Spectral Signatures of Proton-Transfer Dynamics ... - ACS Publications

Subscriber access provided by UNIVERSITY OF TOLEDO LIBRARIES

Spectroscopy and Photochemistry; General Theory

Spectral Signatures of Proton-Transfer Dynamics at the Cusp of Low-Barrier Hydrogen Bonding Zachary Nelson Vealey, Lidor Foguel, and Patrick Henry Vaccaro J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.8b02199 • Publication Date (Web): 13 Aug 2018 Downloaded from http://pubs.acs.org on August 14, 2018

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 19 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

The Journal of Physical Chemistry Letters

Spectral Signatures of Proton-Transfer Dynamics at the Cusp of Low-Barrier Hydrogen Bonding Zachary N. Vealey, Lidor Foguel, and Patrick H. Vaccaro* Department of Chemistry, Yale University, New Haven, CT 06520-8107 USA

ABSTRACT: Despite their importance in diverse chemical and biochemical processes, lowbarrier hydrogen bonds remain elusive targets to classify and interpret spectroscopically. Here the correlated nature of hydrogen bonding and proton transfer in the low-barrier regime has been probed for the ground and excited electronic states of 6-hydroxy-2-formylfulvene by acquiring jet-cooled fluorescence spectra of the parent and monodeuterated isotopologs. While excitedstate profiles reveal regular vibronic patterns devoid of obvious dynamical signatures, their ground-state counterparts display a radically altered energy landscape characterized by spectral bifurcations comparable in magnitude to typical vibrational spacings ( > 100 cm −1 ). Quantitative analyses yield unusual deuterium kinetic isotope effects that straddle limiting values attributed to above-barrier vibration and below-barrier tunneling of the proton adjoining donor/acceptor sites. Our findings provide compelling experimental evidence for ultrafast hydron-migration events commensurate with the onset of low-barrier hydrogen bonding and afford a trenchant glimpse of molecular phenomena taking place at the “tipping point” between disparate dynamical regimes.

ACS Paragon Plus Environment 1

The Journal of Physical Chemistry Letters 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 2 of 19

TOC GRAPHIC

Reactions involving the transfer of protons between distinct sites, as often directed by attendant hydrogen bonds, are among the most prevalent of chemical transformations.1,2 An extensive body of work has established the basic paradigms that govern these processes,2-5 yet important questions regarding their origins continue to emerge, spawning efforts to formulate a more comprehensive and inclusive definition for the hydrogen bond.6 Further evidence for this assertion can be found in the lively debate that erupted over the biochemical relevance of lowbarrier hydrogen bonding (LBHBing),7,8 where a vanishing potential barrier to hydron migration was presumed to imbue great (bond) strength over short (donor-acceptor) distances.9-11 These entities were implicated in diverse catalytic pathways, with the putative formation of a lone LBHB imparting vital stabilization to an enzyme-bound transition state;12-14 however, prevailing critics adopted a radically opposite stance, claiming such motifs to be anti-catalytic and advancing alternate explanations for the efficacy of biological catalysis.15-18 Model compounds have played crucial roles for elaborating the geometric/electronic properties that govern proton transfer, permitting the progression from below-barrier quantum tunneling to above-barrier classical transport to be explored through both experiment and theory. Symmetrical systems, characterized by potential-energy landscapes that support structurally and energetically equivalent nuclear configurations along an intramolecular reaction coordinate, have proven


Page 3 of 19 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


especially useful owing, in part, to the ability to extract dynamical information directly from spectroscopic measurements of tunneling-induced signatures.19 The present analyses focus on a key member of this family, 6-hydroxy-2-formylfulvene (HFF), that has been earmarked as a prototypical example of LBHBing in neutral species, despite the paucity of isolated-molecule data to validate this assertion and the seemingly contradictory claims emerging from condensedphase measurements. By interrogating HFF and its monodeuterated isotopolog (HFF-d) under cryogenic molecular-beam conditions, ultrafast proton-transfer rates commensurate with the onset of such low-barrier phenomena (as encoded in remarkably large spectral bifurcations) have been observed and quantified for the first time.

Figure 1. The ground-state reaction coordinate for HFF is depicted using quantum-chemical [CCSD(T)/aug-cc-pVDZ] predictions of minimum-energy (Cs) and transition-state (C2v) geometries (color code: red≡oxygen, black≡carbon, gray≡hydrogen), the latter of which has the inertial-axis system superimposed. The potential barrier of ΔEpt height that separates the two equivalent tautomers induces a dynamical (tunneling-induced) bifurcation of the vibrationless ( v = 0 ) zero-point level (which straddles the barrier crest as drawn) into symmetric ( 0+ ) and antisymmetric ( 0− ) components having the distinct wavefunction probability distributions !

shown, with the attendant energy separation, Δ 0X , encoding the rate of proton transfer.



Page 4 of 19

The predicted equilibrium geometry for the X! 1 A1 ground electronic state20 of HFF in Fig. 1 highlights the atypical quasi-linear O − H !O bonding motif created by proximity of hydroxymethylene (proton-donating) and formyl (proton-accepting) moieties on adjacent carbons of the cyclopentadiene ring. While dual connectivity of the central hydrogen suggests a symmetric double-minimum potential in which a barrier of finite height, ΔEpt , separates two equivalent tautomers, the nature of the ensuing proton-transfer process depends critically on the relative location of the zero-point energy, EZPE .21 The classical above-barrier behavior engendered by EZPE ≫ ΔEpt should manifest regular vibrational energy level patterns (nominally with minimal perturbation) and a probability density for the vibrationless ( v = 0 ) level that peaks midway between donor and acceptor sites in keeping with the complete H-atom dislocation of an effective single-well potential (viz., proton shared equitably between donor and acceptor sites). For the classically hindered situation anticipated when EZPE ≪ ΔEpt , rapid interconversion between the resulting pair of degenerate structures can take place by quantum-mechanical (proton) tunneling, thereby producing a characteristic bifurcation of all rovibronic features into components having symmetric (+) and antisymmetric (–) wavefunctions (with respect to the reaction coordinate) that skew probability towards the stable minimum-energy configurations. This measurable tunneling-induced energy splitting, Δυ (for vibration υ ), is directly proportional to the rate of proton transfer,22 kptυ ≈ 2Δυ h where h is the Planck constant, and reflects the detailed mechanism (nuclear displacements and electron redistribution) of unimolecular transformation. Although typically small enough to be treated as a negligible perturbation on vibrational spectra, the magnitude of Δυ is expected to grow rapidly as the energy approaches and surmounts the barrier crest,21 heralding the transition from below-barrier to above-barrier dynamics.23 In particular, the progressive decrease of barrier height brought


Page 5 of 19 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


about by shortening of the donor-acceptor distance,24 dO!O , is believed to culminate in LBHBing ( EZPE ≈ ΔEpt ) for 2.5Å ≤ dO!O ≤ 2.6Å .25 Previous HFF experiments have focused primarily on the ground-state equilibrium structure. While early NMR work advocated a symmetrical (C2v) configuration,26 analysis of the solutionphase IR spectrum implicated rapidly interconverting asymmetric (Cs) tautomers.26,27 Solid-state neutron and X-ray diffraction data28 gave a slightly asymmetrical hydrogen-bonding motif characterized by O!H distances of 1.214Å and 1.343Å that spanned an O!O gap of

2.550Å . This was supported by gas-phase photoelectron spectroscopy,29 where two overlapping peaks in the O1s ionization region indicated distinct oxygen environments (an inference questioned by new work on kindred systems).30 In contrast, the inertial constants and nuclearspin statistics reported by bulk-gas microwave studies31 of HFF and HFF-d were deemed to be most consistent with a C2v geometry, although the existence of a Cs framework having an O!O separation of ~ 2.5Å was not discounted. The absence of discernable tunneling signatures, combined with the relative intensities of hot-band transitions, placed a lower limit of 150cm −1 on the dynamical bifurcation of vibrationless HFF. Theoretical treatments of proton-transfer dynamics conducted on potential-energy surfaces of reduced dimensionality by Millefiori and Alparone32 and by Tayyari et al.33 reinforced this experimental estimate, reporting Δ 0X !

parameters of 142 and 181cm −1 , respectively, where the latter dropped to 43cm −1 upon deuteration. Recent NMR probes of chemical shifts and isotopic perturbations,34,35 as well as earlier measurements of deuterium electric-field gradients and quadrupole coupling constants,36 have been interpreted in terms of facile exchange processes between asymmetric forms that might reflect local disorder arising from differential solvation effects.37,38 Prior spectroscopic



Page 6 of 19

investigations of electronically excited HFF appear to have been limited to solution-phase ultraviolet absorption traces of low resolution.39,40

Figure 2. The top panel illustrates the initial portion of the A! − X! LIF spectrum acquired for jetcooled HFF (seeded in He) while the bottom panel presents the DF signals obtained under similar conditions by selectively pumping the symmetric origin band ( 00+ ) of the parent (positive 0+ trace) and monodeuterated (inverted negative trace) species. Labeled LIF vibronic progressions entail increasing quanta in two excited-state modes, ν 4 ( a1 ) and ν 7 ( b2 ) , and do not exhibit obvious dynamical signatures. Their DF counterparts show spectral shifts and bifurcations that


Page 7 of 19 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


!

depend on the nature of the underlying vibrational motion, with the ΔυX metrics measured for several ground-state features highlighted [ ν 0 ( a1 ) denotes the zero-point level]. These results build on laser-spectroscopic techniques41 and HFF-synthesis procedures34,42 reported elsewhere (cf. supporting information for details). Chart 1. Nuclear Displacements of HFF Vibrational Modes.a

a

Mass-weighted normal coordinates for the CCSD/aug-cc-pVDZ equilibrium geometry of the ground state. Displacement vectors obtained at the CCSD(T)/aug-cc-pVDZ level of theory differ negligibly from those shown, as do their excited-state counterparts for the two depicted modes. The top panel of Fig. 2 highlights the laser-induced fluorescence (LIF) spectrum recorded under rovibrationally cold ( Trot

Spectral Signatures of Proton-Transfer Dynamics ... - ACS Publications

Recommend Documents