A Sustainable Slit Jet FTIR Spectrometer for Hydrate Complexes and Beyond

Abstract

Fourier transform infrared (FTIR) absorption spectroscopy of cold molecules and clusters in supersonic slit jet expansions complements and extends more sensitive action spectroscopy techniques and provides important reference data for the latter. We describe how its major drawback, large substance and carrier gas consumption, can be alleviated by one to two orders of magnitude via direct and continuous recycling of the gas mixture. This is achieved by a combination of dry rotary lobe and screw pump compression. The signal-to-noise ratio is boosted by the established buffered giant gas pulse technique with full interferogram synchronization. The buildup of water impurities typically limits the recycling gain, but is turned into a feature for the study of hydrate complexes of volatile molecules. Continuous operation with a single gas filling over several days becomes practical and useful. Decadic absorbances in the low ppm range are detectable and the mid infrared range can be recorded simultaneously with the near infrared. The less straightforward hydration number assignment of spectral features in direct absorption spectroscopy is supported by a gradual water buildup at a rate of less than 0.5 mg/h. A recent reassignment proposal for the water dimer OH stretching spectrum is refuted and vibrational spectra of vacuum-isolated 18O-water clusters are presented for the first time. Methanol docking on asymmetric ketones is used to illustrate the advantages and limitations of the recycling concept. Previous assignments of the hydrate complex of 1-phenylethanol are confirmed. Additional features of the setup await testing and refinement, but the recycling technique already substantially widens the applicability of direct absorption spectroscopy of neutral molecular clusters. It may be attractive for other high-throughput jet spectrometers.

The Hydrates of TEMPO: Water Vibrations Reveal Radical Microsolvation

“…The experimental OH stretching spectrum of TEMCO + water (trace A of Figure 2) fits rather well into what one would expect from other ketone hydrate clusters [39] . It is obtained by a newly developed, compound‐economic FTIR slit jet cluster spectrometer with gas recycling (see the Supporting Information for experimental details and scaling factors) [40] . There is a dominant peak (p⋅⋅⋅HOH) due to water docking to one of the equivalent lone electron pairs of the keto group, embedded into the two alpha‐methyl groups which shield this lone pair, roughly in the ring plane.…”

“…Each of them could furthermore undergo a b2lib‐like resonance reported to be active in this spectral range for monohydrates, [39] so up to 6 peaks might be explainable based on TEMCO and the different torsional potential for a water molecule around the accepting group. Replacement of the carrier gas helium by neon (trace D), affordable due to the recycling concept of the jet spectrometer, [40] removes a single peak from the manifold by better conformational cooling of the heavier rare gas collision partner, but leaves the other 4 more or less unchanged. This shows that the disappearing peak must belong to a metastable isomer (comparison to the scaled harmonic prediction tentatively indicates o⋅⋅⋅HOH, which is the second‐lowest structure after CCSD(T) correction and separated by moderate barriers from its torsional isomers, see the Supporting Information).…”

The Hydrates of TEMPO: Water Vibrations Reveal Radical Microsolvation

“… [39] It is obtained by a newly developed, compound‐economic FTIR slit jet cluster spectrometer with gas recycling (see the Supporting Information for experimental details and scaling factors). [40] There is a dominant peak (p⋅⋅⋅ H OH) due to water docking to one of the equivalent lone electron pairs of the keto group, embedded into the two alpha‐methyl groups which shield this lone pair, roughly in the ring plane. Interestingly, there is a little satellite peak (b2lib) which we attribute to a recently discovered universal Darling‐Dennison resonance involving the bending overtone of the water molecule (b2) and a hydrogen bond librational motion (lib).…”

Halogens in Acetophenones Direct the Hydrogen Bond Docking Preference of Phenol via Stacking Interactions

“…The downshifts are quite sizeable, but in contrast to the hydrate case [ 17 ], there is no pronounced resonance coupling pattern evident in the spectra, thus facilitating the assignment. The monomer C=O overtone is predicted/found close to 3400 (see Table S7 and [ 40 ] for 2F). In some cases, it overlaps with the methyl docking signal of the complex; in others, it does not.…”