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A 1 + A 2 + A 3 + A 4

VIEWS: 6 PAGES: 22

									                                                              C 2004 Barry Linkletter, UPEI




                                     Lecture 2

                            Describing molecular structures
                               using quantum chemistry




Chemistry 243 - Lecture 1                                                                1
                                                 C 2004 Barry Linkletter, UPEI




                                       Methane
             • Major component of
               natural gas
             • Simplest hydrocarbon
             • CH4

                     – Draw Lewis and Kekulé
                       structures for methane
                     – How can we draw the
                       molecular bonding
                       orbitals?




Chemistry 243 - Lecture 1                                                   2
                                                                    C 2004 Barry Linkletter, UPEI




                             Methane Structure

             • VESPR theory predicts a tetrahedral
               structure. Why?
                     – Experimental evidence confirms this stucture.
             • Molecular orbitals are mathematical
               functions dependant on the exact position
               of nuclei.
                     – We are not computers, so there is a simple
                       method for estimating shapes of simple
                       molecular orbitals in simple molecules.



Chemistry 243 - Lecture 1                                                                      3
                                                  C 2004 Barry Linkletter, UPEI




                                Orbitals Needed

             • Carbon
                     – 2S, 2Px, Py and Pz




             • 4 Hydrogens
                     – 4 x 1S




Chemistry 243 - Lecture 1                                                    4
                                                 C 2004 Barry Linkletter, UPEI




                            Geometry Conundrum

             • The H atoms are
               104 apart
             • The p-orbitals are
               90 apart
             • How can I avoid
               trigonometry?
                     – Answer: Do all
                       trigonometry in
                       advance.



Chemistry 243 - Lecture 1                                                   5
                                                     C 2004 Barry Linkletter, UPEI




                            Dependency on Angle

             • The 1S orbital of
               the hydrogen with
               overlap with the
               2S and the 2Px,y,z
               orbitals of carbon


             • Bonding Orbital               AX = Cos(fx)
               is…


Chemistry 243 - Lecture 1                                                       6
                                                  C 2004 Barry Linkletter, UPEI




                            Dependency on Angle
        Orbital combination is
        dependant on the angle of
        the two orbitals w.r.t. their
        axes of mutual symmetry



          Cosine (0) = 1 or maximum

          Cosine (90) = 0 or minimum

          Cosine (some other angle) =
          some number between 1 and
          0 (in between)




Chemistry 243 - Lecture 1                                                    7
                                                                             C 2004 Barry Linkletter, UPEI




                                That’s a Lot of Math…

             • Lets simplify.
                     – Let us combine all the terms for the carbon
                       atomic orbitals in advance.
                     – We will create a new orbital from the
                       combination of fractions of the 4 atomic
                       orbitals in play.
                     – This new atomic orbital will be called an SP3
                       orbital
                            • Made from fractions of an S and 3 P orbitals




Chemistry 243 - Lecture 1                                                                               8
                                                       C 2004 Barry Linkletter, UPEI




                            Combining an SP3 Orbital




           A1                + A2       + A3    + A4


                                    =

Chemistry 243 - Lecture 1                                                         9
                                                         C 2004 Barry Linkletter, UPEI




                                       The SP3 Orbital
             • 4 atomic orbitals
                     – S, Px ,Py ,Pz
             • Combine to make 4
               hybrid atomic orbitals
                     – 4 x SP3
             • Each SP3 alignes with
               axes of C-H covalent
               bonds
                     – We created them for
                       that very purpose




Chemistry 243 - Lecture 1                                                          10
                                                    C 2004 Barry Linkletter, UPEI




                            Hybridization Summary




Chemistry 243 - Lecture 1                                                     11
                                                       C 2004 Barry Linkletter, UPEI




                            Addition and Subtraction




Chemistry 243 - Lecture 1                                                        12
                                                  C 2004 Barry Linkletter, UPEI




                            Orbital Combination




Chemistry 243 - Lecture 1                                                   13
                                                               C 2004 Barry Linkletter, UPEI




                                Combining Orbitals

                                   C-H antibonding MOs




                                                         Hydrogen
                                                         1S AOs
                 Carbon
                 S and Px,y,z
                 AOs
                                   C-H bonding MOs

Chemistry 243 - Lecture 1                                                                14
                                                                  C 2004 Barry Linkletter, UPEI




                                 Combining Orbitals

                                      C-H antibonding MOs




               Carbon                                       Hydrogen
               SP3 AOs                                      1S AOs

           •Exercise
                    –Fill the MOs
                     with electrons
                                      C-H bonding MOs

Chemistry 243 - Lecture 1                                                                   15
                                                                                  C 2004 Barry Linkletter, UPEI




                                                 Exercise
             • Draw the molecular orbital
               diagram for acetlyene (C2H2)
                     – Sketch all the atomic orbitals
                       that you would be combining to
                       make bonding molecular orbitals
                     – Sketch all the molecular
                       bonding orbitals created from
                       the combinations of the above
                     – Sketch the antibonding orbitals
                     – Bonus. - Rank the orbitals in
                       energy
                            • Hint:  molecular orbitals will   • Hand in your work at the
                              be the lowest and highest           start of the next class
                              energy orbitals.  molecular        period
                              orbitals will be in between
                              these two extremes.




Chemistry 243 - Lecture 1                                                                                   16
                                                    C 2004 Barry Linkletter, UPEI




                            The C-C Covalent Bond




Chemistry 243 - Lecture 1                                                     17
                                                    C 2004 Barry Linkletter, UPEI




                            The C-H Covalent Bond




Chemistry 243 - Lecture 1                                                     18
                                                          C 2004 Barry Linkletter, UPEI




                            The C-O Polar Covalent Bond




Chemistry 243 - Lecture 1                                                           19
                                               C 2004 Barry Linkletter, UPEI




                            Consider Ethanol
             • Red - More e density
             • Blue - Less e density

             • Observe the C-C, C-O
               and C-H bonds
             • Is the O-H bond
               polar? Why or why
               not? Is it more or less
               polar than the C-O
               bond?



Chemistry 243 - Lecture 1                                                20
                                                   C 2004 Barry Linkletter, UPEI




                            The Na-Cl Ionic Bond




Chemistry 243 - Lecture 1                                                    21
                                                    C 2004 Barry Linkletter, UPEI




                            Ionic Bonds

             • Bonds with little orbital overlap
             • Very little stabilization from sharing
               electrons
             • Stabilization comes from electrostatic
               attraction
             • Occur between atoms of greatly different
               electronegativity




Chemistry 243 - Lecture 1                                                     22

								
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