Rotational energy surfaces of aza- and phospha-1,3-butadienes. A theoretical study

Abstract

7929that except for 2,4,5-trithia[ 1.1.1 ]propellane, single determinant wave function treatments may not be appropriate for these species.B. Germanium and Tin Compounds. Since ECP calculations compare favorably with full ab initio results, only ECP results are reported for the heavier atoms. The most interesting electronic structural features found for C and Si are those in the trioxa species, so we limit ourselves to these and omit discussion of the sulfur analogues. The results of structures, energetics, and the TCSCF NOON of the germanium and tin trioxa[ 1.1 .l]propellane derivatives are listed in Tables I-IV along with the carbon and silicon analogues. Because the essential conclusions drawn for Ge and Sn compounds (3 and 4 with M = Ge, Sn and L = 0) are the same as those discussed above for C and Si, only the key features of these species will be addressed.In both the germanium and tin analogues of 3 (L = 0) extremely short bridgehead M-M distances are found at the RHF, ROHF, and TCSCF levels of theory. These bridgehead M-M distances are not significantly affected by the additions of hydrogens across the bridgehead centers (cf. Tables I and 11). Indeed, the differences between the M-M distances in the propellanes 3 and the bicyclopentane 4 analogues are less than 0.1 A for both M = Ge and Sn. Furthermore, differences among the three levels of theory (RHF, ROHF, and TCSCF) in the corresponding M-M bridgehead distances are within 0.1 A. These results are similar to those found in the silicon analogues of 3 and 4. The RHF/SBK(d) geometric results for 3 (M = Ge, L = 0)and the corresponding bicyclopentane analogue are essentially identical with those calculated by Nagase and Kudo'3a using the 3-21G(d) basis set at the R H F level of theory. The percent diradical character and the singlet-triplet splittings for Sn (2896, 22.0 kcal mol-') and Ge (25%, 29 kcal mol-') are also similar to those discussed above (36%, 20.7 kcal mol-') for the Si analogue of 3. IV. ConclusionsIn this study, ab initio molecular orbital theory has been used to investigate the structure and bonding of sulfur and oxygen propellane derivatives (3) and their bicyclopentane analogues (4) with RHF, ROHF, and TCSCF wave functions. We have found that the M = Si, Ge, and Sn species possess unusually short bridgehead distances. However, this does not result in significant bonding interactions, as shown by the TCSCF calculations and total density plots. For M = C, TCSCF calculations and total density analyses suggest substantial bridgehead bonding only in the L = S system. We have found excellent agreement in structures and energetics between ECP calculations and the 6-3 1G(d) all-electron calculations.Abstract: The rotational energy surfaces of all stereoisomers of I-aza-, 2-aza-, 1,3-diaza-, 1 ,Cdiaza-, and 2,3-diaza-1,3-butadienes and 1 -phospha-, 2-phospha-, 1,3-diphospha-, 1,4-diphospha-, and 2,3-diphospha-1,3-butadienes were calculated at the MP2/6-3 1 G*//HF/6-3 1 G* level. Rotational barriers and all local minima were rigorously located and ...

Synthesis and structural studies of symmetric and unsymmetric adamantylmethyleneazines

“…Its C=N and N-N bond lengths [1.277(2) and 1.418(3) Å ] are longer than those calculated in the present study, and the C-N-N bond angle [111.4(2)°] is smaller if the HF method is considered, but they agree fairly well when methods that take into account electron correlation effects are used ( Table 4) (note that Bachrach and Liu 27 geometries being calculated at the HF/6-31G* level are identical with ours for 14 in Tables 3 and 4). Since MP2 and B3LYP calculations are of comparable quality and owing to the computer time consumed by MP2 calculations for the azines under study, only the HF and B3LYP methods were applied to 15 and 17.…”

“…Previous ab initio studies of azines were limited to formaldazine (14); the potential hypersurface of this compound was calculated by Bachrach and Liu 27 at the MP2/6-31G*//HF/6-31G* level. Other azines have been calculated using semi-empirical methods.…”

Nature of Bonding in Cyclic Conjugated Ylides

“…3 (Barriers for the rotation about the central single bond in case of phosphabutadienes, however, are lower. 4 ) Energies of isodesmic reactions revealed that energy gain in conjugation is nearly the same for λ 3 -PdC and CdC bonded systems, and π-ionization energies of λ 3 -PdC systems are again nearly identical with those of the CdC systems. 5 Aromatic stability of phosphabenzene was found to be just slightly less, than that of benzene.…”

Preparation of 1,3‐diphosphabuta‐1,3‐dienes from sterically encumbered phosphaalkyne and phosphaethenyllithium

“…1. The P C P C skeleton is nearly planar [ (P1 C1 P2 C2) 176.9 (4) • ], which has been determined as the most stable conformer by theoretical calculations [14]. The benzyl group is located trans to the Mes* substituent on C2 to minimize steric repulsion.…”