QTAIM-DI-VISAB computational study on the so-called nonclassical bicyclobutonium cation

Nick H Werstiuk,Daniel A Poulsen,Antigoni Kotali

doi:10.3998/ark.5550190.0010.505

Abstract

QTAIM-DI-VISAB analyses at the CCSD and B3PW91 levels were used to characterize the bonding of the cyclopropylcarbinyl (1) and the so-called ‘nonclassical’ bicyclobutonium and 1methylbicyclobutonium cations, 2 and 3 as well as the transition state for rearrangement of 1 to 2. These analyses involved obtaining QTAIM molecular graphs and delocalization indexes (DIs) for pairs of atoms that were correlated with the proximities of atomic basins (VISAB). This study showed that the supposed nonclassical bicyclobutonium and 1-methylbicyclobutonium cations do not exhibit penta-coordinate carbons at their equilibrium geometries as has been claimed. Both species are best described as distorted cyclobutyl cations that exhibit a single ring critical point in the topology of the charge density.

Highlights

The cyclopropylcarbinyl cation (1) and its isomer, the so-called nonclassical bicyclobutonium cation (2) as well as its 1-methyl analogue (3) whose structures are shown using the usual dashed-line formalism have been the focus of numerous experimental and computational studies over the span of several decades with the latest being the work of Olah and coworkers.[1]
In addition to using QTAIM molecular graphs that unambiguously define molecular structure, we recently showed that QTAIM-delocalization indexes (DIs)-visualized in displays of these basins (VISAB) analyses are useful for characterizing the bonding in molecules at their equilibrium geometries and refining our knowledge of the bonding in hyper-coordinated species.[11,12,13,14]
This paper reports the results of a QTAIM-DIVISAB study on 1, 2, and 3, the goal being to establish whether 2 and 3 are pentacoordinate species as has been claimed

Summary

Introduction

The cyclopropylcarbinyl cation (1) and its isomer, the so-called nonclassical bicyclobutonium cation (2) as well as its 1-methyl analogue (3) whose structures are shown using the usual dashed-line formalism have been the focus of numerous experimental and computational studies over the span of several decades with the latest being the work of Olah and coworkers.[1]. Cation geometries were optimized at B3PW91 and MP2 levels with a range of basis sets – 6-311+G(2d,p), cc-pVTZ, and aug-cc-pVTZ with OPT=TIGHT and OPT=VERYTIGHT and wave functions obtained. Selected inter-nuclear distances of 1-CS, 2-CS, and 3-CS obtained at CCSD(full)/6311+G(2d,p) with OPT=TIGHT are displayed as Figure 1(a), (c), and (d), respectively. That the total charges of the cations obtained at the various levels of theory differed by less than 1% from the expected value of 1.0 confirmed the quality and validity of the QTAIM data (Table 3).

Results

Conclusion