Nam Joon Kim

Resonant two-photon ionization and laser induced fluorescence spectroscopy of jet-cooled adenine

Electronic spectra of the jet-cooled DNA base adenine were obtained by the resonant two-photon ionization (R2PI) and the laser induced fluorescence (LIF) techniques. The 0–0 band to the lowest electronically excited state was found to be located at 35u200a503 cm−1. Well-resolved vibronic structures were observed up to 1100 cm−1 above the 0–0 level, followed by a slow rise of broad structureless absorption. The lowest electronic state was proposed to be of nπ* character, which lies ∼600 cm−1 below the onset of the ππ* state. The broad absorption was attributed to the extensive vibronic mixing between the nπ* state and the high-lying ππ* state.

Dispersed fluorescence spectroscopy of jet-cooled adenine

Dispersed fluorescence spectra of jet-cooled adenine were obtained by exciting the vibronic bands observed in the fluorescence excitation spectrum from an earlier study (Kim et al., J. Chem. Phys., 2000, 113, 10u2006051). The dispersed fluorescence spectra reveal sufficiently well-resolved vibrational bands of the ground electronic state. The excitation of the C′ band at 36u2006062 cm−1 and the D′ band at 36u2006105 cm−1 results in emissions that are associated with out-of-plane and in-plane vibrational modes in the ground state, respectively, which suggests that these bands belong to different electronic states, as first proposed by us and later supported by others. A vibrational analysis yielded an assignment of the C′ band as an out-of-plane vibrational mode of the nπ* state and the D′ band as the 0–0 band of the ππ* state. Combination bands of the out-of-plane and in-plane vibrational modes are observed in the dispersed fluorescence spectrum of the C′ band, which seems to support that there is indeed a strong vibronic coupling between the nπ* and ππ* excited states of adenine.

Ultraviolet Photodepletion Spectroscopy of Dibenzo-18-Crown-6-Ether Complexes with Alkali Metal Cations

Ultraviolet photodepletion spectra of dibenzo-18-crown-6-ether complexes with alkali metal cations (M(+)-DB18C6, M = Cs, Rb, K, Na, and Li) were obtained in the gas phase using electrospray ionization quadrupole ion-trap reflectron time-of-flight mass spectrometry. The spectra exhibited a few distinct absorption bands in the wavenumber region of 35 450-37 800 cm(-1). The lowest-energy band was tentatively assigned to be the origin of the S(0)-S(1) transition, and the second band to a vibronic transition arising from the benzene breathing mode in conjunction with symmetric or asymmetric stretching vibration of the bonds between the metal cation and the oxygen atoms in DB18C6. The red shifts of the origin bands were observed in the spectra as the size of the metal cation in M(+)-DB18C6 increased from Li(+) to Cs(+). We suggested that these red shifts arose mainly from the decrease in the binding energies of larger-sized metal cations to DB18C6 at the electronic ground state. These size effects of the metal cations on the geometric and electronic structures, and the binding properties of the complexes at the S(0) and S(1) states were further elucidated by theoretical calculations using density functional and time-dependent density functional theories.

Hydration of DNA base cations in the gas phase

Abstract Hydration of the adenine and thymine cations was studied in the gas phase. Metastable fragmentation of the cluster ions was used to draw information on the relative strengths of the binding energy of each additional solvent molecule. Both cations of adenine and thymine exhibited a well-defined hydration shell structure, with the first hydration shell complete with four water molecules. On the other hand, the hydration shell structure for the cation dimer of adenine was less evident, but appeared to require seven or eight water molecules for the first hydration shell. An ab initio calculation was carried out at the Hartree–Fock level to address some of these issues.

Ultraviolet–Ultraviolet Hole Burning Spectroscopy in a Quadrupole Ion Trap: Dibenzo[18]crown‐6 Complexes with Alkali Metal Cations

Conformation selective: A new technique of ultraviolet-ultraviolet hole burning spectroscopy that can be applied to ions stored in a quadrupole ion trap (QIT) is developed and used to obtain the conformation-selective electronic spectra of dibenzo[18]crown-6 complexes with alkali metal cations (M(+), see picture; F(+) = fragment).

Ultraviolet Photodepletion Spectroscopy of Dibenzo-18-Crown-6-Ether Complexes with Alkaline Earth Metal Divalent Cations

Ultraviolet photodepletion spectra of dibenzo-18-crown-6-ether complexes with alkaline earth metal divalent cations (A(2+)-DB18C6, A = Ba, Sr, Ca, and Mg) were obtained in the gas phase using electrospray ionization quadrupole ion-trap reflectron time-of-flight mass spectrometry. Each spectrum exhibits the lowest energy absorption band in the wavenumber region of 35u2009400-37u2009800 cm(-1), which is tentatively assigned as the origin of the S(0)-S(1) transition of A(2+)-DB18C6. This origin band shows a red shift as the size of the metal dication increases from Mg(2+) to Ba(2+). The binding energies of the metal dications to DB18C6 at the S(0) state were calculated at the lowest energy structures optimized by the density functional theory and employed with the experimental energies of the origin bands to estimate the binding energies at the S(1) state. We suggest that the red shifts of the origin bands arise from the decrease in the binding energies of the metal dications at the S(1) state by nearly constant ratios with respect to the binding energies at the S(0) state, which decrease with increasing size of the metal dication. This unique relationship of the binding energies between the S(0) and S(1) states gives rise to a linear correlation between the relative shift of the origin band of A(2+)-DB18C6 and the binding energy of the metal dication at the S(0) state. The size effects of the metal cations on the properties of metal-DB18C6 complex ions are also manifested in the linear plot of the relative shift of the origin band as a function of the size to charge ratio of the metal cations, where the shifts of the origin bands for all DB18C6 complexes with alkali and alkaline earth metal cations are fit to the same line.

Laser induced fluorescence and resonant two-photon ionization spectroscopy of jet-cooled 1-hydroxy-9,10-anthraquinone

We carried out laser induced fluorescence and resonance enhanced two-color two-photon ionization spectroscopy of jet-cooled 1-hydroxy-9,10-anthraquinone (1-HAQ). The 0-0 band transition to the lowest electronically excited state was found to be at 461.98 nm (21,646 cm(-1)). A well-resolved vibronic structure was observed up to 1100 cm(-1) above the 0-0 band, followed by a rather broad absorption band in the higher frequency region. Dispersed fluorescence spectra were also obtained. Single vibronic level emissions from the 0-0 band showed Stokes-shifted emission spectra. The peak at 2940 cm(-1) to the red of the origin in the emission spectra was assigned as the OH stretching vibration in the ground state, whose combination bands with the C=O bending and stretching vibrations were also seen in the emission spectra. In contrast to the excitation spectrum, no significant vibronic activity was found for low frequency fundamental vibrations of the ground state in the emission spectrum. The spectral features of the fluorescence excitation and emission spectra indicate that a significant change takes place in the intramolecular hydrogen bonding structure upon transition to the excited state, such as often seen in the excited state proton (or hydrogen) transfer. We suggest that the electronically excited state of interest has a double minimum potential of the 9,10-quinone and the 1,10-quinone forms, the latter of which, the proton-transferred form of 1-HAQ, is lower in energy. On the other hand, ab initio calculations at the B3LYP/6-31G(d,p) level predicted that the electronic ground state has a single minimum potential distorted along the reaction coordinate of tautomerization. The 9,10-quinone form of 1-HAQ is the lowest energy structure in the ground state, with the 1,10-quinone form lying approximately 5000 cm(-1) above it. The intramolecular hydrogen bond of the 9,10-quinone was found to be unusually strong, with an estimated bond energy of approximately 13 kcal/mol (approximately 4500 cm(-1)), probably due to the resonance-assisted nature of the hydrogen bonding involved.

Binding selectivity of dibenzo-18-crown-6 for alkali metal cations in aqueous solution: A density functional theory study using a continuum solvation model

BackgroundDibenzo-18-crown-6 (DB18C6) exhibits the binding selectivity for alkali metal cations in solution phase. In this study, we investigate the main forces that determine the binding selectivity of DB18C6 for the metal cations in aqueous solution using the density functional theory (DFT) and the conductor-like polarizable continuum model (CPCM).ResultsThe bond dissociation free energies (BDFE) of DB18C6 complexes with alkali metal cations (M+-DB18C6, Mu2009=u2009Li, Na, K, Rb, and Cs) in aqueous solution are calculated at the B3LYP/6-311++G(d,p)//B3LYP/6-31u2009+u2009G(d) level using the CPCM. It is found that the theoretical BDFE is the largest for K+-DB18C6 and decreases as the size of the metal cation gets larger or smaller than that of K+, which agrees well with previous experimental results.ConclusionThe solvation energy of M+-DB18C6 in aqueous solution plays a key role in determining the binding selectivity of DB18C6. In particular, the non-electrostatic dispersion interaction between the solute and solvent, which depends strongly on the complex structure, is largely responsible for the different solvation energies of M+-DB18C6. This study shows that the implicit solvation model like the CPCM works reasonably well in predicting the binding selectivity of DB18C6 in aqueous solution.

Conformation-Specific Circular Dichroism Spectroscopy of Cold, Isolated Chiral Molecules†

The CD spectroscopy of a chiral compound in solution yields an average CD value derived from all of the conformations of a chiral molecule. By contrast, CD spectroscopy of cold chiral molecules in the gas phase distinguishes specific conformers of a chiral molecule, but the weak CD effect has limited the practical application of this technique. Reported herein is the first resonant two-photon ionization CD spectra of ephedrines in a supersonic jet using circularly polarized laser pulses, which were generated by synchronizing the oscillation of the photoelastic modulator with the laser firing. The spectra exhibited well-resolved CD bands which were specific for the conformations and vibrational modes of each enantiomer. The CD signs and magnitudes of the jet-cooled chiral molecules were very sensitive to their conformations and thus offered crucial information for determining the three-dimensional structures of chiral species, as conducted in combination with quantum chemical calculations.

Excited-state lifetime of adenine near the first electronic band origin

The excited-state lifetime of supersonically cooled adenine was measured in the gas phase by femtosecond pump-probe transient ionization as a function of excitation energy between 36u2009100 and 37u2009500cm(-1). The excited-state lifetime of adenine is ∼2ps around the 0-0 band of the (1)L(b) ππ(∗) state (36u2009105cm(-1)). The lifetime drops to ∼1ps when adenine is excited to the (1)L(a) ππ(∗) state with the pump energy at 36u2009800cm(-1) and above. The excited-state lifetimes of (1)L(a) and (1)L(b) ππ(∗) states are differentiated in accordance with previous frequency-resolved and computational studies.