Reliable Modeling of Anharmonic Spectra Line-Shapes from VPT2 and Hybrid QM Models: IR Spectrum of Uracil as a Test Case

Hybrid methods combining different levels of electronic
structure
quantum mechanical computations with vibrational perturbation theory
have been increasingly used in anharmonic simulations of vibrational
spectra to achieve accurate results with containable computational
costs. However, energy has often been the main focus of these studies,
so precision in predicting intensities was systematically overlooked.
This situation is largely due to two aspects stemming from theory
and experiment. Theoretically, implementations are fewer, and intensity-specific
resonance analysis methods were not available until very recently
and are still lacking extensive testing. Experimentally, high-resolution
vibrational spectra of suitable molecular systems, which could really
show the effects of different hybrid schemes on the vibrational intensities,
remain scarce. A good candidate in this regard is uracil. Having been
extensively studied experimentally, its IR spectrum is well-known
over a wide range and at high definition. The patterns displayed by
the band shapes represent an excellent challenge to validate and tune
our recently developed automated tool to identify intensity-specific
resonances. In this work, we compare the newly simulated spectra with
state-of-the-art experimental data and propose an extensive analysis
over a wide range covering 300 to 6200 cm–1, including
3-quanta transitions. These will provide valuable guides and references
for further measurements in the mid-infrared (MIR) and near-infrared
(NIR) regions, which have not been reported until now. The methods
and protocols applied in this article can also be used for other molecules
with complex resonance patterns.