Richard Ochran

Effects of ground loop currents on signal intensities in electrospray mass spectrometry

Journal of the American Society for Mass Spectrometry, Dec 1, 2004

Unimolecular Reactions of Proton-Bound Cluster Ions: Competition between Dissociation and Isomerization in the Ethanol−Acetonitrile Dimer

Journal of Physical Chemistry A, Aug 24, 2000

The proton-bound dimer of acetonitrile and ethanol, (CH 3 CN)(CH 3 CH 2 OH)H + , exhibits three u... more The proton-bound dimer of acetonitrile and ethanol, (CH 3 CN)(CH 3 CH 2 OH)H + , exhibits three unimolecular reactions on the microsecond time scale: two simple bond cleavage reactions to form CH 3 CNH + + CH 3 CH 2 OH and CH 3 CH 2 OH 2 + + CH 3 CN, and the loss of water to form CH 3 CNCH 2 CH 3 + . The latter process is preceded by the isomerization of the proton-bound dimer to a second isomer, (CH 3 CNCH 2 CH 3 )(H 2 O) + . The competition between the simple dissociation reactions and the isomerization reaction was modeled with ab initio calculations and RRKM theory to obtain relative energies for the reaction surface. The 0 K binding energy of the (CH 3 CN)(CH 3 CH 2 OH)H + complex was calculated to be 152 kJ mol -1 at the G2(MP2,SVP) level of theory (relative to the dissociation products CH 3 CNH + and CH 3 CH 2 OH). The isomerization barrier for the proton-bound dimer was estimated to be 22 kJ mol -1 lower than CH 3 CNH + + CH 3 CH 2 OH. The greater polarizability of the ethyl group is believed to stabilize this transition structure over that found for the (CH 3 CN)(CH 3 OH)H + ion (which was previously estimated to lie 6 kJ mol -1 below CH 3 CNH + and CH 3 OH).

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SN2 reactions with allylic substrates—Trends in reactivity

International Journal of Mass Spectrometry, Sep 1, 2007

ABSTRACT

How does chlorine substitution on acetonitrile affect the internal SN2 isomerization of proton-bound pairs (ClCH2CN)(ROH)H+ (R=CH3, C2H5, C3H7)?

International Journal of Mass Spectrometry, Jul 1, 2003

The unimolecular reactions of proton-bound pairs of chloroacetonitrile and a variety of alcohols ... more The unimolecular reactions of proton-bound pairs of chloroacetonitrile and a variety of alcohols (methanol, ethanol, n- and i-propanol) were studied in order to probe the impact of chloro-substitution on both the internal SN2 rearrangement leading to dehydration and simple hydrogen-bond cleavage reactions. Chloro-substitution on acetonitrile does not significantly affect the height of the highest energy isomerization barrier which governs the

Stability of Flavin Semiquinones in the Gas Phase: The Electron Affinity, Proton Affinity, and Hydrogen Atom Affinity of Lumiflavin

Journal of Physical Chemistry A, Oct 29, 2013

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Experimental and theoretical studies of unimolecular reactions of gas phase cluster ions

The work presented in the following pages is the culmination of four years of research in the are... more The work presented in the following pages is the culmination of four years of research in the area of gas phase ion chemistry. During this period mass spectrometry and ab initio molecular orbital calculations were employed to investigate the unimolecular decomposition of proton-bound complexes between acetonitrile and methanol, ethanol, n-propanol, i-propanol, n-butanol, s-butanol, i-butanol, and t-butanol. Common to these systems, is a competition between dissociation of the hydrogen bond in the proton-bound dieter and isomerization to (CH3CNR)(H2O)+ (R=CH3, CH2CH3, CH2CH2CH3, CH(CH3)2, butyl). The minimum energy reaction pathways for the isomerization in these systems are presented and compared. The dominant isomerization pathway for these ions is an internal SN2 reaction that proceeds via an intermediate CH3CN &cdots; ROH2+ ion (R=CH3, CH2CH 3, CH2CH2CH3, CH(CH3) 2). The mass spectra for the four butanol containing dimers (n-, s-, i-, and t-butanol) show similar behaviour. The effect of chlorine substitution on the acetonitrile on the four systems methanol, ethanol, n- and i-propanol were also investigated. The theoretical and experimental studies reveal a potential energy surface which is very similar to that obtained for the CH 3CN containing analogues (CH3CN)(CH3OH)H +, (CH3CN)(CH3CH2OH)H+, (CH3CN)(CH3CH2CH2OH)H+ and (CH3CN)((CH3)2CHOH)H +. The effect of chloro-substitution of the acetonitrile does not significantly affect the height of the rate limiting isomerization barrier which governs the water loss channel. The chloro-substitution however, lowers the proton affinity of the chloroacetonitrile and hence where there is competition between simple cleavage and isomerization, the protonated alcohol outcompetes the protonated chloroacetonitrile in the MI mass spectra. The dimer ion of acetonitrile and oxygen exhibit three peaks at m/z 32 (O 2), m/z 41, (CH3CN and ·CH2CNH +) and m/z 56 (-OH) in its MI mass spectrum. When O2 (triplet ground state) encounters a CH3CN+· or ·CH2CNH+ ion, the resulting complex [...]

FOCUS :E LECTROCHEMISTRY COMBINED WITH MS Effects of Ground Loop Currents on Signal Intensities in Electrospray Mass Spectrometry

The occurrence of electrochemical processes during the operation of an electrospray ionization (E... more The occurrence of electrochemical processes during the operation of an electrospray ionization (ESI) source is well established. In the positive ion mode, electrons are drawn from the ESI metal capillary to a high voltage power supply. These electrons are the product of chargebalancing oxidation reactions taking place at the liquid/metal interface of the ion source. In a recent study, (Anal. Chem. 2001, 73, 4836 – 4844), our group has shown that the introduction of a ground loop can dramatically enhance the rate of these oxidation processes. Such a ground loop can be introduced by connecting the sample infusion syringe (or the liquid chromatography column, in the case of LC-MS studies) to ground. The magnitude of the ground loop current can be controlled by the electrolyte concentration in the analyte solution, and by the dimensions of the capillary connecting the syringe needle and the ESI source. Using ferrocene as a model system, it is demonstrated that the introduction of such a...

Experimental and theoretical studies of unimolecular reactions of gas phase cluster ions

The work presented in the following pages is the culmination of four years of research in the are... more The work presented in the following pages is the culmination of four years of research in the area of gas phase ion chemistry. During this period mass spectrometry and ab initio molecular orbital calculations were employed to investigate the unimolecular decomposition of proton-bound complexes between acetonitrile and methanol, ethanol, n-propanol, i-propanol, n-butanol, s-butanol, i-butanol, and t-butanol. Common to these systems, is a competition between dissociation of the hydrogen bond in the proton-bound dieter and isomerization to (CH3CNR)(H2O)+ (R=CH3, CH2CH3, CH2CH2CH3, CH(CH3)2, butyl). The minimum energy reaction pathways for the isomerization in these systems are presented and compared. The dominant isomerization pathway for these ions is an internal SN2 reaction that proceeds via an intermediate CH3CN &cdots; ROH2+ ion (R=CH3, CH2CH 3, CH2CH2CH3, CH(CH3) 2). The mass spectra for the four butanol containing dimers (n-, s-, i-, and t-butanol) show similar behaviour. The effect of chlorine substitution on the acetonitrile on the four systems methanol, ethanol, n- and i-propanol were also investigated. The theoretical and experimental studies reveal a potential energy surface which is very similar to that obtained for the CH 3CN containing analogues (CH3CN)(CH3OH)H +, (CH3CN)(CH3CH2OH)H+, (CH3CN)(CH3CH2CH2OH)H+ and (CH3CN)((CH3)2CHOH)H +. The effect of chloro-substitution of the acetonitrile does not significantly affect the height of the rate limiting isomerization barrier which governs the water loss channel. The chloro-substitution however, lowers the proton affinity of the chloroacetonitrile and hence where there is competition between simple cleavage and isomerization, the protonated alcohol outcompetes the protonated chloroacetonitrile in the MI mass spectra. The dimer ion of acetonitrile and oxygen exhibit three peaks at m/z 32 (O 2), m/z 41, (CH3CN and ·CH2CNH +) and m/z 56 (-OH) in its MI mass spectrum. When O2 (triplet ground state) encounters a CH3CN+· or ·CH2CNH+ ion, the resulting complex [...]

Physicochemical characteristics of castor oil from local wild castor plant in Ghana

Ghana Journal of Science, 2006

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Unimolecular Reactions of Proton-Bound Cluster Ions: Competition between Dissociation and Isomerization in the Methanol−Acetonitrile Dimer

The Journal of Physical Chemistry A, 1999

The proton-bound dimer of acetonitrile and ethanol, (CH 3 CN)(CH 3 CH 2 OH)H + , exhibits three u... more The proton-bound dimer of acetonitrile and ethanol, (CH 3 CN)(CH 3 CH 2 OH)H + , exhibits three unimolecular reactions on the microsecond time scale: two simple bond cleavage reactions to form CH 3 CNH + + CH 3 CH 2 OH and CH 3 CH 2 OH 2 + + CH 3 CN, and the loss of water to form CH 3 CNCH 2 CH 3 + . The latter process is preceded by the isomerization of the proton-bound dimer to a second isomer, (CH 3 CNCH 2 CH 3 )(H 2 O) + . The competition between the simple dissociation reactions and the isomerization reaction was modeled with ab initio calculations and RRKM theory to obtain relative energies for the reaction surface. The 0 K binding energy of the (CH 3 CN)(CH 3 CH 2 OH)H + complex was calculated to be 152 kJ mol -1 at the G2(MP2,SVP) level of theory (relative to the dissociation products CH 3 CNH + and CH 3 CH 2 OH). The isomerization barrier for the proton-bound dimer was estimated to be 22 kJ mol -1 lower than CH 3 CNH + + CH 3 CH 2 OH. The greater polarizability of the ethyl group is believed to stabilize this transition structure over that found for the (CH 3 CN)(CH 3 OH)H + ion (which was previously estimated to lie 6 kJ mol -1 below CH 3 CNH + and CH 3 OH).

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Effects of ground loop currents on signal intensities in electrospray mass spectrometry

Journal of the American Society for Mass Spectrometry, 2004

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SN2 reactions with allylic substrates—Trends in reactivity

International Journal of Mass Spectrometry, 2007

ABSTRACT

How does chlorine substitution on acetonitrile affect the internal SN2 isomerization of proton-bound pairs (ClCH2CN)(ROH)H+ (R=CH3, C2H5, C3H7)?

International Journal of Mass Spectrometry, 2003

The unimolecular reactions of proton-bound pairs of chloroacetonitrile and a variety of alcohols ... more The unimolecular reactions of proton-bound pairs of chloroacetonitrile and a variety of alcohols (methanol, ethanol, n- and i-propanol) were studied in order to probe the impact of chloro-substitution on both the internal SN2 rearrangement leading to dehydration and simple hydrogen-bond cleavage reactions. Chloro-substitution on acetonitrile does not significantly affect the height of the highest energy isomerization barrier which governs the

Stability of Flavin Semiquinones in the Gas Phase: The Electron Affinity, Proton Affinity, and Hydrogen Atom Affinity of Lumiflavin

The Journal of Physical Chemistry A, 2013

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Swelling of coal particles irradiated with well-characterized laser pulses

Fuel, May 1, 1994

Abstract The unique ability of the laser to simulate the flame heating fluxes in conventional and... more Abstract The unique ability of the laser to simulate the flame heating fluxes in conventional and high-intensity combustors was applied to the pyrolysis of single particles of hvA bituminous Pittsburgh No. 8 coal inside an electrodynamic balance (EDB) apparatus. The apparatus together with its associated measuring system provided a continuous and very effective analysis of the swelling behaviour of the coal particles. Evolution of heavy volatiles (condensable) during the heating process was time-resolved using high-speed (5000 frames per second) cinematography. An imaging system capable of analysing images from both a microscope and a video cassette player input was used to study the particle size changes. The swelling behaviour under these heating conditions appeared to be bimodal. Some of the particles swelled relatively little, with a maximum swelling ratio of 1.3, but for the majority the maximum swelling ratio was between 1.65 and 1.8. Neither the heating flux intensity nor the initial particle diameter appeared to influence the final swelling ratio significantly.

Swelling of coal particles irradiated with well-characterized laser pulses

Fuel, 1994

Abstract The unique ability of the laser to simulate the flame heating fluxes in conventional and... more Abstract The unique ability of the laser to simulate the flame heating fluxes in conventional and high-intensity combustors was applied to the pyrolysis of single particles of hvA bituminous Pittsburgh No. 8 coal inside an electrodynamic balance (EDB) apparatus. The apparatus together with its associated measuring system provided a continuous and very effective analysis of the swelling behaviour of the coal particles. Evolution of heavy volatiles (condensable) during the heating process was time-resolved using high-speed (5000 frames per second) cinematography. An imaging system capable of analysing images from both a microscope and a video cassette player input was used to study the particle size changes. The swelling behaviour under these heating conditions appeared to be bimodal. Some of the particles swelled relatively little, with a maximum swelling ratio of 1.3, but for the majority the maximum swelling ratio was between 1.65 and 1.8. Neither the heating flux intensity nor the initial particle diameter appeared to influence the final swelling ratio significantly.

Ion Rearrangement at the Beginning of Cluster Formation: Methyl Substitution Effects on the Internal SN2 Reaction in the Proton-Bound Dimers of Acetonitrile and Alcohols

European Journal of Mass Spectrometry, 2001

Mass spectrometry and ab initio calculations have been employed to investigate the unimolecular d... more Mass spectrometry and ab initio calculations have been employed to investigate the unimolecular decompositions of proton-bound dimers consisting of acetonitrile with n- and i-propanol. Common to both systems is a competition between dissociation of the hydrogen bond in the proton-bound dimer and isomerization to (CH3CNR)(H2O)+ [R = CH2CH2CH3 and CH(CH3)2]. The minimum-energy-reaction pathways for the isomerization in these two systems, as well as those for the dimers containing methanol and ethanol, are presented and compared. The dominant isomerization pathway for these ions is an internal SN2 reaction that proceeds via a stable intermediate CH3CN•••ROH2+ ion [R = CH3, CH2CH3, CH2CH2CH3 and CH(CH3)2]. The mass spectra for the four butanol-containing dimers ( n-, s-, i- and t-butanol) follow similar behavior.

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