Molecular Geometry and Point Groups

Symmetry Operations and Elements • The goal for this section of the course is to understand how symmetry arguments can be applied to solve physical problems of chemical interest. • To achieve this goal we must identify and catalogue the complete symmetry of a system and subsequently employ the mathematics of groups to simplify and solve the physical problem in question. • A symmetry element is an imaginary geometrical construct about which a symmetry operation is performed. performed • A symmetry operation is a movement of an object about a symmetry element such that the object's orientation and position before and after the operation are indistinguishable. • A symmetry t operation ti carries i every point i t in i the th object bj t into i t an equivalent i l t point i t or the th identical point. Point Group Symmetry • All symmetry elements of a molecule pass through a central point within the molecule. • The symmetry of a molecule or ion can be described in terms of the complete collection of symmetry operations it possesses. • The total number of operations may be as few as one or as many as infinity. The more symmetry operations a molecule has, the higher its symmetry is. • Regardless of the number of operations, all will be examples of only five types. The Identity Operation (E) • The simplest of all symmetry operations is identity, given the symbol E. • Every object possesses identity. If it possesses no other symmetry, the object is said to be asymmetric. • As an operation, identity does nothing to the molecule. It exists for every object, because the object itself exists. • The need for such an operation arises from the mathematical requirements of group theory. • In addition, addition identity is often the result of carrying out a particular operation successively a certain number of times, i.e., if you keep doing the same operation repeatedly, eventually you may bring the object back to the identical (not simply equivalent) orientation from which was started. • When identifying the result of multiple or compound symmetry operations they are designated by their most direct single equivalent. • Thus, if a series of repeated operations carries the object back to its starting point, the result would be identified simply as identity. identity The Rotation Operation (C) • The operation of rotation is designated by the symbol Cn . • If a molecule has rotational symmetry Cn , rotation by 2/n = 360°/n brings the object into an equivalent position. • The value of n is the order of an n‐fold rotation. • If the molecule has one or more rotational axes, the one with the highest value of n is the principal axis of rotation. • SSuccessive i C4 clockwise l k i rotations t ti off a planar l MX4 molecule about an axis perpendicular to the plane of the molecule (XA = XB = XC = XD). • Multiple iterations are designated by a superscript, e.g. three successive C4 rotations are identified as C43 • The C42 and C44 operations are preferably identified as the simpler C2 and E operations, operations respectively. respectively • There are four other C2 axes in the place of the molecule. • The C2' and C2" axes of a planar MX4 molecule. molecule • As these twofold axes are not collinear with the principal C4 rotational axis they are distinguished by adding prime (‘) and double prime (‘’) to their symbols. • Only two notations are needed for the four axes, axes because both C2’ axes are said to belong to the same class, while the two C2’’ axes belong to a separate class. i.e., both C2’ axes are geometrically equivalent to each other and distinct from C2’’ . • In listing the complete set of symmetry operations for a molecule, operations of the same class are designated by a single notation preceded by a coefficient indicating the number of equivalent operations comprising the class. e.g. for the square planar structure here discussed of D4h symmetry, discussed, symmetry the rotational operations grouped by class are 2C4 (C4 and C43), C2 (collinear with C4) 2C2’ , and d 2C2’’ . The C2' and C2" axes of a planar MX4 molecule. General Relationships for Cn C nn = E C2nn = C2 (n = 2, 4, 6, 8…etc.) Cnm = Cn/m (n/m = 2, 3, 4, 5…etc.) Cnn‐1 = Cn‐1 Cnn+m = Cnm • Every n‐fold rotational axis has n–1 associated operations (excluding Cnn = E). • Remember, the rotational operation Cnm operation where m/n is an integer value. (m < n) is preferably identified as the simpler Cn/m The Reflection Operation (σ) • The operation of reflection defines bilateral symmetry about a plane, called a mirror plane or reflection plane. • For every point a distance r along a normal to a mirror plane there exists an equivalent point at –r. Two points, equidistant from a mirror plane σ, related by reflection. • For a point (x,y,z), reflection across a mirror plane σxy takes the point into (x,y,–z). • Each mirror plane has only one operation associated with it, since σ2 = E. Horizontal, Vertical, and Dihedral Mirror Planes • A h plane is defined as perpendicular to the principal axis of rotation. • If no principal axis of rotation exists, h is defined as the plane of the molecule. • v and d planes are defined so as to contain a principal axis of rotation and to be perpendicular to a h plane. plane • When both v and d planes occur in the same system, the distinction between the types is made by defining v to contain the greater number of atoms or to contain a principal axis of a reference Cartesian coordinate system (x or y axis). • Any d planes typically will contain bond angle bisectors. • The five mirror planes of a square planar molecule MX4 are grouped into three classes (h , 2v , 2d ). The Inversion Operation ( i ) • The operation of inversion is defined relative to the central point within the molecule, through which all symmetry elements must pass, e g typically the origin of the Cartesian coordinate system (x,y,z e.g., (x y z = 0,0,0). 0 0 0) • If inversion symmetry exists, for every point (x,y,z) there is an equivalent point (–x,–y,–z). • Molecules or ions that have inversion symmetry are said to be centrosymmetric. • Each inversion center has only one operation associated with it, since i 2 = E. Effect of inversion (i) on an octahedral MX6 molecule (XA = XB = XC = XD = XE = XF). Inversion Center of Staggered Ethane • Ethane in the staggered configuration. The inversion center is at the midpoint along the C‐C y g atoms related byy inversion are connected byy dotted lines,, which intersect at bond. Hydrogen the inversion center. The two carbon atoms are also related by inversion. The Improper Rotation Operation (Sn) • The improper rotation operation Sn is also known as the rotation‐reflection operation and, as its name suggests, is a compound operation. • Rotation‐reflection consists of a proper rotation followed by reflection in a plane perpendicular to the axis of rotation. • n refers to the improper rotation by 2 / n = 360° / n. • Sn exists i t if the th movements t Cn followed f ll d by b σh (or ( vice i versa)) bring b i the th object bj t to t an equivalent i l t position. • If both Cn and σh exist, then Sn must exist. e.g., S4 collinear with C4 in planar MX4. • Neither Cn nor σh need exist for Sn to exist. e.g., S4 collinear with C2 in tetrahedral MX4. A tetrahedral MX4 molecule inscribed in a cube. A C2 axis, collinear with an S4 axis, passes through the centers of each pair of opposite cube faces and through the center of the molecule. i.e., each axis bisects one of the M‐X bonds. S4 improper rotation of a tetrahedral MX4 molecule (XA = XB = XC = XD). ) The improper axis is perpendicular to the page. Rotation is arbitrarily taken in a clockwise direction. Note that neither C4 nor σh are genuine symmetry operations of tetrahedral MX4. • Successive S4 operations on a tetrahedral MX4 molecule (XA = XB = XC = XD). ) • Rotations are clockwise, except S4‐1 , which is equivalent to the clockwise operation S43. • Successively carrying out two S4 operations is identical to the result of a single C2 operation about the same axis i.e.,, S42 = C2 • Similarly, S44 = E . • Thus, there are only two operations belonging to this class for the tetrahedral MX4 molecule (S4 and S43 ) about this axis. • In the highly symmetric tetrahedral system there are three equivalent and indistinguishable S4 axes. • Consequently, each axes gives rises to two S4 operations resulting in a class designated as 6S4 (3S4 + 3S43 ). ) Non‐Genuine Sn Operations: • S1 = σh • S2 = i General Relations of Sn • Equivalences of successive Sn operations:  If n is even, Snn = E  If n is odd, Snn = σ and Sn2n = E m n when  If m is i even, Snm = Cnm = Cn/m when h m < n and d Snm = Cnm–n h m>n  If Sn with even n exists, then Cn/2 exists  If Sn with odd n exists, then both Cn and σ perpendicular to Cn exist. Examples • Find all symmetry elements and operations in the following: Defining the Coordinate System • Molecules are conventionally oriented relative to a right‐hand Cartesian coordinate system: • The following conventions of axis orientation are usually observed: 1. The origin of the coordinate system is located at the central atom or the center of the molecule. 2. The z axis is collinear with the highest‐order rotational axis (the principal axis). If there are several highest order rotational axes, z is usually taken as the axis passing through the greatest number of atoms. However, for a tetrahedral molecule, the x, y, and z axes are defined as collinear with the three C2 axes (collinear with the three S4 axes). Defining the Coordinate System (contd.) 3. For planar molecules, if the z axis as defined above is perpendicular to the molecular plane, the x axis lies in the plane of the molecule and passes through the greatest number of atoms. If the z axis lies in the plane of the molecule, then the x axis stands perpendicular to the plane. Defining the Coordinate System (contd.) 4. For non‐planar molecules, once the z axis has been defined, the x axis is usually chosen so that the xz plane contains as many atoms as possible. If there are two or more such planes containingg identical sets of atoms,, anyy one mayy be taken as the xz p p plane. Where a decision about the orientation of the x axis cannot be made on this basis, the distinction between x and y is usually not important or is not generally fixed by convention. Combining Symmetry Operations (Multiplication) • Multiplication of symmetry operations is the successive performance of two or more operations to achieve an orientation that could be reached by a single operation e.g., i 2 = E ; S4 S4 = S42 = C2 ; C4 h = S4 etc. • The order in which successive different symmetry operations are performed can affect the result. • Multiplication of symmetry operations is not commutative in general, although certain combinations may be. • In writing multiplications of symmetry operation we use a "right‐to‐left" notation: •  BA = X "Doing A then B has the same result as the operation X."  We cannot assume that reversing the order will have the same result.  It may be that either BA ≠ AB or BA = AB. AB Multiplication of symmetry operations is associative: C(BA) = (CB)A The order of performing S4 and σv , shown here for a tetrahedral MX4 molecule, molecule affects the result. result The final positions in each case are not the same, but they are related to each other by C2 . S4 σv ≠ σv S4 but C2σv S4 = S4 σv • We will now consider the complete set of symmetry operations for a particular molecule and determine all the binary combinations of the symmetry operations it possesses. • The symmetry elements of the CBr2Cl2 molecule are shown below. This molecule has a tetrahedral geometry Note: tetrahedral geometry does not automatically imply tetrahedral symmetry ! • The complete set of symmetry operations are E, C2 , v , v‘ Matrix Notation of the Effects of the Operations • Rather than depict the effect of each operation on the molecule, let us introduce a column matrix notation to indicate the positions of atoms before and after each operation. • As the carbon atom is unaffected by any symmetry operation all matrices need only describe the positions of the bromine and chlorine atoms using a 1x4 column matrix. matrix • The symbols [E], [C2], v and [v’] represent operator matrices. Multiplication Tables • All possible binary combinations of symmetry operations can be summarized in a multiplication table. • Combination order is "top" then "side"; e.g., Multiplication Table for the Operations of CBr2Cl2 • Now let us consider the results for binary combinations of these operations. For this process we can start to build a multiplication table. • We will begin with combinations including the identity operation. operation • Followed by self combinations Step 1: Combinations with identity. • Step 2: Binary self‐combinations. Th we have Thus h th following the f ll i relationships: l ti hi C2E = EC2 = C2 vE = Ev = v v‘E = Ev‘ = v‘ Step 3: Mixed binary combinations. • Let us consider the combination of C2 σv . • First we perform the v operation followed by the C2 operation. • This result is the same as that achieved by σv' alone: • Thus, we can write the following relationship: C2 σv = σv' Complete Multiplication Table • General Features:  The first row of results duplicates the list of operations in the header row.  The first column of results duplicates the list of operations in the label column.  Every row shows every operation once and only once.  Every column shows every operation once and only once.  The order of resultant operations in every row is different from any other row.  The order of resultant operations in every column is different from any other column. Symmetry Point Groups • The complete set of symmetry operations (not symmetry elements) for a molecule or ion satisfies the requirements of a mathematical group. • y y operations p are the elements of a ggroup p and define the symmetry y yp point These symmetry group for that molecule or ion. • The total number of symmetry operations comprising the group is the order of the group, h. • The group formed by the operations of CBr2Cl2 is named C2v where h = 4. 4 • The group of symmetry operations must satisfy the four requirements of a mathematical group, i.e., closure, identity, associativity, and reciprocality. Requirements of a Mathematical Group (1) • Closure: If A and B are in the group G, and AB = X, then X is also in G.  All groups have a self‐contained multiplication table, whose products are members of the group.  For example, inspection of the multiplication table for the operations of CBr2Cl2 shows that all binary combinations equal either E, C2 , v or v‘ .  These four symmetry operations constitute the complete set of elements of a point group called C2v , the order of which is four. Requirements of a Mathematical Group (2) • Identity: In any group G, there is an element E, such that EX = XE = X The symmetry operation of identity is this group element.  This requirement explains the need to define the symmetry operation of identity, which functions at the identity element for every symmetry group. group  As the C2v multiplication table demonstrates, the identity operation does indeed meet the requirements of the identity element of a group. Requirements of a Mathematical Group (3) • Associativity: The associative law of combination is valid for all combinations of elements of the group. If A, B, C, and X are in G, then C(BA) ( ) = X = (CB)A ( ) But commutation is not general (e.g., BAC ≠ CBA).  For example, in the C2v point group C2 vv‘ ) = (C2v )v‘ for the first combination we see C2 vv‘) = C2C2 = E for the second combination we see (C2v )v‘ = v‘v‘ = E  Groups in which all elements do commute are called Abelian (e.g., C2v). Requirements of a Mathematical Group (4) • Reciprocality: In any group G, every element A has an inverse A‐1, such that AA–1 = A–1A = E An element may be its own inverse (e.g., all operations of C2v).  For the C2v point group every symmetry operation is its own inverse. Subgroups • Within all groups there are smaller collections of elements, called subgroups, which also obey the criteria for a group. note: one exception is the trivial asymmetric group, group C1 = {E} • The order of any subgroup, g, relative to the order of its parent group, h, must be h/g = n ( n = 1,2,3,4,….etc.) • Not every allowed value of g is always represented among a group’s subgroups. • For the C2v point group (h = 4) subgroups with g = 1, 2 are possible and each exist. • From the multiplication table of C2v we can identify the following subgroups Point Groups of Molecules • Chemists in general and spectroscopists in particular use the Schönflies notation. • In contrast , crystallographers prefer to use the Hermann‐Mauguin notation, which is best suited for designating the 32 crystallographic point groups and the space groups used to describe crystal structures. • Familiar Schönflies labels and their corresponding Hermann‐Mauguin notation are • All of the chemically important point groups fall within one of four general categories: 1. Non‐rotational 2. Single‐axis rotational 3. Dihedral 4. Cubic Non‐Rotational Point Groups • With their low orders (h = 1,2) and lack of an axis of symmetry, the non‐rotational point groups represent the lowest symmetry point groups.  C1 is the point group of asymmetric molecules which only possess the identity element E.  The Cs point group describes the symmetry of bilateral objects that lack any symmetry other than E and h .  The Ci point group is not commonly encountered as most molecules which posses the i element also possess other complimentary symmetry elements. Single‐Axis Rotational Point Groups • The simplest family of this group are the Cn point groups, which consist of operations generated by an n‐fold rotation Cn applied successively n times. • point ggroups p are an example p of the important p cyclic y groups. g p These p • A cyclic group of order h is generated by taking a single element X through all its powers up to Xh = E. G = { X, X2 , ... , Xh = E } • All cyclic groups are Abelian, since all of their multiplications commute. • The Cn and S2n groups are cyclic groups; e.g., C4 = { C4 , C42 , C43 , E } S4 = { S4, C2 , S43 , E } • The multiplication tables of cyclic groups "scroll" from row to row and column to column: e.g., • To the rotations of the corresponding Cn groups the family of Cnv groups adds n vertical mirror planes which intersect at the Cn axis. planes, axis • The point group Cv , which has a infinite‐fold C rotational axis, is an important member of this family. It is the point group of all non‐centrosymmetric linear molecules. e g H‐Cl, e.g., H Cl CO. C O • To generate any of the Cnh groups, we need only add a horizontal mirror plane to the series of Cn rotations of the appropriate cyclic Cn group. • Since Cnh = Sn and C2h = S2 = i , these groups also have n‐fold improper axes when n > 2, and they are centrosymmetric when n is even. • The S2n series are not common. Dihedral Point Groups • The dihedral groups have n twofold axes perpendicular to the principal n‐fold axis. These C2 axes are called the dihedral axes. • The number and arrangement of the dihedral axes are dictated by the n‐fold order of the principal axis. axis e.g. the staggered conformation of ethane is of D3d symmetry and possesses 3C2 dihedral axes. • There are three families of dihedral groups: Dn , Dnd , Dnh 1) The Dn groups may be thought of as Cn groups to which n dihedral C2 operations have been added. Unlike the Cn groups, the Dn groups are not cyclic. 2) Similarly Similarly, the Dnd groups may be thought of as Cnv groups to which n dihedral C2 operations have been added. In Dnd groups, the combination of rotational operations and vertical mirror reflections (d) ggenerates a series of S2n operations p about an axis collinear with the p principal p axis. 3) The Dnh groups may be thought of as Cnh groups to which n dihedral C2 operations have been added. Like the Cnh groups, groups the Dnh groups include n‐fold improper axis when n>2 and are centrosymmetric. Cubic Point Groups • The cubic groups are associated with polyhedra that are geometrically related to the cube. • All are characterized by the presence of multiple, intersecting, high‐order rotational axes. • p of this type, yp , three of which are frequently q y encountered and highly g y There are seven ggroups relevant in chemistry Cube (Oh) Tetrahedron (T d ) Octahedron (Oh) Icosahedron (I h) • The perfect tetrahedron defines the Td group, comprised of the following 24 operations, listed b classes: by l E , 8C3 (= 4C3 , 4C32 ), 3C2 , 6S4 (= 3S4 , 3S43 ), 6d with h = 24 , Td represents one of the higher symmetries encountered in chemistry. • A three‐fold axis, generating the operations C3 and C32 , emerges from each of the four triangular faces of a tetrahedron. • pp g When a tetrahedron is inscribed inside a cube a C2 axis collinear with the bisector of opposing bond angles emerges from each pair of apposite cube faces. • Three S4 axes, each associated with S4 and S43 operations, are each collinear with these C2 axes. Tetrahedron (T d ) • The octahedron and cube both belong to the point group Oh , which is comprised of the following 48 operations (h = 48) E , 8C3(= 4C3 , 4C32 ), 6C4(= 3C4 , 3C43 ), 6C2 , 3C2(= 3C42), i , 6S4(= 3S4 , 3S43 ), 8S6(= 4S6 , 4S65 ), h(= xy , yz , xz), 6d • In the octahedron a fourfold axis emerges from each pair of opposite apices, whereas a threefold h f ld axis i emerges from f each h pair i off opposite i triangular i l faces. f • In the cube, a fourfold axis emerges from each pair of opposite faces, whereas a threefold axis emerges from each pair of opposite corners, corners extending the diagonals of the cube. cube Cube (Oh) • Both the regular icosahedron and dodecahedron belong to the point group Ih , composed of 120 symmetry operations E , 8C3(= 4C3 , 4C32 ), 6C4(= 3C4 , 3C43 ), 6C2 , 3C2(= 3C42), i , 6S4(= 3S4 , 3S43 ), 8S6(= 4S6 , 4S65 ), h((= xy , yz , xz), 6d • Aside from the Cv and Dh point groups which have an order of h = , Ih represents the highest symmetry one is likely to encounter in structural chemistry. • Buckminsterfullerene C60 is an example of a high‐order polyhedron with Ih symmetry. • A fivefold axis emerges from the face of each five‐membered ring and a threefold axis emerges from the face of each six‐membered ring. Icosahedron (I h) Ih Flow chart for systematically determining the point group of a molecule. Examples for point group classification.