Question

c) The ground state configuration of O2* is ’n, and the next few excited states are *II,,?11,453,4_,*£g, and “ ,. On the basis of the selection rules, which of the excited states can be accessed from the ground state by absorption of UV light? [6] d) Predict the number of unpaired electrons and the ground state term of (i) NO and (ii) CO. [8]

Answer 1

c)

​​​We have mainly two selection rules for the electronic transition from one state to another. They are Laporte Selection rule and Spin selection rule.

Laporte selection rule say that, "electronic transition between states of same parity are forbidden for center of symmetric molecule". ie, gerade (g) to ungerade (u) transition and vice versa allowed transitions. But, gerade to gerade or ungerade to ungerade transitions are forbidden transitions.

Spin selection rule states that, "there must be zero change in overall spin (S) during electronic transition". i.e, $\Delta$ S = 0

So here we have the molecule O₂⁺ which is a centerosymmetric molecule. So Leporte and spin selection rule is applicable in case of electronic transition of O₂⁺ molecule.

d)

Center of Symmetric Molecule Griver term symbol is a Tg Given next few excited states arre, The general form of writing term symbols are ) 2844 + alu ө 1 =) L 2 3 gerade ; u =) ungerade; S=) spin ; from Laporte selection rule sition is forbidden. So the transitions, tran we can eliminat Now remaining transitions are, Eu Here we Here @ Tg use spin selection rule, ie, A8 = 0; → 28+1 = 2 28 = 1 8 = % So, s should be for the excited state to satisfy As=0 sule. So, for * Tu & tu, 25+1 = 4 23 = 3 s=32 So, This transitions are E I also not allowed. So, the allowed transition is, ***

for NO molecule Total number of electrons = 15 (10-), (16*), 26-), (0*)",(17)= 2 Number of unpaired electrons = 1 Term Symbol - 3 - 1% So, 23+1 = = (+182) + (1x1)=2-1= 1 So, term Symbol of No, 2 [No +/- sign since mirror plane along worrit internuclear anis is not considered in case of a [No gle because, no center of symmetry for co molecule Total member of electrons = 14 (10-) / (104)",Q), @*),(6)=(1^)?, Number of unpaired electrons = 1 =) 3 = 1/2 - 1 / 2 = 0 So, ast1= @x0+1=1 1=(4181) + (1x1) = 1-1=0 (lakjes (**) Term Symbol So, term symbol of Co= [t sign because there is symmetric mirror plane worst internuchar axis. [No glu because center of symmetry