(a) Indicating 'Fluorescence' and 'Phosphorescence' processes in Jablonski diagram, distinguish between both processes. (10 marks)

(b) (i) How does water condense onto glass? (5 marks)
(ii) How long will it take for about one-half of a monolayer to

Question

(a) Indicating 'Fluorescence' and 'Phosphorescence' processes in Jablonski diagram, distinguish between both processes. (10 marks)

(b) (i) How does water condense onto glass? (5 marks)
(ii) How long will it take for about one-half of a monolayer to form on the surface if the surface is exposed to a pressure of 2·0 × 10⁻¹¹ torr? Assume that a monolayer formation needs an exposure of 1 Langmuir. Also give reason for using such low pressure in cleaning the surface. (5 marks)

(c) Write the IUPAC nomenclature of the following complexes: (10 marks)
(i) Na₂[ZnCl₄]
(ii) [PtCl₆]²⁻
(iii) [Pt(py)₄] [PtCl₄]
(iv) [(H₃N)₄Co Co(NH₃)₄]⁴⁺
 H₂N
 \
 O
 /
 H
(v) Ni(CO)₄

(d) What is Zeise's salt? Outline its synthesis method. Describe its structure. (10 marks)

(e) (i) Explain lanthanide contraction. (5 marks)
(ii) Hf is placed below Zr in the periodic table. Justify the statement. (5 marks)

UPSC Answer Check · Accepted Answer

(a) Fluorescence and phosphorescence. In a Jablonski diagram, the ground singlet state S0 is drawn at the bottom, the first excited singlet S1 above it, and the first excited triplet T1 below S1 but above S0. The relevant transitions are S0 to S1 for absorption, S1 to S0 for fluorescence, S1 to T1 for intersystem crossing, and T1 to S0 for phosphorescence. Fluorescence is a singlet-to-singlet, spin-allowed emission, whereas phosphorescence is a triplet-to-singlet, spin-forbidden emission. This spin difference is the key property that distinguishes the two: fluorescence can occur readily because the initial and final states have the same spin multiplicity, while phosphorescence has a small transition probability because the spin changes. Consequently, fluorescence has a short lifetime, usually 10⁻⁸ to 10⁻⁴ s, and phosphorescence has a much longer lifetime, usually 10⁻⁴ to 10² s, so phosphorescence may persist after the excitation source is switched off. Fluorescence is generally observed at room temperature and is relatively less temperature dependent; phosphorescence is strongly temperature dependent because heating increases non-radiative decay and quenching of the triplet state. Phosphorescence is also more readily quenched by oxygen and other paramagnetic species. Since T1 lies lower than S1, phosphorescence usually has a larger Stokes shift than fluorescence, and emission often occurs from the lowest vibrational level of T1 to various vibrational levels of S0.

(b) Water condensation and Langmuir exposure. (i) Water condenses onto glass because the glass surface is hydrophilic and contains surface silanol, Si-OH, groups. Water vapour molecules first adsorb by hydrogen bonding to these hydroxyl groups. Once a few molecules are held, they hydrogen bond to one another, forming a monolayer and then a thin liquid film. At higher relative humidity, capillary condensation and multilayer growth spread the film over the surface. (ii) One Langmuir is defined as an exposure of 10⁻⁶ torr s. For about one-half of a monolayer, the required exposure is 0.5 × 10⁻⁶ torr s. At a pressure of 2.0 × 10⁻¹¹ torr, the time is t = exposure/pressure = (0.5 × 10⁻⁶ torr s)/(2.0 × 10⁻¹¹ torr) = 2.5 × 10⁴ s, which is about 6.9 h, i.e. roughly 7 h. Such low pressure is used in cleaning the surface because ultra-high vacuum minimises residual gas impurities, prevents re-contamination of the freshly cleaned surface, gives a long mean free path for controlled adsorption and desorption, and avoids oxidation or adsorption of unwanted species.

(c) IUPAC names. The complexes are named as follows, using chlorido rather than chloro according to current IUPAC. (i) Na2[ZnCl4] is sodium tetrachloridozincate(II), because the four chloride ligands give Zn a +2 oxidation state. (ii) [PtCl6]2- is hexachloridoplatinate(IV). (iii) [Pt(py)4][PtCl4] is tetrapyridineplatinum(II) tetrachloridoplatinate(II); pyridine is neutral, so the cationic Pt is +2, and the anionic Pt is also +2. (iv) The bridged cation shown, with four ammine ligands on each Co, a μ-amido bridge and a μ-hydroxido bridge, is the μ-amido-μ-hydroxido-bis[tetraamminecobalt(III)] ion; the μ prefix shows that each bridge links the two cobalt centres, and the two negative bridging ligands make each Co +3. (v) Ni(CO)4 is tetracarbonylnickel(0), because CO is a neutral ligand.

(d) Zeise's salt. Zeise's salt is potassium trichlorido(ethylene)platinate(II) monohydrate, K[PtCl3(C2H4)]·H2O, and it is historically important as one of the first organometallic compounds. It is prepared by passing ethylene gas through an aqueous solution of potassium tetrachloroplatinate(II), K2[PtCl4], so that ethylene displaces one chloride from the square-planar Pt(II) complex; yellow crystals of the monohydrate separate. The reaction may be written K2[PtCl4] + C2H4 → K[PtCl3(C2H4)]·H2O + KCl. Structurally, Pt is d8 and essentially square planar, coordinated by three chloride ligands and one η2-ethylene ligand. The ethylene is bound side-on through its π bond, with the C=C axis perpendicular to the PtCl3 plane and the midpoint of the C=C bond occupying the fourth coordination site. The bonding is described by the Dewar-Chatt-Duncanson model: the filled C=C π orbital donates electron density to an empty orbital on Pt, while filled Pt dπ orbitals back-donate into the C=C π* orbital. This back-bonding weakens the C=C bond, lengthens it relative to free ethylene, and lowers its stretching frequency.

(e) Lanthanide contraction and Hf-Zr. (i) Lanthanide contraction is the gradual decrease in atomic and ionic radii from La to Lu. Across the lanthanide series the nuclear charge increases steadily, but the added 4f electrons shield the outer electrons poorly. The ineffective shielding allows the valence electrons to experience a greater effective nuclear charge, so the 5d and 6s orbitals contract steadily. The result is a reduction of about 15% in radius from La to Lu and very similar sizes of the lanthanide ions. This contraction also explains similarities among 3d, 4d and 5d transition metal ions. (ii) Hf is placed below Zr in Group 4 because, although Hf is in the 5d series and would normally be expected to be larger than Zr, the lanthanide contraction across the 4f series preceding Hf almost cancels the normal increase in size down the group. The metallic or ionic radii are nearly the same, for example Zr about 160 pm and Hf about 159 pm, and both have the same outer d2s2 valence configuration and common +4 oxidation state. Consequently their chemical properties, complex formation and ionic behaviour are so similar that Hf and Zr are difficult to separate, often requiring ion-exchange or solvent-extraction methods, and Hf is correctly placed directly below Zr.

(a) Indicating 'Fluorescence' and 'Phosphorescence' processes in Jablonski diagram, distinguish between both processes. (10…

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