School of Chemistry - Research Publications
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ItemElectronic spectrum and photodissociation chemistry of the linear methyl propargyl cation H2C4H3+(AMER INST PHYSICS, 2017-01-28)The electronic spectrum of the methyl propargyl cation (2-butyn-1-yl cation, H2C4H3+) is measured over the 230-270 nm range by photodissociating the bare cation and its Ar and N2 tagged complexes in a tandem mass spectrometer. The observed A'1←A'1 band system has an origin at 37 753 cm-1 for H2C4H3+, 37738 cm-1 for H2C4H3+-Ar, and 37 658 cm-1 for H2C4H3+-N2. The methyl propargyl cation photodissociates to produce either C2H3++C2H2 (protonated acetylene + acetylene) or H2C4H++H2 (protonated diacetylene + dihydrogen). Photodissociation spectra of H2C4H3+, H2C4H3+-Ar, and H2C4H3+-N2 exhibit similar vibronic structure, with a strong progression of spacing 630 cm-1 corresponding to excitation of the C-C stretch mode. Interpretation of the spectra is aided by ground and excited state calculations using time dependent density functional theory at the ωB97X-D/aug-cc-pVDZ level of theory. Ab initio calculations and master equation simulations were used to interpret the dissociation of H2C4H3+ on the ground state manifold. These calculations support the experimentally observed product branching ratios in which acetylene elimination dominates and also suggests that channel switching occurs at higher energies to favor H2 elimination.
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ItemElectronic spectrum of the propargyl cation (H2C3H+) tagged with Ne and N2(AMER INST PHYSICS, 2015-11-14)The Ã(1)A1 ← X̃(1)A1 band system of the propargyl cation (H2C3H(+)) is measured over the 230-270 nm range by photodissociation of mass-selected H2C3H(+)-Ne and H2C3H(+)-N2 complexes in a tandem mass spectrometer. The band origin occurs at 37 618 cm(-1) for H2C3H(+)-Ne and 37 703 cm(-1) for H2C3H(+)-N2. Ground and excited state ab initio calculations for H2C3H(+) using the MCSCF and coupled-cluster (CC) response methods show that the ion has C2v symmetry in the ground X̃(1)A1 and excited Ã(1)A1 states and that the strong vibronic progression with a spacing of 630 cm(-1) is due to the C-C stretch vibrational mode, ν 5.
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ItemPhotoisomerization action spectrum of retinal protonated Schiff base in the gas phase(AMER INST PHYSICS, 2014-04-28)The photophysical behaviour of the isolated retinal protonated n-butylamine Schiff base (RPSB) is investigated in the gas phase using a combination of ion mobility spectrometry and laser spectroscopy. The RPSB cations are introduced by electrospray ionisation into an ion mobility mass spectrometer where they are exposed to tunable laser radiation in the region of the S1 ← S0 transition (420-680 nm range). Four peaks are observed in the arrival time distribution of the RPSB ions. On the basis of predicted collision cross sections with nitrogen gas, the dominant peak is assigned to the all-trans isomer, whereas the subsidiary peaks are assigned to various single, double and triple cis geometric isomers. RPSB ions that absorb laser radiation undergo photoisomerization, leading to a detectable change in their drift speed. By monitoring the photoisomer signal as a function of laser wavelength an action spectrum, extending from 480 to 660 nm with a clear peak at 615 ± 5 nm, is obtained. The photoisomerization action spectrum is related to the absorption spectrum of isolated retinal RPSB molecules and should help benchmark future electronic structure calculations.
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ItemIon Mobility Unlocks the Photofragmentation Mechanism of Retinal Protonated Schiff Base(AMER CHEMICAL SOC, 2014-09-18)Retinal protonated Schiff base (RPSB) is a key molecular component of biological photoreceptors and bacterial photosynthetic structures, where its action involves photoisomerization around bonds in the polyene chain. In a vacuum environment, collisional activation or exposure to visible light causes the RPSB molecule to disintegrate, producing charged molecular fragments with m/z = 248 Da that cannot be formed by simple cleavage of the polyene chain. Photofragments resulting from laser excitation of RPSB at a wavelength of 532 nm are analyzed in an ion mobility mass spectrometer (IMMS) and found to be the protonated Schiff base of β-ionone. Density functional theory calculations at the M06-2X/cc-pVDZ level support a fragmentation mechanism in which RPSB undergoes an electrocyclization/fragmentation cascade with the production of protonated Schiff base of β-ionone and toluene.
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ItemGas-phase electronic spectrum of the indole radical cation(TAYLOR & FRANCIS LTD, 2015-08-18)