Conformational Isomerization of 5-Phenyl-1-pentene Probed by SEP-Population Transfer Spectroscopy

Pillsbury, Nathan R.; Zwier, Timothy S.

doi:10.1021/jp806699e

Cited by 5 publications

(6 citation statements)

References 45 publications

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“…There are several important extensions of this work remaining to be carried out. First, SEP-population transfer experiments ,,,, could be used to measure the barriers to isomerization in DPOE and DPE. If successful, such measurements could also provide bounds on the relative energies of the ttt and tgt conformational minima.…”

Section: Discussionmentioning

confidence: 99%

Ground State Conformational Preferences and CH Stretch–Bend Coupling in a Model Alkoxy Chain: 1,2-Diphenoxyethane

2013

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1,2-Diphenoxyethane (C6H5-O-CH2-CH2-O-C6H5, DPOE) is a flexible bichromophore in which the two phenyl rings are separated from one another by an -O-CH2-CH2-O- chain with five flexible dihedral angles about which hindered rotation can occur. As such, it is a phenyl capped analog of dimethoxyethane (DMOE), which has served as a model compound for development of force fields for polyethylene glycol (PEG). The ground state conformational energy landscape of DPOE is explored using a combination of single-conformation spectroscopy of the jet-cooled molecule and calculations of the conformational minima and transition states. In the experimental UV spectrum, ultraviolet hole-burning establishes the presence of just two conformations with significant population in the supersonic jet expansion. Fluorescence dip infrared (FDIR) spectroscopy is used to record infrared spectra of the two conformers in the alkyl CH stretch, CH bend, and CO stretch regions. When compared with harmonic vibrational frequency calculations, the two isomers are determined to be of C2h and C2 symmetry, and labeled ttt and tgt to denote the three central dihedrals as trans or gauche. Infrared population transfer spectroscopy is used to determine fractional abundances for the two conformers (f(ttt) = 0.53 ± 0.01; f(tgt) = 0.47 ± 0.01). Relaxed potential energy curves along the three nonequivalent dihedral angles are used to map out the shape of the potential energy landscape that leads to these preferences. The Fermi resonance in the alkyl CH stretch spectrum is successfully modeled using a recently developed methodology [Buchanan et al., J. Chem. Phys. 2013, 138, 064308] employing a reduced dimension Hamiltonian. The scissor overtones couple to the CH2 symmetric stretch and only indirectly to the asymmetric stretch through symmetric stretch/asymmetric stretch coupling. The presence of the oxygen atoms in the chain shifts the CH scissor overtones to higher frequencies than in pure alkyl chains, qualitatively changing the spectral consequences of the Fermi resonance, with the scissor overtones now appearing as the highest frequency bands in the spectrum. The spectra are contrasted with those in 1,2-diphenylethane, a close analog with a very different appearance to its CH stretch spectrum, in which the scissor overtones appear as the lowest frequency bands.

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Section: Discussionmentioning

confidence: 99%

Ground State Conformational Preferences and CH Stretch–Bend Coupling in a Model Alkoxy Chain: 1,2-Diphenoxyethane

2013

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“…Conformation-specific spectroscopy also can provide an important foundation for studies of isomerization or photodissociation dynamics. 13,[30][31][32][33][34][35][36][37] Having identified unique absorptions due to particular conformations, it is then possible to excite a single-conformer ''reactant'' with the IR laser, giving it sufficient energy to isomerize or dissociate. Here, we illustrate the kinds of questions that can be addressed by considering IR photodissociation of the Ac-g 2 -hPhe-NHMe-H 2 O complexes.…”

Section: Ir-population Transfer Photodissociation Studiesmentioning

confidence: 99%

“…Single-conformation spectroscopy also provides a foundation for conformationspecific studies of isomerization dynamics, 13,[30][31][32][33][34][35][36][37] initiated by selective excitation of a single conformer using IR excitation or stimulated emission pumping (SEP). These methods can determine the fractional abundances of the conformers, isomerization product quantum yields, and provide upper and lower bounds on the energy barriers separating specific conformational minima on the multi-dimensional potential energy surface.…”

Section: Introductionmentioning

confidence: 99%

Single-conformation spectroscopy and population analysis of model γ-peptides: New tests of amide stacking

et al. 2011

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Single-conformation ultraviolet and infrared spectra of a series of model gamma-peptides are reported, with the goal of providing new tests of amide stacking as an amide-amide binding motif. The data also serve to illustrate the power and challenges of carrying out single-conformation spectroscopy of neutral molecules of this size in the gas phase under jet-cooled conditions. Building on recent work on Ac-γ2-hPhe-NHMe (James et al., J. Am. Chem. Soc., 2009, 131, 14243), the effects of derivatization and H2O complexation on amide stacking are studied. Ac-γ2-hPhe-N(Me)2 shows only amide stacked structures, blocking the competing position for formation of an amide-amide H-bond. The Ac-γ2-hPhe-NHMe-H2O complex includes structures in which the H2O molecule forms a bridge between the two stacked amide planes, retaining and enhancing amide stacking. IR population transfer methods are also employed to study the dynamics of photodissociation of the amide stacked-H2O complex. Finally, IR ion-gain spectroscopy is introduced as a means of recording infrared spectra containing contributions from all conformers present, based on IR-induced broadening of the UV absorptions. Its role in estimating fractional abundances is tested on Ac-γ2-hPhe-NHMe.

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“…Several experimental gas-phase techniques have been introduced that have begun to address some of these challenges. Isomer selection of cold, argon-tagged ions and subsequent measurement of single-isomer infrared photodissociation spectra has been demonstrated, , and measurement of isomerization barriers in neutral molecules has been achieved. , IR/UV ion-dip spectroscopy, in which changes in isomer-specific ultraviolet laser-induced photodissociation are measured after irradiation with tunable infrared radiation, has also been used to obtain isomer-specific vibrational spectra of ions. , Isomer populations are not typically obtained using this method in part owing to the difficulties of quantifying isomer populations based on band intensities alone. We recently showed that IRPD kinetics measured using a simple table-top laser system coupled to a Fourier transform ion cyclotron resonance (FT/ICR) mass spectrometer can be used to probe the relative populations of ion isomers at photon energies resonant with only one isomer population. , For ions with two spectroscopically distinguishable ion populations, resonant and nonresonant ion populations dissociate with different first-order rate constants, resulting in biexponential kinetics that can be fitted to obtain relative isomer populations from the pre-exponential factors.…”

Section: Introductionmentioning

confidence: 99%

Isomer Population Analysis of Gaseous Ions From Infrared Multiple Photon Dissociation Kinetics

et al. 2011

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Infrared multiple photon dissociation (IRMPD) kinetics measured with tunable laser radiation from a free electron laser (FEL) are used to probe the relative populations of and interconversions between energetically competitive isomers of gas-phase ions at 298 K. On-resonance IRMPD kinetics of monoisomeric benzoate anion and anilinium (protonated aniline) are measured to determine the extent of overlap of the laser beam with the precursor ion population (∼93%). IRMPD kinetics indicating different photodissociation behavior for different isomers obtained at isomer-specific resonances are used to determine relative populations of salt bridge and charge-solvated isomers for ArgGly·Na(+), Ser·Cs(+), and Arg·Na(+). These values and Gibbs free energy differences obtained from them for thermal precursor populations are compared to values reported using other, less direct population probes. Rapid interconversion of two charge-solvated isomers occurs for ArgGly·Li(+), precluding population analysis for this ion. ArgGly·Na(+), ArgGly·Li(+), and Arg·Na(+) exhibit IRMPD induction periods lasting many seconds for some isomers at the laser photon energies and power used, indicating that IRMPD relative spectral intensities are time-dependent for these ions and that the relative band intensities in IRMPD spectra measured with short irradiation times may not provide meaningful information about relative isomer populations. These results constitute the first direct probe of ion isomer populations using IRMPD kinetics obtained with a FEL and illustrate a number of caveats in interpreting IRMPD spectra measured with just a single irradiation time. These results also indicate that more complete overlap of the laser beam with the ions will be highly advantageous in future instrument designs for IRMPD kinetics and spectroscopy experiments.

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Conformational Isomerization of 5-Phenyl-1-pentene Probed by SEP-Population Transfer Spectroscopy

Cited by 5 publications

References 45 publications

Ground State Conformational Preferences and CH Stretch–Bend Coupling in a Model Alkoxy Chain: 1,2-Diphenoxyethane

Ground State Conformational Preferences and CH Stretch–Bend Coupling in a Model Alkoxy Chain: 1,2-Diphenoxyethane

Single-conformation spectroscopy and population analysis of model γ-peptides: New tests of amide stacking

Isomer Population Analysis of Gaseous Ions From Infrared Multiple Photon Dissociation Kinetics

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