Gas phase complexation

Laali and Lattimer (1989 see also Laali, 1990) observed arenediazonium ion/crown ether complexes in the gas phase by field desorption (FD) and by fast atom bombardment (FAB) mass spectrometry. The FAB-MS spectrum of benzenediazonium ion/18-crown-6 shows a 1 1 complex. In the FD spectrum, apart from the 1 1 complex, a one-cation/two-crown complex is also detected. Dicyclo-hexano-24-crown-6 appears to complex readily in the gas phase, whereas in solution this crown ether is rather poor for complexation (see earlier in this section) the presence of one or three methyl groups in the 2- or 2,4,6-positions respectively has little effect on the gas-phase complexation. With 4-nitrobenzenediazonium ion, 18-crown-6 even forms a 1 3 complex. The authors assume charge-transfer complexes such as 11.13 for all these species. There is also evidence for hydride ion transfer from the crown host within the 1 1 complex, and for either the arenediazonium ion or the aryl cation formed from it under the reaction conditions in the gas phase in tandem mass spectrometry (Laali, 1990). 

We shall show both from experimental evidence about gas-phase complexes and, to a lesser extent, from the results of electronic structure calculations that a parallel definition of the intermolecular halogen bond is appropriate The halogen bond is an attractive interaction between a halogen atom X and an atom or a group of atoms in different molecule(s), when there is evidence of bond formation. ... 

Evidence for a significant contribution from the ionic form [BX] + - -Y in a gas-phase complex B- XY was first deduced from the spectroscopic constants of H3N- -ClF, as obtained by analysis of its rotational spectrum [63]. In particular, the value ka = 34.3 N m 1 of the intermolecular stretching force constant (obtained from the centrifugal distortion constant Dj in the man-... 

The study of electron density distributions resulting from molecular interactions in gas-phase complexes or in molecular crystals, is known [1,2] to facilitate our understanding of the physical mechanisms underlying such interactions. Indeed, the action of these mechanisms is reflected in the interaction density, defined as the difference between the electron density distribution (EDD) of the molecular complex or crystal and that obtained by superimposing the EDDs of free molecules. 

FABMS has been used as a semiquantitative indication of the selectivity of receptors for particular guest metal cations (Johnstone and Rose, 1983). The FABMS competition experiment on [7] with equimolar amounts of the nitrates of sodium, potassium, rubidium and caesium gave gas-phase complex ions of ([7] + K)+ ion (m/z 809) and a minor peak ([7] + Rb)+ ion (m/z 855) exclusively. The relative peak intensities therefore suggested a selectivity order of K+ Rb+ Na+, Cs+, indicative of the bis-crown effect, the ability of bis-crown ether ligands to complex a metal cation of size larger than the cavity of a single crown ether unit, forming a sandwich structure. 

The first data on gas-phase complexes formed between strong acids and the methane molecnle were reported by Barnes in 1983 [15]. The author observed a CH4 HCI complex in a rare gas matrix that was assigned to a van der Waals system. Its C 3v structure has also been confirmed by gas-phase microwave spectroscopy [16]. Other complexes, snch as CH4 HCN, CH4 HF, and CHj HBr, were reported later [17]. Since these complexes represent C-H- H interactions, then-nature is discussed in Chapter 6. 

SbF5 > AsF5 > BF3 > PF5 Gas phase Complexing with F3NO 108... 

Najour, G. C. King, A. D. Solubility of Naphthalene in Compressed Methane, Ethylene, and Carbon Dioxide. Evidence for a Gas-Phase Complex Between Naphthalene and Carbon Dioxide. J. Chem. Phys. 1966, 45, 1915-1921. 

The magnitude of the induced dipole moment that is produced when an indole molecule in its ground Sq and electronically excited Si states is polarized by the attachment of a hydrogen-bonded water molecule in the gas-phase complex indole-H20 have been determined as /i I(So) = 0.7 D atid /t I(Si) = 0.5 D <2005JCP1743011>. The permanent dipole moment values for the complex (/tlW(So) =4.4D and /tlW(Si) = 4.0 D) are substantially different from calculated values based on vector sums of the dipole moments of the component parts. The orientation of the induced moment is also significantly different in the two electronic states. 

Pseudomolecular Ions. In contrast to the traditional MS, the highest mass peaks in ESI/APCI spectra are not always the molecular ion of interest. Instead, pseudomolecular ions, or noncovalent complex ions, are commonly observed. The pseudomolecular ions are generally formed by the analyte-adduct interaction in the solution system that is preserved as a result of the soft ionization of the ESI/APCI process. These ions are also formed by analyte-adduct gas-phase collisions in the spray chamber [49]. The exact mechanisms of how the analyte adducts are formed in ESI/APCI still remain unresolved at this point. More often than not, the adduct ion formation is a major cause for the low detection limit for ESEAPCI MS. However, these associative processes have also created interest in the study of drug-protein/ drug-oligonucleotide gas-phase complexes that benefit from the ability of ESI/APCI MS analysis. 

Alternatively, the chemistry of electronically excited metal atoms (Mn, Fe, Co, Cu, Zn, Ag, Au) has also been studied in low temperature matrices. Reaction with RH has been shown, both experimentally and computationally, to proceed primarily via an insertion mechanism [79]. Such reactions are similar to the studies presented in Section IV, involving photoexcited binary metal-molecule gas-phase complexes. 

Van der Kerk-Van Hoof, A. Heck, A.J.R. Covalent and Non-Covalent Dissociations of Gas-Phase Complexes of Avoparcin and Bacterial Receptor Mimicking Precursor Peptides Studied by Collisionally Activated Decomposition Mass Spectrometry, J. Mass Spectrom. 34, 813-819 (1999). 

Case Study 11.2 Homogeneous Gas-Phase Complex Reaction Oxidation of... 

Najour, G. C., and A. D. King, Jr. 1966. Solubility of naphthalene in compressed methane, ethylene, and carbon dioxide Evidence for a gas-phase complex between naphthalene and carbon dioxide. J. Chem. Phys. 45 1915. 

Halogen bonding also manifests itself in the relative orientations of halogen derivatives in the crystalline state [149]. Indeed, the modes of interaction in many nonhydrogen-bonded noncovalent systems, ranging from gas phase complexes to molecular crystals, can be satisfactorily rationalized in terms of molecular surface electrostatic potentials [44,55,150]. In several instances, we have used this approach to explain anomalously high measured solid densities [151,152]. 

Specific liquid-phase interactions between perhalogenated methanes and organic bases have been inferred from thermodynamic, dielectric, and spectroscopic data. Complex formation between CHF3 and tertiary amines and ethers has been established, using H n.m.r. techniques. Microwave data show that the gas-phase complex between CIF3 and Me3N has the conformation... 

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