Toward Feasible and Comprehensive Computational Protocol for Simulation of the Spectroscopic Properties of Large Molecular Systems: The Anharmonic Infrared Spectrum of Uracil in the Solid State by the Reduced Dimensionality/Hybrid VPT2 Approach

Feasible and comprehensive computational
protocols for simulating
the spectroscopic properties of large and complex molecular systems
are very sought after. Indeed, due to the great variety of intra-
and intermolecular interactions that may take place, the interpretation
of experimental data becomes more and more difficult as the system
under study increases in size or is placed in a complex environment,
such as condensed phases. In this framework, we are actively developing
a comprehensive and robust computational protocol aimed at quantitative
reproduction of the spectra of nucleic acid base complexes, with increasing
complexity toward condensed phases and monolayers of biomolecules
on solid supports. We have resorted to fully anharmonic quantum mechanical
computations within the generalized second-order vibrational perturbation
theory (GVPT2) approach, combined with the cost-effective B3LYP-D3
method, in conjunction with basis sets of double-ζ plus polarization
quality. Such an approach has been validated in a previous work (Phys. Chem. Chem. Phys. 2014, 16, 10112−10128) for simulating the IR spectra of the monomers
of nucleobases and some of their dimers. In the present contribution
we have extended such computational protocol to simulate spectroscopic
properties of a molecular solid, namely polycrystalline uracil. First
we have selected a realistic molecular model for representing the
spectroscopic properties of uracil in the solid state, the uracil
heptamer, and then we have computed the relative anharmonic frequencies
combining less demanding approaches such as the hybrid B3LYP-D3/DFTBA
one, in which the harmonic frequencies are computed at a higher level
of theory (B3LYP-D3/N07D) whereas the anharmonic shifts are evaluated
at a lower level of theory (DFTBA), and the reduced dimensionality
VPT2 (RD-VPT2) approach, where only selected vibrational modes are
computed anharmonically along with the couplings with other modes.
The good agreement between the theoretical results and the experimental
findings allowed us to extend the interpretation of experimental data.
Our results indicate that hybrid and reduced dimensionality models
pave a way for the definition of system-tailored computational protocols
able to provide more and more accurate results for very large molecular
systems, such as molecular solids and molecules adsorbed on solid
supports.