Proton-bound dimer

Collision-induced dissociation mass spectrum of tire proton-bound dimer of isopropanol [(CH2)2CHOH]2H. The mJz 121 ions were first isolated in the trap, followed by resonant excitation of their trajectories to produce CID. Fragment ions include water loss mJz 103), loss of isopropanol mJz 61) and loss of 42 anui mJz 79). (b) Ion-molecule reactions in an ion trap. In this example the mJz 103 ion was first isolated and then resonantly excited in the trap. Endothennic reaction with water inside the trap produces the proton-bound cluster at mJz 121, while CID produces the fragment with mJz 61. 

Similarly, Ervin and co-workers have measured acidities of organic molecules by measuring the energy for endothermic proton transfer reactions between acids and anionic bases." " Alternatively, it is possible to use competitive CID of proton-bound dimer ions." Nominally, these are relative approaches for measuring acidities, as the measured acidities depend on the properties of the reference acids or bases. However, it is usually possible to select references with very accurately known acidities (such as HE, HCN, or HCl), such that the accuracy of the final measurement depends predominantly on the accuracy of the threshold energy determination. 

T. D. Frigden, L. MacAleese, T. B. McMahon, J. Lemaire, and P. Maitre, Gas phase infrared multiple photon dissociation spectra of methanol, ethanol and propanol proton bound dimers, protonated propanol and the propanol/water proton bound dimer. Phys. Chem. Chem. Phys. 8, 955 966 (2006). 

The detection of chiral recognition with a mass spectrometer was reported first in 1977 by Fales and Wright. Their study showed that the chirality of dialkytartrates (T) strongly influences the stability of their diastereomeric proton-bound dimers,... 

More comprehensive CIMS investigations on tartrate systems indicate that the dimer chirality effects disappear when the ester functions of tartrates is replaced by H or an alkyl function, e.g., methyl or cyclohexyl. A similar effect is observed when the proton in the proton-bound dimers is replaced by lithium or ammonium ion. These observations are attributed to a dramatic change in the basket-type... 

PA = 226 kcal moP ), the predominant formation (6.4 to 1) of the (7 ,5 )-di-2-butyl ether over the (R,R) and (5, 5 )-forms is attributed to a simple backside displacement in the proton-bound adduct of the starting 2-butanol enantiomer with inversion of configuration of the reaction site and loss of a molecule of water. When tri-n-propylamine is replaced by the less basic NH3 (PA = 196 kcal moF ), fast neutralization of the proton-bound dimers of the starting 2-butanol is prevented and, therefore, they can grow, producing aggregates that resemble solution microenvironments in which SnI pathways may be accessible as well. In them or in their primary substituted derivatives, consecutive nucleophilic displacements may take place. As a consequence, the stereospecificity of the process is lost and the [(R,S)-di-2-butyl ether]/[(7 ,7 )- and (5, 5 )-di-2-butyl ethers] ratio falls down to 1.2. In this case. 

Dissociation of Proton-Bound Dimers (77JA1279 81JA13I3 83MI3)... 

In this method, proton-bound dimers, such as B,.H - Bz and... 

Contrary to the case of anionic reactions, the formation of a strong proton-bound dimer for alcohols suggests that nucleophilic displacement may actually involve a frontside attack. Recent experiments carried out at atmospheric pressure by Speranza and Angelini (1980) using radiolytic techniques with isolation and glc analysis of neutral products reveal some interesting stereochemistry. For example, the reaction of protonated epoxy-rra/is-but-2-ene with H20 results in 98% inversion of configuration, while a similar reaction with cis-1 -chloro-4-methylcyclohexane results in approximately 80% of tro/is-4-methylcyclohexanol. With the high pressures utilized and with the possible participation of cluster ions a likelihood in this case, the data are consistent with a Walden inversion for these cases. 

Minkwitz and Schneider54 have been able to obtain protonated methanol in the form of hexafluorometalate salts (MeOH2+MF6, M = As, Sb), and these were characterized by X-ray diffraction. Protonated propanol and proton-bound dimers of methanol, ethanol, and propanol with the proton shared between two alcohol molecules (5) have also been characterized by infrared multiphoton dissociation... 

Some of the initial work dealt with the formation of proton-bound dimers in simple amines. Those systems were chosen because the only reaction that occurs is clustering. A simple energy transfer mechanism was proposed by Moet-Ner and Field (1975), and RRKM calculations performed by Olmstead et al. (1977) and Jasinski et al. (1979) seemed to fit the data well. Later, phase space theory was applied (Bass et al. 1979). In applying phase space theory, it is usually assumed that the energy transfer mechanism of reaction (2 ) is valid and that the collisional rate coefficients kx and fc can be calculated from Langevin or ADO theory and equilibrium constants. 

Fig. 18. Overlay of plots from DMS-IMS2 response to several nitrotoluenes [2-mononitrotoluene (2-MNT), 3-MNT, 2,4-dinitrotoluene (2,4-DNT), 3,4-DNT, 2,4,6-trinitrotoluene (TNT)] in positive polarity (left). A reactant ion peak (RIP) is seen at 6.5 V and 1.9 ms, whereas protonated monomers are seen near 3 V and 2.2ms, and proton-bound dimers are at 0V and 3.5ms. TNT does not show a response in positive polarity. Overlay of plots from DMS-IMS2 response to several nitrotoluenes (cf. list in positive polarity) in negative polarity (right). A RIN is seen at 7 V and 1.8 ms, whereas molecular adducts or charge-exchanged ions are seen at 0.5 V and 2.5 ms. MNTs do not show a response in negative polarity. Source Neil D. Paz, NMSU...

The cross-relation (equation (1.6)), has also been applied successfully to transfers of CH3 [31] and to transfers of H" [32-33], while equation (1.4) has been used to treat proton transfers [34] and proton bound dimers [34d]. As already noted, the intersecting parabolas of Fig. 1.3 would not be applicable and so some other treatment was needed to understand the... 

Hydrogen-bonded complexes, including proton-bound dimers, are well-known species. 

Proton affinity determination by the kinetic method, (a) This method is based on competitive dissociation of heterodimer clusters, (b) Potential energy diagram for proton-bound dimer dissociation. 

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