Exploring the G3 method in the study of rotational barrier of some simple molecules

Abstract

The G3 method was used to determine the internal rotation barriers of some simple molecules (H 2 O 2 , H 2 S 2 , N 2 H 4 , CH 3 OH, CH 3 NH 2 , and C 2 H 6 ). The barriers were accurate with deviations lower than 0.5 kcal mol À1 with respect to the experimental data and comparable with other methods like CCSD(T)/aug-cc-pVTZ results. The G3 components showed that the MP4/6-31G(d) energy is quantitatively improved according to: DE G3Large > DE 2df,p > DE þ > DE QCI . However, the relative energies showed that molecules containing lone pairs in neighbor atoms presented high barriers, which were particularly sensitive to polarization and diffuse functions. The G3Large and QCI effects showed no qualitative or quantitative relative contribution. Molecules containing lone pairs in one atom or only bond pairs presented low barriers and were sensitive not only to the polarization and diffuse effects but also depending on the improvement of the basis set through the G3Large correction. The QCI effect showed no qualitative or quantitative relevant contribution in any of the cases studied.

A theoretical study of the inversion and rotation barriers in Methyl-substituted Amines

“…Note that the B3LYP imaginary frequencies differ no more than 80 cm -1 from the CCSD(T)/cc-pVTZ results. All geometric parameters calculated using B3LYP/cc-pVQZ are, in general, in good agreement with the CCSD(T) results and also with previous theoretical calculations [7][8][9][10]13,14 and experimental data. [2][3][4] We have also carried out the IRC calculations using the B3LYP/ cc-pVTZ method and the results confirm that the first-order inversion transition state and the rotation transition state connect to the ground state, as already shown by Kim and Zeroka 13 using a more complete two-dimensional potential energy surface.…”

“…It is important to mention that calculations of T 1 diagnostic were previously carried out 11 at the CCSD/cc-pVQZ level with values equal to 0.008 for all conformations, which are small enough to allow the use of the single reference CCSD(T) wave functions in the study of the present molecular conformations. As already noticed, 11 the increase of the basis set and the inclusion of core correlation reduce both barrier height values, as also observed if one compares the CCSD(T)/cc-pVTZ result for the first-order rotation barrier with the one calculated by Ducati et al 14 with CCSD(T)/aug-cc-pVTZ. The inclusion of ZPE correction decreases the values of barrier heights underestimating the CBS-DT (CBS extrapolation using double and triple basis set) results compared to the experimental data.…”

“…[1][2][3][4][5][6][7] Methylamine is considered a small molecule from the theoretical point of view, therefore it is appropriate to be studied using high level theoretical quantum chemical methodologies, having received many contributions. [7][8][9][10][11][12][13][14] Lee et al 8 investigated the origin of the structural stability of methylamine concluding that the stereo electronic effect is the major interaction force affecting its stability. Csonka and Sztraka 9 using density functional and post-HartreeFock methodologies showed that the density functional BP86/6-311G(d) method provides reliable results for geometry and vibrational frequencies, as compared with the experimental results.…”

“…5, 2011 frequencies, and the barrier heights of the internal rotation and inversion motions of methylamine using the CCSD(T) theory with a systematic improvement via cc-pVDZ, cc-pVTZ, cc-pVQZ, and cc-pCVTZ basis sets and also including the contribution of the inner-shell and valenceshell electrons. 11 More recently, Levi et al 12 carried out accurate calculations of the vibrational frequencies, Kim and Zeroka 13 presented new and important results concerning the inversion and rotational barriers and Ducati et al 14 calculated the rotational barrier.…”

An interpretation of the phenol nitration mechanism in the gas phase using G3(MP2)//B3-CEP theory

“…The G3 theory [28] and computationally less expensive versions of it, such as G3(MP2) [29] and G3(MP2)//B3 [30], are quantum mechanical methods that have been widely used in the study of a variety of compounds [31][32][33][34], rotational barriers [35,36], and mechanisms [37][38][39]. However, the size of molecular systems continues to be a limiting factor that prevents the more general use of Gaussian-n theories.…”