The Raman Spectrum of the Squarate (C4O4-2 ) Anion: An Ab Initio Basis Set Dependence Study

Abstract

O perfil de excitação Raman do anion esquarato, C 4 O 4 -2 , foi calculado por métodos ab initio no nível de teoria Hartree-Fock utilizando a Teoria de Resposta Linear (TRL) para seis frequências de excitação: 632,5, 514,5, 488,0, 457,9, 363,8 e 337,1 nm. Um total de cinco conjuntos de funções de base (6-31G*, 6-31+G*, cc-pVDZ, aug-cc-pVDZ e bases de Sadlej) foi investigado visando comparar o desempenho da base 6-31G*, em termos de convergência numérica dos resultados e custo computacional, com os dos demais conjuntos de base. Todos os conjuntos de base testados reproduzem as principais características do espectro Raman do anion esquarato no intervalo de excitação estudado. A base 6-31G* apresentou, em média, o mesmo desempenho nos resultados numéricos dos conjuntos de base maiores mas a uma fração do custo computacional mostrando-se apropriada para o estudo teórico dos derivados e complexos deste anion.The Raman excitation profile of the squarate anion, C 4 O 4 -2 , was calculated using ab initio methods at the Hartree-Fock using Linear Response Theory (LRT) for six excitation frequencies: 632.5, 514.5, 488.0, 457.9, 363.8 and 337.1 nm. Five basis set functions (6-31G*, 6-31+G*, ccpVDZ, aug-cc-pVDZ and Sadlej's polarizability basis set) were investigated aiming to evaluate the performance of the 6-31G* set for numerical convergence and computational cost in relation to the larger basis sets. All basis sets reproduce the main spectroscopic features of the Raman spectrum of this anion for the excitation interval investigated. The 6-31G* basis set presented, on average, the same accuracy of numerical results as the larger sets but at a fraction of the computational cost showing that it is suitable for the theoretical investigation of the squarate dianion and its complexes and derivatives.Keywords: Raman spectra, squarate anion, vibrational spectroscopy, ab initio Linear Response Theory IntroductionThe advances in computer technology and theoretical methods over the last decades has made possible the ab initio calculation of molecular properties such as equilibrium geometries, infrared absorption intensities, vibrational frequencies and force fields for a large number of molecules and chemical systems of spectroscopic interest.1 These calculated properties proved to be a valuable aid in the interpretation and assignment of the experimental spectroscopic data for a large number of molecules. However, in many cases, the peculiar electronic structure of the system being studied presents Raman spectroscopic features for which no routine computational methods are widely available. One of these cases is the squarate dianion (C 4 O 4 -2 ), which belongs to the monocyclic oxocarbons (C n O n -2 , 3 ≤ n ≤ 6 ) class of organic compounds with a wide range of applications. Since the first vibrational spectroscopic study by West and Ito, 2 the electronic, infrared and Raman spectra of the squarate anion as well as several of its complexes and derivatives have been the subject of several experimental and theoretical studies over...

Determination of ab initio absolute Raman excitation profiles using linear response theory

“…Helgaker and colleagues attributed the discrepancies observed between their calculations and experimental results to the lack of inclusion of electronic correlation (all calculations were done at the HF level), experimental errors, and neglect of anharmonicities. At this level, de Miranda and Vazquez 5 presented a study of the effect of basis sets on the excitation profile of squarate (C 4 O) anion for six excitation frequencies ranging from 632.5 to 337.1 nm. They concluded that all sets tested (i.e., 6‐31G*, 6‐31+G*, cc‐pVDZ, aug‐cc‐pVDZ, and Sadlej's polarizability basis set 6, 7) have correctly reproduced the main spectroscopic features of the squarate Raman spectrum, showing that relative Raman intensities are much less dependent on the basis set than are absolute ones.…”