Letter pubs.acs.org/JPCL
Far-Infrared Signatures of Hydrogen Bonding in Phenol Derivatives Daniel̈ J. Bakker,† Atze Peters,† Vasyl Yatsyna,†,‡ Vitali Zhaunerchyk,‡ and Anouk M. Rijs*,† †
Radboud University, Institute for Molecules and Materials, FELIX Laboratory, Toernooiveld 7c, 6525 ED Nijmegen, The Netherlands ‡ University of Gothenburg, Department of Physics, 412 96 Gothenburg, Sweden S Supporting Information *
ABSTRACT: One of the most direct ways to study the intrinsic properties of the hydrogen-bond interaction is by gas-phase far-infrared (far-IR) spectroscopy because the modes involving hydrogen-bond deformation are excited in this spectral region; however, the far-IR regime is often ignored in molecular structure identification due to the absence of strong far-IR light sources and difficulty in assigning the observed modes by quantum chemical calculations. Far-IR/UV ion-dip spectroscopy using the free electron laser FELIX was applied to directly probe the intramolecular hydrogen-bond interaction in a family of phenol derivatives. Three vibrational modes have been identified, which are expected to be diagnostic for the hydrogen-bond strength: hydrogen-bond stretching and hydrogen-bond-donating and -accepting OH torsion vibrations. Their position is evaluated with respect to the hydrogen bond strength, that is, the length of the hydrogen-bonded OH length. This shows that the hydrogen bond stretching frequency is diagnostic for the size of the ring that is closed by the hydrogen bond, while the strength of the hydrogen bond can be determined from the hydrogen-bond-donating OH torsion frequency. The combination of these two normal modes allows the direct probing of intramolecular hydrogen-bond characteristics using conformation-selective far-IR vibrational spectroscopy.
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ally, mid-IR spectroscopy is applied to probe intramolecular H bonds in isolated (bio)molecules via local vibrations of functional groups such as the OH or NH moieties;18−23 however, the mid-IR spectra of molecules of increasing size offer information only on the direct environment of the addressed oscillator so that the orientation of flexible apolar groups becomes ambiguous, to an extent where only families of structures can be identified.24 It has been shown that far-IR radiation is capable of identifying the precise conformer in such a situation.17 Moreover, the vibrational mode and strength of the hydrogen bonds is not directly observed in the mid-IR but derived from shifts in the measured frequencies of the CO, NH, or OH moiety. The far-IR spectrum, on the contrary, shows direct signatures of H bonds. Because the presented gas-phase experiments are performed in the molecular beam environment with low molecular densities and the induced dipole of the delocalized modes is small, a high-intensity, tunable far-IR source is necessary. The necessity of high-intensity light can be circumvented using Raman spectroscopy, which provides complementary information regarding the low-frequency modes of a molecule.2 Additionally, the anharmonic character of the delocalized modes and their couplings complicate the spectral assignments as most quantum-chemical computation methods do not treat
ntra- and intermolecular hydrogen bonding plays a crucial role in, for example, solvation science, molecular recognition, and structure determination of neutral biomolecules. The wavelengths required to directly and resonantly excite the vibrational modes that are involved in the deformation of the hydrogen bond (H bond) itself can be found in the far-infrared (far-IR, 99%) were prepared in a supersonic jet expansion (seed gas helium or argon, backing pressure 3 bar) using the combination of a heated sample compartment and a heated pulsed valve. The molecular beam travels through a skimmer approximately 5 cm downstream to enter a differentially pumped chamber equipped with a reflectron time-offlight mass spectrometer. Here the molecules are ionized via a (1 + 1) REMPI scheme by a UV beam from a YAG-pumped, frequency-doubled dye laser with a typical output power of 1 mJ per pulse. For both SA and EV a (1 + 1′) scheme was employed, where a 193 nm ArF laser pulse was required to ionize the molecules after excitation. The conformer-selective IR spectra were recorded using IR-UV ion dip spectroscopy. Here the timing of the experiment is chosen such that the end of the FELIX IR macropulse (50 mJ, 6−10 μs duration) overlaps with the nanosecond UV pulse(s). To correct for longterm UV power drifts and changing source conditions, alternating IR-on and IR-off signals are measured by operating FELIX at 10 Hz and the UV laser at 20 Hz. Every IR spectrum reported here is the average of 75 or more referenced measurements. All calculated frequencies and bond lengths reported in the paper are produced using the B3LYP-D3/6-311+G** level of theory.31 All structures were optimized using tight convergence criteria with an ultrafine mesh grid, as is advised for the far-IR spectral region.41 The calculated lines are broadened using a Gaussian line shape with an fwhm of 0.5% of the wavenumber to enable comparison with measured data based on the pulse characteristics of FELIX.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpclett.6b00016. REMPI spectrum of ethylvanillin; overview of the molecule-specific experimental parameters; tabulated Hbond geometry and frequencies of H-bond deforming modes; resonant structures of salicylic acid, rotamer 1; characteristics of open conformers versus H-bonded conformers; and frequency of the H-bonded OH stretching vibration versus the length of the H-bonddonating OH group. (PDF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We gratefully thank the FELIX staff for their experimental support and professor Jos Oomens and Qin Ong for helpful discussions. All authors acknowledge the Stichting voor Fundamenteel Onderzoek der Materie (FOM) for the support of the FELIX laboratory (project no. N2300N). The calculations were sponsored by NWO Physical Sciences 1242
DOI: 10.1021/acs.jpclett.6b00016 J. Phys. Chem. Lett. 2016, 7, 1238−1243
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DOI: 10.1021/acs.jpclett.6b00016 J. Phys. Chem. Lett. 2016, 7, 1238−1243
