Chemistry 125: Lecture 14 October 5, 2009 Checking Hybridization Theory with XH 3 Infrafred and electron spin resonance experiments with three XH 3 molecules confirm our previous theoretical discussion of bonding in terms of hybridization of the central atom. The "Umbrella Vibration" and the associated rehybridization of the central atom is used to illustrate how competition between strong bonds and stable atoms creates differences in molecular structure and discriminates between bonding models. Previous examples of “pathological” bonding are explained, and the BH 3 molecule illustrates how a chemist using localized bonds, vacant atomic orbitals, and unshared pairs to understand molecules slices the molecule’s total electron density pie differently from computational molecular orbitals. For copyright notice see final page of this file

Hybridization Reality Check: Structure and Dynamics of XH 3 BH 3 CH 3 NH 3 valence electrons of X 345

The three X-H Bonds say, “Use 3  sp 2 to maximize overlap” How to Optimize Hybridization of the X Atom in XH 3 ? The X Atom says, “O.K. make 3 bonds, but Maximize s-orbital occupancy” B (3 e - ) N (5 e - ) C (4 e - ) 3  p 2  s2  s (One X-electron in each of 3 bonding AOs; remainder in the 4th AO) 3  sp 2 vacant p Whatever (all sets of four 1-electron valence AOs use up all 100% of 2s)

How to Optimize Hybridization of the X Atom in XH 3 ? The X Atom says, “O.K. make 3 bonds, but Maximize s-orbital occupancy” B (3 e - ) N (5 e - ) C (4 e - ) (One X-electron in each of 3 bonding AOs; remainder in the 4th AO) B (3 e - ) N (5 e - ) C (4 e - ) BH 3 STRONGLY prefers sp 2 bonds (planar) CH 3 Less Strongly prefers sp 2 bonds (planar) NH 3 must compromise sp >2 bonds (pyramidal) 3  sp 2 1  p Whatever The three X-H Bonds say, “Use 3  sp 2 to maximize overlap” 3  sp 2 vacant p 3  p 2  s2  s

Hybridization Reality Check: Structure and Dynamics of XH 3 BH 3 CH 3 NH 3 valence electrons of X 345 Competes with bonds for s-character

BH 3 STRONGLY prefers sp 2 bonds (planar) CH 3 less strongly prefers sp 2 bonds (planar) NH 3 must compromise sp >2 bonds (pyramidal) Are these Predictions True? Experiment: X-Ray? Distortion from plane weakens bonds and deprives electrons of s-character. Distortion from plane weakens bonds and shifts s-character to the lone electron (not wasted). Distortion from plane weakens bonds but shifts s-character from single electrons to a pair of electrons. 2 BH 3  B 2 H 6 2 CH 3  C 2 H 6 Gas IR & ESR Spectroscopy

Infrared: Out-of-Plane Bend X H H H X H H H X H H H X H H H X H H H X H H H X H H H X H H H X H H H This “umbrella” vibration may be treated as a 1-dimensional “Erwin” problem with a fictious “mass” that reflects the amount of motion of the four atoms.

Infrared: Out-of-Plane Bend Weaker Planar Preference Hooke’s Law potential energy adjusted to give proper energy difference Amount of deformation 34.2 Terahertz18.7 THz 34,210,000,000,000 vibrations per sec 1141 cm cm -1 Strong Planar Preference 3.26 kcal/mole 1.73 kcal/mole H H Amount of deformation Stiffer “Spring” H H H H B B

Two closely-spaced absorptions Infrared: Out-of-Plane Bend 932 cm cm cm -1 “Tunnel” Splitting: 1 cm -1 Potential Energy “Inversion” Barrier 3 kcal/mole 2 cal/mole  Ground State Tunneling ~10 11 /sec Umbrella Clock! Not a Hooke’s Law pattern Double Minimum potential energy adjusted to give proper energies 5  sec (kcal) 0 & 1 node 2 & 3 nodes Lect. 9 frame 9

Electron Spin Resonance Spectrum measures s-orbital character of the SOMO electron in CH 3. A line separation due to magnetic interaction between the unpaired electron and the 13 C nucleus occurs only if the electron spends time ON the nucleus, which happens only for s-orbital.

CH 3 SOMO PlanarBent

CH 3 SOMO PlanarBent

Structural Isotope Effect: CH 3 spends more time more bent than CD 3 (thus uses more s-character for SOMO electron) CH 3 38 Gauss  2% s 36 Gauss  less s CD 3 on average

CF 3 Repulsion between F atoms?  Less Bent (flatter) than CH 3 Since Fluorine holds the lion's share of the bonding electron pair, Carbon has less reason to use its valuable s-character in the bonding orbitals. Uses more for the SOMO.  More Bent than CH 3 OR

CF 3 SOMO 271 Gauss!  20% s (vs. 38 for CH 3 )  sp 4

Tension! Differing Goals Computer Chem 125 Student Minimize kinetic plus coulomb energies of electrons and nuclei by “settling down” Minimize total energy using Schrödinger equation with “realistic” constraints Understand structure and reactivity with the simplest “realistic” model Experimental Molecule e.g. limited set of AOs, SCF, some correlation, delocalized MOs e.g. localized bonds, lone pairs; hybridization E-match/overlap HOMO/LUMO Qualitative Insight Useful Predictions of Properties Structure Total e-Density (X-Ray) Energies (IR) Nuclear e-Density (ESR) Dipole Moment, etc. Validation by Experiment Validation by Experiment & Computer

Perspectives: Molecule (Reality) Computer (Approximate Schroedinger) Chemist (Understand Bonds)

Missing Bond ? (e.g. 32 nd of 33 occupied MOs) Cf. Lecture 7 - Dunitz et al. (1981) Experiment: Pathological Bonding Bent Bonds ? Would a Computer’s MOs Provide Understanding? No! Far too complicated to answer “Why?”

But analysis in terms of pairwise bonding overlap of hybrid AOs provides clear explanations. Experiment: Pathological Bonding Missing Bond ? Bent Bonds ? Best Overlap Possible for 60° C-C-C  Very Poor Overlap >90°? p sp 4.1 sp 1.4 Because sp 4.1 extends to give best overlap Why not p orbitals (90°) ? Rehybridizing to strengthen this bond would weaken six others.

Three Views of BH 3 2) Molecular Orbitals 1) Total Electron Density 3) Bonds from Hybrid AOs (Nature) (Computer) (Student) Cf. Course Webpage

B H H H Electron Cloud of by "Spartan"

BH 3 Total e-Density 0.30 e/Å 3 Mostly 1s Core of Boron B H H H

BH 3 Total e-Density 0.15 e/Å 3

BH 3 Total e-Density (0.05 e/Å 3 ) Dimple H atoms take e-density from valence orbitals of B B H + H B

BH 3 Total e-Density 0.02 e/Å 3

BH 3 Total e-Density e/Å 3 van der Waals surface (definition)

BH 3 Total e-Density e/Å 3 Electrostatic Potential Energy of a + probe on the surface low (-) high (+) H  

End of Lecture 14 Oct. 5, 2009 Copyright © J. M. McBride Some rights reserved. Except for cited third-party materials, and those used by visiting speakers, all content is licensed under a Creative Commons License (Attribution-NonCommercial-ShareAlike 3.0).Creative Commons License (Attribution-NonCommercial-ShareAlike 3.0) Use of this content constitutes your acceptance of the noted license and the terms and conditions of use. Materials from Wikimedia Commons are denoted by the symbol. Third party materials may be subject to additional intellectual property notices, information, or restrictions. The following attribution may be used when reusing material that is not identified as third-party content: J. M. McBride, Chem 125. License: Creative Commons BY-NC-SA 3.0