earlier in the literature have been revised.

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.

One-color two-photon mass-analyzed threshold ionization spectroscopy of ethyl bromide through a dissociative intermediate state

Mass-analyzed threshold ionization (MATI) spectra of ethyl bromide were obtained using one-color two-photon ionization through a dissociative intermediate state. Accurate values for the adiabatic ionization energy have been obtained, 83099+/-5 and 85454+/-5 cm(-1) for the X1 2E and X2 2E states of the ethyl bromide cation, respectively, giving a splitting of 2355+/-10 cm(-1). Compared with conventional photoelectron data, the two-photon MATI spectrum exhibited a more extensive vibrational structure with a higher resolution, mainly containing the modes involving the dissociation coordinate. The observed modes were analyzed and discussed in terms of wave packet evolving on the potential-energy surface of the dissociative state.

One-photon mass-analyzed threshold ionization spectroscopy of 2-chloropropene (2-C3H5Cl) and its vibrational assignment based on the density-functional theory calculations

A high-quality mass-analyzed threshold ionization (MATI) spectrum of 2-chloropropene, 2-C3H5Cl, is reported. Its ionization energy determined for the first time from the 0-0 band position was 9.5395+/-0.0006 eV. Almost all the peaks in the MATI spectrum could be vibrationally assigned utilizing the frequencies calculated at the B3LYP6-311++G(3df,3pd) level and the Franck-Condon factors calculated with the molecular parameters obtained at the same level. In particular, the observed methyl torsional progression could be reproduced very well through quantum-mechanical calculations using the molecular parameters obtained at this level. Dramatic lowering of the torsional barrier inferred from the experimental data was entirely compatible with the B3LYP6-311++G(3df,3pd) results. The torsional barrier and the internal rotational constant determined by fits to six torsional peaks were 53.6 and 5.20 cm(-1), respectively. A brief discussion at the level of molecular orbital is presented to account for the dramatic lowering of the torsional barrier upon ionization.

Mass-analyzed threshold ionization spectroscopy of the rotamers of p-n-propylphenol cations and configuration effect

Two-color resonant two-photon mass-analyzed threshold ionization (MATI) spectroscopy was used to record the vibrationally resolved cation spectra of the selected rotamers of p-n-propylphenol. The adiabatic ionization energies of the trans, gauche-A, and gauche-B rotamers are determined to be 65 283+/-5, 65 385+/-5, and 65 369+/-5 cm(-1), which are less than that of phenol by 3342, 3240, and 3256 cm(-1), respectively. This suggests that the n-propyl substitution causes a greater degree in lowering the energy level in the cationic than the neutral ground state. Analysis on the MATI spectra of the selected rotamers of p-n-propylphenol cation shows that the relative orientation of the p-n-alkyl group has little effect on the in-plane ring vibrations. However, the low-frequency C(3)H(7) bending vibrations appear to be active only for the two gauche forms of the cation.

Mass-analyzed threshold ionization spectroscopy of p-methylphenol and p-ethylphenol cations and the alkyl substitution effect

The mass-analyzed threshold ionization (MATI) spectra of p-methylphenol and p-ethylphenol have been recorded by ionizing via various vibronic levels. The adiabatic ionization energies (IEs) of p-methylphenol and p-ethylphenol are determined to be 65918+/-5 and 65628+/-5 cm(-1), which are less than that of phenol by 2707 and 2997 cm(-1), respectively. This redshift indicates that the interaction between the alkyl group and the ring of alkylphenols in the cationic D(0) state is greater than that in the neutral S(0) state. Moreover, a longer alkyl group gives rise to a greater redshift in the IE. Analysis of the MATI spectra shows that most of the active modes are related to the in-plane ring vibrations of these two cations. However, the length of the alkyl group has an insignificant effect on the frequency of the observed ring vibrations. No band with frequency less than 350 cm(-1) is observed for the p-methylphenol cation. In contrast, many low-frequency bands resulting from the characteristic motions (e.g., the C-C(2)H(5) torsion and C-C(2)H(5) and C-OH bending vibrations) appear in the MATI spectra of p-ethylphenol. The present results show that the ethyl group enhances the substituent-sensitive and many large-amplitude vibrations of the cation.