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Establish the rotational type (b-type vs c-type) of the 2753.9 cm−1 (D2O)2 combination band

Ascertain whether the weak combination band of the deuterated water dimer (D2O)2 near 2753.9 cm−1 follows b-type or c-type rotational selection rules, in order to resolve the ambiguity that affects its vibrational assignment.

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Background

A previously observed very weak combination band near 2753.9 cm−1 was tentatively assigned, but the rotational type inferred from partially resolved features conflicted with symmetry-based expectations for one of the proposed vibrational combinations.

Clarifying the rotational type is necessary to settle the band’s vibrational assignment (e.g., acceptor symmetric stretch plus acceptor wag versus donor bound O–D stretch plus donor twist overtone).

References

Our analysis of this band seemed to indicate that it was c-type (based on the shape of the partially resolved Q-branch), which could argue against the assignment just given since the combination of symmetric stretch plus acceptor wag has A" symmetry. In fact, however, this distinction between b- and c-type was not at all well established.6

— Spectra of the D2O dimer in the O-D fundamental stretch region: the acceptor symmetric stretch fundamental and new combination bands (2402.14923 - Barclay et al., 22 Feb 2024) in Section III.B (Combination Bands)