MC‐SCF and CI calculations on the Si₄H₄ system

Abstract

The lowest singlet along with selected high spin states of three isomers of Si,H, have been investigated, using highly correlated wave functions in conjunction with a local pseudopotential approach. Tetrasilabicyclo[ I.. l.O]but-l(3)-ene 1 and tetrasilatetrahedrane 3a are established as true minima by means of the harmonic vibrational frequencies. The local minimum for tetrasilacyclobutadiene is not a planar 2a but a puckered conformer 2b. Isomer 1 was found to be the most stable of the investigated isomers lying 1.96 eV below 3a and 1.04 eV below 2b. A previously examined bond stretch isomer of tetrasilatetrahedrane 3b lies only 0.2 eV above isomer 1. The Si-Si bond energies of 2b, 3a, and 3b are determined as 208, 124, and 228 kJ/mol, respectively. An explanation of the bonding situation in 1 is given.

A Tetrasilicon Analogue of Bicyclo[1.1.0]but‐1(3)‐ene Containing a Si=Si Double Bond with an Inverted Geometry

“…Them olecular structure of 1 was further examined by theoretical calculations.T he structural characteristics of 1, optimized at the B3PW91-D3/B1 (B1: 6 -311G(d) [Si]a nd 6-31G(d) [C,H]) level of theory (1 opt ), is in good agreement with those obtained experimentally:the planar geometry of the Si 4 moiety (dihedral angle Si2-Si1-Si1*-Si2* = 180.08 8)a nd the Si1ÀSi1* (2.4671 ), Si1ÀSi2 (2.2834 ), Si1-Si2* (2.3100 ) distances are close to those obtained from the XRD analysis [180.08 8;2 .4716(11) ,2 .2949 (3) ,a nd 2.3106(4) ,r espectively].T he planar Si 4 framework of 1 should be due to the severe steric demand of the substituents on the saturated silicon atoms as the optimized structures of hydrido-, methyl-, and butan-1,4-diyl-substituted bicyclo[1.1.0]tetrasil-1(3)-enes 1 H , 1 Me ,a nd 1 BD (Figure 4) adopt bent structures at the B3PW91-D3/6-311G(d) level of theory similar to the structures of previously reported 1 H [22] and D. [11] Thedihedral angle increases with increasing steric demand of the substituents on the saturated silicon atoms (1 H :1 42.78 8; Yates et al [22b] Thef ree-energy difference between the bent and planar structures,w hich corresponds to the activation barrier for ring flipping,are very small and decrease with increasing steric demand of the substituents (1 H :1 2.9 kJ mol À1 , 1 Me :7 .3 kJ mol À1 , 1 BD : 5.7 kJ mol À1 ). Thei ntrinsic shallow potential for the bending of the Si 4 moiety in bicyclo[1.1.0]tetrasil-1(3)-ene and the presence of the bulky iPrMe 2 Si groups are likely responsible for the observed planar Si 4 moiety.T he stabilization of transition-state structures for ring flipping due to the presence of bulky substituents has been examined in detail for 1,3dibora-2,4-diphosphabicyclo[1.1.0]butane derivatives.…”

H-Bridged Structures for Tetrahedranes A4H4 (A = C, Si, Ge, Sn, and Pb)

“…Isomer 10 , which is the lowest energy structure for Si 4 H 4 , so far, is also computed in the present study . It is a minimum for C, Ge, Sn, and Pb as well.…”

“…There are many theoretical calculations available in the literature on 1-C and 1- Si . , Structure 1 is a minimum for C to Pb, except for Si, in which it is a second order saddle point (Table ). However, the results at the HF and B3LYP levels are different.…”

“…The triply hydrogen bridged structure, 9 ( C 3 v ), is another candidate considered in the present study. The lowest energy structure suggested for Si 4 H 4 , 10 ( C s ), is also included for comparison …”

The substitution effect on heavy versions of cyclobutadiene

“…With inclusion of electron correlation through the configuration interaction (CI) method, the lowest‐energy isomer is found to be a D 2d puckered ring structure with four equal SiSi bond lengths and an axial arrangement of the hydrogen atoms. This structure is predicted to lay 28 kcal mol −1 below the previously reported tetrasilatetrahedrane 89–98. Detailed study of structures and electron affinities of Si 4 H n ( n = 2–10) and Si ( n = 3–7) have been carried out 98, 99 using density functional theory (DFT).…”