Laser Spectroscopic Study of Cold Host–Guest Complexes of Crown Ethers in the Gas Phase

A laser spectroscopic study on the structure and dynamics of cold host–guest inclusion complexes of crown ethers (CEs) with various neutral and ionic species in the gas phase is presented. The complexes with neutral guest species are formed by using supersonic free jets, and those with ionic species are generated with electrospray ionization combined with a cold 22-pole ion trap. For CEs, various sizes of 3n-crown-n ethers (n = 4, 5, 6, and 8) and their benzene-substituted species are used. For the guest species, water, methanol, ammonia, acetylene, and phenol are employed as neutral guest species, and for charged guest species, alkali metal cations are chosen. The electronic and vibrational spectra of the complexes are measured by using various laser spectroscopic methods; electronic spectra for the neutral complexes are measured by laser-induced fluorescence. Discrimination of different species such as conformers is performed by ultraviolet–ultraviolet hole-burning spectroscopy. The vibrational spectra of selected species are observed by infrared–ultraviolet double-resonance (IR–UV DR) spectroscopy. For the ionic complexes, ultraviolet photodissociation and IR–UV DR spectroscopy are applied. The complex structures are determined by comparing the observed spectra with those of possible structures obtained by density functional theory calculations. How the host CEs change their confor- mation or which conformer prefers to form unique inclusion complexes are discussed. These results reveal the key interactions for forming special complexes leading to molecular recognition.

Electronic and vibrational spectroscopy of cold protonated amino acids in the gas phase

Sébastien Mercier

In this thesis report, we describe a novel home-built instrument designed to study the spectroscopy of biomolecular ions in the gas phase and at low temperature, and we present the first experimental results obtained on protonated aromatic amino acids. The apparatus consists in a tandem mass spectrometer equipped with a nanospray ion source and a cryocooled 22-pole ion trap. The charged species produced by the source traverse a first quadrupole mass filter and mass-selected ions are then accumulated and thermalized in the ion trap, where their vibrational temperature can be lowered to ~ 10 K. Two setups allow us to generate ultraviolet and infrared laser light to spectroscopically probe the trapped ions by photodissociation. The resulting fragment ions are finally released from the trap and mass analyzed by a second quadrupole spectrometer. Photodissociation electronic spectra have been measured for the protonated amino acids tryptophan (TrpH+) and tyrosine (TyrH+), as well as for hydrated complexes of the former. The spectrum of TyrH+ exhibits sharp, fully resolved vibronic transitions, which attests to the low temperature of the ions. In contrast, that of tryptophan shows a broad absorption band that covers several hundreds of wavenumbers. This has been attributed to lifetime broadening related to an ultrafast (< ~ 100 fs) deactivation mechanism of the TrpH+ S1(ππ*) excited state. This phenomenon is caused by a strong coupling between this ππ* state and a nearby dissociative state of πσ* character mainly localized on the charged ammonium group. Solvation of TrpH+ by only two water molecules is sufficient to significantly destabilize the σ* orbitals and reduce that coupling, which results in a longer excited-state lifetime and a fully resolved electronic spectrum. These interpretations are supported by ab initio and density functional theory (DFT) calculations. Infrared-ultraviolet double resonance spectroscopic techniques have been successfully implemented to identify the different conformers of TyrH+ and doubly hydrated protonated tryptophan (TrpH+·W2) by measuring conformer-specific vibrational and electronic spectra for each of them. By comparison with the results of DFT geometry and harmonic frequency calculations, we could confidently determine the structures of the TyrH+ conformers and suggest possible assignments for those of TrpH+·W2. Two important stabilizing interactions could be inferred from these results: (i) the attraction between the charged ammonium group and the π-electron cloud of the aromatic ring; (ii) the hydrogen-bonding interaction of a donor water molecule to the acceptor indole ring of TrpH+·W2.

EPFL2008

UV and IR spectroscopy of cold 1,2-dimethoxybenzene complexes with alkali metal ions

Yoshiya Inokuchi, Thomas Rizzo

We report UV photodissociation (UVPD) and IR-UV double-resonance spectra of 1,2-dimethoxybenzene (DMB) complexes with alkali metal ions, M+•DMB (M = Li, Na, K, Rb, and Cs), in a cold 22-pole ion trap. The UVPD spectrum of the Li+ complex shows a strong origin band. For the K+•DMB, Rb+•DMB, and Cs+•DMB complexes, the origin band is very weak and low-frequency progressions are much more extensive than that of the Li+ ion. In the case of the Na+•DMB complex, the spectral features are similar to those of the K+, Rb+, and Cs+ complexes, but the vibronic bands are not resolved. Geometry optimization with density functional theory indicates that the metal ions are bonded to the oxygen atoms in all the M+•DMB complexes calculated. In the Li+ and Na+ complexes in the S0 state, the metal ions are located almost in the same plane as the benzene ring, while the K+, Rb+, and Cs+ ions are substantially off from the plane. In the S1 state, the Li+ complex has a structure similar to that in the S0 state. In contrast, the other complexes show a large structural change upon S1–S0 excitation to the out-of-plane direction of the benzene ring, which results in the extensive low-frequency progressions in the UVPD spectra. For the Na+•DMB complex, fast charge transfer occurs from Na+ to DMB after the UV excitation, making the bandwidth of the UVPD spectrum much broader than that of the other complexes and producing photofragment DMB+ ion.

2012

Electronic and vibrational spectroscopy of cold protonated amino acids in the gas phase

Sébastien Mercier

EPFL2008