Resonant-enhanced two photon ionization and mass-analyzed threshold ionization spectroscopy of jet-cooled 2-aminopyridines (2AP–NH2,–NHD,–NDH,–ND2)

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.

DFT studies on one-electron oxidation and one-electron reduction for 2- and 4-aminopyridines

Quantum-chemical calculations {DFT(B3LYP)/6-311+G(d,p)} were performed for all possible tautomers (aromatic and nonaromatic) of neutral 2- and 4-aminopyridines and their oxidized and reduced forms. One-electron oxidation has no important effect on the tautomeric preference for 2-aminopyridine. The amine tautomer is favored. However, oxidation increases the stability of the imine NH tautomer, and its contribution in the tautomeric mixture cannot be neglected. In the case of 4-aminopyridine, one-electron oxidation increases the stability of both the amine and imine NH tautomers. Consequently, they possess very close energies. As major tautomers, they dictate the composition of the tautomeric mixture. The CH tautomers may be considered as very rare forms for both neutral and oxidized aminopyridines. A reverse situation takes place for the reduced forms of aminopyridines. One-electron reduction favors the C3 atom for the labile proton for both aminopyridines. This may partially explain the origin of the CH tautomers for the anionic states of nucleobases containing the exo NH₂ group.

Experimental probing of conical intersection dynamics in the photodissociation of thioanisole

Chemical reactions that occur in the ground electronic state are described well by invoking the Born-Oppenheimer approximation, which allows their development to be rationalized by nuclear rearrangements that smoothly traverse an adiabatic potential energy surface. The situation is different, however, for reactions in electronically excited states, where non-adiabatic transitions occur between adiabatic surfaces. The conical intersection, in which two adiabatic surfaces touch, is accepted widely as the dynamic funnel for efficient non-adiabatic transitions, but its direct experimental probing is rare. Here, we investigate the photodissociation of thioanisole and observe a striking dependence of the relative yields of two reaction channels on the photoexcitation energy as indicated by a dynamic resonance in the product branching ratio. This results from the interference of two different adiabatic states that are in close proximity in the region of a conical intersection. The location of the observed resonance on the multidimensional potential energy surface thus reveals the nuclear configuration of the conical intersection and its dynamic role in the non-adiabatic transition.

Pulsed-field ionization spectroscopy of high Rydberg states (n=50–200) of bis(η6-benzene)chromium

The ionization behavior of the high Rydberg states of bis(eta(6)-benzene)chromium in the presence of ac and/or dc fields has been explored. The application of an ac scrambling field at the time of laser excitation lengthens the lifetime of the Rydberg state by almost two orders of magnitude. The lifetime enhancement by the scrambling field is much more effective for n<100 than it is for n>100 Rydberg states. The pulsed-field ionization of Rydberg states of n<100 shows the typical diabatic ionization behavior for low n. The two distinct ionization behaviors observed for the relatively low (n=50-100) and high (n=100-200) Rydberg states suggest that the former originate from the optically accessed nf Rydberg series, whereas the latter are due to np Rydberg series. Based on the understanding of the ionization behavior of bis(eta(6)-benzene)chromium, the accurate ionization potential is deduced to give IP=5.4665+/-0.0003 eV. Optimization of the various electric field conditions greatly enhances the spectral sensitivity of the mass-analyzed threshold ionization (MATI) spectroscopy. The high-resolution MATI spectrum of the title molecule obtained here provides precise cationic vibrational frequencies for many skeletal and benzene ring modes. A number of vibrational modes are newly identified, and the ambiguity regarding to some mode assignments is now clearly resolved through the Frank-Condon analysis based on ab initio calculations.

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.

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.

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.