The generalization of the PICVib approach [M. V. P. dos Santos et al., J. Comput. Chem., 2013, 34, 611] for calculating infrared intensities is shown to be successful and to preserve all interesting features of the procedure such as easiness of implementation and parallelization, flexibility, treatment of large systems and at high theoretical levels. It was tested and validated for very diverse molecular systems: XH3 (D3h), YH4 (D4h), conformers of RDX, S(N)2 and E2 reaction product complexes, the [W(dppe)2(NNC5H10)] complex, carbon nanotubes, and hydrogen-bonded complexes (H2O···HOH, MeHO···HOH, MeOH···OH2, MeOH···OHMe) including the guanine-cytosine pair. The PICVib shows an excellent overall performance for calculating infrared intensities of localized normal modes and even mixed vibrations, whereas care must be taken for vibrations involving intermolecular interactions. DFT functionals are still the best combination with high level ab initio methods such as CCSD and CCSD(T).

As dinâmicas da desidratação do álcool protonado (R)-3,3-dimetilbutan-2-ol (álcool pinacolil), [(CH 3 ) 3 C-CH(OH 2 )CH 3 ] + , e da cicloadição eteno + 1,3-butadieno foram estudadas com a técnica de dinâmica molecular direta de Born-Oppenheimer (BOMD) usando o método AM1. Mais de 10.000 trajetórias foram geradas, em que a maioria foi utilizada no procedimento ainda não explorado de recozimento simulado/fragmentação (SA/F). A energia potencial obtida com o método AM1 (PES-AM1) para o álcool protonado apresenta dois estados de transição associados ao intermediário complexo [(CH 3 ) + foi praticamente inexistente durante a dinâmica. Apesar do caminho concertado (dissociação do fragmento H 2 O e migração do grupo CH 3 ) não ser uma coordenada de reação intrínseca (IRC) no método PES-AM1, um número significativo de trajetórias envolveu este caminho. Para a reação de Diels-Alder, mesmo partindo-se do estado de transição simétrico, e utilizando uma função de onda AM1 restrita, a dinâmica forneceu um número significativo de trajetórias que seguiram caminhos assimétricos, isto é, não-IRC, em direção ao ciclohexeno, independentemente do procedimento utilizado na inicialização. E notável ainda que todas estas trajetórias que seguiram caminhos assimétricos envolveram um caminho de reação concertado.The dynamics of dehydration of the protonated (R)-3,3-dimethylbutan-2-ol (pinacolyl alcohol), [(CH 3 ) 3 C-CH(OH 2 )CH 3 ] + , and of ethene + 1,3-butadiene cycloaddition were studied with the BornOppenheimer molecular dynamics (BOMD) technique for direct dynamics using the AM1 method. More than 10,000 trajectories were generated, most of them related to the unexplored simulated annealing/fragmentation approach. The AM1 potential energy surface (PES) for the protonated pinacolyl alcohol presents two transition states related to the [(CH 3 ) 3 C-CHCH 3 ] + ···OH 2 intermediate complex and to CH 3 migration leading to the [(CH 3 ) 2 C-CH(CH 3 ) 2 ] + ···OH 2 product complex. Direct dynamics yielded negligible trajectories involving these complexes, since the momentum acquired by the H 2 O fragment led to a complete dissociation. Thus, rearrangement of the secondary carbocation [(CH 3 ) 3 C-CHCH 3 ] + was practically inexistent during the dynamics. Despite the concerted path (H 2 O dissociation and CH 3 migration) not being an IRC (intrinsic reaction coordinate) path in AM1-PES, a statistically significant number of trajectories involved this path. As for the Diels-Alder reaction, even when started from a symmetric transition state using the spin restricted AM1 wavefunction, the dynamics yielded a significant number of trajectories that followed asymmetric, i.e. non-IRC, paths toward cyclohexene, independent of the initialization approach. It is noteworthy that all these asymmetric path trajectories led to a concerted reaction mechanism.Keywords: protonated pinacolyl, ethene + 1,3-butadiene, cycloaddition, non-IRC IntroductionRecently, time-dependent methods, 1,2 particularly direct dynamics classical trajectory ones, 3,4 have be...

A new approach Procedure for Investigating Categories of Vibrations (PICVib) for estimating vibrational frequencies of selected modes using only the structure and energy calculations at a more demanding computational level is presented and explored. The PICVib has an excellent performance at only a small fraction of the computational demand required for a complete analytical calculation. The errors are smaller than ca. 0.5% when DFT functionals are combined with high level ab initio methods. The approach is general because it can use any quantum chemical program and electronic structure method. It is very robust because it was validated for a wide range of frequency values (ca. 20-4800 cm(-1)) and systems: XH(3) (D(3h) ) with X = B, Al, Ga, N, P, As, O, S, and Se, YH(4) (D(4h) ) with Y = C, Si, and Ge, conformers of RDX, S(N) 2 and E2 reactions, [W(dppe)(2)(NNC(5)H(10))] complex, carbon nanotubes, and hydrogen-bonded complexes including guanine-cytosine pair.