inorganic chemistry 2 - yazd chemistry 2/inorganic... · inorganic chemistry 2 the electronic...

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Inorganic Chemistry 2

The Electronic Spectra of

Coordination Compounds

یونطیف الکترونی ترکیبات کوئوردیناس

1

Alireza Gorjiagorji@yazd.ac.ir

Department of Chemistry, Yazd University

agorji@yazd.ac.ir2

1d-d transition

Ligand Field transition

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The Electronic Spectra of Coordination Compounds

طیف الکترونی ترکیبات کوئوردیناسیون

3agorji@yazd.ac.ir

The aim of this chapter is to demonstrate how to interpret the origins of the

electronic spectra of coordination comps and to correlate these spectra with bonding.

The spectrum of the d3 complex [Cr(NH3)6] in aqueous solution

The Electronic Spectra of Coordination Compounds

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The classification of microstates

We start the analysis by setting up a table of microstates of the d2

configuration;have been only the microstates allowed by the pauli

principle have been included.The largest value of ML, which for a d2

configuration is +4. This state must belong to a term with L=4 (a G

term).

We can concluded that the terms of a 3d2 configuration are 1G, 3F, 1D, 3P, and 1S. These terms account for all 45 permitted states

Term Number of state1G 9x1 = 93F 7x3 = 211D 5x1 = 53P 3x3 = 91S 1x1 = 1

Total: 45

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It is possible to identify the term of lowest energy by using Hund’s rule

1. For a given configuration, the term with the greatest multiplicity

lies lowest in energy. For the d2 configuration, this rule predicts that

the ground state will be either 3F or 3P.

2. For a term of given multiplicity, the greater value of L, the lower

the energy. In this case, the 3F term is lower in energy than 3P

term.The ground term of a d2 species such as Ti2+ is expected to be 3F.

Thus, for d2 the rules predict the order

3F 3P 1G 1D 1S

but the order observed for Ti2+ from spectroscopy is

3F 1D 3P 1G 1S

The energies of the term

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The Racah Repulsion Parameters

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Energies of d2 free ion terms

3F 1D 3P 1G 1S

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Values for Racah Parameters

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Splitting of d2 free ion terms in Octahedral field

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Splitting of d2 free ion terms in Octahedral field

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Splitting of dn free ion terms in Ligand fields

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Tanabe-Sugano diagram for d2 config. Orgel diagram for d2 config.

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2

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2

3T1g

3T2g

3T1g

3A2g

Electronic Transitions of d2

ion in Octahedral Field

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4T1g

4T2g

4T1g

4A2g

Electronic Transitions of d7

ion in Octahedral Field

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d5+2

Electronic spectrum of [Co(H2O)6]2+

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4T1g

4T2g 4

T1g4

T1g (P)

4T1g

4A2g

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3T2g

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3T1g

3T1g

3A2g

4T1g

4T2g

4T1g

4A2g

d5+2d2

d2 , d7 Oh

Hole Formalism in Electronic Transitions of dn ion

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d7 d3

d2 d8

d2 d7 dn d10-ndn d5+n

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Electronic spectrum of [Cr(OH2)6]3+

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2

3

4A2g

4T2g

4A2g

4T1g

4A2g

4T1g(P)

3

Electronic spectrum of [Ni(OH2)6]2+

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3A2g

3T2g

3A2g

3T1g

3A2g

3T1g (P)

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d2 , d7 Td

d2 , d7 Oh

d3 , d8 Td

d3 , d8 Oh

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d2 , d7 Ohd3 , d8 Oh

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Electronic Transitions of d1 ion in Octahedral FieldThe number of microstates possible for dX configuration is given by formula

)!(!

!

XNX

N

d1 case corresponds to X = 1 and N = 10 (maximum occupancy of the d-level). The number of microstates is then 10 which means that any of the five degenerate d-orbitals may be occupied by an electron with a spin of ½ or - ½.

The orbital angular momentum for Ti3+, L = 2, the spin S = 1/2 and the term is 2D

2T2g

2Eg

Electronic Transitions of d1 ion in Octahedral Field

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lmax

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Hole Formalism in Electronic Transitions of dn ion

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d4 d9

dn d10-n

d1 d6

dn d5+n

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Td Oh

2T2g

2E

Td Oh

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d1 , d6 Td

d1 , d6 Oh

d4 , d9 Td

d4 , d9 Oh

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12T2g

2Eg

2T2g

2Eg

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15T2g

5Eg

Electronic spectrum of [Fe(OH2)6]2+

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5T2g

5Eg

Electronic spectrum of [Cr(H2O)6]2+

15Eg(D) 5T2g

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5Eg(D)

5T2g

5

5

5

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Electronic spectrum of [Cu(OH2)6]2+ 12Eg

2T2g

2Eg

2T2g

2

2

2

d5 metal complexes• Terms of free d5 metal ions are 6S, 4G, 4F, 4D, 4P, 2I, 2H, 2G, 2G, 2F, 2F, 2D, 2D, 2D, 2P, 2S (16 terms, 252

microstates). The lowest energy term is 6S.

• In the octahedral ligand field the 6S term will NOT be split. It gives rise to a single 6A1g term.

• The 6A1g term is the ground state term at weak ligand fields. NO terms of the same multiplicity exists and thus NO spin-allowed e-e transition is possible.

• At strong ligand fields spin pairing occurs (t23e2 t2

5). As a result, the ground state term and the multiplicity change from 6A1g to 2T2g(I)

.

4G

(t2)5

(t2)4(e)1

(t2)2(e)3

(t2)1(e)4

octahedral and tetrahedral d5

2T2

6A1

4P

4T1

4T2

4E

4T1

4T1

4T2

4E

4A2

6S

free ion weak field strong field

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Configuration (example) Ground

state

Excited states w/same S # Abs.bands

d1 oct (Ti(H2O)63+), d9 tetr. 2T2

2E2 1

d2 oct (V(H2O)63+), d8 tetr. 3T1 (F) 3T2,

3T1 (P), 3A2 3

d3 oct (Cr(H2O)63+), d7 tetr. 4A2

4T2, 4T1 (F), 4T1 (P) 3

d4 oct (Cr(H2O)62+), d6 tetr. 5E2

5T2 1

d5 oct (Mn(H2O)62+) or tetr. 6A1 none 0

d6 oct (Fe(H2O)62+), d4 tetr. 5T2

5E2 1

d7 oct (Co(H2O)62+), d3 tetr. 4T1 (F) 4T2,

4T1 (P), 4A2 3

d8 oct (Ni(H2O)62+), d2 tetr. 3A2

3T2, 3T1 (F), 3T1 (P) 3

d9 oct (Cu(NH3)62+), d1 tetr. 2E2

2T2 1

Summary

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Summary

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Electronic Transitions in Low Spin Complexes

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low spin

high spin Orgel Diagram

Tanabe-Sugano Diagram

Tanabe-Sugano Diagram

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Tanabe – Sugano Diagram

d2 A=0 C/B=4.42

E(1S)= A+14B+7C E(1S)= 14B+7C E(1S)/B= 14+7C/B 44.9 52.9E(1G)= A+4B+2C E(1G)= 4B+2C E(1G)/B= 4+2C/B 12.8 20.8

E(1D)= A-3B+2C E(1D)= -3B+2C E(1D)/B= -3+2C/B 5.8 13.8

E(3P)= A+7B E(3P)= +7B E(3P)/B= +7 7 15E(3F)= A-8B E(3F)= -8B E(3F)/B= -8 -8 0

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Tanabe – Sugano Diagram

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low spin high spin low spin high spin

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low spin high spin low spin high spin

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The Nephelauxetic Effect[V(H2O)6]

3+. B = 610 cm-1

V3+(g) B = 861 cm-1

This value indicates that electron repulsions are weaker than in the free ion. This

weakening occurs because the occupied moleculer orbitals are delocalized over the

ligands and away from the metal.

nephelauxetic parameter = B (comp)/ B(free ion)

The values of depend on the metal ion and the ligand. They vary along the

nephelauxetic series:

Br- Cl- CN- NH3 H2O F-

A small value of indicates a large measure of d-electron delocalization on to the

ligands and hence a significant character in the complex.The softer ligand, the

smaller the nephelauxetic parameter.

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Determination of O and B

O

OO

d1, d3, d4, d6, d8, d9 1=O

d2, d7 3 - 1 =O

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CrF63-

14900, 22700 , 34400 cm-1

= 14900 cm-1

2 + 3 - 3 1 = 15B’ = 12400

15B’ = 12400

B’ ≈ 827 cm-1

d3, d81 =

2 = 7.5B’ + 1.5 - 0.5 [225 B’2+2-18B’]1/2

3 = 7.5B’ + 1.5 + 0.5 [225 B’2+2-18B’]1/2

(2 +3 -31)/15=B’

V(H2O)63+ (d2)

1 = 17800 (3T1g 3T2g)

2 = 25700 (3T1g3T1g(P)) cm-1

The third expected transition 3 (3T1g(F) 3A2g) is far in the UV region and is masked by other absorptions. We can calculate the 3.

2/1 = 1.44

2:

2/B = 42(approximately): B= 2/42 = 25700cm-1/42 = 610 cm-1

1:

1/B = 29 (approximately): B= 1/29 = 17800 cm-1/29= 610 cm-1

Since o/B= 31, o= 31xB = 31x 610 cm-1 = 19000cm-1

3 ≈ (60)(610)=37210 cm-1

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2

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UV/VIS spectra of three

chromium(III) complexes:

a) [Cr(en)3]3+

b) [Cr(ox)3]3-

c) [CrF6]3-

look for the shift of the two

absorption peaks 1 and 2

to lower frequencies.

a)

b)

c)

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a) [Cr(en)3]3+

b) [Cr(ox)3]3-

c) [CrF6]3-

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[Ni(en)3]2+(purple)

9000 cm-114000 cm-1

25000 cm-1

[Ni(H2O)6]2+(green)

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B1g B2g

B1g Eg

Free ion term Oh D4h

When degenerate orbitals are asymmetrically occupied, J-T distortions arelikely

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John-Teller Distortion in Spectrum

Eg A1gEg B1g

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12T2g

2Eg

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2- Charge Transfer Transitions

agorji@yazd.ac.ir51Ligand to Metal Charge Transfer Metal to Ligand Charge Transfer

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Ligand to Metal Charge Transfer (LMCT)

Ligand to Metal Charge Transfer

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Ligand to Metal Charge Transfer (LMCT)

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Metal to Ligand Charge Transfer (MLCT)

Metal to Ligand Charge Transfer

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Intensity & Selection Rule

Bear-Lambert

A: جذب

b: cm طول مسیرعبور نور

A = log(I0/I)

: M-1cm-1 ضریب جذب مولی

c: M غلظت

A = bc

A

l

Intensity & Selection Rule

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i

j Transition Moment Integral

0

0 dO ji

Forbiden

Allowed

غیر مجاز

مجاز

اربیتیاسپینی

g g

u u

غیر مجاز

S0غیر مجاز

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Intensity & Selection Rule

اسپین تقارن (M-1cm-1)

d-d (Oh) مجاز

(S=0)

غیرمجاز

g g

20-200

d-d (Td) مجاز

(S=0)

مجاز >250

d-d غیرمجاز

(S0)

<1

CT مجاز

(S=0)

مجاز 1000-50000

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The spectrum of the d3 complex [Cr(NH3)6] in aqueous solution

The Electronic Spectra of Coordination Compounds

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Electronic spectrum of [Mn(H2O)6]2+

Why is absorption by [Mn(H2O)6]2+

so weak?6A1Excited states is no spin-allo-wed absoption, may be very weakforbidden transitions to excited stateof spin multiplicity other than 6

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S0 غیر مجازاسپین

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Vibronic Coupling

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Absorption

, cm-12500012500

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Absorption of a TMC in the UV and visible regions results from transitions of electrons between the energy levels available in the metal complex.

Of our interest will be:

1) The number of absorption bands

2) The energy of absorption bands

3) The intensity of absorption bands

4) The band width of absorption bands

.

1

2

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Lanthanide complexes

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Jablanski Diagram

4A2g

4T2g

4T1g

2Eg

Vio

let

Ab

sorp

tio

n

Gre

en A

bso

rpti

on

Flo

ure

scen

ce

Internal

Conversion

Intersystem

Crossing

Phosphorescence

Light

Amplification by

Stimulated of

Emission

Radiation

LASER

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Luminescence:

Flourescence

Phosphorescence

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Luminescence

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Optical Rotation

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Chiroptic Techniques

• Plane and circularly polarized light

• Definitions of terms

• Optical rotary dispersion (ORD)

• Circular dichroism (CD)

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Types of polarized light

• Plane polarized light consists two circularly polarized components of equal intensity

• Two circularly polarized components are like left- and right-handed springs

• As observed by looking at the source, right-handed circularly polarized light rotates clockwise

• Frequency of rotation is related to the frequency of the light

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optically inactive, nL= nR

Plane Polarized Light

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Plane Polarized Light

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Optical Rotation

optically inactive, nL= nR

optically active, nL nR

n=c/cv

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Optical Rotatory Dispersion (ORD)

moleper gramsin MW theis where,)cm M (deg ][][ rotation Molar

(g/mL)ion concentrat theis and (cm)length path theis where, rotation Specific

line) D Na the589;(normally •

180rotation of angle observed The

1-1-

RL

Mdc

MM

cddc

nnd

lll

ll

l

l

The technique of optical rotatory dispersion (ORD)

examines the wavelength dependence of optical activity.

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• Positive Cotton effect

• Negative Cotton effect

The Cotton Effect in ORD

Circular Dichroism

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Circular Dichroism

• is therefore the angle between the initial plane of polarization and the major axis of the ellipse of the resultant transmitted light

• A quantity is defined such that tan is the ratio of the major and minor axis of the ellipse of the transmitted light

• ’ approximates the ellipticity

• When expressed in degrees, ’ can be converted to a specific ellipticity [] or a molar ellipticity []

• CD is usually plotted as []

θ100.3032εε

10θ y ellipticitmolar

dc' y ellipticit specific

3

rl

2

M

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Circular Dichroism

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)(3300

).( y"ellipticit" as CD express sinstrument commercial reasons, historicalFor

)()()(

l

lll

l

RLCD

Optical rotatory dispersion (ORD) and circular dichroism (CD) are mathematically

related. If you measure one, you can calculate the other by means of functions called

Kronig-Kramers Transforms.

lll

l

lll

l

ll

ll

dM

dM

o

o

o

o

o

o

22

22

][2][

2

CD is more commonly used than ORD

to study molecules.

- Better resolution

- Better sensitivity.

- Easier to assign 79agorji@yazd.ac.ir

ORD, CD and UV of Camphor

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CD Instrumentation

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The Cotton Effect

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Use of CD and ORD spectra

1- Determination of absolute configuration (&)

2- Assignment of Electronic spectra

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1- Determination of absolute configuration (&)

Reference:d6 low spin

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1- Determination of absolute configuration (&)

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1- Determination of absolute configuration (&)

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2- Assignment of Electronic spectra

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d-Orbital splitting of other coordination geometries

89

5-Coordinate: Trigonal Bipyramidal or Square Pyramidal

(90° & 120°) (~100° & 90°)

ML

L L

L

L

L ML

L

L

L

apical

basal

axial

equatorial

L ML

L

3-Coordinate: Trigonal planar (120°)

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trigonal bipyramid90

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Trigonal Bipyramidal

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?

Square Pyramidal

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L=-acceptor L=-donor

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Shape of ML5 complexes

a: low-spin d6 oxyhemoglobinb: low-spin d8 [Ni(CN)5]3-

c: d10 Cu(I)

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Mixed valence compounds

MII -------X-------MIII

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[Mn+ -------X-------Mm+](n+m)+

[M((n+m)/2)+ -------X-------M((n+m)/2)+] (n+m)+

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Robin – Day classification