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

(a) Fluorescence and phosphorescence on a Jablonski diagram

After absorption of a photon (S₀ → S₁ or a higher singlet, about 10⁻¹⁵ s), the molecule loses its excess vibrational energy by collisions (vibrational relaxation, 10⁻¹²–10⁻¹⁰ s) and reaches the lowest vibrational level of S₁. From there it can:

• fluoresce: emit a photon, S₁ → S₀;
• undergo internal conversion: return to S₀ without radiation;
• undergo intersystem crossing (ISC) to T₁: a spin flip made possible by spin–orbit coupling. After relaxing to the bottom of T₁, the molecule can phosphoresce (emit a photon, T₁ → S₀) or return to S₀ by a second ISC.

Distinctions

1. Spin multiplicity:

• Fluorescence is singlet → singlet (ΔS = 0, spin-allowed).
• Phosphorescence is triplet → singlet (ΔS ≠ 0, spin-forbidden). It occurs only because spin–orbit coupling mixes singlet and triplet character, which is why heavy atoms (Br, I, metal centres) enhance it.

2. Lifetime:

• Fluorescence: about 10⁻⁹–10⁻⁷ s; it stops almost as soon as the exciting light is switched off.
• Phosphorescence: about 10⁻⁴ s to many seconds; it gives an afterglow.

3. Route: phosphorescence needs an intermediate ISC step; fluorescence does not.
4. Wavelength: T₁ lies below S₁ (Hund's rule), so phosphorescence appears at longer wavelength, with a larger Stokes shift than fluorescence. Fluorescence is usually a mirror image of the S₀ → S₁ absorption band.
5. Environment:

• The long-lived triplet is easily deactivated by collisions, by heat and by paramagnetic O₂ (itself a triplet). Phosphorescence of organic molecules is therefore normally seen only in rigid media or frozen glasses at 77 K, or in solids.
• Fluorescence is readily observed in fluid solution at room temperature.

6. Examples:

• Fluorescence: fluorescein, quinine sulfate, the green fluorescent protein.
• Phosphorescence: benzophenone at 77 K; ZnS:Cu and SrAl₂O₄:Eu,Dy "glow-in-the-dark" phosphors (in the inorganic phosphors, trap states rather than a molecular triplet produce the long afterglow).

(b)(i) How water condenses on glass

1. Surface chemistry: a clean glass surface is covered with silanol (Si–OH) groups, roughly 4–5 per nm², and adsorbed water. It is therefore a high-energy, hydrophilic surface.
2. Adsorption: water molecules from the vapour are first held by hydrogen bonds to the Si–OH groups (physisorption). As the relative humidity rises, further layers build up on this first layer, since water hydrogen-bonds strongly to adsorbed water (multilayer, BET-type adsorption).
3. Condensation: when the glass is cooled below the dew point of the surrounding air (a cold drinking glass, a car windscreen), the adsorbed film grows into bulk liquid. The surface acts as a site for heterogeneous nucleation, so condensation occurs at much smaller supersaturation than in free air. In fine cracks and pores it starts even below saturation (capillary condensation, Kelvin equation).
4. Film or droplets:

• On clean glass the contact angle is nearly zero, so water spreads into a continuous transparent film.
• On glass contaminated by grease or organic films (a higher contact angle) it condenses as myriad tiny droplets that scatter light: the "fogging" of spectacles and mirrors.
• Anti-fog coatings work by making the surface strongly hydrophilic, so that a film rather than droplets forms.

(b)(ii) Time to form half a monolayer at 2.0 × 10⁻¹¹ torr

1 langmuir (L) = an exposure of 10⁻⁶ torr × 1 s. If 1 L gives a monolayer (sticking probability ≈ 1), half a monolayer needs 0.5 L:

t = exposure/pressure = (0.5 × 10⁻⁶ torr s)/(2.0 × 10⁻¹¹ torr) = 2.5 × 10⁴ s ≈ 6.9 h

Why such low pressures are used

• At 10⁻⁶ torr a monolayer of residual gas (H₂O, CO, H₂) builds up in about 1 s. A surface cleaned by ion sputtering, heating (flashing) or cleaving would be recontaminated almost at once.
• Only in ultra-high vacuum (UHV, below 10⁻⁹ torr) does a clean surface stay clean for hours. That is long enough to characterise it by LEED, XPS, AES or STM, or to study controlled adsorption.
• UHV also gives the electrons and ions used as probes a long mean free path, so that they reach the surface and the detector unscattered.

(c) IUPAC names

1. Na₂[ZnCl₄]: sodium tetrachloridozincate(II). The anion is tetrahedral and Zn is +2.
2. [PtCl₆]²⁻: hexachloridoplatinate(IV) ion (Pt +4; 6Cl⁻ − 4 = 2−).
3. [Pt(py)₄][PtCl₄]: tetrakis(pyridine)platinum(II) tetrachloridoplatinate(II).

• Both Pt centres are +2. "Tetrapyridine" is also seen in textbooks.
• This is a Magnus-type salt. It shows coordination isomerism, e.g. [PtCl(py)₃][PtCl₃(py)], and is a polymerisation isomer of [PtCl₂(py)₂].

4. [(H₃N)₄Co(μ-NH₂)(μ-OH)Co(NH₃)₄]⁴⁺: μ-amido-μ-hydroxido-bis[tetraamminecobalt(III)] ion, equivalently octaammine-μ-amido-μ-hydroxido-dicobalt(III) ion.

• Charge check: 2 Co(III) = +6, and NH₂⁻ + OH⁻ = −2, giving 4+.
• The prefix μ marks a ligand bridging two metal centres.

5. Ni(CO)₄: tetracarbonylnickel(0), a tetrahedral, 18-electron complex.

(Since the 2005 IUPAC recommendations, anionic ligands take the "-ido" ending, so chloride is chlorido and hydroxide is hydroxido. The older "chloro" and "hydroxo" forms remain common in textbooks.)

(d) Zeise's salt

Definition. Zeise's salt is potassium trichlorido(η²-ethene)platinate(II) monohydrate, K[PtCl₃(C₂H₄)]·H₂O, a lemon-yellow crystalline solid. W. C. Zeise prepared it in 1827. It was the first transition-metal–alkene complex and one of the earliest organometallic compounds known, but its structure was not understood until the Dewar–Chatt–Duncanson model (1951–53).

Synthesis

• Modern method: ethylene is bubbled at room temperature through a solution of K₂[PtCl₄] in dilute HCl, with a little SnCl₂ as catalyst. Ethylene replaces one chloride:

K₂[PtCl₄] + C₂H₄ → K[PtCl₃(C₂H₄)] + KCl

On concentration the monohydrate crystallises.

• Zeise's original route: PtCl₄ (or H₂PtCl₆) was boiled with ethanol, which supplies the ethylene, and KCl was then added.

Structure (from X-ray and neutron diffraction)

• The [PtCl₃(C₂H₄)]⁻ anion is square planar about Pt(II) (d⁸). Three positions are occupied by Cl and the fourth by the midpoint of the C=C bond. Ethylene is bound side-on (η²).
• The C=C axis is perpendicular to the PtCl₃ coordination plane. This orientation gives the best π back-bonding overlap and the least steric clash with the cis chlorides.
• The C–C bond lengthens from 134 pm in free ethylene to about 137.5 pm, and the four H atoms are bent back away from the metal. The carbons have acquired some sp³ character.
• The Pt–Cl bond trans to ethylene (≈ 234 pm) is longer than the cis bonds (≈ 230 pm). This reflects ethylene's strong trans influence, the same property behind the trans effect.

Bonding: the Dewar–Chatt–Duncanson model

1. σ-donation: the filled π orbital of C=C donates electron density into an empty dsp² hybrid on Pt.
2. π back-donation: a filled Pt d orbital (dxz) donates into the empty π* orbital of C=C.

Both effects reduce the C=C bond order. As a result the bond lengthens, and the C=C stretching frequency falls from 1623 cm⁻¹ in free ethylene to about 1526 cm⁻¹ in the complex. The two effects are synergic: donation makes the metal richer in electrons, which strengthens back-donation.

(e)(i) Lanthanide contraction

Definition. Lanthanide contraction is the steady decrease in the atomic and ionic radii of the lanthanides with increasing atomic number, from La to Lu. For example, Ln³⁺ falls from 103 pm (La³⁺) to 86 pm (Lu³⁺) for six-coordination.

Cause.

• Across the series each step adds one proton to the nucleus and one electron to the 4f subshell.
• The 4f orbitals are diffuse and poorly penetrating, so they shield one another and the outer 5s, 5p, 5d and 6s electrons very imperfectly.
• The effective nuclear charge felt by the outer electrons therefore rises steadily and pulls the whole electron cloud inward.
• Relativistic effects make a smaller contribution, estimated at about 10–30%.

Consequences:

• The lanthanides are very alike and hard to separate (ion exchange and solvent extraction are needed).
• The basicity of Ln(OH)₃ decreases from La to Lu.
• Y³⁺ (90 pm) is similar in size to Ho³⁺, so yttrium occurs with the heavy lanthanides.
• The 5d (third) transition series has almost the same atomic sizes as the 4d series, as in Zr/Hf, Nb/Ta and Mo/W.
• The 5d metals are very dense (Os and Ir are the densest elements) and have high ionisation energies, which contributes to the nobility of Pt and Au.

(e)(ii) Why Hf is placed below Zr

Electronic basis. Hf ([Xe]4f¹⁴5d²6s²) has the same outer valence configuration, (n−1)d²ns², as Zr ([Kr]4d²5s²) and Ti. It is the first element after the lanthanides to begin filling the 5d subshell beyond one electron, so it belongs to Group 4, directly below Zr.

Size. Moving down a group from the 4d to the 5d period would normally increase the radius substantially. But the 14 intervening lanthanide elements add 32 units of nuclear charge (Z = 40 for Zr, 72 for Hf), including the poorly shielded 4f electrons. The lanthanide contraction cancels the expected increase almost exactly:

• metallic radii: Zr 160 pm, Hf 159 pm;
• ionic radii: Zr⁴⁺ 72 pm, Hf⁴⁺ 71 pm.

Chemical twins. Because Zr and Hf have the same charge, the same valence configuration and virtually the same size, they are chemically almost identical:

• both are stable in the +4 state;
• they form isostructural oxides (ZrO₂/HfO₂) and halides (ZrCl₄/HfCl₄), and similar complex fluorides K₂[MF₆];
• they always occur together in nature (zircon, ZrSiO₄, contains 1–3% Hf).

Hafnium was discovered only in 1923 (by Coster and von Hevesy, using X-ray spectroscopy), precisely because it hides inside zirconium minerals. Their separation still requires multistage solvent extraction or ion exchange. Hf is almost twice as dense as Zr (13.3 against 6.5 g cm⁻³): nearly the same atomic volume holds almost double the mass.

Practical importance: nuclear-grade Zr (used for fuel cladding because of its low neutron-capture cross-section) must be freed of Hf, whose capture cross-section is very high and which is therefore used in control rods.