Journal article

Authors list: Bettinger, HF; Schreiner, PR; Schleyer, PV; Schaefer, HF

Publication year: 1996

Pages: 16147-16154

Journal: Journal of Physical Chemistry

Volume number: 100

Issue number: 40

DOI Link: https://doi.org/10.1021/jp961343f

Publisher: American Chemical Society

Abstract: 

Density functional theory and ab initio quantum mechanical techniques were employed to study the ring opening of singlet cyclopropylidene 1-S and the internal rotation of singlet allene 2-S. The B₃LYP, CISD, and CASSCF(4,4) geometry optimizations used a TZP basis set. Single-point TZ2P energies were evaluated at CCSD(T) and multireference configuration interaction with single and double excitations (MR-CISD) levels. Employing the four most important configurations at the MR-CISD/TZ2P//UB₃LYP/TZP level gave singlet-triplet separations for 1 of 11.1 kcal mol^-1 and for 2 48.9 kcal mol^-1. Asymmetric transition structures for the ring opening of 1-S were obtained at B₃LYP/TZP and CISD/TZP. The barriers are 4.2−4.5 kcal mol^-1 at CCSD(T)/TZ2P with these geometries. In contrast, only one Cs symmetric TS was located at CASSCF(4,4), and the ring opening barrier is only 3.6 and 3.1 kcal mol^-1 at CCSD(T)/TZ2P and MR-CISD/TZ2P, respectively. At CCSD(T)/6-31G* an asymmetric structure for the TS results confirming the predictions of the B₃LYP and CISD methods. The B₃LYP/6-31G* intrinsic reaction coordinate (IRC) delineated the complex modes of rotation of the methylene groups during the ring opening. Although the overall rotation in going from 1-S to 2-S must be conrotatory, the ring opening of 1-S starts with a disrotatory motion of both methylene groups. However, before the C1 TS is reached, the sense of rotation of one of the methylene groups changes. At the B₃LYP and CASSCF(4,4) levels, the transition state for the internal rotation of allene is a planar bent (C_2v) open-shell singlet (¹A₂). The D_2h open-shell singlet is a higher lying second-order stationary point. All D_2h and C_2v closed-shell species are significantly higher in energy. MR-CISD single-point activation barriers obtained from the B₃LYP and CASSCF(4,4)geometries were 44.6 and 45.5 kcal mol^-1, respectively. The B₃LYP/TZP ring opening (4.8 kcal mol^-1) and allene internal rotation (44.6 kcal mol^-1) barriers are in remarkably good agreement with the CCSD(T)/TZ2P//B₃LYP/TZP value (4.5 kcalmol^-1) on the barrier and with the MR-CISD/TZ2P//B₃LYP/TZP barrier for the latter (44.6 kcal mol^-1).

Harvard Citation style: Bettinger, H., Schreiner, P., Schleyer, P. and Schaefer, H. (1996) Ring Opening of Cyclopropylidene and Internal Rotation of Allene, Journal of Physical Chemistry, 100(40), pp. 16147-16154. https://doi.org/10.1021/jp961343f

APA Citation style: Bettinger, H., Schreiner, P., Schleyer, P., & Schaefer, H. (1996). Ring Opening of Cyclopropylidene and Internal Rotation of Allene. Journal of Physical Chemistry. 100(40), 16147-16154. https://doi.org/10.1021/jp961343f