Mass analyzed threshold ionization spectroscopy of 3-aminopyridine cation and vicinal substitution effect

Drastic Change in Electronic Transition upon Hydrogen Bond Network Switching in 3-Aminopyridine-(H2O) n Clusters

The hydration structures of 3-aminopyridine (3AP)-(H₂O) _n ( n = 2-4) in supersonic jets have been investigated by measuring the electronic and vibrational spectra with the aid of quantum chemical calculations. The S₁-S₀ electronic transition of 3AP-(H₂O)₂ is observed at a slightly red-shifted position from 3AP-(H₂O)₁, while further hydration induces drastic red shifts and complicated vibrational structures. We assign the cluster structures of 3AP-(H₂O)₂ as a cyclic structure composed of the homodromic hydrogen bond (H-bond) chain connecting the pyridyl CH bond acting as the proton donor toward a pyridyl nitrogen acceptor. For 3AP-(H₂O) _n ( n = 3, 4), on the other hand, the initial donor site in the H-bond network changes from a pyridyl CH group to an amino group. The observed red shift derived from H-bond network switching can be reproduced very well with the S₁-S₀ origin band estimation obtained by applying geometry optimization and subsequent harmonic vibrational analysis of (TD-)DFT calculations to each electronic state of the isomer structure. It is suggested that the drastic red shift of the electronic transition upon H-bond network switching is due to a much more stabilized "quinoid-like" structure in the ππ* state by the H-bond formation of an amino group.

Solvation effect on the NH stretching vibrations of solvated aminopyrazine, 2-aminopyridine, and 3-aminopyridine clusters

The vibrational spectra of the hydrated and methanol-solvated aminopyrazine, 2-aminopyridine and 3-aminopyridine in supersonic jets have been measured in terms of IR-UV double-resonance spectroscopy. Comparing the IR spectrum of aminopyrazine with those of 2-aminopyridine and 3-aminopyridine clusters, we determine the solvation structure of aminopyrazine to be a similar cyclic structure as hydrated 2-aminopyridine clusters [Wu, et al., Phys. Chem. Chem. Phys. 2004, 6, 515]. In the case of monohydrated aminopyrazine cluster, one of the normal modes composed of the hydrogen-bonded OH and NH stretching local modes gives the anomalously weak IR intensity, which is ascribed to the cancellation of the dipole moment change between the OH and NH stretching local modes. The solvated 3-aminopyridine clusters forms the hydrogen-bond between the pyridyl nitrogen atom and the OH group, but the amino group is indirectly affected to induce slight blue shift of the NH(2) stretches. This phenomenon is explained by inductive effect where the electron withdrawing from the amino group upon the solvation results in a "quinoid-like" structure of the amino group.

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.

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.

Zero kinetic energy photoelectron spectroscopy of p-amino benzoic acid

We report studies of supersonically cooled p-amino benzoic acid using one-color resonantly enhanced multiphoton ionization and two-color zero kinetic energy (ZEKE) photoelectron spectroscopy. With the aid of ab initio and density functional calculations, vibrational modes of the first electronically excited state S(1) of the neutral species and those of the cation have been assigned, and the adiabatic ionization potential has been determined to be 64 540+/-5 cm(-1). A common pattern involving the activation of five vibrational modes of the cation is recognizable among all the ZEKE spectra. A propensity of Deltav=0, where v is the vibrational quantum number of the intermediate vibronic state from S(1), is confirmed, and the origin of this behavior is discussed in the context of electron back donation from the two substituents in the excited state and in the cationic state. A puzzling observation is the doublet splitting of 37 cm(-1) in the ZEKE spectrum obtained via the inversion mode of the S(1) state. This splitting cannot be explained from our density functional calculations.

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.

Resonantly enhanced two photon ionization and zero kinetic energy spectroscopy of jet-cooled 4-aminopyridine

We report studies of supersonically cooled 4-aminopyridine (4-AP) using two-color resonantly enhanced multiphoton ionization (REMPI) and two-color zero kinetic energy (ZEKE) photoelectron spectroscopy. With the aid of ab initio and density functional calculations, vibrational modes of the first electronically excited state (S1) of the neutral species and those of the cation have been assigned, and the adiabatic ionization potential has been determined to be 62291+/-6 cm(-1). The REMPI spectrum of the S1 state is dominated by ring deformation modes and the inversion mode of the amino group, while the ZEKE spectra demonstrate a strong propensity of Deltav=0, where v is the vibrational quantum number of the intermediate vibronic state from S1. In addition, the ZEKE spectra obtained via different vibrational levels of the S1 state contain four common features, corresponding to the activation of four different vibrational modes of the cation. These observations are explained in terms of the structural changes from the ground state to S1 and further to the cation. The vibrational mode distributions in both the REMPI and the ZEKE spectra, the excitation energy of the S1 state, and the ionization potential of 4-AP, are remarkably similar to those of aniline, suggesting that the electronic activity is centered on the ring.