Experiment ll: congeners the same periodic Congeners are compounds that are anal

Question

Experiment ll: congeners the same periodic Congeners are compounds that are analogous except for a change in elements from g., Hzo Has. ir shapes compare? Build the following molecules, perform Equilibrium Geometry calculations on them using Hartree-Fock and their bond distances and angles as listed qu A H-o distance H20 H-o-H angle LOS GI Pair 1 H-s distance 1.27 A H-S-H angle H-N distance I. 01 A NH H-N-H angle Pair 2 H-P distance 1.403 A PH3 H-p-H angle as 40 distance 1.3a l A C-0 H3COCHB C o-C angle Pair 3 Si-O distance I o A H3SiOSiH, s-o si angles luaa a3 Discuss and explain the trends in each pair of congeners.

Explanation / Answer

In all 3 pair, the bond length is small for the compound containing element having small size. In first pair size of oxygen is less than sulfur and hence the bond length will be small in water compared to H2S. Same explanation also suits for the other 2 pair.

In the case of bond angle of first two pairs, the heavier congeners are having lesser bond angle compared to their light congeners. This trend can be explained in different ways. Down the group size increases and electronegativity decreases. Hence the bond pairs are far from the element as we move down the group. Hence the angle between them decreases. Another explanation is that down the group the heavier elements uses pure orbitals rather than hybridised orbitals. Absence of orbitals having similar energy is the reason behind this. Hence here in the first two pairs the heavier congeners uses the pure atomic orbital for bonding. Since P orbitals are mutually perpendicular, the value comes around 90 for heavier congener.

In pair 3 it is those atoms attached to the central oxygen atom is changing. The Si-O-Si bond angle is higher than the carbon analogue. This is due to the higher repulsion of bond pairs of Si-O bond. The large size of silicon compared to carbon also increases the steric repulsion which is minimised by the higher bond angle.

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