BCH2801 CH5-7 - created from Mind Map

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Molecular Orbital Theory

a method for determining molecular structure in which electrons are not assigned to individual bonds between atoms, but are treated as moving under the influence of the nuclei in the whole molecule

bond order

formation illustration

Origin of color

importance

explain fundamental relationship between electronic and molecular structure

allows predict distribution of electrons in a molecule

help predict molecular properties ( electron configurations, shape, magnetic and ionization properties, relative bond lengths & BO)

indicates strength of the bond

calculation:(e in bonding mo - e in antibonding mo)/2

higher=stronger bond

more e in anti-bonding orbital=weaker bond

not more e=stronger bond

assuming that two electrons in a bonding molecular orbital contribute one net bond and that two electrons in an antibonding molecular orbital cancel the effect of one bond

illustration on H2 molecule

σ bond between 2s/2p

π bond between 2 p

head-on overlap

side-way overlap

list of 4 selection rule

Highly conjugated molecules = double bond>7 / ring>3 (lone pair=1)

help of some simple molecular orbital diagram

n to π*

n to σ*

π to π*

(σ to σ*)

possible HOMO to LUMO electronic transition

conjugation HIGHER, ground and excited states of electrons CLOSER in energy

energy difference LOWER=aborb LOWER energy, absorption maximum wavelength LONGER (energy and wavelength have an inverse relationship)

colored when molecule absorbs light at a wavelength in visible light spectrum (400-700 nm)

we see complementary color

X-rays elemental analysis

metal

crystalline

XRF - Fluorescence

Principle

Instrumentation

Sample preparation

Pro

Con

percentage of metal

Higher-energy e drop into "lower level vacancies created in ionization" (by exposed to high-energy, short wavelength radiation (X-rays))

Loss in energy in "transition between specific electron orbitals" release in form of emission of characteristic X-rays (fluorescent radiation) indicating the type of atom present (intensity = abundance)

energy-dispersive system

wavelength-dispersive system

measure all element simultaneously>what element

faster, more sensitive, lower resolution

slower, less sensitive, higher resolution

increase accuracy for interested element

liquid

solid

gas

split plating: uniform layer

press grounded powders into “pucks” using consistent pressure and dies (pellet press)

capture dust particulates on filter paper in gas tube

1. non-destructive

2. little or no sample preparation

3. major and trace element analysis

4. ppm to ppb

1. many overlap element peak

2. difficult to quantify light element (Z

3. cannot distinguish variations among isotopes of an element

4. quantitative limit is 10 to 20 ppm

XRD - Powder Diffraction

Principle

Pro

Con

Instrumentation

Sample preparation

element and molecular structure

atom in crystalline solid arrange in "orderly repeating pattern extending in three dimensions"

act as diffraction grating for x-way

at what angle (2θ) have constructive interfere which produce diffraction ray

using Bragg's Law: 2dsinθ=nλ

unique set of value of d (spacing between layers)

same structure, same diffraction pattern

fingerprint packing characteristic in crystal structure

Reasons of can distinguish diamond from quartz

1. firm mineral determination

2. little or no sample preparation

2. ground into powder if want accuracy

1. not for amorphous material

3. mixed material detection limit is ~2% of sample, trace amount often go undetected

4. need standard reference database JCPDS (Joint Committe on Powder Diffraction Standard)

X-ray diffractometer

source of radiation+monochromator+slits to adjust the shape of the beam+detector

scanning sample through range of 2θ angle

attain all diffraction direction

1. ground well in a mortar and pestle or a ball mill

2. spread 10-20mg material on plastic tape/glass in square hollow space of aluminum sample holder

create uniform particle size

ensure present all possible crystallite orientations randomly

amorphous so no interfere

Separate & identify (&quantify) each component

*Chromatography (20m)

Thin layer TLC

Gas GC

High-performance liquid HPLC

stationary phase: solid adsorbent material - particles of hydrated silica(SiO2) or alumina(Al2O3)

Main principle

Instrumentation

Analysis parameter

separation is obtain when analytes have difference adsorptivity to a solid stationary phase (TLC plate)

Retention factor (Rf)

larger value, lower interaction between compound & stationart phase

value specific for a compound under constant chromatographic conditions

Distance moved by spot/ distance moved by solvent front

In reality, value vary slightly from sample to sample. Interactions & concentration of component in sample will affect the value

similar value suggest that samples could be the same, but is not proof

corroborative evidence for identification

Main principle

Instrumentation

Analysis parameter

volatile mixture

separation is obtain when analytes have difference adsorptivity / partition degree to a solid(GSC) / liquid(GLC) stationary phase (packed column)

and detected at the exit of column by a flow-through detector that measures components amount

Chromatogram

retention time

height and peak area

plot of detector response with time

time a component takes from injection to reach detector

always constant under same chromatographic condition

by comparing to standard (done by finding response for pure substance)

component identity (qualitative)

proportional to concentration of analyte

a peak = a separated component

by comparing to calibration curve (done by finding response for a series of conc of analyte)

component relative concentrations (quantitative)

Main principle

Instrumentation

non-volatile / thermally-unstable mixture

separation is obtain when analyte carried by mobile phase (pressurized liquid by pump) have difference partition degree with to a solid stationary phase (packed column)

6Q 3hr ALWAYS USE diagram

Answer Q with confident first

PT FORM!

BCH2801 CH5-7