IR photodissociation spectroscopy

In the gas phase, ions may be isolated, and properties such as stability, metal-ligand bond energy, or reactivity determined, full structural characterization is not yet possible. There are no complications due to solvent or crystal packing forces and so the intrinsic properties of the ions may be investigated. The effects of solvation may be probed by studying ions such as [M(solvent) ]+. The spectroscopic investigation of ions has been limited to the photoelectron spectroscopy of anions but other methods such as infrared (IR) photodissociation spectroscopy are now available. 

In the case of phenols, either C- or (9-protonated ion 12 or 13 are formed, depending upon the acidity of the medium. In relatively weak acid medium such as H2SO461 only O-protonation is observed. However, in HSO3F only the C-protonated species is formed.62 In a much stronger acid system such as HSC F-SbFs (Magic Acid), both C-or O-protonated dication 14 can be observed. Both the C- and O-protonated ions were shown to exist in the gas phase by IR photodissociation spectroscopy.63... 

O Brien JT, Prell JS, Steill JD, Oomens J, Williams ER. Interactions of mono- and divalent metal ions with aspartic and glutamic acid investigated with IR photodissociation spectroscopy and theory. J Phys Chem A. 2008 112 10823-30. 

Three papers on microsolvation have been published. The effect of microsolvation by 0,1,2, or 3 water molecules on the identity 5 2 reaction between r and CH3-I has been determined using IR photodissociation spectroscopy and theoretical calculations at the MP2/aug-cc VDZ level of theory. The activation energy changes from -1.0 to 6.0 to 15.8 to 27.0 kJ mol with respect to the reactants when 0, 1, 2, or 3 water molecules, respectively, are present. The results indicate that the reaction is exothermic in the gas phase but endothermic in solution. 

Other small molecules for which photodecarbonylation studies have been made include phosgene in both the gas phaseand as a solid,acetyl chloride, and oxalyl chloride though extrusion of CO is only one of the pathways observed for these species. Photodissociations of acetyl and propionyl radicals have been studied by IR emission spectroscopy of the vibrationally excited CO produced, and an ab initio study of the photodissociation of the formyl radical (HCO) has been made." Trifluoroacetylthiol (CF3COSH) and trifluoroacetyl-sulfenyl chloride (CF3COSCI) have been photolysed in noble gas matrices, to produce CF3SH and CO, and CF3SCI, CO and SCO, respectively." ... 

S is greatly stabilized compared to the others, through exciton as well as CT interactions—that is, exactly by the factors responsible for the stability of excimers and exciplexes. (Cf. Sections 5.4.2 and 5.4.3.) The S state has a minimum at the biradicaloid square geometry that may be referred to as the excimer minimum. A strong absorption band in the near-IR region corresponding to a transition between the S and S states of the fluorene excimer has been observed by photodissociation spectroscopy (Sun et al., 1993). 

Provided a tunable laser light source is available, the IRMPD experiment can be performed in a wavelength-dependent manner to reveal the absorption characteristics of gaseous ions. Different from IR spectroscopy of condensed phase samples, in mass spectrometry the occurrence of IR absorption is observed by detection of specific fragment ions and their abundances as a function of laser wavelength. This technique is known as infrared photodissociation spectroscopy (IRPD) [139,156]. 

Fig. 9.35. Setup for infrared photodissociation spectroscopy. In this particular setup (FELIX at Utrecht, Netherlands) the light from a free-electron laser is guided into an open cylindrical ICR cell in a way that the IR light undergoes multiple reflections on the inner surfaces to extend its path for absorption by the ions inside the cell. Reproduced horn Ref. [139] with permission. John Wiley Periodicals, Inc., 2009.

In the recent years, the interaction of PAH with heteroatoms has been studied for Ca [181], Si [98,182,183] Pt, Ag, Pd, Au [184-186], Li [187], Sc, Zn [188], rare gases like Ar [108,189,190], metals like Fe, Y, La, Lu, Cr, Co, V, Ti, Ni, Mn [191 196] or Vn [196,197] or alcah ions Na+, K+, and Cs+ [198]. Many of these studies are performed to characterize the role of a perturbative environement on an observed property. For instance, understanding the effects of argon rare gas matrix on the IR spectra can be investigated from simulations of a PAH with several argon atoms [190]. These studies can also be seen as the first step to investigate a PAH-surface interaction, describing the surface by few atoms [182] or to reproduce some specific experiments like photodissociation spectroscopy of Ar-PAH [108]. In the context of astrophysics, it is also necessary to characterize the role of PAHs in the depletion phenomenon, that is the underabundance of some elements like Fe or Si atoms from the gas phase. One hypothesis is that these elements could be trapped on PAHs [98,183,199,200]. 

We use laser photofragment spectroscopy to study the vibrational and electronic spectroscopy of ions. Our photofragment spectrometer is shown schematically in Eig. 2. Ions are formed by laser ablation of a metal rod, followed by ion molecule reactions, cool in a supersonic expansion and are accelerated into a dual TOE mass spectrometer. When they reach the reflectron, the mass-selected ions of interest are irradiated using one or more lasers operating in the infrared (IR), visible, or UV. Ions that absorb light can photodissociate, producing fragment ions that are mass analyzed and detected. Each of these steps will be discussed in more detail below, with particular emphasis on the ions of interest. 

Vibrationally mediated photodissociation (VMP) can be used to measure the vibrational spectra of small ions, such as V (OCO). Vibrationally mediated photodissociation is a double resonance technique in which a molecule first absorbs an IR photon. Vibrationally excited molecules are then selectively photodissociated following absorption of a second photon in the UV or visible [114—120]. With neutral molecules, VMP experiments are usually used to measure the spectroscopy of regions of the excited-state potential energy surface that are not Franck-Condon accessible from the ground state and to see how different vibrations affect the photodissociation dynamics. In order for VMP to work, there must be some wavelength at which vibrationally excited molecules have an electronic transition and photodissociate, while vibrationally unexcited molecules do not. In practice, this means that the ion has to have a... 

The photolysis of a hexane solution of 1 with a PF3 sparge led ultimately to pure Cp2Ti(PF3)2 (45) as expected due to the PF3 stream sweeping the photodissociated CO from the solution.3 This reaction was conveniently monitored using IR spectroscopy since the intermediate Cp2Ti(CO)(PF3)... 

This interpretation of the microwave spectrum stimulated a flurry of activity to unearth the correct equilibrium geometry. It was suggested, for example, that photoelectron spectroscopy was not inconsistent with a cyclic structure whereas iirfrared photodissociation and matrix infrared measurements suggested the two molecules are not equivalent and supported the microwave equilibrium geometry. Another set of measurements led to the notion that a tunneling motion, similar to that in the HE dimer, which interchanges the roles of proton donor and acceptor, was responsible for the two IR bands observed in the gas phase. State selection in a hexapole electric field indicated that the dimer has a small dipole moment and that it is not a symmetric top structure. ... 

Ab initio calculations gave vibrational wavenumbers for 9 isomers of CNNS.524 A CARS study of the effects of 266 nm. pulsed laser photodissociation of NCNCS showed that the vapour contained both NCNCS and CNCN.525 IR and Raman spectra, with factor group analysis, gave vibrational assignments for crystalline ammonium dicyanamide, NH4[N(CN)2].526 Variable-temperature Raman spectroscopy was used to follow the solid state transformation of NI I4[N(CN)2] into NCN=C(NH2)2.527 Ab initio and/or DFT calculations gave vibrational wavenumbers for CH2=CH-N=C=X (X = O, Se) 528 NN-C(CN)2 529 nitroso-azide, NNN-N=0, and nitro-azide, NNN-N02.530... 

The structure of molecular complexes in their electronic ground state can be obtained from direct IR laser absorption spectroscopy in pulsed supersonic-slit jet expansions [9.47]. This allows one to follow the formation rate of clusters and complexes during the adiabatic expansion [9.48]. Selective photodissociation of van der Waals clusters by infrared lasers may be used for isotope separation [9.49]. 

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