Ring Opening of Cyclopropylidene and Internal Rotation of Allene

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 singlettriplet 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 C s 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 C 1 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 ( 1 A 2 ). 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 kcal mol -1 ) on the barrier and with the MR-CISD/TZ2P//B3LYP/TZP barrier for the latter (44.6 kcal mol -1 ).

Stripping dynamics in the reactions of electronically excited carbon atoms, C(1D), with ethylene and propylene—production of propargyl and methylpropargyl radicals

“…Cyclopropylidene undergoes a ring opening leading to allene 2 via a barrier of 26 kJmol Ϫ1 . The barrier height obtained by the present G2M͑RCC,MP2͒ calculations is slightly higher than the CCSD͑T͒/TZ2P//B3LYP/TZP value of 18.8 kJmol Ϫ1 value reported by Bettinger et al 27 Allene resides in a deep potential well, 699 kJmol Ϫ1 lower in energy than C( 1 D)ϩC 2 H 4 , and can eliminate hydrogen atom to yield the propargyl radical p1, H 2 CCCH(X 1 B 1 ), without exit barrier or molecular hydrogen producing H 2 CCC(X 1 A 1 ), p2, via a barrier of 387 kJmol Ϫ1 . The transition state for the H 2 elimination lies 312 kJmol Ϫ1 below the reactants, while the C( 1 D)ϩC 2 H 4 →H 2 CCCH(X 2 B 1 )ϩH and C( 1 D)ϩC 2 H 4 →H 2 CCC(X 1 A 1 )ϩH 2 reactions are computed to be exothermic by 317 and 351 kJmol Ϫ1 , respectively.…”

Ring inversion barrier of diazepam and derivatives: Anab Initio study

“…Very surprisingly, DFT has proved to be able to handle many of these cases. [18][19][20][21][22] To summarize recent DFT studies devoted to explore the applicability of the method to cases when nondynamical correlation can play an important role, one may mention the following articles. Pulay and coworkers calculated the geometry and vibrational spectra of azulene 20 using DFT and UNO-CAS methods to answer the question whether the minimum has C 2v or C s symmetry.…”

Ring-opening mechanism of disilacyclopropylidenoids and trisilacyclopropylidenoid: A theoretical study

“…Moreover, the ring-opening of 14 to 11 follows a simple disrotatory motion of the H 2 Si groups. The ring opening of cyclopropylidene to allene also starts with a disrotatory motion of the methylene groups, but additional geometry changes are required to reach the final linear geometry of allene [26,40,41]. The overall energy barrier for ring opening of cyclopropylidene to allene is also low, 6.1 and 4.8 kcal/mol, but in this case the reaction is highly exothermic, by 67.9 and 69.3 kcal/mol at the B3LYP/6-31G(d) and B3LYP/TZP levels, respectively [26,40].…”