Applications of density functional theory methods in millimeter‐wave spectroscopy

Abstract

Density functional theory (DFT), using the most common functionals, and ab initio quantum chemistry methods are used to calculate the rotational constants and dipole moments of the astrophysically important molecules HCN, CH 3 CN, CH 3 CNH ϩ , HCCCN, and HCCNC. As far as millimeter-wave spectroscopy is of interest the DFT methods performed well with most functionals, giving results within Ϯ1% of experiments for rotational constants and Ϯ3% for dipole moments. Analyzing the results obtained with all theoretical models, it may be concluded that the Becke's threeparameter exchange functional and the gradient-corrected functional of Lee, Yang, and Paar (B3LYP) and Becke's three-parameter functional with Perdew-Wang correlational functional [B3PW91/6-31G(d, p)] give the best performances. A detailed analysis of the electron correlation effects shows that HCCCN is more stable than is HCCNC, by 1.16 eV, with important contribution arising from triple excitations. This result is also compared with those obtained with DFT methods. Despite occasional difficulties, DFT with the currently available functionals are of great utility in quickly assessing spectroscopic parameters of astrophysical interest.

Diradical Interactions in Ring-Open Isoxazole

“…A similar mechanism is in play as that which makes HCN and HCCCN more stable than their respective HNC and HCCNC isomers. 37,38 The through-space interactions between the O• and •N centers are, of course, present in isoxazole-2 (and are similar in magnitude to the analogous interactions in oxazole-2), but even combined with the through-bond couplings, they are not sufficient to make the singlet state more favorable.…”

Theoretical studies on hydrogen bonding interactions: From small clusters to the liquid phase

“…Some methodological aspects and applications of hydrogen bonding interactions in molecular aggregates and liquid solutions were discussed. In particular, issues that have been research interest of Professor Canuto and his collaborators were briefly reviewed5–33, 104. Moreover, new insights into the fundamental nature of the H‐bonds in molecular complexes were presented34, 35 (Rivelino and Canuto, in preparation).…”

“…Spectroscopic and geometric parameters of HCN have been discussed by us104 using highly correlated levels of theory. Later, we have quantified multiple‐body interaction terms of H‐bonded HCN strings by performing a systematical analysis within MBPT/CC methods19.…”

"Efeitos do meio em propriedades conformacionais e eletrônicas de moléculas"

“…A geometria do DMACA em fase gasosa foi obtida através de otimização de geometria com método DFT híbrido B3PW91/6-311++G(d,p) que é um método bem sucedido em cálculos de geometria (veja por exemplo [118]). Os comprimentos de ligação mais relevantes podem ser vistos na tabela 5.1.…”