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1 Group representations Consider the group C 4v ElementMatrix E C C C Example molecule: SF 5 Cl S F F F F Cl F x y z 3

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2 Group representations Consider the group C 4v ElementMatrix E C C C Example molecule: SF 5 Cl S F F F F Cl F x y z (xyz) (yxz) 3

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3 Group representations Consider the group C 4v Element Matrix E C C v v C d d Example molecule: SF 5 Cl S F F F F Cl F x y z 3 ' '

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4 Group representations These matrices obey all rules for a group when combination rule is matrix multiplication: Identity exists - E Products in group = v C 4 d '

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5 Group representations These matrices obey all rules for a group when combination rule is matrix multiplication: Identity exists - E Products in group = v C 4 d Inverses in group Transpose matrix; determine co-factor matrix of transposed matrix; divide by determinant of original matrix '

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6 Group representations These matrices obey all rules for a group when combination rule is matrix multiplication: Inverses in group Transpose matrix; determine co-factor matrix of transposed matrix ; divide by determinant of original matrix C 4 transpose co-factor matrix det = 1 3

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7 Group representations These matrices obey all rules for a group when combination rule is matrix multiplication: Inverses in group Transpose matrix; determine co-factor matrix of transposed matrix ; divide by determinant of original matrix C 4 transpose inverse = C 4 All matrices listed show these properties 3

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8 Group representations These matrices obey all rules for a group when combination rule is matrix multiplication: Inverses in group Transpose matrix; determine co-factor matrix of transposed matrix ; divide by determinant of original matrix C 4 transpose inverse = C 4 The matrices represent the group Each individual matrix represents an operation 3

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9 Group representations Set of representation matrices that can be block diagonalized termed a reducible representation Ex: trace = trace = 1

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10 Group representations Set of representation matrices that can be block diagonalized termed a reducible representation Ex: trace = trace = 1 Character of matrix is its trace (sum of diagonal elements)

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11 Group representations Consider the group C 4v Element Matrix E1 0 0all matrices can be block diagonalized - all are reducible C C v v C d d ' '

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12 Irreducible Representations 1. Sum of squares of dimensions d i of the irreducible representations of a group = order of group 2. Sum of squares of characters i in any irreducible representation = order of group 3. Any two irreducible representations are orthogonal (sum of products of characters representing each operation = 0) 4. No. of irreducible representations of group = no. of classes in group (class = set of conjugate elements)

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13 Irreducible Representations Ex: C 2h (E, C 2, i, h ) Each operation constitutes a class C 2 – E -1 C 2 E = C 2 (C 2 ) -1 C 2 C 2 = C 2 i -1 C 2 i = C 2 ( h ) -1 C 2 h = C 2 Other elements behave similarly C 2h

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14 Irreducible Representations Ex: C 2h (E, C 2, i, h ) Each operation constitutes a class Must be 4 irreducible representations Order of group = 4: d d d d 4 2 = 4 All d i = ±1 All i = ±1

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15 Irreducible Representations Ex: C 2h (E, C 2, i, h ) Each operation constitutes a class Thus, must be 4 irreducible representations Order of group = 4: d d d d 4 2 = 4 All d i = ±1 All i = ±1 Let 1 = Array 1 of matrices represents the group – thus exhibits all group props. & has same mult. table E = 1 E -1 = = = 1

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16 Irreducible Representations Ex: C 2h (E, C 2, i, h ) Thus, must be 4 irreducible representations Order of group = 4: d d d d 4 2 = 4 All d i = ±1 All i = ±1 4 representations: E C 2 i h –1 –1 3 1 –1 –1 1 4 1 –1 1 –1

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17 Irreducible Representations Ex: C 2h (E, C 2, i, h ) 4 representations: E C 2 i h –1 –1 3 1 –1 –1 1 4 1 –1 1 –1 These irreducible representations are orthogonal Ex: (-1) + 1 (-1) = 0 E C i h

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18 Irreducible Representations Ex: C 3v ([E], [C 3, C 3 ], [ v, v, v,]) 3 classes, 3 representations: Order of group = 6 Dimensions given by d d d 3 2 = 6 ––> E 2C 3 3 v –1 3 2 –1 0 '“

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19 Irreducible Representations Ex: C 3v ([E], [C 3, C 3 ], [ v, v, v,]) 3 classes, 3 representations: Order of group = 6 Dimensions given by d d d 3 2 = 6 ––> E 2C 3 3 v –1 3 2 –1 0 '“

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20 Irreducible Representations Ex: C 3v ([E], [C 3, C 3 ], [ v, v, v,]) 3 classes, 3 representations: Order of group = 6 Dimensions given by d d d 3 2 = 6 ––> E 2C 3 3 v –1 3 2 –1 0 '“ /2 3/2 - 3/2 -1/2

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21 Irreducible Representations Ex: C 2h (E, C 2, i, h ) C 2h E C 2 i h A g R z B g 1 –1 1 –1 R x R y A u 1 1 –1 –1 z B u 1 –1 –1 1 x y 1-D representations called A (+), B(–) 2-D representations called E 2-D representations called T Subscript 1 - symmetric wrt C 2 perpend to rotation axis g, u – character wrt i

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