Noble gas matrices

Techniques other than UV-visible spectroscopy have been used in matrix-isolation studies of Ag see, for example, some early ESR studies by Kasai and McLeod 56). The fluorescence spectra of Ag atoms isolated in noble-gas matrices have been recorded (76,147), and found to show large Stokes shifts when optically excited via a Si j — atomic transition which is threefold split in the matrix by spin-orbit and vibronic interactions. The large Stokes shifts may be explained in terms of an excited state silver atom-matrix cage complex in this... 

However, as in soiid noble gas matrices, the real value of liquefied noble gases becomes apparent when other and more interesting ligands are attached to the transition metal. 

Emission spectra of radical cations are obtained by vacuum UV ionization and subsequent laser excitation in noble-gas matrices (see below), or by electron-impact ionization of a beam of neutral parent molecules at energies above the first ionic excited state. After internal conversion to the first excited state, emission may compete more or less successfully with radiationless deactivation. If the experiment is carried out on a supersonic molecular beam one obtains highly resolved emission spectra which, in the case of small molecules, may contain sufficient information to allow a determination of the molecular structure. 

In order to record excitation spectra, the radical ions must first be thermalized to the electronic ground state, which happens automatically if they are created in condensed phase (e.g. in noble-gas matrices, see below). In the gas-phase experiments where ionization is effected by collision with excited argon atoms (Penning ionization), the unexcited argon atoms serve as a heat bath which may even be cooled to 77 K if desired. After thermalization, excitation spectra may be obtained by laser-induced fluorescence. 

Compounds Complexation of the CuO Molecule by Ar, Kr, and Xe Atoms in Noble Gas Matrices. 

Fluorescence was observed for the TMB family such as 3,5-dimethoxyphenol, l,3-dihydroxy-5-methoxybenzene (5-methoxyresorcinol), l-acetoxy-3,5-dimethoxyben-zene, and l,3,5-trimethoxy-2-methylbenzene. These results indicated that complete symmetry of the substitution on O atoms is not necessary to observe fluorescence from the TMB family, and that the variation of parent molecules of fluorescent radical cation is possibly performed [153]. Fluorescence was also detected from hexamethxybenzene as an example of pseudo-Dgh molecules. The discussion of the symmetry has been described here on the fluorescence from fluorobenzenes in the vapor-phase or noble gas matrices. 

A fourth, often overlooked problem is most prominent in noble gas matrices which are notoriously poor heat sinks because only very low energy lattice phonons are available to accept molecular vibrational quanta. Hence, thermalization is very slow compared to solution, and the excess energy that may be imparted onto an incipient reactive intermediate in the process of its formation (e.g., from a precursor excited state) may therefore be dissipated in secondary chemical processes such as rearrangments or fragmentations, which may make it impossible to generate the primary reactive intermediate. Often, this problem can be alleviated by attaching alkyl groups that serve as internal heat sinks, but sometimes this is not acceptable for other reasons. 

However, there are some exceptions. One of them is the possibility of (photo)-protonation or -deprotonation. If a matrix is doped with sufficient amounts of a proton donor or acceptor, chances are that the substrate will give up or accept a proton already on cocondensation or on subsequent photoexcitation. In fact, the higher noble gases (Ar, Kr, Xe) are themselves good proton acceptors, forming (NG H)+ complexes that can be identified by their characteristic IR vibrations. This feature allows occasionally to observe radicals formed by deprotonation of radical cations formed in noble gas matrices, for example, benzyl radical from ionized toluene. However, we know of no examples where a carbanion was formed by deprotonation in matrices. 

The optical spectra of Ni, Pd and Pt in noble gas matrices have been measured in order to search for complexes of these Group VIII ligands. Changes in the energy levels of the matrix isolated atoms occur because of a weak metal interaction. For platinum the frequency shifts follow the trend Xe > Ar > Kr, but whether this interaction is described as a Van der Waals interaction or a weak coordinate bond is open to speculation.1976... 

Silylenes are short-lived intermediates, and their detection requires fast methods such as ultraviolet (UV)24 or laser-induced fluorescence spectroscopy.25 The characteristic absorption maxima in the UV-visible spectra of these species, which are assigned to n - p transitions of electrons at the silicon atom, were used as a fingerprint to prove the occurrence of silylenes in matrices or solution. In addition, these transient species, which under normal conditions are too short lived to be observed by a slow detection method such as infrared (IR) spectroscopy, can be isolated in inert hydrocarbon or noble gas matrices, thus allowing the accurate measurement of their IR spectra. 

McKean 182> considered the matrix shifts and lattice contributions from a classical electrostatic point of view, using a multipole expansion of the electrostatic energy to represent the vibrating molecule and applied this to the XY4 molecules trapped in noble-gas matrices. Mann and Horrocks 183) discussed the environmental effects on the IR frequencies of polyatomic molecules, using the Buckingham potential 184>, and applied it to HCN in various liquid solvents. Decius, 8S) analyzed the problem of dipolar vibrational coupling in crystals composed of molecules or molecular ions, and applied the derived theory to anisotropic Bravais lattices the case of calcite (which introduces extra complications) is treated separately. Freedman, Shalom and Kimel, 86) discussed the problem of the rotation-translation levels of a tetrahedral molecule in an octahedral cell. 

In the gas phase or isolated in noble gas matrices, they are bent with angles of 90 to 100°. The solids react to complete their octets (e.g., with donors, to produce pyramidal LGeX2 molecules), or with butadiene ... 

Complexation of CAs with dinitrogen will be discnssed in Section VII. Here, we jnst note that such complexation is probably responsible for large shifts in the UV absorption maxima of MeClSi, MeHSi and Me2Si on going from noble gas matrices to nitrogen matrices . The UV spectra of germylenes, stannylenes or plnmbylenes in nitrogen matrices were not reported. 

To find out how free a SiR3 cation would be in contact with inert solvents or even noble gas matrices, we optimized the geometries of complexes of SiH3 with methane, an "inert" aliphatic solvent model, as well as the noble gases. He, Ne, and Ar, and computed the Si chemical shifts. 

Siizer, S. and Andrews, L., FTIR spectra of ammonia clusters in noble gas matrices, J. Chem. Phys. 87,5131-5140(1987). 

In solutions or in frozen matrices the effect of the environment is not negligible any more. The NIR spectra of Cgg were measured in various frozen noble gas matrices [34] and or T)sd distortions were found. The result was the same in the apolar methylcyclohexane matrix, while the distortion was H2h in the polar 2-methyltetrahydrofuran (2-MeTHF) matrix [35]. One might expect the same polarity dependence in solutions, but electrochemically generated Cgg ions showed a... 

The hitherto elusive nitrilimine (13) (Scheme 5) has been generated in noble gas matrices by photolysis of the tetrazole (12), or 1,2,3- or 1,2,4-triazole, and also by flash pyrolysis. On further photolysis, nitrilimine is transformed into other CH2N2 isomers, including diazomethane, and a novel complex between NH and HCN. The identification of the species involved was aided by isotopic substitution and high-level ab initio calculations. 1,3-Diphenylnitrilimine, generated by photolysis or thermolysis, undergoes 1,3-dipolar cycloaddition to [60]fullerene, forming two or more adducts with A -pyrazoline structures. " ... 

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." ... 

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