Physical Chemistry of Cold Gas-Phase Functional Molecules and Clusters

In this chapter, we describe the methods of generating cold neutral and ionic (cation and anion) molecules, their clusters, and metal clusters in the gas-phase. First, a technique of supersonic free-jet or supersonic beam to generate cold neutral molecules and clusters is described. In addition, heating and laser ablation nozzles for the geneation of supersonic free-jet of nonvolatile molecules, such as high melting point and bio-related molecules, are introduced, while the methods of laser ablation and magnetron sputtering to generate metal clusters are also described. We then introduce various laser spectroscopic methods to measure the electronic and vibrational spectra for the jet-cooled molecules. Laser-induced fluorescence (LIF) and resonance-enhanced two-photon ionization (R2PI) spectroscopy is used to measure the electronic spectrum. UV–UV hole-burning (UV-UV HB) spectroscopy is used to discriminate the electronic transitions of different conformers and isomers. For the measurement of the vibrational spectrum of a specific molecule or cluster, we apply infrared-ultraviolet double-resonance (IR-UV DR) spectroscopy. If the molecule has no chromophore, a combination of IR and vacuum UV laser light (IR-VUV) is used to obtain the vibrational spectrum. Second, we describe the generation methods of gas-phase cold ionic molecules and clusters. The gas-phase ions are generated by resonant-enhanced multi-photon ionization, electron impact, electron attachment, matrix-assisted laser disorption/ionization (MALDI), and electrospray ionization (ESI). Cooling of the ions is achieved by supersonic expansion or by the use of cryogenically cooled ion-trap. A time-of-flight (TOF) mass spectrometry or quadrupole mass filter is used for the mass selection, which is also applicable to obtain the single-sized metal clusters selectively. To obtain the electronic and vibrational spectra of the ionic species, we apply UV photodissociation (UVPD) and IR multi-photon dissociation (IRMPD), respectively. IR-UV DR spectroscopy is also used to measure the IR spectrum of a specific ion. In addition to the detection of the ions, a measurement of the photo-ejected electron, called photoelectron spectroscopy, is also described. Finally, we introduce  pump–probe spectroscopy to investigate the dynamics of the vibrationally and electronically excited molecules and clusters.

Crown ethers (CEs) are the typical molecules in the host–guest chemistry. In this chapter, laser spectroscopy and quantum chemical calculations of the cold gas-phase neutral host–guest complexes of crown ethers (CEs) are described. Here, we chose 3n-crown-n (3nCn, n = 4, 5, 6, 8) as the host and substituted benzenes as the guest. The cold gas-phase complexes are produced by a supersonic expansion technique. Electronic spectra of the complexes are observed by laser-induced fluorescence (LIF) spectroscopy and discrimination of different isomers is performed by ultraviolet–ultraviolet hole-burning (UV–UV HB) spectroscopy. Conformer specific vibrational spectra are measured by IR-UV double-resonance (IR-UV DR) spectroscopy. The structures of the complexes are determined by comparing the obtained IR spectra with those of the energy-optimized structures obtained by quantum chemical calculations. We will discuss how the host and guest species change their flexible structures to form a best-fitted stable complex via “key and lock” or “induced-fitting” mechanism, and what kinds of interactions are operated for the stabilization of the complexes. We also will show the effect of complexation on the photophysics of the guest species.

Jet-cooled bichromophoric cyclic dipeptides built on a diketopiperazine (DKP) ring are studied by combining conformer-specific vibrational spectroscopy with quantum chemical calculations. The dependence of the c-LL and c-LD dipeptides structure upon relative absolute configuration, L or D, of the residues is investigated for two residues: phenylalanine (Phe) and tyrosine (Tyr). A folded-extended structure is systematically observed for all systems, like in solution or in the solid. This structure is stabilized by NH…π and CH…π interactions and shows limited stereoselectivity; the only difference between c-LL and c-LD is the nature of the CH…π interaction—C_α…π in c-LD and C_β…π in c-LL—and a stronger NH…π interaction in c-LD. For all the species studied, the electronic excitation and the charge in the radical cation are localized on the extended ring. The c-LL diastereomer of cyclo di-tyrosyl stands out by the existence of a stacked structure, in which formation of an OH…O hydrogen bond stabilizes parallel aromatic rings orientation. In this structure, the electronic excitation and the major part of the charge in the cation are localized on the H-bond donor. The OH…O H-bond is possible in c-LL and not c-LD, which explains the high stereoselectivity.

Infrared spectroscopy was applied to neutral and protonated clusters of fundamental protic molecules, water, methanol (MeOH), and ammonia, in the size range of dozens to hundreds. For neutral (H₂O)_n and (MeOH)_n clusters, a phenol molecule was introduced as a chromophore for the resonant multiphoton ionization detection, and the infrared-ultraviolet double-resonance scheme with mass spectrometry was used to perform moderately size-selective infrared spectroscopy in the OH stretch region. Infrared spectra of neutral (NH₃)_n clusters in the NH stretch region were also measured with the same scheme, but without addition of a chromophore. For protonated clusters, H⁺(H₂O)_n, H⁺(MeOH)_n, and H⁺(NH₃)_n, infrared dissociation spectroscopy was applied to observe definitely size-selective spectra of these clusters in the large size region. The size dependence of the hydrogen bond networks of the neutral and protonated clusters is discussed on the basis of the observed spectra. Convergence of the spectra of the protonated clusters to those of the neutral clusters is seen with increasing size, and the convergence size tells us the maximum size range of the influence of the excess proton to the surrounding hydrogen bond network. Convergence of the spectra of large-sized clusters to those of the bulk is also seen, and the dependence of the convergence size on the hydrogen bond network is discussed.

UV and IR spectra of benzo-crown ether complexes with metal ions are obtained under cold, gas-phase conditions with an electrospray ion source and cryogenically cooled ion traps. UV photodissociation spectroscopy is performed to obtain the UV spectra of the complexes. Conformer-specific UV and IR spectra are measured by UV-UV hole-burning and IR-UV double-resonance spectroscopy. Geometric and electronic structures of the crown ether complexes in addition to the number of the conformers are determined on the basis of the UV and IR spectra with the aid of quantum chemical calculations. We examine solvation effects on the encapsulation of metal ions by crown ethers by using microsolvated complexes, which contain several numbers of solvent molecules such as water and methanol. The relation between the function characteristic of crown ethers such as ion selectivity and the structures is discussed.

Transition metal cation–molecular complexes are produced in the gas phase environment of a molecular beam using laser ablation in a supersonic expansion. Complexes with carbon monoxide, carbon dioxide, water, acetylene, or benzene are produced by entraining small partial pressures of these molecules into an expansion of either argon or helium. A specially designed time-of-flight mass spectrometer is used to analyze the ions produced and to mass-select them for spectroscopy. Mass-selected ions are excited in the infrared region of the spectrum with a tunable IR optical parametric oscillator laser system to measure photodissociation spectroscopy in the 2000–4500 cm⁻¹ region. Infrared band patterns, combined with structures and spectra predicted by density functional theory, reveal the coordination and solvation interactions in these systems, and how binding to metal distorts the structures of small molecules.

This chapter describes two superatoms, each comprising a central atom and a silicon or aluminum cage. Binary nanoclusters (NCs) at optimized mixing ratios are key components in designing the functionalities relevant to their electronic properties. To form chemically robust functional NCs, it is important to design the cooperatively synergistic effects between the electronic and geometric structures because these stabilize the individual NCs not only against charge transfer into the corresponding cations or anions but also against structural perturbations in their assemblies. Among binary NCs, synergistic effects are particularly expected when one central atom encapsulating cage structure completes a specific electron shell because electronic and geometric factors can operate simultaneously. Although the term “superatom” is widely used when the valence electrons in NCs complete an electron shell, more synergistic effects appear when the superatom adopts a close-packed structure, such as a highly symmetric cage as a binary cage superatom. Representative examples are given for one central atom encapsulated by silicon and aluminum cages, M@Si₁₆ and X@Al₁₂, their formation and characterization are described, and a large-scale synthetic approach is established for M@Si₁₆. The perspectives for binary cage superatom assembly are discussed in terms of theoretical calculations.

Atomically precise Au and Ag clusters protected by organic ligands can be viewed as chemically modified superatoms. These chemically modified Au/Ag superatoms have gained interests as promising building units of functional materials as well as ideal platforms to study the size-dependent evolution of structures and physicochemical properties. Mass spectrometry not only allows us to determine the chemical compositions of the synthesized superatoms but also gives us molecular-level insights into the mechanism of complex processes in solution. A variety of the gas-phase methods including ion-mobility–mass spectrometry, collision- or surface-induced dissociation mass spectrometry, photoelectron spectroscopy, and photodissociation mass spectrometry have been applied to the chemically modified Au/Ag superatom ions isolated in the gas phase. These studies have provided novel and complementary information on their intrinsic geometric and electronic structures that cannot be obtained by conventional characterization methods. This chapter surveys the recent progress in the gas-phase studies on chemically synthesized Au/Ag superatoms.

In this chapter, time-resolved study on the vibrational energy relaxation (VER) of gas-phase aromatic molecules and their clusters cooled in supersonic jets are described. The experiment is performed by picosecond IR-UV pump-probe spectroscopy, where the molecules are excited to a specific vibrational level of the electronic ground state (S₀) by a picosecond IR pulse laser, and the time evolution of the excited level as well as the energy transferred levels are monitored by a picosecond UV pule laser. In the molecule in isolated condition, VER involves intramolecular vibrational energy redistribution (IVR), and in the cluster IVR is followed by vibrational predissociation (VP), if the initial vibrational energy is larger than the binding energy of the cluster. We focus VER of high frequency vibrations; OH, NH and CH stretching vibrations. Among these vibrations, OH and NH starching vibrations are localized motion, while the CH stretching vibration is the collective motion of several CH groups. The molecules studied in this chapter is phenol, aniline, 2-aminopyridine and benzene. We will see how the difference of the vibrational motion affects the IVR process and its speed in these molecules. We also investigate the IVR and VP of the H-bonded clusters and van der Waals clusters. We discuss the route of the energy flow starting from the IR excited vibration to other vibrational levels within the molecules and clusters and the time scales.

Quantitative prediction of branching ratios and photoproduct quantum yields when an excited electronic state has multiple conical intersections (CIs) with other electronic states remains a challenging problem for theoretical chemists. Experimental benchmarking of these observables is thus highly desirable for evaluating the appropriateness of various approximations made in theoretical calculations. In this chapter, we present time-resolved photoelectron spectroscopy (TRPES) using a vacuum-ultraviolet (VUV) laser as a means to observe ultrafast dynamics through CIs in real time. We describe the details of our apparatus, and discuss some representative examples from our recent studies. In the UV photochemistry of furan following ππ* photoexcitation, the ring-puckering CI dominates the dynamics, safely returning more than 90% of the excited molecules to the original ground state. The remaining 10% undergo irreversible isomerization after passing through the puckering CI. In the ultrafast photodissociation of nitromethane, we find that ππ* electronic excitation leads to ultrafast cascading internal conversion (S₃ → S₂ → S₁ + S₀) prior to dissociation into CH₃ + NO₂ fragments, and that the dissociation predominantly proceeds on the S₁ surface, leading to comparable production of NO₂(A) and NO₂(X).

Femtosecond time-resolved photoelectron spectroscopy (TRPES) is a powerful technique to probe the ultrafast excited state dynamics of molecules. TRPES applied to gas-phase molecular anions and clusters is capable of probing not only excited state formation and relaxation but also electron accommodation dynamics upon injection of an excess electron into a solvent or molecule. We review the basics of TRPES as it applies to molecular anions and several applications including the study of electron solvation dynamics in clusters and excited state relaxation in several biomolecules. We then explore in detail the dynamics of electron attachment and photodissociation in iodide–nucleobase clusters studied by TRPES as a model system for examining radiative damage of DNA induced by low-energy electrons. By initiating charge transfer from iodide to the nucleobase and following the dynamics of the resulting transient negative ions with femtosecond time resolution, TRPES provides a novel window into the chemistry triggered by the attachment of low-energy electrons to nucleobases.

In this chapter we will review the processes at play in the excited states of protonated molecules. More specifically, the discussion will be restricted to aromatic molecules, on which the majority of studies have been focused. Rather than going into details of each system, we will highlight the trends and general mechanisms that are involved in the excited state energy and dynamics. The interested reader can find detailed descriptions of each molecule in the individual publications. It appears that the excited state dynamics of many aromatic ions can be understood upon the application of rather elementary and universal concepts, the simplest of which being that the excited electron is attracted to the proton, triggering a large part of the excited state dynamics.

This review describes the development and application of a new experimental approach, namely, picosecond time-resolved pump–probe infrared spectroscopy of size- and isomer-selected aromatic clusters, in which for the first time the dynamics of a single individual solvent molecule can be detected in real time. The intermolecular isomerization reaction is triggered by resonant photoionization, and infrared absorption at variable delay is detected by decrease of parent ion signal due to photodissociation. The advantage of this time-resolved spectroscopy is demonstrated by the isomerization reactions in phenol with nonpolar ligands (rare gas and methane molecule). It gives salient properties of the reaction, including rates, yields, pathways, branching ratios of competing reactions, back reactions, and timescales of energy relaxation processes. Mechanism of the isomerization reaction and its relation to intracluster vibrational relaxation are also discussed.