= Rotational monitoring of deuterium exchange
A distinct experiment in <An Automated, Highly Selective Reaction Monitoring Instrument Using Molecular Rotational Resonance Spectroscopy by Ann Adele Byars and others> (Figure 1 and Figure 3) tracked exchange of <hydrogen> for <deuterium> in <ethyl acetoacetate> mixed with deuterated <methanol>. A pump circulated <chemical solution> through a valve; a calibrated loop injected aliquots into a temperature-controlled inlet, held at room <temperature> for this exchange experiment. <Neon> carried the vaporized sample to a cavity <spectrometer>. Separately recorded broadband spectra and fitted rotational <Hamiltonians> identified transitions for the four isotopic forms before targeted monitoring.<An Automated, Highly Selective Reaction Monitoring Instrument Using Molecular Rotational Resonance Spectroscopy by Ann Adele Byars and others>[apparatus and assignment paragraphs around Figure 1; deuterium-exchange method and Figure 3]{ref}
The readout was each assigned line's signal over reaction time, normalized by the sum of the four signals. Transitions with the same <quantum numbers> reduced response-factor differences. A solvent wash addressed observed sample carryover. The signals distinguished the unsubstituted, two singly substituted, and doubly substituted forms. Only one gas-phase structural form was detected; the authors explicitly caution that their readout does not recover the structural <chemical equilibrium> in <chemical solution>.<An Automated, Highly Selective Reaction Monitoring Instrument Using Molecular Rotational Resonance Spectroscopy by Ann Adele Byars and others>[deuterium-exchange paragraphs surrounding Figure 3; sample carryover and wash paragraph]{ref}
This example uses nonlinear <molecules>, so neither the single-constant <linear rigid rotor> nor the synthetic comb above is a fit to its spectrum. Its connection to the model is the sensitivity of <moments of inertia> to <isotope> placement. Line position helps assign a species; signal size additionally depends on sample transfer, initial <quantum state> populations and instrument response. Matching <quantum numbers> is a useful control, not a proof of identical response factors. These distinctions explain why resolving species and determining their <concentrations> are separate inference steps.
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