A new program for the assignment and fitting of dense rotational spectra based on spectral progressions: Application to the microwave spectrum of pivalic anhydride

A new computerized package for the rapid processing and analysis of dense rotational spectra is described. The package is based on the automated identification of user-specified spectral progressions and interacts with the operator through a simple graphical user interface. In addition to containing an algorithm for obtaining spectral assignments, the package includes a peak-finder with adjustable baseline drift compensation, seamless interfacing with Pickett's SPFIT and SPCAT programs, and a number of visualization features. The utility of the program is illustrated through its application to the analysis of new spectra of pivalic anhydride, (CH₃)₃CCOOCOC(CH₃)₃. Spectra of the parent, as well as all isotopologues with single heavy-atom substitution, are reported, as are calculations for the parent species at the M06-2X/6-311++G(3df,3pd) level of theory. The central oxygen atom is poorly located due to its proximity to the center of mass of the molecule, but a Kraitchman analysis of the fitted rotational constants otherwise provides a detailed structural characterization of the heavy-atom frame. The two carbonyl groups are not coplanar, but rather form a dihedral angle of 53.9(39) deg.