Dissociation of proton-bound dimers

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

Chen, Guodong, Cooks, R. Graham, Bunk, David M., Welch, Michael J., Christie, John R. 1999. "Partitioning of Kinetic Energy to Internal Energy in the Low Energy Dissociation of Proton-Bound Dimers of Polypeptides." International Journal of Mass Spectrometry, 185/186/187 75-90. 

An, X. Eiceman, G.A. Rasanen, R.-M. Rodriguez, J.E. Stone, J.A. Dissociation of proton bound dimers of ketones in asymmetric electric fields with differential mobility spectrometry and in uniform electric fields with ion mobility spectrometry, 2012. (submitted)... 

Another approach, proposed by McLuckey, Cameron and Cooks, that is often used when compounds M and B are of low volatility is based on the observation of the collision-induced dissociation of proton-bound dimer ions, M H+ B, formed in association reactions ... 

McLuckey SA, Cameron D and Cooks RG (1981) Proton affinities from dissociations of proton bound dimers. Journal of the American Chemical Society 103 1313-1317. 

In given concentrations of sample vapor neutrals, spectra for product ions can be altered by control of temperature, and this was seen at cryogenic tanperature, at which ion clusters not usually observed in mobility spectra were formed and appeared in mobility spectra. For example, proton-bound trimers of alcohols were observed when temperatures were decreased to -20°C and dissociated at temperatures from -20°C to +10°C. " Increases in temperatures will lead to dissociation of complex ions, such as proton-bound trimers and proton-bound dimers. As temperature is increased, the intensity of peaks for protonated monomer increase, and the peak abundance of proton-bound dimers decreases. This has been developed and explored for dimethyl methyl phosphonate (DMMP), " amines, and ketones. For example, proton-bound dimers of alkyl amines underwent dissociation above -30°C on a 2- to 20-ms time scale, which is within the range of drift times for these ions. Consequently, the dissociation pathway can be observed as a distortion in the peak shape and baseline of a mobility spectrum since an ion entering the drift region as a proton-bound dimer dissociates to a protonated monomer before arriving at the detector. These studies permitted the determination of kinetics of dissociation for thermalized ions and illustrated that the appearance of an ion in a mobility spectrum is governed by ion lifetimes in comparison to ion residence times in drift tubes, and ion lifetimes are controlled by temperature. 

Cheng, X.H., Wu, Z.C., Fenselau, C. (1993) Collision Energy-dependence of Proton-bound Dimer Dissociation - Entropy Effects, Proton Affinities, and Intramolecular Hydrogen-bonding in Protonated Peptides. J. Am. Chem. Soc. 115 4844-4848. 

Yang B, Wu RR, Berden G, Oomens J, RodgCTs MT (2013) Infrared multiple photon dissociation action spectroscopy of proton-bound dimers of cytosine and modified cytosines effects of modifications on gas-phase conformatitms. J Phys Chem B 117 14191-14201... 

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. 

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). 

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... 

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. 

One of the best-recognized quantitative patterns in mobility spectra is the relationship between protonated monomers and proton-bound dimers, common to several types of compounds, including esters, ketones, alcohols, amines, and organophosphorus compounds. As the vapor concentration of the analyte M is increased in an ion source, a protonated monomer peak first appears with a corresponding decrease in the reactant ion peak intensity. The reaction yields an energetically excited intermediate adduct ion or transition state ([MH+(H20) ] ) that may dissociate back to the reactants or may form product ions through other reaction pathways. Such reaction pathways involve a third body Z to stabilize the products, as shown in Equation 11.4 ... 

An, X. Stone, J.A. Eiceman, G.A., A determination of the effective temperatures for the dissociation of the proton bound dimer of dimethyl methylphosphonate in a planar differential mobility spectrometer, Int. J. Ion Mobil. Spectrom. 2010, 13, 25-36... 

Rate constants for the first-order dissociation of symmetrical proton-bound dimers, M H + —> MH + + M, have been determined for organophosphorus compounds (M = 2,4-dimethylpyridine (DMP) and dimethyl methylphosphonate (DMMP)), where the shapes of the mobility spectra are of the form shown in Figure 13.2d [44]. Some proton-bound dimers decompose in the time taken for the ions to travel between the shutter and the detector plate, and this residence time was varied by changing the electrostatic drift field strength. Typical ion mobility spectra obtained at different field strengths are shown in Figure 13.5 and peaks were mass identified as first peak, H + (DMP), the protonated monomer and second peak H (DMP)2, the proton-bound dimer. The raised baseline between the peaks was due entirely to (DMP)H +, from the decomposition of the proton-bonnd dimer as in Equation 13.25... 

The kinetic method is an approximate scheme to determine relative thermochemical properties based on the rates of competitive dissociation of mass-selected cluster ions. As an example, consider the proton-bound dimer system ... 

Rajabi K, Pridgen TD (2008) Structures of aliphatic amino acid proton-bound dimers by infrared multiple photon dissociation spectroscopy in the 700-2000 cm(—1) region. J Phys Chem A 112 23-30... 

Oomens J, Moehlig AR, Morton TH (2010) Infrared multiple photon dissociation (IRMPD) spectroscopy of the proton-bound dimer of 1-methylcytosine in the gas phase. J Phys Chem Lett 1 2891-2897... 

Dissociation rate constants of ions at atmospheric pressure have been obtained by ion mobility spectrometry. The time frame of such experiments is in the millisecond range. Dissociation rates of proton-bound amine dimer ions, for example, have been measured recently. 

In the technique called blackbody infrared radiation dissociation (BIRD), developed in E.R. Williams laboratory, ions trapped in an ICR cell at very low pressure absorb infrared photons emitted by the cell walls. Dissociation decays in the second to minute time frame can be followed. In both techniques, the analysis of the temperature dependence of the rate constant leads to activation parameters. BIRD has been applied to biologically important non-covalent complexes like proton-bound dimers of amino acids or double-stranded DNA complexes. 

A. Thermal Unimolecular Dissociation of the Proton-Bound Methoxide Dimer... 

In order to better understand the detailed dynamics of this system, an investigation of the unimolecular dissociation of the proton-bound methoxide dimer was undertaken. The data are readily obtained from high-pressure mass spectrometric determinations of the temperature dependence of the association equilibrium constant, coupled with measurements of the temperature dependence of the bimolecular rate constant for formation of the association adduct. These latter measurements have been shown previously to be an excellent method for elucidating the details of potential energy surfaces that have intermediate barriers near the energy of separated reactants. The interpretation of the bimolecular rate data in terms of reaction scheme (3) is most revealing. Application of the steady-state approximation to the chemically activated intermediate, [(CH30)2lT"], shows that. 

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