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.

Setting up the HyDRA blind challenge for the microhydration of organic molecules

“…It was anticipated that one member of the test set (PCD) would only be characterised by an action technique (IR/UV double resonance), whereas the others would be mostly accessed by direct infrared absorption. For the latter, a recent instrumental development 104 is very helpful for monohydrates, because its gas recycling allows for the use of expensive isotopologues, solutes and carrier gases. In some cases, where the water vibration of interest has a low infrared intensity or may be distributed over several OH stretching modes in the complex, linear Raman spectroscopy 14,87 could provide the required information.…”

Setting up the HyDRA blind challenge for the microhydration of organic molecules

“…It is anticipated that one member of the test set (PCD) will only be characterised by an action technique (IR/UV double resonance), whereas the others will be mostly accessed by direct infrared absorption. For the latter, a recent instrumental development 99 is very helpful for monohydrates, because its gas recycling allows for the use of expensive isotopologues, solutes and carrier gases. In some cases, where the water vibration of interest has a low infrared intensity or may be distributed over several OH stretching modes in the complex, linear Raman spectroscopy 14,85 can provide the required information.…”